Collective effects in a system of Pearl vortices and the magnetization of a thin superconducting film

The magnetization curves of a thin superconducting monolayer are calculated. It is found that for sufficiently high fields and temperatures the magnetization of a monolayer exhibits the same features as the reversible magnetization of layered superconductors in high fields. It is shown that these features are due to instability with respect to the dissociation of pairs of Pearl vortices into a gas of free vortices and to collective effects in a system of free vortices. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 6, 460–465 (25 September 1998)     

Multi-vortex versus interstitial vortices scenario in superconducting antidot arrays

In superconducting thin films, engineered lattice of antidots (holes) act as an array of columnar pinning sites for the vortices and thus lead to vortex matching phenomena at commensurate fields guided by the lattice spacing. The strength and nature of vortex pinning is determined by the geometrical characteristics of the antidot lattice (such as the lattice spacing a₀, antidot diameter d, lattice symmetry, and orientation) along with the characteristic length scales of the superconducting thin films, viz., the coherence length (ξ) and the penetration depth (λ). There are at least two competing scenarios: (i) multiple vortices sit on each of the antidots at a higher matching period and (ii) there is nucleation of vortices at the interstitial sites at higher matching periods. Furthermore, it is also possible for the nucleated interstitial vortices to reorder under suitable conditions. We present our experimental results on NbN antidot arrays in the light of the above scenarios.     

Structure of the Abrikosov vortex lattice in a thin superconducting film in a parallel magnetic field

The structure of a vortex lattice in thin (d<λ, where d is the film thickness and λ is the London penetration depth) superconducting films is investigated in a magnetic field parallel to the film surface. It is shown that the stable configuration has the form of discrete vortex rows whose number changes discretely with an increase in the applied magnetic field. The entry fields H _c1 ^(N) (d) for vortex rows are calculated for N=1, 2. It is shown that the structural transition in the vortex ensemble is a second-order phase transition. A simpler method (as compared to the Monte Carlo technique) is proposed for calculating the vortex lattice parameters.     

Nonlocal Josephson electrodynamics of electromagnetic excitations in a superconducting magnetic (three-dimensional) thin film

The integro-differential equation for the dynamics of the phase difference with temporal and spatial nonlocality for a Josephson junction in a superconducting three-dimensional magnetic thin film is derived. The equation is valid for any type of magnetic ordering. The magnetic subsystem is shown to significantly renormalize the spectrum of small-amplitude electromagnetic excitations, which, as a result, are damped.     

超导邻近效应在正常金属层中引起的反常小能隙现象

将散射矩阵方法推广到超导双结系统，对正常金属/正常金属/超导结的隧道电导进行了研究.中间正常金属层中的电子和其Andreev反射空穴之间的相位相干性导致了隧道电导出现反常小能隙现象.综合考虑相干和顺序遂穿，计算结果理想的解释了最近Gupta等人在实验中观测到的反常小能隙现象.     

One-and two-front states of equilibrium in a thin superconducting film

Two-phase states of equilibrium of a thin superconducting film carrying a current under conditions of convective heat exchange at the free surface of the film are studied. It is shown that for a classical superconductor the two-phase state of the film remains a single-front state over a wide range of parameters of the system. For high-T _c superconductors there exists a maximum value of the Steckl number above which weakly nonequilibrium stationary states can only be multifront states. The solutions of the boundary-value problem modeling a two-front state of equilibrium are investigated, and the conditions under which they obtain are examined. Zh. Tekh. Fiz. 68, 84–87 (March 1998)     

Lower critical field and vortices configuration in 2D superconducting junction arrays

Summary In order to describe properly the magnetic status of a 2D superconducting junction array, one has to consider not only the effect of the screening currents but also that of the particular experimental protocol followed to measure the physical quantities of interest like, for example, the magnetization. We show that the value of the lower critical field,f _c1, of the junction array depends strongly on the intensity of the screening currents,i.e. on the strength of the junction coupling,E _j, and that reliable results can be obtained only by considering the full-inductance matrix. We also show that the magnetic configuration of the vortices may depend on the particular experimental approach followed (static or dynamic) and, even, in some cases, on the initial configuration of the phases. Paper presented at the ?VII Congresso SATT?, Torino, 4–7 October 1994.     

Thin film arrays of weakly coupled superconducting particles

Thin film, macroscopic arrays of superconducting particles 100–10,000 Å in diameter can be produced by evaporating the superconductor in a gas at low pressure. The arrays have a critical supercurrent, I_c, which is a sensitive, multiply periodic function of the applied magnetic field. A simple model of multiple current paths through the array is proposed to explain both this observation and the general shape of the current-voltage characteristics.     

Laser induced fractal structure on magnetic dielectric thin film

The Richardson plots method is employed to measure the fractal dimensions D of the surface of magnetic dielectric film fractured by excimer laser irradiation near the ablation threshold. It is shown that the fractured surfaces are fractal character. The value of D decreases while the laser pulse number increases. This relation may reflect how the fractured surface changes from irregular structure to regular structure with laser pulse number.     

Surface induced mixed state of a superconducting film

Presented in this paper is a theoretical analysis of a planar surface induced mixed state for a superconducting film in parallel applied field. An analytical solution of the internal magnetic field is obtained based on Saint-James and de Gennes' order parameter in a film. An expression of Gibbs free energy per unit volume without restriction of a geometry is derived from non-linear Ginzburg-Landau (GL) equation in terms of a renormalized GL parameter and a modified geometric factor. Based on the Gibbs free energy, a phase diagram of distinguishing a first and second order phase transition for a type I superconducting film is calculated. The numerical results for exact solutions of spatial variation of order parameter, current density and internal magnetic field in the film geometry in parallel applied field case are presented. Near the upper critical field, the first entry of an applied field in the film exhibits a laminar structure.     

Kosterlitz-Thouless transition and radiation defects in a superconducting thin film

The thermodynamics of a system of Pearl vortices in a superconducting thin film containing radiation defects is studied. It is shown that three phase transition scenarios are possible, depending on the defect density. At low densities there is one stable state of the system. When the first critical density n _d 1 is reached, there appears a temperature interval in which the system can be in two stable states. If the density exceeds n _d 2, then the lower limit of stability of the metastable states shifts abruptly to zero. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 9, 643–648 (10 May 1999)     

Interstitial vortex in superconducting film with periodic hole arrays

The response of superconducting Nb films with a diluted triangular and square array of holes to a perpendicular magnetic field are investigated.Due to small edge-to-edge separation of the holes,the patterned films are similar to multi-connected superconducting islands.Two regions in the magnetoresistance R(H) curves can be identified according to the field intervals of the resistance minima.Moreover,in between these two regions,variation of the minima spacing was observed.Our results provide strong evidence of the coexistence of interstitial vortices in the islands and fluxoids in the holes.     

Thermally induced magnetic flux in a superconducting ring

The thermally induced magnetic flux in a bi-metallic superconducting loop of Nb and Ta has been measured and compared with theory. The predicted logarithmic divergence of the flux near the T_c of Ta is found, however the magnitude of the flux is larger than predicted.     

Reversible magnetization of a thin superconducting film with radiation defects

The thermodynamic potential of a system of Peierls vortices in a thin superconducting film, containing radiation defects, in a perpendicular external magnetic field is calculated. The equilibrium temperature dependences of the densities of free vortices and vortices trapped by defects are found in the mean-field approximation for various magnitudes of the external field. It is shown that the equilibrium magnetization of a thin superconducting film exhibits the same features that were observed experimentally in the reversible magnetization of high-temperature superconductors. An asymptotic expression is obtained for the difference of the magnetizations of perfect and irradiated films. According to this expression, the difference depends on the pinning energy of a vortex on a defect and the density of defects.