The Influence of Pinning Centers in the Magnetization of the Mesoscopic Superconductors

In this work, we study the mesoscopic superconducting samples with pinning centers using the time-dependent Ginzburg-Landau theory (TDGL). The pinning centers are introduced via the phenomenological function f(r) Sorensen et al. (Physica C 533, 40–43 2017). We calculated the magnetization curves M(H) for some distances d from the boundary of the sample to the position of the pinning center. From this investigation arises the relation between the first magnetic field H _p and the distance d. It shows that the pinning centers located close to the boundary of the sample decrease H _p , and also the existence of two regimes of the penetration of the vortices. The magnetization curves revel the existence of ruddle of jumps for low magnetic fields for small distances d, indicating a complex vortex penetration.     

Secondary Peak on Asymmetric Magnetization Loop of Type-II Superconductors

Asymmetric magnetization loops with a second peak effect were parameterized by the extended critical-state model. The magnetic field distribution in a sample is considered. An expression is suggested for a peak of the critical current density and corresponding depression on field dependence of the depth of surface layer with equilibrium magnetization. These functions determine the width and the asymmetry of a magnetization loop. The asymmetry of the secondary peak height on magnetization branches for increasing and decreasing field is reproduced on the computed magnetization curves.     

Dissipation in Mesoscopic Superconductors with Ac Magnetic Fields

The response of mesoscopic superconductors to an ac magnetic field is investigated both experimentally and with numerical simulations. We study small square samples with dimensions of the order of the penetration depth. We obtain the ac susceptibitity χ=χ′+iχ″ at microwave frequencies as a function of the dc magnetic field H _dc. We find that the dissipation, given by x″, has a non monotonous behavior in mesoscopic samples. In the numerical simulations we obtain that the dissipation increases before the penetration of vortices and then it decreases abruptly after vortices have entered, the sample. This is verified experimentally, where we find that χ″ has strong oscillations as a function of H _dc in small squares of Pb.     

Vortex Interaction with Mesoscopic Surface Cavities in Superconductors

The conformal mapping method is used to study the problem of flux line interaction with surface cavities having cylindrical profile and characteristic size , i.e., within mesoscopic scale, where λ is the penetration length. It is shown that the metastable states are achieved when the dimensions of the surface irregularities do not exceed the coherence length ξ. Our study shows that the surface barrier may vanish at some weak point at which the surface irregularities have mesoscopic scales. On the other hand, a remarkable decrease in the surface barrier occurs when the surface defects size . Our results are compared with the available experimental data and theoretical results.     

Magnetization Anisotropy in YBCO Superconductors Prepared MPMG Method

We have studied magnetization anisotropy in a YBa₂Cu₃O₇ sample synthesized by a modified melt powder melt growth (MPMG) method. Magnetization of the sample upon ZFC process was measured as a function of magnetic field oriented both parallel and perpendicular to the c-axis. Magnetization anisotropy parameter γ is determined from remanent magnetization as a function of temperature. Magnetic relaxation measurement has been carried out in selected magnetic fields in both field increasing and decreasing case. At 20 kOe and 25 K for the field increasing case, the activation energy was found to be 53.5 meV and 51.7 meV when H∥c and H⊥c, respectively. Calculations based on the critical-state model can reproduce quite well the experimental data for H∥c and H⊥c where we have chosen n=0.15 and n=0.3, respectively.     

Scaling of the Resistance and Magnetization for High Temperature Superconductors under Pressure

The scaling relation of single parameter scaling hypothesis is applied to the study of the scaling behavior of high temperature superconductors under pressure. The data of resistance and magnetization under various pressures are scaled onto a universal curve according to this scaling relation. The scaling parameters are pressure dependent while temperature independent. It is found that the controlling parameter B _i equals the relative critical temperature t _cP , which indicates that the superconducting energy gap at the zero temperature 2Δ _s0 is the controlling parameter in this scaling.      

Scaling of the Equilibrium Magnetization in the Mixed State of Type-II Superconductors

No Heading We discuss the analysis of mixed-state magnetization data of type-II superconductors using a recently developed scaling procedure. It is based on the fact that, if the Ginzburg-Landau parameter does not depend on temperature, the magnetic susceptibility (H, T) is a universal function of H/H_c2(T), leading to a simple relation between magnetizations at different temperatures. Although this scaling procedure does not provide absolute values of the upper critical field H_c2(T), its temperature variation can be established rather accurately. This provides an opportunity to validate theoretical models that are usually employed for the evaluation of H_c2(T) from equilibrium magnetization data. In the second part of the paper we apply this scaling procedure for a discussion of the notorious first order phase transition in the mixed state of high-Tc superconductors. Our analysis, based on experimental magnetization data available in the literature, shows that the shift of the magnetization accross the transition may adopt either sign, depending on the particular chosen sample. We argue that this observation is inconsistent with the interpretation that this transition always represents the melting transition of the vortex lattice.PACS numbers: 74.60.-w, 74.60.Ec, 74.60.Ge, 74.72.-h     

The Effectiveness of Pulsed-Field Magnetization with Respect to Different Performance Bulk Superconductors

Pulsed-field magnetization (PFM) is one of the practical activation techniques for bulk applications, especially as an inexpensive, small-volume, and mobile experimental setup. To utilize the method, its effectiveness to excite the bulk superconductors was examined by four samples with different trapped-flux performance as representatives. The PFM experiments were conducted with a pair of vortex-type copper coils at liquid nitrogen temperature (77 K). The utmost trapped-flux performance of the four samples was obtained by a multi-PFM method with progressively increased applied fields. By comparison with their trapped-flux results of static field-cooling magnetization (FCM), the effectiveness of PFM according to different performance bulk superconductors was analyzed. It was found that with the improvement of the bulk performance, the PFM results start to deviate from the FCM ones more and more, indicating a harder tendency to excite the bulk samples by the PFM method. The possible explanations from heat generation during the pulse and some suggestions to the improvement of the PFM process for practical applications are discussed.     

Magnetization Study of Hg and Tl based High Temperature Superconductors in Mixed State

A characteristic study of the temperature and field dependence magnetization of high temperature superconductors (HTSCs) HgBa₂CuO₄ and Tl₂Ba₂Ca₂Cu₃O₁₀ have been carried out in this present work. The field dependence of dM/dlnB has been discussed and compared using few existing models. The findings of variation of magnetization with temperature for different applied fields are found to give correct behavior as observed in experiments.     

Observation of the Curved and Entangled Vortex Tubes in 3D Mesoscopic Cubic Superconductors

The time-dependent Ginzburg-Landau equations have been solved numerically by a finite element analysis for the superconducting samples with a cubic shape in a uniform external magnetic field. We obtain the different vortex patterns as a function of the magnetic field perpendicular to its surface. The vortex tubes must reach the surface perpendicularly in order to avoid a supercurrent component pointing outwards the surface. At the same time, we observed the arrangement of spiral vortices in the cubic superconductor. These results show that our approach is an effective and useful to interpret experimental data on vortex states in the mesoscopic superconductors.     

Relaxation Properties of the Trapped Flux of Bulk High-Temperature Superconductors at Different Magnetization Levels

The magnetic relaxation phenomenon is a crucial subject for the engineering applications of bulk high-temperature superconductors (HTS) in which the trapped field or levitation force behaves with a time-dependent decay due to the intrinsic flux creep inside the HTS materials. To fully exploit the high trapped field of bulk HTS, we have experimentally investigated the trapped flux relaxation properties, especially at different magnetization levels. With different excitation fields, the dependence between trapped field and relaxation rate was analyzed and compared in both field-cooling magnetization (FCM) and zero-field-cooling magnetization (ZFCM) conditions. In parallel, the relaxation rates for different trapped flux can be measured all at once during a single magnetization process. The relaxation rate is closely correlated to the trapped field and useful to reflect the effect of the following remagnetization processes. To suppress the relaxation, we further checked possible methods from the three aspects of material improvement, working temperature and flux annealing effect.     

The Particle Size Effect on the Irreversible Magnetization and Critical Current Density in Low Fields for Polycrystalline SmFeAsO0.91F0.09 Superconductors

Journal of Superconductivity and Novel Magnetism - The particle size dependence of the isothermal magnetization loops, critical current density, and the irreversibility field were determined for a...     

Vortex States and Magnetization Properties in Mesoscopic Superconducting Ring Structures: A Finite-Element Analysis

The time-dependent Ginzburg–Landau equations have been solved numerically by a finite-element analysis for mesoscopic superconducting ring structures with different inner radii. For given applied magnetic fields, we have studied the influences of the inner radius on the vortex states and the magnetization properties of these systems. Our results show that the multivortex states can be stabilized in the mesoscopic superconducting ring with proper inner radius. Magnetization curves show that the magnetic vortices penetrate easily into the superconductor, and the system is magnetized easily for the superconducting ring with smaller inner radius.     

Andreev Reflection and Granular Superconductivity Features Observed in Mesoscopic Samples Using Amorphous Tungsten Carbide Superconductors

Point-contact Andreev reflection and magnetoresistance measurements were done using amorphous tungsten carbide (WC _x ) superconductors. Superconducting tips as well as microwires were grown directly under a focused Ga⁺-ion beam (FIB) on pre-patterned samples. Using e-beam lithography, the electrical contacts were prepared later using a special geometry. Current?Cvoltage measurements as a function of temperature and magnetic field clearly showed the signatures of Andreev reflection. We observed anomalies in the differential conductivity at voltages above the energy gap values. These anomalies can be well understood as due to a weak-link formation with narrow band properties contributing in parallel at the interfaces of the contacts. We also observed Andreev oscillations as a function of magnetic field similar to those found recently in specially prepared normal?Csuperconducting?Cnormal nanostructures and multigraphene samples. In some of the structures, we were able to produce interfaces in which clear granular behavior in a certain temperature region was observed, such as, for example, an anomalous field hysteresis loop compatible with the existence of granular superconductivity, similar to that found also in thin mesoscopic graphite samples.     

Pressure Effect in High-Temperature Superconductors

The model of the pressure effect on the critical temperature T _c in high-temperature superconductors [11], earlier proposed by the present author and consisting in the increase of the pair wave function overlap with increase in pressure p that in its manifestations is equivalent to an increase in doping, is compared with the recent experimental data [1–4]. By making this model more precise for inhomogeneous specimens, we obtained the doping region near the optimal one where the critical temperature is independent of doping, and the pressure region where it is independent of pressure. It is shown that, as observed in [1,2], the dependence of dT _c/dp on doping in HgBa₂CuO₄₊ after excluding the influence of the specimen's inhomogeneous structure is explained satisfactorily by the model of [11]. So the introduction in [1,2] of a term in T _c proportional to p ² is unnecessary. An analogous comparison is made for YBa₂Cu₃O_6+y .     

Dynamics of Driven Vortices in the Presence of a Regular Array of Columnar Defects

The effects of pinning by a square lattice of columnar defects (CD) on the low-temperature dynamics of vortex lines driven by a uniform force, F _d , oriented in the (1,0) CD-lattice direction, are investigated by Langevin dynamics simulations of a London model for straight vortex lines interacting with CD of radius comparable to the vortex-core radius. Starting the simulations with the vortex lines in the zero-drive ground-state, it is found that for F _d Fp (F _p = single CD pinning force) reordering into a new state occurs. The spatial order in this state is identified with that of the ground-state of stationary vortex lines interacting with the potential obtained by averaging the vortex-line CD-lattice interaction in the direction of motion. The dynamical phase diagram is investigated by starting the simulations from this ground-state and with F _d F _p , and progressively decreasing F _d . Evidence is reported for the existence of two dynamical phases: high-drive elastic flow and low-drive plastic flow. These are characterized by studying the behavior of the vortex lines density-density correlation function and velocity distribution as a function of F _d .