**VII Training Course in the Physics of**

**Correlated Electron Systems and High-Tc
Superconductors**

*Vietri sul Mare (Salerno) Italy*

*14 - 25 October 2002*

*Participant Seminar
Abstracts*

*
15/10/2002*

Dr.LuigiAmico

Dipartimento di Metodologie Fisiche e Chimiche per l'Ingegneria, Università di Catania

**Scaling of entanglement close to quantum phase transitions**

**Abstract:**
We discuss the entanglement near a quantum phase transition by analyzing the properties of the concurrence for a class of exactly solvable models in one dimension. We find that entanglement can be classified in the framework of scaling theory. Further, we reveal a profound difference between classical correlations and the non-local quantum correlation, entanglement: the correlation length diverges at the phase transition, whereas entanglement in general remains short ranged.[ Nature 416, 608 (2002)]

*
15/10/2002*

Prof.VictorYarzhemsky

Institute of General and Inorganic Chemistry of RAS

**Spece -group approach to the wavefunction of a Cooper pair and its applications high-temperature superconductors**

**Abstract:**
A standart theory of space-groups based on the induced representation method is applied to construct Cooper pair wavefuctions as zero-total
momentum states obeying Pusi exclusion principle. It is shown that in many case the results are similar to that of point-group approach.
The diffences of the space-group approach and point group approach are duscussed. The method is applied to UPt3, for which E2u symmetry
of superconduction order parameter is obtained and and to perovskite systems as high-temperature supercunductors and Sr2RuO4.
The influence of different types of time reversal symmetry violation on the SOP structure is discuseed (see papers 1,2 and 5)

*
16/10/2002*

Dr.BalazsDora

Department of Physics, Budapest University of Technology and Economics

**Unconventional density waves in quasi-one dimensional systems**

**Abstract:**
We consider the possibility of formation of unconventional charge and spin density waves (UCDW, USDW) in quasi-one dimensional electronic systems. In analogy with unconventional superconductivity, we develop a mean field theory of UDW allowing for momentum dependent gap on the Fermi surface. Conditions for the appearance of such a low temperature phase are investigated. The thermodynamic properties are found to be very similar to those of d-wave superconductors. The linear (optical conductivity) and nonlinear (threshold electric field) response is calculated. These theoretical results describe convincingly the low temperature phase of the $\alpha$-(BEDT-TTF)$_2$KHg(SCN)$_4$ salt.

*
18/10/2002*

Dr.AnnaPosazhennikova

Katholieke Universiteit Leuven

**On the toy model of pseudogap**

**Abstract:**
The problem of pseudo-gap formation in an electronic system,
induced by the fluctuations of the order parameter is revisited.
We make the observation that a large class of current theories are
theoretically equivalent to averaging the Free energy of the
pseudo-gap system over quenched-disordered distribution of the
order parameter. We examine the cases of both infinite and finite
correlation length, showing how the interplay of pseudo-gap
formation and superconductivity can be treated in this approach.

*
21/10/2002*

Mr.MarcinRaczkowski

Institute of Physics, Jagellonian University

**Competition between Vertical and Diagonal Static Stripes in the HF approximation**

**Abstract:**
The charge localization and tendencies of doped holes towards
self-oganization into striped patterns is one of the most interesting
topics in the physics of high-$T_c$ superconductors. Qualitative picture of
stable static stripe phases can be given within the single band Hubbard model
using Hartree-Fock approximation. Here we investigate the
properties and stability of the filled (one doped hole per stripe site)
vertical stripes (VS) and diagonal stripes (DS) by varying the on-site
Coulomb repulsion $U$ for two representative doping levels $x=1/8$ and
$x=1/6$, and reveal the microscopic reasons of the observed
transition from VS to DS with increasing $U$. In the weak coupling regime
of U=4t, where t is the hopping element, the stability of VS
is best explained by the solitonic mechanism which leads to the kinetic
energy gain due to the hopping perpendicular to the stripes. On the
contrary, the stability of DS in the strong coupling regime of U=6t
is less obvious. We show that the charge densities along DS (m_i^z=0)
are lower than along VS, and the nonequivalent atoms within antiferromagnetic
domains in the case of DS have larger site magnetization densities
and consequently lower probabilities of double occupancy. Hence, DS have
a more favorable potential energy which explains their stability in the
large $U$ regime.

*
22/10/2002*

Mr.MarcosRigol Madrazo

Institut für Theoretische Physik III, Universität Stuttgart

**Quantum Monte Carlo study of confined fermions in 1-D optical lattices**

**Abstract:**
Quantum Monte Carlo simulations are used to study the ground state of the one dimensional fermionic Hubbard Model in a harmonic trap. Local phases appear in the system and a local order parameter is defined to characterize them. The establishment of the Mott phase does not proceed via the traditional quantum phase transition. Important implications for the experimental study of these systems are deduced.

*
23/10/2002*

Mr.AdamRycerz

Marian Smoluchowski Institute of Physics, Jagellonian University

**On metal-insulator transition for a one-dimensional correlated nanoscopic chain**

**Abstract:**
We have applied our novel numerical scheme combining Lanczos diagonalization
in the Fock space with an ab initio renormalization of the single-particle
(Wannier) functions, to study the ground state properties of the Extended
Hubbard Model. Through the finite-size scaling we determine the discontinuity
of the momentum distribution Fermi surface. Our results imply Fermi-liquid
behavior for lattice parameter a < 3 a0 (a0 is the Bohr radius) and
zero-temperature transition to the localized spin system for larger a.
Future applications of the method are listed. The talk will be complemented
by possible experimental verifications of the presented theoretical results,
in respect to recently discussed limitations of ARPES experiments for
one-dimensional systems.

*
24/10/2002*

Dr.YasuhiroSaiga

Department of Physics, Tokyo Institute of Technology

**Two-Dimensional t-J Model in a Staggered Field**

**Abstract:**
The two-dimensional t-J model in a staggered field is studied by numerically exact
diagonalization up to 20 sites.
For the low-hole-density region and a realistic value of J/t, it is found that the presence of
staggered field strengthens the attraction between two holes.
With increasing field, the d_{x^2-y^2}-wave superconducting correlations are enhanced while the
extended-s-wave ones hardly change.
This implies that coexistence of the d_{x^2-y^2}-wave superconducting order and the commensurate
antiferromagnetic order occurs in a staggered field.