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Anisotropy of the antiferromagnetic spin correlations in the superconducting state of YBa_2Cu_3O_7 and YBa_2Cu_4O_8

A. Uldry·M. Mali·J. Roos·P. F. Meier
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Abstract

We present evidence that the antiferromagnetic spin correlations in optimally doped YBa_2Cu_3O_7 and underdoped YBa_2Cu_4O_8 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 T_c, 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 YBa_2Cu_4O_8. In contrast, the out-of-plane correlations do not change appreciably between T=T_c 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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A. Uldry, M. Mali, J. Roos, P. F. Meier. 2005-10-12. Anisotropy of the antiferromagnetic spin correlations in the superconducting state of YBa_2Cu_3O_7 and YBa_2Cu_4O_8. https://doi.org/10.1088/0953-8984%2F17%2F46%2Fl03

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