You-Quan Li

Entanglement and Quantum Phase Transition in the Extended Hubbard Model

We study quantum entanglement in a one-dimensional correlated fermionic system. Our results show, for the first time, that entanglement can be used to identify quantum phase transitions in fermionic systems.

Entanglement, quantum phase transition, and scaling in the XXZ chain

Motivated by recent development in quantum entanglement, we study relations among concurrence C, SU{sub q}(2) algebra, quantum phase transition and correlation length at the zero temperature for the XXZ chain. We find that at the SU(2) point, the ground state possesses the maximum concurrence. When the anisotropic parameter {delta} is deformed, however, its value decreases. Its dependence on {delta} scales as C=C{sub 0}-C{sub 1}({delta}-1){sup 2} in the XY metallic phase and near the critical point (i.e., 1<{delta}<1.3) of the Ising-like insulating phase. We also study the dependence of C on the correlation length {xi}, and show that it satisfies C=C{sub 0}-1/2{xi} near the critical point. For different sizes of the system, we show that there exists a universal scaling function of C with respect to the correlation length {xi}.

SU(4) Theory for Spin Systems with Orbital Degeneracy

The isotropic limit of spin systems with orbital degeneracy has global SU(4) symmetry. On many 2D lattices, the ground state does not possess long-range order, which may explain the observed spin liquid properties of LiNiO{sub 2} . In the SU(4) N{acute e}el-ordered state, spin-spin correlations can be antiferromagnetic between two neighboring sites with parallel magnetic moments. {copyright} {ital 1998} {ital The American Physical Society }

SU(2) × U(1) unified theory for charge, orbit and spin currents

Spin and charge currents in systems with Rashba or Dresselhaus spin–orbit couplings are formulated in a unified version of four-dimensional SU(2) × U(1) gauge theory, with U(1) being the Maxwell field and SU(2) being the Yang–Mills field. While the bare spin current is non-conserved, it is compensated by a contribution from the SU(2) gauge field, which gives rise to a spin torque in the spin transport, consistent with the semi-classical theory of Culcer et al. Orbit current is shown to be non-conserved in the presence of electromagnetic fields. Similar to the Maxwell field inducing forces on charge and charge current, we derive forces acting on spin and spin current induced by the Yang–Mills fields such as the Rashba and Dresselhaus fields and the sheer strain field. The spin density and spin current may be considered as a source generating Yang–Mills field in certain condensed matter systems.

Entanglement of the heisenberg chain with the next-nearest-neighbor interaction

The features of the concurrences between the nearest-neighbors and that of the next-nearest-neighbors for the one-dimensional Heisenberg model with next-nearest-neighbor coupling J are studied as functions of temperature and J. The two concurrences exhibit a different dependence on J at the ground state, which could be interpreted from the point of view of the correlation functions. The threshold temperature at which the concurrence is zero and the temperature effect on the two concurrences for systems up to 12 sites are studied numerically.

Drift of spiral waves controlled by a polarized electric field

The drift behavior of spiral waves under the influence of a polarized electric field is investigated in the light that both the polarized electric field and the spiral waves possess rotation symmetry. Numerical simulations of a reaction-diffusion model show that the drift velocity of the spiral tip can be controlled by changing the polarization mode of the polarized electric field and some interesting drift phenomena are observed. When the electric field is circularly polarized and its rotation follows that of the spiral, the drift speed of the spiral tip reaches its maximal value. On the contrary, opposite rotation between the spiral and electric field locks the drift of the spiral tip. Analytical results based on the weak deformation approximation are consistent with the numerical results. We hope that our theoretical results will be observed in experiments, such as the Belousov-Zhabotinsky reaction.

Exact results of the ground state and excitation properties of a two-component interacting Bose system

We study a one-dimensional Bose system with repulsive δ-function interaction in the presence of an SU(2) intrinsic degree of freedom on the basis of the coordinate Bethe ansatz. The ground state and the low-lying excitations are determined by both numerical and analytical methods. It is shown that the ground state is an isospin-ferromagnetic state, and the excitations are composed of three elementary particles: holons, antiholons, and isospinons. The isospinon is a triplet coupled to the ferromagnetic background anti-parallelly.

Entanglement in spin-1 Heisenberg chains

By using the concept of negativity, we study entanglement in spin-1 Heisenberg chains. Both the bilinear chain and the bilinear–biquadratic chain are considered. Due to the SU(2) symmetry, the negativity can be determined by two correlators, which greatly facilitate the study of entanglement properties. Analytical results of negativity are obtained in the bilinear model up to four spins and in the 2-spin bilinear–biquadratic model, and numerical results of negativity are presented. We determine the threshold temperature before which the thermal state is doomed to be entangled.

A mechanism to pin skyrmions in chiral magnets.

We propose a mechanism to pin skyrmions in chiral magnetic thin films by introducing local maxima of magnetic exchange strength as pinning centers. The local maxima can be realized by engineering the local density of itinerant electrons. The stationary properties and the dynamical pinning and depinning processes of an isolated skyrmion around a pinning center are studied. We carry out numerical simulations of the Landau-Lifshitz-Gilbert (LLG) equation and find a way to control the position of an isolated skyrmion in a pinning center lattice using electric current pulses. The results are verified by a Thiele equation analysis. We also find that the critical current to depin a skyrmion, which is estimated to have order of magnitude 10(7)-10(8) A m(-2), has linear dependence on the pinning strength.

Entanglement in dimerized and frustrated spin-one Heisenberg chains

We investigate entanglement properties in dimerized and frustrated spin-one models by applying the concept of negativity. On the basis of the relation between negativity and correlators, negativities are numerically calculated. We study the effects of dimerization and frustration on entanglement. We also consider the case of finite temperature, and determine the threshold temperature before which the thermal state is doomed to be entangled.