The electronic structure of a vortex line trapped by an insulating columnar defect in a type-II superconductor is analysed within the Bogolubov-de Gennes theory. For quasiparticle trajectories with small impact parameters defined with respect to the vortex axis the normal reflection of electrons and holes at the defect surface results in the formation of an additional subgap spectral branch. The increase in the impact parameter at this branch is accompanied by the decrease of the excitation energy. When the impact parameter exceeds the radius of the defect this branch transforms into the Caroli--de Gennes--Matricon one. As a result, the minigap in the quasiparticle spectrum increases with the increase in the defect radius. The scenario of the spectrum transformation is generalized for the case of arbitrary vorticity.
The electronic structure of a vortex pinned by an insulating columnar inclusion in a type-II chiral $p_x \pm i p_y$ superconductor is studied within the Bogolubov-de Gennes theory. The structure of the anomalous spectral branch is shown to be strongly affected by the mutual orientations of the angular momenta of the center of mass and the relative motion of the two electrons in the Cooper pair. Being only slightly perturbed by the scattering at the defect for the zero sum of these angular momenta the anomalous spectral branch appears to change dramatically in the absence of such compensation. …
Equilibrium vortex configuration in conventional type II superconductors containing columnar defects is theoretically investigated. Near the upper critical field a single defect causes a strong local deformation of the vortex lattice. This deformation has $C_3$ or $C_6$ point symmetry, whose character strongly depends on the vortex-defect interaction. If the interaction is attractive, the vortices can collapse onto defect, while in the case of repulsion the regions free of vortices appear near a defect. Increasing the applied magnetic field results in an abrupt change of the configuration of v…
Short-range columnar defects essentially influence the magnetic properties of a mesoscopic superconducting disc.They help the penetration of vortices into the sample, thereby decrease the sample magnetization and reduce the upper critical field. Even the presence of weak defects split a giant vortex state (usually appearing in a clean disc in the vicinity of the transition to a normal state) into a number of vortices with smaller topological charges. In a disc with a sufficient number of strong enough defects vortices are always placed onto defects. The presence of defects lead to the appearan…
In the present work we investigate the behavior of a vortex in a long superconducting cylinder near to a columnar defect at the center. The derivations of the local magnetic field distribution and the Gibbs free energy will be carried out for a cylinder and a cavity of arbitrary sizes. From the general expressions, it considered two particular limits: one in which the radius of the cavity is very small but the radius of the superconducting cylinder is kept finite; and one in which the radius of the superconducting cylinder is taken very large (infinite) but the radius of the cavity is kept fin…
We use numerical minimization of a model free energy functional to study the effects of columnar pinning centers on the structure and thermodynamics of a system of pancake vortices in the mixed phase of highly anisotropic layered superconductors. The magnetic field and the columnar pins are assumed to be perpendicular to the layers. Our methods allow us to study in detail the density distribution of vortices in real space. We present results for the dependence of the average number of vortices trapped at a pinning center on temperature and pinning strength, and for the effective interaction be…
The finite temperature dynamical phases of vortices in films driven by a uniform force and interacting with the periodic pinning potential of a square lattice of columnar defects are investigated by Langevin dynamics simulations of a London model. Vortices driven along the [0,1] direction and at densities for which there are more vortices than columnar defects ($B>B_ϕ$) are considered. At low temperatures, two new dynamical phases, elastic flow and plastic flow, and a sharp transition between them are identified and characterized according to the behavior of the vortex spatial order, velocity …
We present experiments on weakly-pinned vortices, which exhibit a large critical transverse depinning force. These results are obtained in the superconducting metallic glasses Fe$_{x}$Ni$_{1-x}$Zr$_{2}$ using crossed ac and dc driving currents. We study the vortex depinning force due to the transverse ac drive as a function of a longitudinal dc drive; the ac/dc combination permits the separation of the transverse drive from the longitudinal one. We show that the force required for depinning in the transverse direction is greatly enhanced by the longitudinal drive, which demonstrates the existe…
We show that gapless superconductivity of a strongly type-II superconductor in a high magnetic field prevails in the presence of disorder, suggesting a topological nature. We calculate the density of states of the Bogoliubov-de Gennes quasiparticles for a two-dimensional inhomogeneous system in both cases of weak and strong disorder. In the limit of very weak disorder, the effect is very small and the density of states is not appreciably changed. As the disorder increases, the density of states at low energies increases and the ratio of the low-energy density of states to its maximum increases…
We analyse the thermally activated depinning of an elastic string (line tension $ε$) governed by Hall dynamics from a columnar defect modelled as a cylindrical potential well of depth $V_{0}$ for the case of a small external force $F.$ An effective 1D field Hamiltonian is derived in order to describe the 2D string motion. At high temperatures the decay rate is proportional to $F^{{5}/{2}}T^{-{1}/{2}} \exp{\left [{F_{0}}/{F}-{U(F)}/{T}\right ]},$ with $F_{0}$ a constant of order of the critical force and $U(F) \sim{\left ({εV_{0}})}^{{1}/{2}}{V_{0}/{F}}$ the activation energy. The results are a…
Using a two-coil mutual inductance method, we have measured the complex resistivity, rho_v(T,Be), of pinned vortices in c-axis pulsed laser deposited YBa2Cu3O7-d films with magnetic field Be applied perpendicular to the film. At low frequencies, (<100 kHz), rho_v is inductive and is inversely proportional to the Labusch parameter, the average vortex pinning force constant, kappa_exp. The observed weakening of kappa_exp with Be is consistent with a simple model based on linear pinning defects. Adding classical thermal fluctuations to the model in a simple way describes the observed linear T dep…
Low-temperature dynamics of flux lines in high temperature, type II, superconductors in the presence of correlated disorder in the form of columnar defects is discussed. The effect of tilting the applied magnetic field with respect to the column's directions is considered, using the non-Hermitian quantum mechanics technique evaluated by Hatano and Nelson. It is shown that the critical current, as well as the vortex transport properties below this current, may be determined by ``surface excitations'', i.e., by the roughness of the flux line near the edges of the sample, which dominated the bulk…
We present the results of three-dimensional molecular dynamics simulations of vortices which indicate that, for B greater than the matching field, the enhanced pinning effectiveness of splayed columnar defects relative to vertical columnar defects can be explained in terms of the existence or absence of channels through which the vortices can flow without encountering defects.
Using a differential magneto-optical technique to visualize flow of transport currents, we reveal a new delocalization line within the reversible vortex liquid region in the presence of a low density of columnar defects. This line separates a homogeneous vortex liquid, in which all the vortices are delocalized, from a heterogeneous "nanoliquid" phase, in which interconnected nanodroplets of vortex liquid are caged in the pores of a solid skeleton formed by vortices pinned on columnar defects. The nanoliquid phase displays high correlation along the columnar defects but no transverse critical c…
Hall anomaly, as appears in the mixed-state Hall resistivity of type-II superconductors, has had numerous theories but yet a consensus on its origin. In this work, we conducted a quantitative analysis of the magnetotransport measurements on BSCCO thin films by Zhao et al. [Phys. Rev. Lett. 122, 247001 (2019)] and validate a previously proposed vacancy mechanism [cf. J. Phys. Condens. Matter. 10, L677 (1998)] with many-body vortex correlations for the phenomenon. The model attributes the Hall anomaly to the motion of vacancies in pinned fragments of vortex lattice. Its validity is first examine…
We investigate theoretically the eigenmodes and the stability of one and two arbitrary fractional vortices pinned at one and two $κ$-phase discontinuities in a long Josephson junction. In the particular case of a single $κ$-discontinuity, a vortex is spontaneously created and pinned at the boundary between the 0 and $κ$-regions. In this work we show that only two of four possible vortices are stable. A single vortex has an oscillatory eigenmode with a frequency within the plasma gap. We calculate this eigenfrequency as a function of the fractional flux carried by a vortex. For the case of two …
We calculate the local density of states of a vortex in a two-dimensional s-wave superconductor, in the presence of a uniform applied supercurrent. The supercurrent induces changes in the electronic structure for the isolated vortex as well as the vortex lattice, which agree with the recent measurements in 2H-NbSe2 [Maldonado et al., Phys. Rev. B 88, 064518 (2013)]. We find that the supercurrent polarizes the core states when the vortices are pinned. This shows that the transfer of momentum from the supercurrent to the bound states and the rigidity of the wave functions must be considered for …
We present experimental evidence for the existence of a large critical transverse depinning force. These results are obtained in the weakly-pinned superconducting metal glasses Fe$_{x}$Ni$_{1-x}$Zr$_{2}$ using crossed ac and dc driving currents. We study the depinning force due to the transverse ac drive as a function of a longitudinal dc drive. The ac/dc combination allows us to separate the transverse drive from the longitudinal one. We show that the force required for depinning in the transverse direction is enhanced by a longitudinal drive, which leads to the existence of a large transvers…
We discuss the main features of Hall tunneling of pancake vortices in superclean high-$T_{c}$ superconductors. The general formalism for the calculation of the lifetime of a vortex pinned in a metastable configuration is described. The results are applied to the problem of quantum tunneling of a pancake vortex from a columnar defect in the limit of a small driving current.
From magneto-optical imaging performed on heavy-ion irradiated YBaCuO single crystals, it is found that at fields and temperatures where strong single vortex pinning by individual irradiation-induced amorphous columnar defects is to be expected, vortex motion is limited by the nucleation of vortex kinks at the specimen surface rather than by half-loop nucleation in the bulk. In the material bulk, vortex motion occurs through (easy) kink sliding. Depinning in the bulk determines the screening current only at fields comparable to or larger than the matching field, at which the majority of moving…
The ratchet effect is demonstrated theoretically for the simple model of a vortex in a thin superconducting film interacting with a periodic array of magnetic dipoles placed in the vicinity of the film . The pinning potential for the vortex is calculated in the London limit and found to break spatial inversion symmetry and to depend on the orientation of the magnetic dipole moments. The motion of the vortex at zero temperature driven by a force oscillating periodically in time is investigated numerically. Drift vortex motion consisting of displacements by a translation vector of the dipole arr…
Within a Green's function formalism we calculate the electronic structure around static extended magnetic and non-magnetic perturbations in a d-wave superconductor. In partucular, we discuss recent elastic neutron scattering and scanning tunneling experiments on High-T_c cuprates exposed to an applied magnetic field. A physical picture consisting of antiferromagnetic vortex cores operating as pinning centers for surrounding stripes is qualitatively consistent with the neutron data provided the stripes have the usual antiphase modulation. The low energy electronic structure in such a region rev…
In a Type II superconductor, the vortex core behaves like a normal metal. Consequently, the single-particle density of states in the vortex core of a conventional Type II superconductor remains either flat or (for very clean single crystals) exhibits a peak at zero bias due to the formation of Caroli-de Gennes-Matricon bound state inside the core. Here we report an unusual observation from scanning tunneling spectroscopy measurements in a weakly pinned thin film of the conventional s-wave superconductor a-MoGe, namely, that a soft gap in the local density of states continues to exist even at t…
We report on measurements of the angular dependence of the irreversibility temperature $T_{irr}(θ) $ in $YBa_2Cu_3O_{7-δ}$ thin films, defined by the onset of a third harmonic signal and measured by a miniature Hall probe. From the functional form of $T_{irr}(θ)$ we conclude that the origin of the irreversibility line in unirradiated films is a dynamic crossover from an unpinned to a pinned vortex liquid. In irradiated films the irreversibility temperature is determined by the trapping angle.
We report measurements of the irreversible magnetization M_i of a large number of YBa_2Cu_3O_7 single crystals with columnar defects (CD). Some of them exhibit a maximum in M_i when the density of vortices equals the density of tracks, at temperatures above 40K. We show that the observation of these matching field effects is constrained to those crystals where the orientational and pinning energy dispersion of the CD system lies below a certain threshold. The amount of such dispersion is determined by the mass and energy of the irradiation ions, and by the crystal thickness. Time relaxation me…
Using the time-dependent Ginzburg-Landau equation with the complex relaxation time and the Maxwell equation, we systematically examine transverse motion of vortex dynamics in the presence of pinning disorders. Consequently, in a plastic flow phase in which moving and pinned vortices coexist, we find that the Hall voltage generally changes its sign. The origin of the sign change is ascribed to a fact that moving vortices are strongly drifted by circular current of pinned vortices and the enforced transverse moving direction becomes opposite to that by transport current. This suggests that the H…
We investigate the influence of columnar defects in layered superconductors on the thermally activated penetration of pancake vortices through the surface barrier. Columnar defects, located near the surface, facilitate penetration of vortices through the surface barrier, by creating ``weak spots'', through which pancakes can penetrate into the superconductor. Penetration of a pancake mediated by an isolated column, located near the surface, is a two-stage process involving hopping from the surface to the column and the detachment from the column into the bulk; each stage is controlled by its o…
We examine the melting of commensurate and incommensurate vortex lattices interacting with square pinning arrays through the use of numerical simulations. For weak pinning strength in the commensurate case we observe an order-order transition from a commensurate square vortex lattice to a triangular floating solid phase as a function of temperature. This floating solid phase melts into a liquid at still higher temperature. For strong pinning there is only a single transition from the square pinned lattice to the liquid state. For strong pinning in the incommensurate case, we observe a multi-st…
We present a microscopic derivation, without electrodynamical assumptions, of $B(x,y,H(t))$, $M(H(t))$, and $J_c(H(t))$, in agreement with experiments on strongly pinned superconductors, for a range of values of the density and strength of the pinning sites. We numerically solve the overdamped equations of motion % dynamics of these flux-gradient-driven vortices which can be temporarily trapped at pinning centers. The field is increased (decreased) by the addition (removal) of flux lines at the sample boundary, and complete hysteresis loops can be achieved by using flux lines with opposite ori…
Strong pinning of superconducting flux quanta by a square array of 1 $μ$m-sized ferromagnetic dots in a magnetic-vortex state was visualized by low-temperature magnetic force microscopy (LT-MFM). A direct correlation of the superconducting flux lines with the positions of the dots was derived. The force that the MFM tip exerts on the individual vortex in the depinning process was used to estimate the spatial modulation of the pinning potential. It was found, that the superconducting vortices which are preferably located on top of the Py dots experience about 15 times stronger pinning forces as…
The mixed phase of layered superconductors with no magnetic screening is studied through a partial duality analysis of the corresponding frustrated XY model in the presence of random columnar pins. A small fraction of pinned vortex lines is assumed. Thermally induced plastic creep of the vortex lattice within isolated layers results in an intermediate Bose glass phase that exhibits weak superconductivity across layers in the limit of weak Josephson coupling. The correlation volume of the vortex lattice is estimated in the strongly-coupled Bose-glass regime at lower temperature. In the absence …
The structure and equilibrium properties of a two-dimensional system of superconducting vortices in a periodic pinning potential with square symmetry are studied numerically. For a range of the strength of the pinning potential, the low-temperature crystalline state exhibits only one of the two basic periodicities (in the $x$- and $y$-directions) of the pinning potential. This ``partially pinned'' solid undergoes a continuous melting transition to a weakly modulated liquid as the temperature is increased. A spin model, constructed using symmetry arguments, is shown to reproduce the critical be…
This is an analytical study of pinning and spontaneous vortex phase is a system consisting of a superconducting thin film pierced by a long ferromagnetic columnar defect of finite radius $R$. The magnetic fields, screening currents, energy and pinning forces for this system are calculated. The interaction between the magnetic field of vortices and the magnetization outside the plane of the film and its close proximity enhances vortex pinning significantly. Spontaneous vortex phase appears when the magnetization of the columnar defect is increased above a critical value. Transitions between pha…
When an electron beam strikes a superconducting thin film near a pinned vortex, it locally increases the temperature-dependent London penetration depth and perturbs the circulating supercurrent, thereby distorting the vortex's magnetic field toward the heated spot. This phenomenon has been used to visualize vortices pinned in SQUIDs using low-temperature scanning electron microscopy. In this paper I develop a quantitative theory to calculate the displacement of the vortex-generated magnetic-flux distribution as a function of the distance of the beam spot from the vortex core. The results are c…
Poisson brackets for the Hamiltonian dynamics of vortices are discussed for 3 regimes, in which the dissipation can be neglected and the vortex dynamics is reversible: (i) The superclean regime when the spectral flow is suppressed. (ii) The regime when the fermions are pinned by crystal lattice. This includes also the regime of the extreme spectral flow of fermions in the vortex core: these fermions are effectively pinned by the normal component. (iii) The case when the vortices are strongly pinned by the normal component. All these limits are described by the single parameter $C_0$, which phy…
The effects of pinning between fluxoids and vortices in the core of a neutron star, on the dynamics of the core neutron superfluid are considered. The pinning impedes, but does not absolutely block, any radial as well as {\em azimuthal} motion of the neutron vortices with respect to the lattice of fluxoids. The time scale for the coupling of rotation of the core superfluid to the rest of the star is calculated, allowing for the effect of the finite frictional force on the neutron vortices due to their pinning with the fluxoids. This turns out to be the dominant mechanism for the coupling of th…
By analytically solving the Bogoliubov-de Gennes equations we study the fermion bound states at the center of the core of a vortex in a two-dimensional superconductor. We consider three kinds of 2D superconducting models: (a) a standard type II superconductor in the mixed state with low density of vortex lines, (b) a superconductor with strong spin-orbit coupling locking the spin parallel to the momentum and (c) a superconductor with strong spin-orbit coupling locking the spin perpendicular to the momentum. The 2D superconducting states are induced via proximity effect between an $s$-wave supe…
We explore in detail the angular dependent vortex dynamics in type II superconductors with aligned columnar defects introduced by irradiation with very energetic heavy-ions. We use dc magnetization measurements deep in the vortex solid phase, and ac susceptibility near the solid-liquid transition. We show that aligned columnar defects are an excellent tool to test models for vortex dynamics, particularly if they are tilted with respect to the crystallographic axes, so their effects can be easily distinguished from those arising from mass anisotropy, sample geometry, twin boundaries and intrins…
In order to enhance the critical current density ($J_c$) of superconductors, introduction of columnar defects (CDs) through swift-particle irradiation is effective. By dispersing the direction of CDs (splayed CDs), not only further enhancement of $J_c$ has been confirmed but also an anomalous peak effect (APE) in $J_c$ at a certain magnetic field determined by the irradiation dose was observed. It has been proposed that the APE arises from the suppression of kink motion and/or the effects of vortex entanglement in systems with splayed CDs. In this study, we measure $J_c$ properties of optimall…
The defect-rich morphology of YBa$_{2}$Cu$_{3}$O$_{7-δ}$ (YBCO) thin films leads to a glass-like arrangement of Abrikosov vortices which causes the resistance to disappear in vanishing current densities. This vortex glass consists of entangled vortex lines and is identified by a characteristic scaling of the voltage-current isotherms. Randomly distributed columnar defects stratify the vortex lines and lead to a Bose glass. Here, we report on the observation of an ordered Bose glass in a YBCO thin film with a hexagonal array of columnar defects with 30 nm spacings. The periodic pinning landscap…
The effect of columnar pins on the flux-lines melting transition in high-temperature superconductors is studied using Path Integral Monte Carlo simulations. We highlight the similarities and differences in the effects of columnar disorder on the melting transition in YBa$_2$Cu$_3$O$_{7-δ}$ (YBCO) and the highly anisotropic Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ (BSCCO) at magnetic fields such that the mean separation between flux-lines is smaller than the penetration length. For pure systems, a first order transition from a flux-line solid to a liquid phase is seen as the temperature is increased. When…
We investigate the scaling properties of single flux lines in a random pinning landscape consisting of splayed columnar defects. Such correlated defects can be injected into Type II superconductors by inducing nuclear fission or via direct heavy ion irradiation. The result is often very efficient pinning of the vortices which gives, e.g., a strongly enhanced critical current. The wandering exponent ζand the free energy exponent ωof a single flux line in such a disordered environment are obtained analytically from scaling arguments combined with extreme-value statistics. In contrast to the case…
We present a novel method, based on vortex imaging by low-temperature scanning electron microscopy (LTSEM), to directly image the sheet-current distribution in YBa2Cu3O7 dc SQUID washers. We show that the LTSEM vortex signals are simply related to the scalar stream function describing the vortex-free circulating sheet-current distribution J. Unlike previous inversion methods that infer the current distribution from the measured magnetic field, our method uses pinned vortices as local detectors for J. Our experimental results are in very good agreement with numerical calculations of J.
In the mixed state of type II superconductors penetrated by an external magnetic field in the form of a lattice of Abrikosov vortices the dc resistance is known to increase with increasing velocity of the vortex lattice. Accordingly, vortex pinning sites impeding the vortex motion are widely used to preserve the low-dissipative response of the system. Here, while subjecting superconducting Nb films with nanogrooves to a combination of dc and ac current stimuli and tuning the number of mobile and pinned vortices by varying the magnetic field around the so-called matching values, we observe a co…
We report here the observation of anomalous paramagnetic signal(s) in the isofield field-cooled cool-down magnetization scans (MFCC(T)) recorded for a single crystal of a low Tc superconductor Ca3Ir4Sn13. Novel features emanating from the MFCC(T) response include an oscillatory magnetization behaviour below Tc and a rich multiplicity (non-uniqueness) in magnetization ranging from diamagnetism to paramagnetism at a given H, T value. The metastability in MFCC(T) has been ascribed to non-unique coexistence of multi-quanta vortex states and single quantum (Abrikosov) vortices. Additionally, the is…
We study theoretically the simultaneous effect of a regular and a random pinning potentials on the vortex lattice structure at filling factor of 1. This structure is determined by a competition between the square symmetry of regular pinning array, by the intervortex interaction favoring a triangular symmetry, and by the randomness trying to depin vortices from their regular positions. Both analytical and molecular-dynamics approaches are used. We construct a phase diagram of the system in the plane of regular and random pinning strengths and determine typical vortex lattice defects appearing i…
Triangular arrays of Ni nanotriangles embedded in superconducting Nb films exhibit unexpected dynamical vortex effects. Collective pinning with a vortex lattice configuration different from the expected fundamental triangular "Abrikosov state" is found. The vortex motion which prevails against the triangular periodic potential is produced by channelling effects between triangles. Interstitial vortices coexisting with pinned vortices in this asymmetric potential, lead to ratchet reversal, i.e. a DC output voltage which changes sign with the amplitude of an applied alternating drive current. In …
We use magnetic force microscopy to both image and manipulate individual vortex lines threading single crystalline YBa$_2$Cu$_3$O$_{6.4}$, a layered superconductor. We find that when we pull the top of a pinned vortex, it may not tilt smoothly. Sometimes, we observe a vortex to break into discrete segments that can be described as short stacks of pancake vortices, similar to the "kinked" structure proposed by Benkraouda and Clem. Quantitative analysis gives an estimate of the pinning force and the coupling between the stacks. Our measurements highlight the discrete nature of stacks of pancake …
Angularly Selective Enhanced Vortex Screening in Extremely Layered Superconductors with Tilted Columnar Defects
Low Temperatures Lab, Centro Atómico Bariloche, CNEA（低温实验室，巴里洛切原子中心，国家原子能委员会）；Instituto de Nanociencia y Nanotecnología, CONICET-CNEA, Nodo Bariloche（纳米科学与技术研究所，CONICET-CNEA，巴里洛切节点）；Instituto Balseiro, CNEA and Universidad Nacional de Cuyo（巴尔塞罗研究所，国家原子能委员会和库约国立大学）；Neutrons Beam National Lab, CNEA（中子束国家实验室，国家原子能委员会）；Vmnetic SAS；Laboratoire des Solides Irradiés, CEA/DRF/IRAMIS, École Polytechnique, CNRS, Institut Polytechnique de Paris（辐照固体实验室，CEA/DRF/IRAMIS，巴黎综合理工学院，CNRS，巴黎理工学院）
Maximizing the sustainable supercurrent density, Jc, is crucial to high current applications of superconductivity and, to achieve this, preventing dissipative motion of quantized vortices is key. Irradiation of superconductors with high-energy heavy ions can be used to create nanoscale defects that act as deep pinning potentials for vortices. This approach holds unique promise for high current applications of iron-based superconductors because Jc amplification persists to much higher radiation doses than in cuprate superconductors without significantly altering the superconducting critical tem…
Vortices confined to superconducting easy flow channels with periodic constrictions exhibit reversible oscillations in the critical current at which vortices begin moving as the external magnetic field is varied. This commensurability scales with the channel shape and arrangement, although screening effects play an important role. For large magnetic fields, some of the vortices become pinned outside of the channels, leading to magnetic hysteresis in the critical current. Some channel configurations also exhibit a dynamical hysteresis in the flux-flow regime near the matching fields.
We study the effect of a single columnar pin on a $(1+1)$ dimensional array of vortex lines in planar type II superconductors in the presence of point disorder. In large samples, the pinning is most effective right at the temperature of the vortex glass transition. In particular, there is a pronounced maximum in the number of vortices which are prevented from tilting by the columnar defect in a weak transverse magnetic field. Using renormalization group techniques we show that the columnar pin is irrelevant at long length scales both above and below the transition, but due to very different me…
Quantum fluids refer to a class of systems that remain in fluid state down to absolute zero temperature. In this letter, using a combination of magnetotransport and scanning tunneling spectroscopy down to 300 mK, we show that vortices in a very weakly pinned a-MoGe thin film can form a quantum vortex fluid. Under the application of a magnetic field perpendicular to the plane of the film, the vortex state transforms from a vortex solid to a hexatic vortex fluid and eventually to an isotropic vortex liquid. The fact that the two latter states remain fluid down to absolute zero temperature is evi…
We study effects of a chiral helimagnet (CHM) on vortex states in a superconductor, solving the Ginzburg-Landau equations in a chiral helimagnet/superconductor bilayer system. We found that vortices form a periodically modulated triangular lattice, because the magnetic field from the chiral helimagnet $H_{CHM}$ oscillates spatially. An increase of a critical current is expected, because vortices are pinned by the $H_{CHM}$.
\\A simple model for a superconductor with tunable critical current is studied theoretically. The model consists of a thin superconducting film with one vortex interacting with one magnetic dipole, whose magnetic moment is free to rotate, in the presence of a magnetic field applied parallel to the film surfaces. The pinning potential for the vortex is calculated exactly in the London limit. It is found that, due to the dipole freedom to rotate, the dependence of the pinning potential on the applied field is non-trivial, and allows both the spatial dependence and strength of the pinning potenti…
We investigate the vortex phase diagram in untwinned YBaCuO single crystals with columnar defects. These randomly distributed defects, produced by heavy ion irradiation, are expected to induce a ``Bose Glass'' phase of localized vortices characterized by a vanishing resistance and a Meissner effect for magnetic fields transverse to the defect axis. We directly observe the transverse Meissner effect using an array of Hall probe magnetometers. As predicted, the Meissner state breaks down at temperatures Ts that decrease linearly with increasing transverse magnetic field. However, Ts falls well b…
The properties of vortices in superconducting thin films are revisited. The interaction between two Pearl vortices in an infinite film is approximated at all distances by a simple expression. The interaction of a vortex with a regular lattice of real or image vortices is given. The two spring constants are calculated that one vortex in the vortex lattice feels when the surrounding vortices are rigidly pinned or are free. The modification of these London results by the finite size of real films is obtained. In finite films, the interaction force between two vortices is not a central force but d…
