共查询到20条相似文献,搜索用时 15 毫秒
1.
L. Miu 《Physica C: Superconductivity and its Applications》2008,468(15-20):1254-1257
Analyses of standard current–voltage (I–V) characteristics of disordered high-temperature superconductors (HTS) indicate that the vortex phase at high magnetic fields H should be an elastic vortex glass, where the vortex pinning barriers diverge at low current densities, whereas dc magnetization relaxation measurements reveal the presence of nondiverging (plastic) pinning barriers in a wide H–T domain. We show that the different conclusions concerning the nature of the vortex phase at high H in disordered HTS seem to be due to the ordering effect of the driving force existing in various experiments. 相似文献
2.
Slow Vortex Creep Induced by Strong Grain Boundary Pinning in Advanced Ba122 Superconducting Tapes
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We report the temperature, magnetic field and time dependences of magnetization in advanced Ba122 superconducting tapes. The sample exhibits peculiar vortex creep behavior. Below 10 K, the normalized magnetization relaxation rate S = d ln(-M)/d ln(t) shows a temperature-insensitive plateau with a value comparable to that of low-temperature superconductors, which can be explained within the framework of collective creep theory. It then enters into a second collective creep regime when the temperature increases. Interestingly, the relaxation rate below 20 K tends to reach saturation with increasing the field. However, it changes to a power law dependence on the field at a higher temperature. A vortex phase diagram composed of the collective and the plastic creep regions is shown. Benefiting from the strong grain boundary pinning, the advanced Bal22 superconducting tape has potential to be applied not only in liquid helium but also in liquid hydrogen or at temperatures accessible with cryocoolers. 相似文献
3.
H.A. Radovan H.H. Wen P. Ziemann 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,7(4):533-540
Using a specially designed SQUID magnetometer we measured the temperature dependence of the critical current density in a
ring patterned thin film for magnetic fields parallel to the c-axis. In addition, the temporal relaxation of the remanent state as prepared by field cooling in an external field of 100
Oe at different temperatures is determined. The j
c
(
T
) data show a field-dependent anomalous kink close to Tc pointing to reduced dissipation with increasing temperature allowing to construct a corresponding H-T borderline. A similar behavior is observed for the normalized relaxation rate S
(
T
) as extracted from the temporal behavior of the remanent state, which, at low temperatures, exhibits the expected increase
for increasing T-values, while an anomalous decrease of S
(
T
) is found for temperatures above 85 K. While the low-T regime is attributed to creep of 2D pinned single vortex lines, the high-T behavior is suggested to be dominated by collective motion with a more sluggish dynamics. This change in dynamics is also
reflected by the activation barriers for flux creep U
(
j
), which show a corresponding crossover in μ from 0.06 to 0.99. An additional scaling analysis of the E-j characteristics for according to vortex glass theory reveals quasi-2D collective creep behavior with .
Received: 8 April 1998 / Revised: 15 July 1998 / Accepted: 2 September 1998 相似文献
4.
R. Prozorov M.A. Tanatar E.C. Blomberg P. Prommapan R.T. Gordon N. Ni S.L. Bud’ko P.C. Canfield 《Physica C: Superconductivity and its Applications》2009,469(9-12):667-673
We discuss the irreversible magnetic properties of self-flux grown Ba(Fe1?xCox)2As2 single crystals for a wide range of concentrations covering the whole phase diagram from the underdoped to the overdoped regime, x = 0.038, 0.047, 0.058, 0.071, 0.074, 0.10, 0.106 and 0.118. Samples were characterized by a magneto-optical method and show excellent spatial uniformity of the superconducting state down to at least the micrometer scale. The in-plane properties are isotropic, as expected for the tetragonal symmetry, and the overall behavior closely follows classical Bean model of the critical state. The field-dependent magnetization exhibits second peak at a temperature and doping – dependent magnetic field, Hp(T, x). The evolution of this fishtail feature with doping is discussed. In particular we find that Hp, measured at the same reduced temperature for different x, is a unique monotonic function of the superconducting transition temperature, Tc(x), across all dopings. Magnetic relaxation is time-logarithmic and unusually fast. Similar to cuprates, there is an apparent crossover from collective elastic to plastic flux creep above Hp. At high fields, the field dependence of the relaxation rate becomes doping independent. We discuss our results in the framework of the weak collective pinning and show that vortex physics in iron-based pnictide crystals is much closer to high-Tc cuprates than to conventional s-wave (including MgB2) superconductors. 相似文献
5.
I. I. Sannikov A. A. Ivanov A. V. Kuznetsov A. P. Menushenkov O. A. Churkin 《Bulletin of the Lebedev Physics Institute》2014,41(8):215-217
In weak magnetic fields, precision measurements of the magnetic moment relaxation in thin epitaxial films YBa2Cu3O7?δ are performed. The exponents µ of the dependence of the vortex motion activation energy on the current are determined in a wide temperature range. Despite the low vortex density, the data obtained indicate the collective nature of their creep, which is probably caused by the strong interaction of vortices through the space surrounding the film. 相似文献
6.
In granular superconductors, the grain boundaries are closely related to the vortex dynamics and the macroscopic superconducting properties. In our research, Pb films with different grain sizes were prepared by tuning the substrate temperature. With the grain size decreasing, Pb films are prone to feature the anomalous magnetization jumps in the M − T curves, while in the M − H curves flux avalanche happens. Both phenomena appear in the same region of the H − T phase diagram and thus are considered to have the same origin. The further theoretical analysis shows that with grain size decreasing the pinning mechanism evolves from a mixed δTc and δl pinning to the δl pinning mechanism. The results shed light on the study of pinning mechanism for granular superconductors and is beneficial to the potential application of manipulating vortex pinning by regulation of intrinsic defects. 相似文献
7.
Critical current densities and magnetic relaxation have been measured for Bi2Sr2CaCu2Ox (Bi-2212) single crystals and melt-processed ceramic samples. Strongly nonlogarithmic decay of the magnetization is observed down to 10 K in Bi-2212 single crystals over a time range of more than 3 decades. Different decay behaviours are found in Bi-2212 single crystals, bulk ceramics and powdered ceramics. The experimental results are discussed within the pinning distribution model of Hagen and Griessen. A pinning distribution function yielding the power law decay of magnetization as an analytical solution is found. A careful analysis of the pinning energy distribution reveals high energy tails which can hardly be associated with single vortex pinning mechanisms. These high pinning energies can, however, be explained by reinterpreting the Hagen-Griessen model in terms of collective pinning theory. 相似文献
8.
The newly discovered superconductor MgB2 has a transition temperature Tc of about 40 K, which is touching the upper line of that predicted by the phonon-mediated Bardeen--Cooper--Schrieffer (BCS) theory. It is interesting to investigate the flux creep in MgB2 and compare it with other superconductors. We have measured the magnetization relaxation of MgB2 sintered at high temperature and high pressure. It is found that the quantum tunnelling and the thermally activated flux creep are very weak, implying a strong pinning in this material. 相似文献
9.
《Current Applied Physics》2002,2(2):113-116
Superparamagnetic properties of the assembly of carbon encapsulated Ni nanoparticles (Ni(C) nanoparticles) with an average particle size of 10.5 nm are studied, the blocking temperature (TB) is determined to around 115 K at l000 Gs applied field. Above TB, the magnetization M(H,T) can be described by the standard Langevin function L using the relation M/MS(T=0)=coth(μH/kT)−kT/μH. Magnetization measurements suggest, this assembly of carbon encapsulated Ni nanoparticles have been exhibited typical single-domain, field-dependent superparamagnetic relaxation properties. 相似文献
10.
The effect of structural inhomogeneities in a superconductor on a vortex medium flow in weak magnetic fields at temperatures varying from 78 to 83 K for various bias current densities is investigated by using transport measurements of Bi2Sr2CaCu2O8+x thin-film microbridges. The results obtained are analyzed on the basis of the theories of flux creep and the regular flow of vortices. It is shown that the current dependences of the effective potential for vortex pinning can be satisfactorily described in the framework of two statistical models, one of which was proposed earlier by the authors. Both models cover the regimes of thermally activated and regular flow of vortices as limiting cases. The wide transition region in which the creep and regular vortex flow processes simultaneously occur due to a large dispersion in the pinning energy distribution. It is found that when the magnetic field exceeds a certain value, the average value and dispersion of the pinning potential decrease sharply, so that the conditions of regular flow set in even for small values of the bias current. This fact is attributed to the destruction of vortex lines into two-dimensional segments. 相似文献
11.
Y. Tanabe T. Adachi H. Sato T. Sasaki Y. Koike 《Journal of Physics and Chemistry of Solids》2008,69(12):3217-3220
We have measured the temperature dependence of the magnetic susceptibility, χ vs. T, and the magnetization curve, M vs. H, for NbSe2 single crystals, in order to compare the superconducting (SC) state in the overdoped regime of La2−xSrxCuO4 (LSCO) with the SC state of the layered conventional superconductor NbSe2. While a plateau in χ vs. T in a moderate magnetic field and a so-called second peak in M vs. H, which is due to the marked enhancement of vortex pinning, have been observed in the overdoped regime of LSCO, these behaviors have not been observed in NbSe2. The present results indicate that the anomalously marked enhancement of vortex pinning is a characteristic feature in the overdoped LSCO where a microscopic phase separation into SC and normal-state regions takes place. 相似文献
12.
The temperature dependence of the magnetization reversal dynamics of the chiral molecular ferrimagnet [Mn{(R/S)-pn}]2[Mn{(R/S)-pn}2(H2O)][Cr(CN)6]2 has been studied at low frequencies of 1–1400 Hz, which are characteristic of the domain wall motion. It has been found from the Cole-Cole plots that domain walls undergo relaxation (at temperatures T > 10 K) and creep (at T < 10 K), and the main parameters determining these modes and the transition between them have been determined. It has been shown that the quantitative regularities of the transition between the modes of the domain wall motion correspond to the concepts of the competition between the contributions of two mechanisms to the domain wall retardation (the periodic Peierls relief and random structural defects). 相似文献
13.
《Magnetic resonance imaging》1995,13(3):379-385
Spin-lattice relaxation time T1 and relaxation parameters in magnetization transfer (MT) imaging were measured in 11 intracranial tumors before and after injection of Gd-DTPA at 0.1 T by using the inversion recovery method and the saturation transfer technique, respectively. Preinjection T1 relaxation times of the tumors were longer than those of white matter, but after Gd-enhancement the relaxation times of most tumors were in the same range as those of white matter. Gd-DTPA shortened the apparent relaxation time in the presence of off-resonance saturation pulse (T1α) due to marked shortening of the relaxation time of mobile water (T1w). Gd-DTPA decreased the magnetization transfer contrast (MTC) but did not influence on the magnetization transfer rate (Rwm). The parameters MTC and Rwm differed clearly between Gd-enhanced tumors and normal brain, whereas the relaxation time T1α was in many Gd-enhanced tumors in the same range as in normal brain. 相似文献
14.
Qing-Bao Ren Tong-Wei Lai Meng-Bo Luo 《Physica C: Superconductivity and its Applications》2011,471(13-14):385-388
The dynamics of a two-dimensional vortex system with strong periodic square columnar pins is investigated. For the case vortex number matching pinning number, we find that the vortex liquid is frozen into square lattice via a continuous transition, and the freezing (melting) temperature Tm is the same as the thermal depinning temperature of vortices, which are different from the first-order phase transition at weak pinning. The zero-temperature critical depinning force Fc0 is exactly the same as the maximum pinning force, and the depinning property at T = 0 can be expressed by scaling v ~ (F ? Fc0)β with the exponent β close to 0.5. The v–F curves at temperatures below Tm show that vortices are pinned at small driving force. 相似文献
15.
Gaia Grimaldi Antonio Leo Angela Nigro Sandro Pace 《Physica C: Superconductivity and its Applications》2010,470(19):911-913
We measure current–voltage characteristics at high driving currents for different magnetic fields and temperatures in Nb thin films of rather strong pinning. In a definite range of the B–T phase diagram we find that a current induced transition occurs in the flux flow motion of the vortex lattice, namely a dynamic ordering (DO). Contrary to the case of weaker pinning materials, DO is observed only at low fields, due to the stronger intrinsic disorder that can deform plastically the moving vortex lattice even for small applied fields. 相似文献
16.
J.P. Rodriguez E.J. Oswald 《Physica C: Superconductivity and its Applications》2009,469(23-24):2077-2082
Theoretical work predicts that the strong dependence of Tc on pure shear strain within the a–b plane of optimally doped YBa2Cu3O7?δ 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 antipin component of the heterogenous columnar pins contributes substantially to the net in-field critical current. 相似文献
17.
T. Taen Y. Tsuchiya Y. Nakajima T. Tamegai 《Physica C: Superconductivity and its Applications》2010,470(20):1106-1108
The critical current density and the normalized relaxation rate are reported in FeTe0.59Se0.41 single crystal. Critical current density is of order of 105 A/cm2, which is comparable to that in Co-doped BaFe2As2. In low temperature and low field region, the vortex dynamics of this system is well defined by the collective creep theory, which is quite similar to Co-doped BaFe2As2 reported before. We also discuss the origin of the anomaly in the field dependence of the relaxation rate. 相似文献
18.
19.
N. Moutalbi A. Ouerghi E. Djurado J.G. Noudem A. M’chirgui 《Physica C: Superconductivity and its Applications》2011,471(3-4):97-103
Crystalline defects on the nano-scale were successfully introduced into YBCO high-temperature superconductors (HTS) by ZrO2 nanometer particles addition in order to strongly pin the quantized vortices. Three batches of ZrO2 nano-particles with different particle size distributions were used. The corresponding mean nano-particle diameters are respectively, 287, 536 and 764 nm. Serving as artificial pinning centers (APC), non-superconducting nano-particles cause a remarkable enhancement of critical current density (Jc) at T = 77 K. This improvement has been shown to depend on the size of APC. The pinning strength of nano-particles inclusions has been found to be greater with wide size dispersed nano-particles. Our results indicate that pinning properties and vortex dynamics depend on the size of APCs. The introduction of APCs with controlled size is indispensable to achieve a high Jc. 相似文献
20.
N.J.O. Silva 《Journal of magnetism and magnetic materials》2011,323(24):3259-3264
We present simplified expressions for the out-of-phase component of the dynamic susceptibility χ″ of lognormal-sized magnetic nanoparticles under Brownian rotation. These expressions are based on transforming the general integral functions used for χ″ in the convolution of gaussian functions. χ″ can thus be expressed as a sum of gaussians with parameters directly related to those of the size distribution and to the saturation magnetization. The gaussian fit of χ″(ω) (where ω is the ac field frequency) is a simpler way to determine these structural and magnetic parameters as it avoids fitting χ″(ω) to an integral function. The expressions derived for χ″ suggest that χ″T data collapses in a ωη(T)/T scale (where T is the temperature and η the fluids viscosity), which is confirmed by numerical calculations. We also discuss the limits of validity of these approximations in real systems where both Néel and Brownian relaxation mechanisms coexist and we present further approximations for the relation of ωχ with the average volume (being ωχ the frequency at which χ″ is maximum). The ωη(T)/T scale can be used to qualitatively evaluate the dominance of the Brownian relaxation mechanism. 相似文献