Momentum-dependent snapshots of a melting charge density wave

TL;DR

This study uses ultrafast spectroscopy to reveal that in 1T-TaS2, charge density wave order can rapidly melt independently of the lattice structure, challenging existing theories of CDW formation.

Contribution

It provides direct momentum-resolved evidence of electronic decoupling from lattice distortions during CDW melting in a prototypical material.

Findings

Charge density wave order melts before structural relaxation.

Electronic structure changes are momentum-dependent.

Decoupling challenges Fermi surface nesting as the primary CDW mechanism.

Abstract

Charge density waves (CDWs) underpin the electronic properties of many complex materials. Near-equilibrium CDW order is linearly coupled to a periodic, atomic-structural distortion, and the dynamics is understood in terms of amplitude and phase modes. However, at the shortest timescales lattice and charge order may become de-coupled, highlighting the electronic nature of this many-body broken symmetry ground state. Using time and angle resolved photoemission spectroscopy with sub-30-fs XUV pulses, we have mapped the time- and momentum-dependent electronic structure in photo-stimulated 1T-TaS2, a prototypical two-dimensional charge density wave compound. We find that CDW order, observed as a splitting of the uppermost electronic bands at the Brillouin zone boundary, melts well before relaxation of the underlying structural distortion. Decoupled charge and lattice modulations challenge the view of Fermi Surface nesting as a driving force for charge density wave formation in 1T-TaS2.