arXiv subjects

S. Sajad Dabiri

Publications and source records attributed to S. Sajad Dabiri.

9 recordsLinked to original sources

arXiv2026

Time-dependent Berry curvature and quantum metric of Floquet-Bloch states

The quantum geometry of Bloch bands, characterized by the Berry curvature and the quantum metric, underpins a wide range of linear and nonlinear responses in static systems. Here, we extend this framework to periodically driven (Floquet) systems by introducing a time-dependent Berry curvature and quantum metric defined directly in the Floquet-Bloch basis. We derive optical sum rules that relate the Fourier components of these geometric quantities to the optical conductivity and demonstrate that, under ideal Floquet-band occupations, the first-order DC Hall and longitudinal responses at harmonic frequencies vanish identically. We further introduce a mixed Berry curvature involving time and momentum derivatives, which gives rise to a non-adiabatic quantized charge-pumping mechanism that occurs naturally during each driving period without requiring adiabatic evolution. In addition, we identify the time-domain quantum metric as a measure of the energy fluctuations of a Floquet band and interpret its mixed components as quantifying polarization-energy correlations. A comprehensive symmetry analysis reveals how time-reversal, sublattice (chiral), particle-hole, inversion, rotational, and reflection symmetries constrain the time-dependent quantum geometric tensor and its associated topological invariant. Numerical simulations of the Rudner-Lindner-Berg-Levin model and a fully symmetric Floquet model confirm the analytical predictions. These results establish the time-dependent quantum geometric tensor as a unified framework for describing the geometric, topological, and dynamical properties of periodically driven quantum systems, with direct implications for optical spectroscopy, quantum transport, and topological charge-pumping experiments.

cond-mat.mes-hall↗

arXiv2025

Nonlinear Optical Response in Pseudo-Hermitian Systems at Steady State

We establish a steady-state theory for nonlinear optical conductivity in pseudo-Hermitian systems. We derive compact formulas for the first and second order conductivity tensors in both the velocity and length gauges and prove their exact equivalence through generalized sum rules and Berry connection identities by formulating the nonlinear response in terms of a biorthogonal density matrix. Utilizing the formalism on parity-time symmetric two-level systems reveals nonlinear phenomena that are not present in Hermitian systems, such as extra terms in the conductivity, corrections to the velocity operator, photocurrent, and resonance structures with higher-order poles at one-photon transitions. These features yield qualitatively distinct harmonic generation responses like real second-order conductivities and nonzero DC limits. These results provide new insights into nonlinear light-matter interactions in active media characterized by balanced gain and loss, with implications for non-Hermitian photonics, dissipative topological systems, and quantum devices designed with engineered dissipation.

cond-mat.mes-hall↗

arXiv2025

Velocity gauge formulation of nonlinear optical response in Floquet-driven systems

Using the velocity gauge formalism, we develop a theoretical framework for computing the nonlinear optical responses of time-periodic quantum systems. This approach complements the length gauge formulation and offers distinct advantages in both numerical and analytical treatments, particularly for atomic and solid-state systems with well-defined momentum-space structures. By applying our framework to the Rabi model, we derive numerical solutions in the velocity gauge and compare them with the length gauge, demonstrating full agreement between the two formulations. Our findings reveal rich optical phenomena, including photon-assisted transitions, frequency mixing effects, and emergent Floquet-induced photocurrents that are absent in static systems. We demonstrate that nonlinear responses in Floquet-driven systems exhibit resonances at integer multiples of the driving frequency, providing insights into ultrafast spectroscopy and Floquet engineering of quantum materials. The present formulation establishes a bridge between theoretical models and experimental observations in driven quantum systems, with potential applications in quantum optics, photonics, and next-generation optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2025

Dynamical nonlinear optical response in time-periodic quantum systems

We present a comprehensive theoretical framework for calculating the linear and nonlinear optical responses of time-periodic quantum systems. Using density matrix evolution in the Floquet basis and adopting the length gauge, our approach incorporates both interband and intraband contributions of the position operator, enabling detailed insights into photon-assisted transitions and their associated optical phenomena. Notably, we identify a divergent ac response to dc fields in Floquet systems, reminiscent of a Drude peak at finite frequencies. This framework generalizes to optical tensor conductivity calculations at arbitrary perturbation orders and captures various DC photocurrents, including shift current, injection current, and Berry dipole contributions, under specific limits. To demonstrate the versatility of our method, we compute linear and nonlinear optical conductivities for one- and two-dimensional systems, revealing phenomena such as band inversions, j-photon-assisted transitions, and high harmonic generation. These results highlight the interplay between periodic driving and nonlinear optical effects, offering different avenues for exploring dynamical topological properties and their applications in ultrafast spintronics, optoelectronics, and strongly correlated systems. Our findings provide a robust platform for analyzing the complex optical behavior of driven quantum systems and guiding experimental investigations in this rapidly evolving field.

cond-mat.mtrl-sci↗

arXiv2024

Modulating dichroism and optical conductivity in bilayer graphene under intense electromagnetic field irradiation

This study explores the impact of a strong perpendicular laser field on the electronic structure and optical conductivity of bilayer graphene. Employing the Floquet-Bloch theorem and a four-band Hamiltonian model, we calculate the optical conductivity, unveiling modified optical properties due to the altered band structure. We investigate the effects of both circularly and linearly polarized dressing fields on the electronic structure and optical conductivity in the system. Under linear polarization, we observe a notable anisotropy in the band dispersion and optical conductivity, resulting in linear dichroism. In the case of circular polarization, we anticipate the emergence of induced Berry curvature and circular dichroism, especially close to the dynamical gaps. When circularly polarized light is applied alongside a bias potential, the band structure differs for right-handed and left-handed polarization. In this case, the longitudinal optical conductivity remains the same for both, while the transversal optical conductivity exhibits distinct results. Furthermore, the induced Berry curvature and valley asymmetry introduce the potential for generating a valley-polarized current, enabling valley-selective pumping and leading to circular dichroism.

cond-mat.mes-hall↗

arXiv2023

Electric Circuit Simulation of Floquet Topological Insulators

We present a method for simulating any non-interacting and time-periodic tight-binding Hamiltonian in Fourier space using electric circuits made of inductors and capacitors. We first map the time-periodic Hamiltonian to a Floquet Hamiltonian, which converts the time dimension into a Floquet dimension. In electric circuits, this Floquet dimension is simulated as an extra spatial dimension without any time dependency in the electrical elements. The number of replicas needed in the Floquet Hamiltonian depends on the frequency and strength of the drive. We also demonstrate that we can detect the topological edge states (including the anomalous edge states in the dynamical gap) in an electric circuit by measuring the two-point impedance between the nodes. Our method paves a simple and promising way to explore and control Floquet topological phases in electric circuits.

cond-mat.mes-hall↗

arXiv2023

Floquet states and optical conductivity of an irradiated two dimensional topological insulator

We study the topology of the Floquet states and time-averaged optical conductivity of the lattice model of a thin topological insulator subject to a circularly polarized light using the extended Kubo formalism. Two driving regimes, the off-resonant and on-resonant, and two models for the occupation of the Floquet states, the ideal and mean-energy occupation, are considered. In the ideal occupation, the real part of DC optical Hall conductivity is shown to be quantized while it is not quantized for the mean energy distribution. The optical transitions in the Floquet band structure depend strongly on the occupation and also the optical weight which consequently affect all components of optical conductivity. At high frequency regime, we present an analytical calculation of the effective Hamiltonian and also its phase diagram which depends on the tunneling energy between two surfaces. The topology of the system shows rich phases when it is irradiated by a weak on-resonant drive giving rise to emergence of anomalous edge states.

cond-mat.mes-hall↗

arXiv2021

Light-induced topological phases in thin films of magnetically doped topological insulators

We study the photon-dressed electronic band structure of topological insulator thin films which could be also doped doped by magnetic impurities in response to an off-resonance time-periodic electromagnetic field. The thin films irradiated by a circularly polarized light undergo phase transition, in a driven system made by and a fascinating feature of distinct phases emerges in the phase diagram depending on the parameters such as frequency, intensity and polarization of the light. As a particular case, quantum anomalous Hall insulator phase is induced purely by the light-induced mass term with no need to any external magnetic field or even magnetization arising from the doped-doping magnetic impurities. Moreover, a novel phase, quantum pseudo-spin Hall insulator, emerges in the phase diagram leading to anisotropic helical edge states with zero total Chern number. We verify these achievements in the phase diagrams are supported by numerical calculations for a nanoribbon of the thin film for which the edge mode behavior is observed at several points on the phase diagram. The emergence of the mentioned topological phases and the edge modes are further confirmed by both calculating the Hall conductivity by means of the Kubo formula and the Chern number of each band. The effect of light parameters on the Landau level fan diagram in the presence of a perpendicular magnetic field indicates various topological phases occurring at higher Chern numbers.

cond-mat.mes-hall↗

arXiv2021

Engineering of topological phases in driven thin topological insulator: Structure inversion asymmetry effect

We investigate the effect of a high frequency electromagnetic field with both of circularly and linearly polarization, on the emergence of quantum phases on thin topological insulators. Simultaneously, the influence of the system parameters (such as magnetic impurity, thickness engineering and structural inversion asymmetry of the potential) on emergence of topological phases is studied. We take our attention to the high frequency regime in which it is possible to consider an expansion for the Floquet Hamiltonian in terms of orders of 1/Ω. The topological invariants are determined and it is demonstrated that some phase transitions between quantum anomalous Hall insulator, quantum pseudospin Hall insulator, quantum spin Hall insulator and normal insulator can be induced by altering the aforementioned parameters of the system. To avoid heating process, tuning of the system parameters gives us the opportunity to observe these phase transitions at small intensities of light.

cond-mat.mes-hall↗