Amplitude modes in three-dimensional spin dimers away from quantum critical point

Magnetic amplitude mode excitations have been generally observed in quantum spin dimers near a quantum critical point. This paper presents an unusual observation and explanation of such magnetic excitations in three-dimensional $S$ = 3/2 spin dimers Cr${}_{2}$TeO${}_{6}$ and Cr${}_{2}$WO${}_{6}$, even though these two compounds are away from the quantum critical point with the ordered moment reduced by only ~24%.