Using a THz-pump/THz-probe scheme in an STM, the team directly excited and detected coherent phonons in semiconducting 2H-MoTe₂ with atomic resolution. They observed long-lived oscillations corresponding to distinct lattice vibrational modes. Interestingly, the relative strength of in-plane shear and out-of-plane breathings modes varied in the vicinity of atomic defects. This variation indicates that local differences in the electronic structure, including electronic band bending, induced by defects, modulates the coupling between the THz near-field and phonon modes.

“By probing coherent phonons with atomic precision, we can see how individual defects influence ultrafast vibrational dynamics in ways that cannot be accessed by typical measurements of space-averaging techniques,” says Vibhuti Rai, postdoctoral researcher in the Franke group. “This opens up exciting possibilities for defect-engineered control of transient material properties and phase transitions at the nanoscale.”

The research was carried out within the framework of TRR227, where the researchers have intensively worked on advancing high-resolution techniques for studying ultrafast phenomena in quantum materials.

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Figure caption: 

Measurement scheme of THz STM: Two time-delayed THz pulses are coupled into an STM junction where the dc tunneling current is measured as a function of delay time.

Original publication:

Influence of atomic-scale defects on coherent phonon excitations by THz near fields in an STM
V. N. Rai, J. Sim, F. Faaber, N. Bogdanoff, S. Thrishin, P. Wiechers, T. S. Seifert, T. Kampfrath, C. Lotze, and K. J. Franke
Sci. Adv. 11 50, eadz6549 (2025) - DOI: 10.1126/sciadv.adz6549