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. Luigi Amico

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. Victor Yarzhemsky

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. Balazs Dora

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. Anna Posazhennikova

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. Marcin Raczkowski

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

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

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. Yasuhiro Saiga

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.

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