高温超导体磁通钉扎和磁通动力学研究简介

ψ=ψ0eiφ行为用统一的波函数进行描述,其相位φ在宏观尺度上是相同的.当磁场低于一定值的时候,在超Φ0=h/2e保证最大的界面面积,降低系统能量.该最小的磁通束被称为磁通量子,其磁通量是(h为普朗克超导态是一个宏观量子相干态,其载流子是库珀对.在没有外加磁场和电流的时候,这些库珀对的运动导体的边界处穿透深度内会出现一...     

Finite length effect on supercurrents between trivial and topological superconductors

We numerically analyze the effect of finite length of the superconducting regions on the low-energy spectrum, current-phase curves, and critical currents in junctions between trivial and topological superconductors. Such junctions are assumed to arise in nanowires with strong spin-orbit coupling under external magnetic fields and proximity-induced superconductivity. We show that all these quantities exhibit a strong dependence on the length of the topological sector in the topological phase and serve as indicators of the topological phase and thus the emergence of Majorana bound states at the end of the topological superconductor.     

Critical currents of Nb3Sn and V3Ga in transverse magnetic fields up to 150000 oersteds

The quenching curves of some high-field superconductors have been measured for the first time in steady transverse magnetic fields up to 150 kOe. The quenching curves of Nb₃Sn diffusion-layers on single niobium wires show the same slope in the high-field region as they do in lower fields. On the other hand, the quenching curves of “Nb-Sn multiwires” with a synthetic filamentary structure of many interior Nb₃Sn diffusion-layers in the core of this material are nearly horizontal in the high-field region with very high critical current values. The quenching curves of V₃Ga diffusion-layers and of V₃Ga core-wires show an unexpected inversion at about 90 and 100 kOe respectively with increasing critical currents in higher fields.     

Absence of orbital currents in superconducting YBa2Cu4O8 using a zeeman-perturbed nuclear-quadrupole-resonance technique

Zeeman perturbed nuclear quadrupole resonance was applied to evaluate weak magnetic fields in the context of orbital currents in cuprate superconductors. The magnetic environment of the barium atom in c-axis oriented powder samples of YBa(2)Cu(4)O(8) was investigated in the pseudogap phase at 90 K. No evidence for orbital currents was found: any static and dynamic field must be less than 0.07 and 0.7 mT, respectively.     

Calculation of AC losses in HTSC wires with arbitrary current voltage characteristics

A method for the calculation of magnetic field dynamics and AC losses in superconductors with smooth current–voltage characteristics is described. It is based on an integral equation for the current density, recently used by Brandt for magnetic relaxation. Brandt's equation is generalized to include arbitrary external magnetic fields and transport currents. One of the benefits of the integral equation formulation is that no boundary conditions ‘at infinity' are required, thus restricting the calculation region to the conductor cross section. The method is applied to superconducting tapes in oblique external fields. A further extension of the theory is shown to be applicable to the calculation of coupling losses in twisted multifilamentary superconductors.     

On the magnetic properties of conjugated molecules

Recent criticisms of the gauge factors usually employed in the ‘test-dipole’ method for the calculation of ‘ring current’ chemical shifts in conjugated molecules, are discussed. It is shown that, in a simple semi-empirical scheme of the London-McWeeny type, insertion of a dipole contribution into the vector potential appearing in the gauge factor, whilst having no effect on the calculated ‘ring current’ intensities, is algebraically analogous (and, at large distances from ring centres, numerically equivalent) to estimating the secondary field at the origin due to a set of classical line currents, as discussed originally by Salem; these ‘line currents’ are of the same magnitude as the quantum-mechanical ‘bond currents’ implicit in the ‘ring currents’ calculated using the simpler gauge factors originally due to London, but their contributions to the secondary magnetic field experienced by the peripheral protons, are estimated classically. Extensive numerical comparison is made between experimentally-observed proton chemical shifts in some conjugated hydrocarbons, and secondary fields estimated by this semi-classical formalism, and its predictions are found to correlate as well with experiment as do those of the original McWeeny approach. It is concluded that any further illegitimacy involved in the procedure of inserting a dipole contribution into the gauge factor, is evidently quite simply compensated for, numerically, by an appropriate empirical parametrization. Such empirical parametrizations are also thought to absorb errors due to all the other various approximations of the ‘ring current’ theories (with apparently unwarranted efficiency), and they should, therefore, be treated with more scepticism than has previously been thought necessary.     

A quantum of history

The critical temperature, T _c, for all presently known superconductors does not exceed 20°K. This fact obviously limits the range of applications of superconductivity in technology in a very fundamental way. On the whole, the reason why the value of T _c for ‘ordinary’ superconductors should not exceed 20–40 °K is fairly well understood on the basis of the existing theory of superconductivity. At the same time, there apparently could exist high temperature superconductors for which the temperature T _c would reach hundreds of degrees, or at least liquid air temperature. Possible means of producing high temperature superconductors are considered in this article. Special attention is paid to what can be called the exciton mechanism of superconductivity.     

Remarks on Small Superconducting Specimens in Magnetic Field and Measurements of the Lower Critical Field

We shall discuss the magnetic behaviors associated with the boundary effects for type-Ⅱ superconductors in general orientations. Using previous results, we speculate that the relation among the lower critical field H_c1, the thermodynamic critical field H_c and the upper critical field H_c2, namely H_c²～H_c1H_c2, is still approximately valid for general experimental orientations. More precisely, the magnitudes of H_c1, H_c and H_c2 are consistent with the formulae given for bulk superconductors when the typical size of the superconducting specimen is greater than the coherence length ε, provided the penetration depth λ is substituted by a modified value λ_m. Consequently, the criterion for observing type-Ⅱ superconducting magnetic behaviors also becomes λ_m>ε, although the London equation is still governed by λ. In the second part of the paper, we shall also suggest an experiment, based on the fact of H_c1. of superconducting thin films in parallel magnetic field being much higher than that of bulk superconductors, to measure H_c1 precisely, especially for extreme type-Ⅱ superconductors,without influence of the penetration barrier.     

The Hall effect and oscillating decay of a magnetic field

The decay of a strong magnetic field in a conducting matter is considered. It is shown that nondissipative Hall currents can considerably change the behavior of the field when it decays. The nonlinear character of the Hall effect leads to the generation of fields of high multipolarity even for most simple initial magnetic configurations. In particular, the evolution of an initially dipole configuration may give rise not only to quadrupole or higher poloidal harmonics but also to a toroidal field that is other than zero only inside the conductor. The nonlinear Hall currents relate different harmonics to each other and, in a sufficiently strong field, may provide efficient energy exchange between them. Due to this redistribution of the magnetic energy, the evolution of different harmonics has an oscillating character. The oscillation period is determined by the characteristic time of Hall drift and may be fairly short in strong magnetic fields.     

Imaging with polarized neutrons

While polarized neutrons have long proved to be an outstanding tool for the investigation of magnetic structures by scattering, we report on their potential for real space investigations of magnetic fields on a macroscopic scale by neutron imaging. Due to the ability of neutrons to penetrate thick layers of matter and their high sensitivity to magnetic fields owed to their magnetic moment, neutron imaging enables the investigation of magnetic fields even in bulk samples of condensed matter. We demonstrate how neutrons provide images of magnetic fields trapped in or expelled by superconductors or even reveal the path of electric currents due to the corresponding magnetic fields.     

Measurement of the magnetic field of resin-impregnated bulk superconductor annuli

Large single-grain bulk RE–Ba–Cu–O (RE: rare earth elements) superconductors can trap large fields exceeding several teslas and thus can function as very strong quasi-permanent magnets. We have found that the resin can penetrate into a bulk superconductor, when the sample was immersed in molten resin. Hence, resin impregnation was effective in improving mechanical properties. Three bulk superconductors‘ annuli with resin impregnation, each 50-mm i.d. and 80-mm o.d. was built and energized, by a field-cool method, to generate, in a bath of liquid nitrogen, a persistent trapped field of 1.62 T.     

High magnetic field behavior of strongly correlated uranium-based compounds

Magnetic fields are now available to 100?T (pulsed), 45?T (static) at temperatures below 0.3?K. Such technical developments allow the study and tuning of (quantum) phase transitions, unusual magnetic structures and (high-temperature) superconductors in a variety of quantum materials. An especially important class of strongly correlated electron materials is the heavy Fermi liquids (HFLs) displaying numerous reduced-moment antiferromagnets, quantum critical points, unconventional superconductivity, hidden order (HO) and other mysterious ground states. Among the ‘heavy fermions’, the duality of 5f electrons in uranium-based compounds introduces interesting behavior that can be affected by large magnetic fields. I list a few such heavy fermion materials to be considered: URu₂Si₂ and its tunable hidden state, UBe₁₃ and UPt₃ as very HFL paramagnets that become superconducting, the magnetic superconductors UPd₂Al₃ and UNi₂Al₃, and the ferromagnetic s UGe₂, URhGe and UCoGe. There are also the suggested metamagnetic Fermi-surface reconstructed intermetallic compounds such as UPt₂Si₂ and UCo₂Si₂. Present research attention focuses on the high-field behavior (30–40?T) of URu₂Si₂ and its destruction of HO. These and other U-based systems, e.g. UAu₂Si₂, UIrGe, etc., expand the opportunities of high magnetic field studies far into the future.     

Domain boundary engineering

We review the idea that domain boundaries, rather than domains, can carry information and act as memory devices. Domains are bulk objects; their large response to changing external fields is related to their change in volume, which implies the movement of domain boundaries. In many cases, the design of ‘optimal’ domain structures corresponds to ‘optimal’ domain boundaries with parameters such as the domain boundary mobility, pinning properties and the formation of specific boundaries such as curved boundaries or needle domains. This argument is enhanced further in this review: domain boundaries themselves can host properties which are absent in the bulk, they can be multiferroic, super- or semi-conductors while the matrix shows none of these properties. It is agued that multiferroic walls can be described formally as chiral whereby the chirality relates to state-vectors such as polarisation and magnetic moment and their (non-linear) coupling. Once such walls can be generated reliably, a new generation of devices with much higher storage density than ever produced before can be envisaged.     

高速飞行器磁控阻力特性

采用低磁雷诺数磁流体数学模型,对外加磁场下的高超声速半球体流场进行数值模拟.选取三种简单理想磁场(轴向、径向、周向均布磁场),分析了不同磁场类型对流场结构、气动阻力与洛伦兹阻力的影响及作用机理.研究发现,轴向磁场径向"挤压"效应使得激波外形凸出,且壁面静压存在"饱和现象";径向磁场存在轴向"外推"效应,较大的磁场强度会导致肩部形成高温区;周向磁场下感应电场的存在导致增阻效果很差.进而对比了两种相同驻点磁感应强度特殊分布磁场(偶极子磁场、螺线管磁场)下的流场,发现了不同于理想磁场的径向"扩张"效应.按增阻效果从大到小依次为径向磁场、螺线管磁场、轴向磁场、偶极子磁场、周向磁场.     

Nuclear magnetic resonance studies of vortices in high temperature superconductors

The distinct distribution of local magnetic fields due to superconducting vortices can be detected with nuclear magnetic resonance (NMR) and used to investigate vortices and related physical properties of extreme type II superconductivity. This review summarizes work on high temperature superconductors (HTS) including cuprates and pnictide materials. Recent experimental results are presented which reveal the nature of vortex matter and novel electronic states. For example, the NMR spectrum has been found to provide a sharp indication of the vortex melting transition. In the vortex solid a frequency dependent spin–lattice relaxation has been reported in cuprates, including YBa₂Cu₃O_7-x, Bi₂SrCa₂Cu₂O_8+δ, and Tl₂Ba₂CuO_6+δ. These results have initiated a new spectroscopy via Doppler shifted nodal quasiparticles for the investigation of vortices. At very high magnetic fields this approach is a promising method for the study of vortex core excitations. These measurements have been used to quantify an induced spin density wave near the vortex cores in Bi₂SrCa₂Cu₂O_8+δ. Although the cuprates have a different superconducting order parameter than the iron arsenide superconductors there are, nonetheless, some striking similarities between them regarding vortex dynamics and frequency dependent relaxation.     

Anisotropic pinning in macroscopic electrodynamics of superconductors

Anisotropy of critical currents and electric fields in superconductors with strong pinning has been ascribed in the macroscopic model to features of the material equation system relating the electric field to the current density in a superconductor. The anisotropy of the pinning proper is described by an operator relating the pinning force density to the vectors of magnetic induction and Lorentz force. In the approximation of an extended critical state model, a feasible expression of this operator is given in the form of an algorithm based on the concept of a collective anisotropic potential well containing fluxoids. The current-carrying capacity of a strongly anisotropic niobium-titanium foil as a function of the orientation of the current density and applied field with respect to the principal axes of the material has been investigated in detail. Given measurements of the transverse electric fields in the foil under magnetic fields normal to the foil plane, we can plot cross sections of surfaces describing the pinning force density in the space of magnetic induction and Lorentz force. Zh. éksp. Teor. Fiz. 112, 1055–1081 (September 1997)     

Mössbauer studies of the peculiar magnetism in parent compounds of the iron-based superconductors

A review of the magnetism in the parent compounds of the iron-based superconductors is given based on the transmission Mössbauer spectroscopy of ⁵⁷Fe and ¹⁵¹Eu. It was found that the 3d magnetism is of the itinerant character with varying admixture of the spin-polarized covalent bonds. For the ‘122’ compounds, a longitudinal spin density wave (SDW) develops. In the case of the EuFe₂As₂, a divalent europium arranges in an anti-ferromagnetical order at a much lower temperature as compared with the onset of SDW. These two magnetic systems remain almost uncoupled one to another. For the non-stoichiometric Fe_1+xTe parent of the ‘11’ family, one has a transversal SDW and magnetic order of the interstitial iron with relatively high and localized magnetic moments. These two systems are strongly coupled one to another. For the ‘grand parent’ of the iron-based superconductors FeAs, one observes two mutually orthogonal phase-related transversal SDW on the iron sites. There are two sets of such spin arrangements due to two crystallographic iron sites. The FeAs exhibits the highest covalency among the compounds studied, but it has still a metallic character.     

Calculation of electric fields induced by body and head motion in high-field MRI

In modern magnetic resonance imaging (MRI), patients are exposed to strong, nonuniform static magnetic fields outside the central imaging region, in which the movement of the body may be able to induce electric currents in tissues which could be possibly harmful. This paper presents theoretical investigations into the spatial distribution of induced electric fields and currents in the patient when moving into the MRI scanner and also for head motion at various positions in the magnet. The numerical calculations are based on an efficient, quasi-static, finite-difference scheme and an anatomically realistic, full-body, male model. 3D field profiles from an actively shielded 4T magnet system are used and the body model projected through the field profile with a range of velocities. The simulation shows that it possible to induce electric fields/currents near the level of physiological significance under some circumstances and provides insight into the spatial characteristics of the induced fields. The results are extrapolated to very high field strengths and tabulated data shows the expected induced currents and fields with both movement velocity and field strength.     

对超热电子诱生的磁场分布的估算

利用简化模型估算了电荷分离场及由超热电子逃逸在等离子体表面产生的自生磁场的大小和空间分布.受电荷分离场的影响以及超热电子逃逸数的限制,超热电子产生的环形磁场主要分布于等离子体表面附近的焦斑半径内,仅当超热电子束流很强时(在1μm半径截面内达到10³A量级),环形磁场才可以达到10²T量级.一般情况下,由超热电子产生的磁场极小. 关键词： 磁场 超热电子     

Paramagnetism and reentrant behavior in quasi-one-dimensional superconductors at high magnetic fields

The thermodynamics of quasi-one-dimensional superconductors in the presence of large magnetic fields is studied. When the quantum effects of the magnetic field are taken into account, several reentrant superconducting phases persist at very high fields. In the last reentrant phase the free-energy change, the specific-heat jump and the excess magnetization are estimated near the critical temperature. In particular, the excess magnetization is found to be paramagnetic as opposed to diamagnetic in weak fields and its sign is controlled by the slope of H_c2(T). We further generalize this result to the entire phase diagram (including all quantum phases) and to different physical systems using general thermodynamic relations which show that the sign of the excess magnetization ΔM of the superconducting state near H_c2(T) follows dH_c2(T)/dT. These relations provide a scenario for the evolution of the sign of ΔM from weak fields to strong fields.