arXiv subjects

Bashab Dey

Publications and source records attributed to Bashab Dey.

16 recordsLinked to original sources

arXiv2026

Generalized s-d model for Wannier-Mott excitons in layered magnetic semiconductors

The recent discovery of excitons coupled to the magnetic order, and the consequent strong magneto-optic responses, in some van der Waals magnetic semiconductors has triggered intense activity at the interface of magnetism and semiconductor optics. Here, we present an analytically tractable minimal model that describes magnetic order, electrons, holes, and excitons within a unified framework, thereby capturing a wide range of phenomena. It treats the magnetic order and itinerant carriers to be comprised by distinct electronic orbitals that are mutually coupled via orbital-dependent onsite exchange, similar to the treatment of metallic magnets using an s-d model. Investigating CrSBr bilayer as a case study, we benchmark our model and its predictions against recent experimental and ab-initio results finding good agreement as well as new insights enabled by the model's simplicity. Examining the optical selection rules, we find the conservation of a quantum number formed from a combination of spin and layer pseudospin to be a useful guiding principle, even in noncollinear magnetic configurations. Our analysis finds a series of bright and dark excitonic states in such layered A-type antiferromagnets. The presented framework should be valuable in achieving intuitive understanding of recently discovered excitonic phenomena and guiding the discovery of other excitonic states in layered magnetic semiconductors.

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arXiv2026

Quantum sensing composite excitations in an anisotropic ferromagnet via a qubit

Ordered magnets harbor intrinsically squeezed ground states and magnonic excitations characterized by entanglement between spins and nonclassical magnon number composition. A pathway to detecting the superpositions of noneigenmode magnon number states underlying these nonclassical magnetic ground states has recently been demonstrated by utilizing a qubit coupled to the magnon mode via a direct dispersive interaction. Here, we theoretically develop this qubit spectroscopy further delineating the capabilities and limitations of this qubit spectroscopy for sensing the quantum superpositions that underlie the excited states. We demonstrate that the spectroscopy lends itself naturally to unraveling the superpositions that underlie the various quantized squeezed-magnon number states. However, excited states comprising superpositions of multiple squeezed Fock states become increasingly hard due to the large number of possible transitions, and resulting peaks, in the qubit spectroscopy thereby requiring qubits with narrower linewidths. Along the same lines, we theoretically demonstrate the qubit spectroscopy of a low amplitude coherent squeezed-magnon state analyzing the tradeoff between frequency crowding due to multiple transitions and peak linewidths. Our work lays the groundwork and design equations for deploying high-quality qubits towards sensing the composite nature of spin excitations in magnetic systems.

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arXiv2025

Quantum sensing magnonic number states using a bosonic mode as the probe

Sensing number states of a magnonic mode has been accomplished using a superconducting qubit by realizing an effective dispersive interaction between the two systems. Here, we theoretically demonstrate that a seemingly classical bosonic mode can be utilized as a probe for resolving the number states of a magnon mode, while outperforming a qubit in various regards as the sensor. Considering another magnon mode in an antiferromagnet as the probe mode, we delineate the required dispersive coupling emerging directly from antiferromagnetic exchange interaction. When a phonon is used as the probe mode, we derive the effective dispersive coupling emerging from the lowest-order nonlinear magnon-phonon interactions. Our two considered examples provide the general design principles for identifying and utilizing a bosonic probe mode for sensing magnonic superpositions in a physical platform of interest.

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arXiv2024

Role of anisotropic confining potential and elliptical driving in dynamics of a Ge hole qubit

The squeezing of a Ge planar quantum dot enhances the Rabi frequency of electric dipole spin resonance by several orders of magnitude due to a strong Direct Rashba spin-orbit interaction in such geometries (Bosco et al 2021 Phys. Rev. B 104 115425). We investigate the geometric effect of an elliptical (squeezed) confinement and its interplay with the polarization of driving field in determining the Rabi frequency of a heavy-hole qubit in a planar Ge quantum dot. To calculate the Rabi frequency, we consider only the p-linear SOIs viz. electron-like Rashba, hole-like Rashba and hole-like Dresselhaus which are claimed to be the dominant ones by recent studies on planar Ge heterostructures. We derive approximate analytical expressions of the Rabi frequency using a Schrieffer-Wolff transformation for small SOI and driving strengths. Firstly, for an out-of-plane magnetic field with magnitude B, we get an operating region with respect to B, squeezing and polarization parameters where the qubit can be operated to obtain 'clean' Rabi flips. On and close to the boundaries of the region, the higher orbital levels strongly interfere with the two-level qubit subspace and destroy the Rabi oscillations, thereby putting a limitation on squeezing of the confinement. The Rabi frequency shows different behaviour for electron-like and hole-like Rashba SOIs. It vanishes for right (left) circular polarization in presence of purely electron-like (hole-like) Rashba SOI in a circular confinement. For both in- and out-of-plane magnetic fields, higher Rabi frequencies are achieved for squeezed configurations when the ellipses of polarization and the confinement equipotential have their major axes aligned but with different eccentricities. We also deduce a simple formula to calculate the effective heavy hole mass by measuring the Rabi frequencies using this setup.

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arXiv2024

Photodriven germanium hole qubit

Hole qubits in germanium quantum dots are promising candidates for coherent control and manipulation of the spin degree of freedom through electric dipole spin resonance. We theoretically study the time dynamics of a single heavy-hole qubit in a laser-driven planar germanium quantum dot confined laterally by a harmonic potential in presence of linear and cubic Rashba spin-orbit couplings and an out-of-plane magnetic field. We obtain an approximate analytical formula of the Rabi frequency using a Schrieffer-Wolff transformation and establish a connection of our model with the ESDR results obtained for this system. For stronger beams, we employ different methods such as unitary transformation and Floquet theory to study the time evolution numerically. We observe that high radiation intensity is not suitable for the qubit rotation due to the presence of high frequency noise superimposed on the Rabi oscillations. We display the Floquet spectrum and highlight the quasienergy levels responsible for the Rabi oscillations in the Floquet picture. We study the interplay of both the types of Rashba couplings and show that the Rabi oscillations, which are brought about by the linear Rashba coupling, vanish for typical values of the cubic Rashba coupling in this system.

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arXiv2022

Nonlinear magnetotransport in a two-dimensional system with merging Dirac points

We study the linear, second-order nonlinear (NL) current and voltage responses of a two-dimensional gapped semi-Dirac system with merging Dirac nodes along the $x$ direction under the influence of a weak magnetic field ($B$), using the semiclassical Boltzmann formalism. We investigate the effect of band geometric quantities like Berry curvature and orbital magnetic moment in the responses up to linear order in $B$. We derive exact analytical expressions of the linear magnetoconductivities, second-harmonic NL anomalous Hall (NAH), and anomalous velocity and Lorentz force induced (NAL) conductivities, unveiling their dependence on Fermi energy and a gap parameter $\delta_0$. For $\delta_0 > 0$, the Fermi surface topology changes at a particular Fermi energy, which is reflected in the nature of conductivities through a kink. The ratio of NAL and NAH conductivities is found to be independent of $\delta_0$ and inversely related to Fermi energy. The NL dc current exhibits distinct orientations depending on the Fermi energy, magnetic field, polarization of the electromagnetic wave. In the presence of magnetic field, the NL dc current vector can be rotated through large angles on variation of Fermi energy. For high Fermi energies, the NL dc current is directed nearly along the $y$-axis for $x$-polarized and low-frequency circularly polarized light, whereas it aligns close to $x$-axis for high-frequency circularly polarized light. These orientations of the NL dc current are predominantly governed by the mirror symmetry of the system along the $x$ direction. Additionally, we also study the NL voltage responses of the system by applying current along the $x$ and $y$ directions. The system exhibits asymmetry in the $B$-dependencies of the NL resistivities, which may serve as an experimentally relevant signature for band geometric quantities and merging Dirac nodes in such systems.

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arXiv2022

Berry curvature induced anisotropic magnetotransport in a quadratic triple-component fermionic system

Triple-component fermions are pseudospin-1 quasiparticles hosted by certain three-band semimetals in the vicinity of their band-touching nodes [Phys. Rev. B {\bf 100}, 235201 (2019)]. The excitations comprise of a flat band and two dispersive bands. The energies of the dispersive bands are $E_{\pm}=\pm\sqrt{α^2_n k^{2n}_\perp+v^2_z k^2_z}$ with $k_\perp=\sqrt{k^2_x+k^2_y}$ and $n=1,2,3$. In this work, we obtain the exact expression of Berry curvature, approximate form of density of states and Fermi energy as a function of carrier density for any value of $n$. In particular, we study the Berry curvature induced electrical and thermal magnetotransport properties of quadratic $(n=2)$ triple-component fermions using semiclassical Boltzmann transport formalism. Since the energy spectrum is anisotropic, we consider two orientations of magnetic field (${\bf B}$): (i) ${\bf B}$ applied in the $x$-$y$ plane and (ii) ${\bf B}$ applied in the $x$-$z$ plane. For both the orientations, the longitudinal and planar magnetoelectric/magnetothermal conductivities show the usual quadratic-$B$ dependence and oscillatory behaviour with respect to the angle between the applied electric field/temperature gradient and magnetic field as observed in other topological semimetals. However, the out-of-plane magnetoconductivity has an oscillatory dependence on angle between the applied fields for the second orientation but is angle-independent for the first one. We observe large differences in the magnitudes of transport coefficients for the two orientations at a given Fermi energy. A noteworthy feature of quadratic triple-component fermions which is typically absent in conventional systems is that certain transport coefficients and their ratios are independent of Fermi energy within the low-energy model.

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arXiv2021

Berry curvature induced magnetotransport in 3D noncentrosymmetric metals

We study the magnetoelectric and magnetothermal transport properties of noncentrosymmetric metals using semiclassical Boltzmann transport formalism by incorporating the effects of Berry curvature and orbital magnetic moment. These effects impart quadratic-B dependence to the magnetoelectric and magnetothermal conductivities, leading to intriguing phenomena such as planar Hall effect, negative magnetoresistance, planar Nernst effect and negative Seebeck effect. The transport coefficients associated with these effects show the usual oscillatory behavior with respect to the angle between the applied electric field and magnetic field. The bands of noncentrosymmetric metals are split by Rashba spin-orbit coupling except at a band touching point. For Fermi energy below (above) the band touching point, giant (diminished) negative magnetoresistance is observed. This difference in the nature of magnetoresistance is related to the magnitudes of the velocities, Berry curvature and orbital magnetic moment on the respective Fermi surfaces, where the orbital magnetic moment plays the dominant role. The absolute magnetoresistance and planar Hall conductivity show a decreasing (increasing) trend with Rashba coupling parameter for Fermi energy below (above) the band touching point.

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arXiv2021

Magnus spin Hall and spin Nernst effects in gapped 2D Rashba systems

We study the Magnus transport in a gapped 2D electron gas with Rashba spin-orbit coupling using semiclassical Boltzmann transport formalism. Apart from its signature in the charge transport coefficients, the inclusion of Magnus velocity in the spin current operator enables us to study Magnus spin transport in the system. In particular, we study the roles of mass gap and Fermi surface topology on the behavior of Magnus Hall and Nernst conductivities and their spin counterparts. We find that the Magnus spin Hall conductivity vanishes in the limit of zero gap, unlike the universal spin Hall conductivity $\sigma_s=e/(8\pi)$. The Magnus spin currents with spin polarization perpendicular to the applied bias (electrical/thermal) are finite while with polarization along the bias vanishes. Each Magnus conductivity displays a plateau as Fermi energy sweeps through the gap and has peaks (whose magnitudes decrease with the gap) when the Fermi energy is at the gap edges.

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arXiv2021

Dynamical polarization, optical conductivity and plasmon mode of a linear triple component fermionic system

We investigate the density and optical responses of a linear triple component fermionic system in both non-interacting and interacting regimes by computing its dynamical polarization function, RPA dielectric function, plasmon mode and long wavelength optical conductivity and compare the results with those of Weyl fermions and three-dimensional free electron gas. Linear triple component fermions are pseudospin-1 generalization of Weyl fermions, consisting of two linearly dispersive bands and a flat band. The presence of flat band brings about notable modifications in the response properties with respect to Weyl fermions such as induction of a new region in the particle-hole continuum, increased static polarization, reduced plasmon gap, shift in absorption edge, enhanced rate of increase in energy absorption with frequency and highly suppressed intercone transitions in the long wavelength limit. The plasmon dispersion follows the usual $\omega \sim \omega_0+ \omega_1 q^2$ nature as observed in other three-dimensional systems.

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arXiv2021

Role of Berry curvature in the generation of spin currents in Rashba systems

We study the background (equilibrium), linear and nonlinear spin currents in 2D Rashba spin-orbit coupled systems with Zeeman splitting and in 3D noncentrosymmetric metals using modified spin current operator by inclusion of the anomalous velocity. The linear spin Hall current arises due to the anomalous velocity of charge carriers induced by the Berry curvature. The nonlinear spin current occurs due to the band velocity and/or the anomalous velocity. For 2D Rashba systems, the background spin current saturates at high Fermi energy (independent of the Zeeman coupling), linear spin current exhibits a plateau at the Zeeman gap and nonlinear spin currents are peaked at the gap edges. The magnitude of the nonlinear spin current peaks enhances with the strength of Zeeman interaction. The linear spin current is polarized out of plane, while the nonlinear ones are polarized in-plane. We witness pure anomalous nonlinear spin current with spin polarization along the direction of propagation. In 3D noncentrosymmetric metals, background and linear spin currents are monotonically increasing functions of Fermi energy, while nonlinear spin currents vary non-monotonically as a function of Fermi energy and are independent of the Berry curvature. These findings may provide useful information to manipulate spin currents in Rashba spin-orbit coupled systems.

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arXiv2020

Topological phase transition induced by band structure modulation in a Chern insulator

Here we study the systematic evolution of the topological properties of a Chern insulator in presence of an electronic dispersion that can be tuned smoothly from being Dirac-like till a semi-Dirac one and beyond. The band structure under such controlled deformation shows that the two Dirac points approach each other, merge at an intermediate point (the ${\mathbf{M}}$ point), where the low energy spectrum turns gapless, shows anisotropic Dirac features in the $k$-space and is denoted as the semi-Dirac limit, however a gap eventually opens up again in the spectrum. The Chern number phase diagram obtained via integrating the Berry curvature over the Brillouin zone (BZ) shows a gradual shrinking of the 'topological' lobes, and vanishes just beyond the semi-Dirac limit of the electronic dispersion. Thus there is a phase transition from a topological phase to a trivial phase across the semi-Dirac point. The vanishing of the anomalous Hall conductivity plateau and the merger of the chiral edge states with the bulk bands near the ${\mathbf{M}}$ point provide robust support of the observed phase transition.

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arXiv2020

Valley caloritronics in a photodriven hetero-junction of Dirac materials

We consider a lateral hetero-junction where the left and right leads are made of monolayer graphene and the middle region is made of a gapped tilted Dirac material (borophene or quinoid graphene) illuminated with off-resonant circularly polarized radiation. The tilt parameter $v_t$ makes the band gap indirect and smaller in magnitude as compared to Dirac materials without tilt. Exposure to radiation makes the band gaps of the central region valley-dependent which show their signatures as valley polarized charge and thermal currents, thereby causing a valley Seebeck effect. We study the variation of the valley polarized electrical conductance, thermal conductance, thermopower and figure of merit of this junction with chemical potential $μ$ and a tunable gap parameter $η$. For non-zero $η$, all the valley polarized quantities are peaked at certain values of chemical potential and then vanish asymptotically. Increase in gap parameter enhances the valley thermopower and valley figure of merit, whereas the valley conductances (electrical and thermal) show non monotonic behavior with $η$. We also compare the valley polarized quantities with their corresponding charge counterparts (effective contribution from both the valleys). The charge thermopower and the charge figure of merit behave non monotonically with $η$ and the charge conductances (electrical and thermal) depict a decreasing trend with $η$. Furthermore, the tilt parameter reduces the effective transmission of carriers through the junction, thereby diminishing all the charge and valley polarized quantities. As the gaps in the dispersion can be adjusted by varying the intensity of light as well as the Semenoff mass, the tunability of this junction with regard to its thermoelectric properties may be experimentally realizable.

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arXiv2020

Unconventional phases in a Haldane model of dice lattice

We propose a Haldane-like model of dice lattice analogous to graphene and explore its topological properties within the tight-binding formalism. The topological phase boundary of the system is identical to that of Haldane model of graphene but the phase diagram is richer than the latter due to existence of a distorted flat band. The system supports phases which have a "gapped-out" valence (conduction) band and an indirect overlap between the conduction (valence) band and the distorted flat band. The overlap of bands imparts metallic character to the system. These phases may be further divided into topologically trivial and nontrivial ones depending on the Chern number of the "gapped-out" band. The semimetallic phases exist as distinct points that are well separated from each other in the phase diagram and exhibit spin-1 Dirac-Weyl dispersion at low energies. The Chern numbers of the bands in the Chern-insulating phases are $0$ and $\pm2$. This qualifies the system to be candidate for quantum anomalous Hall effect with two chiral channels per edge. Counterpropagating edge states emanate from the flat band in certain topologically trivial phases. The system displays beating pattern in Shubnikov de Haas oscillations for unequal magnitude of mass terms in the two valleys. We show that the chemical potential and ratio of topological parameters of the system viz. Semenoff mass and next-neighbor hopping amplitude may be experimentally determined from the number of oscillations between the beating nodes and the beat frequency, respectively.

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arXiv2019

Floquet topological phase transition in $α$-$\mathcal{T}_3$ lattice

We investigate topological characteristics of the photon-dressed band structure of $α$-$\mathcal{T}_3$ lattice on being driven by off-resonant circularly polarized radiation. We obtain exact analytical expressions of the quasienergy bands over the first Brillouin zone. The broken time-reversal symmetry caused by the circularly polarized light lifts the triple point degeneracy completely at both the Dirac points. The gaps become unequal at $ {\bf K}$ and ${\bf K}^{\prime}$ (except at $α=0$ and 1), which reveals the absence of inversion symmetry in the system. At $α=1/\sqrt{2}$, the gap between flat and valence bands closes at ${\bf K}$, while that between conduction and flat bands closes at ${\bf K}^{\prime}$, thereby restoring a semimetalic phase. At the gap closing point ($α=1/\sqrt{2}$) which is independent of the radiation amplitude, there is a reappearance of low-energy Dirac cones around ${\bf K}$ and ${\bf K}^{\prime}$ points. Under the influence of the circularly polarized radiation, the $α$-$\mathcal{T}_3$ lattice is transformed from semimetal to a Haldane-like Chern insulator characterized by non-zero Chern number. The system undergoes a topological phase transition from $\mathcal{C} = 1 (-1)$ to $\mathcal{C}=2 (-2)$ at $α=1/\sqrt{2}$, where $\mathcal{C}$ is the Chern number of the valence (conduction) band. This sets an example of a multiband system having larger Chern number. These results are supported by the appearance of chiral edge states in irradiated $α$-$\mathcal{T}_3$ nanoribbon.

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arXiv2018

Photoinduced valley and electron-hole symmetry breaking in $ α$-$T_3$ lattice: The role of a variable Berry phase

We consider $α$-$T_3$ lattice illuminated by intense circularly polarized radiation in terahertz regime. We present quasienergy band structure, time-averaged energy spectrum and time-averaged density of states of $α$-$T_3$ lattice by solving the Floquet Hamiltonian numerically. We obtain exact analytical expressions of the quasienergies at the Dirac points for all values of $α$ and field strength. We find that the quasienergy band gaps at the Dirac point decrease with increase of $α$. Approximate forms of quasienergy and band gaps at single and multi-photon resonant points are derived using rotating wave approximation. The expressions reveal a stark dependence of quasienergy on the Berry phase of the charge carrier. The quasi energy flat band remains unaltered in presence of radiation for dice lattice ($α=1$). However, it acquires a dispersion in and around the Dirac and even-photon resonant points when $0<α<1$. The valley degeneracy and electron-hole symmetry in the quasienergy spectrum are broken for $0<α<1$. Unlike graphene, the mean energy follows closely the linear dispersion of the Dirac cones till near the single-photon resonant point in dice lattice. There are additional peaks in the time-averaged density of states at the Dirac point for $0 < α\leq 1$.

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