Unconventional spin excitations in the S=32 triangular antiferromagnet RbAg2Cr[VO4]2

Abstract

We present muon spin relaxation ($\ensuremath{\mu}\mathrm{SR}$) measurements of the $S=3/2$ undistorted triangular lattice established in ${\mathrm{RbAg}}_{2}\mathrm{Cr}{[{\mathrm{VO}}_{4}]}_{2}$. The zero- (ZF) and longitudinal-field $\ensuremath{\mu}\mathrm{SR}$ spectra evidence the absence of spin freezing and long-range magnetic ordering down to $T=25$ mK, supporting the formation of a dynamic ground state. Noticeably, we observe an anomalous temperature dependence of the ZF muon spin relaxation rate ${\ensuremath{\lambda}}_{Z}F(T)$, featuring a decrease below $T=20$ K. This suggests the alteration of the dominant relaxation mechanism by the development of short-range magnetic correlations. A subsequent leveling off of ${\ensuremath{\lambda}}_{Z}F(T)$ below $T=2$ K indicates persistent spin dynamics and reveals the presence of exotic magnetic excitations. The field dependence of the muon spin relaxation rate at $T=25$ mK is well described by a diffusive spin transport model with algebraic spin-spin correlations. The suppressed long-range order and the peculiar temperature-dependent behavior of ${\ensuremath{\lambda}}_{Z}F(T)$ will be discussed in terms of the exchange interaction between ${\mathrm{Cr}}^{3+}$ moments via nonmagnetic ${[{\mathrm{VO}}_{4}]}^{3\ensuremath{-}}$ entities. In the title compound, the degeneracy of the ${t}_{2g}$-orbital set is not lifted by a space group symmetry reduction or subject to significant anisotropy resulting from spin-orbit coupling.