Glassy Exponent Research Articles

Strong pinning and slow flux creep relaxation in Co-doped CaFe2As2 single crystals

We report on measurements of critical current densities Jc and flux creep rates S of freestanding Ca(Fe1−xCox)2As2 (x ≈ 0.033) single crystals with Tc ≈ 15.7 K by performing magnetization measurements. The magnetic field dependences of Jc at low temperature display features related to strong pinning. In addition, we find that the system displays small flux creep rates. The characteristic glassy exponent, μ, and the pinning energy, U0, display exceptional high values for pristine crystals. We find that for magnetic fields between 0.3 T and 1 T, μ decreases from ≈ 2.8 to ≈ 2 and U0 remains ≈ 300 K. Analysis of the pinning force indicates that the mechanism is similar to the observed in polycrystalline systems in which grain boundaries and random disorder produce the vortex pinning. Considering the large U0 observed in the single crystal, we attempt to improve the pinning by adding random point disorder by 3 MeV proton irradiation with a fluence of 2 × 1016 proton/cm2. The results show that, unlike other iron-based superconductors, the superconducting fraction is sharply reduced by irradiation. This fact indicates that the superconductivity in the system is extremely fragile to an increment in the disorder. The superconducting volume fraction in the irradiated crystal systematically recovers after removal disorder by thermal annealing, which evidences as to the observation of critical state in curves of magnetization versus magnetic field. No features related to a reentrant antiferromagnetic transition are observed for the irradiated sample.

Collective vortex pinning and merging of the irreversibility line and second peak effect in optimally doped Ba[formula omitted]KxBiO3 single crystals

Measurements on magnetization and relaxation have been carried out on an optimally doped Ba1−xKxBiO3+δ single crystal with Tc = 31.3 K. Detailed analysis is undertaken on the data. Both the dynamical relaxation and conventional relaxation have been measured leading to the self-consistent determination of the magnetization relaxation rate. It is found that the data are well described by the collective pinning model leading to the glassy exponent of about μ ≈ 1.64–1.68 with the magnetic fields of 1 and 3 T. The analysis based on Maley’s method combining with the conventional relaxation data allows us to determine the current dependent activation energy U which yields a μ value of about 1.23–1.29 for the magnetic fields of 1 and 3 T. The second magnetization peaks appear in wide temperature region from 2 K to 24 K. The separation between the second peak field and the irreversibility field becomes narrow when temperature is increased. When the two fields are close to each other, we find that the second peak evolves into a step-like transition of magnetization. Finally, we present a vortex phase diagram and demonstrate that the vortex dynamics in Ba1−xKxBiO3 can be used as a model system for studying the collective vortex pining.

Magnetization relaxation, critical current density, and vortex dynamics in aBa0.66K0.32BiO3+δsingle crystal

We have conducted extensive investigations on the magnetization and its dynamical relaxation on a Ba$_{0.66}$K$_{0.32}$BiO$_{3+\delta}$ single crystal. It is found that the magnetization relaxation rate is rather weak compared with that in the cuprate superconductors, indicating a higher collective vortex pinning potential (or activation energy), although the intrinsic pinning potential $U_\mathrm{c}$ is weaker. Detailed analysis leads to the following discoveries: (1) A second-peak effect on the magnetization-hysteresis-loop was observed in a very wide temperature region, ranging from 2K to 24K. Its general behavior looks like that in YBa$_2$Cu$_3$O$_7$; (2) Associated with the second peak effect, the magnetization relaxation rate is inversely related to the transient superconducting current density $J_\mathrm{s}$ revealing a quite general and similar mechanism for the second peak effect in many high temperature superconductors; (3) A detailed analysis based on the collective creep model reveals a large glassy exponent $\mu$ and a small intrinsic pinning potential $U_\mathrm{c}$; (4) Investigation on the volume pinning force density shows that the data can be scaled to the formula $F_{p}\propto b^p(1-b)^q$ with $p=2.79$ and $q=3.14$, here $b$ is the reduced magnetic field to the irreversible magnetic field. The maximum normalized pinning force density appears near $b\approx0.47$. Finally, a vortex phase diagram is drawn for showing the phase transitions or crossovers between different vortex phases.

Magnetic relaxation and collective vortex creep in FeTe0.6Se0.4 single crystal

We study the vortex dynamics in a high-quality FeTe0.6Se0.4 single crystal by performing magnetization measurements of the screening current density Js and flux creep rate S. The temperature dependence of S shows a plateau in the intermediate-temperature region with a high creep rate ∼ 0.03, which is interpreted in the framework of the collective creep theory. A crossover from elastic to plastic creep is observed. The glassy exponent and barrier height for the flux creep are directly determined by the extended Maley's method. Js with flux creep, obtained from magnetic hysteresis loops, is successfully reproduced based on the collective creep analysis. We also approach the critical current density without flux creep by means of the generalized inversion scheme, which proves that the and pinning coexist in the FeTe0.6Se0.4 single crystal.

Enhancement of critical current density and vortex activation energy in proton-irradiated Co-dopedBaFe2As2

The effect of proton irradiation in Ba(Fe0.93Co0.07)2As2 single crystals is reported. We analyze temperature dependence of current density and normalized flux relaxation rate in the framework of collective creep model. Glassy exponent and barrier height for flux creep are directly determined by Maley's method. Our model functions for barrier height and critical current density in the absence of flux creep are explained by the superposition of \deltaTc- and \deltal-pinning. We also approach true critical current density by means of generalized inversion scheme, and the obtained result is in reasonable agreement with our model function. Proton irradiation effect on temperature dependence of current density and normalized relaxation rate can be summarized as doubling of barrier height at the beginning of flux creep.

Strong pinning and elastic to plastic vortex crossover in Na-doped CaFe2As2single crystals

We study the vortex dynamics of NaxCa1-xFe2As2 single crystals with x = 0.5 (underdoped) and x = 0.75 (optimally doped), having T-c approximate to 19.4 and 33.4 K, respectively, by performing magnetization measurements of the critical current density J(c) and flux creep rate S. We find that the J(c) versus temperature, T, dependence is consistent with delta T-c pinning, indicating strong pinning associated with randomly distributed defects larger than the coherence length xi. The temperature dependence of S shows a crossover between glassy (elastic) and plastic creep regimes. The boundary T-cr(H) between both creep regimes coincides with the upper limit of the regime of strong pinning by nanoparticles. The glassy exponent mu in the optimally doped crystal is consistent with the thermal collective creep theory previously applied to cuprate superconductors, but in the underdoped sample the plateau in S(T) indicates that mu approximate to 3-3.3, a value larger than the existing theoretical predictions. We discuss the quantum creep contributions in both samples.

Vortex glass state in superconductors with fractal clusters of normal phase

The influence of fractal clusters of a normal phase on the dynamics of vortices in a percolation superconductor is considered. It is established that a vortex glass state is formed in superconductors with a fractal cluster structure. The vortex glass phase can be identified by the initial part of its current-voltage characteristic. The dependence of the glassy exponent on the fractal dimension of normal phase cluster boundaries is found.

Determination of the Glassy Exponent of Melt Textured YBa2Cu3O7−x by Magnetic Relaxation

By magnetic relaxation method we obtained the glassy exponent μ (T, H) of the melt textured Yba2Cu3O7−x sample. The results have a similar behavior compared with ones using the method of magnetic field sweep, and are compatible with the model of collective pinning theory.

Comparative study of theI−Vscaling in highly anisotropicBi2Sr2CaCu2O8+δand isotropic(K,Ba)BiO3heavy-ion-irradiated single crystals

The Bose-glass transition has been investigated by transport measurements on heavy-ion irradiated ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ and isotropic $(\mathrm{K},\mathrm{B}\mathrm{a}){\mathrm{BiO}}_{3}$ single crystals. When a magnetic field was applied parallel to the defects, we show that, for ${B}_{\ensuremath{\varphi}}/10l{\ensuremath{\mu}}_{0}Hl{B}_{\ensuremath{\varphi}},$ the same scaling functions could be used to describe the transition in these two systems emphasizing the universality of this transition. The field and sample-independent critical exponents which were found to be ${\mathbf{\ensuremath{\nu}}}_{\ensuremath{\perp}}=1.1\ifmmode\pm\else\textpm\fi{}0.1,$ $z=5.3\ifmmode\pm\else\textpm\fi{}0.2,$ and $\ensuremath{\alpha}(\ensuremath{\equiv}{\ensuremath{\nu}}_{\ensuremath{\parallel}}/{\mathbf{\ensuremath{\nu}}}_{\ensuremath{\perp}})=2,$ are consistent with Monte Carlo simulations with screened vortex interactions and columnar defects. In addition, we found that in both systems, the creep process in the Bose glass phase could be described by the variable-range hopping mechanism of flux lines with the glassy exponent value of $\ensuremath{\mu}=1/3$ which is close to the transition and the creep exponent $\ensuremath{\mu}\ensuremath{\approx}1$ at lower temperatures suggests that creep is then dominated by a generation of half loops expanding along the track direction.

The vortex soliton and its quantum tunneling in anisotropic superconductors in intermediate magnetic fields

Exact explicit soliton solutions are derived for two-coupled non-linear equations of motion relevant to vortex dynamics in anisotropic superconductors. The crossover temperature from quantum to classical behavior and Bose-glass exponent are expressed as a function of induction, but not as an explicit function of angle. The exponent has maximum value at accommodation field, separating a single-vortex regime from the collective pinning regime. The quantum magnetization relaxation rate is expressed in terms of pinning energy, angle, anisotropy, coupling constant, dissipation and glassy exponent. The results are qualitatively consistent with the experimental observations.

Dynamic regimes in the ac response ofYBa2Cu3O7with columnar defects: Intra- and inter-valley vortex motion

We use ac susceptibility measurements to explore the dynamic response of vortices in YBCO single crystals pinned by aligned columnar defects at low dc fields. Using a combination of techniques for the analysis of the data, we investigate the vortex motion as a function of the amplitude and frequency of the applied ac field hac , identifying the influence of both intra- and inter-valley motion. We build up a dynamic diagram in the hac-T plane indicating the crossover lines among several regimes in the solid phase. At low hac , a linear response with very low dissipation arises from the oscillation of pinned vortices inside the tracks ~Campbell regime! .A t high hac a critical state develops. The large influence of thermal fluctuations in this limit is indicated by the frequency dependence, which allows us to obtain the current density dependence of the activation energy and the glassy exponent. We also discuss the origin of the large nonlinear transition regime observed at intermediate amplitudes. @S0163-1829~99!02613-2#

Dependence of the Glassy Exponent on Field and Temperature in Textured YBCO Specimen

The glassy exponent Μ of a melt-textured YBa2Cu3O7-x sample is derived from measurements of the magnetic hysteresis loops with various applied field sweep rates and at different temperatures and magnetic fields. Thus, the dependences of the exponent Μ on temperature and field are obtained. Furthermore, we report for the first time the crossover-line of the vortex creep regime from a single vortex to a small bundle.

Glassy exponent of melt textured YBCO extracted from magnetic hysteresis loops with various field sweep rates

The glassy exponent μ of the melt-textured YBCO sample is extracted from the magnetic hysteresis loops with the various applied field sweep rates at the different temperatures. Our results show that the field and temperature dependence of μ is consistent with the predictions of the collective pinning theory.

Superfast Vortex Creep inYBa2Cu3O7−δCrystals with Columnar Defects: Evidence for Variable-Range Vortex Hopping

We observe a large peak in the vortex creep rate of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ crystals with columnar defects. The peak appears at the crossover from the regime where vortices are well localized on columnar pins to a regime where vortex-vortex interactions dominate, and it is most prominent (nearly $12%$) at low fields, near $T\ensuremath{\sim}40\mathrm{K}$. Near this crossover, the current dependent creep activation energy $U(J)$ shows distinct and unusual behavior (``pinch''). On the high- $J$ side of the pinch, the glassy exponent $\ensuremath{\mu}\ensuremath{\sim}1/3$ and large creep rate are characteristic of variable range vortex hopping.