Bumsoo Kyung

Density-Induced Breaking of Pairs in the Attractive Hubbard Model

A conserving T-matrix approximation is applied to the two-dimensional attractive Hubbard model in the low-density regime. A set of self-consistent equations is solved in the real-frequency domain to avoid the analytic continuation procedure. By tuning the chemical potential the particle density was varied in the limits 0.01 < n < 0.18. For the value of the attractive potential U=8t the binding energy of pairs monotonically decreases with increasing n, from its zero-density limit 2.3t and vanishes at a critical density n=0.19. A pairing-induced pseudogap in the single-particle density of states is found at low densities and temperatures.

Evolution of the Fermi surface geometry in high Tc superconductors: from small hole pockets to a large tight binding Fermi surface

Abstract The evolution of the Fermi surface geometry in high T c superconductors is studied within the t – J model with increasing hole doping concentration. The drastic change of the Fermi surface geometry from small hole pockets to a large tight binding Fermi surface happens at about 10% doping concentration much faster than a rigid band picture predicts. This rapid change is caused by the strong dynamical effect of interacting doped holes on the spin–polaron quasiparticle band. The decreasing spectral weight of the quasiparticle band is compensated by the significant band flattening near the Fermi energy for an optimal doping case so that the high density of states is still preserved in agreement with experiments.

Resonance state in the 2D attractive Hubbard model

The systematic change of a resonance state with high momenta is studied with increasing particle density in the 2D attractive Hubbard model. Within the conserving self-consistent T-matrix approximation, we present the spectral functions for the one and two particle Greens functions as well as the self-energy. In the small density limit, the resonant state becomes stable and the result from the self-consistent calculations shows a good agreement with that from a simple analytical calculation. As particle density is increased, the resonance state acquires a short lifetime due to the increasing decay into two free particles.

The pseudogap and photoemission spectra in the attractive Hubbard model

Angle-resolved photoemission spectra are calculated microscopically for the two-dimensional attractive Hubbard model. A system of self-consistent T-matrix equations are solved numerically in the real-time domain. The single-particle spectral function has a two-peak structure resulting from the presence of bound states. The spectral function is suppressed at the chemical potential, leading to a pseudogap-like behaviour. At high temperatures and densities the pseudogap diminishes and finally disappears; these findings are similar to the experimental observations for the cuprates.

PRECURSOR OF ANTIFERROMAGNETIC BANDS IN THE HALF-FILLED TWO-DIMENSIONAL HUBBARD MODEL

We study the half-filled two-dimensional Hubbard model in the intermediate coupling regime. As temperature is decreased for a two-dimensional half-filled band, two-dimensional critical fluctuations give rise to a strong local maximum in |Im Σ(kF, ω)| at ω=0, leading to a split peak with a pseudogap inside in the spectral function and the density of states. The calculated energy dispersion is in good agreement with that of spin density wave approximation.

Evolution of hole and spin dynamics in high-Tc superconductors within the small hole density limit of the t - J model

Abstract The evolution of hole and spin dynamics in high Tc superconductors is studied within the self-consistent noncrossing approximation of the t - J model in the small hole density limit. As the doping concentration is increased, long-range electron correlations disappear rapidly and the quasiparticle energy band becomes considerably narrow. At a small hole density of a few percent, long-range antiferromagnetic order is destroyed leading to the inadequacy of spin wave basis approximation near small wave vectors. Spin excitations near the antiferromagnetic zone boundary are strongly renormalized and damped, but they are still well described within spin wave basis approximation.