arXiv subjects

Tarun Kanti Ghosh

Publications and source records attributed to Tarun Kanti Ghosh.

At least 19 recordsLinked to original sources

arXiv2026

Zeeman spin quantum geometry and intrinsic gyrotropic spin current

We uncover an intrinsic gyrotropic spin current (IGSC) driven solely by an oscillating magnetic field, arising from the spin version of Zeeman quantum geometry. Using the density matrix formalism, we derive the IGSC in the low-frequency clean limit, in terms of the Zeeman spin quantum metric and the Zeeman spin Berry curvature. The effect is studied in a minimal Rashba-Dresselhaus model that also supports intrinsic gyrotropic charge currents from the Zeeman Berry curvature. The spin and charge currents exhibit a $π/2$ phase shift and are found to be perpendicular to each other for certain parameter ranges. This not only enables spin current detection but also opens pure spin current channels for energy-efficient spintronics. We further observe that these currents switch directions between longitudinal and transverse relative to the applied field, depending on the relative strength of Rashba and Dresselhaus interactions.

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arXiv2026

Non-linear spin current in the surface states of topological insulators

Spin Hall effect is one of the primary sources of pure spin current in spintronic devices, observed in spin-orbit coupled materials. In this work, we investigate linear and non-linear pure spin currents in the hexagonally warped surface states of topological insulators like $\rm Bi_2Te_3$, using the formalism of conserved spin transport. The hexagonal warping effect enables the application of this formalism by providing a well-defined Berry curvature without breaking the time-reversal symmetry. The absence of in-plane spin currents in the linear response motivates the extension of the formalism to the non-linear regime, by combining the perturbation of Bloch states with the Boltzmann transport equation. The second-order response exhibits both intrinsic and extrinsic in-plane spin currents induced by the spin-weighted Berry curvature, Berry connection polarizability (BCP) dipole and Berry curvature dipole (BCD) respectively. The band anisotropy due to the $C_{3v}$ symmetry of the material ($\rm Bi_2Te_3$) propagates through the spin textures and the band geometric quantities, resulting in the selection of certain components of the spin conductivity tensors.

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arXiv2026

Non-linear in-plane spin current in spin-orbit coupled 2D hole gases

The non-linear transport of charge and spin due to the emergence of band geometric effects has garnered much interest in recent years. In this work, we show that a linear in-plane spin current vanishes, whereas a non-linear (second-order) in-plane spin current exists for a generic two-dimensional system having time-reversal symmetry. The intrinsic second-order spin current originates from the spin Berry curvature polarizability. The formulation when applied to 2D hole gases with the $k^3$ Rashba spin-orbit coupling reveals the existence of both transverse and longitudinal second-order spin currents normal to the spin orientation. Interestingly, anisotropic spin-orbit couplings can generate collinearly polarized spin current (spins polarized in the direction of spin current) in the second-order. The effects of anisotropy are explored by introducing an additional Dresselhaus spin-orbit coupling and electromagnetic radiation over the isotropic Rashba system. The generation and control over the multiple in-plane spin currents may have important applications in spintronic devices.

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arXiv2026

Resonant spin Hall and Nernst effect in a nanoribbon of a spin-orbit coupled electronic system

We present a theoretical study of spin Hall phenomenon in a nanoribbon of a two-dimensional electronic system with Rashba and Dresselhaus spin-orbit coupling. We model the electronic system by a square lattice in real space. We show that such nanoribbon can give rise to a number of additional spin degeneracy points as well as anticrossing points, apart from the $Γ$ point, between two opposite spin subbands. We compute the SHC and demonstrate that it diverges and gives rise to a resonance when the chemical potential passes through those spin degenerate or anticrossing points. Contrary to the previous studies, here such resonance emerges even without any external perturbation like magnetic field or light. We also examine the spin Nernst effect and find that it shows clear peaks at the anticrossing and spin degeneracy points, consistent with the Mott relation at low temperature. Finally, we also investigate the signature of such additional spin degeneracy and anticrossing points in the longitudinal conductance by using the retarded Green function approach in lattice model. The finite width induced subbands are reflected in the longitudinal conductance, which takes quantized values of $2n e^{2}/{h}$ where $n$ denotes the number of bands occupied by the chemical potential with each band having spin split subbands. We also note that anticrossing that occurs at low energy between two opposite spin subbands could be also detected via longitudinal conductance.

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arXiv2024

Bulk-boundary correspondence in extended trimer Su-Schrieffer-Heeger model

We consider an extended trimer Su-Schrieffer-Heeger (SSH) tight-binding Hamiltonian keeping up to next-nearest-neighbor (NNN) hopping terms and on-site potential energy. The Bloch Hamiltonian can be expressed in terms of all the eight generators (i.e. Gell-Mann matrices) of the SU(3) group. We provide exact analytical expressions of three dispersive energy bands and the corresponding eigenstates for any choices of the system parameters. The system lacks full chiral symmetry since the energy spectrum is not symmetric around zero, except at isolated Bloch wavevectors. We explore parity, time reversal, and certain special chiral symmetries for various system parameters. We discuss the bulk-boundary correspondence by numerically computing the Zak phase for all the bands and the boundary modes in the open boundary condition. There are three different kinds of topological phase transitions, which are classified based on the gap closing points in the Brillouin zone (BZ) while tuning the nearest-neighbor (NN) and NNN hopping terms. We find that quantized changes (in units of $π$) in two out of three Zak phases characterize these topological phase transitions. We propose another bulk topological invariant, namely the {\it sub-lattice winding number}, which also characterizes the topological phase transitions changing from $ ν^α = 0 \leftrightarrow 2 $ and $ ν^α = 0 \leftrightarrow 1 \leftrightarrow 2 $ ($α$: sub-lattice index). The sub-lattice winding number not only provides a relatively simple analytical understanding of topological phases but also successfully establishes bulk-boundary correspondence in the absence of inversion symmetry, which may help in characterizing the bulk-boundary correspondence of systems without chiral and inversion symmetry.

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arXiv2024

Polarization and third-order Hall effect in III-V semiconductor heterojunctions

We study Berry connection polarizability (BCP) induced electric polarization and third-order Hall (TOH) effect in a two-dimensional electron/hole gas (2DEG/2DHG) with Rashba-Dresselhaus (RD) spin-orbit couplings in III-V semiconductor heterostructures. The electric polarization decreases with the increase of the Fermi energy and is responsive to the electric field orientation in the presence of RD spin-orbit couplings for both systems. We determine the BCP-induced TOH conductivity ($χ_{\perp}^{\text{I}}$) along with the TOH conductivity associated with the band velocity ($χ_{\perp}^{\text{II}}$). We find that the presence of an infinitesimal amount of Dresselhaus coupling in addition to the dominant Rashba coupling results in finite TOH responses. These conductivities vanish when the field is aligned with and/or orthogonal to the symmetry lines $k_x\pm k_y=0$ in both systems. For typical system parameters in a 2DEG with $k$-linear RD interactions, the magnitude of $χ_{\perp}^{\text{I}}$ is smaller than that of $χ_{\perp}^{\text{II}}$. On the other hand, when both the SO couplings are comparable, $χ_{\perp}^{\text{I}}$ shows a notable increase in magnitude, owing to the distinctive characteristics of BCP. The TOH conductivity of 2DEG remains unchanged when Rashba and Dresselhaus spin-orbit couplings are exchanged. For 2DHG with $k$-cubic RD interactions, $χ_{\perp}^{\text{I},h}$ exhibits a larger magnitude compared to $χ_{\perp}^{\text{II},h}$. Unlike the electron case, the BCP induced $χ_{\perp}^{\text{I},h}$ alters under the exchange of spin-orbit coupling parameters, whereas $χ_{\perp}^{\text{II},h}$ remains the same.

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arXiv2023

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 $δ_0$. For $δ_0 > 0$, 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 $δ_0$ 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

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 $σ_s=e/(8π)$. 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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arXiv2022

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 $ω\sim ω_0+ ω_1 q^2$ nature as observed in other three-dimensional 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

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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arXiv2021

Multi-class probabilistic atlas-based whole heart segmentation method in cardiac CT and MRI

Accurate and robust whole heart substructure segmentation is crucial in developing clinical applications, such as computer-aided diagnosis and computer-aided surgery. However, segmentation of different heart substructures is challenging because of inadequate edge or boundary information, the complexity of the background and texture, and the diversity in different substructures' sizes and shapes. This article proposes a framework for multi-class whole heart segmentation employing non-rigid registration-based probabilistic atlas incorporating the Bayesian framework. We also propose a non-rigid registration pipeline utilizing a multi-resolution strategy for obtaining the highest attainable mutual information between the moving and fixed images. We further incorporate non-rigid registration into the expectation-maximization algorithm and implement different deep convolutional neural network-based encoder-decoder networks for ablation studies. All the extensive experiments are conducted utilizing the publicly available dataset for the whole heart segmentation containing 20 MRI and 20 CT cardiac images. The proposed approach exhibits an encouraging achievement, yielding a mean volume overlapping error of 14.5 % for CT scans exceeding the state-of-the-art results by a margin of 1.3 % in terms of the same metric. As the proposed approach provides better-results to delineate the different substructures of the heart, it can be a medical diagnostic aiding tool for helping experts with quicker and more accurate results.

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arXiv2020

Dynamical polarization and plasmons in noncentrosymmetric metals

We study the dynamical polarization function and plasmon modes for spin-orbit coupled noncentrosymmetric metals (NCMs). These systems have different Fermi surface topology for Fermi energies above and below the spin degenerate point which is also known as the band touching point (BTP). We calculate the exact dynamical polarization function numerically and also provide its analytical expression in the long wavelength limit. We obtain the plasmon dispersion within the framework of random phase approximation. In NCMs, there is a finite energy gap in between intra and interband particle hole continuum (PHC) for vanishing excitation wavevector. In the long wavelength limit, the width of interband PHC behaves differently for Fermi energies below and above the BTP as a clear signature of the Fermi surface topology change. We find a single undamped optical plasmon mode lying in between the intra and interband PHC for Fermi energies above and below the BTP. The plasmon mode below the BTP has smaller velocity than that of above the BTP. It is interesting to find that as we tune the Fermi energy around the BTP, the plasmon mode becomes damped within a range of e-e interaction strength. For Fermi energies above and below the BTP, we also obtain an approximate analytical result of plasma frequency and plasmon dispersion which match well with their numerical counterparts in the long wavelength limit. The plasmon dispersion is $\propto q^2$ with $q$ being the wave vector for plasmon excitation in the long wavelength limit. We find that varying the carrier density with fixed e-e interaction strength or vice versa does not change the number of undamped plasmon mode, although damped plasmon modes can be more in number for some values of these parameters. We demonstrate our results by calculating the loss function and optical conductivity which can be measured in experiments.

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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

Thermoelectric and optical probes for a Fermi surface topology change in noncentrosymmetric metals

Noncentrosymmetric metals such as Li$_2$(Pd$_{1-x}$Pt$_x$)$_3$B have different Fermi surface topology below and above the band touching point where spin-degeneracy is not lifted by the spin-orbit coupling. We investigate thermoelectric and optical response as probes for this Fermi surface topology change. We show that the chemical potential displays a dimensional crossover from a three-dimensional to one-dimensional characteristics as the descending Fermi energy crosses the band touching point. This dimensional crossover is due to the existence of different Fermi surface topology above and below the band touching point. We obtain an exact expression of relaxation time due to short-range scatterer by solving Boltzmann transport equations self-consistently. The thermoelctric power and figure of merit are significantly enhanced as the Fermi energy goes below the band touching point owing to the underlying one-dimensional-like nature of noncentrosymmteric bulk metals. The value of thermoelectric figure of merit goes beyond two as the Fermi energy approaches to the van Hove singularity for lower spin-orbit coupling. Similarly, the studies of the zero-frequency and finite-frequency optical conductivities in the zero-momentum limit reflect the nature of topological change of the Fermi surface. The Hall coefficient and optical absorption width exhibit distinct signatures in response to the changes in Fermi surface topology.

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