The π–π* 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 π–π* transitions.

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].

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.

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 π–π* 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.

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.

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.