Field Induced Vanishing of the Vortex Glass Temperature inTl2Ba2CaCu2

Abstract

The current density j s and relaxation rate Q have been measured on a ring shaped Tl 2 Ba 2 CaCu 2 O 8 film between 4.2 K and the irreversibility temperature in magnetic fields up to 7 T. The temperature and field dependent exponent m͑T , B e ͒ used to parametrize the activation energy U in the form U͑ j s , T , B e ͒ ͑U c ͞m͒ ͓͑ j c ͞j s ͒ m 2 1͔ is determined for many different fields. In the B e -T phase diagram we associate the m 0 line with the glass temperature T g and find that T g essentially drops to 0 K at B e ഠ 0.7 T, which indicates a dimensional crossover at this field. The m . 0 region ͑B e , 0.7 T, T # 55 K͒ corresponds to 3D elastic vortex motin while the m , 0 region corresponds to 2D dislocation mediated flux creep. [S0031-9007(97) The small coherence length, the presence of weak and dense point pinning centers, and the layered structure of high-T c superconductors are at the origin of the intricate nature of the mixed state phase diagram of these superconductors [1][2][3][4]. A lot of effort has been put into investigations of various phases and phase transitions [5][6][7][8]. It is generally believed that below the irreversibility line B irr ͑T ͒ there exists a vortex solid state which is usually described in terms of collective flux pinning (CFP) [3,4] or vortex glass (VG) [2,9] models. Both the CFP and VG models are based on elastic vortex motion. Dissipation due to the thermally activated motion of vortices is associated with an electric field E y 0 B e exp͓2U͑ j s , T , B e ͒͞k B T ͔, where U͑ j s , T, B e ͒ ͑U c ͞m͒ ͓͑ j c ͞j s ͒ m 2 1͔ is the activation energy at the external magnetic field B e and temperature T, y 0 an attempt hopping velocity, and U c the characteristic pinning energy. The superconducting current density j s is always smaller than the critical current j c for which U vanishes. Many different values have been predicted theoretically for the parameter m on the basis of the CFP and VG models. For all regimes and dimensionalities [1-4,9], however, it is found that m $ 0. A direct conclusion then is that the activation energy diverges when the current j s approaches 0. This is reasonable for a 3D elastic vortex system, but for a 2D vortex system, it may not be true [9]. For example, for a 2D system, the vortex glass state does not exist at finite temperatures [1,10]. As mentioned by Vinokur et al. [4], in strongly layered systems such as Bi 2 Sr 2 CaCu 2 O 8 and Tl 2 Ba 2 CaCu 2 O 8 pinning is too strong at high magnetic fields to allow vortices to creep collectively. Instead, dislocation-mediated (plastic) creep is expected to be predominant. However, as far as we know, no direct evidence has been found for plastic creep except in a small part of the phase diagram ͑T . 81 K and B e , 6 T͒ of a YBa 2 Cu 3 O 7 single crystal with a fishtail effect [11]. In this Letter, we conclude from a detailed investigation of the vortex dynamics in high quality Tl 2 Ba 2 CaCu 2 O 8 thin films that the B e -T plane below the irreversibility line is separated into two regions by a m 0 lin...

Absence of a true vortex-glass phase above the Bragg glass line in Bi2Sr2CaCu2O8+δ

“…Hence both of the dip on ∆M(H) and the peak on Q(H) have dynamical features and should disappear above the SP field in the small current limit in low temperature region. After decreasing in region III, Q upturns and increases monotonously with the field up to the irreversibility field H irr (Q ≈ 1 [13])in region IV. Due to the dynamical feature of the dip and peak mentioned above, it can be expected that the region IV will dominate the whole H − T phase diagram above the SP field in long time or small current limit.…”

Dimensional crossover of vortex dynamics induced by Gd substitution on Bi2212 single crystals

“…where C = −(dlnj s /dT )(T /S)| T =0 = lnv 0 B/lnE is a parameter independent of T. For four samples studied here, the value of C estimated by Maley's method is about 10 to 15 [2,31].…”

“…Since a plateau will only appear when µCT exceeds U c /k B , the sharp transition may manifest that there is a rapid variation of either µ, C or U c . Considering C is independent of T as stated above, and U c is weakly dependent on temperature in the temperature region studied here [2], the only possibility is that µ rises suddenly at a certain temperature.…”

“…For highly anisotropic HTSC, such as Tl and Bi compounds, it has been found that the vortex dynamics can be driven from 3D to 2D at relatively high fields and high temperatures [1,2]. Though it is generally believed that the doping will strongly affect the coupling between the Cu-O layers and further influence the vortex dynamics, there is few such research [3].…”

“…Since U c is weakly temperature dependent when T << T irr in highly anisotropic superconductors [2], we can get the general U c = U c (0) from the low temperature limit of T/S.…”