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arXiv:2202.04322 · Physical Review Research

Glassy dynamics of the one-dimensional Mott insulator excited by a strong terahertz pulse

Kazuya Shinjo, Shigetoshi Sota, Takami Tohyama

Published Aug 3, 2022• 8 citations
arXivHTMLPDF

Abstract

The elucidation of nonequilibrium states in strongly correlated systems holds the key to emergence of novel quantum phases.The nonequilibrium-induced insulator-to-metal transition is particularly interesting since it reflects the fundamental nature of competition between itinerancy and localization of the charge degrees of freedom.We investigate pulse-excited insulator-to-metal transition of the half-filled one-dimensional extended Hubbard model.Calculating the time-dependent optical conductivity with the time-dependent density-matrix renormalization group, we find that strong mono-and half-cycle pulses inducing quantum tunneling strongly suppress spectral weights contributing to the Drude weight σ D , even if we introduce a large number of carriers n d .This is in contrast to a metallic behavior of σ D ∝ n d induced by photon absorption and chemical doping.The strong suppression of σ D in quantum tunneling is a result of the emergence of the Hilbert-space fragmentation, which makes pulse-excited states glassy.

DOI
10.1103/physrevresearch.4.l032019

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The elucidation of nonequilibrium states in strongly correlated systems holds the key to emergence of novel quantum phases.The nonequilibrium-induced insulator-to-metal transition is particularly interesting since it reflects the fundamental nature of competition between itinerancy and localization of the charge degrees of freedom.We investigate pulse-excited insulator-to-metal transition of the half-filled one-dimensional extended Hubbard model.Calculating the time-dependent optical conductivity with the time-dependent density-matrix renormalization group, we find that strong mono-and half-cycle pulses inducing quantum tunneling strongly suppress spectral weights contributing to the Drude weight σ D , even if we introduce a large number of carriers n d .This is in contrast to a metallic behavior of σ D ∝ n d induced by photon absorption and chemical doping.The strong suppression of σ D in quantum tunneling is a result of the emergence of the Hilbert-space fragmentation, which makes pulse-excited states glassy.
Recent advancements in pulse laser technology have facilitated the exploration of nonequilibrium spectroscopy of electronic states in the presence of strong electric fields across a broad range of photon energies. The Keldysh crossover serves as an indicator that distinguishes between excitations resulting from photon absorption triggered by near-infrared multicycle pulses and those arising from quantum tunneling induced by terahertz pulses. Using a time-dependent density-matrix renormalization group, we investigate the emergence of the Keldysh crossover in a one-dimensional (1D) Mott insulator. We find that the Drude weight is proportional to photo-doped doublon density when a pump pulse induces photon absorption. In contrast, the Drude weight is suppressed when a terahertz pulse introduces doublons and holons via quantum tunneling. The suppressed Drude weight accompanies glassy dynamics with suppressed diffusion, which is a consequence of strong correlations and exhibits finite polarization decaying slowly after pulse irradiation. In the quantum tunneling region, entanglement entropy slowly grows logarithmically. These contrasting behaviors between the photon-absorption and quantum tunneling regions are a manifestation of the Keldysh crossover in 1D Mott insulators and provide a novel methodology for designing the localization and symmetry of electronic states called subcycle-pulse engineering.
Photoinduced nonequilibrium states in the Mott insulators reflect the fundamental nature of competition between itinerancy and localization of the charge degrees of freedom. The spin degrees of freedom will also contribute to the competition in a different manner depending on lattice geometry. We investigate pulse-excited optical responses of a half-filled two-leg Hubbard ladder and compare them with those of a one-dimensional extended Hubbard chain. Calculating the time-dependent optical conductivity, we find that strong monocycle pulse inducing quantum tunneling gives rise to anomalous suppression of photo-induced in-gap weight, leading to negative weight. This is in contrast to finite positive weight in the Hubbard chain. The origin of this anomalous behavior in the two-leg ladder is attributed to photoinduced localized exciton that reflects strong spin-singlet dimer correlation in the ground state.
Abstract Motivated by the recent progress in time-resolved nonequilibrium spectroscopy in condensed matter, we study an optically excited one-dimensional ionic Hubbard model by exact diagonalization. The model is relevant to organic crystals, transition metal oxides, or ultracold atoms in optical lattices. We implement numerical pump-probe measurements to calculate time-dependent conductivity and single-particle spectral functions. In general, short optical excitation induces a metallic behavior imprinted as a Drude peak in conductivity or an in-gap density of states. In a Mott insulator, we find that the induced Drude peak oscillates at the pump frequency and its second harmonic. The former comes from the oscillation of currents, and the latter from the interference of single- and three-photon excited states. In a band insulator, the Drude peak oscillates only at the pump frequency, and quantities such as the double occupancy do not oscillate. The absence of the second harmonic oscillation is due to the degeneracy of multi-photon excited states. The in-gap density of states in both insulators correlates with the Drude weight and the energy absorption for weak pumping. Strong pumping leads to saturation of the in-gap density of states and to suppression of the Drude weight in the Mott regime. We have also checked that the above features are robust for insulators in the intermediate parameter range. Our study demonstrates the distinct natures of the multi-photon excited states in two different insulators.
We define, compute, and analyze the nonequilibrium differential optical conductivity of the one-dimensional extended Hubbard model at half-filling after applying a pump pulse, using the time-dependent density matrix renormalization group method. The melting of the Mott insulator is accompanied by a suppression of the local magnetic moment and ensuing photogeneration of doublon-holon pairs. The differential optical conductivity reveals (i) mid-gap states related to parity-forbidden optical states and (ii) strong renormalization and hybridization of the excitonic resonance and the absorption band, yielding a Fano resonance. We offer evidence and interpret such a resonance as a signature of nonequilibrium optical excitations resembling excitonic strings, (bi)excitons, and unbound doublon-holon pairs, depending on the magnitude of the intersite Coulomb repulsion. We discuss our results in the context of pump and probe spectroscopy experiments on organic Mott insulators.
We study photoinduced optical responses of one-dimensional strongly correlated electron systems. The optical conductivity spectra are calculated for the ground state and a photoexcited state in the one-dimensional Hubbard model at half filling by using the exact diagonalization method. It is found that, in the Mott insulator phase, the photoexcited state has large spectral weights including the Drude weight below the optical gap. As a consequence, the spectral weight above the optical gap is largely reduced. These results imply that a metallic state is induced by photoexcitation. Comparison between the photoexcited and hole-doped states shows that the photoexcitation is similar to chemical doping.
Utilizing the exact diagonalization method, we investigate the one-dimensional Peierls-Hubbard model at quarter filling, where it manifests as an antiferromagnetic Mott insulator in units of dimers. By increasing the on-site Coulomb repulsion UUU, we observe a significant suppression of the Drude peak, based on a nonequilibrium linear-response theory capable of capturing the zero-frequency (Drude) weight of the optical conductivity under periodic boundary conditions. However, after the ultrafast photoirradiation of this model with large UUU, we detect a distinct enhancement of the Drude peak, signifying the onset of a photoinduced insulator-metal transition. Comparing these dynamics with the half-filled Hubbard model and a noninteracting spinless half-filled Su-Schrieffer-Heeger model (corresponding to the quarter-filled Peierls-Hubbard model with infinite UUU), we propose a mechanism for the photoinduced metallic state: The empty-occupied and double-occupied dimers serve as the photoinduced charge carriers, akin to the holons and doublons in the Hubbard model.