Critical currents of LaMo6S8 thin films

The thin films of LaMo₆S₈ were prepared by d.c. getter sputtering method. Critical current have been measured vs applied magnetic field B_⊥ up to 8 T at the temperature ranged from 1.7 to 4.2 K. The highest critical current density reached was J_c = 1.7 × 10⁷ Am^?2 at B = 0 and T = 1.7 K. Based on the measurements performed, it was possible to indicate that depending on the heat treatment applied, the pinning forces created in the LaMo₆S₈ thin films obey the scaling law.     

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基于当前应用较广泛的应变下临界电流测量装置Pacman,利用有限元方法对应变盘及超导线进行受力分析,得到了超导线轴向及横向的应变分布情况。结合Nb3Sn超导线偏应变临界电流定标律,分析了在两种极限假设情况(低丝间电阻和高丝间电阻)下超导线截面应变分布对临界电流的影响。     

Bi—2223带材在拉伸应力下的临界电流

研究了不锈钢加强的Bi-2223/Ag高温超导带材在不同的拉伸应力下临界电流的变化情况。采用改造后的拉伸设备,在温度77K下对超导带材施加拉伸应力的同时,利用四引线法测量带材的临界电流,得到了超导带材的应力—应变曲线以及临界电流—应变曲线。利用理论公式计算了应力对超导带材临界电流的影响,与实验结果进行对比,并对两者的差别进行了分析。     

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介绍了Pacman装置的设计原理,用Pacman装置测量了OST公司通过内锡法工艺制备的Nb3Sn超导股线在12T,4.2K条件下单轴方向的临界电流与应变的关系,应变的范围在-0.8%～+0.6%。用偏量比例模型模拟了临界电流与轴向应变之间的规律,分析了随应变的改变n值的变化,得出了Nb3Sn超导股线对应变的敏感程度。     

2H-Nb0.9Ta0.1Se2中磁通涡旋输运的标度行为

测量了2H-Nb_0.9Ta_0.1Se₂单晶样品在不同电流下电压随磁场变化的曲线V(H),并从V(H)曲线得到V(I)数据.使用标度关系V=α(I-I_c)^β进行了拟合,得到了临界电流随磁场I_c(H)和微分电阻随磁场R_d的变化关系.在微分电阻随磁场变化的曲线中,电流较大时,靠近上临界磁场H_c2附近出现一个强峰,而在低电流下,该峰消失.同 关键词： 磁通涡旋 标度定律 临界电流峰效应     

ITER超导股线的力学性能测试

介绍了Pacman装置的设计原理,用Pacman装置测量了OST公司通过内锡法工艺制备的Nb₃Sn超导股线在12T,4.2K条件下单轴方向的临界电流与应变的关系,应变的范围在-0.8%～+0.6%。用偏量比例模型模拟了临界电流与轴向应变之间的规律,分析了随应变的改变n值的变化,得出了Nb₃Sn超导股线对应变的敏感程度。     

Anomalous behaviour of the proton spin-lattice relaxation time near the phase transition of the layered antiferroelectric squaric acid

The temperature dependence of the proton spin-lattice relaxation time has been measured at 51, 70 and 300 MHz by different pulse sequences. Besides a strongly frequency dependent background relaxation rate, a frequency independent critical relaxation rate could be resolved particularly in a broad temperature interval above T_c. The temperature dependence of the critical relaxation rate could not be represented by a power law with a single exponent. Rather it most favourably could be fitted by a logarithmic law. The results are discussed with respect to the phase transition mechanism of squaric acid.     

Some superconducting properties of 25%Nb–75%Zr alloy

The manner of preparation of superconducting 25% wt Nb–75% wt Zr wires is described. Short samples of these wires were measured in static magnetic fields up to 80·5 kG and the authors describe the method of these measurements. The paper gives the results of measuring the critical current density dependence on the external perpendicular magnetic field for both cold-worked wires with different deformations and heat-treated wires. The dependence of the critical current density on the heat treatment temperature after wire deformation for different magnetic fields was obtained. The optimum heat treatment temperature (vacuum better than 10^–3 torr, 1 hour) is 450–600C for magnetic fields 0–80·5 kG. The values ofi _c of these wires in magnetic fields up to 60 kG are the same or higher than those of 75% Nb-25% Zr wires, and in fields above 60 kG they are much higher.     

Critical behavior of the heat capacity of the manganite La<Subscript>0.87</Subscript>K<Subscript>0.13</Subscript>MnO<Subscript>3</Subscript>

The heat capacity of the manganite La_0.87K_0.13MnO₃ has been measured in the temperature range 80–350 K. The nature of the ferromagnetic phase transition and the critical properties of heat capacity near the Curie temperature have been studied. The regularities of variations in the universal critical parameters near the phase transition point have been established. The calculated critical exponent and amplitudes of the heat capacity with allowance for corrections on the scaling (α = −0.13 and A ⁺/A ⁻ = 1.178) correspond to the critical behavior of the 3D Heizenberg model.     

Experimental investigation of the behavior of the specific heat in finite systems in the vicinity of the critical mixing point

The specific heat of a 2,6-lutidine-water mixture is measured in the vicinity of the lower critical mixing point in the bulk and in porous media. The results of the measurements are interpreted by finite-size scaling. In particular, a universal function of the ratio t/t* [t is the dimensionless deviation of the temperature from the specific-heat maximum T _m (L), and t* is the characteristic dimensionless temperature, which depends on the pore size] describing the behavior of the specific heat in the vicinity of the critical point in porous media with different pore sizes is obtained. The results obtained are consistent with the predictions of finite-size scaling and with the data from a numerical calculation of the specific heat of the finite three-dimensional Ising model. Zh. éksp. Teor. Fiz. 113, 1071–1080 (March 1998)     

Anomalous behavior of the specific heat and upper critical magnetic field in the superconducting single crystal La1.85Sr0.15CuO4

The specific heat and resistive upper critical magnetic field of the single crystal La_1.85Sr_0.15CuO₄ are investigated in the temperature range 2–50 K in magnetic fields up to 8 T for two directions of the magnetic field, parallel and normal to the ab crystalline plane. For both orientations a nonlinear (close to square root) magnetic field dependence of the mixed-state specific heat and a positive curvature of the temperature dependence of the upper critical magnetic field are observed. Neither of these anomalies is described by standard theories of superconductivity. Within the framework of the thermodynamic relations it is shown that in a type-II superconductor a relationship exists between the temperature dependence of the critical magnetic field and the field dependence of the specific heat. The anomalies observed in these phenomena are interrelated. Zh. éksp. Teor. Fiz. 112, 1386–1395 (October 1997)     

非晶态Co70Cr20Zr10合金在居里温度附近的磁性

本文研究非晶态Co₇₀Cr₂₀Zr₁₀合金在居里温度附近的磁性,样品的磁特性符合二级相变规律。得到临界指数β=0.45±0.02,γ=1.9±0.1,δ=5.13±0.05。Co₇₀Cr₂₀Zr₁₀合金的居里温度为T_c=(186.7±0.2)K。临界指数β,γ,δ满足γ=β(δ-1)关系,但临界指数值都偏离三维Heisenberg模型的理论值,这种行为可能起源于非晶态合金磁的不均匀性。讨论了Kouvel-Fisher(K-F)参数γ(T)对温度T的依赖关系。 关键词：     

Propagation of longitudinal ultrasonic waves in RbH2PO4 near its ferroelectric phase transition temperature

Ferroelectric phase transition in RbH₂PO₄ has been investigated using propagation of longitudinal acoustic waves along the polar axis near the transition temperature. The velocity of this mode is continuous across the transition temperature. Velocity data in the ferroelectric phase are analyzed in terms of coupled soft modeacoustic mode model of Pytte to obtain the temperature dependence of the soft mode frequency. The attenuation data in the ferroelectric phase show power law dependence. It follows scaling behaviour of the type predicted by Kawasaki from the mode-mode coupling theory and the dynamical scaling.     

Magnetic phases of amorphous transition metal-metalloid alloys

Hysteresis loop and ac susceptibility measurements were performed on three series of amorphous alloys: (A_wB_1-w)₇₅P₁₆B₆Al₃, where (A, B) are (Fe, Ni), (Co, Ni) and (Fe, Mn). Upon cooling, low w alloys undergo paramagne t to spin glass transitions. Alloys with higher w first experience a Curie transition to a ferromagnetic state, and then a spin freezing transition to a spin glass state. the T dependence of the width of the ac hysteresis loop is used to determine the spin freezing transition temperature. A magnetic phase diagram is presented for each alloy series and the value of w required for ferromagnetism, w_C, is determined. When measured in the presence of small constant fields, the ac susceptibility of alloys with w just above w_C has maxima near both transition temperatures. The field and temperature dependences of the peaks are explained by scaling arguments, used to determine the critical exponent δ for the Curie transition, and suggest that a similar scaling law holds for the ferromagnet to spin glass transition.     

Short-time behavior of the kinetic spherical model with long-ranged interactions

The kinetic spherical model with long-ranged interactions and an arbitrary initial order m₀ quenched from a very high temperature to T is solved. In the short-time regime, the bulk order increases with a power law in both the critical and phase-ordering dynamics. To the latter dynamics, a power law for the relative order is found in the intermediate time-regime. The short-time scaling relations of small m₀ are generalized to an arbitrary m₀ and all the time larger than . The characteristic functions for the scaling of m₀ and for are obtained. The crossover between scaling regimes is discussed in detail. Received 17 September 1999     

Relation between the internal stress measured in creep and the stress generated by the dislocation structure in the fcc metals

From the steady-state creep rate data treated as a function of the applied and the measured effective stress and temperature, a phenomenological dependence of the internal stress on the applied stress and temperature was derived. The result determined the expected character of the applied stress- and temperature dependences of the dislocation density, which was considered the microstructure parameter of the internal stress σ _i?=?α MGbρ ^1/2 (σ_i is the internal stress, α the dislocation interaction factor, M the Taylor factor, G the shear modulus, b the Burgers vector length and ρ the dislocation density). A scaling of the expected dislocation density by fitting it to measured dislocation density data yielded reasonable values of the parameter α in the Taylor formula, but the experimental data indicated a weaker applied stress dependence of the measured dislocation density than that of the expected dependence. An admission of an empirical formula fitting the dependence of dislocation density on the applied stress and temperature leads to a suggestion that the parameter α might be dependent on applied stress and temperature.     

Specific heat and critical behavior of Tl0.3MoO3 near the Peierls phase transition

The specific heat of the quasi-one-dimensional charge-density-wave (CDW) compound Tl_0.3MoO₃ has been measured using an adiabatic continuous heating method from 100 to 220 K. A specific heat jump associated with the Peierls phase transition occurs at 172.3 K. A good scaling relation between the excess specific heat and the susceptibility is found between 140 K and 190 K. Further analysis indicates that the width of the critical region of Tl_0.3MoO₃ is about 10 K and the specific-heat critical behavior can be well described by the three-dimensional XY model.     

Possible observation of the coexistence of superconductivity and long-range magnetic order in NdRh4B4

The ternary rare earth compound NdRh₄B₄ has been studied by means of critical field, low temperature heat capacity, and static magnetic susceptibility measurements. Features in the upper critical field and heat capacity data at 1.31 K and 0.89 K suggest the occurrence of long-range magnetic order in the superconducting state. The temperature dependence of the static magnetic susceptibility follows a Curie-Weiss law with an effective magnetic moment μ_eff = 3.58 ± 0.05 μ_B and a Curie-Weiss temperature θ_p = ?6.2 ± 1.0 K between 20 K and room temperature. However,, magnetization vs. applied magnetic field isotherms suggest the development of a ferromagnetic component in the Nd³⁺ magnetization at low temperatures.     

Ullah-Dorsey Scaling of Fluctuation Conductivity Near the Superconducting Transition in (LaSe)1.14(NbSe2)

Magnetotransport measurements with the quasi two-dimensional misfit-layer compound (LaSe)_1.14(NbSe₂) with critical temperature T _c = 1.23 K are presented. The temperature dependence of the upper critical magnetic field H _c2 has been determined from an analysis of magnetoresistance data through the scaling procedure obtained theroretically by Ullah and Dorsey for the fluctuation conductivity. We show that in contrast to a direct determination of H _c2(T) with a positive curvature the temperature dependence of H _c2 obtained via the scaling procedure reveals a negative curvature in agreement with the Werthammer-Helfand-Hohenberg theory for conventional type-II superconductors.     

Josephson结的动力学行为(Ⅱ)

本文对射频电流驱动下的包含干涉项电流的Josephson结系统的阵发混沌行为进行了研究,得到的标度关系与理论预言的相符。此外,我们计算了一些参数下的里亚普诺夫指数以及由此引入定义的吸引子的维数d_L,并且还用盒子计数法计算了几组参数下奇异吸引子的柯尔莫哥洛夫容量d_c,其值与d_L相近。 关键词：