Thermal entanglement in two-qutrit spin-1 anisotropic Heisenberg model with inhomogeneous magnetic field

The thermal entanglement of a two-qutrit spin-1 anisotropic Heisenberg XXZ chain in an inhomogeneous magnetic field is studied in detail. The effects of the external magnetic field (B), a parameter b which controls the inhomogeneity of B, and the bilinear interaction parameters J_{x=Jy≠Jz on the thermal variation of the negativity are studied in detail. It is found that negativity N decreases when the values of magnetic field, inhomogeneity b and temperature are increasing. In addition, N remains at higher temperatures for higher values of Jz and lower values of B and b.}     

SQUID法和Campbell法测量超导材料的研究

本文对比研究了超导材料磁测量中的SQUID法和Campbell法; 并用高压PIT法制备的超导材料MgB₂作为测量样品,用两种方法,测量了超导样品的临界电流密度, 分别得到了样品的J_c-B关系曲线; SQUID法测量样品的外磁场可以达到6 T, 此时材料已经处于失超状态,此方法测得的结果是样品各个小区域结果的平均值, SQUID还可以用来进一步标度材料的钉扎力行为,研究材料磁特性. Campbell法测量只能测量到外磁场强度为0.4 T,外磁场的交流部分的频率可以达到800 Hz, 用这种测量方法得到的是整块样品的电流,由于测量计及材料内部微观结构缺陷等影响电流传输因素, 所测结果小于直流磁化法,但更切近材料实际电流,能用来深入研究材料内部结构差别对材料电性能的影响.     

Nuclear-induced time evolution of entanglement of two-electron spins in anisotropically coupled quantum dot

We study the time evolution of entanglement of two spins in an anisotropically coupled quantum dot interacting with the unpolarised nuclear spins environment. We assume that the exchange coupling strength in the z direction J _z is different from the lateral one J _l . We observe that the entanglement decays as a result of the coupling to the nuclear environment and reaches a saturation value, which depends on the value of the exchange interaction difference J = ‖J _l ? J _z ‖ between the two spins and the strength of the applied external magnetic field. We find that the entanglement exhibits a critical behaviour controlled by the competition between the exchange interaction J and the external magnetic field. The entanglement shows a quasi-symmetric behaviour above and below a critical value of the exchange interaction. It becomes more symmetric as the external magnetic field increases. The entanglement reaches a large saturation value, close to unity, when the exchange interaction is far above or below its critical value and a small one as it closely approaches the critical value. Furthermore, we find that the decay rate profile of entanglement is linear when the exchange interaction is much higher or lower than the critical value but converts to a power law and finally to a Gaussian as the critical value is approached from both directions. The dynamics of entanglement is found to be independent of the exchange interaction for an isotropically coupled quantum dot.     

Electronic structure of paramagnetic In<Subscript>1-x</Subscript>Mn<Subscript>x</Subscript> As nanowires

The electronic structure, spin splitting energies, and g factors of paramagnetic In_1-xMn_xAs nanowires under magnetic and electric fields are investigated theoretically including the sp-d exchange interaction between the carriers and the magnetic ion. We find that the effective g factor changes dramatically with the magnetic field. The spin splitting due to the sp-d exchange interaction counteracts the Zeeman spin splitting. The effective g factor can be tuned to zero by the external magnetic field. There is also spin splitting under an electric field due to the Rashba spin-orbit coupling which is a relativistic effect. The spin-degenerated bands split at nonzero k_z (k_z is the wave vector in the wire direction), and the spin-splitting bands cross at k_z = 0, whose k_z-positive part and negative part are symmetrical. A proper magnetic field makes the k_z-positive part and negative part of the bands asymmetrical, and the bands cross at nonzero k_z. In the absence of magnetic field, the electron Rashba coefficient increases almost linearly with the electric field, while the hole Rashba coefficient increases at first and then decreases as the electric field increases. The hole Rashba coefficient can be tuned to zero by the electric field.     

Entanglement in spin-1 Heisenberg <Emphasis Type="Italic">XY</Emphasis> chain

We investigated the quantum entanglement in spin-1 Heisenberg XY chain for two-spin-qutrit and multi-particle systems. As a measure of the entanglement, the negativity of this state was analyzed as a function of the temperature and the magnetic field. We gave some numerical results and discussed them in detail. We found that the negativity increases monotonously with the coupling constants |J ₁| and |J ₂|, and it showed a symmetry with respect to the point of J ₁ = 0 and J ₂ = 0. In addition to the above features, there is evidence that the critical temperature is independent of the length of the chain.     

Thermal entanglement in the anisotropic Heisenberg model with Dzyaloshinskii-Moriya interaction in an inhomogeneous magnetic field

The thermal entanglement of a two-qubit anisotropic Heisenberg XXZ chain with Dzyaloshinskii-Moriya (DM) interaction in an inhomogeneous magnetic field was studied in detail. The effects of the DM parameter, external magnetic field (B), a parameter b which controls the inhomogeneity of B and the bilinear interaction parameters J_x = J_y ≠ J_z (the anisotropic case) on the concurrence (C) was formulated and studied in detail. The behaviors of the concurrences for the cases between (J = J_z = 1) and (J = -1,J_z = 1) and, (J = J_z = -1) and (J = 1,J_z = -1) at the ground state and at the thermal equilibrium are exactly the same. It was found that for the antiferromagnetic (AFM) case the entanglements persist to higher temperatures in comparison with the ferromagnetic (FM) case. In addition, the AFM case presents a special point at which the nonzero concurrences are all equal at some special temperatures. The further properties will be given in the text.     

Dynamic phase transitions and dynamic phase diagrams of the Blume–Emery–Griffiths model in an oscillating field: the effective-field theory based on the Glauber-type stochastic dynamics

Using the effective-field theory based on the Glauber-type stochastic dynamics (DEFT), we investigate dynamic phase transitions and dynamic phase diagrams of the Blume–Emery–Griffiths model under an oscillating magnetic field. We presented the dynamic phase diagrams in (T/J, h₀/J), (D/J, T/J) and (K/J, T/J) planes, where T, h₀, D, K and z are the temperature, magnetic field amplitude, crystal–field interaction, biquadratic interaction and the coordination number. The dynamic phase diagrams exhibit several ordered phases, coexistence phase regions and special critical points, as well as re-entrant behavior depending on interaction parameters. We also compare and discuss the results with the results of the same system within the mean-field theory based on the Glauber-type stochastic dynamics and find that some of the dynamic first-order phase lines and special dynamic critical points disappeared in the DEFT calculation.     

Memorandum on non-curl-free geomagnetic field

Summary In 1939 Schmidt denied the presence of nonpotential geomagnetic field observed on the Earth’s surface, after stating that the nonvanishment of curl _z B must be merely a consequence of remaining errors in geomagnetic observations. His paper was so influential that the discussion on nonpotential magnetic field disappeared ever since. The apparent nonvanishment of curl _z B (calculated from the differences of observed magnetic-field values at neighbouring meshpoint stations) would be, however, more reasonably attributable to a possible small-scale inhomogeneity in underground magnetic susceptibility or/and electric conductivity, whose spatial wavelength is shorter than the distance between observation meshpoints. To speed up publication, the proofs were not sent to the authors and were supervised by the Scientific Committee.     

Bipartite entanglement of a two-qubit system with anisotropic couplings under nonuniform magnetic fields

This paper investigates bipartite entanglement of a two-qubit system with anisotropic couplings under an inhomogeneous magnetic field. This work is mainly to investigate the characteristics of a Heisenberg XYZ chain and obtains some meaningful results. By the concept of negativity, it finds that the inhomogeneity of magnetic field may induce entanglement and the critical magnetic field is independent of J_z. The inhomogeneous magnetic field can increase the value of critical magnetic field B_c. It also finds that the magnetic field not only suppresses the entanglement but also can induce it to revival for some time.     

Fast iterative approach to the difference scattering from a dielectric target above a rough surface

1 Introduction Electromagnetic scattering from the target above or beneath a rough surface has at- tracted much interest during recent years, because of extensive applications to radar surveillance, target detection, ground radar probing, and so on[1―3]. In order to numeri- cally simulate scattering from composite model of the target and underlying rough sur- face, some fast numerical methods, such as general forward backward method and spec- trum acceleration algorithm (GFBM/SAA)[4,5], fin…     

Fast iterative approach to the difference scattering from a dielectric target above a rough surface

The difference field RCS (d-RCS) has been defined to analyze the scattering from the target above a rough surface. The electric field integral equations (EFIEs) of the difference induced currentJ _sd on the rough surface, the induced electric currentJ _o and magnetic currentK _o on the dielectric target under a TE wave incidence are derived. A small portion of the rough surface towards the target along the specular direction is taken to compute the scattering contributionE _s0 from the rough surface towards the target, which improves the computation speed. A numerical iterative approach is developed to solve the EFIEs and bistatic d-RCS. The surface length for iterations is dependent on the scattering angle and discussed for comparison with Johnson’s method. Using the Monte-Carlo method to generate the Pierson-Morkowitz (P-M) ocean-like rough surface, bistatic d-RCS of the dielectric target, e.g. a cylinder or a square column, above the rough surface is numerically simulated. The induced electric and magnetic currents on the dielectric target and the difference induced current on the rough surface are numerically discussed.     

Magnetoelastic excitations and structural phase transitions in tetragonal paramagnets

The systematic discussion of the elastic and structural properties of the tetragonal paramagnetic model with the maximum value of the total angular momentumJ=1 in an external magnetic field along thez orx direction is given. The sound velocities of all the symmetry modes are calculated by using the standard perturbation theory and Green's function method. It is shown that only in the presence of the external field directed along thez-axis, for some values of this field and temperature one of the symmetry elastic constantsc ₄₄ orc _B becomes zero and the phase transition to monoclinic (triclinic) or volume change transition can occur.     

自旋为1的三粒子Heisenberg XXX链中杂质对热纠缠的影响

采用纠缠的度量方法Negativity研究了匀强磁场下自旋为1的含杂质三粒子Heisenberg XXX链的热纠缠特性.通过计算系统的杂质位与其余部分间的两体纠缠N_1-23及正常位与其余部分间的两体纠缠N_12-3,发现纠缠存在的临界温度T_c的改变来自杂质参数J₁的变化,随杂质参数J₁的增加而增加,外界磁场B< 关键词： 热纠缠 XXX链')" href="#">Heisenberg XXX链 杂质     

A new model analysis of the third harmonic voltage in inductive measurement for critical current density of superconducting films

The critical current density J c is one of the most important parameters of high temperature superconducting films in superconducting applications,such as superconducting filter and superconducting Josephson devices.This paper presents a new model to describe inhomogeneous current distribution throughout the thickness of superconducting films applying magnetic field by solving the differential equation derived from Maxwell equation and the second London equation.Using this model,it accurately calculates the inductive third-harmonic voltage when the film applying magnetic field with the inductive measurement for J c.The theoretic curve is consistent with the experimental results about measuring superconducting film,especially when the third-harmonic voltage just exceeds zero.The J c value of superconducting films determined by the inductive method is also compared with results measured by four-probe transport method.The agreements between inductive method and transport method are very good.     

Thermal Entanglement in Anisotropic Heisenberg XYZ Chain with External Magnetic Field at Any Finite <Emphasis Type="Italic">T</Emphasis>

The thermal entanglement of a Two-qubit anisotropic Heisenberg XYZ chain in thermal equilibrium at temperature T in the presence of external magnetic field is investigated, the combined influence of anisotropic interaction and the inhomogeneous magnetic field on the thermal entanglement of the system is examined. Our results show that, the thermal entanglement can be produced at any finite T by adjusting the magnetic field strength, and the critical magnetic field ε for which the concurrence C vanishes is increased by introducing the interaction of the z-component of two neighboring spins J _z , and by the increasing of the anisotropic parameter γ.     

Quantum Discord Behavior for Two Qubits Heisenberg XYZ Chain with Inhomogeneous Magnetic Field

We report the quantum correlation behavior (quantum discord (QD)) of a two-qubit anisotropic Heisenberg XYZ chain under an inhomogeneous magnetic field. It is shown that in the lower region of b QD can be enhanced evidently through increasing anisotropic parameter γ, while the effects of γ are disappear when b is strong enough. The role of J _z is nearly opposite to γ, there is a critical value of b _c , with b>b _c QD is improved with decreasing interaction J _z , while the value of QD is nearly invariable whatever changing J _z when b<b _c . In addition, one can get non-zero QD by properly tuning J _z and γ even at a higher temperature. When inhomogeneity is increased to b _c , the QD can exhibit a regrowth procession, and the regrowth value of QD can be larger than that of before dropping in the region of weak J _z . We also obtain the ground state QD properties. These investigations can imply us more control parameters on quantum correlation in solid state systems.     

Transport critical-current and voltage-current characteristics of Bi(2223) silver-clamped thick films

The transport critical current properties of Bi(2223) silver-clamped thick films are studied by the measurement of its dependence on magnetic field and temperature close to T _c. It is found that the transport critical current follows a power law J _c(1–T/T _c)^3/2 for the sample with J _c>2.0×10⁴ A/cm² (77K, zero field) and that J _c(H) is basically reversible for increasing and decreasing magnetic field. After the transport current exceeds the critical current, the voltage-current (V-I) characteristics show a flux-creep-like behaviour until they smoothly join the flux-flow state. From the measurement of V-I curves, the pinning property of the sample may be estimated using the flux-creep mode. The flux-flow resistance is found to have a nonlinear magnetic field dependence.     

Level-anticrossing effect on the magnetoluminescence of the triplet indirect bound exciton in GaSe

The presence of level-anticrossing in the triplet state of the indirect impurity-bound exciton in GaSe has been found for the first time in the magnetic field of 1.5 kOe applied along the c-axis. Measurements on the magnetic circular dichroism (MCD) of the photoluminescence and on the spin dependent formation rate (SDFR) show that the |S_z| = 1 level lies 0.016 meV higher than the S_z = 0 level in the absence of the magnetic field. The resonance-like behavior in these two types of polarization correlation measurements may be explained in terms of weak perturbation between |S_z| = 1 and S_z = 0 levels.     

Global and local critical current density in superconducting SmFeAsO1−xFx measured by two methods

The critical current densities of polycrystalline bulk SmFeAsO_1−xF_x prepared by the powder-in-tube (PIT) method and by a conventional solid-state reaction were investigated using the remnant magnetic moment method and Campbell’s method. Two types of shielding current, corresponding to global and local critical current densities J_c were observed using both measurement methods. The global and local J_c were on the order of 10⁷ A/m² and 10¹⁰ A/m² at 5 K, respectively. The local J_c decreased slightly with increasing magnetic field. The global J_c was independent of the preparation method, while the local J_c was larger for samples prepared by PIT than for those prepared by solid-state reaction.     

Crystallographic anisotropy and distortions in helicoidal magnetic structure

The appearance and development of aperiodic distortions in a helicoidal, layered magnetic structure with increasing crystallographic magnetic anisotropy in the magnetization rotation plane have been theoretically studied. A simple phase diagram for this system is proposed. It is established that, at a weak anisotropy, the spiral splits into regions of various lengths with an approximately uniform rotation of the magnetization in each region and a deviation from uniformity at the boundaries; the stronger the anisotropy, the shorter the regions and the greater the deviations. In the limit of high anisotropy, the minimum energy of the system corresponds (depending on the ratio of interlayer exchange integrals J ₁ and J ₂) to either a spiral with constant angular pitch (a multiple of the angle between easy axes) or a double antiferromagnetic structure with a four-layer period. In the case of sixth-order anisotropy with |J ₁| = −J ₂, the energies of phases with different periods (four and six layers for J ₁ > 0; four and three layers for J ₁ < 0) coincide and the excess boundary energy vanishes. In the case of a fourth- and second-order anisotropy, the analogous anomalies appear at |J ₁| = −2J ₂. As a result, the magnetic structure at these points becomes unstable and the phase diagram exhibits the corresponding singularities.