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...

A cation-exchange process has been developed for growth of high-quality epitaxial thin films of highly volatile mercury-based high-temperature superconductors. By selecting epitaxial precursor matrices of TlyBa2CaCu2Ox (y=1,2) and annealing them in Hg vapor, c-axis-oriented epitaxial HgBa2CaCu2O6+δ thin films with superior quality were formed through Tl and Hg-cation exchange. At 110 K, these films can carry supercurrent density close to 1 MA/cm2 which is nearly an order of magnitude higher than the best reported previously. This cation-exchange technique also provides a general scheme for synthesis of other volatile compounds with predesigned structure and composition.

The authors have explored the coupling mechanism of a millimeter wave radiation to bicrystal Josephson junction arrays embedded in a hemispherical Fabry-Pérot resonator. The authors have found that their high-temperature superconductor array can be modeled as a thin film grid polarizer. In agreement with this model, a strong dependence of the coupling of the Josephson junctions on the polarization of the electric filed in the resonator was observed. The authors achieved a maximum Josephson voltage of 28mV for an array of 182 bicrystal junctions at a temperature of 75K and a frequency of 74.4GHz.

No abstract

High quality superconducting HgBa2CaCu2Ox (Hg-1212) thin films have been reproducibly fabricated using cation-exchange method. The thin films have pure Hg-1212 phase and have smooth surface morphology. The superconducting transition temperatures of these films are in the range of 120–124 K. The critical current density Jc is up to 3.2×106 A/cm2 at 77 K and drops only by a factor of 2 at 100 K and self field. At 110 K, a Jc of 7.8×105 A/cm2 has been obtained. X-ray diffraction pole figures show that these films are epitaxially grown on LaAlO3(001) substrates, which is consistent with a χmin of 19% obtained using Rutherford backscattering/channeling analysis.