63Cu NMR shift and linewidth anomalies in the Tc=60 K phase of Y-Ba-Cu-O.

{sup 63}Cu NMR shifts and spectra are presented for both Cu(1) and Cu(2) sites in a specimen of YBa{sub {ital z}}Cu{sub 3}O{sub 6.64} ({ital T}{sub {ital c}}{similar to}60 K). For {bold H} along the {ital c} axis, the Cu(2) shift is not distinguishable from that reported for YBa{sub 2}Cu{sub 3}O{sub 7.0}. For {bold H} in the {ital a}-{ital b} plane, the spin component of Cu(2) shift declines sharply as {ital T}{r arrow}{ital T}{sub {ital c}}. The Cu(1) shift behaves similarly, showing also a dramatic broadening at low temperatures in clearly nonmetallic behavior. Combined with anomalous {ital T}{sub 1} data reported earlier for the 60-K phase of YBa{sub 2}Cu{sub 3}O{sub 6+{ital x}}, these results are shown to be consistent with strong and monotonically increasing antiferromagnetic correlations as {ital T} is decreased.