SQUID nature of microwave absorption in a high-Tc superconducting Ho-Ba-Cu-0 single crystal

Abstract

A high-Tc, superconducting Ho-Ba-Cu-O single crystal with Tc=86.8 K is investigated on a modified ESR spectrometer at temperatures T>78 K. A signal is observed, whose spectrum in the range of magnetic fields H<0.7 Oe has the form of equivalent absorption lines with an interval Δ= 0.009 Oe. Detailed measurements of the dependence of the spectrum on the amplitude of the microwave field, the temperature, and the orientation of the single crystal are reported. It is established that the absorption spectrum exhibits a typical behavior inherent in SQUIDs with a critical current that decays linearly with increasing T. Three different techniques are used to determine the so-called hysteresis parameter of the postulated SQUID: Li_c /Φ_0 (L and i_c are the inductance and critical current of the SQUID, and Φ_0 is the quantum of magnetic flux). All three techniques give the same result: Li_c (T=79 K)/Φ_0= 3.1\pm 0.3, attesting to the existence of a SQUID-like structure in the single crystal.