Upon excitation by a laser pulse, broken-symmetry phases of a wide variety of solids demonstrate similar order parameter dynamics characterized by a dramatic slowing down of relaxation for stronger pump fluences. Motivated by this recurrent phenomenology, we develop a simple nonperturbative effective model for photoinduced dynamics of collective bosonic excitations. We find that as the system recovers after photoexcitation, it shows universal prethermalized dynamics manifesting a power-law, as opposed to exponential, relaxation, explaining the slowing down of the recovery process. For strong quenches, long-wavelength overpopulated transverse modes dominate the long-time dynamics; their distribution function exhibits universal scaling in time and space, whose universal exponents can be computed analytically. Our model offers a unifying description of order parameter fluctuations in a regime far from equilibrium, and our predictions can be tested with available time-resolved techniques.
FIG. 2. Light-induced modification of the electronic structure. a,b Snapshots of the trARPES spectra along the kx momentum direction and for kz = 0 Å−1 . The data have been measured at 11 K with a probe photon energy of 10.75 eV and an absorbed pump fluence of 0.4 mJ/cm 2 . (a) Snapshot before photoexcitation (t < 0). At the Γ point of the Brillouin zone (kx = 0 Å−1 ), the flat anti-bonding VB is located around -0.16 eV, whereas the bonding VB appears around -0.65 eV. (b) Snapshot measured at the maximum of the pump-probe response (t = 0.3 ps). Upon photoexcitation, the VB is depleted in intensity and broadens significantly. Spectral weight is transfered above EF and accumulates close to Γ. c-f, Evolution of the photoexcited state (at t = 0.3 ps) along kx at representative kz momenta, as indicated in the labels. Note that the color scale is different from that of panels (a,b). The spectral weight above EF assumes a W-like shape consistent with the dispersion of the CB. The VB and CB never crosses each other and thus the gap size remains finite in the whole kx-kz momentum space around Γ. The white lines denote the energy-momentum dispersion calculated at the GW level (Fig. 1b). A rigid shift of -84 meV has been applied to the VB to account for the underestimated gap resulting from the GW method. The calculated dispersions have an excellent match with the experimental findings.
Nonlinear interactions between collective modes play a definitive role in far out of equilibrium dynamics of strongly correlated electron systems. Understanding and utilizing these interactions is crucial to photocontrol of quantum many-body states. One of the most surprising examples of strong mode coupling is the interaction between apical oxygen phonons and Josephson plasmons in bilayer YBa 2 Cu 3 O 6+x superconductors. Experiments by Hu et al. [Nat. Mater. 13, 705 (2014)] and Kaiser et al. [Phys. Rev. B 89, 184516 (2014)] showed that below T c , photoexcitation of phonons leads to enhancement and frequency shifts of Josephson plasmon edges, while above T c , photoexcited phonons induce plasmon edges even when there are no discernible features in the equilibrium reflectivity spectrum. Recent experiments by von Hoegen et al. (arXiv:1911.08284) also observed parametric generation of Josephson plasmons from photoexcited phonons both below T c and in the pseudogap phase. In this paper, we present a theoretical model of three-wave phonon-plasmon interaction arising from changes of the in-plane superfluid stiffness caused by the apical oxygen motion. Analysis of the parametric instability of plasmons based on this model gives frequencies of the most unstable plasmons that are in agreement with experimental observations. We also discuss how strong parametric excitation of Josephson plasmons can explain pump-induced changes in the terahertz reflectivity of YBa 2 Cu 3 O 6+x in the superconducting state, including frequency shifts and sharpening of Josephson plasmon edges, as well as appearance of a new peak around 2 THz. An interesting feature of this model is that overdamped Josephson plasmons do not give any discernible features in reflectivity in equilibrium, but can develop plasmon edges when parametrically excited. We suggest that this mechanism explains photoinduced plasmon edges in the pseudogap phase of YBa 2 Cu 3 O 6+x .
Recent experimental realizations of uniform confining potentials for ultracold atoms make it possible to create quantum acoustic resonators and explore nonequilibrium dynamics of quantum field theories. These systems offer a promising new platform for studying the dynamical Casimir effect, since they allow to achieve relativistic, i.e. near sonic, velocities of the boundaries. In comparison to previously studied optical and classical hydrodynamic systems ultracold atoms allow to realize a broader class of dynamical experiments combining both classical driving and vacuum squeezing. In this paper we discuss theoretically two types of experiments with interacting one dimensional condensates with moving boundaries. Our analysis is based on the Luttinger liquid model which utilizes the emergent conformal symmetry of the low energy sector of the Lieb-Liniger model. The first system we consider is a variable length interferometer with two Y-junctions connected back to back. We demonstrate that dynamics of the relative phase between the two arms of the interferometer can be analyzed using the formalism developed by Moore in the problem of electro-magnetic vacuum squeezing in a cavity with moving mirrors. The second system we discuss is a single condensate in a box potential with periodically moving walls. This system exhibits classical excitation of the mode resonant with the drive as well as nonlinear generation of off-resonant modes. In addition we find strong parametric multimode squeezing between modes whose energy difference matches integer multiples of the drive frequency. arXiv:1812.05114v1 [quant-ph]
The excitonic insulator is an electronically driven phase of matter that emerges upon the spontaneous formation and Bose condensation of excitons. Detecting this exotic order in candidate materials is a subject of paramount importance, as the size of the excitonic gap in the band structure establishes the potential of this collective state for superfluid energy transport. However, the identification of this phase in real solids is hindered by the coexistence of a structural order parameter with the same symmetry as the excitonic order. Only a few materials are currently believed to host a dominant excitonic phase, Ta 2 NiSe 5 being the most promising. Here, we test this scenario by using an ultrashort laser pulse to quench the broken-symmetry phase of this transition metal chalcogenide. Tracking the dynamics of the material’s electronic and crystal structure after light excitation reveals spectroscopic fingerprints that are compatible only with a primary order parameter of phononic nature. We rationalize our findings through state-of-the-art calculations, confirming that the structural order accounts for most of the gap opening. Our results suggest that the spontaneous symmetry breaking in Ta 2 NiSe 5 is mostly of structural character, hampering the possibility to realize quasi-dissipationless energy transport.
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