Effect of columnar defects on the pinning properties of NdFeAsO0.85 conglomerate particles
J. D. Moore, L. F. Cohen, Y. Yeshurun, A. D. Caplin, K. Morrison, K. A. Yates, C. M. McGilvery, J. M. Perkins, D. W. McComb, C. Trautmann, Z. A. Ren, J. Yang, W… 展开作者
J. D. Moore, L. F. Cohen, Y. Yeshurun, A. D. Caplin, K. Morrison, K. A. Yates, C. M. McGilvery, J. M. Perkins, D. W. McComb, C. Trautmann, Z. A. Ren, J. Yang, W. Lu, X. L. Dong, Z. X. Zhao
Oxypnictide superconductor NdFeAsO0.85 sample was irradiated with 2 GeV Ta ions at a fluence of 5x10^10 ions/cm2. High resolution transmission electron microscopy study revealed that the irradiation produced columnar-like defects. The effect of these defects on the irreversible magnetisation in polycrystalline randomly oriented fragments was studied as a function of field angle and field sweep rate. We find that the critical current density is enhanced at fields below the matching field (~1 Tesla) but only marginally. The pinning enhancement is anisotropic and maximum along the defect direction at high temperatures but the pinning then becomes more isotropic at low temperatures. The creep rate is suppressed at high temperatures and at fields below the matching field, indicating the columnar defects are efficient pinning sites at these H and T conditions.
We identify a phase transition in the vortex system of a high-temperature superconductor with nano-columnar stacks of precipitates as strong vortex pinning centers. Above a particular, temperature-dependent field $B_X(T)$ the vortex response is no longer determined by the nano-columns, and is instead determined by point-like pinning. This phase transition leads to the change in the measured critical current density as a function of angle between the applied magnetic field and the nano-columns. Below the unbinding, there is a strong maximum in $J_C$ when field is aligned parallel to the columns…
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…
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…
We study the vortex matter phase diagram of a layered superconductor in the presence of columnar pinning defects, {\it tilted} with respect to the normal to the layers. We use numerical minimization of the free energy written as a functional of the time averaged vortex density of the Ramakrishnan-Yussouff form, supplemented by the appropriate pinning potential. We study the case where the pin density is smaller than the areal vortex density. At lower pin concentrations, we find, for temperatures of the order of the melting temperature of the unpinned lattice, a Bose glass type phase which at l…
We have studied the influence of columnar defects, created by heavy-ion (Kr) irradiation with doses up to 6 10^11 Kr-ions/cm2, on the superconducting critical parameters of single crystalline (YxTm1-x)Ba2Cu3O7. Magnetisation measurements in pulsed fields up to 50 T in the temperature range 4.2 - 90 K revealed that: (i) in fields up to T the critical current Jc(H,T) is considerably enhanced and (ii) down to temperatures T ~ 40 K the irreversibility field Hirr(T) is strongly increased. The field range and magnitude of the Jc(H,T) and Hirr(T) enhancement increase with increasing irradiation dose.…
Extensive work has shown that irradiation with 0.8 GeV protons can produce randomly oriented columnar defects (CD's) in a large number of HTS materials, specifically those cuprates containing Hg, Tl, Pb, Bi, and similar heavy elements. Absorbing the incident proton causes the nucleus of these species to fission, and the recoiling fission fragments create amorphous tracks, i.e., CD's. The superconductive transition temperature Tc decreases linearly with proton fluence and we analyze how the rate depends on the family of superconductors. In a study of Tl-2212 materials, adding defects decreases …
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…
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…
We analyze the angular dependence of the irreversible magnetization of YBa$_2$Cu$_3$O$_7$ crystals with columnar defects inclined from the c-axis. At high fields a sharp maximum centered at the tracks' direction is observed. At low fields we identify a lock-in phase characterized by an angle-independent pinning strength and observe an angular shift of the peak towards the c-axis that originates in the material anisotropy. The interplay among columnar defects, twins and ab-planes generates a variety of staircase structures. We show that correlated pinning dominates for all field orientations.
We present the results of a detailed investigation of the low-temperature properties of the vortex system in strongly anisotropic layered superconductors with a random array of columnar pinning centers. Our method involves numerical minimization of a free energy functional in terms of the time-averaged local vortex density. It yields the detailed vortex density distribution for all local free-energy minima, and therefore allows the computation of any desired correlation function of the time-averaged local vortex density. Results for the phase diagram in the temperature vs. pin concentration pl…
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 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…
We have measured an angle-dependent contribution to the equilibrium magnetization of a YBa_2Cu_3O_{7-d} single crystal with columnar defects created by 5.8GeV Pb ions. This contribution manifests itself as a jump in the equilibrium torque signal, when the magnetic field crosses that of the defects. The magnitude of the signal, which is observed in a narrow temperature interval less than 2K and for fields up to about twice the irradiation field, is used to estimate the the energy gained by vortex pinning on the defects. The vanishing of the effective pinning energy at a temperature below Tc is …
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…
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…
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 …
It is argued that most of the RTSC hydrides are intrinsic hard type-II superconductors with strong pinning effects. The pinning centers are long columnar-like defects, with the radius of the order of the superconducting coherence length. The core and electromagnetic pinning are both equally operative, thus giving maximal pinning potential when vortices are oriented along the columnar axis. The theory predicts: (i) large critical currents, (ii) huge decrease of the temperature broadening of the resistance in magnetic field compared with standard superconductors, (iii) large magnetization hyster…
We study the elasticity, fluctuations and pinning of a putative spontaneous vortex solid in ferromagnetic superconductors. Using a rigorous thermodynamic argument, we show that in the idealized case of vanishing crystalline pinning anisotropy the long-wavelength tilt modulus of such a vortex solid vanishes identically, as guaranteed by the underlying rotational invariance. The vanishing of the tilt modulus means that, to lowest order, the associated tension elasticity is replaced by the softer, curvature elasticity. The effect of this is to make the spontaneous vortex solid qualitatively more …
We report significant alteration of the equilibrium properties of the superconductor HgBa$_2$Ca$_2$Cu$_3$O$_x$ when correlated disorder in the form of randomly oriented columnar tracks is introduced via induced fission of Hg-nuclei. From studies of the equilibrium magnetization $M_{eq}$ and the persistent current density over a wide range of temperatures, applied magnetic fields, and track densities up to a ``matching field'' of 3.4 Tesla, we observe that the addition of more columnar tracks acting as pinning centers is progressively offset by reductions in the magnitude of $M_{eq}$. Invoking …
We consider layered superconductors with a flux lattice perpendicular to the layers and random columnar defects parallel to the magnetic field B. We show that the decoupling transition temperature Td, at which the Josephson coupling vanishes, is enhanced by columnar defects by an amount ~B^2 relative to Td. Decoupling by increasing field can be followed by a reentrant recoupling transition for strong disorder. We also consider a commensurate component of the columnar density and show that its pinning potential is renormalized to zero above a critical long wavelength disorder. This decommnesura…
We consider the melting of the vortex solid in highly anisotropic layered superconductors with a small concentration of random columnar pinning centers. Using large-scale numerical minimization of a free-energy functional, we find that melting of the low-temperature, nearly crystalline vortex solid (Bragg glass) into a vortex liquid occurs in two steps as the temperature increases: the Bragg glass and liquid phases are separated by an intermediate Bose glass phase. A suitably defined local melting temperature exhibits spatial variation similar to that observed in experiments.
Within the Ginzburg-Landau theory we consider Abrikosov vortex pinning on a columnar defect with the characteristic size of the cross-section much smaller than the coherence length. We present an extension of the electrostatic analogies method, which proved to be useful for calculations of the pinning force for large cavities, to the case of small defects. The pinning potential for an elliptic cavity is derived analytically. Also, we determine the depinning current for a circular defect.
Phase diagram of vortex states of high-$T_{\rm c}$ superconductors with {\it sparse and weak} columnar defects is obtained by large-scale Monte Carlo simulations of the three-dimensional anisotropic, frustrated XY model. The Bragg-Bose glass phase characterized by hexagonal Bragg spots and the diverging tilt modulus is observed numerically for the first time at low density of columnar defects. As the density of defects increases, the melting temperature increases owing to "selected pinning" of flux lines. When the density of defects further increases, the transition to the Bose glass phase occ…
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 study the melting transition of the low-temperature vortex solid in strongly anisotropic layered superconductors with a concentration of random columnar pinning centers small enough so that the areal density of the pins is much less than that of the vortex lines. Both the external magnetic field and the columnar pins are assumed to be oriented perpendicular to the layers Our method, involving numerical minimization of a model free energy functional, yields not only the free energy values at the local minima of the functional but also the detailed density distribution of the system at each m…
We explore the interplay between the mass anisotropy and the uniaxial pinning of tilted columnar defects (CD) in a YBa2Cu3O7 single crystal. At high temperatures T and fields H a sharp peak in the irreversible magnetization Mi at the direction of the tracks signals the presence of the CD. At low T such a peak is not observed, and the influence of the CD appears only as a perturbation to the angular dependence due to the anisotropy. We show that at low T and high H the uniaxial effects due to CD can be recovered by performing an anisotropic rescaling. As expected, this scaling fails at low H.
Recently, the existence of superconductivity in $\mathrm{H_{3}S}$ and other high-pressure hydrides is called into question, because of some flawed magnetic measurements. In Ref.[1] the SCPC model is proposed, where strong pinning of vortices is due to long columnar defects in $\mathrm{H_{3}S}$ - with lengths $\mathrm{L}$ of the order of vortex lengths $\mathrm{L_{v}}$. Two relevant type of experiments in magnetic fields are explained by this model: (1) Reduction (with respect to standard superconductors) of the thermal broadening of resistance ($\mathit{TBR}$) in magnetic field $\mathrm{h=H/H_…
Epitaxial MgB2 thick films were grown on Al2O3 substrates at 600 oC by using the hybrid physical chemical vapor deposition (HPCVD) technique. In order to obtain a high magnesium vapor pressure around the substrates, we used a special susceptor having a susceptor cap and achieved a very high growth rate of 0.17 um/min. Hexgonal-shaped columnar structures were observed by cross-sectional and planar view transmission electron microscope (TEM) images. For the 1.7-um-thick film, the Tc was observed to be 40.5 K with a Jc of 1.5 x 10^6 A/cm^2 at 30 K. The vortex pinning mechanism by intercolumnar bo…
Through 2.6 GeV U irradiations, we have induced bimodal splayed columnar defects in Ba$_{1-x}$K$_x$Fe$_2$As$_2$ single crystals with splay angles, $\pm 5 ^\circ$, $\pm 10 ^\circ$, $\pm 15 ^\circ$, and $\pm 20 ^\circ$. Critical current densities through magnetization measurements were carefully evaluated, where a splay angle of $\pm 5 ^\circ$ brought about the highest $J_\mathrm{c}$. Mageto-optical images close to $T_\mathrm{c}$ indicates highly anisotropic discontinuity lines in the remnant state, and with anisotropy increasing with greater splay angles. Moreover, amongst those with splayed co…
The U/n method is a well-established means of improving flux pinning and critical current performance in cuprate superconductors. The method involves the doping of the superconductor with 235U followed by irradiation with thermal neutrons to promote fission. The resultant columnar damage tracks produced by the energetic fission products pin flux vortices and improve critical current performance in magnetic fields. No such improvement could be observed when the U/n method was applied to MgB2 superconductor. No fission tracks could be observed in TEM, even for samples that were irradiated at the…
Intermodulation distortion (IMD) of microwave signals incident upon a superconducting YBa2Cu3O7 thin film sample on a LaAlO3 substrate is shown to relax as the sample magnetization also relaxes, revealing the signature of fluxon decay in the microwave distortion. The sample contained a microchannel of columnar defects that was introduced using heavy ion beam irradiation. After exposure to and then removal of a low strength static magnetic field, IMD relaxed due to thermal activation of fluxons over the surface barrier. This process influenced second and third order IMD differently, visible in …
A flux line in a Type-II superconductor with a single nonuniform columnar defect is studied by a perturbative diagrammatic expansion around an annealed approximation. The system undergoes a finite temperature depinning transition for the (rather unphysical) on-the-average repulsive columnar defect, provided that the fluctuations along the axis are sufficiently large to cause some portions of the column to become attractive. The perturbative expansion is convergent throughout the weak pinning regime and becomes exact as the depinning transition is approached, providing an exact determination of…
We report measurements of the field and angular dependences of Jc of truly epitaxial Co-doped BaFe2As2 thin films grown on SrTiO3/(La,Sr)(Al,Ta)O3 with different SrTiO3 template thicknesses. The films show Jc comparable to Jc of single crystals and a maximum pinning force Fp(0.6Tc) > 5 GN/m3 at H/Hirr ~ 0.5 indicative of strong vortex pinning effective up to high fields. Due to the strong correlated c-axis pinning, Jc for field along the c-axis exceeds Jc for H//ab plane, inverting the expectation of the Hc2 anisotropy. HRTEM reveals that the strong vortex pinning is due to a high density of n…
We present the experimental observation of magnetic field line curvature at the surface of a superconducting film by local quantitative magneto-optics. In addition to the knowledge of the full induction field at the superconductor surface yielding the quantitative observation of the flux line curvature, our analysis method allows also local value measurements of the electrical current density inside the sample. Thus, we study the interplay between the electrodynamic constraints dictated by the film geometry and the pinning properties of the superconductor. In particular, we investigate the ani…
We use the Feynman--Kleinert variational method [R.P. Feynman and H. Kleinert, Phys. Rev. A {\bf 34}, 5080, (1986)] to calculate the partition function and effective pinning energy of a single flux line near a columnar pin in type II superconductors. It is found that there is a phase transition between a low temperature phase where the flux line is localized near the columnar pin and where its internal modes fluctuations are bounded, and a high temperature, depinned phase where the flux line is delocalized and its internal fluctuations are those of a free line.
The transverse (Hall) voltage in thin films of the high-temperature superconductor YBa$_{2}$Cu$_{3}$O$_{7-δ}$ with an artificial periodic pinning array is investigated. Columnar defect regions along the crystallographic $c$ axis, in which superconductivity is suppressed, are created by irradiation with He$^+$ ions through a silicon stencil mask. The commensurate arrangement of magnetic flux quanta with the artificial defect lattice is confirmed by maxima of the critical current and minima of the resistance, respectively. The magnetic field dependence of the transverse voltage reveals a commens…
We have deposited c-axis oriented thin films of La1.5Dy0.5CaBa2Cu5Oz (La-2125) tetragonal superconductor on LaAlO3 (001) by Pulsed Laser Deposition. These films were irradiated with 200 MeV Ag+15 ions. Atomic Force Microscopy and Elastic Recoil Detection Analysis indicate that the irradiation has created columnar defects along the entire thickness (2000 A) of these films. With ion irradiation up to 1*10^11 ions/cm2, the critical current density (Jc(H)) is found to enhance by almost five folds, which is attributed to the augmented flux pinning by the columnar defects. A further increase in irra…
Theoretical work predicts that the strong dependence of Tc on pure shear strain within the a-b plane of optimally doped YBa2Cu3O{7-delta} results in heterogenous columnar pins of vortex lines about dislocation lines and about nano-columns inclusions aligned in parallel to the c axis. The critical current of a rigid vortex lattice driven by the Lorentz force in the presence of such clusters of pin/antipin lines is computed using two-dimensional (2D) collective pinning theory and by numerical simulation of the corresponding 2D vortex dynamics. Both theory and computer calculation find that the a…
Flux pinning is still the main limiting factor for the critical current of the high-temperature superconductors in high fields. In this paper we model the field dependence of the critical current in thin films with columnar defects aligned with the field. The characteristic shape of the critical current of the BaZrO$_3$ doped YBa$_2$Cu$_3$O$_{6+x}$ thin films is reproduced and explained. The model is based on solving the Ginzburg-Landau equations with columnar defects present in the lattice. The size of the columnar defect is found to be of key importance in explaining the rounded shape of the…
We report the effects of heavy-ion irradiation on FeSe single crystals by irradiating Uranium up to a dose equivalent matching field of $B_ϕ$ = 16 T. Almost continuous columnar defects along the $c$-axis with a diameter $\sim$10 nm are confirmed by high-resolution transmission electron microscopy. $T_c$ is found to be suppressed by introducing columnar defects at a rate of d$T_c$/d$B_ϕ$ $\sim$ -0.29KT$^{-1}$, which is much larger than those observed in iron pnictides. This unexpected large suppression of $T_c$ in FeSe is discussed in relation to the large diameter of the columnar defects as we…
We consider the problem of the depinning of a weakly driven ($F\ll F_{c}$) pancake vortex from a columnar defect in a Josephson-coupled superconductor, where $F$ denotes the force acting on the vortex ($F_{c}$ is the critical force). The dynamics of the vortex is supposed to be of the Hall type. The Euclidean action $S_{Eucl}(T)$ is calculated in the entire temperature range; the result is universal and does not depend on the detailed form of the pinning potential. We show that the transition from quantum to classical behavior is second-order like with the temperature $T_{c}$ of the transition…
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.
The two-dimensional (2D) vortex lattice in the extreme type-II limit is studied by Monte Carlo simulation of the corresponding 2D Coulomb gas, with identical pins placed at sites coinciding with the zero-temperature triangular vortex lattice. At weak pinning we find evidence for 2D melting into an intermediate hexatic phase. The strong pinning regime shows a Kosterlitz-Thouless transition, driven by interstitial vortex/anti-vortex excitations. A stack of such identical layers with a weak Josephson coupling models a layered superconductor with a triangular arrangement of columnar pins at the ma…
The competition between intrinsic disorder in superconducting YBa$_{2}$Cu$_{3}$O$_{7-δ}$ (YBCO) thin films and an ultradense triangular lattice of cylindrical pinning centers spaced at 30 nm intervals results in an ordered Bose glass phase of vortices. The samples were created by scanning the focused beam of a helium-ion microscope over the surface of the YBCO thin film to form columns of point defects where superconductivity was locally suppressed. The voltage-current isotherms reveal critical behavior and scale in the vicinity of the second-order glass transition. The latter exhibits a disti…
The angular dependence of magnetic-field commensurability effects in thin films of the cuprate high-critical-temperature superconductor YBa$_{2}$Cu$_{3}$O$_{7-δ}$ (YBCO) with an artificial pinning landscape is investigated. Columns of point defects are fabricated by two different methods of ion irradiation -- scanning the focused 30 keV ion beam in a helium ion microscope or employing the wide-field 75 keV He$^+$ beam of an ion implanter through a stencil mask. Simulations of the ion-target interactions and the resulting collision cascades reveal that with both methods square arrays of defect …
The results of a linear elasticity analysis yields that nano-rod inclusions aligned along the c axis of a thin film of YBa2Cu3O{7-delta}, such as BaZrO3 and BaSnO3, squeeze that matrix by pure shear. The sensitivity of the superconducting critical temperature in that material to the latter implies that the phase boundary separating the nano-rod inclusion from the superconductor acts as a collective pinning center for the vortex lattice that appears in external magnetic field. A dominant contribution to the in-field critical current can result. The elasticity analysis also finds that the growth…
In order to characterize flux flow through disordered type-II superconductors, we investigate the effects of columnar and point defects on the vortex velocity / voltage power spectrum in the driven non-equilibrium steady state. We employ three-dimensional Metropolis Monte Carlo simulations to measure relevant physical observables including the force-velocity / current-voltage (I-V) characteristics, vortex spatial arrangement and structure factor, and mean flux line radius of gyration. Our simulation results compare well to earlier findings and physical intuition. We focus specifically on the v…
Square arrays of sub-micrometer columnar defects in thin YBa$_{2}$Cu$_{3}$O$_{7-δ}$ (YBCO) films with spacings down to 300 nm have been fabricated by a He ion beam projection technique. Pronounced peaks in the critical current and corresponding minima in the resistance demonstrate the commensurate arrangement of flux quanta with the artificial pinning landscape, despite the strong intrinsic pinning in epitaxial YBCO films. Whereas these vortex matching signatures are exactly at predicted values in field-cooled experiments, they are displaced in zero-field cooled, magnetic-field ramped experime…
We use 3D numerical simulations to explore the phase diagram of driven flux line lattices in presence of weak random columnar disorder at finite temperature and high driving force. We show that the moving Bose glass phase exists in a large range of temperature, up to its melting into a moving vortex liquid. It is also remarkably stable upon increasing velocity : the dynamical transition to the correlated moving glass expected at a critical velocity is not found at any velocity accessible to our simulations. Furthermore, we show the existence of an effective static tin roof pinning potential in…
The magnetoresistivity and critical current density of well characterized Si-nanoparticle doped and undoped Cu-sheathed MgB$_{2}$ tapes have been measured at temperatures $T\geq 28$ K in magnetic fields $B\leq 0.9$ T. The irreversibility line $B_{irr}(T)$ for doped tape shows a stepwise variation with a kink around 0.3 T. Such $B_{irr}(T)$ variation is typical for high-temperature superconductors with columnar defects (a kink occurs near the matching field $% B_ϕ$) and is very different from a smooth $B_{irr}(T)$ variation in undoped MgB$_{2}$ samples. The microstructure studies of nanoparticl…
As a model of thermally excited flux liquids connected by a weak link, we study the effect of a single line defect on vortex filaments oriented parallel to the surface of a thin planar superconductor. This problem can be mapped onto the physics of a Luttinger liquid of interacting bosons in 1 spatial dimension with a point impurity. When the applied magnetic field is tilted relative to the line defect, the corresponding quantum boson Hamiltonian is non-Hermitian. We analyze this problem using a combination of analytic and numerical (density matrix renormalization group) methods, uncovering a d…
We present simulations of flux-gradient-driven superconducting rigid vortices interacting with square and triangular arrays of columnar pinning sites in an increasing external magnetic field. These simulations allow us to quantitatively relate spatio-temporal microscopic information of the vortex lattice with typically measured macroscopic quantities, such as the magnetization $M(H)$. The flux lattice does not become completely commensurate with the pinning sites throughout the sample at the magnetization matching peaks, but forms a commensurate lattice in a region close to the edge of the sam…
We study, by means of three-dimensional Monte Carlo simulations, the current-voltage (IV) characteristics and the voltage noise spectrum at low temperatures of driven magnetic flux lines interacting with randomly placed point or columnar defects, as well as with periodically arranged linear pinning centers. Near the depinning current J_c, the voltage noise spectrum S(w) universally follows a 1/w^a power law. For currents J > J_c, distinct peaks appear in S(w) which are considerably more pronounced for extended as compared to point defects, and reflect the spatial distribution of the correlated…
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）
The boson Hubbard model has been extensively studied as a model of the zero temperature superfluid/insulator transition in Helium-4 on periodic substrates. It can also serve as a model for vortex lines in superconductors with a magnetic field parallel to a periodic array of columnar pins, due to a formal analogy between the vortex lines and the statistical mechanics of quantum bosons. When the magnetic field has a component perpendicular to the pins, this analogy yields a non-Hermitian boson Hubbard model. At integer filling, we find that for small transverse fields, the insulating phase is pr…
This paper reports the nanoscale texture and microstructure of a high-performance NdFeAs(O,F) superconducting thin film grown by molecular beam epitaxy on a textured MgO/Y$_2$O$_3$/Hastelloy substrate. The NdFeAs(O,F) film forms a highly textured columnar grain structure by epitaxial growth on the MgO template. Although the film contains stacking faults along the $ab$-plane as well as grain boundaries perpendicular to the $ab$-plane, good superconducting properties are measured: a critical temperature, $T _{\rm c}$, of 46 K and a self-field critical current density, $J_{\rm c}$, of $2 \times 1…
We present simulations of flux-gradient-driven superconducting vortices interacting with strong columnar pinning defects as an external field $H(t)$ is quasi-statically swept from zero through a matching field $B_ϕ$. We analyze several measurable quantities, including the local flux density $ B(x,y,H(t))$, magnetization $M(H(t))$, critical current $J_{c}(B(t))$, and the individual vortex flow paths. We find a significant change in the behavior of these quantities as the local flux density crosses $B_ϕ$, and quantify it for many microscopic pinning parameters. Further, we find that for a given …
The most effective way to enhance the dissipation-free supercurrent in presence of magnetic field for type II superconductors is the introduction of defects that acts as artificial pinning centres (APCs) for the vortices. For instance, the in-field critical current density of doped BaFe2As2 (Ba122), one of the most technologically important Fe-based superconductors, has been improved over the last decade by APCs created by ion-irradiation. The technique of ion-irradiation has been commonly implemented to determine the ultimate superconducting properties. However, this method is rather complica…
The equilibrium properties of flux lines pinned by columnar disorder are studied, using the analogy with the time evolution of a diffusing scalar density in a randomly amplifying medium. Near H_{c1}, the physical features of the vortices in the localized phase are shown to be determined by the density of states near the band edge. As a result, H_{c1} is inversely proportional to the logarithm of the sample size, and the screening length of the perpendicular magnetic field decreases with temperature. For large tilt the extended ground state turns out to wander in the plane perpendicular to the …
We study the effects of rapid temperature and magnetic field changes on the non-equilibrium relaxation dynamics of magnetic vortex lines in disordered type-II superconductors by employing an elastic line model and performing Langevin molecular dynamics simulations. In a previously equilibrated system, either the temperature is suddenly changed, or the magnetic field is instantaneously altered which is reflected in adding or removing flux lines to or from the system. The subsequent aging properties are investigated in samples with either randomly distributed point-like or extended columnar defe…
