Signatures of exciton condensation in a transition metal dichalcogenide

TL;DR

This paper provides experimental evidence for exciton condensation in a transition metal dichalcogenide using momentum-resolved electron energy-loss spectroscopy, revealing a phase transition at 190 K.

Contribution

It demonstrates the first direct observation of exciton condensation in a 3D solid using M-EELS, highlighting a new method for studying quantum phase transitions.

Findings

Electronic mode energy drops to zero at nonzero momentum near T_c

Dynamical slowing down of plasma fluctuations observed

Valence electrons crystallize into an exciton condensate

Abstract

Bose condensation has shaped our understanding of macroscopic quantum phenomena, having been realized in superconductors, atomic gases, and liquid helium. Excitons are bosons that have been predicted to condense into either a superfluid or an insulating electronic crystal. Using the recently developed momentum-resolved electron energy-loss spectroscopy (M-EELS), we study electronic collective modes in the transition metal dichalcogenide semimetal, 1T-TiSe$_2$. Near the phase transition temperature, T$_c$ = 190 K, the energy of the electronic mode falls to zero at nonzero momentum, indicating dynamical slowing down of plasma fluctuations and crystallization of the valence electrons into an exciton condensate. Our study provides compelling evidence for exciton condensation in a three-dimensional solid and establishes M-EELS as a versatile technique sensitive to valence band excitations in quantum materials.