Anisotropy of the antiferromagnetic spin correlations in the superconducting state of YBa2Cu3O7 and YBa2Cu4O8
Abstract
We present evidence that the antiferromagnetic spin correlations in optimally doped YBa2Cu3O7 and underdoped YBa2Cu4O8 develop a surprisingly strong anisotropy in the superconducting state. Comparing the ratio of the nuclear spin-lattice relaxation rates of the planar copper and oxygen, measured at the lowest and highest temperatures as well as at Tc, we conclude that the antiferromagnetic in-plane correlations vanish as the temperature goes to zero. This observation is corroborated by the measurement of the copper linewidth in YBa2Cu4O8. In contrast, the out-of-plane correlations do not change appreciably between T=Tc and T=0. Within a model of fluctuating fields this extreme anisotropy of the antiferromagnetic correlations also explains the observed temperature dependence of the anisotropy of the copper relaxation measured in a low external magnetic field.
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