Ultrafast electron calorimetry uncovers a new long-lived metastable state in 1T-TaSe$_2$ mediated by mode-selective electron-phonon coupling

TL;DR

This paper introduces a novel ultrafast electron calorimetry method using time-resolved photoemission spectroscopy to discover a long-lived metastable state in 1T-TaSe$_2$, revealing mode-selective electron-phonon interactions that alter phase stability.

Contribution

It presents a new technique for probing phase changes in quantum materials and uncovers a previously unknown metastable state with unique heat capacity properties.

Findings

Discovered a long-lived metastable state in 1T-TaSe$_2$

Identified mode-selective electron-phonon coupling effects

Showed reduced heat capacity facilitates phase transition

Abstract

Quantum materials represent one of the most promising frontiers in the quest for faster, lightweight, energy efficient technologies. However, their inherent complexity and rich phase landscape make them challenging to understand or manipulate in useful ways. Here we present a new ultrafast electron calorimetry technique that can systematically uncover new phases of quantum matter. Using time- and angle-resolved photoemission spectroscopy, we measure the dynamic electron temperature, band structure and heat capacity. We then show that this is a very sensitive probe of phase changes in materials, because electrons react very quickly, and moreover generally are the smallest component of the total heat capacity. This allows us to uncover a new long-lived metastable state in the charge density wave material 1T-TaSe$_2$, that is distinct from all of the known equilibrium phases: it is characterized by a significantly reduced effective heat capacity that is only 30% of the normal value, due to selective electron-phonon coupling to a subset of phonon modes. As a result, significantly less energy is required to melt the charge order and transform the state of the material than under thermal equilibrium conditions.