Scaling of entanglement entropy for spin chain with Dzyaloshinskii-Moriya interaction

This paper investigates the entanglement in an XX-type spin chain with Dzyaloshinskii-Moriya interaction under an external magnetic field.The von Neumann entropy of entanglement between two blocks for the ground state of the system is evaluated.It analyses and discusses the scaling behaviour of the entanglement entropy.     

利用信息流方法优化多激发自旋链中的量子态传输

研究了量子态在一条均匀耦合的反铁磁自旋链中传输时, 信道中自旋激发数变化对其传输性质的影响. 利用信息流方法分析输出端粒子的算符演化动力学, 获得了量子态传输的平均保真度与信道自旋初态之间的关系. 结果表明, 平均保真度的大小只依赖于信道中自旋激发数的奇偶性. 通过比较在奇偶激发信道中获得的最大平均保真度, 构建了优化信道来提升量子态在自旋链中的传输质量. 进一步分析了纠缠在激发信道中的传输情况, 发现纠缠的传输质量不仅和信道中自旋激发的具体个数有关, 还取决于激发自旋的初始排列. 特别地, 当信道中自旋无激发或全部激发时, 纠缠传输的大小和持续时间都优于其他的激发信道. 上述研究结果有助于在实际系统中搭建适合量子态和纠缠传输的量子信道.     

Thermal Entanglement in a Two-Qubit Heisenberg XXZ Spin Chain Under a Single Pulse Magnetic Field

The thermal entanglement in a two-qubit Heisenberg XXZ spin chain is investigated under a single pulse magnetic field Bsinθ. The paper shows that the greater the contribution of the inhomogeneity on the exchange interaction, the higher thermal entanglement will be attained at the fixed temperature except the case that $\theta=2k\pi+\frac{3}{2}\pi$ . J _z  significantly disturb the thermal entanglement in uniform case. When the structure of the spin system are given, changing the external magnetic field B can induce controllable entanglement. Our study may provide a useful tool to change the entanglement of spin chain system.     

Entanglement of electronic subbands and coherent superposition of spin states in a Rashba nanoloop

The present work is concerned with an analysis of the entanglement between the electronic coherent superpositions of spin states and subbands in a quasi-one-dimensional Rashba nanoloop acted upon by a strong perpendicular magnetic field. We explicitly include the confining potential and the Rashba spin-orbit coupling into the Hamiltonian and then proceed to calculate the von Neumann entropy, a measure of entanglement, as a function of time. An analysis of the von Neumann entropy demonstrates that, as expected, the dynamics of entanglement strongly depends upon the initial state and electronic subband excitations. When the initial state is a pure one formed by a subband excitation and the z-component of spin states, the entanglement exhibits periodic oscillations with local minima (dips). On the other hand, when the initial state is formed by the subband states and a coherent superposition of spin states, the entanglement still periodically oscillates, exhibiting stronger correlations, along with elimination of the dips. Moreover, in the long run, the entanglement for the latter case undergoes the phenomenon of collapse-revivals. This behaviour is absent for the first case of the initial states. We also show that the degree of entanglement strongly depends upon the electronic subband excitations in both cases.     

Enlarged omnidirectional photonic gap in one dimensional ternary plasma photonic crystals that contain metamaterials

We study the dynamics of multipartite quantum correlations measured by the lower bound of concurrence and quantum discord in a three-qubit system coupled to an XY spin chain. For the initial pure GHZ and W state, we find the lower bound of entanglement is more robust than the quantum discord against the decoherence induced by the spin environment. But for the Werner state, the sudden death of discord is not observed even in the presence of entanglement sudden death. By comparing the evolutions for the GHZ and W states, we show that the W state preserves more quantum correlations than the GHZ state. In addition, we put research emphasis on the relation between the dynamics of multipartite quantum correlations and the quantum phase transition of the spin environment.     

Spin waves on chains of YIG particles: dispersion relations,Faraday rotation,and power transmission

We calculate the dispersion relations for spin waves on a periodic chain of spherical or cylindrical Yttrium Iron Garnet (YIG) particles. We use the quasistatic approximation, appropriate when kd ? 1, where k is the wave number and d the interparticle spacing. In this regime, because of the magnetic dipole-dipole interaction between the localized magnetic excitations on neighboring particles, dispersive spin waves can propagate along the chain. The waves are analogous to plasmonic waves generated by electric dipole-dipole interactions between plasmons on neighboring metallic particles. The spin waves can be longitudinal (L), transverse (T), or elliptically polarized. We find that a linearly polarized spin wave undergoes a Faraday rotation as it propagates along the chain. The amount of Faraday rotation can be tuned by varying the off-diagonal component of the permeability tensor. We also discuss the possibility of wireless power transmission along the chain using these coupled spin waves.     

Pseudospin approach for planar ferromagnets

A pseudospin approach is developed for the study of quantum spin systems. The method is based on generalized coherent states in the symmetric representations of the unitary algebra U(2s+1), where s is the total spin at each site, and on the associated Holstein-Primakoff theory. A systematic 1/n éxpansion is derived which enables one to study magnets with weak as well as strong single-site anisotropies. While the method applies to a variety of quantum spin systems in arbitrary lattice dimensions, particular emphasis is placed here on the spin-1 planar ferromagnetic chain observed in CsNiF₃. The 1/n expansion for that system yields a twofold spectrum of elementary excitations, the usual magnon and a collective mode. It is also found that a sharp crossover transition occurs for intermediate couplings in analogy with the Gross-Witten third-order transition in the large-n limit of simple gauge models.     

The ground-state entanglement in the spin-XX chain with a magnetization current

The ground-state entanglement in a transverse spin-1/2 XX chain with a magnetization current is studied.By introducing a magnetization current to the system,a quantum phase transition to current-carrying phase may be presented with the variation of the driving field λ for the magnetic field h > 1;and the ground-state entanglement arises simultaneously at the critical point of quantum phase transition.In our model,the introduction of magnetization current may result in more entanglement between any two nearest-neighbour spins.     

Effects of Dzyaloshinskii-Moriya interaction on thermal entanglement and teleportation via a two-qubit Heisenberg XXZ spin chain under external magnetic field

This paper mainly investigates the effects of different Dzyaloshinskii-Moriya (DM) anisotropic antisymmetric interactions on thermal entanglement and teleportation of one-qubit state in both the standard and non-standard protocols as well as the partial teleportation of an entangled state via a two-qubit Heisenberg XXZ spin chain in the presence of external magnetic fields. The dependency of the thermal entanglement and average fidelity on various system parameters is analyzed. The interplay of the different parameters on the teleportation is discussed. The DM interaction is found to be effective for the thermal entanglement in the spin chain both with and without external magnetic fields. However, it turned out to be destructive for the teleportation in the standard protocol, whereas is found constructive for single qubit teleportation when the spin chain with the z-direction parameters is used as the channel in the non-standard protocol. Moreover, the results show that, for teleporting one-qubit state, the antiferromagnetic (AFM) chain is the only qualified candidate in the standard protocol, while both the AFM and ferromagnetic (FM) chains with the parameters along the z-axis are all suitable in the non-standard protocol when the parameters are chosen appropriately. For the partial teleportation of entanglement, both the AFM and FM chains are eligible as long as the appropriate combinations of parameters are chosen. In addition, the comparison of the effects of the same, fixed x- and z-component parameters of the DM interaction (D_x and D_z) on teleportation is presented.     

低温下自旋为1/2的一维亚铁磁棱型链系统的元激发谱

利用不变本征算符法计算了低温下自旋为1/2的一维亚铁磁棱型链系统的元激发谱,并讨论了在此系统中不同的特殊情形下的元激发能量及不变本征算符法的优点与不足.     

Low-lying excitations in the S = 1 antiferromagnetic Heisenberg chain

In order to confirm the picture of domain-wall excitations in the hidden antiferromagnetic order of the Haldane phase, the structure of the low-lying excitations in the S = 1 antiferromagnetic Heisenberg chain is studied by a quantum Monte Carlo method. It is confirmed that there exists a finite energy gap between the first- and the second-excited states at k = π as well as between the ground state and the first-excited state at k = π. In the thermodynamic limit, the second-excited state at k = π is separated from the ground state by the gap which is three times as large as the Haldane gap. From the size dependences of the low-lying-excitation energies, the interactions between the elementary excitations in the excited states are concluded to be repulsive.     

Soliton dynamics in a spin nematic

One-dimensional localized waves, which can be considered as soliton elementary excitations, exist in a magnet with a unit spin and comparable bilinear and biquadratic spin-spin interactions, with which the state of spin nematic is realized. These excitations are characterized by a certain momentum P and a certain energy E. The structure of these solitons has been found, and the E = E(P) dependence, which plays the role of the dispersion law of these soliton elementary excitations, has been constructed. The energy of a soliton with a certain momentum is shown to be lower than that of the quasiparticles of a linear theory. At small momenta, these E = E(P) dependences of the soliton and quasiparticles coincide asymptotically. The dependence of the soliton energy on the soliton momentum is a periodic function with a period P ₀ = π?/a, whose value does not depend on exchange integrals and depends only on a single crystal parameter, namely, the interatomic distance a. These soliton excitations have common features with the so-called Lieb states, which are well known in many condensed matter models.     

Dzyaloshinskii-Moriya相互作用对量子XY链中热纠缠的影响

研究了Dzyaloshinskii-Moriya(DM)相互作用对混合自旋(1/2,3/2)XY链以及自旋为1的XY链热纠缠的影响.通过计算两粒子之间的纠缠,发现它不仅能够增强纠缠,而且能使两粒子之间的纠缠度达到一稳定值;当温度较高时,要使热纠缠达到稳定值需要更强的这种相互作用.在相同的条件下,自旋s=1的两粒子之间的纠缠要小于混合自旋两粒子之间的纠缠.粒子之间的交换耦合相互作用有助于加强粒子之间的热纠缠,因此可以与DM相互作用一起调节纠缠度.当交换耦合相互作用比 关键词： 量子纠缠 XY 模型')" href="#">XY 模型 negativity Dzyaloshinskii-Moriya相互作用     

Magnetization studied on spin alignments in organic molecule-based ferrimagnetics

A physics picture of spin alignments in molecule-based ferrimagnets is presented from studying the temperature dependence of the effective sublattice magnetic moments and the total reduced magnetization by means of Green’s function theory combined with the Jordan-Wigner transformation. The ferrimagnetic chain includes an S=1 biradical and an S=1/2 monoradical with antiferromagnetic alternating interactions, and the S=1 site in the chain is composed of two S=1/2 spins coupled by a finite ferromagnetic interaction. From the calculations of the sublattice magnetic moments, the magnetic moment of the S=1 biradical is negative, while that of the S=1/2 monoradical is positive, leading to a ferrimagnetic ground state. With the different kinds of the elementary excitations and the competition between the magnetic interactions and thermal fluctuations, the temperature dependence of the magnetization displays rich thermodynamic properties. Meanwhile, the external magnetic field dependence of the magnetization has a clear plateau at one third of the saturation magnetization, which can be compared with the possible experimental findings.     

Quantum entanglement in a molecular magnet with itinerant electrons

We study the behaviors of pairwise and multipartite entanglement in a molecular magnet with itinerant electrons. In different ground states, the ratio of pairwise to multipartite entanglement is different. The monogamy of quantum entanglement is shown. Both charge correlation and spin correlation play important roles in the entanglement. The entanglements are generally suppressed by the on-site repulsion U and are mainly determined by spin correlation for large U and by charge correlation for small U. At finite temperature, in general, the thermal fluctuation suppresses the entanglements. However, in some cases, the multipartite entanglement can be enhanced by increasing temperature. Comparing the Heisenberg model with the Hubbard model, it is found that thermal entanglement in the itinerant electron system is more robust because charge correlation can survive at much higher temperature than spin correlation.     

Enhancement of next-nearest-neighbouring entanglement in quantum Ising spin chain

Using the method of the Jordan--Wigner transformation for solving different spin--spin correlation functions, we have investigated the generation of next-nearest-neighbouring entanglement in a one-dimensional quantum Ising spin chain with the Gaussian distribution impurities of exchange couplings and external magnetic fields taken into account. The maximal value of entanglement between the next-nearest-neighbouring qubits in the transverse Ising model was analysed in detail by varying the effectively controlled parameters such as interchange coupling, magnetic field and the system impurity. For such systems, where both exchange couplings and external magnetic field disorder appear, we show that it is possible to achieve next-nearest-neighbouring entanglement better than the previously discussed pure Ising spin chain case. We also show that the Gaussian distribution impurity can induce next-nearest-neighbouring entanglement, which can be used as a means to characterize quantum phase transition.     

Low-temperature heat transport of spin-gapped quantum magnets

This article reviews low-temperature heat transport studies of spin-gapped quantum magnets in the last few decades. Quantum magnets with small spins and low dimensionality exhibit a variety of novel phenomena. Among them, some systems are characteristic of having quantum-mechanism spin gap in their magnetic excitation spectra, including spin-Peierls systems, S=1 Haldane chains, S= 1/2 spin ladders, and spin dimmers. In some particular spin-gapped systems, the XY-type antiferromagnetic state induced by magnetic field that closes the spin gap can be described as a magnon Bose-Einstein condensation (BEC). Heat transport is effective in probing the magnetic excitations and magnetic phase transitions, and has been extensively studied for the spin-gapped systems. A large and ballistic spin thermal conductivity was observed in the two-leg Heisenberg S=1/2 ladder compounds. The characteristic of magnetic thermal transport of the Haldane chain systems is quite controversial on both the theoretical and experimental results. For the spin-Peierls system, the spin excitations can also act as heat carriers. In spin-dimer compounds, the magnetic excitations mainly play a role of scattering phonons. The magnetic excitations in the magnon BEC systems displayed dual roles, carrying heat or scattering phonons, in different materials.     

Quantum communication through anisotropic Heisenberg XY spin chains

We study quantum communication through an anisotropic Heisenberg XY chain in a transverse magnetic field. We find that for some time t and anisotropy parameter γ, one can transfer a state with a relatively high fidelity. In the strong-field regime, the anisotropy does not significantly affect the fidelity while in the weak-field regime the affect is quite pronounced. The most interesting case is the intermediate regime where the oscillation of the fidelity with time is low and the high-fidelity peaks are relatively broad. This would, in principle, allow for quantum communication in realistic circumstances. Moreover, we calculate the purity, or tangle, as a measure of the entanglement between one spin and all the other spins in the chain and find that the stronger the anisotropy and exchange interaction, the more entanglement will be generated for a given time.     

Correlation functions for the Heisenberg and biquadratic pair interactions

Using the diagrammatic technique worked out for spin one Hamiltonians, one particle Green functions and corresponding elementary excitations as well as the expressions for the free energy and order parameters within the Heisenberg model with biquadratic exchange are obtained. The calculations are performed in the (1/z)¹ approximation, where z is the number of spins interacting with any given spin.