79Br nuclear-quadrupole-resonance line shape and Raman-induced spin-lattice relaxation in the incommensurate phase of beta -ThBr4.

Abstract

We performed $^{79}\mathrm{Br}$ nuclear-quadrupole-resonance (NQR) line-shape and spin-lattice relaxation time measurements in the incommensurate (I) system \ensuremath{\beta}-${\mathrm{ThBr}}_{4}$ over the temperature range 293\char21{}2.5 K. In addition, we extended the theory of the effects of Raman processes on amplitudon and phason spin-lattice relaxation in incommensurate systems by obtaining general expressions for the spectral densities and phason gap ${\ensuremath{\Delta}}_{\ensuremath{\varphi}}$ that are valid at all temperatures in the I phase. By measuring ${T}_{1}$ selectively for the different parts of the broadened NQR line in \ensuremath{\beta}-${\mathrm{ThBr}}_{4}$, we separately obtained the phason and amplitudon contributions, ${T}_{1\ensuremath{\varphi}}$ and ${T}_{1A}$, respectively. A comparison between theory and the experimental data shows excellent agreement and demonstrates that spin-lattice relaxation in \ensuremath{\beta}-${\mathrm{ThBr}}_{4}$ is dominated by Raman processes. The phason gap ${\ensuremath{\Delta}}_{\ensuremath{\varphi}}$ was determined to be 0.072\ifmmode\pm\else\textpm\fi{}0.020 THz.