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

TL;DR

This study reveals that in certain high-temperature superconductors, antiferromagnetic spin correlations become highly anisotropic in the superconducting state, with in-plane correlations diminishing at low temperatures while out-of-plane correlations remain stable.

Contribution

It provides new experimental evidence of anisotropic antiferromagnetic correlations in YBa_2Cu_3O_7 and YBa_2Cu_4O_8 during superconductivity, highlighting the directional dependence of magnetic fluctuations.

Findings

In-plane antiferromagnetic correlations vanish as temperature approaches zero.

Out-of-plane correlations remain largely unchanged between T_c and zero.

The anisotropy explains the temperature dependence of copper relaxation measurements.

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.