Anomalous NMR frequency shift in the low-field phase of solidHe3

We present results of the first orientation-dependent study of the spin dynamics of the low-field spin-ordered phase of solid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$, the U2D2 phase. The results of a hydrodynamical treatment of the spin dynamics describe the observed behavior exceedingly well; however, only if a field- and orientation-independent negative shift, not provided by the theory, is properly incorporated into the equations. This shift does not arise from a lattice distortion as had been expected, and its measured temperature dependence suggests an unknown nonhydrodynamical origin.