th  - 1988 - 51st ENC Conference

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T[-~-UE ~ t~ve 8:00! 63,65Cu NQR STUDIES OF HIGH T OXIDE SUPERCONDUCTORS:
W. W. Warren, Jr.*, R. E. Walstedt, G. F. Brennert, R. F c. Bell, R. J. Cava, G. P. Espinosa,
and J. P. Remeika, AT&T Bell Laboratories, Murray Hill, NJ 07974
Nuclear quadrupole resonance provides a local, site selective probe which has proven highly
informative for studies of oxide superconductors. In this talk I will review our investigations
using the 6~,6SCu resonances in the 90 K superconductor YBa2Cu3OT_8" The crystal structure
of YBa2Cu307 contains two inequivalent Cu sites (Cu-O ".chains" and "planes") whose NQR
lines fall at roughly 22 MI-Iz and 31.5 MHz. Although electron band theory calculations imply
similar electronic structure at the two sites, spin-lattice relaxation time measurements reveal
strikingly different behavior in both the normal and superconducting states. In the normal
states, the 22 MHz sites relax by an enhanced Korringa process that exhibits the usual roughly
linear temperature dependence for the relaxation rate. At 31 MHz, in contrast, the rate
increases only slightly with increasing temperature. Below To, spin-lattice relaxation reflects
the excitation spectrum of unpaired electrons ("quasiparticles"). Again, the relaxation
behavior is dramatically different for the two sites. An exponential representation of the
relaxation rate yields a value 2A / kT c -- 2.4 for the 22 MI-Iz sites (A is the superconducting
energy gap) while for 31.5 MHz, the corresponding value is 2A / kT c -- 8.3. Reduced oxygen
content (8 = 0.3) leads to relaxation at the 31.5 MHz sites reduced by a factor - 1/1500 but
only a modest 30 ~o effect at 22 MHz. This indicates that the former sites carry almost no
conduction electron density and, by implication, do not participate in the 60 K
superconductivity observed in this phase. The effects of extrinsic paramagnetic moments, the
assignment of the NQR lines to the respective Cu sites, and the implications of these results of
models of superconductivity will be discussed.
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