**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.