Showing papers by "C. A. Balseiro published in 1992"
TL;DR: It is found that the four-spin exchange competes with the effects of frustration and this competition leads to a very rich phase diagram in the frustration-cyclic exchange plane.
Abstract: We study the effect of four-spin cyclic-exchange interactions on the magnetic properties of the spin-1/2 Heisenberg antiferromagnetic on a two-dimensional square lattice with the two-spin interactions between nearest and next-nearest neighbors. We found that the four-spin exchange competes with the effects of frustration and this competition leads to a very rich phase diagram. We use the standard spin-wave formalism, the perturbative expansions around various dimerized states and finite-cluster calculations to study the locations of different phases of the model. The phase diagram in the frustration-cyclic exchange plane involves two collinear phases [(\ensuremath{\pi},\ensuremath{\pi}) and (0,\ensuremath{\pi})], three canted states, and two different dimerized configurations.
28 citations
TL;DR: The negative-{ital U} Hubbard model for arbitrary values of the attractive interaction and band filling is studied by means of the slave-boson mean-field theory and shows that for the superconducting state the approach gives results similar to those obtained in the usual BCS mean field.
Abstract: We study the negative-U Hubbard model for arbitrary values of the attractive interaction U and band filling by means of the slave-boson mean-field theory. We analyze both the normal and the superconducting states. We show that for the superconducting state our approach gives results similar to those obtained in the usual BCS mean field; only small renormalization of the bare parameters are obtained for U of the order of the bandwidth W. The energy difference between the normal and the superconducting states is maximum for U\ensuremath{\simeq}W. This energy difference is a measure of the superconducting critical temperature, which increases exponentially for small U and decreases as 1/U for large U. We discuss briefly the pair-breaking excitations.
16 citations
TL;DR: The collective excitations of a system with local attractive interactions as a function of the coupling strength are studied, and the effect of these excitations in the thermodynamic properties of the system is briefly discussed.
Abstract: We study the collective excitations of a system with local attractive interactions as a function of the coupling strength. The two-particle excitations are obtained in the random-phase approximation. For the strong-coupling limit we reproduce the excitation spectrum of a hard-core Bose gas, while in the weak-coupling limit we obtained a spectrum consistent with the BCS picture. The evolution of the excitation spectrum from the weak- to the strong-coupling limits is continuous. We briefly discuss the effect of these excitations in the thermodynamic properties of the system.
15 citations
TL;DR: The Fermi surface obtained from the one-particle spectral functions is consistent with Luttinger's theorem, but the Hall conductivity changes from electron like for a dilute system to holelike for a nearly half-filled system, indicating the deconfinement of charge and spin excitations in a two-dimensional system.
Abstract: We calculate exactly the one-particle spectral densities and Hall conductivity for the two-dimensional Hubbard model in the large-U limit, for two different finite clusters. The Fermi surface obtained from the one-particle spectral functions is consistent with Luttinger's theorem, but the Hall conductivity changes from electronlike for a dilute system to holelike for a nearly half-filled system, indicating the deconfinement of charge and spin excitations in a two-dimensional system.
15 citations
12 citations
TL;DR: In this article, the t−J model is studied in one dimension for different particle densities using exact diagonalizations of finite chains, and a phase diagram is presented, which includes a large region where two-particle bound states are stable.
4 citations
TL;DR: It is shown that the BCS approximation gives results in good agreement with the exact ones for the ground-state energy and wave function in the whole range of {ital U}, in the one-particle excitation spectrum.
Abstract: We study the negative-{ital U} Hubbard model by exact diagonalization of a 4{times}4 cluster with different numbers of particles. We calculate the ground-state energy and wave function as well as the one-particle, charge- and spin-excitation spectra. We show that the BCS approximation gives results in good agreement with the exact ones for the ground-state energy and wave function in the whole range of {ital U}. In the one-particle excitation spectrum the BCS dispersion relation gives the position of the most intense peaks of the exact calculation. Lower-energy (quasiparticle) excitations, which are interpreted as strongly dressed Bogoliubov quasiparticles, also appear. The occurrence of pair-breaking and collective excitations in the charge- and spin-excitation spectra and the interplay between one-particle and collective modes are discussed.
4 citations
2 citations
TL;DR: For larger values of [ital t][prime] the superconducting critical temperature increases almost linearly with the interplane coupling, and the critical temperature for systems consisting of weakly coupled planes is calculated.
Abstract: We calculate the superconducting critical temperature for systems consisting of weakly coupled planes. We consider an attractive-[ital U] Hubbard model in a tetragonal structure. The anisotropy is characterized by the ratio between the hopping matrix elements within the planes ([ital t]) and between them ([ital t][prime]). We calculate the critical temperature [ital T][sub [ital c]] for the onset of off-diagonal long-range order by calculating fluctuations around the mean-field solution. For independent planes ([ital t][prime]=0) [ital T][sub [ital c]] is zero and each plane has a Kosterlitz-Thouless transition at a temperature [ital T][sub KT]. As the interplane coupling increases [ital T][sub [ital c]] increases very rapidly and for [ital t][prime]/[ital t][congruent]0.03 we obtain [ital T][sub [ital c]][congruent][ital T][sub KT]. For larger values of [ital t][prime] the superconducting critical temperature increases almost linearly with the interplane coupling.
1 citations