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arXiv:2404.14411 · Physical review. B./Physical review. B

High harmonic tracking of ultrafast electron dynamics across the Mott to charge-density-wave phase transition

Marlena Dziurawiec, J. O. de Almeida, Mohit Lal Bera, Marcin Płodzień, Maciej Lewenstein, Tobias Graß, Ravindra W. Chhajlany, Maciej M. Maśka, Utso Bhattacharya

Published Jan 5, 2026• 0 citations
arXivHTMLPDF

Abstract

Different insulator phases compete with each other in strongly correlated materials with simultaneous local and non-local interactions. It is known that the homogeneous Mott insulator converts into a charge density wave (CDW) phase when the non-local interactions are increased, but there is ongoing debate on whether and in which parameter regimes this transition is of first order, or of second order with an intermediate bond-order wave phase. Here we show that strong-field optics applied to an extended Fermi-Hubbard system can serve as a powerful tool to reveal the nature of the quantum phase transition. Specifically, we show that in the strongly interacting regime characteristic excitations such as excitons, biexcitons, excitonic strings, and charge droplets can be tracked by the non-linear optical response to an ultrafast and intense laser pulse. Subcycle analysis of high harmonic spectra unravels the ultrafast dynamics of these increasingly complex objects, which partially escape the scrutiny of linear optics. Their appearance in the high harmonic spectrum provides striking evidence of a first-order transition into the CDW phase, and makes a strong case for using strong-field optics as a powerful tool to reveal the nature of quasiparticles in strongly correlated matter, and to track the electron dynamics during a first-order quantum phase transition.

DOI
10.1103/bl2s-cv34

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Different insulator phases compete with each other in strongly correlated materials with simultaneous local and non-local interactions. It is known that the homogeneous Mott insulator converts into a charge density wave (CDW) phase when the non-local interactions are increased, but there is ongoing debate on whether and in which parameter regimes this transition is of first order, or of second order with an intermediate bond-order wave phase. Here we show that strong-field optics applied to an extended Fermi-Hubbard system can serve as a powerful tool to reveal the nature of the quantum phase transition. Specifically, we show that in the strongly interacting regime characteristic excitations such as excitons, biexcitons, excitonic strings, and charge droplets can be tracked by the non-linear optical response to an ultrafast and intense laser pulse. Subcycle analysis of high harmonic spectra unravels the ultrafast dynamics of these increasingly complex objects, which partially escape the scrutiny of linear optics. Their appearance in the high harmonic spectrum provides striking evidence of a first-order transition into the CDW phase, and makes a strong case for using strong-field optics as a powerful tool to reveal the nature of quasiparticles in strongly correlated matter, and to track the electron dynamics during a first-order quantum phase transition.
By employing the exact diagonalization method, we investigate the high-harmonic generation (HHG) of the correlated systems under the strong laser irradiation. For the extended Hubbard model on a periodic chain, HHG close to the quantum critical point (QCP) is more significant compared to two neighboring gapped phases (i.e., charge-density-wave and spin-density wave states), especially in low frequencies. We confirm that the systems in the vicinity of the QCP are supersensitive to the external field and more optical-transition channels via excited states are responsible for HHG. This feature holds the potential of obtaining high-efficiency harmonics by making use of materials approaching QCP. Based on the two-dimensional Haldane model, we further propose that the even- or odd-order components of generated harmonics can be promisingly regarded as spectral signals to distinguish the topologically ordered phases from locally ordered ones. Our findings in this Letter pave the way to achieve ultrafast light source from HHG in strongly correlated materials and to study quantum phase transition by nonlinear optics in strong laser fields.
Abstract The use of intense tailored light fields is the perfect tool to achieve ultrafast control of electronic properties in quantum materials. Among them, Mott insulators are materials in which strong electron-electron interactions drive the material into an insulating phase. When shinning a Mott insulator with a strong laser pulse, the electric field may induce the creation of doublon-hole pairs, triggering an insulator-to-metal phase transition. In this work, we take advantage of the threshold character of this insulator-to-metal transition and we propose a pump-probe scheme that consists of a mid-infrared laser pulse and a train of short pulses separated by half-period of the mid-infrared with alternating phases. By varying the time-delay between the two pulses and the internal carrier envelope phase of the short pulses, we achieve control of the phase transition, that leaves it fingerprint at its high harmonic spectrum.
Strongly correlated materials exhibit some of the most complex and technologically useful phenomena in condensed matter. We use the world line and determinant quantum Monte Carlo techniques to explore the phases of the Hubbard Hamiltonian, a model describing strong interactions, in a number of different contexts. We first consider the spin, charge, and bond order correlations of the one-dimensional extended fermion Hubbard model in the presence of a coupling to the lattice. A static alternating lattice distortion leads to enhanced charge density wave correlations at the expense of antiferromagnetic order. When the lattice degrees of freedom are dynamic, we show that a similar effect occurs even though the charge asymmetry must arise spontaneously. Although the evolution of the total energy with lattice coupling is smooth, the individual components exhibit sharp crossovers at the phase boundaries. Finally, we observe a tendency for bond order in the region between the charge and spin density wave phases. Second, we examine mixtures of bosons and fermions in one-dimensional optical lattices. We evaluate the density profiles and bosonic visibility [special characters omitted] resolving the discrepancy between theory and experiment by identifying parameter regimes where [special characters omitted] is reduced and increased. We present a simple qualitative picture of the different response to the fermion admixture in terms of the superfluid and Mott-insulating domains before and after the fermions are included. Finally, we show that [special characters omitted] exhibits kinks which are tied to the domain evolution present in the pure bosonic case, and also additional structure arising from the formation of boson-fermion molecules, a prediction for future experiments. Third, we report large scale calculations of the effective bandwidth, momentum distribution, and magnetic correlations of the square lattice fermion Hubbard Hamiltonian. The sharp Fermi surface of the non-interacting limit is significantly broadened by the electronic correlations, but retains signatures of the approach to the edges of the first Brillouin zone as the density increases. Finite size scaling of simulations on large lattices allows us to extract the interaction dependence of the antiferromagnetic order parameter, exhibiting its evolution from weak-coupling to the strong-coupling Heisenberg limit. Our lattices provide improved resolution of the momentum distribution, allowing a more quantitative comparison with time-of-flight optical lattice experiments.
We investigate the laser-induced dynamics of electronically driven charge-density-wave (CDW) order. A comprehensive mean-field analysis of the attractive Hubbard model in the weak-coupling regime reveals ultrafast switching and ultrafast melting of the order via a nonthermal pathway. The resulting nonequilibrium phase diagram exhibits multiple distinct regimes of the order parameter dynamics upon increasing field strength, indicative of multiple dynamical phase transitions. Using an intuitive pseudospin picture, we show how the distinct dynamical regimes can be connected to the spin precession angle. We furthermore study the effects of electron-electron interactions beyond mean field to show that the main features of the phase diagram are robust against scattering or thermalization processes. For example, the nonthermal state with switched order is characterized by a particularly slow relaxation. The nonthermal phases can be stabilized by tailoring the pulse shape with optimal control theory. We also demonstrate how the dynamics allows to distinguish an electron-electron interaction driven CDW from an electron-phonon interaction driven CDW.
By employing an unbiased numerical method, we investigate the ultrafast dynamics of the correlated systmes in one and two dimensions. For the extended Hubbard model on a periodic chain, the high-harmonic generation (HHG) of system close to the quantum critical point (QCP) is more intense than HHG in the neighboring two gapped phases, i.e., charge-density-wave and spin-density-wave states, especially in low-frequencies. The reason can be attributed to that the system in QCP is more sensitive to the external field and there are more optical-allowed excited states available to furnish photon-absorbing channels for this nonlinear processes. This feature provides a possibility of detecting the quantum phase transition by means of high-harmonic spectroscopy. We also expect to generate stronger HHG by irradiating materials with parameter settings close to QCP. In addition, we find that the even- or odd-order components of HHG can be used to distinguish the topologically ordered phases from locally ordered ones in a two-dimensional interacting Haldane model.
The competition of interactions implies the compensation of standard mechanisms, which leads to the emergence of exotic phases between conventional phases. The extended Hubbard model (EHM) is a fundamental example for the competition of the local Hubbard interaction and the nearest-neighbor density-density interaction, which at half-filling and in one dimension leads to a bond-order wave (BOW) between a charge-density wave (CDW) and a quasi-long-range order Mott insulator. We study the full momentum-resolved excitation spectrum of the one-dimensional EHM in the CDW phase, and we clarify the relation between different elementary energy gaps. We show that the CDW-to-BOW transition is driven by the softening of a singlet exciton at momentum $\ensuremath{\pi}$. The BOW is realized as the condensate of this singlet exciton.