YbRh2Si2: Spin Fluctuations in the Vicinity of a Quantum Critical Point at Low Magnetic Field

We report a ${}^{\mathrm{29}}\mathrm{S}\mathrm{i}$ NMR study on aligned single crystals of ${\mathrm{Y}\mathrm{b}\mathrm{R}\mathrm{h}}_2{\mathrm{S}\mathrm{i}}_2$ which shows behavior characteristic of a quantum critical point (QCP: $T_{\mathrm{N}}\to 0$). The Knight shift $K$ and the nuclear spin-lattice relaxation rate $1/T_1$ of Si show a strong dependence on the external field $H$, especially below 5 kOe. At the lowest $H$ used in this measurement ($H\sim 1.5\mathrm{k}\mathrm{O}\mathrm{e}$), it was found that $1/T_{1}T$ continues to increase down to 50 mK, whereas $K$ stays constant with a large magnitude below 200 mK. This result strongly suggests the development of antiferromagnetic fluctuations with finite $\mathit{q}$ vectors that compete with $\mathit{q}=0$ spin fluctuations in the vicinity of the QCP near $H=0.5\mathrm{k}\mathrm{O}\mathrm{e}$.