Magnetic field dependent microwave absorption in high-Tc superconducting cuprates

Abstract

A strong microwave absorption at low magnetic fields is observed for a variety of metallic cuprates below their critical superconducting transition temperatures (T_c) using conventional electron paramagnetic resonance (EPR) instrumentation. The low-field microwave signal was investigated in the following high-temperature superconductors, YBa₂Cu₃O_7–x(T_c= 95 K), GdBa₂Cu₃O_7–x(T_c= 95 K), YBa₂Cu₄O₈(T_c= 75 K), (Bi_1.6Pb_0.4)Sr₂Ca₂Cu₃O_y(T_c= 115 K), Pb_0.5Bi₂Sr₂Ca_2.5Cu_4.5O_y(T_c= 118 K)(Tl_0.5Pb_0.5)Sr₂(Ca_0.8Er_0.2)Cu₂O_y(T_c= 115 K), Tl₂Ba₂Ca₂Cu₃O_y(T_c= 118 K), TbTl₂Ba₂Cu₃O_y(T_c= 84 K), Tl₂Ba₂Ca₂Cu₃O₁₀(single-crystal, T_c= 118 K) and Bi₂Sr₂CaCu₂O₈(single-crystal, T_c= 80 K). We have investigated the usefulness of the technique as a practical non-intrusive method for screening potentially superconducting samples, with particular emphasis on sensitivity, the effect of microwave power and the inherent problems of studying signals at small magnetic fields. In the latter regard, we have also carried out studies using a radiofrequency EPR spectrometer operating at 300 MHz, which facilitated detailed calibration of the 0–200 G region, where the signal is most intense.