Mesoscopic superconducting disc with short-range columnar defects (original) (raw)
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Magnetically induced splitting of a giant vortex state in a mesoscopic superconducting disk
Physical Review B, 2005
The nucleation of superconductivity in a superconducting disk with a Co/Pt magnetic triangle was studied. We demonstrate that when the applied magnetic field is parallel to the magnetization of the triangle, the giant vortex state of vorticity three splits into three individual Φ0-vortices, due to a pronounced influence of the C3 symmetry of the magnetic triangle. As a result of a strong pinning of the three vortices by the triangle, their configuration remains stable in a broad range of applied magnetic fields. For sufficiently high fields, Φ0-vortices merge and the nucleation occurs through the giant vortex state. The theoretical analysis of this novel reentrant behaviour at the phase boundary, obtained within the Ginzburg -Landau formalism, is in excellent agreement with the experimental data. PACS numbers: 74.78.Na., 75.75.+a, 74.25.Dw
Effect of an columnar defect on vortex configuration in a superconducting mesoscopic sample
Brazilian Journal of Physics, 2009
In this work we investigate the vortex dynamics in a square mesoscopic superconducting cylinder in the presence of an applied magnetic field parallel to its axis. The rectangular cross-section of the sample is L 2 and an engineered columnar defect of area d 2 at the center is taken into account; L = 12ξ(0) for all simulations while d varies discretely from 1ξ(0) to 10ξ(0). We study the magnetization and the vortex configuration, increasing the magnetic field from zero to the normal state field. We found that for d ≥ 7ξ(0) no vortices in the superconductor area are possible. Also, if the size of the defect is reduced, the nucleation fields decrease.
Physical Review B, 2007
A computational study of mixed states in mesoscopic type-II superconducting cylinders is presented. The dependence of transient behaviors and steady-state configurations on the value of the applied magnetic field is examined as are the effects of sample size and cross-sectional shape on vortex nucleation and penetration. As is well known, more vortices enter the sample as the sample size grows. It is also found that if a small indentation is made on the sample boundary, vortices can be made to enter the system one by one from the tip of the indentation. An efficient scheme to determine, for any applied field, the equilibrium vortex configuration in mesoscopic samples in a way that is not constrained by sample symmetries, is devised and demonstrated.
Anomalous behavior of superconducting samples with a fixed number of vortices
Physical Review B, 1987
We report an anomalous reversible temperature dependence of the magnetization of amorphous type-II superconductors. The experimental results were obtained studying the vortex pinning in samples with very low vortex concentration. The anomaly is shown to be a consequence of the lack of flux quantization in finite samples. Pinning forces in homogeneous amorphous superconductors have received recent attention, ' among other reasons because the high degree of structural homogeneity makes these materials appropriate to investigate the influence of disorder in ideal periodic vortex lattices. The weak collective pinning concept introduced by Larkin and Ovchinnikov to predict the behavior of superconducting vortices is also used to discuss the properties of other periodic systems. Luzuriaga recently remarked that the understanding of pinning interactions can provide new information concerning the nature and density of defects in disordered matter. In this work we are concerned with the behavior of very dilute vortices. We define the elementary interaction as that between a single vortex and defect distribution. When studying this elementary pinning interaction in Zr70CU3O we have found that vortex pinning induces an anomalous increase in the magnetic flux through the sample, when decreasing temperature at constant field.
Vortex matter in superconducting mesoscopic disks: Structure, magnetization, and phase transitions
Physical Review B
The dense vortex matter structure and associated magnetization are calculated for type-II superconducting mesoscopic disks. The magnetization exhibits generically first-order phase transitions as the number of vortices changes by one and presents two well-defined regimes: A non-monotonous evolution of the magnitude of the magnetization jumps signals the presence of a vortex glass structure which is separated by a second-order phase transition at Hc2 from a condensed state of vortices (giant vortex) where the magnitude of the jumps changes monotonously. We compare our results with Hall magnetometry measurements by Geim et al. (Nature 390, 259 (1997)) and claim that the magnetization exhibits clear traces of the presence of these vortex glass states.
Vortex matter in mesoscopic superconductors
Physica B: Condensed Matter, 1998
Superconducting mesoscopic devices in magnetic fields present novel properties which can only be accounted for by both the quantum confinement of the Cooper pairs and by the interaction between the magnetic-field-induced vortices. Sub-micrometer disks, much the same as their semiconductor counterparts known as quantum dots, are being subject to experimental investigation by measuring their conducting properties and, more recently, their magnetization by using state-of-the-art ballistic Hall magnetometry. In this work I review the main results obtained in these two types of experiments as well as the current theoretical developments which are contributing to our understanding of the superconducting condensate in these systems.
Effects of Internal Defect on the Vortex Entrance in Mesoscopic Superconductor
Modern Physics Letters B, 2013
In this paper, we report on the influence of an internal defect on the vortex entrance in a mesoscopic superconducting sample. Effects associated to the pinning force of the defect on the configuration and on the vortex entry fields are studied for a very thin disk. We calculate the supercurrent, magnetization, vorticity, free energy and Cooper pairs density for a disk in presence of external magnetic field applied perpendicular to the disk plane. Due to vortex-defect attraction (repulsion), the vortices always (never) are found to be sitting on the defect position.
Vortex phase separation in mesoscopic superconductors
Scientific Reports, 2013
We demonstrate that in mesoscopic type II superconductors with the lateral size commensurate with London penetration depth, the ground state of vortices pinned by homogeneously distributed columnar defects can form a hierarchical nested domain structure. Each domain is characterized by an average number of vortices trapped at a single pinning site within a given domain. Our study marks a radical departure from the current understanding of the ground state in disordered macroscopic systems and provides an insight into the interplay between disorder, vortex-vortex interaction, and confinement within finite system size. The observed vortex phase segregation implies the existence of the soliton solution for the vortex density in the finite superconductors and establishes a new class of nonlinear systems that exhibit the soliton phenomenon. V ortex matter in the presence of structural defects forms a wide variety of phases with specific properties depending on the relation between the vortex-vortex and vortex-defect interactions 1,2. The findings of Refs. 3, 4, which revealed significant enhancement of vortex pinning in high-temperature superconductors by ion irradiation, broke ground for a new direction in vortex physics. Heavy ions leave the tracks of the damaged amorphous material where superconductivity is suppressed. Thus the vortices penetrating the sample occupy columnar defects where the vortex energy is appreciably less than in the undamaged material. A theory of the resulting vortex Bose glass phase was developed in Refs. 5, 6, where the physics of flux lines in superconductors pinned by columnar defects was mapped onto boson localization in two dimensions. The distribution of vortices in the Bose glass state that forms in the infinite (i.e. thermodynamically large) samples, containing columnar defects, is a uniform one. A question about what happens to the Bose glass in the finite samples is most natural in view of explosively developing studies of small superconductors, i.e. superconductors with the lateral sizes R s comparable to the London screening length l or even with the coherence length j. Indeed even the samples without columnar defects reveal that the properties of the homogeneous vortex state change dramatically as R s l. The boundaries start to affect the distribution of vortices and makes it nonuniform. Experimental study of mesoscopic superconducting discs with the total vorticity L , 40 revealed formation of the concentric shells of vortices 7 in accord with the results of numerical simulations 8. The analysis of shell filling with increasing L allowed the authors of Ref. 7 to identify magic numbers corresponding to the appearance of consecutive new shells. At the same time, vortex distribution over the sample remains ''quasi-homogeneous'' with the vortex density gradually changing with the distance from the sample center. For example, the experimental and numerical studies of the samples containing a macroscopic number of vortices showed that, almost everywhere, vortices arrange themselves into a nearly perfect Abrikosov lattice, containing the few disclinations necessary to match the cylindrical symmetry of the sample. Only within a few, 2-3, shells adjacent to the surface, vortex distribution differs noticeably from that in the bulk. At the same time, theoretical consideration of the critical state in a superconducting slab containing a lattice of strong pins 9 predicted that instead of the expected in the critical state constant gradient in the vortex density a terraced piecewise vortex structure structure can form. This terraced vortex distribution, unexpected from the viewpoint of an orthodox concept of the critical state, is, formally, nothing but a standard soliton solution for the one-dimensional commensurate structures, which appeared first as a 1D model for dislocations 10,11. The physical reason for emerging such a structure is the competition between the effect of the critical current flowing uniformly through the slab and thus implying the constant gradient of the vortex density across the sample and the action of the lattice of strong pinning sites that tend to trap vortices enforcing them into a regular array with the commensurate period. As a result, a metastable structure forms, comprising vortex domains of a piecewise constant vortex density. The originally uniform current is compressed into the current filaments concentrated along the
Vortex configurations and metastability in mesoscopic superconductors
Physica C: Superconductivity, 2004
The vortex dynamics in mesoscopic superconducting cylinders with rectangular cross section under an axially applied magnetic field is investigated in the multivortex London regime. The rectangles considered range from a square up to an infinite slab. The flux distribution and total flux carried by a vortex placed in an arbitrary position of the sample is calculated analytically by assuming Clem's solution for the vortex core. The Bean-Livingston energy barrier is also analytically calculated in this framework. A Langevin algorithm simulates the flux penetration and dynamical evolution of the vortices as the external field is slowly cycled. The simulated magnetization process is governed by metastable states. The magnetization curves are hysteretic, with paramagnetic response in part of the downward branch, and present a series of peaks corresponding to the entry or expulsion of a single vortex. For elongated rectangles, the vortices arrange themselves into parallel vortex chains and an additional modulation of the magnetization, corresponding to creation or destruction of a vortex chain, comes out.
Vortices in mesoscopic superconductors
arXiv (Cornell University), 2000
We present an analysis of the magnetic response of a mesoscopic superconductor, i.e. a system of sizes comparable to the coherence length and to the London penetration depth. Our approach is based on special properties of the two dimensional Ginzburg-Landau equations, satisfied at the dual point (κ = 1 √ 2). Closed expressions for the free energy and the magnetization of the superconductor are derived. A perturbative analysis in the vicinity of the dual point allows us to take into account vortex interactions, using a new scaling result for the free energy. In order to characterize the vortex/current interactions, we study vortex configurations that are out of thermodynamical equilibrium. Our predictions agree with the results of recent experiments performed on mesoscopic aluminium disks.