NQR Study of Spin-Freezing in Superconducting La2-xSrxCuO4: The Example of x=0.06

TL;DR

This study uses NQR measurements to investigate spin dynamics and the coexistence of spin-glass and superconducting phases in La2-xSrxCuO4 with x=0.06, revealing persistent magnetic fluctuations below Tc.

Contribution

It provides detailed NQR evidence of spin-freezing and phase coexistence in underdoped cuprate superconductors, highlighting the persistence of spin-glass behavior within the superconducting state.

Findings

Spin fluctuations slow down as temperature decreases.

Peak in 139La relaxation rate at Tg indicates spin-glass freezing.

Coexistence of spin-glass and superconducting phases is confirmed.

Abstract

NQR 139La and 63Cu spin-lattice relaxation rate (1/T_1) measurements in a La1.94Sr0.06CuO4 single crystal are described. Slowing-down of Cu2+ spin fluctuations is evidenced through a dramatic increase of 139La 1/T_1 on cooling. While the onset of diamagnetism occurs at Tc=8 K, 1/^{139}T_1 has a peak at Tg~5 K, when the characteristic frequency of magnetic fluctuations reaches the NQR frequency nu_Q~19 MHz. In agreement with a number of previous studies, these results show that the so-called "cluster spin-glass" phase persists in the superconducting regime. Issues concerning the coexistence of the two phases are discussed.