Pump-probe spectroscopy of ultrathin 1T'-MoTe₂

Type II Weyl semimetal candidate MoTe₂ in bulk form undergoes a transition at ~250 K from the monoclinic phase (1T’-MoTe₂) to the inversion-symmetry breaking, orthorhombic phase (T_d-MoTe2). Previous transport and Raman measurements on thin MoTe₂ flakes have shown a dimensionally driven transition to the orthorhombic phase at room temperature for flakes less than ~12 nm. Using femtosecond broadband pump-probe spectroscopy, we impulsively excited MoTe₂ flakes of varying thickness that are protected from oxidation. The temporal evolution of the vibrational wave packet causes spectral modulations of electronic transitions. A probe pulse following the pump reveals any pump-induced time dependence of these signals, including oscillations of the initially excited coherent nuclear motion. Using some novel data analysis tools, and after background subtraction, we were able to follow characteristic phonon modes of each phase resolved in time and frequency domains.