Pinned Vortex Liquid Research Articles

Low-energy (40 keV) proton irradiation of YBa2Cu3O7−x thin films: Micro-Raman characterization and electrical transport properties**Project supported by the National Natural Science Foundation of China (Grant No. 61473023), the Aerospace Science and Technology Innovation Fund, CASC and International S & T Cooperation Program of China (ISTCP) (Grant No. 2015DFR80190).

To investigate the damage profiles of high-fluence low-energy proton irradiation on superconducting materials and related devices, Raman characterization and electrical transport measurement of 40-keV-proton irradiated YBa2Cu3O7−x (YBCO) thin films are carried out. From micro-Raman spectroscopy and x-ray diffraction studies, the main component of proton-radiation-induced defects is found to be the partial transition of superconducting orthorhombic phase to the semiconducting tetragonal phase and non-superconducting secondary phase. The results indicate that the defects induced in the conducting CuO2 planes, such as increased oxygen vacancies and interstitials, can result in an increase in the resistivity but a decrease in the transition temperature TC with the increase in the fluence of proton irradiation, which is confirmed in the electrical transport measurements. Especially, zero-resistance temperature TCO is not observed at a fluence of 1015 p/cm2. Furthermore, the variation of activation energy U0 can be explained by the plastic-flux creep theory, which indicates that the plastic deformation and entanglement of vortices in a weakly pinned vortex liquid are caused by disorders of point-like defects. Point-like disorders are demonstrated to be the main contribution to the low-energy proton radiation damage in YBCO thin films. These disorders are likely to cause flux creep by thermally assisted flux flow, which may increase noise and reduce the precision of superconducting devices.

Phase diagram of randomly pinned vortex matter in layered superconductors: Dependence on the details of the point pinning

We study the thermodynamic and structural properties of the superconducting vortex system in high-temperature layered superconductors, with magnetic field normal to the layers, in the presence of a small concentration of strong random point pinning defects via numerical minimization of a model free-energy functional in terms of the time-averaged local density of pancake vortices. Working at constant magnetic induction and point pinning center concentration, we find that the equilibrium phase at low temperature $(T)$ and small pinning strength $(s)$ is a topologically ordered Bragg glass. As $T$ or $s$ is increased, the Bragg glass undergoes a first-order transition to a disordered phase which we characterize as a ``vortex slush'' with polycrystalline structure within the layers and interlayer correlations extending to about 20 layers. This is in contrast with the pinned vortex liquid phase into which the Bragg glass was found to melt, using the same methods, in the case of a large concentration of weak pinning centers: That phase was amorphous with very little interlayer correlation. The value of the second moment of the random pinning potential at which the Bragg glass melts for a fixed temperature is very different in the two systems. These results imply that the effects of random point pinning cannot be described only in terms of the second moment of the pinning potential and that some of the unresolved contradictions in the literature concerning the nature of the low $T$ and high $s$ phase in this system are likely to arise from differences in the nature of the pinning in different samples, or from assumptions made about the pinning potential.

Phase diagram of vortex matter in layered high-temperature superconductors with random point pinning

We study the phase diagram of the superconducting vortex system in layered high-temperature superconductors in the presence of a magnetic field perpendicular to the layers and of random atomic scale point pinning centers. We consider the highly anisotropic limit where the pancake vortices on different layer are coupled only by their electromagnetic interaction. The free energy of the vortex system is then represented as a Ramakrishnan-Yussouff free energy functional of the time averaged vortex density. We numerically minimize this functional and examine the properties of the resulting phases. We find that, in the temperature ($T$) -- pinning strength ($s$) plane at constant magnetic induction, the equilibrium phase at low $T$ and $s$ is a Bragg glass. As one increases $s$ or $T$ a first order phase transition occurs to another phase that we characterize as a pinned vortex liquid. The weakly pinned vortex liquid obtained for high $T$ and small $s$ smoothly crosses over to the strongly pinned vortex liquid as $T$ is decreased or $s$ increased -- we do not find evidence for the existence, in thermodynamic equilibrium, of a distinct vortex glass phase in the range of pinning parameters considered here. %cdr We present results for the density correlation functions, the density and defect distributions, and the local field distribution accessible via $\mu$SR experiments. These results are compared with those of existing theoretical, numerical and experimental studies.

Flux pinning in ternary (Nd–Eu–Gd)Ba2Cu3O7−δ: Optimization for the highest pinning

The optimization of high-quality melt-processed ternary (Nd–Eu–Gd)Ba2Cu3O7−δ composite samples has been carried out to achieve the largest pinning. Our results suggest that the interaction between the twin planes and the fine second-phase precipitate forms a strongly pinned entangled vortex liquid in which vortices are correlated along the c axis. This pinned vortex liquid freezes into a glassy state at low temperature rendering high critical current density, Jc. The maximum Jc can be achieved in as-grown optimally annealed twinned sample with second phase precipitate without disturbing the twin influence.

Vortex liquid properties in optimally doped and oxygen-deficient YBa 2Cu 3O 7− δ single crystals

The electrical anisotropy, γ=( m c / m ab ) 1/2, can easily be varied by changing the oxygen content in single crystals of YBa 2Cu 3O 7− δ (YBCO), thus making this system useful as a test system for vortex dynamics in high-temperature superconductors (HTSC). Here, some recent results are reviewed. Measurements of the c-axis resistivity with the magnetic field B∥ ab-plane is shown to be a powerful tool in studying vortex velocity correlations both parallel and perpendicular to the applied field. In optimally doped crystals, we find evidence that the in-plane vortex lattice melts at T m into a disentangled vortex liquid, which looses its correlation along the c-axis at a higher temperature T br> T m. For weakly disordered crystals and magnetic fields B∥ c-axis, a relation between the vortex glass line and the magnetic field dependence of the characteristic energy, U*, in the pinned vortex liquid is proposed. This approach is shown to give support to our idea of describing the vortex glass resistivity in the liquid regime by a modified glass correlation length, ξ g= ξ 0| k B T/ U 0−1| − ν , which depends on the average pinning energy, U 0, in the system [A. Rydh, Ö. Rapp, M. Andersson, Phys. Rev. Lett. 83 (1999) 1850].

Critical current density and flux pinning in vortex liquid regime for YBa 2Cu 3O 7− δ epitaxial thin films

The critical current density J c for epitaxial YBa 2Cu 3O 7− δ thin films as a function of applied magnetic field, temperature and angle between the magnetic field and the ab plane has been measured in detail. We observed an evidence for the dramatic change of the critical current density J c above and below the vortex–glass transition line. In the vortex solid state, the field dependence of J c follows an exponential dependence was argued to be from the intrinsic pinning in layered structural superconductors. But in the pinned vortex–liquid state, the J c decreases more rapidly with increasing magnetic field. It was suggested that the nonlinear temperature dependence of J c( T) near T c was from the vortex–liquid pinning. The temperature dependence of the critical current density exhibited an exponential law, J c∝exp(− T/ T 0), in the pinned vortex liquid regime.

Pinned Vortex Liquid above the Critical Point of the First-Order Melting Transition: A Consequence of Pointlike Disorder

D. Lopez,1,* L. Krusin-Elbaum,1 H. Safar,2 E. Righi,3 F. de la Cruz,3 S. Grigera,3 C. Feild,1 W. K. Kwok,4 L. Paulius,5 and G. W. Crabtree4 1IBM T. J. Watson Research Center, Yorktown Heights, New York 10598 2University of Illinois, Chicago, Illinois 60607 3Centro Atomico Bariloche, Comision Nacional de Energia Atomica, S. C. de Bariloche, 8400 R.N., Argentina 4Materials Science and Science and Technology Center for Superconductivity, Argonne National Lab, Argonne, Illinois 60435 5Western Michigan University, Kalamazoo, Michigan 49008 (Received 7 July 1997)

The property of the vortex glass in the vicinity of the pinned vortex liquid for textured bulk YBa2Cu3O7

We have performed AC susceptibility measurement and shown the property of the vortex glass in the region near the pinned vortex liquid, where it is proposed that the velocity of flux diffusion, ν=ν0(j/jc)exp(-U/T), instead of ν=ν0exp(-U/T) in the region far away from the glass melting line for providing a gradual crossover to the viscous flux flow regime (ν ∝j, where j is current density). It is shown that the additional factor (j/j0) in the velocity is necessity to explain the experimental finding of the dependence of time scale t0 on the ac field amplitude.

Origin of the irreversibility line in thin YBa2Cu3O7- delta films with and without columnar defects.

We report on measurements of the angular dependence of the irreversibility temperature $T_{irr}(\theta) $ in $YBa_2Cu_3O_{7-\delta }$ thin films, defined by the onset of a third harmonic signal and measured by a miniature Hall probe. From the functional form of $T_{irr}(\theta)$ 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.