Relaxation dynamics in type-II superconductors with point-like and correlated disorder

We employ an elastic line model to investigate the steady-state properties and non-equilibrium relaxation kinetics of magnetic vortex lines in disordered type-II superconductors using Langevin molecular dynamics (LMD). We extract the dependence of the mean vortex line velocity and gyration radius as well as the mean-square displacement in the steady state on the driving current, and measure the vortex density and height autocorrelations in the aging regime. We study samples with either randomly distributed point-like or columnar attractive pinning centers, which allows us to distinguish the complex relaxation features of interacting flux lines subject to extended vs. uncorrelated disorder. Additionally, we find that our new LMD findings match earlier Monte Carlo (MC) simulation data well, verifying that these two microscopically quite distinct simulation methods lead to macroscopically very similar results for non-equilibrium vortex matter.     

Effects of kinked linear defects on planar flux line arrays

In the hard core limit, interacting vortices in planar type II superconductors can be modeled as non-interacting one dimensional fermions propagating in imaginary time. We use this analogy to derive analytical expressions for the probability density and imaginary current of vortex lines interacting with an isolated bent line defect and to understand the pinning properties of such systems. When there is an abrupt change of the direction of the pinning defect, we find a sinusoidal modulation of the vortex density in directions both parallel and perpendicular to the defect.     

Effects of crossed columnar defects on vortex pinning in Bi2Sr2CaCu2O8

To investigate the existence of a splay effect in Bi₂Sr₂CaCu₂O₈ (Bi-2212), vortex pinning has been studied in different configurations of strongly inclined columnar defects (75 from the c axis), installed by heavy-ion irradiation. It is shown that the symmetry of the track setting with respect to the field direction is a more influent parameter than the presence of a dispersion in the track directions. We claim that the enhanced pinning efficiency which is observed in some splayed configurations of columnar defects in Bi-2212 can be interpreted without invoking a splay effect. Received 27 December 1999     

Vortex pinning in superconducting Nb thin films deposited on nanoporous alumina templates

We present a study of magnetization and transport properties of superconducting Nb thin films deposited on nanoporous aluminium oxide templates. Periodic oscillations in the critical temperature vs. field, matching effects in fields up to 700 mT and strongly enhanced critical currents were observed. These fields are considerably higher than those typical for periodic pinning arrays made by lithographic techniques, which reflects the benefits of nanostructuring superconductors by using self-organized growth. This method provides a periodic pinning potential with sub-100 nm spacing between the pinning centers, which enhances vortex pinning in broad field and temperature ranges.     

A possible role of amorphous regions at grain-boundary vertex points as linear defects on flux pinning in MgB2 films with columnar grain structure

We measured the transport properties of MgB₂ films having columnar grain structure with their axis normal to the substrate. When an external magnetic field was applied parallel to the grain axis, an enhanced critical current density has been observed, and this result has been ascribed to flux pinning induced by grain boundaries. The shape of the angular dependence of critical current density and its magnetic field dependence showed a quite similar resemblance to those of YBa₂Cu₃O_x films containing columnar defects, implying a possible existence of linear defects in MgB₂ films of columnar structure. We propose that the amorphous regions at the vertex points of three or more grain boundaries observed in microstructural studies correspond to the linear defects and these linear defects anchor the end points of the flux line dislocations of Frank-Read sources, by which the shear in the flux line lattice is actuated. This assumed mechanism is found to reasonably explain the magnetic field dependence of the flux pinning force density of MgB₂ films with columnar grain structure.     

Scanning tunneling microscopy study of pinning-induced vortex lattice distortion in ion-irradiated NbSe2

We observe vortex pinning in 2.2 GeV Au-ion irradiated NbSe₂ by scanning tunneling microscopy (STM) at 3K. The ion irradiation generates columnar defects which act as pinning sites. At various external magnetic fields the vortex arrangement is clearly resolved but shows strong distortion. The location of individual defects is extracted from STM data and compared to the vortex arrangement.     

Sudden critical current drops induced in S/F structures

In the search for new physical properties of S/F structures, we have found that the superconductor critical current can be controlled by the domain state of the neighboring ferromagnet. The superconductor is a thin wire of thickness d_s ≈2ξ_S. Nb/Co and Nb/Py (Permalloy Ni₈₀Fe₂₀) bilayer structures were grown with a significant magnetic anisotropy. Critical current measurements of Nb/Co structures with ferromagnet thickness d_F > 30 nm show sudden drops in two very defined steps when the measurements are made along the hard axes direction (i.e. current track parallel to hard anisotropy axes direction). These drops disappear when they are made along the easy axis direction or when the ferromagnet thickness is below 30 nm. The drops are accompanied by vortex flux flow. In addition magnetorestistance measurements close to T_C show a sharp increase near saturation fields of the ferromagnet. Similar results are reproduced in Nb/Py bilayer structure with the ferromagnet thickness d_F ~ 50 nm along the easy anisotropy axes. These results are explained as being due to spontaneous vortex formation and flow induced by Bloch domain walls of the ferromagnet underneath. We argue these Bloch domain walls produce a 2D vortex-antivortex lattice structure.     

Influence of non-uniform J<Subscript>c</Subscript> distributions on flux jumps in high-temperature superconductors

The influence of non-uniform J_c distributions on flux jumps in high-temperature superconductors is investigated with the simplified models in the form of a composite superconducting slab consisting of different pinning regions. The magnetization loops and flux jumps for the simplified models are calculated with the modified recursion formulas which could predict the flux-jump fields. The valid ranges of the temperature and sweep rate for B_fj1 are specified and the fields of the subsequent jumps after the first flux jump of the inhomogeneous models are obviously lower than that of the homogeneous models. The results indicate that flux jump instability could be stimulated by non-uniform J_c distributions and with the increase of the discrepancy of J_c between different pinning regions the instability also increase.     

Weak to strong pinning crossover

Material defects in hard type II superconductors pin the flux lines and thus establish the dissipation-free current transport in the presence of a finite magnetic field. Depending on the density and pinning force of the defects and the vortex density, pinning is either weak collective or strong. We analyze the weak to strong pinning crossover of vortex matter in disordered superconductors and discuss the peak effect appearing naturally in this context.     

磁通密度对第Ⅱ类超导体磁通动力学的影响

计算了二维无序钉扎系统中磁通运动的平均速度、微分电阻、纵向电压噪声和静态结构因子.通过在不同磁通密度下的磁通运动形式,给出了磁通运动的动力学相图.研究表明,磁通晶格存在钉扎相、塑性流相、近晶流相,和运动玻璃相.在运动玻璃相中,随着驱动力的进一步增加,横向玻璃态和运动Bragg玻璃态相继出现.磁通密度增大有利于有序相的出现.当磁通密度增大到一定程度时,近晶流动相会消失.磁通运动随着外加驱动电流增大发生从塑性流动相到运动玻璃相的转变 关键词： Ⅱ类超导体 磁通动力学 运动玻璃     

Ginzburg-Landau simulation for a vortex around a columnar defect in a superconducting film

By means of numerical simulations based on Ginzburg-Landau theory, we study the vortex depinning from a columnar defect in a superconducting film. We evaluate the limiting thickness of the film, below which the depinning does not occur even under an application of the magnetic field perpendicular to the columnar defect. The limiting thickness is a measure of the pinning strength of the columnar defect. The dependence of this limiting thickness on the magnitude of the applied field is obtained for two types of columnar defects.     

Transport of magnetic vortices by surface acoustic waves

In a thin film of superconducting Y Ba₂Cu₃O₇ the impact of surface acoustic waves (SAWs) traveling on the piezoelectric substrate is investigated. A pronounced interaction between the ultrasonic waves and the vortex system in the type II superconductor is observed. The occurrence of a SAW-induced dc voltage perpendicular to the sound path is interpreted as dragging of vortices by the piezoacoustic SAW, which acts as a conveyor for the flux quanta. The antisymmetry of this voltage with respect to the magnetic field directly evidences the induced, directed flux motion. This dynamic manipulation of vortices can be seen as an important step towards flux-based electronic devices.     

Anisotropic pinning in Nb thin films with triangular pinning arrays

A superconducting thin film with regular triangular arrays has been fabricated to explore the anisotropic vortex pinning behavior. We found that the critical currents for these films depend strongly on the current directions. Some interesting temperature-dependent phenomena are observed and discussed. We also made molecular dynamic simulations to study the dynamics of the vortex motion. The simulation results confirm the anisotropic features we found in the experiments.     

Angular dependence of vortex matter phase transition in MgB2 single crystal

The vortex matter phase transitions and intrinsic pinning effect were investigated in an MgB₂ single crystal using the torque magnetometry. For the field directions apart from the ab plane, we succeed in the observation of the vortex lattice melting transitions, which are transformed from the order-disorder transitions at low temperatures. Both transition fields with field directions can be describe by the GL effective mass model. For the field direction along the ab plane, these transitions become unobservable. Instead, the sudden increase in the hysteresis of magnetization curve occurs, indicating the existence of the intrinsic pinning coming from the layer structure.     

200 MeV Ag ion irradiation created columnar defects and enhanced critical current density of La-2125 type superconducting thin films

We have deposited c-axis oriented thin films of La_1.5Dy_0.5CaBa₂Cu₅O_z (La-2125) tetragonal superconductor on LaAlO₃(001) substrates 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 through the entire thickness (2000 Å) of these films. With ion irradiation up to 1×10¹¹ ions/cm², the critical current density (J_c(H)) enhances by fivefold, which is attributed to the augmented flux pinning by the columnar defects. A further increase in irradiation to 1×10¹² ions/cm² causes reduction in J_c(H) due to distorted morphology of the film. Our work shows that the enhancement in J_c(H) of the irradiated La-2125 film is comparable to that in irradiated RE-123 (RE = rare earth ion). Also, as the La-2125 type films have greater chemical stability than RE-123, La-2125 type superconductors are potential candidates for applications. It is interesting to note that there are partial flux jumps observed to occur symmetrically in the magnetic hysteresis of irradiated La-2125 thin films with enhanced J_c(H).     

Electrodynamics of the vortex lattice in untwinned YBaCuO by complex impedance measurements

We report complex impedance measurements in an untwinned YBaCuO crystal. Our broad frequency range covers both the quasi static response and the resistive response of the vortex lattice. It allow us to characterize the irreversibility line without the need of any frequency dependent pinning parameters. We confirm the validity of the two modes model of vortex dynamic, and extract both the surface critical current and the flux flow resistivity around the first order transition T_m. This latter is identified by the abrupt loss of pinning and by an unexpected step of (T) at T_m. Received 22 November 2002 / Received in final form 17 February 2003 Published online 20 June 2003 RID="a" ID="a"e-mail: alain.pautrat@ismra.fr RID="b" ID="b"UMR 6508 associée au CNRS     

Effect of grain-boundary flux pinning in MgB2 with columnar structure

We studied the flux pinning properties by grain boundaries in MgB₂ films prepared by using a hybrid physical chemical vapor deposition method on the c-axis oriented sapphire substrates. All the films we report here had the columnar grains with the growth direction perpendicular to the substrates and the grain sizes in the range of a few hundred nanometers. At very low magnetic fields, no discernable grain-boundary (GB) pinning effect was observed in all measuring temperatures, but above those fields, the effect of GB flux pinning was observed as enhanced critical current densities (J_cs) and reduced resistances when an external magnetic field (B) was aligned parallel to the c-axis. We interpret the B dependence of J_c in the terms of flux line lattice shear inside the columnar grains activated by dislocations of Frank–Read source while the flux lines pinned by GB act as anchors for dislocations. Magnetic field dependence of flux pinning force density for B parallel to the c-axis was reasonably explained by the above model.     

第Ⅱ类超导体中磁通运动的低频电压噪声研究

用有限温度下的分子动力学方法模拟二维无序钉扎磁通系统的低频宽带电压噪声.计算了磁通运动的电压噪声谱密度，研究了宽带噪声（BBN）随驱动电流、钉扎强度和温度的变化规律.BBN随钉扎强度的增加而增大，反映了BBN是磁通运动受体钉扎阻碍而产生的内部耗散. BBN随温度的升高而减小，表明热运动部分抵消了体钉扎以及磁通之间相互作用，软化了磁通线格子，使磁通运动BBN减小.以上结论与实验相符，并能解释磁通运动的微观图像. 关键词： 第Ⅱ类超导体 电压噪声 动力学模拟     

Flux trapping in a macroscopic cylinderical hole drilled in Bi-Sr-Ca-Cu-O

The flux dynamics in a polycrystalline sample of Bi_1.7Pb_0.3Sr₂Ca₂Cu₃O_x with a macroscopic cylindrical hole (CH) drilled was investigated by slow transport relaxation (V-t curves) and magnetovoltage measurements (V-H curves). It was monitored that there are several discontinuities in the time evolution of quenched state in V-t curves, which was attributed to the leaving of quantized flux lines trapped through CH together with surface superconducting effects. We observed that asymmetric V-H curves demonstrate unusual remarkable counter clockwise hysteresis effects upon cycling of field. This interesting result was correlated mainly to the flux trapping inside the CH that acts as a macroscopic attractive pinning center for flux lines. Further, the hysteresis effects in V-H curves for a fixed transport current provide a direct evidence that the number of flux lines, measured dissipation and relative decrease/increase in irreversibilities could be determined by sweeping rate of external magnetic field (dH/dt) which leads also to peculiar time effects.