Publications

2021
Mantela M, Morphis A, Lambropoulos K, Simserides C, Felice RD. Effects of Structural Dynamics on Charge Carrier Transfer in B-DNA: A Combined MD and RT-TDDFT Study. The Journal of Physical Chemistry B [Internet]. 2021;125(16):3986-4003. Publisher's VersionAbstract
Hole transfer along the axis of duplex DNA has been the focus of physical chemistry research for decades, with implications in diversefields, from nanotechnology to cell oxidative damage. Computational approaches are particularly amenable for this problem,to complement experimental data for interpretation of transfer mechanisms. To be predictive, computational results need to account for the inherent mobility of biological molecules during the time frame of experimental measurements. Here, we address the structural variability of B-DNA and its effects on hole transfer in a combined molecular dynamics (MD) and real-time time-dependent density functional theory (RT-TDDFT) study. Our results show that quantities that characterize the charge transfer process, such as the time-dependent dipole moment and hole population at a specific site, are sensitive to structural changes that occur on the nanosecond time scale. We extend the range of physical properties for which such a correlation has been observed, further establishing the fact that quantitative computational data on charge transfer properties should include statistical averages.Furthermore, we use the RT-TDDFT results to assess an efficient tight-binding method suitable for high-throughput predictions. We demonstrate that charge transfer, although affected by structural variability, on average, remains strong in AA and GG dimers.
2020
Lambropoulos K, Chatzieleftheriou M, Morphis A, Kaklamanis K, Lopp R, Theodorakou M, Tassi M, Simserides C. Erratum: Electronic structure and carrier transfer in B-DNA monomer polymers and dimer polymers: Stationary and time-dependent aspects of a wire model versus an extended ladder model (Physical Review E (2016) 94 (062403) DOI: 10.1103/PhysRevE.94.062403). Physical Review E [Internet]. 2020;102:019901. Publisher's VersionAbstract
This paper was published online on 12 December 2016 with an incorrect panel in Fig. 2. Figure 2 has been replaced as of 20 July 2020. The figure is incorrect in the printed version of the journal.
Simserides C, Morphis A, Lambropoulos K. Hole Transfer in Cumulenic and Polyynic Carbynes. Journal of Physical Chemistry C [Internet]. 2020;124:12834-12849. Publisher's VersionAbstract
We study hole transfer in open cumulenic and polyynic nanowires made of N carbon atoms, using real-time time-dependent density functional theory (RT-TDDFT) and tight-binding (TB) wire models. For RT-TDDFT, we mainly use functional B3LYP and basis sets cc-pVDZ, cc-pVTZ, and cc-pVQZ, obtaining clear convergence; cc-pVTZ is the smallest basis set of sufficient quality; cc-pVQZ is better with a higher computational cost. For TB, we use a simplistic wire model where all sites are equivalent (TBI) and models with modified initial and final sites, mimicking the existence of one or two or three hydrogens at edge sites (TBImod, TBImodt4times). We compare the ground state energy, EGS, obtained by density functional theory (DFT) for cumulenic molecules with coplanar (co) or perpendicular (pe) methylene end groups as well as polyynic molecules starting with short (sl) or with long (ls) C–C bonds. For odd N, cumulenic pe molecules have lower EGS than cumulenic co molecules, that are probably transition states. We examine energy spectra, density of states, energy gap, charge oscillations, mean over time probabilities to find the hole at each site, coherent transfer rates, electric dipole moment, and frequency content. DFT shows that due to the impact of end groups, there exists a cumulenic energy gap, smaller than the polyynic one. TBI and TBImod reproduce approximately the magnitude of the energy gap in the polyynic case at the limit of large N. TBImod is capable of predicting the same site occupations with the nicely converged RT-TDDFT ones for the cumulenic case. However, charge and dipole moment oscillations as well as transfer rates by RT-TDDFT are approximately four times faster than those by TBImod. The site occupations of polyynic sl and of polyynic ls molecules are modified relative to cumulenic molecules; the trends can be explained qualitatively.
Simserides C, Morphis A, Lambropoulos K. Hole transfer in open carbynes. Materials [Internet]. 2020;13:3979. Publisher's VersionAbstract
We investigate hole transfer in open carbynes, i.e., carbon atomic nanowires, using Real-Time Time-Dependent Density Functional Theory (RT-TDDFT). The nanowire is made of N carbon atoms. We use the functional B3LYP and the basis sets 3-21G, 6-31G*, cc-pVDZ, cc-pVTZ, cc-pVQZ. We also utilize a few Tight-Binding (TB) wire models, a very simple model with all sites equivalent and transfer integrals given by the Harrison ppπ">ppπ expression (TBI) as well as a model with modified initial and final sites (TBImod) to take into account the presence of one or two or three hydrogen atoms at the edge sites. To achieve similar site occupations in cumulenes with those obtained by converged RT-TDDFT, TBImod is sufficient. However, to achieve similar frequency content of charge and dipole moment oscillations and similar coherent transfer rates, the TBImod transfer integrals have to be multiplied by a factor of four (TBImodt4times). An explanation for this is given. Full geometry optimization at the B3LYP/6-31G* level of theory shows that in cumulenes bond length alternation (BLA) is not strictly zero and is not constant, although it is symmetrical relative to the molecule center. BLA in cumulenic cases is much smaller than in polyynic cases, so, although not strictly, the separation to cumulenes and polyynes, approximately, holds. Vibrational analysis confirms that for N even all cumulenes with coplanar methylene end groups are stable, for N odd all cumulenes with perpendicular methylene end groups are stable, and the number of hydrogen atoms at the end groups is clearly seen in all cumulenic and polyynic cases. We calculate and discuss the Density Functional Theory (DFT) ground state energy of neutral molecules, the CDFT (Constrained DFT) “ground state energy” of molecules with a hole at one end group, energy spectra, density of states, energy gap, charge and dipole moment oscillations, mean over time probabilities to find the hole at each site, coherent transfer rates, and frequency content, in general. We also compare RT-TDDFT with TB results.
2019
Mantela M, Lambropoulos K, Bilia P, Morphis A, Simserides C, Vantaraki C. Frequency Content of Carrier Oscillations along B-DNA Aperiodic and Periodic Polymers. In: Progress in Electromagnetics Research Symposium. Vol. 2019-June. ; 2019. pp. 831-838. Publisher's VersionAbstract
We study the frequency content of an extra carrier oscillation along B-DNA aperiodic and periodic polymers and oligomers made of N monomers. In our work, we employ two variants of the Tight-Binding (TB) approach: a wire model and an extended ladder model including diagonal hoppings, as well as Real-Time Time-Dependent Density Functional Theory (RT-TDDFT). In the wire model, the site is a monomer, i.e., a base pair, while, in the extended ladder model, the site is a base. Initially, we focus on the Fourier Spectra of the probabilities to find the extra carrier at each monomer, having placed it at time zero at a specific monomer. We define the weighted mean frequency (WMF) of each site, a measure of its frequency content, using as weight the Fourier amplitude of each component of its frequency spectrum. The large-N limits of the WMFs are constants in the THz domain. To obtain a measure of the overall frequency content of carrier oscillations in the polymer, we define the total weighted mean frequency (TWMF), averaging the WMFs of all sites weighting over the mean over time probabilities of finding the extra carrier at each site. The large-N limit of the TWMFs are also constants in the THz domain. Generally, the frequency content of coherent carrier oscillations along B-DNA aperiodic and periodic polymers is in the THz domain.
Lambropoulos K, Simserides C. Periodic, quasiperiodic, fractal, Kolakoski, and random binary polymers: Energy structure and carrier transport. Physical Review E [Internet]. 2019;99:032415. Publisher's VersionAbstract
We study periodic, quasiperiodic (Thue-Morse, Fibonacci, period doubling, Rudin-Shapiro), fractal (Cantor, generalized Cantor), Kolakoski, and random binary sequences using a tight-binding wire model, where a site is a monomer (e.g., in DNA, a base pair). We use B-DNA as our prototype system. All sequences have purines, guanine (G) or adenine (A), on the same strand, i.e., our prototype binary alphabet is {G,A}. Our aim is to examine the influence of sequence intricacy and magnitude of parameters on energy structure, localization, and charge transport. We study quantities such as autocorrelation function, eigenspectra, density of states, Lyapunov exponents, transmission coefficients, and current-voltage curves. We show that the degree of sequence intricacy and the presence of correlations decisively affect the aforementioned physical properties. Periodic segments have enhanced transport properties. Specifically, in homogeneous sequences transport efficiency is maximum. There are several deterministic aperiodic sequences that can support significant currents, depending on the Fermi level of the leads. Random sequences is the less efficient category.
Mantela M, Lambropoulos K, Theodorakou M, Simserides C. Quasi-periodic and fractal polymers: Energy structure and carrier transfer. Materials [Internet]. 2019;12:2177. Publisher's VersionAbstract
We study the energy structure and the coherent transfer of an extra electron or hole along aperiodic polymers made of N monomers, with fixed boundaries, using B-DNA as our prototype system. We use a Tight-Binding wire model, where a site is a monomer (e.g., in DNA, a base pair). We consider quasi-periodic (Fibonacci, Thue–Morse, Double-Period, Rudin–Shapiro) and fractal (Cantor Set, Asymmetric Cantor Set) polymers made of the same monomer (I polymers) or made of different monomers (D polymers). For all types of such polymers, we calculate the highest occupied molecular orbital (HOMO) eigenspectrum and the lowest unoccupied molecular orbital (LUMO) eigenspectrum, the HOMO–LUMO gap and the density of states. We examine the mean over time probability to find the carrier at each monomer, the frequency content of carrier transfer (Fourier spectra, weighted mean frequency of each monomer, total weighted mean frequency of the polymer), and the pure mean transfer rate k. Our results reveal that there is a correspondence between the degree of structural complexity and the transfer properties. I polymers are more favorable for charge transfer than D polymers. We compare k(N)">k(N) of quasi-periodic and fractal sequences with that of periodic sequences (including homopolymers) as well as with randomly shuffled sequences. Finally, we discuss aspects of experimental results on charge transfer rates in DNA with respect to our coherent pure mean transfer rates.
Lambropoulos K, Simserides C. Tight-binding modeling of nucleic acid sequences: Interplay between various types of order or disorder and charge transport. Symmetry [Internet]. 2019;11:968. Publisher's VersionAbstract
This review is devoted to tight-binding (TB) modeling of nucleic acid sequences like DNA and RNA. It addresses how various types of order (periodic, quasiperiodic, fractal) or disorder (diagonal, non-diagonal, random, methylation et cetera) affect charge transport. We include an introduction to TB and a discussion of its various submodels [wire, ladder, extended ladder, fishbone (wire), fishbone ladder] and of the process of renormalization. We proceed to a discussion of aperiodicity, quasicrystals and the mathematics of aperiodic substitutional sequences: primitive substitutions, Perron–Frobenius eigenvalue, induced substitutions, and Pisot property. We discuss the energy structure of nucleic acid wires, the coupling to the leads, the transmission coefficients and the current–voltage curves. We also summarize efforts aiming to examine the potentiality to utilize the charge transport characteristics of nucleic acids as a tool to probe several diseases or disorders.
2018
Lambropoulos K, Vantaraki C, Bilia P, Mantela M, Simserides C. Periodic polymers with increasing repetition unit: Energy structure and carrier transfer. Physical Review E [Internet]. 2018;98:032412. Publisher's VersionAbstract
We study the energy structure and the transfer of an extra electron or hole along periodic polymers made of N monomers, with a repetition unit made of P monomers, using a tight-binding wire model, where a site is a monomer (e.g., in DNA, a base pair), for P even, and deal with two categories of such polymers: made of the same monomer (GC…, GGCC…, etc.) and made of different monomers (GA…, GGAA…, etc.). We calculate the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) eigenspectra, density of states, and HOMO-LUMO gap and find some limiting properties these categories possess, as P increases. We further examine the properties of the mean over time probability to find the carrier at each monomer. We introduce the weighted mean frequency of each monomer and the total weighted mean frequency of the whole polymer, as a measure of the overall transfer frequency content. We study the pure mean transfer rates. These rates can be increased by many orders of magnitude with appropriate sequence choice. Generally, homopolymers display the most efficient charge transfer. Finally, we compare the pure mean transfer rates with experimental transfer rates obtained by time-resolved spectroscopy.
Lambropoulos K, Simserides C. Spectral and transmission properties of periodic 1d tight-binding lattices with a generic unit cell: an analysis within the transfer matrix approach. Journal of Physics Communications [Internet]. 2018;2:035013. Publisher's VersionAbstract
We report on the electronic structure, density of states and transmission properties of the periodic one-dimensional tight-binding (TB) lattice with a single orbital per site and nearest-neighbor hoppings, with a generic unit cell of u sites. The determination of the eigenvalues is equivalent to the diagonalization of a real tridiagonal symmetric u-Toeplitz matrix with (cyclic boundaries) or without (fixed boundaries) perturbed upper right and lower left corners. We solve the TB equations via the Transfer Matrix Method, producing analytical solutions and recursive relations for its eigenvalues, closely related to the Chebyshev polynomials. We examine the density of states and provide relevant analytical relations. We attach semi-infinite leads, determine and discuss the transmission coefficient at zero bias and investigate the peaks number and position, and the effect of the coupling strength and asymmetry as well as of the lead properties on the transmission profiles. We introduce a generic optimal coupling condition and demonstrate its physical meaning.
2017
Lambropoulos K, Mantela M, Simserides C. Frequency content of carrier oscillations along B-DNA polymers. In: Progress in Electromagnetics Research Symposium. Vol. 2017-November. ; 2017. pp. 186-193. Publisher's VersionAbstract
We study the frequency content of an extra carrier oscillation along characteristic periodic B-DNA polymers made of N monomers. We employ two variants of the Tight-Binding approach, a wire model and an extended ladder model including diagonal hoppings. In the former, the site is a monomer, i.e., a base pair, while, in the latter, the site is a base. Initially, we focus on the Fourier Spectra of the probabilities to find the extra carrier at each monomer, having placed it at time zero at a specific monomer. Using the Fourier amplitude of each component of the frequency spectrum, we define the weighted mean frequency (WMF) for each site, a measure of its frequency content. To obtain a measure of the overall frequency content of carrier oscillations in the polymer, we define the total weighted mean frequency (TWMF), averaging the WMFs of all sites weighting over the probabilities of finding the extra carrier at each site. The frequency content is generally in the THz domain. Finally, we also give an example of an aperiodic sequence, the (A, T) Cantor dust.
Lambropoulos K, Simserides C. Electronic structure and charge transport properties of atomic carbon wires. Physical Chemistry Chemical Physics [Internet]. 2017;19:26890-26897. Publisher's VersionAbstract
Atomic carbon wires represent the ultimate one-atom-thick one-dimensional structure. We use a Tight-binding (TB) approach to determine the electronic structure of polyynic and cumulenic carbynes, in terms of their dispersion relations (for cyclic boundaries), eigenspectra (for fixed boundaries) and density of states (DOS). We further derive the transmission coefficient at zero-bias by attaching the carbynes to semi-infinite metallic leads, and demonstrate the effect of the coupling strength and asymmetry to the transparency of the system to incident carriers. Finally, we determine the current–voltage (IV) characteristics of carbynes and study the effect of factors such as the weakening of the coupling of the system to one of the leads, the relative position of the Fermi levels of the carbyne and the leads, the leads' bandwidth and, finally, the difference in the energy structure between the leads. Our results confirm and reproduce some of the most recent experimental findings.
2016
Lambropoulos K, Chatzieleftheriou M, Morphis A, Kaklamanis K, Lopp R, Theodorakou M, Tassi M, Simserides C. Electronic structure and carrier transfer in B-DNA monomer polymers and dimer polymers: Stationary and time-dependent aspects of a wire model versus an extended ladder model. Physical Review E [Internet]. 2016;94:062403. Publisher's VersionAbstract
We employ two tight-binding (TB) approaches to systematically study the electronic structure and hole or electron transfer in B-DNA monomer polymers and dimer polymers made up of N monomers (base pairs): (I) at the base-pair level, using the onsite energies of base pairs and the hopping integrals between successive base pairs, i.e., a wire model and (II) at the single-base level, using the onsite energies of the bases and the hopping integrals between neighboring bases, i.e., an extended ladder model since we also include diagonal hoppings. We solve a system of M (matrix dimension) coupled equations [(I) M=N, (II) M=2N] for the time-independent problem, and a system of M coupled first order differential equations for the time-dependent problem. We perform a comparative study of stationary and time-dependent aspects of the two TB variants, using realistic sets of parameters. The studied properties include HOMO and LUMO eigenspectra, occupation probabilities, density of states and HOMO-LUMO gaps as well as mean over time probabilities to find the carrier at each site [(I) base pair or (II) base], Fourier spectra, which reflect the frequency content of charge transfer, and pure mean transfer rates from a certain site to another. The two TB approaches give coherent, complementary aspects of electronic properties and charge transfer in B-DNA monomer polymers and dimer polymers.
Mantela M, Morphis A, Tassi M, Simserides C. Lowest ionisation and excitation energies of biologically important heterocyclic planar molecules. Molecular Physics [Internet]. 2016;114:709-718. Publisher's VersionAbstract
We calculate the lowest ionisation and excitation energies in a variety of biologically important molecules, i.e. π-conjugated systems like DNA and RNA bases and isomers plus related heterocyclic molecules. For approximately half of these molecules, there are no experimental and theoretical/numerical data in the literature, as far as we know. These electronic transitions are mainly but not exclusively of π and π–π* character, respectively. We perform symmetry-constrained density functional theory (DFT) geometry optimisation at the B3LYP/6-311++G** level of theory. At the DFT-obtained ground-state geometries, we calculate vertical ionisation energies with ionisation potential coupled cluster with singles and doubles (IP-EOMCCSD) and vertical excitation energies with the completely renormalised equation of motion coupled cluster with singles, doubles, and non-iterative triples (CR-EOMCCSD(T)) method. We also investigate whether a simple semi-empirical Hückel-type model approach with novel parametrisation could provide reasonable estimates of the lowest π ionisation and π–π* excitation energies. Our coupled cluster (CC) results are in very good agreement with experimental data, while the Hückel-type model predictions generally follow the trends with some deviation. Finally, we investigate the effect of basis set in IP-EOMCCSD energies and we compare our CR-EOMCCSD(T) results with time-dependent DFT (TDDFT) ones.
Lambropoulos K, Kaklamanis K, Morphis A, Tassi M, Lopp R, Georgiadis G, Theodorakou M, Chatzieleftheriou M, Simserides C. Wire and extended ladder model predict THz oscillations in DNA monomers, dimers and trimers. Journal of Physics Condensed Matter [Internet]. 2016;28:495101. Publisher's VersionAbstract
We call monomer a B-DNA base pair and study, analytically and numerically, electron or hole oscillations in monomers, dimers and trimers. We employ two tight binding (TB) approaches: (I) at the base-pair level, using the on-site energies of the base pairs and the hopping parameters between successive base pairs i.e. a wire model, and (II) at the single-base level, using the on-site energies of the bases and the hopping parameters between neighbouring bases, specifically between (a) two successive bases in the same strand, (b) complementary bases that define a base pair, and (c) diagonally located bases of successive base pairs, i.e. an extended ladder model since it also includes the diagonal hoppings (c). For monomers, with TB II, we predict periodic carrier oscillations with frequency –550 THz. For dimers, with TB I, we predict periodic carrier oscillations with –100 THz. For trimers made of identical monomers, with TB I, we predict periodic carrier oscillations with –33 THz. In other cases, either with TB I or TB II, the oscillations may be not strictly periodic, but Fourier analysis shows similar frequency content. For dimers and trimers, TB I and TB II are successfully compared giving complementary aspects of the oscillations.
2015
Lambropoulos K, Kaklamanis K, Georgiadis G, Theodorakou M, Chatzieleftheriou M, Morphis A, Tassi M, Simserides C. THz oscillations in DNA monomers, dimers and trimers. In: Progress in Electromagnetics Research Symposium. Vol. 2015-January. ; 2015. pp. 879-883. Publisher's VersionAbstract
We call monomer a B-DNA base-pair and study electron or hole oscillations in monomers, dimers and trimers. We employ two Tight Binding (TB) approaches: (I) at the base-pair level, using the on-site energies of the base-pairs and the hopping parameters between successive base-pairs and (II) at the single-base level, using the on-site energies of the bases and the hopping parameters between neighboring bases. With (II), for monomers, we predict oscillations with frequency f ≈ 50-550 THz. With (I), for dimers, we predict oscillations with f ≈ 0.25-100THz, for trimers made of identical monomers f ≈ 0.5-33 THz. In other cases, the oscillations may be not strictly periodic, but Fourier analysis shows similar frequency content. For dimers, we compare approaches (I) and (II). Finally, we present calculations with (III) RealTime Time-Dependent Density Functional Theory (RT-TDDFT) for the adenine-thymine (A-T) and the guanine-cytosine (G-C) base-pairs. It seems that a non conventional source or receiver of electromagnetic radiation with f from fractions to THz to just below PHz could be envisaged. 
  • ISSN: 15599450
  • ISBN: 978-193414230-1
Lambropoulos K, Chatzieleftheriou M, Morphis A, Kaklamanis K, Theodorakou M, Simserides C. Unbiased charge oscillations in B-DNA: Monomer polymers and dimer polymers. Physical Review E - Statistical, Nonlinear, and Soft Matter Physics [Internet]. 2015;92:032725. Publisher's VersionAbstract
We call monomer a B-DNA base pair and examine, analytically and numerically, electron or hole oscillations in monomer and dimer polymers, i.e., periodic sequences with repetition unit made of one or two monomers. We employ a tight-binding (TB) approach at the base-pair level to readily determine the spatiotemporal evolution of a single extra carrier along a N base-pair B-DNA segment. We study highest occupied molecular orbital and lowest unoccupied molecular orbital eigenspectra as well as the mean over time probabilities to find the carrier at a particular monomer. We use the pure mean transfer rate k to evaluate the easiness of charge transfer. The inverse decay length β for exponential fits k(d), where d is the charge transfer distance, and the exponent η for power-law fits k(N) are computed; generally power-law fits are better. We illustrate that increasing the number of different parameters involved in the TB description, the fall of k(d) or k(N) becomes steeper and show the range covered by β and η. Finally, for both the time-independent and the time-dependent problems, we analyze the palindromicity and the degree of eigenspectrum dependence of the probabilities to find the carrier at a particular monomer.
2014
Simserides C, Majewski JA, Trohidou KN, Dietl T. Theory of ferromagnetism driven by superexchange in dilute magnetic semi-conductors. In: EPJ Web of Conferences. Vol. 75. ; 2014. pp. 01003. Publisher's VersionAbstract
Magnetic properties of Ga1−xMnxN are studied theoretically by employing a tight binding approach to determine exchange integrals Jij characterizing the coupling between Mn spin pairs located at distances Rij up to the 16th cation coordination sphere in zinc-blende GaN. It is shown that for a set of experimentally determined input parameters there are no itinerant carriers and the coupling between localized Mn3+ spins in GaN proceeds via superexchange that is ferromagnetic for all explored Rij values. Extensive Monte Carlo simulations serve to evaluate the magnitudes of Curie temperature TC by the cumulant crossing method. The theoretical values of TC(x) are in quantitative agreement with the experimental data that are available for Ga1−xMnxN with randomly distributed Mn3+ ions with the concentrations 0.01 ≤ x ≤ 0.1.
Simserides C. A systematic study of electron or hole transfer along DNA dimers, trimers and polymers. Chemical Physics [Internet]. 2014;440:31-41. Publisher's VersionAbstract
A systematic study of carrier transfer along DNA dimers, trimers and polymers including poly(dG)-poly(dC), poly(dA)-poly(dT), GCGCGC..., ATATAT... is presented allowing to determine the spatiotemporal evolution of electrons or holes along a N base-pair DNA segment. Physical quantities are defined including maximum transfer percentage p and pure maximum transfer rate p/T when a period T is defined; pure mean transfer rate k and speed u=kd, where d is the charge transfer distance. The inverse decay length β for the exponential fit k=k0exp(-βd) and the exponent η for the power-law fit k=k0′N-η are computed. β≈0.2-2 Å-1, k0 is usually 10-2-10-1 PHz, generally ≈10-4-10 PHz. η≈1.7-17, k0′ is usually 10-2-10-1 PHz, generally ≈10-4-10 3 PHz. The results are compared with theoretical and experimental works. This method allows to assess the extent at which a specific DNA segment can serve for charge transfer.
Lambropoulos K, Kaklamanis K, Georgiadis G, Simserides C. THz and above THz electron or hole oscillations in DNA dimers and trimers. Annalen der Physik [Internet]. 2014;526:249-258. Publisher's VersionAbstract
A non conventional source or receiver of THz and above THz electromagnetic radiation is proposed. Specifically, electron or hole oscillations in DNA dimers (two interacting DNA base‐pairs or monomers) are predicted, with frequency in the range f ~ 0.25 - 100 THz (period T ~ 10 - 4000 fs) i.e. potentially absorbing or emitting electromagnetic radiation mainly in the mid‐ and far‐infrared with wavelengths ~ 3 - 1200 μm. The efficiency of charge transfer between the two monomers which make up the dimer is described with the maximum transfer percentage p  and the pure maximum transfer rate pf. For dimers made of identical monomers p = 1, but for dimers made of different monomers p < 1. The investigation is extended to DNA trimers (three interacting DNA base‐pairs or monomers). For trimers made of identical monomers the carrier oscillates periodically with f ~ 0.5 - 33 THz (T ~ 30 - 2000 fs); for 0 times crosswise purines p = 1, for 1 or 2 times crosswise purines p < 1. For trimers made of different monomers the carrier movement may be non periodic. Generally, increasing the number of monomers above three, the system becomes more complex and periodicity is lost; even for the simplest tetramer the carrier movement is not periodic.
2013
Dimakogianni M, Simserides C, Triberis GP. Density of states and extent of wave function: Two crucial factors for small polaron hopping conductivity in 1D. Philosophical Magazine [Internet]. 2013;93:2729-2748. Publisher's VersionAbstract
We introduce a theoretical model to scrutinize the conductivity of small polarons in 1D disordered systems, focusing on two crucial – as will be demonstrated – factors: the density of states and the spatial extent of the electronic wave function. The investigation is performed for any temperature up to 300 K and under electric field of arbitrary strength up to the polaron dissociation limit. To accomplish this task, we combine analytical work with numerical calculations.
Stefanowicz S, Kunert G, Simserides C, Majewski JA, Stefanowicz W, Kruse C, Figge S, Li T, Jakieła R, Trohidou KN, et al. Phase diagram and critical behavior of the random ferromagnet Ga1 -xMnxN. Physical Review B - Condensed Matter and Materials Physics [Internet]. 2013;88:081201(R). Publisher's VersionAbstract
Molecular beam epitaxy has been employed to obtain Ga1−xMnxN films with x up to 10% and Curie temperatures TC up to 13 K. The magnitudes of TC and their dependence on x, TC(x)∝xm, where m=2.2±0.2, are quantitatively described by a tight-binding model of superexchange interactions and Monte Carlo simulations of TC. The critical behavior of this dilute magnetic insulator shows strong deviations from the magnetically clean case (x=1), in particular, (i) an apparent breakdown of the Harris criterion, (ii) a nonmonotonic crossover in the values of the susceptibility critical exponent γeff between the high temperature and critical regimes, and (iii) a smearing of the critical region, which can be explained either by the Griffiths effects or by macroscopic inhomogeneities in the spin distribution with a variance Δx=(0.2±0.1)%.
2012
Sawicki M, Devillers T, Gałȩski S, Simserides C, Dobkowska S, Faina B, Grois A, Navarro-Quezada A, Trohidou KN, Majewski JA, et al. Origin of low-temperature magnetic ordering in Ga 1-xMn xN. Physical Review B - Condensed Matter and Materials Physics [Internet]. 2012;85:205204. Publisher's VersionAbstract
By using highly sensitive millikelvin superconducting quantum interference device magnetometry, the magnitude of the Curie temperature as a function of the Mn concentration x is determined for thoroughly characterized Ga1−xMnxN. The interpretation of the results in the frame of tight-binding theory and of Monte Carlo simulations allows us to assign the spin interaction to ferromagnetic superexchange and to point out the limited accuracy of state-of-the-art ab initio methods in predicting the magnetic characteristics of dilute magnetic insulators.
2011
Hawke LGD, Kalosakas G, Simserides C. Erratum: Electronic parameters for charge transfer along DNA (European Physical Journal E (2010) 32 (291) DOI:10.1140/epje/i2010-10650-y). European Physical Journal E [Internet]. 2011;34:118. Publisher's VersionAbstract
In our discussion of electronic parameters for charge (hole or electron) transfer along DNA we have omitted to mention that, regarding the tight-binding description of hole transport, the corresponding tight-binding parameters should be taken with the opposite sign of the calculated on-site energies and transfer hopping integrals. This means that for describing hole transport at the base-pair level, the on-site energies EbpH presented in the second row of table 2 and the hopping transfer integrals tbpH presented in the second column of table 3 should be used with opposite signs in order to provide the tight-binding parameters of eq. (10). Similarly, for describing hole transport at the single-base level, the on-site energies EbH presented in the eleventh row of table 1 and the hopping transfer integrals tbH presentedin the second column of tables 4–7 should be used with opposite signs in order to provide the tight-binding parameters of eq. (13). Moreover, on p. 300, 8 lines below eq. (14), in the calculation of charge transfer hopping parameters the separation between adjacent base-pairs in B-DNA should read 3.4 ̊A, instead of 3.14 ̊A.
Kosionis SG, Terzis AF, Simserides C, Paspalakis E. Intrinsic optical bistability in a two-subband system in a semiconductor quantum well: Analytical results. Journal of Applied Physics [Internet]. 2011;109:063109. Publisher's VersionAbstract
We study theoretically the conditions under which optical bistability is achievable in a two-subband system in a semiconductor quantum well. We consider the interaction of the two-subband system with a continuous wave electromagnetic field, which induces intersubband transitions. For the description of the system dynamics we use the effective nonlinear density matrix equations. We solve these equations analytically, in the steady state, for a GaAs/AlGaAs quantum well structure. For several combinations of the values of the parameters three real solutions of the population inversion arise and the phenomenon of optical bistability prevails.
Zora A, Triberis GP, Simserides C. Near-field optical properties of quantum dots, applications and perspectives. Recent Patents on Nanotechnology [Internet]. 2011;5:188-224. Publisher's VersionAbstract
Recent years have witnessed tremendous research in quantum dots as excellent models of quantum physics at the nanoscale and as excellent candidates for various applications based on their optoelectronic properties. This review intends to present theoretical and experimental investigations of the near-field optical properties of these structures, and their multimodal applications such as biosensors, biological labels, optical fibers, switches and sensors, visual displays, photovoltaic devices and related patents.
2010
Blekos K, Simserides C, Terzis AF, Paspalakis E. Effects of detuning on control of intersubband quantum well transitions with chirped electromagnetic pulses. In: AIP Conference Proceedings. Vol. 1288. ; 2010. pp. 137-141. Publisher's VersionAbstract
We study the interaction of a chirped electromagnetic pulse with intersubband transitions of a double semiconductor quantum well. We specifically consider the interaction of the ground and first excited subbands with the electromagnetic field and use the nonlinear density matrix equations for the description of the system dynamics. These equations are solved numerically for various values of the electron sheet density for a realistic double GaAs/AlGaAs quantum well, and the efficiency of population transfer is discussed with emphasis given to the effects of the detuning of the central frequency of the electromagnetic field from resonance.
Zora A, Simserides C, Triberis GP. Temperature dependence of the emission spectra of individual self-assembled quantum dots. In: Journal of Physics: Conference Series. Vol. 244. ; 2010. pp. 012037. Publisher's VersionAbstract
We have developed a quantum-mechanical theory for the interaction of light and electron-hole excitations in semiconductor quantum dots. Our theoretical analysis results in an expression for the photoluminescence intensity in the non-linear regime. The validity of the theoretical results is tested analyzing experimental data reported for the temperature dependence of the emission spectra of an individual lens-shaped In0.4Ga0.6As self-assembled quantum dot in a wide temperature range up to 300 K. Our theoretical predictions for the redshift of the emission peak with increasing temperature, in the range 2–300 K, agree with the experiment.
Paspalakis E, Simserides C, Terzis AF. Control of intersubband quantum well transitions with chirped electromagnetic pulses. Journal of Applied Physics [Internet]. 2010;107:064306. Publisher's VersionAbstract
We study the interaction of chirped electromagnetic pulses with intersubband transitions of a double semiconductor quantum well. We consider the ground and first excited subbands and give emphasis to controlled intersubband population transfer. The system dynamics is described by the nonlinear density matrix equations that include the effects of electron-electron interactions. These equations are solved numerically for various values of the electron sheet density for a realistic double GaAs/AlGaAs quantum well, and the efficiency of population transfer is discussed.
Hawke LGD, Kalosakas G, Simserides C. Electronic parameters for charge transfer along DNA. European Physical Journal E [Internet]. 2010;32:291-305. Publisher's VersionAbstract
We systematically examine all the tight-binding parameters pertinent to charge transfer along DNA. The pi molecular structure of the four DNA bases (adenine, thymine, cytosine, and guanine) is investigated by using the linear combination of atomic orbitals method with a recently introduced parametrization. The HOMO and LUMO wave functions and energies of DNA bases are discussed and then used for calculating the corresponding wave functions of the two B-DNA base-pairs (adenine-thymine and guanine-cytosine). The obtained HOMO and LUMO energies of the bases are in good agreement with available experimental values. Our results are then used for estimating the complete set of charge transfer parameters between neighboring bases and also between successive base-pairs, considering all possible combinations between them, for both electrons and holes. The calculated microscopic quantities can be used in mesoscopic theoretical models of electron or hole transfer along the DNA double helix, as they provide the necessary parameters for a tight-binding phenomenological description based on the pi molecular overlap. We find that usually the hopping parameters for holes are higher in magnitude compared to the ones for electrons. Our findings are also compared with existing calculations from first principles.
Kosionis SG, Terzis AF, Simserides C, Paspalakis E. Linear and nonlinear optical properties of a two-subband system in a symmetric semiconductor quantum well. Journal of Applied Physics [Internet]. 2010;108:034316. Publisher's VersionAbstract
We study the linear and nonlinear optical response of intersubband transitions in a semiconductor quantum well. We describe the coupling of the quantum well structure with the electromagnetic field by using the nonlinear density matrix equations, in the two-subband approximation. We provide proper approximate analytical solutions to these equations that are used for the closed-form determination of the optical susceptibilities &#x3C7;(1)">χ(1), &#x3C7;(3)">χ(3), and &#x3C7;(5)">χ(5). We also explore the dependence of &#x3C7;(1)">χ(1), &#x3C7;(3)">χ(3), and &#x3C7;(5)">χ(5) on the electron sheet density for a specific double GaAs/AlGaAs quantum well.
Simserides C, Zora A, Triberis GP. Low-dimensional carriers under in-plane magnetic field: Novel phenomena. 2010 by Nova Science Publishers, Inc. ; 2010 pp. 1-165. Publisher's VersionAbstract
Under the influence of different external stimuli condensed matter reveals its magnificent properties. The electric field, the temperature, the concentration gradients and the light are the basic “forces” responsible for processes such as the electrical, the thermal, the diffusion transport or optical phenomena. The action of the magnetic field brings about the galvanomagnetic or the thermomagnetic effects. New alloy semiconductors and the development of artificial semiconductor heterostructures led to the confinement of carriers in two, one or zero dimensions, opening a new window in condensed matter research. The application of a perpendicular magnetic field upon two-dimensional carriers, led to the discovering of astonishing phenomena, namely, the integer or the fractional quantum Hall effects and inspired radical theoretical interpretations. The reduced symmetry of low dimensional structures enhances decisively the role of the magnetic field orientation, bringing to light novel and unexpected phenomena. In the present book the effect of the application of an in-plane magnetic field upon low dimensional carriers, giving rise to impressive novel phenomena, is presented and discussed. Specifically, whenever a quantum well is subjected to an in-plane or tilted magnetic field, the elegant concept of Landau levels must be modified, because the carriers move under the competing influence of the Lorentz force and the force due to the quantum well confining potential. Under these conditions, the equal-energy surfaces or equivalently, the density of states (DOS), are qualitatively and quantitatively modified. The DOS diverges significantly from the ideal step-like two-dimensional carrier form. The book discusses various physical properties which are affected by the DOS modification.
Simserides C, Lipińska A, Trohidou KN, Dietl T. Reducing influence of antiferromagnetic interactions on ferromagnetic properties of p-(Cd,Mn)Te quantum wells. Physica E: Low-Dimensional Systems and Nanostructures [Internet]. 2010;42:2694-2697. Publisher's VersionAbstract
In order to explain the absence of hysteresis in ferromagnetic p-type (Cd,Mn)Te quantum wells (QWs), spin dynamics was previously investigated by Monte Carlo simulations combining the Metropolis algorithm with the determination of hole eigenfunctions at each Monte Carlo sweep. Short-range antiferromagnetic superexchange interactions between Mn spins—which compete with the hole-mediated long-range ferromagnetic coupling—were found to accelerate magnetization dynamics if the layer containing Mn spins is wider than the vertical range of the hole wave function. Employing this approach it is shown here that appreciate magnitudes of remanence and coercivity can be obtained if Mn ions are introduced to the quantum well in a delta-like fashion.
2009
Hawke LGD, Kalosakas G, Simserides C. Empirical LCAO parameters π for molecular orbitals in planar organic molecules. Molecular Physics [Internet]. 2009;107:1755-1771. Publisher's VersionAbstract
A novel parametrization within a simplified LCAO model (a type of Hückel model) is presented for the description of π molecular orbitals in organic molecules containing π-bonds between carbon, nitrogen, or oxygen atoms with sp2 hybridization. It is shown that the model is quite accurate in predicting the energy of the highest occupied π orbital and the first ππ* transition energy for a large set of organic compounds. Four empirical parameter values are provided for the diagonal matrix elements of the LCAO description, corresponding to atoms of carbon, nitrogen with one pz electron, nitrogen with two pz electrons, and oxygen. The bond-length dependent formula of Harrison (proportional to 1/d2) is used for the non-diagonal matrix elements between neighbouring atoms. The predictions of our calculations have been tested against available experimental results in more than sixty organic molecules, including benzene and its derivatives, polyacenes, aromatic hydrocarbons of various geometries, polyenes, ketones, aldehydes, azabenzenes, nucleic acid bases and others. The comparison is rather successful, taking into account the small number of parameters and the simplicity of the LCAO method, involving only pz atomic-like orbitals, which leads even to analytical calculations in some cases.
Lipińska A, Simserides C, Trohidou KN, Goryca M, Kossacki P, Majhofer A, Dietl T. Ferromagnetic properties of p-(Cd,Mn)Te quantum wells: Interpretation of magneto-optical measurements by Monte Carlo simulations. Physical Review B - Condensed Matter and Materials Physics [Internet]. 2009;79:235322. Publisher's VersionAbstract
In order to single out dominant phenomena that account for carrier-controlled magnetism in p-Cd1-xMnxTe quantum wells we have carried out magneto-optical measurements and Monte Carlo simulations of time-dependent magnetization. The experimental results show that magnetization relaxation is faster than 20 ns in the paramagnetic state. Decreasing temperature below the Curie temperature TC results in an increase of the relaxation time but to less than 10 μs. This fast relaxation may explain why the spontaneous spin splitting of electronic states is not accompanied by the presence of nonzero macroscopic magnetization below TC. Our Monte Carlo results reproduce the relative change of the relaxation time on decreasing temperature. At the same time, the numerical calculations demonstrate that antiferromagnetic spin-spin interactions, which compete with the hole-mediated long-range ferromagnetic coupling, play an important role in magnetization relaxation of the system. We find, in particular, that magnetization dynamics is largely accelerated by the presence of antiferromagnetic couplings to the Mn spins located outside the region, where the holes reside. This suggests that macroscopic spontaneous magnetization should be observable if the thickness of the layer containing localized spins will be smaller than the extension of the hole wave function. Furthermore, we study how a spin-independent part of the Mn potential affects TC. Our findings show that the alloy disorder potential tends to reduce TC, the effect being particularly strong for the attractive potential that leads to hole localization.
Simserides C, Koumpouras K. Multi-spin-subband structure of dilute magnetic semiconductor quantum wells: Feedback mechanism. Superlattices and Microstructures [Internet]. 2009;46:889-899. Publisher's VersionAbstract
Using a fully self-consistent envelope function approach, we focus on wide conduction band NMS (non-magnetic semiconductor)/DMS (dilute magnetic semiconductor)/NMS quantum wells, under weak external parallel magnetic field, where many spin-subbands are usually present. We concentrate on small values of the magnetic field because we want to investigate the influence of the feedback mechanism due to the difference of the concentrations of spin-up and spin-down carriers which could induce spontaneous spin-polarization i.e. in the absence of a magnetic field. We study the spin-subband structure, the spin-subband populations and the spin-polarization as functions of the sheet carrier concentration, Ns, for different values of the magnitude of the exchange interaction, | J |, between the itinerant carriers and the magnetic impurities. Our calculations for 0.01 T show that at 20 K the values of | J | necessary to make this feedback mechanism sufficiently strong are too high compared to the | J | values of common Mn-doped systems in the conduction band. However, the feedback mechanism will be sufficiently strong at low enough temperatures below 20 K for realistic values of | J |. Moreover, we explain how increasing the sheet carrier concentration the heterostructure is transformed from an almost square quantum well to a system of two coupled heterojunctions with an intermediate soft barrier.
Simserides C. Principal thermodynamic properties of quasi-two-dimensional carriers under in-plane magnetic field. Journal of Physics Condensed Matter [Internet]. 2009;21:015304. Publisher's VersionAbstract
An external magnetic field, H, applied parallel to a quasi-two-dimensional carrier system modifies quantitatively and qualitatively the density of states. We examine how this affects primary thermodynamic properties, namely, the entropy, S, the internal and free energy, U and F, the magnetization, M, and the magnetic susceptibility, χm, using a self-consistent numerical approach. Although M is mainly in the opposite direction to H, the system is not linear. Hence, surprisingly, δM/δH swings between negative and positive values, i.e. a diamagnetic to paramagnetic transition of entirely orbital origin is predicted. This phenomenon is important compared to the ideal de Haas–van Alphen effect, i.e. the corresponding phenomenon under perpendicular magnetic field. By augmenting temperature, the diamagnetic to paramagnetic transition fades away. The overall behaviour of entropy is also foreseen and consistently interpreted. While the entropy contribution to the free energy is very small at low temperatures, entropy shows a clear dependence on the external magnetic field.
Simserides C. Quantum wells under in-plane magnetic field: Density of states and novel phenomena in thermodynamic properties, magnetization and spin-polarization.; 2009 pp. 481-516. Publisher's VersionAbstract
The elegant concept of Landau levels must be abandoned, whenever a quantum well (QW) is subjected to an in-plane or tilted magnetic field, because carriers move under the competing influence of the Lorentz force and the force due to the QW confining potential. The equal-energy surfaces [1, 2] or equivalently the density of states (DOS) [3, 4] are qualitatively and quantitatively modified because the spatial and the magnetic confinement compete. In the general case, handling of such problems involves self-consistent computations [2, 4, 5] of the energy dispersion, Ei,σ(kx), where i is the subband index, σ denotes the spin and kx is the in-plane wave vector perpendicular to the external in-plane magnetic field (applied along y), H. The envelope functions along the "growth" z-axis depend on kx, i.e., ψi,σ,kx,ky (r) κ ζi,σ,kx (z)eikxxeikyy. The impact on the physical properties was initially realized in transport experiments [6, 7, 8, 9]. The character of plasmons in single [10] and double [11] QWs is also affected. The N-type kink was theoretically predicted [12] and recently verified in photoluminescence experiments [13]. In this chapter, employing a self-consistent envelope function approach: (a) I summarize fundamental quantum mechanical relations pertinent to QWs under in-plane magnetic field, and I stage a compact DOS formula which holds for any type of interplay between spatial and magnetic confinement [4, 5]. (b) I describe the influence of an in-plane magnetic field on fundamental thermodynamic properties of dilutemagnetic- semiconductor narrow to wide single QWs. I discuss the entropy, S, the internal energy, U, the free energy, F, and the magnetization, M. (c) I examine the spin-subband populations and the spin-polarization of dilute-magnetic-semiconductor narrow to wide single QWs [5, 14].
Hawke LGD, Simserides C, Kalosakas G. The π-π* molecular structure of flavin of FADH- enzymatic cofactor using the LCAO method. Materials Science and Engineering B: Solid-State Materials for Advanced Technology [Internet]. 2009;165:266-269. Publisher's VersionAbstract
The ππ* molecular structure (eigenenergies and eigenfunctions) of flavin tricyclic ring is calculated, using the linear combination of atomic orbitals (LCAO) method containing only pz atomic orbitals. In respect to FADH− position opposite to DNA lesion in photolyase, flavin's HOMO is found to be distributed in the central and distal side, while LUMO is localized in the distal side of flavin (the side that is closer to the adenine part of FADH− and farther than the DNA dimer lesion). LUMO1 as well as LUMO2 are mainly distributed in the proximal side of flavin (the side that is closer to the DNA dimer). Our findings are compared with previous theoretical results as well as with experimental values of known ππ* transitions.
2008
Zora A, Simserides C, Triberis GP. The effect of the size of self-assembled individual quantum dots on their PL spectra. Physica Status Solidi (C) Current Topics in Solid State Physics [Internet]. 2008;5:3806-3808. Publisher's VersionAbstract
Based on an analytical expression for the photoluminescence (PL) intensity of individual quantum dots (QDs) in the linear regime, we investigate its dependence upon the size of self‐assembled InGaAs QDs. We prove that decreasing the QD size, the PL‐emission spectrum moves to higher energy, due to the confinement‐induced blue‐shift of the electronic levels and the redshift from the increased Coulomb interaction caused by the compression of the exciton radii. This shift is in agreement with experimental results. Moreover, we show that larger dots provide more intense PL spectra.
König R, Sieluzycki C, Simserides C, Heil P, Scheich H. Effects of the task of categorizing FM direction on auditory evoked magnetic fields in the human auditory cortex. Brain Research [Internet]. 2008;1220:102-117. Publisher's VersionAbstract
We examined effects of the task of categorizing linear frequency-modulated (FM) sweeps into rising and falling on auditory evoked magnetic fields (AEFs) from the human auditory cortex, recorded by means of whole-head magnetoencephalography. AEFs in this task condition were compared with those in a passive condition where subjects had been asked to just passively listen to the same stimulus material. We found that the M100-peak latency was significantly shorter for the task condition than for the passive condition in the left but not in the right hemisphere. Furthermore, the M100-peak latency was significantly shorter in the right than in the left hemisphere for the passive and the task conditions. In contrast, the M100-peak amplitude did not differ significantly between conditions, nor between hemispheres. We also analyzed the activation strength derived from the integral of the absolute magnetic field over constant time windows between stimulus onset and 260 ms. We isolated an early, narrow time range between about 60 ms and 80 ms that showed larger values in the task condition, most prominently in the right hemisphere. These results add to other imaging and lesion studies which suggest a specific role of the right auditory cortex in identifying FM sweep direction and thus in categorizing FM sweeps into rising and falling.
Paspalakis E, Simserides C, Baskoutas S, Terzis AF. Electromagnetically induced population transfer between two quantum well subbands. Physica E: Low-Dimensional Systems and Nanostructures [Internet]. 2008;40:1301-1304. Publisher's VersionAbstract
We study theoretically the potential for control of the electron population in a single GaAs/AlGaAs quantum well that is coupled by strong pulsed electromagnetic fields. Using numerical calculations we present the conditions that lead to high-efficiency intersubband population inversion.
Simserides C, Galanakis I. Quasi-two-dimensional carriers in dilute-magnetic-semiconductor quantum wells under in-plane magnetic field. Physica E: Low-Dimensional Systems and Nanostructures [Internet]. 2008;40:1214-1216. Publisher's VersionAbstract
Due to the competition between spatial and magnetic confinement, the density of states (DOS) of a quasi-two-dimensional system deviates from the ideal step-like form both quantitatively and qualitatively. We study how this affects the spin-subband populations and the spin polarization as functions of the temperature, T, and the in-plane magnetic field, B, for narrow to wide dilute-magnetic-semiconductor quantum wells (QWs). We focus on the QW width, the magnitude of the spin–spin exchange interaction, and the sheet carrier concentration dependence. We look for ranges where the system is completely spin polarized. Increasing T, the carrier spin splitting, U0σU0&#x3C3;">, decreases, while on increasing B, U0&#x3C3;"> U0σ increases. Moreover, due to the DOS modification, all energetically higher subbands become gradually depopulated.
Zora A, Simserides C, Triberis GP. Room temperature photoluminescence of individual self-assembled quantum dots. Physica E: Low-Dimensional Systems and Nanostructures [Internet]. 2008;40:1687-1689. Publisher's VersionAbstract
We investigate the emission spectra of individual lens-shaped self-assembled quantum dots (QDs) in the high-temperature regime in order to contribute to the fine structural analysis and to the appreciation of the QDs’ optical response. Our theoretical analysis results in an expression for the photoluminescence (PL) intensity of QDs in the linear regime, which reproduces satisfactorily the experimentally observed PL signal of individual lens-shaped In0.5Ga0.5As self-assembled QDs. Using the appropriate material parameters, the theoretical predictions for the interlevel spacing as well as for the dephasing time caused by electron-longitudinal optical phonon interactions are in good agreement with experiment.
2007
Paspalakis E, Simserides C, Terzis AF. Controlled intersubband population dynamics in a semiconductor quantum well. In: AIP Conference Proceedings. Vol. 963. ; 2007. pp. 533-540. Publisher's VersionAbstract
We examine the intersubband transition dynamics of a single semiconductor quantum well when the ground and the first excited subbands are coupled by strong electromagnetic fields, with emphasis given to controlled intersubband population inversion. The system dynamics is described by the nonlinear density matrix equations that include the effects of electron‐electron interactions. We present analytical results for the electromagnetic field that can lead to high‐efficiency population inversion in the system. The validity of the analytical results is tested with numerical solutions of the density matrix equations for various values of the electron sheet density for a realistic GaAs/AlGaAs quantum well.
Zora A, Simserides C, Triberis G. Near-field magneto-optics of quantum dots. In: AIP Conference Proceedings. Vol. 893. ; 2007. pp. 893-894. Publisher's VersionAbstract
Encouraged by the latest experimental developments as well as by the theoretical interest on the near‐field (NF) optics of semiconductor quantum dots (QDs), we present our most recent theoretical results on the NF optical absorption and photoluminescence (PL) of single and coupled III‐V QDs subjected additionally to an external magnetic field of variable orientation and magnitude. The zero‐magnetic‐field “structural” QD symmetry can be destroyed varying the magnetic field orientation. The asymmetry induced by the magnetic field ‐except for specific orientations along symmetry axes‐ can be uncovered in the near‐field but not in the far‐field spectra. Hence, we predict that NF magnetoabsorption experiments, of realistic spatial resolution, will be in the position to bring to light the QD symmetry.
König R, Sieluzycki C, Simserides C, Scheich H. The effect of a categorical discrimination task on the auditory M100-peak latency. International Congress Series [Internet]. 2007;1300:45-48. Publisher's VersionAbstract
The rationale of this magnetoencephalographic (MEG) study was the quest for temporal aspects of the fMRI-characterized hemispheric asymmetries of auditory cortex functions during the processing of simple linearly frequency-modulated (FM) tones. We searched for parameters which distinguish a stimulus-related task condition—the categorical discrimination of direction (upward versus downward) of frequency modulation—from mere exposure to the same FM tones. We found that the M100-peak latency after FM tones was significantly shorter in the task condition than in the exposure condition, in the left but not in the right hemisphere.
Simserides C, Zora A, Triberis G. Magneto-optics of quantum dots in the near field. International Journal of Modern Physics B [Internet]. 2007;21:1649-1653. Publisher's VersionAbstract
We examine a quantum dot (QD) illuminated in the near field with subwavelength spatial resolution, while simultaneously it is subjected to a magnetic field of variable orientation and magnitude. The magnetic field orientation can conserve or destroy the zero-magnetic-field ("structural") symmetry. The asymmetry induced by the magnetic field -except for specific orientations along symmetry axes- can be uncovered in the near-field (NF) but not in the far-field (FF) spectra. We predict that NF magnetoabsorption experiments of realistic spatial resolution could reveal the QD symmetry. This exceptional symmetry-resolving power of the near-field optics, is lost in the far field.
Simserides C. Spin-subband populations and spin polarization of quasi-two-dimensional carriers under an in-plane magnetic field. Physical Review B - Condensed Matter and Materials Physics [Internet]. 2007;75:195344. Publisher's VersionAbstract
Under an in-plane magnetic field, the density of states of quasi-two-dimensional carriers deviates from the occasionally stereotypic step-like form both quantitatively and qualitatively. Here we study how this affects the spin-subband populations and the spin-polarization as functions of the temperature, T, and the in-plane magnetic field, B, for narrow to wide dilute-magnetic-semiconductor quantum wells. We examine a wide range of material and structural parameters, focusing on the quantum well width, the magnitude of the spin-spin exchange interaction, and the sheet carrier concentration. Generally, increasing T, the carrier spin-splitting, Uoσ, decreases, augmenting the influence of the “minority”-spin carriers. Increasing B, Uoσ, increases and, accordingly, carriers populate “majority”-spin subbands while they abandon “minority”-spin subbands. Furthermore, in line with the density of states modification, all energetically higher subbands become gradually depopulated. We also indicate the ranges where the system is completely spin-polarized.
Zora A, Simserides C, Triberis GP. Theory of spontaneous emission of quantum dots in the linear regime. Journal of Physics Condensed Matter [Internet]. 2007;19:406201. Publisher's VersionAbstract
We develop a fully quantum-mechanical theory for the interaction of light and electron–hole excitations in semiconductor quantum dots. Our theoretical analysis results in an expression for the photoluminescence intensity of quantum dots in the linear regime. Taking into account the single-particle Hamiltonian, the free-photon Hamiltonian, the electron–hole interaction Hamiltonian, and the interaction of carriers with light, and applying the Heisenberg equation of motion to the photon number expectation values, to the carrier distribution functions and to the correlation term between the photon generation (destruction) and electron–hole pair, we obtain a set of luminescence equations. Under quasi-equilibrium conditions, these equations become a closed-set of equations. We solve them analytically, in the linear regime, and we find an approximate solution of the incoherent photoluminescence intensity. The validity of the theoretical analysis is tested by investigating the emission spectra in the high-temperature regime, interpreting the experimental findings for the emission spectra of a lens-shaped In0.5Ga0.5As self-assembled quantum dot. Our theoretical predictions for the interlevel spacing as well as for the dephasing time caused by electron–longitudinal optical phonon interactions are in good agreement with the experimental results.
2006
Simserides C, Zora A, Triberis G. Near-field magnetoabsorption of quantum dots. Physical Review B - Condensed Matter and Materials Physics [Internet]. 2006;73:155313. Publisher's VersionAbstract
We investigate the effect of an external magnetic field of variable orientation and magnitude (up to 20T) on the linear near-field optical absorption spectra of single and coupled III-V semiconductor quantum dots. We focus on the spatial as well as on the magnetic confinement, varying the dimensions of the quantum dots and the magnetic field. We show that the ground-state exciton binding energy can be manipulated utilizing the spatial and magnetic confinement. The effect of the magnetic field on the absorption spectra, increasing the near-field illumination spot, is also investigated. The zero-magnetic-field “structural” symmetry can be destroyed varying the magnetic field orientation and this affects the near-field spectra. The asymmetry induced (except for specific orientations along symmetry axes) by the magnetic field can be revealed in the near-field but not in the far-field spectra. We predict that near-field magnetoabsorption experiments, of realistic spatial resolution, will be in the position to bring to light the quantum dot symmetry. This exceptional symmetry-resolving power of the near-field magnetoabsorption is lost in the far field. The influence of the Coulomb interactions on the absorption spectra is also discussed. Finally, we show that certain modifications of the magnetoexcitonic structure can be uncovered using a realistically acute near-field probe of ≈20nm.
Triberis G, Simserides C, Karavolas V. A small polaron hopping model for multiphonon-assisted transport along DNA molecules, in the presence of disorder. Physica E: Low-Dimensional Systems and Nanostructures [Internet]. 2006;32:592-595. Publisher's VersionAbstract
We discuss a small polaron hopping model, in order to explain the intense temperature (T) dependence of the electrical conductivity (σ) observed at high temperatures along the DNA molecules. The model takes into account the one-dimensional character of the system as well as the presence of disorder in the DNA double helix. Theoretical considerations based on percolation lead to analytical expressions for the high temperature multiphonon-assisted small polaron hopping conductivity, the maximum hopping distance and their temperature dependence. For example, experimental data for the lambda-phage DNA, the poly(dA)-poly(dT) DNA, and the native wet-spun calf thymus Li-DNA, follow nicely the theoretically predicted behavior, EQUATION, over wide high-T ranges. In contrast to some previously presented theoretical suggestions, our model leads to realistic values for the maximum hopping distances, supporting the idea of multiphonon-assisted hopping of small polarons between next nearest neighbors of the DNA molecular “wire”. We also examine the low temperature case.
2005
Triberis GP, Karavolas VC, Simserides CD. High temperature electrical conductivity due to small polaron hopping motion in DNA molecules. In: Journal of Physics: Conference Series. Vol. 10. ; 2005. pp. 210-213. Publisher's VersionAbstract
We present a small polaron hopping model to interpret the high-temperature electrical conductivity measured along the DNA molecules. The model takes into account the one-dimensional character of the system and the presence of disorder in the DNA double helix. The experimental data for the lambda phage DNA (λ-DNA) and the poly(dA)-poly(dT) DNA follow nicely the theoretically predicted behavior leading to realistic values of the maximum hopping distances supporting the idea of multiphonon-assisted hopping of small polarons between next nearest neighbors of the DNA molecular "wire".
Simserides C. Spin polarization and magnetization of conduction-band dilute-magnetic- semiconductor quantum wells with non-step-like density of states. In: Journal of Physics: Conference Series. Vol. 10. ; 2005. pp. 143-146. Publisher's VersionAbstract
We study the magnetization, M, and the spin polarization, ζ, of n-doped non-magnetic-semiconductor (NMS)/narrow to wide dilute-magnetic-semiconductor (DMS)/n-doped NMS quantum wells, as a function of the temperature, T, and the in-plane magnetic field, B. Under such conditions the density of states (DOS) deviates from the occasionally stereotypic step-like form, both quantitatively and qualitatively. The DOS modification causes an impressive fluctuation of M in cases of vigorous competition between spatial and magnetic confinement. At low T, the enhanced electron spin-splitting, Uoσ, acquires its bigger value. At higher T, Uoσ decreases, augmenting the influence of the spin-up electrons. Increasing B, Uoσ increases and accordingly electrons populate spin-down subbands while they abandon spin-up subbands. Furthermore, due to the DOS modification, all energetically higher subbands become gradually depopulated.
Simserides C. Temperature dependent magnetization and magnetic phases of conduction-band dilute-magnetic-semiconductor quantum wells with non-step-like density of states. In: AIP Conference Proceedings. Vol. 772. ; 2005. pp. 341-342. Publisher's VersionAbstract
We investigate the magnetization of II–VI non‐magnetic‐semiconductor (NMS) / narrow to wide dilute‐magnetic‐semiconductor (DMS) / NMS quantum wells. These structures are appropriate for conduction‐band spintronics. We employ an in‐plane magnetic field, B, in order to induce non‐step‐like density of states. Finally, we tune the spin polarization by varying the temperature, T, and B, i.e. we investigate the magnetic phases of these NMS/DMS/NMS structures.
Zora A, Simserides C, Triberis G. Magnetic field effects on the near field spectra of quantum dots. Physica Status Solidi (A) Applications and Materials Science [Internet]. 2005;202:619-624. Publisher's VersionAbstract
We theoretically investigate the linear near field absorption spectra of semiconductor quantum dots under magnetic field of variable orientation. We examine if the application of the magnetic field alone is sufficient to cause – increasing the spot illuminated by the near field probe – “unexpected” features to the absorption spectra.
Triberis GP, Simserides C, Karavolas VC. Small polaron hopping transport along DNA molecules. Journal of Physics Condensed Matter [Internet]. 2005;17:2681-2690. Publisher's VersionAbstract
We present a small polaron hopping model for interpreting the strong temperature (T) dependence of the electrical conductivity, σ, observed at high (h) temperatures along DNA molecules. The model takes into account the one-dimensional character of the system and the presence of disorder in the DNA double helix. Percolation-theoretical considerations lead to analytical expressions for the high temperature multiphonon-assisted small polaron hopping conductivity, the hopping distance and their temperature dependence. The experimental data for lambda phage DNA (λ-DNA) and poly(dA)–poly(dT) DNA follow nicely the theoretically predicted behaviour (EQUATION). Moreover, our model leads to realistic values of the maximum hopping distances, supporting the idea of multiphonon-assisted hopping of small polarons between next nearest neighbours of the DNA molecular 'wire'. The low temperature case is also investigated.
2004
Simserides C. Magnetization and magnetic phases of conduction-band dilute-magnetic- semiconductor quantum wells with non-step-like density of states. International Journal of Modern Physics B [Internet]. 2004;18:3745-3748. Publisher's VersionAbstract
We study the magnetization and the magnetic phases of II-VI-based n-doped non-magnetic-semiconductor (NMS) / narrow to wide dilute-magnetic-semiconductor (DMS) / n-doped NMS quantum wells under in-plane magnetic field. The parallel magnetic field is used as a tool, in order to achieve non-step-like density of states in these -appropriate for conduction-band spintronics- structures.
Zora A, Simserides C, Triberis G. Near field spectroscopy of quantum dots under magnetic field. International Journal of Modern Physics B [Internet]. 2004;18:3717-3721. Publisher's VersionAbstract
We present the basic steps for the study of the linear near field absorption spectra of semiconductor quantum dots under magnetic field of variable orientation. We show that the application of the magnetic field alone is sufficient to induce -increasing the spot illuminated by the near field probe- interesting features to the absorption spectra.
Simserides C. Not-step-like density of states and carrier distribution of conduction-band, narrow-to-wide dilute magnetic semiconductor quantum wells under in-plane magnetic field. Physica E: Low-Dimensional Systems and Nanostructures [Internet]. 2004;21:956-960. Publisher's VersionAbstract
We analyze the important changes induced in the density of states of narrow-to-wide conduction-band dilute magnetic semiconductor quantum wells subjected to an in-plane magnetic field, B. We show quantitatively that the DOS diverges significantly from the famous step-like two-dimensional electron gas form, by providing results for many values of B and grades of spatial localization. This introduces changes in the pertinent electronic properties. The self-consistent approach is indispensable and the eigenvalue problem has to be solved for each subband index i, spin σ, and in-plane wave vector, e.g. kx. We can select the appropriate parameters so that the structure is populated by carriers of spin-down or exploit the effect of the depopulation of the higher spin-subbands to eliminate carriers with spin-up.
Simserides C. Properties of conduction-band dilute-magnetic-semiconductor quantum wells in an in-plane magnetic field: A density of states profile that is not steplik. Physical Review B - Condensed Matter and Materials Physics [Internet]. 2004;69:113302. Publisher's VersionAbstract
We examine how an in-plane magnetic field B modifies the density of states (DOS) in narrow-to-wide, conduction-band dilute-magnetic semiconductor quantum wells. We demonstrate that the DOS diverges significantly from the ideal steplike two-dimensional electron gas form and this causes severe changes to the physical properties, e.g., to the spin-subband populations, the internal and free energy, the Shannon entropy, and the in-plane magnetization M. We predict a considerable fluctuation of M in cases of vigorous competition between spatial and magnetic confinement.
2003
Simserides C. The Density of States and the Pertinent Electronic Properties of the Quasi 2DEG in Simple and DMS Structures Subjected to an In-Plane Magnetic Field. Journal of Computational Electronics [Internet]. 2003;2:459-463. Publisher's VersionAbstract
We analyze the important changes induced to the density of states (DOS) of a quasi two-dimensional-electron-gas (2DEG), when it is subjected to an in-plane magnetic field, B. The DOS diverges significantly from the famous step-like form and this introduces changes to the pertinent electronic properties. In order to calculate the DOS it is indispensable to use a self-consistent approach. The eigenvalue problem has to be solved for each subband index i and in-plane wavevector, k x, when the quasi 2DEG is parallel to the xy-plane and B is applied along the y-axis. Although the modification of the DOS is usually ignored, we show here that not only the general shape of the DOS varies, but this effect is also quantitative.
2001
Simserides CD, Hohenester U, Goldoni G, Molinari E. Local absorption spectra of single and coupled semiconductor quantum dots. Materials Science and Engineering B: Solid-State Materials for Advanced Technology [Internet]. 2001;80:266-269. Publisher's VersionAbstract
We study theoretically the local absorption spectra of single and double semiconductor quantum dots (QDs), in the linear regime. The three-dimensional confinement leads to an enhancement of the Coulomb correlations, while the spectra depend crucially on the size of the ‘local’ probe. We show that because of such Coulomb correlations the intensity of certain optical peaks as a function of the resolution can exhibit an unexpected non-monotonic behavior for spatial resolutions comparable with the excitonic Bohr radius. We finally discuss the optical near-field properties of coupled QDs for different coupling strengths.
Simserides CD, Hohenester U, Goldoni G, Molinari E. Local optical absorption by confined excitons in single and coupled quantum dots. Physica Status Solidi (B) Basic Research [Internet]. 2001;224:745-749. Publisher's VersionAbstract
We investigate optical near‐field spectra of single and coupled semiconductor quantum dots. An enhanced role for the Coulomb correlations is predicted, and it is shown that the spectra depend crucially on the spatial resolution of the “local” probe. The intensity of certain optical peaks as a function of the resolution exhibits an unexpected non‐monotonic behavior, which is identified as a fingerprint of Coulomb interactions in zero‐dimensional nanostructures.
2000
Simserides CD, Hohenester U, Goldoni G, Molinari E. Local absorption spectra of artificial atoms and molecules. Physical Review B - Condensed Matter and Materials Physics [Internet]. 2000;62:13657-13666. Publisher's VersionAbstract
We investigate theoretically the spatial dependence of the linear absorption spectra of single and coupled semiconductor quantum dots, where the strong three-dimensional quantum confinement leads to an overall enhancement of Coulomb interaction and, in turn, to a pronounced renormalization of the excitonic properties. We show that—because of such Coulomb correlations and the spatial interference of the exciton wave functions—unexpected spectral features appear whose intensity depends on spatial resolution in a highly nonmonotonic way when the spatial resolution is comparable with the excitonic Bohr radius. We finally discuss how the optical near-field properties of double quantum dots are affected by their coupling.
Hartmann A, Ducommun Y, Kapon E, Hohenester U, Simserides C, Molinari E. Optical spectra of single quantum dots: Influence of impurities and few-particle effects. Physica Status Solidi (A) Applied Research [Internet]. 2000;178:283-290. Publisher's VersionAbstract
The evolution of photoluminescence (PL) spectra of single GaAs/AlGaAs quantum dots (QD) is studied as a function of laser excitation power and temperature. At very low powers, where multi‐exciton occupation of the QD can be excluded, an unexpected and pronounced spectral evolution is observed (large energy shifts and appearance of multiple emission lines). A similar evolution is observed at low excitation powers with increasing temperature. A model, taking into account the influence of the shallow, residual impurities in the environment of each QD, explains the observed spectral evolutions in terms of photo‐depletion of the QD and hopping of impurity‐bound carriers back into the QD. Theoretical calculations of the PL due to Nelectrons + 1 hole (Ne + 1h) QD states allow us to attribute the ≈︂2 meV spaced lines in the experimental spectra to the different charge states Ne + 1h, (N — 1) e + 1h, … of the QD.
1999
Simserides CD. The density of states and the electron concentration of a double-heterojunction system subjected to an in-plane magnetic field. Journal of Physics Condensed Matter [Internet]. 1999;11:5131-5141. Publisher's VersionAbstract
We calculate the electronic states of AlxGa1-xAs/GaAs/AlxGa1-xAs double heterojunctions subjected to a magnetic field parallel to the quasi-two-dimensional electron gas layer. We study the energy dispersion curves, the density of states, the electron concentration and the distribution of the electrons in the subbands. The parallel magnetic field induces severe changes in the density of states, which are of crucial importance for the explanation of the magnetoconductivity in these structures. However, to our knowledge, there has been no systematic study of the density of states under these circumstances. We attempt a contribution in this direction. For symmetric heterostructures, the depopulation of the higher subbands, the transition from a single-layer to a bilayer electron system and the domination of the bulk Landau levels in the centre of the wide quantum well, as the magnetic field is continuously increased, are presented in the `energy dispersion picture' as well as in the `electron concentration picture' and in the `density-of-states picture'.
1997
Zianni X, Simserides C, Triberis G. Electron scattering by optical phonons iAs/GaAAs quantum wells. Physical Review B - Condensed Matter and Materials Physics [Internet]. 1997;55:16324-16330. Publisher's VersionAbstract
The scattering of a quasi two-dimensional electron gas by optical phonons in selectively doped AlxGa1−xAs/GaAs/AlxGa1−xAs quantum wells is systematically studied in order to determine the effect of phonon confinement. The electron states are calculated solving self-consistently Schrödinger and Poisson equations to obtain an accurate dependence upon the structure parameters and the temperature. We study the way the scattering is affected by the form of the phonons calculating the mobility using three models for the phonons. They are considered:m(a) as three dimensional (3D), (b) as a set of confined and interface phonons, and (c) as the normal modes of the heterostructure. The relaxation times for the electron energy subbands are calculated solving the system of Boltzmann equations. The effect of the temperature and the well width variation is also investigated. The results are in a good agreement with experimental measurements. The agreement is only slightly dependent on the model used for the phonons and becomes best when the effect of the heterostructure on the phonon modes is taken into account.
1996
Simserides CD, Triberis GP. Looking for the maximum low-temperature conductivity in selectively doped AlxGa1-xAs-GaAs-AlxGa1-xAs double heterojunctions. Journal of Physics Condensed Matter [Internet]. 1996;8:L421-L426. Publisher's VersionAbstract
We use self-consistent numerical calculations to study the sheet electron concentration and the mobility as functions of the doping concentration, the spacer thickness, the well width and the Al mole fraction of a selectively doped AlxGa1−xAs/GaAs/AlxGa1−xAs double heterojunction, using no arbitrary, a priori, assumptions, at low temperatures.For the first time we take into account two kinds of donor (shallow and deep) that coexist in the Si-doped AlxGa1−xAs. We study all the significant scattering mechanisms. The model, based exclusively upon the knowledge of the material and structural parameters involved, allows us to obtain the maximum conductivity for any specific structure. Our results are in a very good agreement with experiment.
1995
Simserides CD, Triberis GP. A study on the temperature dependence of the quasi-two-dimensional electron concentration and mobility in AlxGa1-xAs/GaAs selectively doped heterostructures. Journal of Physics: Condensed Matter [Internet]. 1995;7:6317-6326. Publisher's VersionAbstract
We present a quantitative analysis of the temperature dependence of the quasi-two-dimensional electron concentration in AlxGa1-xAs/GaAs heterostructures taking into account the fact that in the bulk Si-doped AlxGa1-xAs two types of donor coexist, i.e. deep and shallow, which independently, and by different mechanisms, provide electrons to the bulk AlxGa1-xAs different conduction band minima and to the quasi-two-dimensional electron gas (Q2DEG). We calculate the electronic states, the ionized-donor concentrations, the Q2DEG and the bulk-electron concentrations and the corresponding mobilities as a function of temperature. Our numerical results are in very good agreement with the experimental data.
1994
Simserides CD, Triberis GP. Comments on the Efficiency of Selectively Doped Double Heterojunction (SD‐DH) Structures. physica status solidi (b) [Internet]. 1994;184:K49-K52. Publisher's VersionAbstract
[No abstract available]
1993
Simserides CD, Triberis GP. A systematic study of electronic states in n-AlxGa 1-xAs/GaAs/n-AlxGa1-xAs selectively doped double-heterojunction structures. Journal of Physics: Condensed Matter [Internet]. 1993;5:6437-6446. Publisher's VersionAbstract
The electron concentration, the wavefunctions and the energy levels of a n-AlxGa1-xAs/GaAs/n-AlxGa1-xAs double heterojunction are evaluated by solving Schrodinger and Poisson equations self-consistently. The authors investigate, at zero temperature, the dependence of the sheet electron concentration, and the subband populations on the well width, spacer thickness and doping concentrations, for Al mole fraction x=0.3. They give physical interpretations of some interesting characteristics observed. The transition from a 'perfect' square well to a system of 'two separated heterojunctions' is systematically studied. The results are in excellent agreement with previous experiments.
Simserides CD, Triberis GP. On the temperature dependence of the electronic states and the mobility in AlGaAs/GaAs heterostructures. Superlattices and Microstructures [Internet]. 1993;14:277. Publisher's VersionAbstract
Experiment shows that in AlGaAs/GaAs heterostructures the sheet electron concentration remains almost constant up to a certain temperature, while it increases at higher temperatures. We attempt an interpretation of this temperature dependence, taking into account the fact that in the bulk, n-AlGaAs deep and shallow donors exist, which independently and by different mechanisms provide electrons to the different conduction band minima of the bulk n-AlGaAs, and contribute to the formation of the Q2DEG. We calculate the electronic states of this structure, the Q2DEG and the bulk concentrations, and the corresponding mobilities as a function of temperature. Our numerical results are in an excellent agreement with experimental data.