Publications by Year: 2006

2006
Pissas M, Stamopoulos D, Ma YC, Wang NL. Vortex matter properties of Tl2Ba2CaCu2O8 superconductor. PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS. 2006;437-38(4th International Conferene on Vortex Matter in Nanostructured Superconductors (VORTEX IV):267 - 269.Abstract
The vortex matter phase diagram for H parallel to c of the thallium-based cuprate with formula Tl2Ba2CaCu2O8 (Tl-2212) has been studied using magnetization measurements. We observed a second magnetization peak monotonically decreasing with temperature in the temperature interval 20 K < T < 40 K. At 40 K the second peak splits into two peaks. The field location of the lower part of the second peak decreases with temperature and for 80 K <= T <= T-c it is transformed to a sudden drop. The corresponding upper part of the second peak forms a broad maximum and subsequently develops a negative curvature as it approaches the melting line. In the magnetic field interval [H-LCP,H-UCP] we observed a first-order melting transition (FOT) of the vortex lattice. (c) 2006 Elsevier B.V. All rights reserved.
Stamopoulos D, Manios E, Pissas M, Niarchos D. Modulation of the properties of a low-T-c superconductor by anisotropic ferromagnetic particles. PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS. 2006;437-38(4th International Conferene on Vortex Matter in Nanostructured Superconductors (VORTEX IV):289 - 292.Abstract
In this work we study the modulation of superconductivity in combinatorial films (CFs) that are constructed by anisotropic CoPt ferromagnetic particles (FPs) randomly embedded in high-quality Nb layers of thickness 200 nm. In the constructed CFs the FPs are embedded in only half of the Nb layers. In this way we studied the modulation of superconductivity directly by performing transport measurements on the hybrid and pure superconducting areas of the CFs simultaneously under the application of the same dc current. Modulation of superconductivity and magnetic memory effects appeared only at the hybrid part of the CFs. This proves that superconductivity can be noticeably controlled under the action of the dipolar fields of FPs. The enhancement of the superconducting regime on the H-T operational diagram suggests that such hybrids can be attractive for current-carrying applications. In addition, magnetoresistive elemental superconducting devices could be based on our CFs. (c) 2005 Elsevier B.V. All rights reserved.
Pissas M, Stamopoulos D, Psycharis V, Ma YC, Wang NL. Splitting of the second magnetization peak in the superconductor Tl2Ba2CaCu2O8+x. PHYSICAL REVIEW B. 2006;73(17).Abstract
The vortex-matter phase diagram of Tl2Ba2CaCu2O8+x single crystal superconductor has been studied, using bulk and local magnetization measurements. We observed a second magnetization peak in the temperature interval 20 K < T < 40 K. At 40 K the second peak splits into two peaks, termed the upper and lower second magnetization peaks (USP and LSP), respectively. In the magnetic field interval [H-LCP,H-UCP] we observed the clear signature of a first-order melting transition of the vortex lattice. This particular behavior is similar to a situation with two critical endpoints of the first-order melting line and one coexistence point, where the topological transition line ends on the first-order melting line.
Stamopoulos D, Pissas M. Manipulating superconductivity through the domain structure of a ferromagnet: Experimental aspects and theoretical implications. PHYSICAL REVIEW B. 2006;73(13).Abstract
In the present work we study experimentally the influence that the domain structure of a ferromagnet (FM) has on the properties of a superconductor (SC) in bilayers and multilayers of La0.60Ca0.40MnO3/Nb and FePt/Nb proximity hybrids. Specific experimental protocols that were employed in the performed magnetization measurements enabled us to directly uncover a generic property of FM/SC hybrids: In the absence of an external magnetic field, the multidomain structure of the FM promotes the nucleation of superconductivity, while its monodomain state strongly suppresses it. Our experimental findings support recent theoretical studies [A. I. Buzdin and A. S. Mel'nikov, Phys. Rev. B 67, 020503(R) (2003); T. Champel and M. Eschrig, Phys. Rev. B 71, 220506(R) (2005)] that suggest the formation of the so-called domain-wall superconductivity and propose that when an inhomogeneous exchange field is offered by the FM to the SC the superconducting pairs are not susceptible to pairbreaking. In contrast, when magnetic homogeneity is restored in the FM the SC's properties are strongly suppressed.
Papavassiliou G, Pissas M, Diamantopoulos G, Belesi M, Fardis M, Stamopoulos D, Kontos AG, Hennion M, Dolinsek J, Ansermet JP, et al. Low temperature charge and orbital textures in La0.875Sr0.125MnO3. PHYSICAL REVIEW LETTERS. 2006;96(9).Abstract
By using nuclear magnetic resonance techniques we show that for T < 30 K the La0.875Sr0.125MnO3 compound displays a nonuniform charge distribution, comprised of two interconnected Mn ion subsystems with different spin, orbital, and charge couplings. The NMR results agree very well with the two spin wave stiffness constants observed at small q values in the spin wave dispersion curves [Phys. Rev. B 67, 214430 (2003)]. This picture is probably related to a yet undetermined charge and orbital superstructure occurring in the ferromagnetic insulating state of the La0.875Sr0.125MnO3 compound.
Pissas M, Psycharis V, Stamopoulos D, Papavassiliou G, Sanakis Y, Simopoulos A. Mossbauer study of (Na0.82Co0.99Fe0.01O2)-Fe-57. SOLID STATE COMMUNICATIONS. 2006;137(12):668 - 672.Abstract
We studied by Mossbauer spectroscopy the Na0.82CoO2 compound using 1% 57 Fe as a local probe which substitutes for the Co ions. Mossbauer spectra at T = 300 K revealed two sites which correspond to Fe3+ and Fe4+. The existence of two distinct values of the quadrupole splitting instead of a continuous distribution should be related with the charge ordering of Co+3, Co+4 ions and ion ordering of Na(1) and Na(2). Below T = 10 K part of the spectrum area, corresponding to Fe4+ and all of Fe3+, displays broad magnetically split spectra arising either from short-range magnetic correlations or from slow electronic spin relaxation. (c) 2006 Elsevier Ltd. All rights reserved.
Sandim MJR, Stamopoulos D, Sandim HRZ, Ghivelder L, Thilly L, Vidal V, Lecouturier F, Raabe D. Size effects on the magnetic properties of Cu-Nb nanofilamentary wires processed by severe plastic deformation. SUPERCONDUCTOR SCIENCE & TECHNOLOGY. 2006;19(12):1233 - 1239.Abstract
We report on the influence of the microstructure on the AC and DC magnetic properties of Cu-3.5% Nb nanofilamentary wires. Samples obtained from a single Cu-3.5% Nb wire were subsequently submitted to different plastic strain levels via drawing so that their microstructure was altered. Noticeable differences are observed in their isothermal DC magnetization curves that present a double-peak structure. The first peak, which occurs at low magnetic fields, is attributed to superconductivity induced in the Cu matrix due to the proximity effect. It is argued that the second peak is related exclusively to niobium. The dependence of these two distinct peaks on the characteristic nanometre length scales of the samples, i.e. dimension of the Nb filaments and interfilamentary spacing, are discussed.
Bourlinos AB, Dallas P, Sanakis Y, Stamopoulos D, Trapalis C, Niarchos D. Synthesis and characterization of a pi-conjugate, covalent layered network derived from condensation polymerization of the 4,4 '-bipyridine-cyanuric chloride couple. EUROPEAN POLYMER JOURNAL. 2006;42(11):2940 - 2948.Abstract
Nucleophilic substitutions of the reactive chlorine atoms in cyanuric chloride by the bridging 4,4'-bipyridine in refluxing toluene lead to quaternarization of the latter and the subsequent formation of a pi-conjugate, covalent layered network. The network is composed of central 1,3,5-triazine units with 4,4'-bipyridinium rings covalently attached and balanced by the released chloride ions. Due to the extremely high electron deficiency of the triazine rings, the material undergoes partial reduction by its compensated chloride ions resulting in a radical concentration of 1 X 1020 spin g(-1), according to EPR quantitative analysis. In this instance, the radicals provide stability to the organic network by minimizing its electron deficiency. The material exhibits thermal and electrochemical stability, as evidenced by thermal gravimetric analysis (TGA) and cyclic voltammetry techniques. As such, the pi-conjugate organic material displays low band gaps and electrical conductivity in the range of 10(-4)-10(-5) S cm(-1) at room temperature. (c) 2006 Elsevier Ltd. All rights reserved.
Stamopoulos D, Pissas M, Sandim MJR, Sandim HRZ. Proximity induced superconductivity in bulk Cu-Nb composites: The influence of interface's structural quality. PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS. 2006;442(1):45 - 54.Abstract
The influence of the structural quality of a normal metal-superconductor interface on the strength of the proximity effect is studied for the cylindrical geometry of bulk Cu-Nb multifilamentary composite samples. In as-drawn Cu-Nb samples the superconducting properties induced to the Cu matrix due to its proximity with Nb filaments are manifested in the magnetization measurements through a pronounced peak positioned in the low-field regime. By systematic annealing in an extended temperature regime for various durations we changed the structural quality of the Cu-Nb interfaces and as a result their transparency to the superconducting carrier. The direct comparison of scanning electron microscopy images with magnetization data revealed that the distortion of the Cu-Nb interfaces is responsible for the suppression of the proximity effect. (c) 2006 Elsevier B.V. All rights reserved.
Sandim MJR, Sandim HRZ, Stamopoulos D, Renzetti RA, Das Virgens MG, Ghivelder L. Spheroidization effects on the electrical and magnetic properties of Cu-Nb composites. IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY. 2006;16(19th International Conference on Magnet Technology2):1692 - 1695.Abstract
We report on the microstructure evolution in Cu-15% Nb multifilamentary wires upon annealing and the corresponding effects on their electrical and magnetic properties. During annealing at temperatures higher than 800 degrees C thermal instability mechanisms take place in the microstructure of the Cu-15%Nb composite, leading to spheroidization, followed by coarsening of niobium filaments. Such microstructural changes exert considerable influence on the magnitude of the normal electrical resistivity for the Cu-Nb composite. In addition, the annealing process leads to a double-peak structure in the isothermal DC magnetization curves of the composite, which is strongly dependent on the annealing time. This behavior suggests that the spheroidization kinetics can be obtained from M(H) curves.
Stamopoulos D. Superconducting properties of LaCaMnO/Nb hybrids: a complete experimental study and comparison with theory. SUPERCONDUCTOR SCIENCE & TECHNOLOGY. 2006;19(6):652 - 666.Abstract
In the present work we study extensively the manipulation of superconductivity through ferromagnetism in a new category of hybrids. The studied hybrids consist of antiferromagnetic/ ferromagnetic (AF/FM) [La0.33Ca0.67MnO3/La0.60Ca0.40MnO3](15) multilayers (MLs) in contact with a low-T-c Nb superconductor (SC). In these hybrids a relatively thick FM buffer layer was used as a reservoir for the generation of stray fields that influence the SC intensively. Our results show that in the parallel field configuration the SC becomes ferromagnetically coupled to the ML when field-cooled through its T-c(SC). Thus, although the SC should behave diamagnetically in respect to the externally applied magnetic field, its bulk magnetization behaves ferromagnetically and switches together with the magnetization of the ML when its coercive field is exceeded (switching effect). By employing specific experiments, where the ML was selectively exchange biased or not, we clearly demonstrate that the ML structure, separating the FM buffer and the SC layers, inflicts its magnetic properties on the whole hybrid. Thus, in such ML/SC hybrids the exchange bias mechanism can be used for regulating the switching of the SC magnetization. By employing specific experimental protocols for our magnetization measurements we directly uncover that the multidomain magnetic state of the ML strongly suppresses both the transition's height and the critical temperature of the SC. Simple FM/SC bilayers have also been studied. In these samples the switching effect is observed only for zero external field, while it is absent when a magnetic field is applied. This indicates that the ML structure is an essential ingredient for the generic observation of the switching effect. Our experimental results support recent theoretical studies referring to the mutual proximity effect (Kharitonov et al 2006 Phys. Rev. B 73 054511), to the possible formation of spin-triplet superconductivity (Bergeret et al 2001 Phys. Rev. Lett. 86 4096), and to the influence of an FM domain state on the properties of an SC (Buzdin and Mel'nikov 2003 Phys. Rev. B 67 020503( R)) in relevant hybrids. Apart from their importance for theory, our results are valuable for the design of spin-valve devices that recently have attracted great interest.