Manipulation of Rashba splitting and thermoelectric performance of MTe (M = Ge, Sn, Pb) via Te off-centering distortion.

The Rashba effect is an essential phenomenon in asymmetrical ferroelectric materials with local dipole fields. Rashba spin splitting due to spin-orbit coupling in the asymmetrical 2D ferroelectricity MTe (M = Ge, Sn, Pb) has provided a promising arena for achieving an ultra-high power factor to improve their thermoelectric performance. Herein, we show that the hidden Rashba effect may exist in centrosymmetric rock-salt MTe because of the emerging macroscopic electric dipoles caused by the local Te off-centering distortion. Using first-principles calculations, we prove that Te off-centering causes a change in the atomic reciprocal displacement and induces a ferroelectric Rashba effect in rock-salt MTe. Further analyses of the energy evaluation, lattice vibration and chemical orbital confirm that the atomic off-centering behavior manipulates Rashba spin splitting and stereochemical irregularities of the s2 lone electron pair of the cation, leading to the formation of strong anharmonic bonds in the original high-symmetry crystalline solids. The changes of the phonon dispersion and electronic structure also affect the electron-phonon scattering and scattering mechanism of MTe, which then manipulates the electrical transport properties. This work unravels the underlying physics on how the local Te off-centering distortion manipulates Rashba spin splitting and improves the thermoelectric performance of MTe.