Low-temperature magnetic order and spin dynamics inYbRh2Si2

Abstract

Muon spin rotation and relaxation experiments have been carried out in single crystals of ${\mathrm{YbRh}}_{2}{\mathrm{Si}}_{2},$ a compound that exhibits non-Fermi-liquid (NFL) behavior associated with a quantum critical point (QCP) at $T=0.$ The zero-field muon relaxation rate is found to be independent of temperature down to 100 mK but to increase below $\ensuremath{\sim}70\mathrm{mK},$ which suggests magnetic order at low temperatures. From the relation between the internal field at the ${\ensuremath{\mu}}^{+}$ stopping site and the hyperfine coupling constant the ordered ${\mathrm{Yb}}^{3+}$ moment is very small, $\ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}{\ensuremath{\mu}}_{B}.$ Muon spin rotation linewidths in a transverse field of 6 kOe indicate a homogeneous susceptibility down to 2 K, which is an order of magnitude lower than the characteristic (Kondo) temperature ${T}_{K}\ensuremath{\approx}25\mathrm{K}.$ This is evidence against the importance of disorder-driven NFL mechanisms in ${\mathrm{YbRh}}_{2}{\mathrm{Si}}_{2}.$ In longitudinal magnetic fields the muon spin-lattice relaxation function $G(t)$ is exponential, again indicative of a homogeneous system. The relaxation obeys the time-field scaling relation $G(t,H)=G(t/H),$ which suggests long-lived spin correlations at low temperatures. The ${\mathrm{Yb}}^{3+}$ spin dynamics derived from muon spin relaxation appear to be intimately related to critical magnetic fluctuations near the QCP.