VI Training Course in the Physics of
Correlated Electron Systems and High-Tc Superconductors
Vietri sul Mare (Salerno) Italy
8 - 19 October 2001
Participant Seminar Abstracts
Laboratorium voor Vaste-Stoffysica en Magnetisme, Katholieke Universiteit, Leuven, Belgium
Fluctuation Conductivity in Superconductivity in Strong Electric Field
Abstract: We study the effect of a strong electric field on the fluctuation conductivity within the time-dependent Ginsburg-Landau theory for the case of arbitrary dimension. Our results are based on the analytical derivation of the velocity distribution law for the fluctuation Cooper pairs, from the Boltzmann equation. Special attention is drawn to the case of small nonlinearity of conductivity, which can be investigated experimentally. We obtain a general relation between the nonlinear conductivity and the temperature derivative of the linear Aslamazov-Larkin conductivity, applicable to any superconductor. For the important case of layered superconductors we derive an analogous relation between the small nonlinear correction for the conductivity and the fluctuational magnetoconductivity. On the basis of these relations we provide new experimental methods for determining both the lifetime constant of metastable Cooper pairs above Tc and the coherence length. A systematic investigation of the 3rd harmonic of the electric field generated by a harmonic current can serve as an alternative method for the examination of the metastable Cooper-pair relaxation time.
Institute of Physics, Jagellonian University
Magnetic and orbital ordering in doped manganites
Abstract: I present the quantum corrections to the order parameter and to the ground state of energy in some spins systems as function of antiferromagnetic interaction size and dimension. In the second part I consider the correlation between magnetic order and orbital structure in doped manganites La1-x(Sr,Ca)xMnO3.
Institute of Solid State and Material Research Dresden
Calculated de Haas-van Alphen data and Plasma Frequencies of MgB2 and TaB2
Abstract: The de Haas van Alphen frequencies as well as the effective masses for a magnetic field parallel to the crystallographic C direction are calculated for MgB2 and TaB2. In addition, we analyze the plasma frequencies. We find a large anisotropy of Fermi velocities in MgB2 in difference to the more isotropic behavior in TaB2.
Institute for Solid State Research, IFW-Dresden
High resolution ARPES of BSCCO using circularly polarized light
Abstract: First high resolution ARPES experiment on BSCCO using circularly polarized light reveals some new details in the low lying electronic structure. Electrons in the final state are spin-polarized, strongly suggesting the excitation from the bands splitted by the spin-orbit interaction. Clearly antisymmetric behavior of the asymmetry, detected experimentally, together with the assumption that the time reversal symmetry is not broken implies the presence of the center of inversion, i.e. proves the bulk nature of the studied phenomenon.
Marian Smoluchowski Institute of Physics, Jagellonian University, Krakow
ELECTRON LOCALIZACTION FROM THE COMBINED EXACT DIAGONALIZATION - AB INITIO APPROACH
Abstract: Exact ground state properties are presented by combining the exact diagonalization of the Hamiltonian (including ALL hopping integrals and ALL two-site interactions) with ab initio optimization of the single-particle (Wannier) functions. Such a procedure is carried out for linear systems up to N=12 atoms. We study the system evolution as a function of lattice parameters. Electrons are essentially localized for the interatomic distance R > 2.2 Å. The statistical distribution evolves from the Fermi-liquid limit (at R ≈ 1-2 Å) to continuous momentum distribution for R> 2.2 Å. The Mott and the Hubbard criteria of localization are compared.
MPI-PKS Dresden
A new spinwave expansion for the ordered Kondo lattice
Abstract: A new spinwave expasion scheme for magnetically ordered Kondo lattice models is described, together with its application to quantum effects in double exchange systems.
Mr. Alejandro Vladimiro Silhanek
Centro Atómico Bariloche-Instituto Balseiro
Effects of a Non-local Electrodynamics on the Reversible and Irreversible Properties of Borocarbides
Abstract: After Kogan and co-workers showed in 1996 that nonlocal electrodynamics effects are relevant in high-k superconductors in a broad H-T domain, several works demonstrated that nonlocality give rise to unusual properties of the vortex matter. Those studies were done mainly on the family of compounds {R}Ni_2B_2C, where {R} = Lu, Y, Tm, Er, Ho, Dy. These borocarbides are very well suited to study nonlocal effects due to their intermediate k values (10 - 20) that result in a wide field range where the extended London description holds, their high transition temperature T_c and the possibility to fabricate clean single crystals with large electronic mean free path. Although the consequences of nonlocality on the equilibrium properties of the superconducting vortex matter is by now convincingly established, very little is known about its effects on the nonequilibrium vortex response. In this talk I will show that the pinning force density F_p for (Lu-Y)Ni_2B_2C exhibits a rich anisotropic behaviour that sharply contrasts with its small mass anisotropy. Beside this, we observe that nonlocal electrodynamics influences pinning via the unusual behavior of the shear modulus in non-hexagonal lattices, either through continuous variations (fourfold basal plane anisotropy) or abrupt jumps (the kink at H_1).
Max-Planck-Institute for the Chemical Physics of Solids
Superconductivity in the new germanium clathrates
Abstract: We investigate the low temperature properties of the recently discovered clathrates Ba6Ge25 and Na2Ba4Ge25 by tuning both materials with hydrostatic pressure. At ambient pressure, Ba6Ge25 undergoes a two-step structural phase transition between 230K and 180K from metallic behavior to a high-resistivity state. A superconducting transition occurs at K out of the resulting bad metal ( ). With increasing pressure, the structural phase transition is shifted to lower temperature but increases drastically. reaches a maximum value of 3.85K at the critical pressure GPa, where the structural distortion is completely suppressed and the system exhibits metallic behavior. Higher pressures above lead to a slight decrease of . On replacing 1/3 of the Ba atoms with Na (Na2Ba4Ge25), no structural transformation is observed below room temperature, and the superconducting transition temperature is higher ( K) than in the undoped case at ambient pressure. The pressure-dependence of Na2Ba4Ge25 resembles the case of Ba6Ge25 above . Keywords: high pressure; clathrates; superconductivity; structural phase transition.
