*1PO1-10 Density response of the t-J model and phonon renormalization
Peter Horsch, Giniyat Khaliullin, and Viktor Oudovenko, Max-Planck- Institut fur Festkoerperforschung, Heisenbergstr. 1, D-70569 Stuttgart, Germany
Presenting Author: P. Horsch
The density fluctuation spectrum, the physical electron Green's function, and the momentum distribution are calculated for the t-J model in the low-doping regime using a slave fermion method for the constrained fermions. The results are in good agreement with diagonalization results for the different quantities. The density response is characterized by incoherent, momentum dependent spectral functions reaching up to energies ~8t and a low-energy structure at energy ~J due to transitions within the quasiparticle band. The latter feature is shown to lead to an anomalous renormalization of planar bond-stretching and breathing phonon modes for intermediate and large momentum transfers. Our results are consistent with recent neutron scattering data showing the peculiar behavior of these modes [1]. Finally we present a comparison of the results for the density response with an earlier study based on the slave boson method [2], and discuss how the doping induced change of the Fermi surface topology affects the density fluctuation spectrum.
[1] R.J. McQueney et al., Phys. Rev. Lett. 82, 628 (1999).
[2] G. Khaliullin and P. Horsch, Phys. Rev. B 54, R9600 (1996).
1PO1-11 Physics of the Three Band Model in the Limit of Large S
George F. Reiter, TCSUH and Physics Department, University of Houston, Houston, TX 77204-5506
Presenting Author: G.F. Reiter
In the limit that the holes on the oxygen remain spin 1/2 but the spin on the copper is taken to be large, the three band model of the copper oxygen plane in the cuprates becomes a model describing a weakly coupled fermion and boson gas. The conducting polarons that form have small fermi surfaces at a low density of oxygen holes. We show that in this limit, the system is a d-wave BCS superconductor, the pairing being due to the exchange of single magnons, and that the fixed point describing this state is stable for large S values. Since the BCS theory does not give the correct phenomenology for the spin 1/2 limit of the model, we conjecture that there is another stable fixed point at smaller values of S, where the tendency to phase separation leads to a fixed point for which the fermion density fluctuations have become a relevant perturbation, and that this fixed point controls the spin 1/2 behavior. Some evidence for this is provided.
1PO1-12 Some Properties of Two-Band Superconductors: Role of k-Dependent Hybridization
V.P. Ramunni 1, G.M. Japiassu 2, and A. Troper 1. 1 Centro Brasileiro de Pesquisas Fisicas, Rue Xavier Sigaud, 150, Rio de Janeiro, 20550-103, RJ, Brazil. 2 Instituto de Fisica, Universidade Federal do Rio de Janiero, Caixa Postal 68528, Rio de Janeiro, 21945-970, RJ, Brazil.
Presenting Author: A. Troper
We discuss the influence of a k-dependent hybridization on a superconducting material described by a two-band model, within an extended BCS-picture. The effect of hybridization is to create in the normal state new bands with mixed features and the k-dependence arises from its non-local character. We consider a broad s-p conduction band hybridized with a narrow d-band, assuming that the pairing occurs at the d-band (singlet d-d pairing). We calculate self-consistently the BCS type order parameter D and the critical temperature Tc. We verify that Tc is stable when hybridization is turned on until a critical value ov V=Vc and both the "phononic" and electronic parts of Tc are renormalized by a parameter a , which gives the ratio of the effective band masses. We calculate the value of 2D (0)/kBTc and compare with standard BCS results. The isotope effect is almost the same in the whole range of 0£ V£ Vc. We also discuss the features of a constant hybridization between the overlapping of s-p and d-bands close to the Fermi level.
1PO1-13 Relating scaling and pseudogap behaviour in strongly correlated superconductors
Jan R. Engelbrecht and Alexander Nazarenko, Physics Dept, Boston College, Chestnut Hill, MA 02167
Presenting Author: J.R. Engelbrecht
We use self-consistent diagrammatics to study the superconducting transition in the attractive Hubbard model in two and three dimensions. Remarkably, we find that this approximate method captures the expected critical scaling in the correlation length and pair susceptibility. In 2D the critical regime is very large, extending to several times the KTB transition temperature and coincides with the temperature range over which pseudogap behaviour develops in the normal state. This behaviour is very similar to what is observed for underdoped cuprates and our results suggest that the increased anisotropy of underdoped cuprates contributes to their enhanced pseudogap behaviour.
1PO1-14 Competition Between Superconductivity and SDW in the 2D Hubbard Model Studied by the Variational Monte Carlo Calculations
K. Yamaji, T. Yanagisawa, and S. Koike, Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba 305-8568, Japan
Presenting Author: K. Yamaji
Using finite-size scaling we obtained the bulk-limit of the superconducting (SC) condensation energy in the titled model with next-nearest-site transfer energy t'. When on-site Coulomb energy U was fixed at 8, the bulk-limit values were established to be finite for the electron density equal to or greater than 0.84 and t' in the range between -.25 and -.10 and found comparable to the experimental value for YBCO7 (the energy unit is nearest-neighbor transfer energy t). We have also computed the condensation energy for the antiferromagnetic (SDW) phase with the commensurate wave vector in the same parameter region. Unexpectedly, it increases when negative t' increases its absolute value and overwhelms the SC condensation energy. Now we are searching for parameter regions with realistically smaller values of U where the SC condensation energy wins over the SDW one. The results will be shown in the presentation. In view of sharp increase of the SDW condensation energy with decreasing hole-doping the superconductivity in the underdope regime is argued to survive only in such spatially restricted regions as made by SDW discommensurations.
1PO1-15 Non-Fermi liquid states in an extended Hubbard model
J.L. Smith and Qimiao Si, Physics Dept., Rice University, Houston, TX 77005-1892
Presenting Author: J.L. Smith
We study the two-band extended Hubbard model using an extended dynamical mean field theory (DMFT). This extended DMFT goes beyond the usual infinite-dimensional DMFT by taking into account both local and intersite interactions on an equal footing. In the mixed valence regime the model displays both Fermi liquid and non-Fermi liquid phases, depending on the strength of certain density-density interaction parameters. In particular, there exists an intermediate phase with spin-charge separation: it has coherent spin excitations but incoherent charge excitations.
1PO1-16 A new analysis of the 2-layer Hubbard model
Adolfo Avella, Ferdinando Mancini, and Volodymyr Turkowski, Dipartimento di Scienze Fisiche "E.R. Caianiello", Universita' di Salerno, 84081 Baronissi (SA), Italy
Presenting Author: A. Avella
The 2-layer Hubbard model is studied by means of the composite operator method in the static approximation, by using open and periodic boundary conditions. Local and thermodynamics properties are in good agreement with the results of numerical simulation. The calculation of specific heat and spin magnetic susceptibility gives evidence that the model can be used to describe the yttrium family of high Tc cuprates.
1PO1-17 The Hubbard model: 4-pole approximation
Adolfo Avella and Ferdinando Mancini, Dipartimento di Scienze Fisiche "E.R. Caianiello", Universita' di Salerno, 84081 Baronissi (SA), Italy
Presenting Author: A. Avella
The Hubbard model has been studied, by means of the Composite Operator Method in the static approximation, with a four pole structure for the electronic Green's function. Eigenoperators of the on-site Coulomb interaction have been chosen as basis. Constraints, coming from the Pauli principle, have been used to find a self-consistent solution. A comparison with Hubbard III solution and QMC data, where available, have been done with a resulting good agreement.
1PO1-18 Superconducting phase fluctuations in the negative -U Hubbard model
Tadeusz K. Kopec, Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, 50-950 Wroclaw, Poland
Presenting Author: T.K. Kopec
We explore the role of phase fluctuations in the pseudogap regime. Starting from the order parameter phase fluctuations (and deferring the issue of the origin of the pairing) we elaborate on mechanism that would destroy phase coherence while preserving the gap in the excitation spectrum. We use the methods of functional integration for self-consistent formulation of the superconducting phase transition of pre-formed composite bosons in the extended negative-U Hubbard model in the intermediate t/U limit. By employing the representation in terms of the amplitude and phase of the bound pair we derive the effective phase Hamiltonian and exploit the effects associated with coherent bound pair tunneling at the microscopic level - responsible for the phase coherent state.
1PO1-19 d-wave hole pairing and its thermodynamics in generalized Hubbard systems
Luis A. Perez and Chumin Wang, Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México, Apartado Postal 70-360, 04510, México D.F., Mexico
Presenting Author: L.A. Perez
A dx2-y2 paired ground state is found within a two-dimensional generalized Hubbard model, in which a next-nearest-neighbor correlated-hopping interaction is considered. In spite of its smaller strength in comparison with other terms of the model, we have found its key participation in the formation of the d-channel hole pairs. Moreover, the on-site repulsive Coulomb interaction favors the formation of these pairs. A clear pairing asymmetry between electrons and holes is also observed, where the hole pairing occurs in a realistic regime of interactions [1]. Finally, the thermodynamics of the d-channel pairing states, with and without antiferromagnetic background, are comparatively analyzed by using the variational method. We have calculated the critical temperature as a function of the hole density and the superconducting gap as a function of temperature, where consistent behaviors are observed in comparison with the experimental data obtained from the high Tc materials.
[1] L.A. Pérez and C. Wang, Solid State Commun. 108, 215 (1998).
This work was supported partially by CONACyT-25455E, and UNAM-CRAY-SC008697. L.A.P. acknowledges the UNAM Ph.D. scholarship and support from PAEP-003304.