79Br nuclear-quadrupole-resonance lineshape and Raman-induced spin-lattice relaxation in the incommensurate phase of β- ThBr4

We performed 7 9Br nuclear-quadrupole-resonance (NQR) line-shape and spin-lattice relaxation time measurements in the incommensurate (D system j8-ThBr4 over the temperature range 293-2.5 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 A^ that are valid at all temperatures in the / phase. By measuring Tx selectively for the different parts of the broadened NQR line in /3-ThBr4, we separately obtained the phason and amplitudon contributions, and TXA, respectively. A comparison between theory and the experimental data shows excellent agreement and demonstrates that spin-lattice relaxation in /?-ThBr4 is dominated by Raman processes. The phason gap A^ was determined to be 0.072+0.020 THz.