Vortex quasi-crystals in mesoscopic superconducting samples

There seems to be a one to one correspondence between the phases of atomic and molecular matter(AMOM) and vortex matter(VM) in superfluids and superconductors. Crystals, liquids, and glasses have been experimentally observed in both AMOM and VM. Here, we propose a vortex quasi-crystal state which can be stabilized due to boundary and surface energy effects for samples of special shapes and sizes. For finite sized pentagonal samples, it is proposed that a phase transition between a vortex crystal and a vortex quasi-crystal occurs as a function of magnetic field and temperature as the sample size is reduced.     

Superconductivity in mesoscopic high-Tc superconducting particles

Based on the Hubbard model in the framework of non-phonon kinematical mechanism and taking into account the discreetness of an electronic energy spectrum, the superconducting critical temperature of a mesoscopic high-T_c sphere is analyzed as a function of doping and as a function of particle's radius. The critical temperature T_c is found to be an oscillating function of the radius of a particle. The size-dependent doping regime is revealed in high-T_c nanoparticles. Our analysis shows that each oscillation in T_c corresponds to the increase in a number of the energy levels in the sphere by 1. The amplitude of oscillations of T_c increases with decreasing R and can reach a value of 6 K for nanoparticles with sizes about 25 nm, in good agreement with experimental studies of YBa₂Cu₃O_7−δ nanoparticles.     

三维介观超导环的涡旋结构

在Ginzburg-Landan理论的框架下, 运用有限差分法研究了在圆环电流产生磁场下的介观超导圆环内的涡旋结构, 讨论了超导圆环尺寸和不同空间分布的磁场对涡旋形成的影响, 得到在一般超导圆环体内的基态多是巨涡旋态、而多涡旋态多以激发态形式存在的结论, 说明磁场一般从超导圆环的环孔穿过, 而很难穿过超导圆环体.     

Critical fluctuations in a mesoscopic superconducting ring

The nonlocal magnetoconductivity fluctuations in a superconducting submicron ring, with radius comparable to the Ginzburg-Landau coherence length, are studied. The order parameter mode separation yields to the solution of the time-dependent Ginzburg-Landau equation and the paraconductivity Fourier components are calculated in the vicinity of the critical temperature, including the critical fluctuation region. The homogeneous component has a logarithmic singularity at T(c) while the other components are found to be not singular.     

Metastability and paramagnetism in superconducting mesoscopic disks

A projected order parameter is used to calculate not only local minima of the Ginzburg-Landau energy functional but also saddle points or energy barriers responsible for the metastabilities observed in superconducting mesoscopic disks [A. K. Geim et al., Nature (London) 396, 144 (1998)]. We calculate the local minima magnetization and find the energetic instability points between vortex configurations with different vorticity. We also find that, for any vorticity, the supercurrent can reverse its flow direction on decreasing the magnetic field before one vortex can escape.     

Vortex properties of mesoscopic superconducting samples

In this work we investigated theoretically the vortex properties of mesoscopic samples of different geometries, submitted to an external magnetic field. We use both London and Ginzburg–Landau theories and also solve the non-linear Time Dependent Ginzburg–Landau equations to obtain vortex configurations, equilibrium states and the spatial distribution of the superconducting electron density in a mesoscopic superconducting triangle and long prisms with square cross-section. For a mesoscopic triangle with the magnetic field applied perpendicularly to sample plane the vortex configurations were obtained by using Langevin dynamics simulations. In most of the configurations the vortices sit close to the corners, presenting twofold or three-fold symmetry. A study of different meta-stable configurations with same number of vortices is also presented. Next, by taking into account de Gennes boundary conditions via the extrapolation length, b, we study the properties of a mesoscopic superconducting square surrounded by different metallic materials and in the presence of an external magnetic field applied perpendicularly to the square surface. It is determined the b  -limit for the occurrence of a single vortex in a mesoscopic square of area d²$d^2$, for 4ξ(0)?d?10ξ(0)$4\xi (0)?d?10\xi (0)$.     

Stable vortex-antivortex molecules in mesoscopic superconducting triangles

A thermodynamically stable vortex-antivortex pattern has been revealed in equilateral mesoscopic type I superconducting triangles, contrary to type II superconductors where similar patterns are unstable. The stable vortex-antivortex "molecule" appears due to the interplay between two factors: a repulsive vortex-antivortex interaction in type I superconductors and the vortex confinement in the mesoscopic triangle.     

三维介观超导环涡旋态的研究

通过数值求解非线性金兹堡-朗道(G-L)方程组,研究了三维介观超导环中的涡旋态。发现了在细环中只能存在巨涡旋态,以及存在顺磁、抗磁迈斯纳效应和间隙性超导现象。在粗环中,发现了多涡旋态和巨涡旋态共存的混合态。相应讨论有助于理解介观超导环中涡旋态相变。     

Attojoule calorimetry of mesoscopic superconducting loops

We report the first experimental evidence of nontrivial thermal behavior of the simplest mesoscopic system--a superconducting loop. By measuring the specific heat C of an array of 450,000 noninteracting aluminum loops with very high accuracy of approximately 20 fJ/K, we show that the loops go through a periodic sequence of phase transitions (with a period of an integer number of magnetic flux quanta) as the magnetic flux threading each loop is increased. The transitions are well described by the Ginzburg-Landau theory and are accompanied by discontinuities of C of only several thousands of Boltzmann constants kB.     

Spin-polarized transport in all-metal mesoscopic spin-valve devices

In spintronic devices the spin of the electron as well as its charge is utilized. We have fabricated a spin-valve device consisting of two permalloy electrodes which are connected by a normal-metal strip made of aluminum. By changing the relative alignment of the magnetizations of the electrodes the resistance of the device can be controlled. The domain configurations of the electrodes are imaged by magnetic-force microscopy in external fields at room temperature. In transport measurements at temperatures between 1.8 and 150 K we identify the observed magnetoresistance as a spin-valve effect.     

Vortex dynamics in mesoscopic superconducting square of variable surface

In this paper we apply the time dependent Ginzburg–Landau (TDGL) equations to the study of thin superconducting films of variable surfaces in the presence of an external applied magnetic field. We have made a systematic investigation of both the lower and upper critical fields as a function of the topological parameters which control the shape of the film surfaces. We also make a detailed investigation of the vortex dynamics as they nucleate into the sample.     

Type of phase transitions in a mesoscopic superconducting disc

The Ginzburg–Landau equations coupled with the three-dimensional Maxwell equations are solved for the disc geometry in order to explain recent magnetization experiments. In order to explain the experimental results on a 0.5 m radius Al disc, we have to assume that the superconducting state stays in the lowest angular momentum giant-vortex state even in regions where it is not the lowest energy state.     

Dynamics of kinematic vortices in a mesoscopic superconducting loop

Using the time-dependent Ginzburg–Landau formalism, we study the dynamic properties of a submicron superconducting loop in applied current and in presence of a perpendicular magnetic field. The resistive state of the sample is caused by the motion of kinematic vortex–antivortex pairs. Vortices and antivortices move in opposite directions to each other, perpendicularly to the applied drive, and the periodic creation and annihilation of such pairs results in periodic oscillations of the voltage across the sample. The dynamics of these kinematic pairs is strongly influenced by the applied magnetic field, which for high fields leads to the flow of just vortices. Kinematic vortices can be temporarily pinned inside the loop with observable trace in the voltage vs. time characteristics.