Random singlet physics in the $S = \frac{1}{2}$ pyrochlore antiferromagnet NaCdCu$_2$F$_7$

Abstract: We report a random singlet ground state in the $S = \frac{1}{2}$ Heisenberg pyrochlore antiferromagnet NaCdCu$2$F$_7$. Cationic Na$^+$/Cd$^{2+}$ disorder generates a broad distribution of Cu$^{2+}$-F$^{-}$-Cu$^{2+}$ exchange couplings. Despite strong antiferromagnetic interactions, no magnetic order or freezing occurs, with $\mu$SR measurements confirming dynamics down to 58 mK. $T$-linear specific heat, a Curie-like susceptibility tail, and universal power-law scaling with data collapse in $\chi(T)$, $M(H)$, $C{\rm mag}/T$, $^{23}$Na $(1/T_1T)$ and $\lambda_{\rm LF}$ demonstrate a disorder-driven network of singlets and orphan spins, distinct from a clean quantum spin liquid.