Critical sheet resistance and two-dimensional properties of Bi2Sr2CuOx thin films.

Abstract

Electrical properties of ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CuO}}_{\mathit{x}}$ thin films prepared by the RF magnetron sputtering method have been investigated. By varying the Bi content in the film, the resistance of ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CuO}}_{\mathit{x}}$ changed over a wide range. Samples with low resistance showed superconductivity with a zero-resistance critical temperature of about 5 K, and the temperature dependence of the resistance above the superconducting transition temperature was metallic. On the other hand, samples with high resistance did not show superconducting transition and the temperature dependence of the resistance was semiconductorlike. The critical value of the normal resistance per Cu-O sheet for the appearance of the superconductivity was found to be very close to the quantum resistance (h/4${\mathit{e}}^{2}$). In the case of superconductive films, the power law of current-voltage characteristics (V\ensuremath{\propto}${\mathit{I}}^{\mathrm{\ensuremath{\alpha}}}$) was observed and the temperature dependence of the exponent \ensuremath{\alpha} showed the universal jump, which suggested the dissipation at the superconducting transition due to the Kosterlitz-Thouless (KT) transition. The temperature dependence of the resistance also supported the KT transition. The fluctuation effect above the mean-field BCS transition temperature was interpreted by the 2D Aslamazov-Larkin theory. In the case of insulating films, their behavior was examined by several mechanisms. The Anderson localization could be responsible for the ${\mathit{R}}_{\mathrm{\ensuremath{\square}}}$-T characteristics of the sample of which sheet resistance was just above the critical resistance in the low-temperature region. The intriguing possibility of the KT transition of charges for high-resistance samples was also discussed.