arXiv subjects

Dipendra Dahal

Publications and source records attributed to Dipendra Dahal.

12 recordsLinked to original sources

arXiv2022

Plasmon damping rates in Coulomb-coupled two-dimensional layers in a heterostructure

The Coulomb excitations of charge density oscillation are calculated for a double-layer heterostructure. Specifically, we consider two-dimensional (2D) layers of silicene and graphene on a substrate. From the obtained surface response function, we calculated the plasmon dispersion relations which demonstrate the way in which the Coulomb coupling renormalizes the plasmon frequencies. Additionally, we present a novel result for the damping rates of the plasmons in this Coulomb coupled heterostructure and compare these results as the separation between layers is varied.

cond-mat.mes-hall↗

arXiv2022

Finite-temperature plasmons, damping and collective behavior for $α-\mathcal{T}_3$ model

We have conducted a thorough theoretical and numerical investigation of the electronic susceptibility, polarizability, plasmons, their damping rates, as well as the static screening in pseudospin-1 Dirac cone materials with a flat band, or for a general $α- \mathcal{T}_3$ model, at finite temperatures. This includes calculating the polarization function, plasmon dispersions and their damping rates at arbitrary temperatures and obtaining analytical approximations the long wavelength limit, low and high temperatures. We demonstrate that the integral transformation of the polarization function cannot be used directly for a dice lattice revealing some fundamental properties and important applicability limits of the flat band dispersions model. At $k_B T \ll E_F$, the largest temperature-induced change of the polarization function and plasmons comes from the mismatch between the chemical potential and the Fermi energy. We have also obtained a series of closed-form semi-analytical expressions for the static limit of the polarization function of an arbitrary $α- \mathcal{T}_3$ material at any temperature with exact analytical formulas for the high, low and zero temperature limits which is of tremendous importance for all types of transport and screening calculations for the flat band Dirac materials.

cond-mat.mtrl-sci↗

arXiv2021

Adjustable propagating plasmons in $α-\mathcal{T}_3$ lattice-based armchair nanoribbons

We have obtained and analyzed the electronic states, polarization function and the plasmon excitations for $α- \mathcal{T}_3$-based nanoribbons with armchair termination. The calculated plasmon dispersions strongly depend on the number of the atomic rows across the ribbon, and the presence of the energy gap between the valence and conduction bands which is also determined by the nanoribbon geometry. The bandgap was proven to have the strongest effect on both the plasmon dispersions and their Landau damping. We have also demonstrated that for a small electron doping the plasmon dispersions do not depend on the relative hopping parameter $α$ of the considered $α- \mathcal{T}_3$ material in the long-wave limit and investigated the conditions when $α$ becomes an important factor which strongly affects the plasmons. We believe that our new uncovered electronic and collective properties of nano-size $α- \mathcal{T}_3$ribbons will find their applications in the field of modern electronics and nanodevices.

cond-mat.mes-hall↗

arXiv2020

Magnetoplasmons for the $α$-T$_3$ model with filled Landau levels

Using the $α$-T$_3$ model, we carried out analytical and numerical calculations for the static and dynamic polarization functions in the presence of a perpendicular magnetic field. These results were employed to determine the longitudinal dielectric function and the magnetoplasmon dispersion relation. The magnetic field splits the continuous valence, conduction and flat energy subband into discrete Landau levels which present significant effects on the polarization function and magnetoplasmon dispersion. We present results for a doped layer in the integer quantum Hall regime for fixed hopping parameter $α$ and various magnetic fields as well as chosen magnetic field and different $α$ in the random phase approximation.

cond-mat.mes-hall↗

arXiv2019

Quantum-statistical theory for laser-tuned transport and optical conductivities of dressed electrons in $α-\mc{T}_3$ materials

In the presence of external off-resonance and circularly-polarized irradiation, we have derived a many-body formalism and performed a detailed numerical analysis for both the conduction and optical currents in $α-\mc{T}_3$ lattices. The calculated complex many-body dielectric function, as well as conductivities of displacement and transport currents, display strong dependence on the lattice-structure parameter $α$, especially approaching the graphene limit with $α\to 0$. Unique features in dispersion and damping of plasmon modes are observed with different $α$ values, which are further accompanied by a reduced transport conductivity under irradiation. The discovery in this paper can be used for designing novel multi-functional nanoelectronic and nanoplasmonic devices.

cond-mat.mes-hall↗

arXiv2018

Polarizability and impurity screening for phosphorene

Using a tight-binding Hamiltonian for phosphorene, we have calculated the real part of the polarizability and the corresponding dielectric function, Re$[ε(\textbf{q},ω)]$, at zero temperature (T = 0) with free carrier density $10^{13}$/ $cm^2$. We present results showing the real part of dielectric function in different directions of the transferred momentum $\bf{q}$. When $q$ is larger than a particular value which is twice the Fermi momentum $k_F$, Re$[ε(\textbf{q},ω)]$ becomes strongly dependent on the direction of $\bf{q}$. We also discuss the case at room temperature (T = 300K). These results which are similar to those previously reported by other authors are then employed to determine the static shielding of an impurity in the vicinity of phosphorene.

physics.comp-ph↗

arXiv2018

Role played by strain on Plasmons, screening and energy loss in Graphene/substrate contacts

The combined effect due to mechanical strain, coupling to the plasmons in a doped conducting substrate, the plasmon-phonon scattering in conjunction with the role played by encapsulation of a secondary two-dimensional (2D) layer is investigated both theoretically and numerically. The calculations are based on the random-phase approximation (RPA) for the surface response function which yields the plasmon dispersion equation that is applicable in the presence or absence of an applied uniaxial strain. We present results showing the dependence of the frequency of the charge density oscillations on the strain modulus and direction of the wave vector in the Brillouin zone. The shielding of a dilute distribution of charges as well as the rate of loss of energy for impinging charges is investigated for this hybrid layered structure.

cond-mat.mes-hall↗

arXiv2017

Effect of Temperature and Doping on Plasmon Excitations for an Encapsulated Double-Layer Graphene Heterostructure

We perform a comprehensive analysis of the spectrum of graphene plasmons which arise when a pair of sheets are confined between conducting materials. The associated enhanced local fields may be employed in the manipulation of light on the nanoscale by adjusting the separation between the graphene layers, the energy band gap as well as the concentration of charge carriers in the conducting media surrounding the two-dimensional (2D) layers. We present a theoretical formalism, based on the calculation of the surface response function, for determining the plasmon spectrum of an encapsulated pair of 2D layers and apply it to graphene. We solve the coupled equations involving the continuity of the electric potential and discontinuity of the electric field at the interfaces separating the constituents of the hybrid structure. We have compared the plasmon modes for encapsulated gapped and gapless graphene. The associated nonlocal graphene plasmon spectrum coupled to the "sandwich" system show a linear acoustic plasmon mode as well as a low-frequency mode corresponding to in-phase oscillations of the adjacent 2D charge densities. These calculations are relevant to the study of energy transfer via plasmon excitations when graphene is confined by a pair of thick conducting materials.

cond-mat.mes-hall↗

arXiv2017

Adatom Doping-Enriched Geometric and Electronic Properties of Pristine Graphene: a Method to Modify the Band Gap

We have investigated the way in which the concentration and distribution of adatoms affect the geometric and electronic properties of graphene. Our calculations were based on the use of first principle under the density functional theory which reveal various types of $π$-bonding. The energy band structure of this doped graphene material may be explored experimentally by employing angle-resolved photo-emission spectroscopy (ARPES) for electronic band structure measurements and scanning tunneling spectroscopy (STS) for the density-of-states (DOS) both of which have been calculated and reported in this paper. Our calculations show that such adatom doping is responsible for the destruction or appearance of the Dirac cone structure.

cond-mat.mes-hall↗

arXiv2016

Effect of Energy Band Gap in Graphene on Negative Refraction through the Veselago Lens and Electron Conductance

A remarkable property of intrinsic graphene is that upon doping, electrons and holes travel through the monolayer thick material with constant velocity which does not depend on energy up to about $0.3$ eV (Dirac fermions), as though the electrons and holes are massless particles and antiparticles which move at the Fermi velocity $v_F$. Consequently, there is Klein tunneling at a $p-n$ junction, in which there is no backscattering at normal incidence of massless Dirac fermions. However, this process yielding perfect transmission at normal incidence is expected to be affected when the group velocity of the charge carriers is energy dependent and there is non-zero effective mass for the target particle. We investigate how away from normal incidence the combined effect of incident electron energy $ε$ and band gap parameter $Δ$ can determine whether a $p-n$ junction would allow focusing of an electron beam by behaving like a Veselago lens with negative refractive index. We demonstrate that there is a specific region in $ε-Δ$ space where the index of refraction is negative, i.e., where monolayer graphene behaves as a metamaterial. Outside this region, the refractive index may be positive or there may be no refraction at all. We compute the ballistic conductance across a $p-n$ junction as a function of $Δ$ and $ε$ and compare our results with those for a single electrostatic potential barrier and multiple barriers.

cond-mat.mes-hall↗

arXiv2016

Plasmon Excitations for Encapsulated Graphene

We have developed an analytical formulation to calculate the plasmon dispersion relation for a two-dimensional layer which is encapsulated within a narrow spatial gap between two bulk half-space plasmas. This is based on a solution of the inverse dielectric function integral equation within the random-phase approximation (RPA). We take into account the nonlocality of the plasmon dispersion relation for both gapped and gapless graphene as the sandwiched two-dimensional (2D) semiconductor plasma. The associated nonlocal graphene plasmon spectrum coupled to the "sandwich" system is exhibited in density plots, which show a linear mode and a pair of depolarization modes shifted from the bulk plasma frequency.

cond-mat.mes-hall↗

arXiv2016

Thermal smearing of the magneto-Kohn anomaly for Dirac materials and comparison with the two-dimensional electron liquid

We compute and compare the effects due to a uniform perpendicular magnetic field as well as temperature on the static polarization functions for monolayer graphene (MLG), associated with the Dirac point, with that for the two-dimensional electron liquid (2DEL) with the use of comprehensive numerical calculations. Previous results for the 2DEL are discussed and, in particular, we point out a flaw in a reported analytic derivation which was carried out to exhibit the smearing of the Fermi surface for 2DEL. The relevance of our study to the Kohn anomaly in low-dimensional structures and the Friedel oscillations for the screening of the potential for a dilute distribution of impurities is reported. Our results show substantial differences due to screening for the 2DEL and MLG which have not been given adequate attention previously.

cond-mat.mes-hall↗