Abstract
A rapidly-growing interest in WTe2 has been triggered by the giant magnetoresistance effect discovered in this unique system. While many efforts have been made towards uncovering the electron- and spin-relevant mechanisms, the role of lattice vibration remains poorly understood. Here, we study the coherent vibrational dynamics in WTe2 crystals by using ultrafast pump-probe spectroscopy. The oscillation signal in time domain in WTe2 has been ascribed as due to the coherent dynamics of the lowest energy A1 optical phonons with polarization- and wavelength-dependent measurements. With increasing temperature, the phonon energy decreases due to anharmonic decay of the optical phonons into acoustic phonons. Moreover, a significant drop (15%) of the phonon energy with increasing pump power is observed which is possibly caused by the lattice anharmonicity induced by electronic excitation and phonon-phonon interaction.
Highlights
Mode as verified by polarization- and wavelength-dependent measurements
The asymmetric phonon spectral profile and a redshift of phonon frequency have been observed with increasing excitation density, implying that the softening of optical phonons is caused by the lattice anharmonicity induced by intense electronic excitation
The results suggest a high degree of lattice anharmonicity under intense excitation, implying possible involvement of electronic softening effect
Summary
The oscillation frequency decreases due to the decay of optical phonons into acoustic phonons. The asymmetric phonon spectral profile and a redshift of phonon frequency have been observed with increasing excitation density, implying that the softening of optical phonons is caused by the lattice anharmonicity induced by intense electronic excitation. The frequency of oscillation remains nearly identical (Fig. 2(a)) but the amplitude shows strong polarization dependence with a two-fold symmetry (Fig. 2(b)) which is expected for A1 optical phonon.
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