Magnetophonon oscillations in the transverse and longitudinal magnetoresistance of Hg1-xCdxTe

The transverse and longitudinal magnetoresistance (MR) as well as the longitudinal magneto-thermoelectric coefficient of n-type Hg1-xCdxTe (MCT) (0.20 < x < 0.33) have been measured at various temperatures (40 less than or equal to T less than or equal to 140K) as a function of magnetic field (0 less than or equal to B less than or equal to 18 kG). Both the transverse and the longitudinal MR clearly exhibit oscillations which are described in terms of magnetophonon (MP) transitions involving the HgTe-like and the CdTe-like longitudinal optical (LO) phonons of MCT. The field positions of the transverse MR maxima agree with the calculated MP resonances taking into account nonparabolic bands (k . p model for narrow-gap zinc-blende-type semiconductors) and the polaron effect. Those of the longitudinal MR minima are found to coincide with the oscillation minima in the longitudinal magneto-thermoelectric coefficient. However, these minima are shifted by pi/2 to lower fields with respect to the positions of the MP resonances. This phase shift was predicted by Barker(13,14) for the case of strong Landau level damping but has not been previously observed. In contrast, the MP oscillation minima of the longitudinal MR and the oscillation maxima of the transverse MR of n-type InSb (investigated here for comparison) occur exactly at the fields of the MP resonances. Only the oscillation minima of the longitudinal magneto-thermoelectric coefficient are slightly shifted to the side below the MP resonance fields. With regard to the band parameters and the dominant polar optical mode scattering of charge carriers InSb very much resembles MCT. InSb, however; is a binary compound whereas MCT is a solid solution. Thus, the phase shift by pi/2 to lower fields observed for the oscillation minima in the longitudinal MR and magneto-thermoelectric coefficient of MCT may be due to alloy scattering. The temperature coefficients of the MP resonance fields of MCT are found to be substantially smaller than those reported by Takita et al.(11) and McClure et al.(10) The larger temperature coefficients are presumably due to unresolved two-phonon structures of the MP oscillations.