title: Non-thermal separation of electronic and structural orders in a persisting charge density wave.
abstract: The simultaneous ordering of different degrees of freedom in complex materials undergoing spontaneous symmetry-breaking transitions often involves intricate couplings that have remained elusive in phenomena as wide ranging as stripe formation, unconventional superconductivity or colossal magnetoresistance. Ultrafast optical, X-ray and electron pulses can elucidate the microscopic interplay between these orders by probing the electronic and lattice dynamics separately, but a simultaneous direct observation of multiple orders on the femtosecond scale has been challenging. Here we show that ultrabroadband terahertz pulses can simultaneously trace the ultrafast evolution of coexisting lattice and electronic orders. For the example of a charge density wave (CDW) in 1T-TiSe2, we demonstrate that two components of the CDW order parameter--excitonic correlations and a periodic lattice distortion (PLD)--respond very differently to 12-fs optical excitation. Even when the excitonic order of the CDW is quenched, the PLD can persist in a coherently excited state. This observation proves that excitonic correlations are not the sole driving force of the CDW transition in 1T-TiSe2, and exemplifies the sort of profound insight that disentangling strongly coupled components of order parameters in the time domain may provide for the understanding of a broad class of phase transitions.
authors.affiliation: Department of Physics, University of Regensburg, 93040 Regensburg, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Porer
authors.affiliation: Department of Physics, University of Regensburg, 93040 Regensburg, Germany.
authors.forename: U
authors.initials: U
authors.lastname: Leierseder
authors.affiliation: Department of Physics, University of Regensburg, 93040 Regensburg, Germany.
authors.forename: J-M
authors.initials: JM
authors.lastname: Ménard
authors.affiliation: 1] Molecular and Surface Physics, University of Bielefeld, 33615 Bielefeld, Germany [2].
authors.forename: H
authors.initials: H
authors.lastname: Dachraoui
authors.affiliation: Department of Physics, University of Crete and FORTH/IESL, Heraklion, Crete 71110, Greece.
authors.forename: L
authors.initials: L
authors.lastname: Mouchliadis
authors.affiliation: Department of Physics, University of Crete and FORTH/IESL, Heraklion, Crete 71110, Greece.
authors.forename: I E
authors.initials: IE
authors.lastname: Perakis
authors.affiliation: Molecular and Surface Physics, University of Bielefeld, 33615 Bielefeld, Germany.
authors.forename: U
authors.initials: U
authors.lastname: Heinzmann
authors.affiliation: Institute of Physics, Ilmenau University of Technology, 98684 Ilmenau, Germany.
authors.forename: J
authors.initials: J
authors.lastname: Demsar
authors.affiliation: Institute of Experimental and Applied Physics, University of Kiel, 24098 Kiel, Germany.
authors.forename: K
authors.initials: K
authors.lastname: Rossnagel
authors.affiliation: Department of Physics, University of Regensburg, 93040 Regensburg, Germany.
authors.forename: R
authors.initials: R
authors.lastname: Huber