Probing the coherent amplitude mode in 1T-TaS2 with THz-scanning tunneling microscopy

Dr. Luis Enrique Parra Lopez1, Ms. Alkisti Vaitsi1, Ms. Vivien Sleziona1, Dr. Fabian Schulz2, Prof. Martin Wolf1, Dr. Melanie Müller1
1Fritz Haber Institute of the Max Planck Society, Berlin, Germany. 2CIC nanoGUNE, Donostia-San Sebastian, Spain


Interest in lightwave driven electronics has increased over the years due to its promising potential for application in novel quantum technologies. Moreover, its application to scanning tunneling microscopy (STM) has been key for the recent development of ultrafast STM. In particular, the use of single-cycle THz-pulses as lightwave bias in STM (THz-STM) has bridged the experimental gap between simultaneously achieving atomic spatial and femtosecond temporal resolution.  

A particularly interesting application for THz-STM is the study of the non-equilibrium dynamics of correlated quantum materials. An important and widely studied example is 1T-TaS2 which exhibits a commensurate charge density wave (C-CDW) phase leading to the opening of a Mott gap. Furthermore, it exhibits a metastable metallic (hidden) state characterized by the emergence of an amorphous-like phase of nanometer-sized CDW domains. Local investigation of the non-equilibrium dynamics of such states can offer valuable insights into the underlying interactions that govern the properties of quantum materials. 

Here we use THz-STM combined with ultrafast optical excitation to investigate the photoinduced dynamics of the C-CDW state in 1T-TaS2. Upon photoexcitation, we observe a 3% periodic modulation of the rectified current at a frequency of 2.5 THz which corresponds to the amplitude phonon mode in 1T-TaS2 .Furthermore, we discuss to what extent transient electronic temperatures, ultrafast Mott gap collapse and coherent AM oscillations can be probed by the rectified current in THz-STM. Our observations are a hallmark step towards the application of THz-STM to study the non-equilibrium dynamics of quantum materials at the atomic scale.