Thermal fluctuations and coherence transition in superconductor-isolator composites

Abstract

HoBa2Cu3O7−δ–SrTiO3 composites were produced with nominal isolator volume percentages from 0% up to 10%. Resistivity measurements were carried out for all composites and for the HoBa2Cu3O7−δ with different measurement current. All composites evidenced a superconducting behavior with critical temperature TC ≈ 92 K with wider transitions when isolator concentrations are incremented. By using the logarithmic derivative method, characteristic fluctuation exponents were determined close to TC. Above TC, typical gaussian and critical fluctuation regimes are observed. However, still closer to TC an atypical exponent about 0.14 is observed for all samples. Below TC, exponent of the coherence transition increases rapidly with SrTiO3 concentrations. Results for different currents reveal wider transitions as the current increases but characteristic exponents of the coherence transition do not change. Our results suggest a vortex–glass behavior for different current measurements and a gauge–glass feature for different insulator concentrations.