高温超导YBCO薄膜中的传输交流损耗

基于Norris方程和Bean临界态模型,考虑薄膜超导体内的磁场和电流密度分布特性,通过解析求解的方法推导出薄膜超导体在传输外加电流时其内部的磁场和电流密度以及传输交流损耗的解析表达式,从定量的角度研究超导体截面几何形状对传输交流损耗的影响.结果表明薄膜超导体边缘处的剧烈变化的磁场和电流的分布以及无场区的电流承载能力是...     

Demagnetizing fields of crystallites and a method for measuring the thermodynamic fields of quasi-single-crystal and polycrystalline thin YBa2Cu3O7 − x disks

The role of the demagnetizing fields of crystallites in HTSC samples is studied. An increase in the crystallite size is shown to suppress the intra-and intercrystalline critical currents of the sample in lower fields. The demagnetizing fields of crystallites are shown to be one of the main causes of the fact that the Bean model is invalid for HTSC samples. A method is proposed to measure the thermodynamic field of a superconductor; this method allows the first thermodynamic critical magnetic fields of the sample and its crystallites and “subcrystallites” to be measured with a high accuracy. The first thermodynamic critical magnetic fields are used to estimate the critical current density J _c of the sample, crystallites, and subcrystallites.     

Pr替代YPBCO超导单晶的峰效应研究

采用TSSG法在1 atm氧气氛下生长出Y1-xPrxBa2Cu3O7(x=0.07215)超导单晶样品;测量了该单晶样品在不同温度下的磁特性;根据Bean临界态模型,计算出不同温度下的临界电流密度Jc对磁场H的依赖关系;在对该样品的直流磁化研究中观测到了峰效应。随着温度的降低,样品的临界电流密度明显增加。     

Magnetic field penetration into a 3D ordered Josephson medium and applicability of the bean model

The results of calculation of penetration of an external magnetic field into a 3D ordered Josephson medium, based on analysis of modification of the configuration in the direction of the decrease in its Gibbs potential, are reported. When the external field slightly exceeds the stability threshold, the Meissner configuration is transformed into a periodic sequence of linear vortices, which are parallel to the boundary of the medium and are located at a certain distance from it. There exists a critical value I _C separating two possible regimes of penetration of the external magnetic field into the medium. For I > I _C , for any value of the external field, a finite-length boundary current configuration appears, which completely compensates the external field in the bulk of the sample. At the sample boundary, the field decreases with increasing depth almost linearly. The values of the slope of the magnetic field dependence are rational fractions, which remain constant in finite intervals of I. When the value of I exceeds the upper boundary of such an interval, the slope increases and assumes the value of another rational fraction. If, however, I < I _C , such a situation takes place only up to a certain value of external field H _max. For higher values, the field penetrates into the medium to an infinite depth. These results lead to the conclusion that the Bean assumptions are violated and that Bean’s model is inapplicable for analyzing the processes considered here.     

A comparative study of magnetic relaxation in YBa2Cu3−x Fe x O7−y single crystals

The zero-field-cooled magnetization relaxation has been studied over wide temperature and field ranges on iron-doped YBa₂Cu₃O_7–y single crystals. The relationship between the twin boundary density, which depends on the iron composition, and the pinning strength of the flux lines is determined systematically. For magnetic fields up to 1 T the inductive critical current density decreases as the iron composition increases. The Bean critical state model and the Anderson-Kim thermally activated flux-creep model are utilized to analyze and interpret the experimental results.     

The immutability of the critical current under any sample cross-sectional form modification

The temperature and size dependences of the critical currents in bismuth-based high-temperature superconducting ceramic samples consisting of randomly oriented grains have been studied in zero magnetic field. It is shown that the critical current is a function of the sample cross-sectional area only. At constant sample cross-sectional area the value of the critical current is independent of any variation of a sample cross-sectional shape. At the same time the distributions of the critical current density and induced magnetic field in ceramics are functions of the sample cross-sectional shape. These values very in accordance with a change of the sample cross-sectional shape (at constant cross-sectional area i.e. at constant value of the critical current). The field and current distributions in the rectangular sample are reported.     

Magnetization of two-dimensional superconductors with defects

A new method is developed for numerical simulation of the magnetization of layered superconductors with defects that is based on the Monte Carlo algorithm. The minimization of the free energy functional of a two-dimensional vortex system enables one to obtain equilibrium configurations of vortex density and calculate the magnetization of a superconductor with arbitrary distribution of defects in a wide temperature range. Magnetization curves are obtained for the first time for a defective superconductor under conditions of cyclic variation of the external magnetic field for different temperatures. The magnetic induction profiles and the magnetic flux distribution inside a superconductor are calculated, which support the validity of Bean’s model. It is demonstrated that the process of magnetization reversal is accompanied by the emergence of an annihilation wave, i.e., the motion of a zone with zero magnetic induction at the leading front of the incoming magnetic flux.     

On the peak-effect near Hc1 in niobium

Significant peak-effect is observed in the magnetic field near H_c1 on Nb bicrystals with macro faceted grain boundary. In the region of peak-effect the increasing of the magnetic field is accompanied by more than double growth of the critical current. Critical current reaches the maximum at the intensity of the magnetic field corresponding the boundary between an intermediate mixed state and mixed state for the given sample. The geometry of the experiment let to suppose that the peak effect observed is correlated with the flux line lattice deformation.     

Low temperature magnetocaloric effects in Dy1Ba2Cu3O7−δ and Y1Ba2Cu3O7−δ high-Tc single crystals

The heat release has been measured in high temperature superconductors during and after a change of the external magnetic field μ₀H_a ≤ 0.5 T applied parallel to the c-axis at 0.1 K ≤ T ≤ 0.45 K. Two Y123 and one Dy123 twinned single crystals were investigated. An analysis of the heat release based on the Bean model and on a model for thin superconductors in perpendicular geometry recently proposed by Brandt et al. has been carried out. During field sweep we measured ∝ H_a²dH_a/dt. This dependence can be understood within the Bean model for longitudinal geometry. The critical current densities obtained with this method are in good agreement with those from magnetization measurements. We also present measurements, and a first quantitative analysis, of dissipation due to the time relaxation of the flux line lattice after a field sweep. For the heat release a t⁻¹ dependence has been found which corresponds to a logarithmic time dependence of the magnetization. The normalized relaxation rate we obtained is in agreement with literature values.     

Dynamics of a magnetic flux jump in a composite superconductor

An attempt is made at direct experimental verification of the theory of thermomagnetic instability in composite superconductors under conditions of external magnetic field or transport current variations. The development of thermomagnetic instability in the form of a magnetic flux jump is experimentally studied in a bulk low-temperature composite niobium-tin superconductor. The liquid-helium-cooled sample representing a compressed tape helix (helicoid) is placed in an external magnetic field orthogonal to the turn plane and varying with a constant rate. For the first time, both the magnetic induction inside the sample and its temperature are simultaneously detected in experiments. The sample overheat preceding the magnetic flux jump is measured to be 0.23 + 0.02 K. This value is found to be independent of the rate of the external magnetic field variation and the value of the jump itself and coincides, within the experimental accuracy, with the temperature parameter of electric field buildup involved in the general exponential I-V characteristic of the composite superconductor, which depends on temperature and magnetic induction.     

样品电流模式下外磁场引起的X射线吸收谱强度变化

在用样品电流模式的测量过程中发现，磁场强度、磁场与样品表面的夹角以及光斑在样品表面的位置都会对吸收谱强度产生影响；在光斑、磁场和样品的不同几何配置下,测量并分析了表面均匀氧化的铝箔中氧的K边吸收谱，指出外磁场下吸收谱强度随各种条件变化的趋势，并对实验结果给出了合理解释；结果表明所用的模型分析与实验数据符合得很好；所得到的信息对于XMCD实验的设计安排、相应数据的分析以及物理信息的提取具有重要意义.     

Critical supercurrents and self-organization in quantum Hall bilayers

We present a theory of interlayer tunneling in a disordered quantum Hall bilayer at total filling factor one, allowing for the effect of static vortices. In agreement with recent experiments [Phys. Rev. B 80, 165120 (2009); Phys. Rev. B 78, 075302 (2008)], we find that the critical current is proportional to the sample area and is comparable in magnitude to observed values. This reflects the formation of a Bean critical state as a result of current injection at the boundary. We predict a crossover to a critical current proportional to the square-root of the area in smaller samples. We also predict a peak in the critical current as the electron density varies at fixed layer separation.     

Magnetic field analysis in a diamond anvil cell for Meissner effect measurement by using the diamond NV~- center

Diamond negatively charged nitrogen-vacancy(NV-) centers provide an opportunity for the measurement of the Meissner effect on extremely small samples in a diamond anvil cell(DAC) due to their high sensitivity in detecting the tiny change of magnetic field. We report on the variation of magnetic field distribution in a DAC as a sample transforms from normal to superconducting state by using finite element analysis. The results show that the magnetic flux density has the largest change on the sidewall of the sample, where NV-centers can detect the strongest signal variation of the magnetic field. In addition, we study the effect of magnetic coil placement on the magnetic field variation. It is found that the optimal position for the coil to generate the greatest change in magnetic field strength is at the place as close to the sample as possible.     

Scanning ESR-microprobe studies of the flux distribution in type-II superconductors

A new method for the study of the distribution of the magnetic field in a superconducting sample is developed. The field profile in the interior of the sample is mapped from the data on the spin probe ESR-signal shift measured at different points over the plane surface. The main attention is focused to the penetration of a perpendicular magnetic field into single crystal strips. The penetration of magnetic flux into an YBCO crystal is shown to be delayed and this gives rise to formation of the steep fronts in the profile of the field near the center of the strip. The profile observed differs considerably from the well-known Bean picture. It is highly dependent on the magnetic history. For the case of a Bi:2212 rectangular crystal, the crossover of the magnetic field profile from a dome shape, typical for a geometrical barrier (T > 30 K), to a bulk pinning picture (T < 25 K) was observed upon variation of temperature. The analysis of the field distribution made it possible to determine the temperature dependence of the lower critical field.     

Time‐decay of magnetisation in high-Tc superconductors

An extension of Bean's model, incorporating the observed exponential decay, with increasing field, of critical current density in the high-T_c superconductors, is developed. It is used for explaining the various features of the isothermal magnetisation curves. Using this model, a calculation is presented for the time-decay of magnetisation after the sample is exposed to a magnetic field, and also after this applied field is switched off. The field dependence of the decay rate in these two conditions is calculated, and then compared with available results on the 90 K and the 40 K class of oxide superconductors. The results, in the limit of field-independent current density, also agree qualitatively with experimental results for the heavy fermion superconductor CeCu₂Si₂.     

Pinning of planar vortices and magnetic field penetration in a three-dimensional Josephson medium

The behavior of planar (laminar) vortices in a three-dimensional, ordered Josephson medium as a function of the parameter I, which is proportional to the critical junction current and the cell size, is investigated with allowance for pinning due to the cellular structure of the medium. The minimum possible distances between two isolated vortices are calculated. A system of vortices formed in a sample in a monotonically increasing external magnetic field is analyzed. The minimum distance from the outermost vortex to the nearest neighbor is proportional to I ^−1.1. For I⩽1.3 each vortex contains a single flux quantum Φ₀, and the distance between them does not decrease in closer proximity to the boundary but remains approximately constant, implying that the magnetic field does not depend on the coordinate in the region penetrated by vortices. These facts contradict the generally accepted Bean model. The sample magnetization curve has a form typical of type II superconductors. Allowance for pinning raises the critical field H _c and induces a sudden jump in the curve at H=H _c. Zh. Tekh. Fiz. 67, 38–46 (September 1997)     

Exact time-dependent nonlinear dispersive wave solutions in compressible magnetized plasmas exhibiting collapse

Compressional waves in a magnetized plasma of arbitrary resistivity are treated with the lagrangian fluid approach. An exact nonlinear solution with a nontrivial space and time dependence is obtained with boundary conditions as in Harris' current sheet. The solution shows competition among hydrodynamic convection, magnetic field diffusion, and dispersion. This results in a collapse of density and the magnetic field in the absence of dispersion. The dispersion effects arrest the collapse of density but not of the magnetic field. A possible application is in the early stage of magnetic star formation.     

霍尔推力器磁场位形及其优化的数值研究

基于麦克斯韦方程,在轴对称假设下建立了霍尔推力器磁场的数学模型.用有限差分方法对模型进行了离散.给出了数值求解模型的迭代法.通过对模型的数值求解,得到了相应的数值结果.通过对所得数值结果的分析,研究了磁场线圈电流变化对霍尔推力器磁场位形的影响.通过调整磁场线圈电流的大小找到了理想磁场位形.研究表明,对于理想磁场位形,内通道的磁镜比在3—3.5之间,外通道的磁镜比在0.4—0.9之间;增加磁场线圈的电流,出口的磁场强度随着增加,但不能增加磁镜比.通道内部的磁场强度几乎不随着磁场线圈电流的变化而变化. 关键词： 霍尔推力器 磁场位形 磁场线圈电流 磁镜比     

Spatial distribution of magnetic field and persistent currents in Condon domain phase

The distributions of magnetic field above the surface of the sample due to existence of the diamagnetic domain structure are found. It is shown, that the constant magnetic induction splitting inside of a sample is caused by the magnetization current density, localised in the boundaries between adjacent domains, close to the sample surface. The properties of this current are studied. The influence of the domain wall thickness on the spatial distribution of magnetic field and magnetization current density is present. A possibility of detection of the changes in the magnetic field distribution in vacuum, close to the surface of the sample, by means of Hall probes, is discussed. The measurement of the spatial distribution of magnetic field can give lacking information about characteristic sizes of magnetic domain formation at the conditions of the strong de Haas van Alphen effect.