G. Kriza

Potential and current distribution in strongly anisotropic Bi 2 Sr 2 CaCu 2 O 8 single crystals at current breakdown

Experiments on potential differences in the low-temperature vortex solid phase of monocrystalline platelets of superconducting Bi 2 Sr 2 CaCu 2 O 8 subjected to currents driven either through an ab surface or from one such surface to another show evidence of a resistive/nonresistive front moving progressively out from the current contacts as the current increases. The depth of the resistive region has been measured by an in-depth voltage probe contact. The position of the front associated with an injection point appears to depend only on the current magnitude and not on its withdrawal point. It is argued that enhanced nonresistive superconducting anisotropy limits current penetration to depths less than the London length and results in a flat rectangular resistive region with simultaneous ab and c current breakdown which moves progressively out from the injection point with increasing current. Measurements in ab or c configurations are seen to give the same information, involving both ab-plane and c-axis conduction properties.

Linear Current-Field Relation of Charge Density Waves near the Depinning Threshold in Alkali-Metal Blue Bronzes A0.3MoO3

The charge-density-wave current density-voltage (jCDW-V) relation is determined near the depinning threshold Vc in the blue bronzes K0.3MoO3 and Rb0.3MoO3 by measuring the narrow-band voltage noise frequency of single CDW domains. A fit to the formula jCDW = σCDW(Vc/l)[(V/Vc) - 1]γ yields a linear relation γ = 1.0(± 0.1) for fields Vc < V < 2Vc within the temperature range of 51 to 77 K. This finding is qualitatively different from theoretical results based on the mean-field approximation of the deformable medium model (γ = 3/2) and also from the classical model of a finite domain in the potential of a single impurity (γ = 1/2). The CDW conductivity, σCDW, near threshold is activated similarly to the findings of FLEMING et al. (Phys. Rev. B, 33 (1986) 5450) at higher fields.