NMR study for tunnelling NH4+ions: non-exponential relaxation in the rotating frame

Abstract

The quantum effect of NH₄⁺rotation on spin-lattice relaxation in the rotating frame has been investigated in NH₄IO₄and NH₄ReO₄. It is shown that the relaxation becomes of non-exponential type for Lambda₀>> omega₁and Lambda₀tau_t>1, because the effect of the spin diffusion is not enough to prepare a common spin temperature, where Lambda₀is the tunnelling frequency of NH₄⁺, omega₁the Larmor frequency in the rotating frame tau_tthe correlation time. The three-bath model was adopted in the explanation, in good accordance with experiment. The tunnelling frequencies Lambda₀/2 pi =1.5 MHz at 68K and 800 kHz at 65K have been determined in NH₄IO₄and NH₄ReO₄, respectively.