Mapping the nonequilibrium order parameter of a quasi-two dimensional charge density wave system

The driving force of a charge density wave (CDW) transition in quasi-two dimensional systems is still debated, while being crucial in understanding electronic correlation in such materials. Here we use femtosecond time- and angle-resolved photoemission spectroscopy combined with computational methods to investigate the coherent lattice dynamics of a prototypical CDW system. The photo-induced temporal evolution of the periodic lattice distortion associated with the amplitude mode reveals the dynamics of the free energy functional governing the order parameter. Our approach establishes that optically-induced screening rather than CDW melting at the electronic level leads to a transiently modified potential which explains the anharmonic behaviour of the amplitude mode and discloses the structural origin of the symmetry-breaking phase transition.. Authors: Sayers, C. J. [Politecnico di Milano (Italy)] (ORCID:0000000237775155); Zhang, Y. [Harwell Science and Innovation Campus, Didcot (United Kingdom). STFC Central Laser Facility] (ORCID:0000000161844870); Sanders, C. E. [Harwell Science and Innovation Campus, Didcot (United Kingdom). STFC Central Laser Facility]; Chapman, R. T. [Harwell Science and Innovation Campus, Didcot (United Kingdom). STFC Central Laser Facility]; Wyatt, A. S. [Harwell Science and Innovation Campus, Didcot (United Kingdom). STFC Central Laser Facility] (ORCID:0000000186256709). DOE Contract: AC02-76SF00515. Subjects: electronic properties and materials; phase transitions and critical phenomena; quantum fluids and solids