$^{133}$Cs and $^{75}$As NMR investigation of the normal metallic state of the quasi-one-dimensional Cs$_2$Cr$_3$As$_3$

TL;DR

This study uses $^{133}$Cs NMR and $^{75}$As NQR to investigate the normal metallic state of the quasi-one-dimensional superconductor Cs$_2$Cr$_3$As$_3$, revealing how spin susceptibility and fluctuations evolve with temperature and ion substitution.

Contribution

It provides the first detailed NMR/NQR analysis of Cs$_2$Cr$_3$As$_3$, showing how spin dynamics and susceptibility relate to superconductivity and how they are affected by alkali ion substitution.

Findings

Spin susceptibility increases from 295K to 60K, then mildly suppresses and levels off below 10K.

Cs2 sites contribute little to conduction and exchange interactions.

Cr spin fluctuations decrease with larger alkali ions, correlating with lower $T_c$.

Abstract

We report $^{133}$Cs NMR and $^{75}$As Nuclear Quadrupole Resonance (NQR) measurements on the normal metallic state above $T_c$ of a quasi-one-dimensional superconductor Cs$_2$Cr$_3$As$_3$ ($T_c < 1.6$~K). From the $^{133}$Cs NMR Knight shift $^{133}K$ measured at the Cs1 site, we show that the uniform spin susceptibility $\chi_{spin}$ increases from 295~K to $\sim$ 60~K, followed by a mild suppression; $\chi_{spin}$ then levels off below $\sim$10~K. In contrast, a vanishingly small magnitude of $^{133}K$ indicates that Cs2 sites contribute very little to electrical conduction and the exchange interactions between 3d electrons at Cr sites. Low frequency Cr spin dynamics, reflected on $^{75}$As $1/T_1T$ (the nuclear spin-lattice relaxation rate $1/T_1$ divided by temperature $T$), shows an analogous trend as $\chi_{spin}$. Comparison with the results of $1/T_1T$ near $T_c$ with Rb$_2$Cr$_3$As$_3$ ($T_c=6.1$~K) and Rb$_2$Cr$_3$As$_3$ ($T_c=4.8$~K) establishes a systematic trend that substitution of K$^{+}$ ions with larger alkali ions progressively suppresses Cr spin fluctuations together with $T_c$.