POSTER SESSION 1PO1: HTS Theory

Monday, Feb. 21, 2:30 p.m. – 4:30 p.m., Hall D (GRB)

*1PO1-1 Pairing Correlation of t-J Type Models Studied by the Power-Lanczos Method

C.T. Shih 1, Y.C. Chen 2, T.K. Lee 3. 1 Division of Science Application, National Center for High-Performance Computing, P.O. Box 19-136, Hsinchu, Taiwan. 2 Department of Physics, Tunghai University, Taichung, Taiwan. 3 Institute of Physics, Academia Sinica, Nankang, Taipei, Taiwan, and National Center for Theoretical Sciences, Hsinchu, Taiwan.

Presenting Author: T.K. Lee

We reported that the pair-pair correlation function of the two-dimensional t-J model does not have long-range d-wave superconducting correlation in the physical parameter range of J/t £ 0.5 [1]. The power-Lanczos method is used under the assumption of monotonic behavior, which has been well checked in the two-dimensional t-J and attractive Hubbard model. Here we examine this criterion for the t-J model in one dimension and two-leg ladders. The exponent of the power law decay of the pairing correlation behaves monotonically up to 12 powers and well in agreement with the DMRG results. The criterion is applied on the two-dimensional t-t'-J model. The results show that negative t'/t suppresses the pairing correlation further, and we conclude that there is also no long-range pairing correlation in the t-t'-J model with t'/t<0.

[1] C.T. Shih, Y.C. Chen, H.Q. Lin, and T.K. Lee, Phys. Rev. Lett 81, 1294, 1998.

*1PO1-2 Interplay of superconductivity with structural phases in a generalized t-J model

Roland Zeyher, MPI-FKF, Postfach 800665, 70506 Stuttgart, Germany, and Emmanuele Cappelluti, Dipart. di Fisica, Univ. di Roma I "La Sapienza", Piazzale A. Moro 2, 00185 Roma, Italy

Presenting Author: R. Zeyher

Based on 1/N expansions the interplay of superconductivity and structural phases such as flux, bond-order, and charge-density wave phases are dicussed within a t-J model including also Coulomb interactions. It is found that the optimal T_c for superconductivity as well as the fast decrease of T_c and the appearance of a pseudogap phase in the underdoped region can be understood as the result of a competition between an incommensurate flux and the superconducting phase, both having order parameters with d-wave symmetry. Theoretical results on the influence of non-magnetic impurities on the phase diagram agree well with recent experimental data in Zn-substituted La-214 and (Y,Ca)-123 systems on optimal doping and the behavior of the pseudogap phase. Including also spin fluctuation terms of order 1/N² it is found that spin fluctuations, often considered to be vital for the phenomena of high- T_c superconductivity, influence only little T_c. Finally, we comment on the occurrence of striped phases.

*1PO1-3 Hall response of strongly correlated electrons

P. Prelovsek 1,2, M. Long 3, D. Veberic 1, and X. Zotos 4. 1 Dept. of Theoretical Physics, Jozef Stefan Institute, Ljubljana, Slovenia. 2 Faculty of Mathematics and Physics, University of Ljubljana, Ljubljana, Slovenia. 3 Dept. of Physics, University of Birmingham, Edgbaston, United Kingdom. 4 Institut Romand de Recherche Numerique en Physique des Materiaux, Lausanne, Switzerland.

Presenting Author: P. Prelovsek

Theoretical understanding and the calculation of the Hall response of strongly correlated electrons is addressed. It is shown that the Hall constant R_H in a tight-binding model of correlated electrons on a ladder at T=0 can be expressed in terms of derivatives of the ground state energy with respect to external magnetic and electric fields. This novel method is used for the analysis of the t-J model on finite size ladders. It is found that for a single hole R_H is hole-like and close to the semiclassical value, while for two holes it can vary with ladder geometry. In odd-leg ladders, R_H behaves quite regularly changing sign as a function of doping, the variation being quantitatively close to experimental results in cuprates. The approach can be generalized to quasi-1D systems where R_H can be simply related to charge stiffness of the system. The temperature and frequency variation of R_H(w ) on ladders will be also discussed.

*1PO1-5 Microscopic Origin of the Pseudogap

Hiroshi Kamimura 1, Tsuyoshi Hamada 1, Masaru, Nishimura 1, and Hideki Ushio 2. 1 Institute of Physics, Science University of Tokyo, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan. 2 Tokyo National College of Technology, 1220-2 Kunugida-chou, Hachioji 193-8610, Japan.

Presenting Author: H. Kamimura

By paying attention to the contraction effect of the Cu-apical oxygen distance in a CuO₆ octahedron or a CuO5 pyramid in the underdoped cuprates, Kamimura and his coworkers [1] have recently clarified the key features of the many-electron electronic structures of underdoped cuprates from the first principles cluster calculations. According to them, a metallic state of the underdoped cuprates is characterized by the altenating appearance of the Zhang-Rice spin-singlet multiplet and the Hund's coupling spin-triplet multiplet in the spin-correlated region of local antiferromagnetic ordering due to the localized spins around Cu sites. This differs from the ordinary Fermi liquid theory. As a result the area of the Fermi surfaces is proportional to the doping concentration. When the hole concentration increases beyond the optimum doping to the overdoped region, the spin-correlation length decreases so that the local antiferromagnetic ordering disappears. Thus the Fermi surfaces change from small ones to larger ones in the overdoped region, where the latter is treated by the ordinary Fermi liquid. We will show that the pseudogap whose concept has been introduced by Loram et al.[2] from the standpoint of the Fermi liquid corresponds to the transition from the non-Fermi liquid nature in the underdoped regime to the Fermi liquid nature in the overdoped regime and that the superconducting gap at the d-wave scales with T_c. In this context the puzzling phenomena recently observed by tunnelling experiment for Bi2212 compounds by Miyakawa et al.[3] can be explained in terms of the pseudogap but not the superconducting gap. We will also discuss the origins for the change in the temperature dependences of the spin-susceptibility and electronic entropy including the electronic specific heat across the optimum doping concentration.

[1] H. Kamimura, et al. J. Phys. Condensed Matter 10, 11345 (1998); related references therein.

[2] J.W. Loram, et al. Phys. Rev. Lett. 71, 1740 (1993).

[3] N. Miyakawa, et al. Phys. Rev. Lett. 83, 1018 (1999).

1PO1-6 Electron momentum distribution of a single hole in planar antiferromagnet

A. Ramsak 1,2 and I. Sega 2. 1 Faculty of Mathematics and Physics, University of Ljubljana, 1000 Ljubljana, Slovenia. 2 J. Stefan Institute, University of Ljubljana, SI-1001 Ljubljana, Slovenia.

Presenting Author: A. Ramsak

We investigate the electron momentum distribution function for the two dimensional t-t'-J model doped with one hole on finite clusters by the method of twisted boundary conditions which enable the system to set into the true, zero-current ground state. The results are shown to quantitatively agree with our analytical results for a single hole in the antiferromagnetic background, based on the self-consistent Born approximation for the self-energy and the wave function. Moreover, within the self-consistent Born approximation an anomalous momentum dependence of momentum distribution is found, pointing to an emerging large Fermi surface. The analysis showed that the presence of next-nearest-neighbor hopping terms changes electron momentum distribution only quantitatively if the ground state momentum is at (1/2,1/2), and qualitatively if it is shifted at (1,0).

1PO1-7 Resonant Inelastic X-ray Scattering in Copper Oxides

Kenji Tsutsui, Hiroshi Kondo, Takami Tohyama, and Sadamichi Maekawa, Institute for Materials Research, Sendai 980-8577, Japan

Presenting Author: K. Tsutsui

Momentum dependence of resonant inelasitc x-ray scattering (RIXS) in one- (1D) and two-dimensional (2D) copper oxides is examined theoretically. Cu K-edge RIXS spectrum is calculated by using the exact diagonalization techniques on small clusters in 1D and 2D Hubbard models with 1s-core bands. In 1D case, we find the following characteristic features that can be detectable by RIXS experiments [1]: The spectrum with large momentum transfer indicates the formation of excitons, i.e., bound states of holon and doublon. The spectrum with small momentum transfer depends on the incident photon energy. In 2D case, we find that the long-range hoppings in the Hubbard model cause anisotropic momentum dependence in the spectrum [2]: The energy of the threshold of the spectrum at (p /2, p /2) is larger than that at (0,0), whereas the energy of the threshold at (p /2,0) is smaller than (0,0). The dependence is explained by the particle-hole exciations modified by the long-range hoppings. We propose that the RIXS provides a unique opportunity to study the upper Hubbard band in cuprates. We also discuss the doping dependence of the RIXS spectrum.

[1] K. Tsutsui, et al., cond-mat/-9909346.

[2] K. Tsutsui, et al., cond-mat/-9905372, to be published in PRL.

1PO1-9 Quasiparticle states near the surfaces with d-wave superconductivity and antiferromagnetism

Y. Tanuma 1, Y. Tanaka 2, M. Ogata 3, and S. Kashiwaya 4. 1 Graduate School of Science and Technology, Niigata University, Ikarashi, Niigata 950-2181, Japan. 2 Department of Applied Physics, Nagoya University, Nagoya 464-8063, Japan. 3 Department of Basic Science, Graduate School of Arts and Sciences, University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-0041, Japan. 4 Electrotechnical Laboratory, Tsukuba, Ibaraki 305-0045, Japan.

Presenting Author: Y. Tanuma

We study the quasiparticle states near the surfaces with the d-wave superconductivity and antiferromagnetism in the two-dimensional t-J model. By using the modified Gutzwiller approximation, we investigate the probability of the time-reversal symmetry breaking state near the surface in coexisting of the antiferromagnetism and d-wave superconductivity.