Local-magnetic-moment variation in pure and impure phases of Tl-Ca-Ba-Cu-O superconductors

Abstract

In this work, we have successfully prepared Tl-Ca-Ba-Cu-O compounds with on-set temperature of 155 K and zero-resistance temperature of 123 K. Transmission electron microscopy and electron energy dispersive spectrometry analyses indicate that the major phase in this compound is tetragonal Tl 1Ca 2Ba 3Cu 4O x of P4/ mmm space-group symmetry with a = b = 0.394 nm and c = 1.97nm. Electron spin resonance (ESR) spectrometry was used to explore the local moments and hyperfine interaction of copper ions. The hyperfine lines are judged to be due to the CuO with Ca-Cu compounds intermixed between the grains. In pure samples all the spins are paired-off to give the singlet state. The vacuum annealed pure samples of Tl-Ca-Ba-Cu-O and YBa 2Cu 3O 7−δ have similar ESR lineshapes suggesting that the copper ions retrieve their original local moment due to the suppression of the super-exchange force by the decrease of oxygen atoms near the Cu-O planes.