Ground State of the Kagomé-Like S = 1 / 2 Antiferromagnet Volborthite C$u_3$ $V_2$ $O_7$ (OH$)_2$ · 2$H_2$O

Abstract : The volborthite compound is one of the very few realizations of S=1/2 quantum spins on a highly frustrated kagomé-like lattice. Low-{\it T} SQUID measurements reveal a broad magnetic transition below 2K which is further confirmed by a peak in the $^{51}$V nuclear spin relaxation rate (1/$T_1$) at 1.4 K \pm 0.2 K. Through $^{51}$V NMR, the ground state (GS) appears to be a mixture of different spin configurations, among which 20% corresponds to a well defined short-range order, possibly of the $\sqrt 3$ x $\sqrt 3$ type. While the freezing involves all the C$u_{2,+}$ spins, only 40% of the copper moment is actually frozen which suggests that quantum fluctuations strongly renormalize the GS.