ESR studies of high critical-temperature superconductors. Absorption at low magnetic fields

One of the most intriguing aspects of ESR studies of oxide superconductors is the presence of a broad, and intense, low-field microwave absorption which grows in intensity upon cooling the sample through the superconducting transition temperature (Tc). In the present investigations we exploit the low-field response as a screening technique for superconductivity in a range of oxide materials. The materials investigated, derivatives of the 90 K superconductor YBa2Cu3O7, were specifically chosen so that Tc could be drastically modified either by oxygen loss or cation substitution. In all of these materials a clear correlation exists between the onset temperature of the low-field absorption and the superconducting transition temperature, as gauged by both a.c. inductive and electrical resistivity measurements. The temperature range over which the superconductivity onset was detected by these complementary methods extended from ca. 90 K down to ca. 30 K in the case of a Zn-substituted YBa2Cu3O7. Measurements on the corresponding Bi-Sr-Ca-Cu-O system, which becomes superconducting for temperatures below 110 K, also illustrate that this absorption phenomenon is not unique to YBa2Cu3O7 and hence will have widespread applicability in detecting superconductivity