Wang Yu-Peng

The Minimum Model for the Iron-Based Superconductors

A single band t-U-J1-J2 model is proposed as the minimum model to describe the superconductivity of the newly discovered iron-based superconductors R(O1-xFx)FeAs and RO1-xFeAs (R = La, Ce, Sm, Pr, Nd, Gd). With the mean-field approach, it is found that the pairing occurs in the d-wave channel. In the likely parameter region of the real materials, by lowering temperature, the system enters firstly the dxy superconducting phase with D4h-symmetry and then enters the time-reversal-symmetry-broken dxy + idx2-y2 superconducting phase with C4h-symmetry.

Effects of Conduction Electron Band Structure on Transport of Quantum Dot Systems

We study the effects of the energy band structure of conduction-electron on the transport properties of an interacting quantum dot system. By applying the nonequilibrium Keldysh Green function technique, we show that the finite width of electron band in leads causes the negative differential conductance in some regions of the applied voltage. We also show that the van Hove singularities in the density of states of conduction-electron do not qualitatively change the differential conductance of the system, and hence can be safely ignored. Therefore, the wide band approximation used in the previous investigations is partially justified.

Integrability of the Inozemtsev Spin Chain with Open Boundary Conditions

A long range interacting quantum spin chain with open boundary conditions is proposed. By constructing the reflection spin-Dunkl operators, the integrability of the model is proven. The model is therefore exactly solvable via the asymptotic Bethe ansatz method and falls into the universal class of the usual Heisenberg spin chain with boundary fields.

Low-temperature specific heat of superconducting MgB2

The specific heat of the recently discovered superconductor MgB2 has been measured at temperatures ranging from 4.5 to 80 K. The superconducting anomaly ΔC at Tc is clearly observed. The total specific heat in the normal state can be well fitted by electronic and phonon contributions. The Debye temperature θD is found to be 737 K, much larger than other intermetallic superconductors. The normal-state electronic specific heat coefficient γ is found to be 2.48±0.5 mJ/molK2 and ΔC/γTc is between 1.41 and 2.15.

Exclusion statistics: a generalized description

By constructing the fused-particle representation of the free boson gas, we propose a description of the exclusion statistics which allows us to connect the Bose-Einstein statistics and the Fermi-Dirac statistics smoothly. With an proper choice of the exclusion factors gamma(t), Hadane-Wus fractional statistics is retrieved in this representation.

Critical behaviour of the one-dimensional ferromagnetic t-J model

The one-dimensional super-symmetric ferromagnetic t-J model is studied via the thermal Bethe ansatz. Analytic expressions of the free energy for T→0 are obtained. A new critical behaviour beyond the universal class of Luttinger liquids is found in this system.

Boundary Quantum Entanglement of the XXZ Spin Chain with Boundary Impurities

The boundary quantum entanglement for the s = 1/2 XXZ spin chain with boundary impurities is studied via the density matrix renormalization group (DMRG) method. It is shown that the entanglement entropy of the boundary bond (the impurity and the chain spin next to it) behaves differently in different phases. The relationship between the singular points of the boundary entropy and boundary quantum critical points is discussed.

Entanglement in Anisotropic Heisenberg Model with Non-Uniform External Fields

We study entanglement properties of the three-qubit anisotropic Heisenberg model with both uniform and non-uniform external magnetic fields. Analytic expressions for the measures of entanglement at the ground state are obtained. We show that the pairwise entanglement and global entanglement of the system at the ground state clearly depend on the strength and configuration of external fields. The entanglement between some pairs can be enhanced by non-uniform external fields.

The Green Function Approach to the Two-Dimensional t-J model

We investigate the two-dimensional t-J model by the Green function approach with the Hubbard operator. The phase diagram, including the Neel temperature and the superconducting transition temperature simultaneously, is derived. The results are consistent with experiments of cuprate compounds.

D-Wave Kondo Problem in a Square Tight-Bounding Lattice

A Kondo model in a square tight-bonding lattice of conduction electrons is considered. The Kondo interaction takes a d-wave. Using mean-field theory and numerical simulations, it is found that there is a pseudo-gap for the impurity density of states at the Fermi level. Numerical results also show that the exponents of the low-temperature thermodynamic quantities are non-universal but dependent on the interaction.