Lateral magnetic stiffness under different parameters in a high-temperature superconductor levitation system

Magnetic stiffness determines the stability of a high-temperature superconductor(HTS) magnetic levitation system.The quantitative properties of the physical and geometrical parameters that affect the stiffness of HTS levitation systems should be identified for improving the stiffness by some effective methods. The magnetic stiffness is directly related to the first-order derivative of the magnetic force with respect to the corresponding displacement, which indicates that the effects of the parameters on the stiffness should be different from the relationships between the forces and the same parameters.In this paper, we study the influences of some physical and geometrical parameters, including the strength of the external magnetic field(B0) produced by a rectangular permanent magnet(PM), critical current density(Jc), the PM-to-HTS area ratio(α), and thickness ratio(β), on the lateral stiffness by using a numerical approach under zero-field cooling(ZFC)and field cooling(FC) conditions. In the first and second passes of the PM, the lateral stiffness at most of lateral positions essentially increases with B0 increasing and decreases with β increasing in ZFC and FC. The largest lateral stiffness at every lateral position is almost produced by the minimum value of Jc, which is obviously different from the lateral force–Jc relation. The α-dependent lateral stiffness changes with some parameters, which include the cooling conditions of the bulk HTS, lateral displacement, and movement history of the PM. These findings can provide some suggestions for improving the lateral stiffness of the HTS levitation system.     

Properties of strong-coupling magneto-bipolaron qubit in quantum dot under magnetic field

Based on the variational method of Pekar type, we study the energies and the wave-functions of the ground and the first-excited states of magneto-bipolaron, which is strongly coupled to the LO phonon in a parabolic potential quantum dot under an applied magnetic field, thus built up a quantum dot magneto-bipolaron qubit. The results show that the oscillation period of the probability density of the two electrons in the qubit decreases with increasing electron–phonon coupling strength α, resonant frequency of the magnetic field ω_c, confinement strength of the quantum dot ω_0, and dielectric constant ratio of the medium η; the probability density of the two electrons in the qubit oscillates periodically with increasing time t, angular coordinate φ_2, and dielectric constant ratio of the medium η; the probability of electron appearing near the center of the quantum dot is larger, and the probability of electron appearing away from the center of the quantum dot is much smaller.     

电流调控磁涡旋的极性和旋性

提出了一种特殊自旋阀结构, 其极化层(钉扎层)磁矩沿面内方向, 自由层磁矩成磁涡旋结构. 自由层在形状上设计成左右两边厚度不同的阶梯形圆盘. 微磁学模拟研究发现, 通过调控所施加的高斯型脉冲电流的大小、方向和脉冲宽度, 可以实现磁涡旋的不同旋性、不同极性的组态控制. 分析了该结构中电流调控磁涡旋旋性和极性的物理原因和微观机理.     

准周期层状铁磁超晶格的实空间重整化群方法

本文用实空间重整化群方法讨论了准周期层状铁磁超晶格的磁自旋波,用Reduce语言推导了decimation变换公式,从而求得了局域格林函数、局域态密度和约化磁矩。发现局域态密度的带宽和约化磁矩与最近邻相互作用J₁、J₂及格点自旋s_a、s_b密切相关。     

Steered coherence and entanglement in the Heisenberg XX chain under twisted boundary conditions

We study steered coherence(SC) and entanglement in a three-spin Heisenberg XX model under twisted boundary conditions and show that their strengths can be significantly enhanced by tuning the twist angle. The optimal twist angle θ_(opt) for achieving the maximum l_1 norm of SC is π in the region of weak field B and decreases gradually from π to 0 when B increases after a critical value, while for the relative entropy of SC, θ_(opt) equals π in the weak field region and 0 otherwise.The entanglement and the critical temperature above which the entanglement vanishes can also be significantly enhanced by tuning the twist angle from 0 to π.     

Proper In deposition amount for on-demand epitaxy of InAs/GaAs single quantum dots

The test-QD in-situ annealing method could surmount the critical nucleation condition of InAs/GaAs single quantum dots(SQDs) to raise the growth repeatability.Here,through many growth tests on rotating substrates,we develop a proper In deposition amount(θ) for SQD growth,according to the measured critical θ for test QD nucleation(θ_c).The proper ratio θ/θ_c,with a large tolerance of the variation of the real substrate temperature(T_(sub)),is 0.964-0.971 at the edge and 0.989 but 0.996 in the center of a 1/4-piece semi-insulating wafer,and around 0.9709 but 0.9714 in the center of a 1/4-piece N~+ wafer as shown in the evolution of QD size and density as θ/θ_c varies.Bright SQDs with spectral lines at 905 nm-935 nm nucleate at the edge and correlate with individual 7 nm-8 nm-height QDs in atomic force microscopy,among dense 1 nm-5 nm-height small QDs with a strong spectral profile around 860 nm-880 nm.The higher T_(sub) in the center forms diluter,taller and uniform QDs,and very dilute SQDs for a proper θ/θ_c:only one 7-nm-height SQD in25 μm~2.On a 2-inch(1 inch = 2.54 cm) semi-insulating wafer,by using θ/θ_c = 0.961,SQDs nucleate in a circle in 22%of the whole area.More SQDs will form in the broad high-T_(sub) region in the center by using a proper θ/θ_c.     

Influence of the Tl- and Hg-content on magnetic and transport properties of the Pb, Sr-doped Tl-1223 and Hg-1223 superconductors

The effects of the thallium and mercury content x on the as-sintered and post annealed samples of M_xPb_0.4Sr_1.6Ba_0.4Ca₂Cu₃O_8+δ {M: Tl (0.32≤x≤0.74) or Hg (0.18≤x≤0.68)} have been studied by magnetization and transport measurements. For Tl-1223 we have found the optimum Tl doping level to be x=0.53 regarding the grain properties, the content of superconducting phase, the first penetration field H_pl^wl, the transport (J_c^tr), magnetic intergrain (J_c^M) and intragrain (J_c^g) critical current densities. The critical temperature T_c of the as-sintered Tl-1223 sample decreased with increasing Tl content. Post-annealing in oxygen improved the T_c for Tl contents of x≥0.53 and had generally positive effects on the critical current densities. The intergrain properties of the Hg-1223 samples were much worse than those of the Tl-based superconductors.     

Dynamic contribution to the fishtail effect in a twin-free DyBa2Cu3O7−δ single crystal

Superconducting current densities j_s and dynamic relaxation rates Q d ln j_s/d In(dB_e/dt), where dB_e/dt is the sweep rate of the external magnetic field B_e, were measured as a function of temperature (5 K < T < 65 K) in magnetic fields up to 7 T on a twin-free DyBa₂Cu₃O_7−δ single crystal by means of a high-sensitivity capacitance torque magnetometer. Above 15 K, we observe a “fishtail” effect, i.e. a pronounced minimum in the j_s(B_e) curve at fields around B_e = 1 T. The relaxation rate Q shows an anomalous increase at low fields which is correlated to the minimum in the j_s(B_e) curve. Both the j_s versus B_e and Q versus B_e data are used as input parameters into the generalized inversion scheme developed by Schnack et al. [Phys. Rev. B 48 (1993) 13178] to calculate the true critical current density j_c which is by definition independent of relaxation effects. Interestingly, the j_c(B_e, T) curves derived in this way do not show a minimum. This points clearly to a dynamic contribution to the fishtail effect. The true critical current density j_c(B_e, T) decreases weakly with increasing B_e over the entire measured temperature and field range, as expected for single-vortex pinning. This indicates that the observed fishtail effect is not caused by a crossover from single-vortex pinning to pinning of flux bundles. The temperature dependence of j_c is in good agreement with the predictions of a model based on single-vortex pinning caused by spatial fluctuations in the charge-carrier mean free path.     

Magnetic dynamics of two-dimensional itinerant ferromagnet Fe_3GeTe_2

Among the layered two-dimensional ferromagnetic materials(2 D FMs),due to a relatively high T_C,the van der Waals(vdW) Fe_3 GeTe_2(FGT) crystal is of great importance for investigating its distinct magnetic properties.Here,we have carried out static and dynamic magnetization measurements of the FGT crystal with a Curie temperature TC ≈ 204 K.The M-H hysteresis loops with in-plane and out-of-plane orientations show that FGT has a strong perpendicular magnetic anisotropy with the easy axis along its c-axis.Moreover,we have calculated the uniaxial magnetic anisotropy constant(K_1)from the SQUID measurements.The dynamic magnetic properties of FGT have been probed by utilizing the high sensitivity electron-spin-resonance(ESR) spectrometer at cryogenic temperatures.Based on an approximation of single magnetic domain mode,the K_1 and the effective damping constant(α_(eff)) have also been determined from the out-of-plane angular dependence of ferromagnetic resonance(FMR) spectra obtained at the temperature range of 185 K to T_C.We have found large magnetic damping with the effective damping constant α_(eff) ～ 0.58 along with a broad linewidth(ΔH_(pp) 1000 Oe at 9.48 GHz,H ‖ c-axis).Our results provide useful dynamics information for the development of FGT-based spintronic devices.     

易轴取向对Nd2Fe14B/α-Fe双层膜退磁过程影响的微磁学分析

运用一维和三维微磁学模拟探究了易轴与外场存在偏角β情况下Nd₂Fe₁₄B/α-Fe 双层膜的磁矩反转过程, 计算了磁矩反转过程中磁滞回线和磁能积, 并与实验结果进行了对比. 计算结果表明, 在膜面内的易轴偏角β严重影响磁矩反转过程. 当β≠0°时, 磁矩反转过程中无明显成核现象, 随着易轴偏角β的增大, 剩磁显著减小, 磁滞回线方形度变差, 导致磁能积急剧减小. 对于Nd₂Fe₁₄B(10 nm)/α-Fe(8 nm)双层膜, β=10°时, 最大磁能积下降30.3%. 在磁矩反转过程中, 总能量最大时对应的外磁场能随易轴偏角的增大而减小, 交换作用能先增大后减小, 磁晶各向异性能则随着易轴偏角的增大而增大. 软磁相厚度越大, 双层膜的磁能积受易轴偏角影响越大. 在膜面外的易轴偏角对磁矩反转过程也有类似的影响. 关键词： 微磁学模拟 磁晶易轴 磁能积 能量     

Upper critical fields of ferromagnet/superconductor layered structures

The temperature dependence of the upper critical fields, both perpendicular H_c2 and parallel H_c2 to layer planes of ferromagnet/superconductor bi- and multilayers, is theoretically investigated. The secular equation of the superconducting order parameter for determining the phase diagram (H, T) is obtained by solving exactly the linearized Usadel equations in the multimode method taking into account the material parameter values. For the bilayers system, the influence of the boundary resistivity on the critical fields, and the dimensional crossover behavior of H_c2(T) are studied in details. For the multilayered structure, the effects of the π-phase state on both the superconducting transition temperature T_c and the upper critical fields (H_c2, and H_c2) are also considered. The nonmonotonic T_c behaviors are predicted. The interplay between 0- and π-phases leading to the strong oscillations of T_c as well as the temperature dependence of the zero temperature critical fields on the ferromagnetic layer thickness are investigated theoretically.     

自驱动Janus微球近壁运动特性实验与数值模拟研究

自驱动Janus微球是形状规则但表面构成不同的特殊活性颗粒. 针对微米级Pt-SiO₂型Janus 微球近壁面自驱动现象, 实验测得了微球的自驱动速度V_Janus, 并观察到微球运动过程中与垂直方向存在一偏转仰角ψ, 且ψ角随H₂O₂溶液浓度的增大呈减小趋势. 在此基础上, 建立自驱动Janus微球的数值模型, 通过模拟得到了微球在不同浓度H₂O₂溶液中的偏转仰角ψ及距底面的高度δ, 模拟与实验一致. 利用这些数据进一步讨论了壁面效应对微球旋转特征时间τ_R的影响. 这一工作对于理解Janus 微球的运动机理及发展相关应用具有重要意义.     

基于零拍探测和压缩真空光输入增强Sagnac效应

利用量子技术增强Sagnac效应提高陀螺输出精度具有重要的研究意义, 是实现全自主导航的重要途径. 以相干态激光作为输入光源的光学陀螺因真空零点波动使其输出精度限制于散粒噪声极限而难以提高. 为减小真空波动的影响, 提出在激光输入的分束器的另一输入端输入压缩真空光并结合平衡零拍探测技术的方法增强Sagnac效应. 理论分析表明Sagnac效应性能得到有效提升: 干涉输出的灵敏度检测极限和动态范围均随着压缩程度的增加而呈指数级增长. 该方法只需对经典光学陀螺做少量改动就可实现, 是提高光学陀螺输出精度的一种新方法.     

若干高精度恒稳的半显式差分格式

本文构造了解色散方程u₁=au_xxx的若干三层恒稳的半显式差分格式。第Ⅰ、Ⅱ类格式的局部截断误差的阶为O(τ²+h²+(τ²)/(h³));而第Ⅲ、Ⅳ类格式的局部截断误差的阶为O(τ²+h⁴+((τ)/(h))²+τh)。用判别稳定性的Von Neumann准则可证明:第Ⅰ、Ⅱ类格式及当参数α≤1时的第Ⅲ、Ⅳ类格式都是无条件稳定的,并且当必须的边界条件给定时它们可以显式地进行计算。     

Magnetic hysteresis of the magnetoresistance and the critical current density in polycrystalline YBa2Cu3O7−δ–Ag superconductors

A systematic study of the magnetic hysteresis in transport properties of polycrystalline YBa₂Cu₃O_7−δ–Ag compounds has been made based on two kinds of measurements at 77 K and under applied magnetic fields up to 30 mT: critical current density J_c(B_a) and magnetoresistance R(B_a). The R(B_a) curves show a minimum in their decreasing branch occurring at B=B_min which was found to be both the excitation current I_ex and the maximum applied magnetic field B_am dependent. In addition, for a certain value of B_am>5 mT, we have observed that B_min increases with increasing I_ex and reaches a saturation value. The J_c(B_a) curves show a maximum in decreasing applied magnetic fields occurring at B=B_max. We have also found that B_max increases with increasing B_am and reaches a saturation value. The minimum in the R(B_a) and the maximum in J_c(B_a) curves were found to be related to the trapped flux within the grains. All the experimental results are discussed within the context of the flux dynamics and transport mechanisms in these high-T_c materials.     

Magnetic moments and Fe hyperfine field interactions in Y(Fe1−xRux)2 ternaries

Measurements of magnetization and ⁵⁷Fe Mössbauer spectra have been made for Y(Fe_1−xRu_x)₂. The C15 type cubic structure is stabilized for xx 0.7. The C15 compounds is ferromagnetic with T_c200 K and its saturation moment decreases monotonically with increasing x, while the ⁵⁷Fe hyperfine field decreases only slightly with x. From these results, it is deduced that the Ru atoms have an induced moment of ≈1μ_B in the range x 0.2. In the C14 type phase, no magnetic ordering develops even at 4.2 K.     

Effects of short-range attraction on Jamming transition

Enormous progresses to understand the jamming transition have been driven via simulating purely repulsive particles which were somehow idealized in the past two decades. While the attractive systems are both theoretical and practical compared with repulsive systems. By studying the statistics of rigid clusters, we find that the critical packing fraction φ_c varies linearly with attraction μ for different system sizes when the range of attraction is short. While for systems with long-range attractions, however, the slope of φ_c appears significantly different, which means that there are two distinct jamming scenarios. In this paper, we focus our main attention on short-range attractions scenario and define a new quantity named "short-range attraction susceptibility" χ_p, which describes the degree of response of the probability of finding jammed states pjto short-range attraction strength μ. Our central results are that χ_p diverges in the thermodynamic limit as χ_p ∝|φ-φ_c~∞|~(-γ_p), where φ_c~∞ is the packing fraction at the jamming transition for the infinite system in the absence of attraction. χ_p obeys scaling collapse with a scaling function in both two and three dimensions, illuminating that the jamming transition can be considered as a phase transition as proposed in previous work.     

MgO(111)衬底MgB2 超薄膜的制备和性质研究

利用混合物理化学气相沉积法(HPCVD)在MgO(111)衬底上制备了干净的MgB₂超导超薄膜. 在背景气体压强, 载气氢气流量以及沉积时间一定的情况下, 改变B₂H₆的流量, 制备得到不同厚度系列的MgB₂超导薄膜样品, 并测量了其超导转变温度 T_c, 临界电流密度J_c等临界参量. 该系列超导薄膜沿c轴外延生长, 表面具有良好的连接性, 且有很高的超导转变温度T_c(0) ≈ 35-38 K和很小的剩余电阻率ρ(42 K) ≈ 1.8-20.3 μΩ·cm^-1. 随着膜厚的减小而减小, 临界温度变低, 而剩余电阻率变大. 其中20 nm的样品在零磁场, 5K时的临界电流密度J_c ≈ 2.3×10⁷ A/cm². 表明了利用HPCVD在MgO(111)衬底上制备的MgB₂超薄膜有很好的性能, 预示了其在超导电子器件中广阔的应用前景. 关键词： MgO(111)衬底 2超薄膜')" href="#">MgB₂超薄膜 混合物理化学气相沉积     

Electron paramagnetic resonance measurements for (La,Y)2/3(Ca,Ba)1/3MnO3

We have measured the paramagnetic (PM) resonance behavior as a function of temperature for various manganese perovskites (La,Y)_2/3(Ca,Ba)_1/3MnO₃ with an average A-site size r_A basically covering the whole region of ferromagnetic (FM) metallic ground state. We show that at least three regions with different magnetic behaviors can be distinguished by two phase lines: T_onset−r_A and T_c−r_A. For each given r_A, the complete PM and long-range FM behaviors appear above T_onset and below T_c, respectively; while some anomalous PM behaviors appear for the intermediate temperature range of T_onset<T<T_c . A possible magnetic transition process is discussed in order of decreasing temperature from high-T complete paramagnetism to low-T long-range ferromagnetism.