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arXiv:2303.17085 · Physical review. B./Physical review. B

Photoinduced inhomogeneous melting of charge order by ultrashort pulsed light excitation

Hitoshi Seo, Yasuhiro Tanaka

Published Aug 8, 2024• 2 citations
arXivHTMLPDF

Abstract

We numerically investigate photoresponses of the charge ordered state upon stimuli of pulsed laser light, especially paying attention to the differences in the pulse width, whose shortness has been a key to experimentally realize large photoinduced effects. As a model for charge ordering, we consider an interacting spinless fermion model on a one-dimensional chain coupled to classical phonons and numerically simulate its real-time dynamics. First, we demonstrate the case where spatially uniform dynamics take place, and show the delayed response of phonons after the photoirradiation causing destabilization of charge order. When a local impurity potential is introduced in the initial state, an inhomogeneous charge order melting occurs and the delay in the timescale also happens in the real-space melting process. Finally, by investigating wider parameter ranges using different pulse widths, we find that the use of ultrashort pulses considerably expands the frequency window for charge order melting; it is indispensable in realizing the photoinduced phase transition with off-resonant conditions.

DOI
10.1103/physrevb.110.085118

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We numerically investigate photoresponses of the charge ordered state upon stimuli of pulsed laser light, especially paying attention to the differences in the pulse width, whose shortness has been a key to experimentally realize large photoinduced effects. As a model for charge ordering, we consider an interacting spinless fermion model on a one-dimensional chain coupled to classical phonons and numerically simulate its real-time dynamics. First, we demonstrate the case where spatially uniform dynamics take place, and show the delayed response of phonons after the photoirradiation causing destabilization of charge order. When a local impurity potential is introduced in the initial state, an inhomogeneous charge order melting occurs and the delay in the timescale also happens in the real-space melting process. Finally, by investigating wider parameter ranges using different pulse widths, we find that the use of ultrashort pulses considerably expands the frequency window for charge order melting; it is indispensable in realizing the photoinduced phase transition with off-resonant conditions.
We theoretically investigate photoresponses of a correlated electron system upon stimuli of a pulsed laser light. Real-time dynamics of an interacting spinless fermion model on a one-dimensional chain, as a model of charge order (CO), are numerically simulated using the time-dependent Hartree-Fock method. In particular, we discuss the differences between two situations as the initial state: the homogeneous order and the presence of a domain wall, i.e., a kink structure embedded in the CO bulk. Coherent dynamics are seen in the former case: When the frequency of the pump light ${\ensuremath{\omega}}_{p}$ is varied, along with single particle excitations across the CO gap (${\mathrm{\ensuremath{\Delta}}}_{\mathrm{CO}}$), the resonantly-excited collective phase mode near ${\ensuremath{\omega}}_{p}\ensuremath{\simeq}{\mathrm{\ensuremath{\Delta}}}_{\mathrm{CO}}/2$ efficiently destabilizes CO. In clear contrast, in the latter case, when ${\ensuremath{\omega}}_{p}$ is tuned at such in-gap frequencies and the intensity of light is sufficiently large, inhomogeneity spreads out from the kink to the bulk region through kink creations. Moreover, even stronger intensity induces the inhomogeneous melting of CO where the CO gap is destroyed.
We theoretically investigate the transient spectral function during the photoinduced melting of charge order in a correlated electron system, to unravel the dynamical processes triggered by different initial excitations. We employ a one-dimensional interacting spinless fermion model introducing a pulsed laser light, and perform a comparative study by the Hartree-Fock approximation and by the exact diagonalization method to numerically solve the time-dependent Schrödinger equation. We find characteristic behavior in the transient spectral function, whose features strongly depend on the pump light frequency ωpω_pωp​. When ωpω_pωp​ is resonant with the collective phase mode of frequency Ωc≃ΔCO/2Ω_c\simeq Δ_{\rm CO}/2Ωc​≃ΔCO​/2, where ΔCOΔ_{\rm CO}ΔCO​ is the charge gap, the transient spectral function exhibits a photoinduced in-gap weight which triggers large responses. With increasing the laser intensity, the development of in-gap weight directly turns into the collapse of the gap. This charge-order destabilization process is in sharp contrast to the case of ωp>ΔCOω_p>Δ_{\rm CO}ωp​>ΔCO​, where the photoirradiation induces interband electron-hole excitations giving rise to a shrinkage of the gap. The impact of quantum fluctuations and spatial inhomogeneity on the photoinduced dynamics is also discussed.
Transient quantum dynamics in an interacting fermion-phonon system are investigated with a focus on a charge order (CO) melting after a short optical-pulse irradiation and the roles of the quantum phonons in the transient dynamics. A spinless-fermion model in a one-dimensional chain coupled with local phonons is analyzed numerically. The infinite time-evolving block decimation algorithm is adopted as a reliable numerical method for one-dimensional quantum many-body systems. Numerical results for the photoinduced CO melting dynamics without phonons are well interpreted by the soliton picture for the CO domains. This interpretation is confirmed by numerical simulation of an artificial local excitation and the classical soliton model. In the case of large phonon frequencies corresponding to the antiadiabatic condition, CO melting is induced by propagations of the polaronic solitons with the renormalized soliton velocity. On the other hand, in the case of small phonon frequencies corresponding to the adiabatic condition, the first stage of the CO melting dynamics occurs due to the energy transfer from the fermionic to phononic systems, and the second stage is brought about by the soliton motions around the bottom of the soliton band. The analyses provide a standard reference for photoinduced CO melting dynamics in one-dimensional many-body quantum systems.
We investigate the out-of-equilibrium dynamics of a photoexcited charge-density-wave (CDW) state in the square-lattice Holstein model, in a setup similar to a pump-probe experiment. At half-filling, the ground state of this system is characterized by a checkerboard modulation of particle densities, accompanied by a concomitant lattice distortion. An efficient real-space dynamics method integrating the von Neumann equation for electron density matrix and Newton equation for classical lattice dynamics is developed to simulate the dynamical evolution of a photoexcited Holstein system. We find that the energy injected by a short pump pulse results in the reduction of the CDW order and the generation of coherent phonons. At strong photoexcitations, while the CDW order is melted in the sense that the time-averaged order parameter vanishes, a dynamical CDW state is self-sustained by the strong coherent phonon oscillations. Our large-scale simulations further uncover a dynamical regime of intermediate fluence where complex pattern formation is induced by the pump pulse. The emergent spatial textures are characterized by super density modulations on top of the short-range checkerboard CDW order. Our demonstration of pattern formation highlights the significance of dynamical inhomogeneity in quantum many-body systems in pump-probe experiments.
Lattice effects on photo-excited states in interacting charge frustrated system are examined. Real time dynamics in the interacting spinless fermion model on a triangular lattice coupled to lattice vibration are analyzed by applying the exact diagonalization method combined with the classical equation of motion. A photo-induced phase transition from the horizontal stripe-type charge order (CO) to the 3-fold CO occurs through a characteristic intermediate time domain. By analyzing the time evolution in detail, we find that this characteristic dynamics are seen when the electron and lattice sectors are not complementary to each other but show cooperative time evolutions. We also find that there are threshold values in the optical fluorescence and electron-lattice coupling for emergence of the photo-induced 3-fold CO. The dynamics are distinct from those from the vertical stripe-type CO, in which a monotonic CO melting occurs. A scenario of the photo-induced CO phase transition with lattice degree of freedom is presented from a view point of charge frustration.
We investigate the real-time dynamics of the half-filled one-dimensional extended Hubbard model in the strong-coupling regime, when driven by a transient laser pulse. Starting from a wide regime displaying a charge-density wave in equilibrium, a robust photoinduced in-gap state appears in the optical conductivity, depending on the parameters of the pulse. Here, by tuning its conditions, we maximize the overlap of the time-evolving wave function with excited states displaying the elusive bond-ordered wave of this model. Finally, we make a clear connection between the emergence of this order and the formation of the aforementioned in-gap state, suggesting the potential observation of purely electronic (i.e., not associated with a Peierls instability) bond-ordered waves in experiments involving molecular crystals.