# Collapse of layer dimerization in the photo-induced hidden state of 1T-TaS2

## TL;DR

This paper reveals that the photo-induced insulator-to-metal transition in 1T-TaS2 involves the collapse of inter-layer molecular orbital dimers, providing insight into the microscopic mechanism of the hidden quantum state.

## Contribution

It identifies the collapse of inter-layer molecular orbital dimers as the key mechanism behind the photo-induced transition in 1T-TaS2, advancing understanding of its hidden quantum state.

## Key findings

- Collapse of inter-layer molecular orbital dimers occurs during transition
- Transition involves rearrangement of charge and orbital order
- Transition is a collective change between long-range ordered electronic states

## Abstract

Photo-induced switching between collective quantum states of matter is a fascinating rising field with exciting opportunities for novel technologies. Presently very intensively studied examples in this regard are nanometer-thick single crystals of the layered material 1T-TaS2 , where picosecond laser pulses can trigger a fully reversible insulator-to-metal transition (IMT). This IMT is believed to be connected to the switching between metastable collective quantum states, but the microscopic nature of this so-called hidden quantum state remained largely elusive up to now. Here we determine the latter by means of state-of-the-art x-ray diffraction 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. More specifically, we identify the collapse of inter-layer molecular orbital dimers, which are a characteristic feature of the insulating phase, as a key mechanism for the non-thermal IMT in 1T-TaS2, which indeed involves a collective transition between two truly long-range ordered electronic crystals.