NQR study of spin-freezing in superconducting La2−x Sr x CuO4: the example ofx = 0.06

Abstract

Nuclear quadrupole resonance (NQR) "9La and 63 Cu spin-lattice relaxation rate Ti ' measurements in a La 194 Sr006CuO4 single crystal are described. Slowing-down of Cu" spin fluctuations is evidenced through a dramatic increase of 139 T1 on cooling. While the onset of diamagnetism occurs at T = 8 K, 19 T1 ' has a peak at TT 5 K, when the characteristic frequency of magnetic fluctuations reaches the NQR frequency VQ = 9 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.

Competition between spin ordering and superconductivity near the pseudogap boundary in La2−xSrxCuO4 : Insights from NMR

“…An alternative explanation for the relatively slow divergence of τ c is that it is an intrinsic consequence of the quasi-two-dimensional, nearly isotropic (Heisenberg) nature of the spin system that orders only at T = 0 [ 45]. While this latter view appears to be at odds with three sets of experimental observations (traditional glass-like properties at low doping [51,52], evidence that disorder plays a role [53,54] and the three dimensionality of spin correlations for p ≃ 0.12 [55,56]), it is interesting to note the similarity with the renormalized classical regime of the 2D Heisenberg AFM already pointed out in several NMR papers [16,[28][29][30][31]41,42]: in the limit T > T peak , ω L τ c ≪ 1 and thus Eq. ( 2) reduces to T , which resembles the expression derived by Chakravarty and Orbach [57] for the renormalized classical regime, 1…”

“…NMR studies of spin-freezing in superconducting cuprates [5,12,13,15,16,18,[28][29][30][31][32][33]41,42] generally find that the relaxation curves are stretched, which indicates that there is a distribution of relaxation rates (in the absence of any significant quadrupolar relaxation mechanism, which is true at low T ). In this situation, a standard procedure is to fit the relaxation curves with a stretched exponential form, which yields a "typical" relaxation rate T −1 1 , equivalent to the median of the distribution, and a stretching exponent β that quantifies the breadth of the distribution [18,33,34].…”