  The relation between the polar structural instability and superconductivity
in a Weyl semimetal candidate MoTe2 has been clarified by finely controlled
physical and chemical pressure. The physical pressure as well as the chemical
pressure, i.e., the Se substitution for Te, enhances the superconducting
transition temperature Tc at around the critical pressure where the polar
structure transition disappears. From the heat capacity and thermopower
measurements, we ascribe the significant enhancement of Tc at the critical
pressure to a subtle modification of the phonon dispersion or the semimetallic
band structure upon the polar-to-nonpolar transition. On the other hand, the
physical pressure, which strongly reduces the interlayer distance, is more
effective on the suppression of the polar structural transition and the
enhancement of Tc as compared with the chemical pressure, which emphasizes the
importance of the interlayer coupling on the structural and superconducting
instability in MoTe2.
