arXiv subjects

Partha Goswami

Publications and source records attributed to Partha Goswami.

At least 37 records · Page 2Linked to original sources

arXiv2019

On Topological Kondo Insulator Plasmonics

We study surface state plasmonics in the periodic Anderson model of a bulk topological Kondo insulator system, involving strongly correlated f electrons screened by weakly correlated d electrons, performing a mean-field theoretic calculation within the frame-work of slave-boson protocol. We include the effect of magnetic impurities as well which do not act as scattering agent in our scheme; the exchange interaction due to them is included in the band dispersion. The interaction opens a gap at the surface states with gapless Dirac dispersion. We find that bulk insulation together with the surface metallicity is possible for the system albeit only under the condition demanding strong f-electron localization. The possibility of the intra-band and the inter-band acoustic plasmons for the gapped surface states exists in our report. The dominance of the bulk metallicity and the strong incarceration capability of these plasmons are other notable outcome of the present work.

cond-mat.str-el↗

arXiv2018

Intra-band and Inter-band Plasmons in Transition Metal Dichalcogenide-Graphene van der Waals Heterostructures

In an earlier work, it was shown that the dispersion of the van der Waals heterostructures (vdWHs) of graphene monolayer on 2D transition metal dichalcogenide (GrTMD) substrate comprises of the spin-split, gapped bands. Using these it was also shown that the intra-band plasmon dispersion for the finite doping and the long wavelength limit involves the q2/3 (unconventional) behaviour and not the well known q1/2 behaviour. In this paper the optical conductivity and the optical properties of this vdWH are reported. We find that there are no (inter-band) plasmons for electron doping as long as the Gr-TMD hetero-structure is surrounded by the positive dielectric constant materials above and below. However, if the heterostructure is surrounded by negative dielectric constant (NDC) materials, one finds that there would be acoustic plasmons. For the hole doping case NDC material is not required. We find that the gate voltage tunable intra-band acoustic plasmons also emerge when the carrier density is greater than a moderately critical value. The stronger incarceration capability of GrTMD plasmon compared to that of doped graphene is another notable outcome of the present work. One finds that the intra-band absorbance is decreases with the frequency at a given gate voltage and increases with the gate voltage at a given frequency; the inter-band transmittance, however, is a decreasing function of frequency and the gate voltage.

cond-mat.mes-hall↗

arXiv2017

Optical properties of van der Waals heterostructure of uniaxially strained graphene on TMD

The spin and valley polarizations and plasmonics in Van der Waals heterostructures of strained graphene monolayer on 2D transition metal dichalcogenide (GrTMD) substrate are reported in this communication. The substrate induced interactions (SII) involve sub-lattice-resolved, and enhanced intrinsic spin-orbit couplings, the extrinsic Rashba spin-orbit coupling (RSOC), and the orbital gap related to the transfer of the electronic charge from graphene to the substrate. Furthermore, magnetic impurity atoms are deposited to the graphene surface and the corresponding exchange field is included in the band dispersion. A Rashba coupling dependent pseudo Zeeman term arising due to the interplay of SIIs was found to be responsible for the spin degeneracy lifting and the spin polarization. The latter turns out to be electrostatic doping and the exchange field tunable and inversely proportional to the square root of the carrier concentration. The strain field, on the other hand, brings about the valley polarization. The intra-band plasmon dispersion for the finite doping and the long wavelength limit has also been obtained. The dispersion involves the q2/3 behavior and not the well known q1/2 behavior. The uniform, uniaxial strain does not bring about any change in this behavior. However, the plasmon dispersion gets steeper for the wavevector perpendicular to the direction of strain and is flattened for wave vectors along the direction of the strain with the term responsible for the flattening proportional to the strain field. The stronger confinement capability of GrTMD Plasmon compared to that of standalone, doped graphene is an important outcome of the present work. One finds that whereas the intra-band absorbance of GrTMD is decreasing function of the frequency at a given strain field, it is an increasing function of the strain field at a given frequency.

cond-mat.mes-hall↗

arXiv2017

Optical cooling of interacting atoms in a tightly confined trap

In a recent paper, we have proposed a novel laser cooling scheme for reducing collisional energy of a pair of atoms by using photoassociative transitions. In that paper, we considered two atoms in free space, that is we have not considered the effects of trap on the cooling process. Here in this paper, we qualitatively discuss the possibility of extending this idea for Raman sideband cooling of a pair of interacting atoms trapped in Lamb-Dicke (LD) regime. Apart from cooling, our method may be important for manipulating on-site interaction of atoms in an optical lattice.

physics.atom-ph↗

arXiv2017

Dielectric properties of graphene on transition metal dichalcogenide substrate

The tunability of the dielectric properties induced by the substrate driven interactions (SDI) and the exchange field (M) due to the ferro-magnetic impurities in graphene monolayer on transition metal dichalcogenide (TMDC) (viz., XY2 , X = Mo, W; Y = S, Se) around K and K prime points is reported here. The cavalcade of interactions involve sub-lattice-resolved, strongly enhanced intrinsic spin-orbit couplings(SOC), the extrinsic Rashba spin-orbit coupling (RSOC), and the orbital gap related to the transfer of the electronic charge from graphene to XY2. The RSOC allows for external tuning of the band gap in graphene and connects the nearest neighbors with spin-flip. We obtain the usual gapped bands with an effective, RSOC-dependent Zeeman field due to the interplay of SDI. Using these bands we obtain the closed analytical expression of the dielectric function in the finite doping case. The zero of the dielectric function corresponds to the collective mode. Upon including the full dispersion of graphene on TMDC, we find that there is in fact only one collective mode and it corresponds to charge plasmons. The dispersion of the latter yields the q^2/3 behavior and not the well known q^1/2 behavior. We also find that the plasmon frequency could be changed by the tuning of the chemical potential.

cond-mat.mes-hall↗

arXiv2017

Ultra-confined q2/3- plasmons and optical properties of graphene-transition metal dichalcogenide heterostructures

We report the interesting possibilities related to the plasmonics and the key parameters of optoelectronics, such as the absorbance and the transmittance, in Van der Waals heterostructures of graphene monolayer on 2D transition metal dichalcogenide (GrTMD) substrate here. We obtain the gapped bands with a Rashba spin-orbit coupling dependent pseudo Zeeman field due to the interplay of substrate induced interactions. This enables us to obtain an expression of the dielectric function in the finite doping case ignoring the spin-flip scattering events completely. We observe that the relative strength of screening is nearly a constant with relative to the changes in the the carrier density.The stronger confinement capability of GrTMD Plasmon(tunable)compared to that of standalone graphene is a major outcome of our analysis;the plasmon dispersion yields the q2/3 behavior and not the well known q1/2 behavior. We also find that the absorbance and the transmittance are increasing functions of the frequency and the gate voltage. This outcome is useful for devices utilizing photoconductivity and the photo-electric effect.

cond-mat.mes-hall↗

arXiv2016

A model study on a pair of trapped particles interacting with an arbitrary effective range

We study the effects of the effective range of interaction on the eigenvalues and eigenstates of two particles confined in a three-dimensional (3D) isotropic as well as one- or quasi-one dimensional harmonic (1D) traps. For this we employ model potentials which mimic finite-range s-wave interactions over a wide range of s-wave scattering length $a_s$ including the unitarity limits $a_s \rightarrow \pm\infty$. Our results show that when the range is larger than the 3D or 1D harmonic oscillator length scale, the eigenvalues and eigenstates are nearly similar to those of noninteracting two particles in the 3D or 1D trap, respectively. In case of 3D, we find that when the range goes to zero, the results of contact potential as derived by Busch {\it et al.} [Foundations of Physics, {\bf28}, 549 (1998)] are reproduced. However, in the case of 1D, such reproducibility does not occur as the range goes to zero. We have calculated the eigenvalues and eigenstates in 1D harmonic trap taking one-dimensional finite- range model potential. We have also calculated bound state properties of two particles confined in a highly anisotropic quasi-1D trap taking three-dimensional finite-range model potential, and examined whether these quasi-1D results approach towards 1D ones as the aspect ratio $η$ of the radial to axial frequency of the trap increases. We find that if the range is very small compared to the axial size of the trap, then one can reach 1D regime for $η\ge 10000$. However, for large range, one can nearly get 1D results for smaller values of $η$. This study will be important for exploration of two-body or many body physics of trapped ultracold atoms interacting with narrow Feshbach resonance for which the effective range can be large.

physics.atom-ph↗

arXiv2016

Effect of ferromagnetic exchange field on band-gap and spin-polarization of graphene on a TMD substrate

We calculate the electronic band dispersion of graphene monolayer on a two dimensional transition metal dichalcogenide substrate (GTMD) (viz., XY2 , X = Mo, W; Y = S, Se) around K and K prime points taking into account the interplay of the exchange field due to the ferromagnetic impurities and the substrate induced, sub-lattice-resolved, strongly enhanced intrinsic spin-orbit couplings(SOC). There are extrinsic Rashba spin-orbit coupling(RSOC) and the orbital gap related to the transfer of the electronic charge from graphene to XY2 as well. The former allows for external tuning of the band gap in GTMD and connects the nearest neighbors with spin-flip. On account of the strong SOC, the system acts as a quantum spin Hall insulator. We introduce the exchange field (M) in the Hamiltonian to take into account the deposition of Fe atoms on the graphene surface. The cavalcade of the perturbations yield particle-hole symmetric bands with an effective Zeeman field due to the interplay of the substrate induced interactions with RSOC as the prime player. Our graphical analysis with extremely low-lying states strongly suggests the following: The GTMDs like WY2 exhibit band gap narrowing/widening (Moss-Burs-tein(MB)gap shift)including the spin-polarization inversion(SPI) at finite but low temperature (T = 1 K) due to the increase in the exchange field (M) at the Dirac point K. For graphene on MoY2, on the other hand, the occurrence of the MB-shift and the SPI at higher temperature (T = 10 K) take place as M is increased at the Dirac point K prime. Finally, there is anti-crossing of non-parabolic bands with opposite spins around Dirac points. A direct electric field control of magnetism at the nano-scale is needed here. The magnetic multi-ferroics, like BiFeO3 (BFO), are useful for this purpose due to the coupling between the magnetic and electric order parameters.

cond-mat.mes-hall↗

arXiv2016

Casimir-Polder attraction-repulsion crossover criterion

The mutual electromagnetic correlations between two spatially separated systems gives rise to Casimir and Casimir-Polder effect. The corresponding forces, which are generally attractive for most vacuum-separated metallic or dielectric geometries, are due to the contribution to the ground-state energy of the coupled system. We investigate here the Casimir-Polder free energy corresponding to interactions of a magnetically and electrically polarizable micro-particle with a magneto-dielectric sheet. Our semi-phenomenological study shows that such an interaction is reversibly tunable in strength and sign.The latter, particularly, is true provided we look for the exotic materials fabricated at scales between the micron and the nanometer. The crossover between attractive and repulsive behavior is found to depend on the polarizability ratio of the micro-particle and the electromagnetic impedance of the magneto-dielectric sheet.

cond-mat.mes-hall↗

arXiv2016

2+1 dimensional Fermions on the low-buckled honey-comb structured lattice plane and classical Casimir-Polder force

We have calculated the Casimir-Polder interaction (CPI) of a micro-particle with a sheet on the basis of the Klimchitskaya-Mostepanenko theory. We find the result that for non-trivial susceptibility values of the sheet and micro-particle, there is crossover between attractive and repulsive behavior. The transition depends only on the impedance, involving permeability and permittivity, apart from the ratio of the film thickness and the micro-particle separation (D/d) and temperature. The approach to calculate CPI of a micro-particle with a silicene sheet involves replacing the dielectric constant of the sample by the static dielectric function obtained using the expressions for the polarization function. The silicene is described by the low-energy Liu-Yao-Feng-Ezawa (LYFE)Model Hamiltonian involving the Dirac matrices in the chiral representation obeying the Clifford algebra.We find that the collective charge excitations at zero doping, i.e., intrinsic plasmons, in this system, are absent in the Dirac limit. The valley-spin-split intrinsic plasmons, however, come into being in the case of the massive Dirac particles with characteristic frequency close to 10 THz.Furthermore, there is a longitudinal electric field induced topological insulator(TI) to spin-valley polarized metal (SVPM) transition in silicene, which is also referred to as the topological phase transition (TPT). The low-energy SVP carriers at TPT possess gap-less (mass-less) and gapped (massive) energy spectra close to the two nodal points in the Brillouin zone with maximum spin-polarization. We find that the magnitude of the Casimir-Polder force at a given ratio of the film thickness and the separation between the micro-particle and the film is greater at TPT than at the topological insulator and trivial insulator phases.

cond-mat.mes-hall↗

arXiv2015

A model study on atom-atom interactions with large scattering length in quasi-two dimensional traps

We carry out a model study on two-atom interactions and bound states in quasi-two dimensional traps. The interactions are modeled by two-parameter potentials with parameters being the range $r_0$ and the $s$-wave scattering length $a_s$. We show that one can make use of two forms of finite-range model potentials, one for $a_s > 0$ and the other for $a_s < 0$. Both potentials reduce to same form in the limits $a_s \rightarrow \pm \infty $. We investigate into the dependence of the binding energies and the wave functions of two-atom trap-bound states on $a_s$ and $r_0$. In particular, we study the effects of $a_s$ ranging from large negative to large positive values on the bound state properties. Our results show that long-range interactions with infinite scattering length significantly alter the ground-state energy of the two atoms in a quasi-two or two dimensional trap. In contrast, short-range interactions can not significantly change the ground-state energy of two atoms in a 2D harmonic trap.

physics.atom-ph↗

arXiv2014

Probing topological phase transition in silicene via Klein tunneling in a normal/ferromagnetic/normal silicene junction

The two apparently disparate phenomena, viz. the Klein tunneling and the electric field driven topological phase transition(TPT), exhibited by the silicene converge on the issue of the no change in the pseudo-spin. An ensuing possibility is that the former, in the case of a normal/magnetic/normal silicene junction, may be considered theoretically as a probe to ascertain the onset of the latter. In this communication, we explore this option and calculate the total transmission probability(TTP). We find that TPT is characterized by a cusp in a plot of TTP as a function of the electric field.

cond-mat.mes-hall↗

arXiv2014

Theoretical elucidation of possibility of Majorana modes in a two dimensional Dirac system

Here we present the theoretical clarification of possibility of eight Majorana-like modes (quasi-particles which are self-conjugate) close to the experimentally inaccessible Dirac points of a two-dimensional monolayer Dirac system. The valley-mixing and the spin-degeneracy lifting are the main requirements. These are possible by wedging in the requisite ingredients in the description, viz. the atomically sharp scatterers and the strong spin-orbit coupling (SOC). The latter can possibly be achieved in graphene folding a sheet; the higher curvature of deformations correspond to stronger values of the coupling. In silicene, the buckled structure of the system generates a staggered sub-lattice potential between silicon atoms at A sites and B sites for an applied electric field perpendicular to its plane. The stronger SOC in silicene has its origin also in the buckled structure of the system. Tuning of electric field, allows for rich behavior varying from a topological insulator to a normal insulator with a valley spin-polarized metal (VSPM) at a critical value in between. The VSPM stage is characterized by the valley-spin locking, i.e. the opposite spin polarization at different valleys. We shall see that in this phase, if the inter-valley scattering process and the real spin-flip process in moderation are allowed to take place, we have the right condition for capturing Majoranas in the proximity of a s-wave superconductor.

cond-mat.mes-hall↗

arXiv2014

Effect of disorder in the transition from topological insulator to valley-spin polarized state in silicene and Germanene

We start with the silicene or germanene single-particle Hamiltonian in buckled 2D hexagonal lattices expressed in terms of Dirac matrices in the Weyl basis. The Hamiltonian of these systems comprises of the Dirac kinetic energy, a mass gap term, and the spin-orbit coupling. The second term breaks the sub-lattice symmetry of the honey-comb lattice structure and generates a gap. The buckled structure generates a staggered sub-lattice potential between silicon atoms at A sites and B sites for an applied electric field perpendicular to its plane. Tuning of the electric field, allows for rich behavior varying from a topological insulator to a band insulator with a valley spin-polarized metal at a critical value in between. Thus, the mobile electrons in silicene or germanene are coupled differently compared to graphene to an external electric field. Our preliminary investigation have shown that, as long as the non-magnetic impurity scattering strength is moderate, i.e. the strength is of the same order as the intrinsic spin-orbit coupling of 4 meV, valley-spin polarized metal phase is protected. The effective "two-component Dirac physics" remains valid in this phase. The increase in the electric field beyond the critical value leads to the valley magnetic moment reversal. The enhancement in the impurity scattering strength, however, leads to the disappearance of the polarized metal phase due to accentuated intra- and inter-valley scattering processes. This disappearance does not occur due to the increase in Rashba spin-orbit coupling effect.

cond-mat.mes-hall↗

arXiv2013

Investigation of pseudogap and superconducting transitions in hole-doped cuprates

We consider the id-density wave (DDW) order, representing the pseudo-gap (PG) state, and the d-wave superconductivity (DSC)within the BCS framework for the two-dimensional (2D) fermion system on a square lattice starting with a mean-field Hamiltonian involving the singlet DDW and the DSC pairings. The absence of nesting in the normal state dispersion leads to the particle-hole asymmetry in the single-particle excitation spectrum of the pure DDW state. This is reflected in the coexisting DDW and DSC states though the latter is characterized by the Bogoluibov quasi-particle bands- a characteristic feature of SC state. We solve the coupled gap equations self-consistently together with the equation to determine the chemical potential. We also calculate the thermodynamic and transport properties in the PG phase.The electronic specific heat displays non-Fermi liquid feature. We show that the PG and DSC are representing two competing orders as the former brings about a depletion of the spectral weight available for pairing in the anti-nodal region of momentum space. We also show the depletion of the spectral weight below Tc at energies larger than the gap amplitude. This is an important hallmark of the strong coupling superconductivity. Furthermore, the passage from normal state to the PG state at a fixed hole under-doping is shown to correspond to a non-sharp thermal phase transition.

cond-mat.supr-con↗

arXiv2013

On d+id density wave and superconducting orderings in hole-doped cuprates

The Chiral d-density wave (CDDW) order, at the anti-ferromagnetic wave vector (pi,pi), is assumed to represent the pseudo-gap (PG) state of a hole-doped cuprate superconductor. The pairing interaction required for the PG phase corresponds to a repulsive interaction of Coulombic origin. The d-wave superconductivity (DSC), driven by an appropriate assumed attractive interaction, is discussed within the mean-field framework together with the CDDW ordering. The single-particle excitation spectrum in the CDDW and DSC state is characterized by the Bogolubov quasi-particle bands-a characteristic feature of SC state. The coupled gap equations are solved self-consistently together with the equation to determine the chemical potential. With the pinning of the van Hove-singularities close to the chemical potential, one is able to calculate the thermodynamic and transport properties of the under-doped cuprates in a consistent manner. The electronic specific heat displays non-Fermi liquid feature in the CDDW state. The CDDW and DSC are found to represent two competing orders as the former brings about a depletion of the spectral weight (and Raman response function density) available for pairing in the anti-nodal region of momentum space.This is not in agreement with a preformed pairing scenario and justifies the different structures we have assumed for the pseudo-gap and the superconducting gap. Furthermore, these gaps in the excitation spectrum do not merge into a single quadrature gap. It is also shown that the depletion of the spectral weight below Tc at energies larger than the gap amplitude occurs. This is an indication of the strong-coupling superconductivity in cuprates. The calculation of the ratio of the quasi-particle thermal conductivity and temperature in the superconductiong phase is found to be constant in the limit of near-zero quasi-particle scattering rate.

cond-mat.supr-con↗

arXiv2012

Rashba coupling and Lifshitz transition in monolayer graphene

We take a wide-angle view of the problem of monolayer graphene (MLG) where spin-degeneracy lifting is assumed to be possible by wedging in the tunable Rashba spin-orbit coupling(RSOC) and the sub-lattice staggered potential. We next consider the AB-stacked bi-layer graphene (BLG) system (the A-carbon of the upper sheet lying on top of the B-carbon of the lower one) and assume that a perpendicular electric field is created by the external gates deposited on the BLG surface. This system exhibits the occurrence of trigonal warping due to a (skew) interlayer hopping leading to the well-known Lifshitz transition(LT)[Y. Lemonik, I.L. Aleiner, C. Toke, and V.I. Fal'ko; arXiv:1006.1399]. We do not observe the replication of the features associated with BLG-LT in MLG in the presence of RSOC.

cond-mat.mes-hall↗

arXiv2012

Monolayer graphene panorama, Majorana modes and Longitudinal conductivity

We take a wide-angle view of the problem of monolayer graphene where the valley-mixing and the spin-degeneracy lifting are assumed to be possible by wedging in the requisite ingredients, viz. the atomically sharp scatterers and the strong Rashba coupling dominating over the intrinsic spin-orbit coupling. This leads to eight Majorana-like modes (quasi-particles which are self-conjugate) close to the experimentally inaccessible Dirac points. Using Kubo formula we also show that the semi-classical diffusive (longitudinal) conductivity is nearly (2.018 e2/h) at room temperature for the disordered system. Though this is an overestimation, we have been, never-the-less, able to qualitatively capture the fact that the room temperature conductivity of graphene is finite and the contribution to the conductivity arises from the momentum very close to the Dirac points.

cond-mat.mes-hall↗