Nonequilibrium charge-density-wave order beyond the thermal limit

Abstract

The interaction of many-body systems with intense light pulses may lead to novel emergent phenomena far from equilibrium. Recent discoveries, such as the optical enhancement of the critical temperature in certain superconductors and the photo-stabilization of hidden phases, have turned this field into an important research frontier. Here, we demonstrate nonthermal charge-density-wave (CDW) order at electronic temperatures far greater than the thermodynamic transition temperature. Using time- and angle-resolved photoemission spectroscopy and time-resolved X-ray diffraction, we investigate the electronic and structural order parameters of an ultrafast photoinduced CDW-to-metal transition. Tracking the dynamical CDW recovery as a function of electronic temperature reveals a behaviour markedly different from equilibrium, which we attribute to the suppression of lattice fluctuations in the transient nonthermal phonon distribution. A complete description of the system’s coherent and incoherent order-parameter dynamics is given by a time-dependent Ginzburg-Landau framework, providing access to the transient potential energy surfaces.

Coherent Modulation of Quasiparticle Scattering Rates in a Photoexcited Charge-Density-Wave System

“…2(bc) show, within 140 fs after optical excitation, the CDW gap is fully suppressed, indicating a transition from the CDW to the metallic phase. However, as shown in a previous work [24], due to the substantial perturbation of the PES, the system subsequently overshoots beyond the metallic phase, followed by several weakly damped oscillations between metallic and CDW order, see the schematic Fig. 1b.…”

“…1(b), which initiates a melting of the CDW, as the system relaxes towards the new minimum corresponding to the metallic phase. However, for sufficiently strong excitation, the system overshoots across the minimum to the other side of the potential, leading to a reemergence of the CDW, followed by several damped oscillation cycles between metallic and CDW order [21][22][23], evident from a transient modulation of the CDW energy gap [24]. Besides the collective CDW excitation, we track the photocarrier population of high-energy states and discover a concurrent oscillation of scattering rates at the exact frequency of the CDW modulation.…”

“…We perform trARPES measurements on single crystals of TbTe 3 as described earlier [24]. The setup includes a tabletop fs XUV source (ℎ probe = 21.7 eV) with a synchronized optical pump laser (ℎ pump = 1.55 eV), using a hemispherical analyzer for photoelectron detection [28].…”

Multi-mode excitation drives disorder during the ultrafast melting of a C4-symmetry-broken phase

“…While the vast majority of ultrafast phase transitions measured to date have been interpreted in this coherent framework 10 – 16 , incoherent transitions have also been reported. The ultrafast phase transition in VO 2 was shown to result from broad distribution of modes, rather than a single OP for the entire system 17 .…”

Excited-state band structure mapping

“…Importantly, the method could also provide access to unoccupied states of quantum materials, to resolve topological features above the Fermi level [6], and for correlated materials, e.g. to access the spectral function of unoccupied states in strongly correlated oxides and charge density wave materials [80][81][82].…”