Narrow Results

The nature of vortex structure in the mixed state of high-temperature superconductors (HTS) is investigated by solving the Bogoliubov-de Gennes equations with consideration of competition between antiferromagnetic (AF) and d-wave superconductivity (DSC) orders. By varying the applied magnetic field and temperature, the geometry of vortex structure can take two different forms: conventional vortex lattice (triangular or square), or vortex stripe phases where all the order parameters including spin density wave, charge density wave and superconducting order exhibit stripe-like behavior. This novel vortex stripe phases may show up at low temperature and adjacent to upper critical field H_c2 Phase diagram of temperature dependence of H_c2 will be presented. Our results may shed light on the understanding of the low-temperature H_c2 anomalies in some HTS. New experiments are proposed to test our predictions.

Based on the extended Hubbard model with the next nearest neighbor hopping term, we study the impurity states in a slightly underdoped (with 0.13 hole) system with competition between d-wave superconductivity and antiferromagnetism by solving the mean field Bogoliubov-de Gennes equations self-consistently. Nickel and Zinc impurities are studied respectively using the potential scattering impurity (PSI) model and the Anderson impurity (AI) model. The significant difference in their local density of states may be useful to predict the impurity states in the stripe phase of the superconductors in the slightly underdoped region.

In this paper, we study phase separation in the two-dimensional single-band Hubbard model with the unrestricted Hartree-Fock(UHF) method and the restricted Hartree-Fock (RHF) method. We perform the calculation for square lattices and rectangle lattices. It is observed that the stripe phase exists and it depends on three aspects: geometry of the lattice, Coulomb interaction U and band filling n. To gain more physical insights, we consider the Hubbard model with spin dependent hoppings: t_↑ and t_↓, and study the effect of varying on the phase separation.

Striking resonances, identified as pinning modes, have recently been observed in the rf spectra of the stripe phases of extremely low disorder two-dimensional electron systems with two or more filled Landau levels. We present spectra which detail the dependence of this resonance on the Landau filling factor ν, in broad ranges which include the stripe phases in the Landau levels with N = 2, 3 and 4.

We use a bond-operator mean-field theory and a generalized spin-wave theory to study the frustrated spin-1/2 dimer Heisenberg model on a square lattice with frustrations in both cardinal and corner directions. We establish the ground state phase diagram with very rich phases. It has been disclosed that the frustrations in cardinal directions can induce exotic superfluid (SF) and supersolid (SS) phases. By introducing the frustrations in corner directions, we found more interesting phases, such as the striped superfluid (S-SF) and the striped supersolid (S-SS). We also investigate the excitation spectra and analyze the anisotropic spin-wave velocities of the stripe phases.