Low-energy spin dynamics and critical hole concentrations in La${}_{2-x}$Sr${}_x$CuO${}_4$ ($0.07\le x\le 0.2$) revealed by ${}^{139}$La and ${}^{63}$Cu nuclear magnetic resonance

Abstract

We report a comprehensive ${}^{139}$La and ${}^{63}$Cu nuclear magnetic resonance study on La${}_{2-x}$Sr${}_x$CuO${}_4$ ($0.07\le x\le 0.2$) single crystals. The ${}^{139}$La spin-lattice relaxation rate ${}^{139}{T}_1^{-1}$ is drastically influenced by Sr doping $x$ at low temperatures. A detailed field dependence of ${}^{139}{T}_1^{-1}$ at $x=1/8$ suggests that charge ordering induces the critical slowing down of spin fluctuations toward glassy spin order and competes with superconductivity. On the other hand, the ${}^{63}$Cu relaxation rate ${}^{63}{T}_1^{-1}$ is well described by a Curie-Weiss law at high temperatures, yielding the Curie-Weiss temperature $\hat{I}˜$ as a function of doping. $\hat{I}˜$ changes sharply through a critical hole concentration $x_c\sim 0.09$. $x_c$ appears to correspond to the delocalization limit of doped holes, above which the bulk nature of superconductivity is established.

7 pages, 4 figure, published in PRB