Research: Laser pulses switch between collective quantum states

Not long ago the discovery of a photo-induced insulator-to-metal transition (IMT) into a so-called hidden quantum state in the layered material 1T-TaS₂ has sparked significant. Fueled by potential applications – such as novel memory or quantum computing devices – large effort has been made to reveal the microscopic mechanism behind this fascinating non-thermal phase transition. Although this IMT is now believed to be connected to the switching between metastable collectively ordered quantum states, a clear picture of the optically driven changes and, in particular, the structure of that novel state remained elusive so far.

In this paper we characterize the hidden quantum state of 1T-TaS₂ by means of high resolution synchrotron x-ray diffraction in combination with picosecond single pulse laser pumping and show that the laser-driven IMT involves a marked rearrangement of the charge and orbital order in the direction perpendicular to the TaS₂-layers. We found that the collapse of interlayer molecular orbital dimers which form along this direction and which are a characteristic feature of the insulating phase, lies at the core of the transition into the hidden state.

Most surprisingly, the studied case shows very clearly that a single picosecond laser pulse can launch a transition between two very complex and truly long-range ordered electronic states.  While this result is highly interesting on its own, as it discloses a widely debated puzzle concerning the hidden state, it also illustrates on a more general level that out-of-plane correlations between the layers in Van der Waals materials can play a crucial role for their physical properties. This is certainly relevant with respect to the down-scaling of the sample thickness on the route towards possible technological applications.

Q. Stahl, M. Kusch, F. Heinsch, G. Garbarino, N. Kretschmar, K. Hanff, K. Rossnagel, J. Geck, T. Ritschel,
Collapse of layer dimerization in the photo-induced hidden state of 1T-TaS₂,
Nat. Commun. 11, 1247 (2020)