Detection of second-order quadrupolar spin-lattice coupling mechanisms in metallic antimony using multifrequency pulsed nuclear quadrupole resonance

Abstract

Semimetals have electronic structures which suggest the possibility of important noncontact terms as well as contact terms in the perturbing Hamiltonian involved in nuclear-spin--lattice relaxation. There is also the possibility of a phonon mechanism becoming important at high temperatures. Experiments using single-frequency and multifrequency pulsed nuclear quadrupole resonance have been carried out on antimony ${(}^{121}$Sb) and have permitted electric quadrupolar interaction terms (\ensuremath{\Delta}m=\ifmmode\pm\else\textpm\fi{}1 and \ifmmode\pm\else\textpm\fi{}2) to be separated from the magnetic (\ensuremath{\Delta}m=\ifmmode\pm\else\textpm\fi{}1) terms. For temperatures much below the Debye temperature ${\mathit{FTHETA}}_{\mathit{D}}$, the results suggest that the quadrupolar interaction involving p electrons contributes 5--10 % to the $^{121}\mathrm{Sb}$ relaxation. At temperatures above ${\mathit{FTHETA}}_{\mathit{D}}$, the importance of the quadrupolar coupling increases dramatically and the ratio of the transition probabilities for \ensuremath{\Delta}m=\ifmmode\pm\else\textpm\fi{}1 and \ensuremath{\Delta}m=\ifmmode\pm\else\textpm\fi{}2 transitions changes, signaling the emergence of a relaxation mechanism which is probably a two-phonon mechanism.