Possible role of Cu2+-Cu4+ pairs in the superconductivity of YBa2Cu3Osub(7-x) from electron spin resonance observations

The authors report some important features of the low-field electron spin resonance spectrum in the superconducting phase of YBa2Cu3Osub(7-x), giving evidence for the dimerization of electrons on copper pairs. An intense low-field resonance appears below the critical temperature, Tsub(c), and exhibits unresolved hyperfine structure at temperatures below 70 K. The resonance field was found to be dependent on both temperature and microwave frequency. These are well-established features characteristic of copper pairs formed due to an exchange interaction. It appears that the individual spins responsible for superconductivity are formed and dimerized only at Tsub(c), with their intensity sharply increasing below Tsub(c). This observation appears to be a pointer towards the possibility of the disproportionation 2Cu3+ → Cu2+ + Cu4+ in adjacent octahedra, facilitated by coupling to local esub(g) vibrations in the superconducting phase. (author)