Thermodynamic behaviour of Rashba quantum dot in the presence of magnetic field

The thermodynamic properties of an In Sb quantum dot have been investigated in the presence of Rashba spin–orbit interaction and a static magnetic field. The energy spectrum and wave-functions for the system are obtained by solving the Schrodinger wave-equation analytically. These energy levels are employed to calculate the specific heat, entropy,magnetization and susceptibility of the quantum dot system using canonical formalism. It is observed that the system is susceptible to maximum heat absorption at a particular value of magnetic field which depends on the Rashba coupling parameter as well as the temperature. The variation of specific heat shows a Schottky-like anomaly in the low temperature limit and rapidly converges to the value of 2kB with the further increase in temperature. The entropy of the quantum dot is found to be inversely proportional to the magnetic field but has a direct variation with temperature. The substantial effect of Rashba spin–orbit interaction on the magnetic properties of quantum dot is observed at low values of magnetic field and temperature.     

Effective-Field Theory on High Spin Systems with Biaxial Crystal Field

Based on the effective-field theory with self-spin correlations and the differential operator technique,physical properties of the spin-2 system with biaxial crystal field on the simple cubic, body-centered cubic, as well as faced-centered lattice have been studied. The influences of the external longitudinal magnetic field on the magnetization,internal energy, specific heat, and susceptibility have been discussed in detail. The phenomenon that the magnetization in the ground state shows quantum effects produced by the biaxial transverse crystal field has been found.     

Effective-Field Theory on High Spin Systems with Biaxial Crystal Field

Based on the effective-field theory with self-spin correlations and the differential operator technique, physical properties of the spin-2 system with biaxial crystal field on the simple cubic, body-centered cubic, as well as faced-centered lattice have been studied. The influences of the external longitudinal magnetic field on the magnetization, internal energy, specific heat, and susceptibility have been discussed in detail. The phenomenon that the magnetization in the ground state shows quantum effects produced by the biaxial transverse crystal field has been found.     

稀释晶场对纳米管上Blume-Capel模型磁化性质的研究

利用有效场理论研究了纳米管上稀释晶场中Blume-Capel模型格点的磁化强度、内能、比热和自由能,得到了系统格点的磁化强度、内能、比热和自由能与温度和稀释晶场的关系.结果表明:稀释晶场取值概率和晶场强度等诸多因素相互竞争,使系统表现出比含恒定晶场作用的Blume-Capel模型更为丰富的磁学特性;稀释晶场会抑制系统格点的磁化强度,导致其基态饱和值小于1;温度较低时,系统格点的磁化强度表现出复杂性;晶场较强时,系统的内能、比热和自由能也呈现出不同的磁学性质.     

PrNi$lt;sub$gt;2$lt;/sub$gt;的磁和磁热性能研究

在一些磁性材料内, 磁性离子间交换作用和磁性离子的自旋涨落对材料磁性有影响. 本文根据磁比热实验值确定了晶场参数后, 利用包含自旋涨落的交换作用有效场H_m= n₀ (1 + γ T + β e^{ω T})M, 计算了PrNi₂晶体晶场能级的Zeeman劈裂. 在温度为3.8 K ≤T≤ 30 K范围内, 计算了该晶体多晶磁矩随外磁场的变化, 以及外磁场H=5000 Oe时磁化率倒数随温度的变化, 计算结果和实验值符合较好. 当外磁场在0–50000 Oe时, 计算的该晶体的磁熵变与已有文献的理论结果相似. 计算结果说明, 提出的包含自旋涨落的交换作用有效场不仅适合亚铁磁性晶体, 而且也适合顺磁性晶体. 关键词： 2')" href="#">PrNi₂ 磁比热 交换作用有效场 磁矩 磁熵变     

Influence of Tb on easy magnetization direction and magnetostriction of ferromagnetic Laves phase GdFe_2 compounds

The crystal structure,magnetization,and spontaneous magnetostriction of ferromagnetic Laves phase Gd Fe2 compound have been investigated.High resolution synchrotron x-ray diffraction(XRD) analysis shows that Gd Fe2 has a lower cubic symmetry with easy magnetization direction(EMD) along [100] below Curie temperature TC.The replacement of Gd with a small amount of Tb changes the EMD to [111].The Curie temperature decreases while the field dependence of the saturation magnetization(Ms) measured in temperature range 5–300 K varies with increasing Tb concentration.Coercivity Hc increases with increasing Tb concentration and decays exponentially as temperature increases.The anisotropy in Gd Fe2 is so weak that some of the rare-earth substitution plays an important role in determining the easy direction of magnetization in GdFe_2.The calculated magnetostrictive constant λ100 shows a small value of 37×10~(-6).This value agrees well with experimental data 30×10~(-6).Under a relatively small magnetic field,GdFe_2 exhibits a V-shaped positive magnetostriction curve.When the field is further increased,the crystal exhibits a negative magnetostriction curve.This phenomenon has been discussed in term of magnetic domain switching.Furthermore,magnetostriction increases with increasing Tb concentration.Our work leads to a simple and unified mesoscopic explanation for magnetostriction in ferromagnets.It may also provide insight for developing novel functional materials.     

外磁场对纳米管上Blume-Capel模型磁热性能的研究

利用有效场理论研究了纳米管上外磁场中Blume-Capel模型格点的磁化强度、磁化率、内能、比热和自由能,得到了系统格点的磁化强度、磁化率、内能、比热和自由能与外磁场和晶场的关系.结果表明:外磁场强度、交换相互作用和晶场强度等诸多因素相互竞争,使系统表现出比没有外磁场作用的Blume-Capel模型更为丰富的磁学特性;外磁场能够增大系统格点的磁化强度,导致系统的二级相变消失;负晶场作用系统时,系统会发生一级相变;晶场强度、外磁场强度不同时,系统的磁化率、内能、比热和自由能也呈现出复杂性.     

Magnetic field effects in the cooperative Jahn-Teller system TbVO4

Effects of a magnetic field on the behaviors of TbVO₄ which shows a cooperative Jahn-Teller phase transition have been investigated. The possibility that the system will return to the undistorted phase at low temperatures in a magnetic field is considered. We show that because of the weak coupling of the system to the magnetic field applied along the c-axis, the magnetic field required to see the transition of the crystal back to the undistorted phase is impractically high. Diluting the crystal with nonmagnetic ions is then a plausible approach in the study of this novel behavior of the system. We have also calculated the effects of a magnetic field applied in the basal plane and shown that the crystal would remain distorted for all temperatures except the cases when the field is applied along the crystallographic axes. Finally, we consider the effects of a magnetic field on the temperature behavior of the specific heat and show that the Schottky anomaly which occurs at a very low temperature (～1°K) will be shifted to a higher temperature by applying a magnetic field.     

Electrical and thermal transport properties of dilute Pr in Pd

We have measured susceptibility, electrical resistivity thermopower, thermal conductivity, specific heat, thermal lattice expansion and Hall effect of dilute alloys of Pr (x = 0.014, 0.03) and of Lu (x = 0.014) in Pd. We find a rather strong valence instability at room temperature from L_III X-ray absorption (v = 3.1), which manifests itself by a concentration-independent characteristic temperature, by a resistivity minimum and by a strong anomally of the thermopower. The susceptibility also indicates strong magnetic polarisation of the surrounding Pd-atoms through the excited crystal field levels but not through the ground state of the impurity. The neutron scattering shows several sharp crystal field excitations between 0 and 1 meV and a very broad quasi-elastic scattering at higher temperatures with the 4f-form factor. We take this also as evidence for a weak coupling of valence instabilities to the low lying crystal field levels of the impurity and a strong coupling to the higher ones.     

Magnetic phase transition in CsVF4

Magnetization and specific heat measurements are reported on the quasi two-dimensional antiferromagnet CsVF₄. Diverging magnetic susceptibility and specific heat at the Néel temperature are observed in a weak field along the crystallographic c-direction, but with increasing field they are progressively smeared. A hysteresis is observed in dc magnetization below the temperature at which M/H reaches a maximum. We present a possible explanation for these phase transition phenomena from the point of view that this compound is composed of two crystallographically inequivalent magnetic sites.     

An introduced effective-field theory study of spin-1 transverse Ising model with crystal field anisotropy in a longitudinal magnetic field

A spin-1 transverse Ising model with longitudinal crystal field in a longitudinal magnetic field is examined by introducing an effective field approximation (IEFT) which includes the correlations between different spins that emerge when expanding the identities. The effects of the crystal field as well as the transverse and longitudinal magnetic fields on the thermal and magnetic properties of the spin system are discussed in detail. The order parameters, Helmholtz free energy, entropy and specific heat curves are calculated numerically as functions of the temperature and Hamiltonian parameters. A number of interesting phenomena such as reentrant phenomena originating from the temperature, crystal field, transverse and longitudinal magnetic fields have been found.     

Investigation of magnetic, thermal and hyperfine properties of Tb in tetragonal sites in SrF2

The crystalline Stark splitting of the electronic ground state and various properties of Tb³⁺ in tetragonal sites in Tb: SrF₂ have been determined with the crystal field parameters of Antipin et al. Two singlets are found to lie lowest, the separation between them being 0.363 cm⁻¹. The χ has a Curie type variation in the range 4–1.0 K below which saturation effects set in. χ has a maximum at 110 K and becomes nearly temperature independent at around 4 K. The magnetic moment turns out to be 9.58 at 300 K which is very close to the free ion value 9.72. There is only a small variation of the magnetic moment with temperature. Two anomalies are found to occur in the Schottky heat capacity curve, one at 100 K, and the other at 0.22 K. The magnetic hyperfine field at the nucleus of Tb³⁺ is obtained as 3.456 MG, neglecting the core polarization. The temperature averaged electric quadrupole splitting of the nuclear ground state of ¹⁵⁹Tb has been calculated at different temperatures and is found to rise smoothly with the decrease of temperature to attain saturation at 10 K. The behavior of the hyperfine heat capacity C_N has been studied theoretically in the range 4.2–0.01 K. A Schottky anomaly appears at 0.1 K. The computations have been carried out in the intermediate coupling approximation.     

磁感应强度和冷却速率对Tb_(0.27)Dy_(0.73)Fe_(1.95)合金凝固过程中取向行为的影响

实验研究了磁感应强度和冷却速率对Tb_(0.27)Dy_(0.73)Fe_(1.95)合金凝固过程中(Tb,Dy)Fe_2相取向行为及合金磁性能的影响.结果表明,将强磁场作用于Tb_(0.27)Dy_(0.73)Fe_(1.95)合金的凝固过程可以制备出(Tb,Dy)Fe_2相沿111取向的组织,同时显著提高了合金的磁致伸缩性能;通过提高磁感应强度可以在更快的冷却速率下得到111取向的组织;在4-10 T范围内,随着冷却速率的增加,(Tb,Dy)Fe_2相沿111取向所需的磁感应强度增加,而发生(110)取向的磁感应强度减小.随着冷却速率的增加,合金的饱和磁化强度增加,而强磁场的施加对合金饱和磁化强度的变化没有明显影响.(Tb,Dy)Fe_2相的取向行为受*Tb,Dy)Fe_3相取向行为的影响,且由磁晶各向异性能与磁场作用时间共同控制.     

两带超导体LaNiC_2上临界磁场的理论分析

非中心对称超导体LaNiC_2是传统BCS超导体还是能隙存在节点又或是两带超导体,目前仍然存在争议.基于此,文章用两带Ginzburg-Landau理论分析了超导体LaNiC_2的上临界磁场随温度的变化关系,计算结果与实验结果在整个温度区间内符合得很好,说明LaNiC_2是两带超导体,和陈健等人的观点一致.文章还分析了两个不同能带对上临界磁场的影响,发现相对较小的相干长度对LaNiC_2的上临界磁场影响较大.     

Magnetic phase transition between ferromagnetic and antiferromagnetic states in Ce(Fe1−xAlx)2

The cubic Laves phase compound Ce(Fe_0.95Al_0.05)₂ exhibits a first order magnetic phase transition from antiferromagnetic to ferromagnetic state at ≈ 100 K with increasing temperature. This note gives the results of the Mössbauer effect of ⁵⁷Fe and the magnetic phase diagram in the external field vs. temperature plane.     

Dy3Al5O12磁热性质研究

用量子理论计算了Dy₃Al₅O₁₂的晶场能谱、Zeeman劈裂能级和波函数. 在外磁场H_e为0<H_e<9 T, 温度为3<T<42 K 范围内, 计算了该晶体的磁矩、磁熵变, 计算结果与相关实验数据吻合较好. 该计算结果表明, Dy₃Al₅O₁₂内磁性离子间的交换作用非常微弱, 可以忽略. 从理论上给出了绝热退磁过程中温度变化ΔT与T的关系, 并与Gd₃Ga₅O₁₂晶体进行了比较, 发现不同外磁场下, Dy₃Al₅O₁₂和Gd₃Ga₅O₁₂的低温制冷性能在不同温区有差别. 在进行低温(T<10 K)制冷时, 若外磁场较低, 选择Dy₃Al₅O₁₂作为磁制冷材料较好; 若外磁场较高, 选择Gd₃Ga₅O₁₂作为磁制冷材料较好.     

{Cu_3}单分子磁体在热平衡和磁场作用下的三体纠缠

本文研究了Na_9[Cu_3Na_3(H_2O)_9(α-As W_9O_(33))_2]·26H_2O(简记为{Cu_3})单分子磁体在热平衡和外加磁场作用下的三体纠缠性质,利用等效自旋模型和实验拟合参数,数值计算了{Cu_3}型三角自旋环中三体负性纠缠度(tripartite negativity).分别考虑沿垂直于三角自旋环方向的磁场、平行于三角自旋环方向的磁场,以及倾斜磁场的情形.结果表明,磁场的方向、大小以及温度对系统三体负性纠缠度有着重要影响.文中给出了在不同磁场方向下,临界温度随磁场强度的变化图,由此可以得到三体纠缠存在的参数区域.同时发现在特定的参数区域,该系统存在纠缠恢复现象.因此适当调节温度、磁场强度大小和磁场方向可以有效调控{Cu_3}型三角自旋环中的三体纠缠性质.     

最近邻原子间交换相互作用对纳米管上Blume-Capel模型磁热性质的影响

利用有效场理论研究了纳米管上最近邻原子间交换相互作用对Blume-Capel模型格点的磁化强度、内能、比热和自由能的影响,得到了系统格点的磁化强度、内能、比热和自由能与交换相互作用和晶场强度的关系.结果表明:最近邻交换相互作用和晶场强度等诸多因素相互竞争,使系统表现出丰富的磁学特性;正晶场对系统磁化强度具有促进作用;负晶场对系统磁化强度具有抑制作用;负晶场作用下,系统发生一级相变;晶场强度参数和交换相互作用不同时,系统的内能、比热和自由能也呈现出奇异性.