Single Crystals: What They Tell About High Temperature Superconductivity
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S BULLETIN/MAY 1991
récent work in which the use of single crystals has answered questions important to the fundamental aspects of the Cu-O-based superconductors, which in turn often affect the potential uses of
Figure 1. Structure of YBCO-123. (After Référence 1).
the materials. The areas covered in this limited spacê dérive strongly from the activities in my laboratory, and do not give full crédit to ail important studies. Most of what follows will be based on YBa2Cu307.x with x = 0.1 because it has been the most studied compound, was the first material with Tc > 77 K, has no intergrowths, and forms with spécifie ionic ratios. However, effects due to oxygen values 6.9 and Tc = 93 K hâve not been well addressed. Significant work has also been carried out with single crystals of other compounds and will also be discussed. AH the Cu-Obased compounds are two-dimensional in structure, so it is interesting to examine anisotropic properties via singlecrystal studies. The added complexity of anisotropy has pushed growth methods and ail models of superconducting behavior beyond previous levels. Growth Methods Most crystals hâve been grown from partly meltéd mixtures where one or more of the components, e.g., CuO may be in excess to act as a flux. In other cases, the material acts as a self-flux. Subséquent long-term annealing in oxygen is usually necessary to bring the oxygen content close to 7.0, i.e., x = 0 (as x drops toward 0.5, Tc drops to 0 K).2 Reoxidation |is not needed for Bi-Sr-CaCu-O (BSCCO-2212,with Tc = 96 K), Tl-Ba-Ca-Cu-O (TBCCO-2212, with Tc = 115 K) or Lai-Cu-O (LCO-214, with Tc = 0 K). For Sr-substituted LCO (0.15 Sr, Tc = 39K), long annealing times are needed to distribute the Sr throughout the crystal. In gênerai, Au or Pt crucibles are satisfactory, although some Au contamination is now being reported for YBCO. Since the YBCO orthorhombic structure generally forms from the higher symmetry, tetragonal phase, samples generally fofm twins, mostly the [110]type. Thus, nearly ail work has been on twinned crystals. Near the middle of 1990, two routes were found for detwinning the material. Thèse are based on (1) quenching samples with ihsufficient oxygen to form the orthorhombic phase, followed by annealing in oxygen toobtain0s x < 0.1, and (2) usinguniaxial compression along [110] of one of the twins, while the crystal is cooled through thé transition in a stream of flowing oxygen.3,4 Although thèse m e t h o d s seem simple, reports based on detwinned crystals hâve corne from only a few laboratories. However, twinned crystals
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Single Crystals: What They Tell About High Température Superconductivity
Diffusion Behavior Despite the small size of available YBCO crystals, the anisotropic nature of diffusion has bêen partly investigated. Rothman et al. hâve reported the anisotropy of diffusion for twinned and untwinned YBCO.5 The diffusion coefficient in the ab-plane is far greater than in the c-direction in both types of crystals. Db >> D„ at 300 K and Dab is approximately equal to Dpoty. Thèse results
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