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Physica Scripta

Spectral weight redistribution and an indirect band gap mott insulator in a shaken optical lattice

Bhargav K. Alavani, Ramesh V. Pai

Published Dec 1, 2025• 0 citations
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Abstract

Abstract In this work, we explore the influence of shaking parameters on the single-particle excitation spectra of bosonic atoms confined in a one-dimensional shaken optical lattice. Using mean-field theory to solve the two-band Bose–Hubbard model, we obtain a phase diagram that qualitatively aligns with experimental observations. We employ the Random Phase Approximation (RPA) based on mean-field states to examine excitation spectra across different phases during the phase transitions. Notably, the gapless superfluid (SF) and π -SF phases exhibit roton-maxon excitations, whereas the gapped Mott insulator (MI) phase can manifest as a direct or indirect band-gap MI. These calculations provide detailed insights into how excitations evolve across different phases and how spectral weights areredistributed during the transitions.

DOI
10.1088/1402-4896/ae2990

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Abstract In this work, we explore the influence of shaking parameters on the single-particle excitation spectra of bosonic atoms confined in a one-dimensional shaken optical lattice. Using mean-field theory to solve the two-band Bose–Hubbard model, we obtain a phase diagram that qualitatively aligns with experimental observations. We employ the Random Phase Approximation (RPA) based on mean-field states to examine excitation spectra across different phases during the phase transitions. Notably, the gapless superfluid (SF) and π -SF phases exhibit roton-maxon excitations, whereas the gapped Mott insulator (MI) phase can manifest as a direct or indirect band-gap MI. These calculations provide detailed insights into how excitations evolve across different phases and how spectral weights areredistributed during the transitions.
We calculate the single-particle spectral function for the one-band Bose-Hubbard model within the random-phase approximation (RPA). In the strongly correlated superfluid, in addition to the gapless phonon excitations, we find extra gapped modes, which become particularly relevant near the superfluid-Mott quantum phase transition (QPT). The strength in one of the gapped modes, a precursor of the Mott phase, grows as the QPT is approached and evolves into a hole (particle) excitation in the Mott insulator depending on whether the chemical potential $\ensuremath{\mu}$ is above (below) the tip of the lobe. The sound velocity ccc of the Goldstone modes remains finite when the transition is approached at constant density; otherwise, it vanishes at the transition. It agrees well with Bogoliubov theory except close to the transition. We also calculate the spatial correlations for bosons in an inhomogeneous trapping potential creating alternating shells of Mott insulator and superfluid. Finally, we discuss the capability of the RPA to correctly account for quantum fluctuations in the vicinity of the QPT.
Motivated by the recent realization of the strongly correlated driven many-body system showing tunable continuous to the discontinuous quantum phase transition betweenMott insulator (MI) and superfluid with staggered phase (π-SF), we study the two-band extended Bose Hubbard model using the density matrix renormalization group with mean-field (CMFT+DMRG) method in one spatial dimension. In this model, the inter-band coupling and effective detuning energy are related to the resonant shaking frequency and amplitude. We calculate the superfluid order parameters and density of bosons in lower and upper bands to identify the underlining MI and π-SF phases. The bosons transfer from the lower band (ground state) to the upper band (first excited state) when the resonant shaking frequency is increased. For small shaking amplitude, the transfer of bosons is sudden giving rise to discontinuous MI to π-SF transition. For the case of large shaking amplitudes. the boson transferscontinuouslyreflect a continuous MI to π-SF transition. We compare our results with the earlier work.
We study an extended Bose-Hubbard model with next-nearest-neighbor (NNN) hopping in a shaken optical lattice. We show how mean-field phase diagram evolves with the change of NNN hopping amplitude t2{t}_{2}t2​, which can be easily tuned via shaking amplitude. As t2{t}_{2}t2​ increases, a Z2{Z}_{2}Z2​-symmetry-breaking superfluid (Z2SF)({Z}_{2}\mathrm{SF})(Z2​SF) phase emerges at the bottom of the Mott lobs. The tricritical points between normal superfluid, Z2SF{Z}_{2}\mathrm{SF}Z2​SF, and Mott insulator (MI) phases are identified. We further demonstrate the tricritical point can be tuned to the tip of the Mott lobe, in which case a new critical behavior has been predicted. Within random-phase approximation, excitation spectra in the three phases are obtained, which indicate how the phase transitions occur.
Abstract We perform the cluster mean-field with density matrix renormalization (CMFT+DMRG) calculation on the two-band extended Bose–Hubbard model to uncover the physics behind the discontinuous transitions observed in the one-dimensional shaken optical lattice with hybridized two lowest Bloch bands. We determine the superfluid, Mott insulator, and staggered superfluid phases associated with this model using the appropriate order parameters. We obtained the phase diagrams for two values of shaking amplitudes and illustrated the phases and nature of phase transitions. The transition from Mott insulator and superfluid to staggered superfluid is discontinuous for small shaking amplitudes. We analyse how the shaking frequency controls the effective chemical potential in the model and, consequently, the ground state energy that drives the discontinuous transition. Finally, we compare our results with those of earlier works.
Abstract Recent studies on ultracold atoms have reported the implementation of laser-assisted tunneling in lattice systems, facilitating the realization of topological phases. Motivated by such advances, we investigate a two-band Bose–Hubbard model with its single-particle energy bands showing gapless nodal lines, which can be realized for ultracold atoms in the cubic optical lattices using laser-assisted tunneling. We analyze the superfluid–Mott insulator (MI) phase transition in this model and obtain a global phase diagram by using the strong-coupling expansion method. The excitation spectra in strong and weak coupling limits inherit the topological properties of single-particle energy bands to show the structures of nodal lines. The topological invariants, the flat surface states and the critical properties are analyzed theoretically in detail. The excitation spectra in the MI and superfluid phases are measurable in ultracold atom experiments to prove our results.
We study ground-state phase diagrams and excitation spectra of Bose-Bose mixtures in an optical lattice by applying the Gutzwiller approximation to the two-component Bose-Hubbard model. A case of equal hoppings and equal intra-component interactions for both components is considered. Due to the existence of inter-component interaction, there appear several quantum phases, such as the superfluid, paired superfluid, and counterflow superfluid phases. We find that the transition from superfluid (SF) to Mott insulator (MI) with even filling factors can be of the first order for a wide range of the chemical potential. We calculate the excitation spectra as a useful probe to identify the quantum phases and the SF-to-MI transitions. In the excitation spectra of the SF phase, there are two gapless modes and a few gapful modes, which respectively correspond to phase and amplitude fluctuations of the order parameters. At the SF-to-MI transition point, we show that the energy gaps of certain amplitude modes reach zero for the second-order transition while they remain finite for the first-order one. Since the excitation spectrum can be measured by the Bragg scattering, we calculate the dynamical structure factor by using the linear response theory. We consider three types of density fluctuations, and show that the density fluctuations are coupled to different excitation branches in different quantum phases.