Near-field infrared spectroscopy of monolayer MnPS3

S. N. Neal, H.-S. Kim, K. A. Smith, A. V. Haglund, D. G. Mandrus, H. A. Bechtel, G. L. Carr, K. Haule, D. Vanderbilt and J. L. Musfeldt

Phys. Rev. B 100, 075428 (2019)
We measured the near-field infrared response of MnPS3 in bulk, few-, and single-layer form and compared our findings with traditional far-field vibrational spectroscopies, a symmetry analysis, and first-principles lattice dynamics calculations. Trends in the Bu mode near 450 cm−1 are striking, with the disappearance of this structure in the thinnest sheets. Combined with the amplified response of the activated Ag mode and analysis of the Au + Bu features, we find that symmetry is unexpectedly increased in few- and single-sheet MnPS3 due to a restoration of the threefold axes of rotation. Monoclinicity in this system is therefore a consequence of the long-range stacking pattern and temperature rather than local structure.