YBa2Cu3O7超导管内磁场与外磁场的关系

本文报道了当外磁场H_e较低时(H_e<4H_rm,其中H_rm是超导管的最大捕获磁场),超导管的内磁场H_i与时间及外磁场的关系可表示为H₀+B₀(lnt+1-exp(-t/τ)),式中B₀Int与磁通蠕动有关,B_c(1-exp(-t/τ))与外磁场引起的磁通扰动有关。扰动随时间很快衰减.H₀,B₀及1/τ随外磁场增加,在H_e>H_rm时,这几个量增加较大。当直流外磁场H_e<1/2H_rm时,超导管可将其完全屏蔽,对交流场不能完全屏蔽,但其屏蔽效果仍优于最好的金属屏蔽筒。 关键词：     

弱场下熔融织构YBa2Cu3O7-δ样品局域磁通蠕动的实验研究

利用半导体霍尔探头测量法在液氮温度下研究了低场下圆柱状熔融织构YBa₂Cu₃O_7-δ样品的局域磁通蠕动行为.对瞬时加减场样品端面局域磁场的磁弛豫,磁滞回线以及不同扫场速率下局域磁场的变化进行了测量,观察到了对数时间磁弛豫的行为,发现由加场和去场数据所得的激活能U₀有明显的不同.磁滞回线测量中,局域点的磁场随外场几乎同步变化,不同扫场速率下局域磁场随外场变化的差别不大.实验结果表明,外场除对样品侧面外,对端面也有明显的穿 关键词： 局域磁通蠕动 激活能 磁弛豫     

Tl2Ba2Ca2Cu3Oy超导体磁通钉扎能的分布

描述一种测量高温超导体磁通钉扎能U₀及其分布的新方法,此方法主要之点是测量磁化后样品的零场临界电流随时间的变化,利用这一方法测量Tl₂Ba₂Ca₂Cu₃O_y的U₀值分布(T=78K)。根据所得结果,对已报道的U₀值的分散性提出一种新解释。 关键词：     

YBa2Cu3O7-δ熔融织构样品的磁响应研究

对YBa₂Cu₃O_7-δ熔融织构样品在不同温度、不同磁场不同外场扫描速率下进行了磁滞回线测量,获得了电流密度J(T,B),lnE-lnJ曲线以及热激活能U₀(T,B)随温度、磁场的变化关系,从而分析了该样品磁响应特性,特别是第二峰(简称峰)受外场扫描速率的影响,探讨了峰形成的真正原因,以及影响峰位置、大小的各种因素 关键词： 磁响应 熔融织构 峰效应 临界电流密度     

高Tc氧化物YBa2Cu3Oy超导体中的磁通蠕动

利用观察磁化随时间变化的方法研究了高T_c氧化物YBa₂Cu₃O_y超导体临界态的磁通蠕动。实验结果表明,零场冷却的样品,加一个所选定场值的磁场,产生的磁化随对数时间线性衰减,同时伴随有磁通跳跃的发生。磁化衰减速率dM/dlnt随外场H的变化在H=3H_P附近有一峰值,并认为与弱连结超导电性的存在有关。在77K以上温度,dM/dlnt随温度的升高而单调减小。讨论了钉扎磁通的热激活能U_{0关键词：}     

磁场对石墨烯中弱耦合极化子性质的影响

研究了磁场作用下石墨烯中电子与表面光学声子弱耦合情况下的极化子的性质。采用线性组合算符和Pekar变分法分别推导出了石墨烯中弱耦合极化子的基态能量E₀、第一激发态能量E₁和跃迁频率ω随磁场强度B和德拜截止波数k_d之间的变化关系。数值计算结果表明,极化子的基态能量E₀随磁场强度B变化的曲线（k_d一定时）和E₀随k_d的变化曲线（B一定时）均会分裂成对称的两条,并且当B一定时E₀的绝对值随k_d的增加而增加。在k_d一定时,极化子的第一激发态能量E₁和跃迁频率ω均为磁场B的增函数;在B一定时,E₁和ω均随k_d的增加而增大。     

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     

Nd1.85Ce0.15CuO4－δ单晶c-轴方向电阻的低场行为

采用自助熔剂缓冷法成功地生长出了Nd_1.85Ce_{0. 15}CuO_4-δ单晶，其零场下零电阻温度约为21K. 在0—0.5T范围内分别测量了磁场平行和垂直样品表面的电阻转变曲线以及0.5T不同角度下的电阻转变曲线. 结果显示磁场平行和垂直样品表面时的转变温度T_p随磁场的变化均服从H=H₀(1-T_p(h)/T_p(0))²关系. 0.5T 关键词： 1.85Ce_0.15CuO_4-δ单晶')" href="#">Nd_1.85Ce_0.15CuO_4-δ单晶 输运性质     

Tl2Ba2Ca2Cu3Oy超导体低场钉扎力的性质

测量了Tl₂Ba₂Ca₂Cu₃O_y超导体多晶样品在不同外磁场和不同温度下的磁滞性质,利用临界态方程求出有效钉扎力随外磁场及温度的变化,有效钉扎力包括晶粒界面处及晶粒内的贡献,发现有效钉扎力有峰效应,峰位置随温度降低向较高磁场移动,峰效应被解释为两种钉扎机制产生的,即两种机制的贡献与外磁场的关系不同,还讨论与这些结果有关的问题。 关键词：     

Yba2Cu3O7-δ不可逆线与钉扎势的关联

本文分别测量了熔融织构YBa₂Cu₃O_7-δ高温超导体不可逆线与钉扎势U₀,发现该样品的不可逆线在温区72—85K范围内呈指数形式B^*-e^(-T*/T_c)函数关系,而且在温度77K,不可逆线并不能与磁测量临界电流密度J_c=0相对应,我们认为高温超导体的不可逆线的函数形式与样品的钉扎势紧密关联。 关键词：     

高Jc Tl系2223相银包套带材的磁通钉扎和耗散

测量了Ｔｌ系２２２３相银包套超导带（Ｊ_c＝１．５×１０^４Ａ·ｃｍ^-2，７７Ｋ，０Ｔ）在０－０．８Ｔ磁场下电阻转变展宽，实验结果引用热激活磁通蠕动模型加以解释。磁场平行于带面（Ｈ∥ａｂ面）和磁场垂直于带面（Ｈ⊥ａｂ面）两种情况下，激活能与磁场之间满足幂指数关系：Ｕ_0∥＝０．２１ H^-0.3(eV),Ｕ_0⊥＝０．１5 H^-0.4(eV)，其中Ｈ的单位为ｋＧ。 关键词：     

Improvement of the intergranular pinning energy in uniaxially compacting (Bi-Pb)<Subscript>2</Subscript>Sr<Subscript>2</Subscript>Ca<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>10+δ</Subscript> ceramic samples

Measurements of the electrical resistivity as a function of temperature, ρ(T), for different values of applied magnetic field, B_a (0 ≤ B_a ≤ 50 mT), were performed in polycrystalline samples of Bi_1.65Pb_0.35Sr₂Ca₂ Cu₃O_10+δ subjected to different uniaxial compacting pressure (UCP). We have found appreciable differences in the grain orientation between samples by using X-ray diffractometry. From the X-ray diffraction patterns performed, in powder and pellet samples, we have estimated the Lotgering factor along the (00l) direction, F_(00l). The results indicate that F_(00l) increases ~23% with increasing UCP suggesting that grains of these samples are preferentially aligned along the c-axis, which is parallel to the compacting direction. The resistive transition of the samples have been interpreted in terms of the thermally activated flux-creep model. In addition, the effective intergranular pinning energy, U₀, have been determined for different applied magnetic field. The magnetic field dependence of U₀, for B_a > 8 mT, was found to follow a H^{- α} dependence with α = 0.5 for all samples. The analysis of the experimental data strongly suggested that increasing UCP results in appreciable changes in both the grain alignment and the grain connectivity of the samples. We have successfully interpreted the data by considering the existence of three different superconducting levels within the samples: the superconducting grains, the weak-links, and the superconducting clusters.     

The effective activation energy Ueff(T,B,J) in Hg-1223 single phase superconductors

We review the methods of calculating the effective activation energy U_eff(T,B,J) for both transport measurements and magnetic decay, together with some theoretical models. Then, we apply these methods to our Hg-1223 single-phase superconductor to obtain the activation energy. Transport results give that the magnetic field and temperature dependence of the U_eff can be well described as U₀B^−α(1−T/T_c)^m. Magnetic relaxation shows that the current density dependence of U(J) can be scaled onto a single curve, which can be considered as the activation energy at some temperature T₀. The pinning mechanism in the measured temperature range does not change, and the activation energy depends separately on the three variables: T, B, and J, are responsible for the magnetic decay data scaling onto a single curve at various temperatures. As temperatures close to zero and near T_c, thermally assisted flux motion model is no longer valid since other processes dominate.     

YBa2Cu3O7-δ外延薄膜的有效钉扎势

在0—7T磁场范围内，测量了不同测量电流密度下YBa₂Cu₃O_7-δ外延薄膜的电阻温度关系.实验结果表明，临界温度以下，混合态的耗散电阻率能很好地用热激活磁通蠕动描述.有效钉扎势的电流密度关系遵守Zeldov等人提出的对数关系，有效钉扎势的温度和磁场关系遵守U∝(1-T/T_c)H^-α关系，其中α=0.63，与热激活磁通点阵位错运动模型相一致，表明样品具有2D涡旋性质. 关键词：     

Deformation of nematic layers in crossed electric and magnetic fields

A parallel nematic layer will show a Fréedericksz-transition in an electric field when the voltage exceeds a threshold value U_c. If a magnetic field is applied parallel to the layer but perpendicular to the molecular orientation the threshold voltage U_c increases with increasing magnetic field field H above a threshold field H_c2. From a plot of U_c vs H all three elastic constants can be obtained. Theoretical results are compared with data of HBAB.     

Magnetic field effects on total and partial conductance histograms in Cu and Ni nanowires

Conductance histograms have become a powerful tool for studying transport properties of metallic nanowires. However, the individual conductance curves display a very rich structure that might be concealed by the statistical procedure of finding preferred conductance values by building conductance occurrence histograms using consecutive nanocontact breakage experiments. This is particularly true when it comes to discerning 1/2G₀=e²/hquantization in magnetic nanowires. The effect of disorder, added to possible magnetic sources of scattering, and different magnetic states of different nanowires, might hide its appearance as histogram peaks. This work analyzes and compares Ni and Cu nanowire experimental histograms at room temperature (RT). Those obtained with no curve selection criteria are basically unaffected by the presence of a magnetic field. A selection of particular sets of conductance curves shows that conductance quantization could occur in steps of e²/h and 2e²/h in Ni as well as in Cu in the presence or absence of a magnetic field. Sorting out curves in sets that present conductance plateaus at half integer and integer values, and compiling statistics on the number of such curves that appear, depending on the applied magnetic field, results in differences between the behaviour of Cu and Ni. While for Cu, the magnetic field keeps the ratio of curves that present plateaus at 1/2G₀with respect those presenting G₀ plateaus unchanged; for Ni, the number of curves which exhibit plateaus at just G₀ almost disappears with the applied field. This experimental fact might indicate that the magnetic field removes spin degeneracy in these magnetic nanowires. PACS 72.25.Ba; 73.40.Jn; 73.63.Rt; 75.75.+a     

Electron paramagnetic resonance of impurities in uranium trichalcogenides

Uranium trichalcogenides US₃ and UTe₃ were examined by EPR. The measurements were performed at the X-band over temperature range 4.2–300K and with the magnetic field applied perpendicular and parallel to the (001) plane. The EPR spectra consist of two lines with g-factors about 2 and 4 respectively. Results are discussed in terms of interactions of the U⁴⁺ ion with impurities. A magnetic ordering in US₃ below 50 K is suggested.     

Self-similar magnetic structures and magnetic flux “Giant” creep

The penetration of a magnetic flux into a type-II high-T _c superconductor occupying the half-space x > 0 is considered. At the superconductor surface, the magnetic field amplitude increases in accordance with the law b(0, t) = b ₀(1 + t)^m (in dimensionless coordinates), where m > 0. The velocity of penetration of vortices is determined in the regime of thermally activated magnetic flux flow: v = v ₀exp?ub;?(U _0/T )(1-b?b/?x)?ub;, where U ₀ is the effective pinning energy and T is the thermal energy of excited vortex filaments (or their bundles). magnetic flux “Giant” creep (for which U ₀/T? 1) is considered. The model Navier-Stokes equation is derived with nonlinear “viscosity” v ∝ U ₀/T and convection velocity v _f ∝ (1 ? U ₀/T). It is shown that motion of vortices is of the diffusion type for j → 0 (j is the current density). For finite current densities 0 < j < j _c, magnetic flux convection takes place, leading to an increase in the amplitude and depth of penetration of the magnetic field into the superconductor. It is shown that the solution to the model equation is finite at each instant (i.e., the magnetic flux penetrates to a finite depth). The penetration depth x _eff ^A (t) ∝ (1 + t)^{(1 + m/2)/2} of the magnetic field in the superconductor and the velocity of the wavefront, which increases linearly in exponent m, exponentially in temperature T, and decreases upon an increase in the effective pinning barrier, are determined. A distinguishing feature of the solutions is their self-similarity; i.e., dissipative magnetic structures emerging in the case of giant creep are invariant to transformations b(x, t) = β^m b(t/β, x/β^{(1 + m/2)/2}), where β > 0.