arXiv subjects

Parijat Sengupta

Publications and source records attributed to Parijat Sengupta.

At least 19 recordsLinked to original sources

arXiv2020

Anomalous Lorenz number in massive and tilted Dirac systems

We analytically calculate the anomalous transverse electric and thermal currents in massive and tilted Dirac systems, using $ β$-borophene as a representative material, and report on conditions under which the corresponding Lorenz number $\left(\mathcal{L}_{an}\right)$ deviates from its classically accepted value $\left(\mathcal{L}_{0}\right)$. The deviations in the high-temperature regime are shown to be an outcome of the quantitative difference in the respective kinetic transport expressions for electric $\left(σ\right)$ and thermal $\left(κ\right)$ conductivity, and are further weighted through a convolution integral with a non-linearly energy-dependent Berry curvature that naturally arises in a Dirac material. In addition, the tilt and anisotropy of the Dirac system that are amenable to change via external stimulus are found to quantitatively influence $ \mathcal{L}_{an} $. The reported deviations from $\mathcal{L}_{0} $ hold practical utility inasmuch as they allow an independent tuning of $σ$ and $ κ$, useful in optimizing the output of thermoelectric devices.

cond-mat.mtrl-sci↗

arXiv2020

Anomalous photo-thermal effects in multi-layered semi-Dirac black phosphorus

The photo-excitation of multi-layered potassium doped black phosphorus that exists as a gapped semi-Dirac two-dimensional normal insulator (NI), transforms it into a time-reversal symmetry broken Chern insulator (CI). This transition from an NI to CI happens under a high-frequency radiation corresponding to the off-resonant regime of the Floquet theory of periodic perturbations, which here is the impinging light beam. This photo-induced topological CI phase manifests as a finite Berry curvature and forms the basis of anomalous transverse heat flow in presence of a longitudinal temperature gradient. The anomalous variants of the Ettinghausen (EE) and Righi-Leduc effects (RLE), via their respective coefficients are quantitatively analyzed in this work. The strength of anomalous EE and RLE coefficients is a direct outcome of the sum of Berry curvatures over the occupied bands and is shown to drop as the Fermi level is positioned high in the conduction states or deep in the valence region. The drop is attributed to the flipped sign of the Berry curvature in the conduction and valence bands. Additionally, when the Fermi level lies in the band gap, the contribution of the conduction and valence bands may numerically outweigh one another for a much larger value of the coefficients. Finally, we point out how beyond the role of Berry curvature, several laboratory accessible methods can be utilized to modulate the EE and RLE coefficients, including an application of strain, variations in the dopant concentration, and frequency of incident radiation.

cond-mat.mtrl-sci↗

arXiv2019

Electron g-factor engineering for non-reciprocal spin photonics

We study the interplay of electron and photon spin in non-reciprocal materials. Traditionally, the primary mechanism to design non-reciprocal photonic devices has been magnetic fields in conjunction with magnetic oxides, such as iron garnets. In this work, we present an alternative paradigm that allows tunability and reconfigurability of the non-reciprocity through spintronic approaches. The proposed design uses the high-spin-orbit coupling of a narrow-band gap semiconductor (InSb) with ferromagnetic dopants. A combination of the intrinsic and a gate-applied electric field gives rise to a strong external Rashba spin-orbit coupling (RSOC) in a magnetically doped InSb film. The RSOC which is gate alterable is shown to adjust the magnetic permeability tensor via the electron g-factor of the medium. We use electronic band structure calculations (k$\cdot$p theory) to show the gate-adjustable RSOC manifest itself in the non-reciprocal coefficient of photon fields via shifts in the Kerr and Faraday rotations. In addition, we show that photon spin properties of dipolar emitters placed in the vicinity of a non-reciprocal electromagnetic environment is distinct from reciprocal counterparts. The Purcell factor (F$_{p}$) of a spin-polarized emitter (right-handed circular dipole) is significantly enhanced due to a larger g-factor while a left-handed dipole remains essentially unaffected. Our work can lead to electron spin controlled reconfigurable non-reciprocal photonic devices.

physics.app-ph↗

arXiv2019

Spin-valley coupled thermoelectric energy converter with strained honeycomb lattices

A caloritronic device setup is proposed that harnesses the intrinsic spin-valley locking of two-dimensional honeycomb lattices with graphene-like valleys, for instance, silicene and stanene. Combining first-principles and analytic calculations, we quantitatively show that when sheets of such materials are placed on a ferromagnetic substrate and held between two contacts at different temperatures, an interplay between the electron degrees-of-freedom of charge, spin, and valley arises. A manifestation of this interplay are finite charge, spin, and valley currents. Uniaxial strain that adjusts the buckling height in silicene-type of lattices, in conjunction with an applied electric field, is shown to further modulate the aforementioned currents. We link these calculations to a Seebeck-like thermopower generator and obtain expressions (and means to optimize them) for two spin-valley polarized performance metrics--the thermodynamic efficiency and thermoelectric figure of merit. A closing summary outlines possible enhancements to presented results through the inherent topological order and substrate-induced external Rashba spin-orbit coupling that exists in silicene-type materials.

cond-mat.mtrl-sci↗

arXiv2018

Photo-caloritronics in the THz regime: Photon-assisted thermopower generators

The generation of thermopower in a miniaturized device modeled as a channel connected to reservoirs/contacts maintained at different temperatures is studied in this work. The left reservoir is also coupled to a periodic THz driving source leading to a rearrangement of its internal energy levels. The modified levels are determined using Floquet theory that governs the dynamics of a periodic Hamiltonian. The starting point for the presented thermopower calculations (mirrored in the related thermal gradient controlled current) is the Landauer formula rooted in the transmission formalism. Primarily, we show that while thermally activated electrons can be pumped from the hot reservoir into the cold side as a straightforward manifestation of the Seebeck effect through a difference in their respective Fermi levels, the quantum of charge flow increases in presence of the periodic perturbation. We explain this phenomenon by noting how the periodic driving makes available a greater number of states in the left reservoir that are able to inject electrons into the channel. The calculations also uncover a useful feature whereby the strength of such a thermally-pumped current is amenable through a joint control of the amplitude and frequency of the signal, offering an additional experimentally-adjustable tool to regulate their flow. The calculated results are shown for two classes of materials defined by prototypical linear and quadratic dispersion, with the former capable of furnishing a larger current by virtue of higher available density-of-modes. We close by pointing out possible improvements to the calculation by accounting for dissipative effects in the channel.

cond-mat.mes-hall↗

arXiv2018

Optical excitation and detection of neuronal activity

Optogenetics has emerged as an exciting tool for manipulating neural activity, which in turn, can modulate behavior in live organisms. However, detecting the response to the optical stimulation requires electrophysiology with physical contact or fluorescent imaging at target locations, which is often limited by photobleaching and phototoxicity. In this paper, we show that phase imaging can report the intracellular transport induced by optogenetic stimulation. We developed a multimodal instrument that can both stimulate cells with high spatial resolution and detect optical pathlength changes with nanometer scale sensitivity. We found that optical pathlength fluctuations following stimulation are consistent with active organelle transport. Furthermore, the results indicate a broadening in the transport velocity distribution, which is significantly higher in stimulated cells compared to optogenetically inactive cells. It is likely that this label-free, contactless measurement of optogenetic response will provide an enabling approach to neuroscience.

physics.bio-ph↗

arXiv2018

Anomalous heat flow in 8-$Pmmn$ borophene with tilted Dirac cones

We analytically establish an anomalous transverse flow of heat in 8-\textit{Pmmn} borophene, one of the several two-dimensional (2D) allotropes of Boron (B). The dispersion of this allotrope contains a pair of anisotropic and tilted Dirac cones which are gapped by placing the 2D \textit{B} sheet under an intense circularly-polarized illumination. A gap in the Dirac dispersion leads to a finite Berry curvature and connected anomalous Hall effects. In the case of thermoelectrics, this manifests as a heat current perpendicular to the temperature gradient - the thermal Hall effect. A quantitative calculation of the attendant thermal Hall conductivity reveals dependence on the intrinsic anisotropy and tilt of the Dirac cone. Further, by estimating the longitudinal thermal conductivity using the Weidemann-Franz law, we also outline steps to compute the thermal Hall angle that gauges the generation efficiency of such transverse heat processes. Finally, we touch upon the idea of thermal rectification wherein the direction of flow of the anomalous heat reverses through a simple switch of the polarization of incident light and is of interest in thermal logic circuits.

cond-mat.mtrl-sci↗

arXiv2018

The electrothermal conductance and heat capacity of black phosphorus

We study a thermal gradient induced current $\left(I_{th}\right)$ flow in potassium-doped two-dimensional anisotropic black phosphorus (BP) with semi-Dirac dispersion. The prototype device is a BP channel clamped between two contacts maintained at unequal temperatures. The choice of BP lies in the predicted efficient thermolectric behaviour. A temperature-induced difference in the Fermi levels of the two contacts drives the current (typified by the electro-thermal conductance) which we calculate using the Landauer transport equation. The current shows an initial rise when the device is operated at lower temperatures. The rise stalls at progressively higher temperatures and $I_{th}$ acquires a plateau-like flat profile indicating a competing effect between a larger number of transmission modes and a corresponding drop in the Fermi level difference between the contacts. The current is computed for both \textit{n}- and \textit{p}-type BP and the difference thereof is attributed to the particle-hole asymmetry. The utility of such calculations lie in conversion of the heat production and an attendant temperature gradient in miniaturized devices to useful electric power and a possible realization of solid-state Peltier cooling. Unlike the flow of $I_{th}$ that removes heat, the ability of a material to maintain a steady temperature is reflected in its heat capacity which is formulated in this work for BP via a Sommerfeld expansion. In the concluding part, we draw a microscopic connection between the two seemingly disparate processes of heat removal and absorption by pinning down their origin to the underlying density of states. Finally, a qualitative analysis of a Carnot-like efficiency of the considered thermoelectric engine is performed drawing upon the previous results on thermal current and heat capacity.

cond-mat.mtrl-sci↗

arXiv2018

Spin-dependent magneto-thermopower of narrow-gap lead chalcogenide quantum wells

A semi-classical analysis of magneto-thermopower behaviour, namely, the \textit{Seebeck} and \textit{Nernst} effect (NE) in quantum wells of IV-VI lead salts with significant extrinsic Rashba spin-orbit coupling (RSOC) is performed in this report. In addition to the spin-dependent Seebeck effect that has been observed before, we also theoretically predict a similar spin-delineated behaviour for its thermal analog, the spin-dependent NE. The choice of lead salts follows from a two-fold advantage they offer, in part, to their superior thermoelectric properties, especially PbTe, while their low band gaps and high spin-orbit coupling make them ideal candidates to study \textit{RSOC} in nanostructures. The calculations show a larger longitudinal magneto-thermopower for the spin-up electrons while the transverse components are nearly identical. In contrast, for a magnetic field free case, the related power factor calculations reveal a significantly higher contribution from the spin-down ensemble and suffer a reduction with an increase in the electron density. We also discuss qualitatively the limitations of the semi-classical approach for the extreme case of a high magnetic field and allude to the observed thermopower behaviour when the quantum Hall regime is operational. Finally, techniques to modulate the thermopower are briefly outlined.

cond-mat.mes-hall↗

arXiv2018

Rashba-driven anomalous Nernst conductivity of lead chalcogenide films

The presence of a finite Berry curvature $\left(Ω\left(k\right)\right)$ leads to anomalous thermal effects. In this letter, we compute the coefficients for the anomalous Nernst effect $\left(ANE\right) $ and its spin analogue, the spin Nernst effect $\left(SNE\right)$ in lead chalcogenide (\textit{PbX}; \textit{X} = S, Se, Te) films. The narrow gapped \textit{PbX} films with a large spin-orbit coupling (\textit{soc}) offer a significant Rashba interaction that gives rise to $ Ω\left(k\right) $ and the attendant anomalous thermal behaviour. In presence of a temperature gradient, the $ ANE $ and $ SNE $ establish a thermal and spin current and are characterized by their respective coefficients which acquire higher values for a stronger Rashba interaction. We further show that an extrinsic \textit{soc} generated by an in-plane electric field offers a gate-like mechanism to control (and turn-off) the anomalous thermal currents. Finally, we conclude by deriving the efficiency of an $ ANE $-driven low-temperature Carnot heat engine and demonstrate that it can be gainfully optimized in systems with a robust intrinsic \textit{soc} resulting in low carrier effective masses.

cond-mat.mtrl-sci↗

arXiv2017

Anisotropy-driven quantum capacitance in multi-layered black phosphorus

We report analytic results on quantum capacitance (C$_{q}$) measurements and their optical tuning in dual-gated device with potassium-doped multi-layered black phosphorous (BP) as the channel material. The two-dimensional (2D) layered BP is highly anisotropic with a semi-Dirac dispersion marked by linear and quadratic contributions. The C$_{q}$ calculations mirror this asymmetric arrangement. A further increase to the asymmetry and consequently C$_{q}$ is predicted by photon-dressing the BP dispersion. To achieve this and tune C$_{q}$ in a field-effect transistor (FET), we suggest a configuration wherein a pair of electrostatic (top) and optical (back) gates clamp a BP channel. The back gate shines an optical pulse to rearrange the dispersion of the 2D BP. Analytic calculations are done with Floquet Hamiltonians in the off-resonant regime. The value of such C$_{q}$ calculations, in addition, to its role in adjusting the current drive of an FET is discussed in context of metal-insulator and topological phase transitions and enhancements to the thermoelectric figure of merit.

cond-mat.mtrl-sci↗

arXiv2017

The intrinsic inter-band optical conductivity of a C$_{2v}$ symmetric topological insulator

In this work we analytically investigate the longitudinal optical conductivity of the C$_{2v}$ symmetric topological insulator. The conductivity expressions at $ T = 0 $ are derived using the Kubo formula and expressed as a function of the ratio of the Dresselhaus and Rashba parameters that characterize the low-energy Hamiltonian. We find that the longitudinal inter-band conductivity vanishes when Dresselhaus and Rashba parameters are equal in strength, also called the persistent spin helix state. The calculations are extended to obtain the frequency-dependent real and imaginary components of the optical conductivity for the topological Kondo insulator SmB$_{6}$ which exhibits C$_{2v}$ symmetric and anisotropic Dirac cones hosting topological states at the $ \bar{X} $ point on the surface Brillouin zone.

cond-mat.mes-hall↗

arXiv2017

Low-temperature thermal transport and thermopower of monolayer transition metal dichalcogenide semiconductors

We study the low temperature thermal conductivity of single-layer transition metal dichalcogenides. In the low temperature regime where heat is carried primarily through transport of electrons, thermal conductivity is linked to electrical conductivity through the Wiedemann-Franz law. Using a \textit{k.p} Hamiltonian that describes the $ K $ and $ K^{'} $ valley edges, we compute the zero-frequency electric (Drude) conductivity using the Kubo formula to obtain a numerical estimate for the thermal conductivity. The impurity scattering determined transit time of electrons which enters the Drude expression is evaluated within the self-consistent Born approximation. The analytic expressions derived show that low temperature thermal conductivity 1) is determined by the band gap at the valley edges in monolayer TMDCs and 2) in presence of disorder which can give rise to the variable range hopping regime, there is a distinct reduction. Additionally, we compute the Mott thermopower and demonstrate that under a high frequency light beam that sets up a Floquet Hamiltonian, a valley-resolved thermopower can be obtained. A closing summary reviews the implications of results followed by a brief discussion on applicability of the Wiedemann-Franz law and its breakdown in context of the presented calculations.

cond-mat.mtrl-sci↗

arXiv2017

Imparity in valley population for single layer transition metal dichalcogenides under elliptical polarization

The illumination of a single-layer transition metal dichalcogenide with an elliptically-polarized light beam is shown to give rise to a differential rate of inter-band carrier excitation between the valence and conduction states around the valley edges, $ K $ and $ K^{'} $. This rate with a linear dependence on the beam ellipticity and inverse of the optical gap manifests as an asymmetric Fermi distribution between the valleys or a non-equilibrium population which under an external field and a Berry curvature induced anomalous velocity results in an externally tunable finite valley Hall current. Surface imperfections that influence the excitation rates are included through the self-consistent Born approximation. Additionally, we show that circular dichroism in the vicinity of the valley edges also exhibits an ellipticity dependence.

cond-mat.mtrl-sci↗

arXiv2016

Spin-orbit coupling mediated tunable electron heat capacity of quantum wells

The heat capacity of conduction electrons obtained from the Sommerfeld expansion is shown to be tunable via the Rashba and Dresselhaus spin-orbit coupling parameters. Using an AlInSb/InSb/AlInSb as a representative heterostructure with alterable well and asymmetric barrier regions, the heat capacity is found to be higher for the spin-down electrons and suffers a reduction for wider wells. A further lowering is obtained through the application of an uniaxial strain. Finally, we suggest a method to determine the spin lifetimes for spins relaxing via the D'yakonov-Perel' mechanism from experimental estimates of thermodynamic potentials such as the Helmholtz free energy and the heat capacity.

cond-mat.mes-hall↗

arXiv2016

Photo-modulation of the spin Hall conductivity of mono-layer transition metal dichalcogenides

We report on a possible optical tuning of the spin Hall conductivity in mono-layer transition metal dichalcogenides. Light beams of frequencies much higher than the energy scale of the system (the \textit{off-resonant} condition) does not excite electrons but rearranges the band structure. The rearrangement is quantitatively established using the Floquet formalism. For such a system of mono-layer transition metal dichalcogenides, the spin Hall conductivity (calculated with the Kubo expression in presence of disorder) exhibits a drop at higher frequencies and lower intensities. Finally, we compare the spin Hall conductivity of the higher spin-orbit coupled WSe$_{2} $ to MoS$_{2} $; the spin Hall conductivity of WSe$_{2}$ was found to be larger.

cond-mat.mes-hall↗

arXiv2016

The optical response of mono-layer transition metal dichalcogenides in a Kerr-type non-linear dielectric environment

We study the optical behaviour of an arrangement in which the interface between a linear and non-linear dielectric media is covered by an embedded mono-layer of transition metal dichalcogenides (TMDC). The optical behaviour is qualitatively obtained through transmission and reflection coefficients which are a function of the third order non-linear susceptibility of the Kerr-type dielectric and the inter-band optical conductivity of the TMDC mono-layer. The inter-band optical conductivity of the TMCD mono-layer is calculated using the Kubo formalism from linear response theory. In particular, we theoretically demonstrate that the optical response of this arrangement can be switched between total internal reflection and a normal transmission regime by controlling the intensity of the incident radiation. The reflection and transmission functions, additionally, are shown to be amenable to further control by altering the inter-band optical conductivity of the embedded TMDC mono-layer. The optical conductivity is an outcome of its energy dispersion; we specifically choose two TMDC mono-layers, MoS$_{2}$ and WSe$_{2}$, which have nearly identical dispersion parameters apart from a much stronger spin-orbit coupling in the latter. The stronger spin-orbit coupling in WSe$_{2}$ does not significantly alter the inter-band optical conductivity to manifest in an enhanced reflection spectrum. However, we find that application of an external perturbation such as strain could be effectively used to modulate the overall optical response. We conclude by offering a brief overview of the applicability of our proposed scheme in devices that employ an all-optical switching mechanism through optical bistability which is the hallmark of a non-linear dielectric.

cond-mat.mtrl-sci↗

arXiv2015

Tunable chirality and circular dichroism of a topological insulator with $ C_{2v} $ symmetry as a function of Rashba and Dresselhaus parameters

Polarization-sensitive devices rely on meta-materials to exhibit varying degrees of absorption of light of a given handedness. The chiral surface states of a topological insulator(TI) selectively absorb right and left circularly polarized light in the vicinity of the Dirac cone reaching its maximum of unity at the $ Γ$ point. In this letter, we demonstrate that a band gap open TI with C$_{2v}$ symmetry which is represented through a combination of Rashba and Dresselhaus Hamiltonians alters the preferential absorption of left and right circularly polarized light allowing a smooth variation of the circular dichroism(CD). This variation in CD, reflected in a range of positive and negative values is shown to be a function of the Rashba and Dresselhaus coupling parameters. Additionally, we draw a parallel between the varying CD and the emerging field of valley-electronics in transition metal dichalcogenides and note the possibility of the chiral states as a basis toward quantum information processing.

cond-mat.mes-hall↗