Papers by Antonios Balassis
Physics Letters A
In the presence of an external vertical electric field and strain, it is evident that 1T-MoS2 exh... more In the presence of an external vertical electric field and strain, it is evident that 1T-MoS2 exhibits tilted Dirac bands which are valley-spin-polarized. Additionally, this material experiences a topological phase change between a topological insulator and band insulator for a critical value of the electric field. Using linear response theory, we calculated the polarization function which is in turn employed to obtain the dielectric function. This latter quantity is subsequently utilized in calculations to determine the plasmons dispersion relation, their decay rate and impurity screening corresponding to various levels of doping, the critical applied vertical electric field strengths and the spin-orbit coupling gap in 1T-MoS2 with tilted Dirac bands.
Nanomaterials
We have investigated the α–T3 model in the presence of a mass term which opens a gap in the energ... more We have investigated the α–T3 model in the presence of a mass term which opens a gap in the energy dispersive spectrum, as well as under a uniform perpendicular quantizing magnetic field. The gap opening mass term plays the role of Zeeman splitting at low magnetic fields for this pseudospin-1 system, and, as a consequence, we are able to compare physical properties of the the α–T3 model at low and high magnetic fields. Specifically, we explore the magnetoplasmon dispersion relation in these two extreme limits. Central to the calculation of these collective modes is the dielectric function which is determined by the polarizability of the system. This latter function is generated by transition energies between subband states, as well as the overlap of their wave functions.
arXiv: Mesoscale and Nanoscale Physics, 2016
We perform a comprehensive analysis of the spectrum of graphene plasmons which arise when a pair ... more We perform a comprehensive analysis of the spectrum of graphene plasmons which arise when a pair of sheets are confined between thick 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 formulation for calculating the plasmon spectrum of an encapsulated pair of 2D layer and apply it to graphene. This calculation is relevant to studies of energy transfer via plasmon excitations when graphene is confined by a pair of thick conducting materials. We employ the random-phase approximation (RPA) integral equation for a system composed of two identical semi-infinite conducting plasmas with planar bounding surfaces at $z = \pm a/2$ enclosing a pair of 2D semiconductor plasma at $z=\pm b/2$ in the narrow gap r...
Undergraduate Research Journal, 2013
Physical Review B, 2021
Using the α-T 3 model, we carried out analytical and numerical calculations for the static and dy... more 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.
Bulletin of the American Physical Society, 2014
Journal of Physics: Condensed Matter, 2020
Using the α-T 3 model, we carried out analytical and numerical calculations for the static and dy... more 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.
Physical Review B, 2017
We compare the two-dimensional (2D) plasmon dispersion relations for monolayer graphene when the ... more We compare the two-dimensional (2D) plasmon dispersion relations for monolayer graphene when the sample is doped with carriers in the conduction band and the temperature T is zero with the case when the temperature is finite and there is no doping. Additionally, we have obtained the plasmon excitations when there is doping at finite temperature. The results were obtained in the random-phase approximation which employs energy electronic bands calculated using ab initio density functional theory. We found that in the undoped case the finite temperature results in appearance in the low-energy region of a 2D plasmon which is absent for the T = 0 case. Its energy is gradually increased with increasing T. It is accompanied by expansion in the momentum range where this mode is observed as well. The 2D plasmon dispersion in the ΓM direction may differ in substantial ways from that along the ΓK direction at sufficiently high temperature and doping concentrations. Moreover, at temperatures exceeding ≈ 300 meV a second mode emerges along the ΓK direction at lower energies like it occurs at a doping level exceeding ≈ 300 meV. Once the temperature exceeds ≈ 0.75 eV this mode ceases to exit whereas the 2D plasmon exists as a well-defined collective excitation up to T = 1.5 eV, a maximal temperature investigated in this work.
physica status solidi (b), 2017
We perform a comprehensive analysis of the spectrum of graphene plasmons which arise when a pair ... more 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.
The European Physical Journal B, 2016
We compute and compare the effects due to a uniform perpendicular magnetic field as well as tempe... more 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. The relevance of our study to the Kohn anomaly in lowdimensional 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.
physica status solidi (b), 2013
We present a formalism for calculating the image potential for a two-dimensional electron gas (2D... more We present a formalism for calculating the image potential for a two-dimensional electron gas (2DEG) with Rashba spin-orbit interaction (SOI) as well as for a 2D topological insulator (TI). The formalism is further generalized for including the Coulomb coupled multiple layers. Roles of broken inversion symmetry near the surface and the dielectric environment are investigated by using a surface-response function. The insignificant role of SOI in 2DEG is dramatically enhanced in TI by selecting a small relative permittivity b for the dielectric environment. Manipulating b is proven to provide an efficient way to drive electrons with opposite spins into two different integral quantum Hall states. The prediction made in this paper is expected to be experimentally observable for a 2DTI system, such as Bi 2 Se 3 , with a helical spin behavior and a dominant linear Rashba SOI-like term in the energy dispersion.
Physical Review B, 2003
We develop a theory of collective plasma excitations in a pair of parallel nonoverlapping cylindr... more We develop a theory of collective plasma excitations in a pair of parallel nonoverlapping cylindrical nanotubes. A general dispersion equation is obtained as a function of the angular momentum and linear momentum transfer. We use our results to calculate the phase velocity of the plasmon excitations when current flows parallel to the axis of the nanotube in either the same or neighboring nanotube as the plasma excitations. We demonstrate that the less energetic undamped plasmon excitations may have a phase velocity which is smaller than the Fermi velocity. The plasmons with the largest frequencies have phase velocities which decrease with increasing wave vector. We discuss the instabilities which may arise when the drift velocity of the electrons lies within a range which is determined by the phase velocity for the plasmon modes of the pair of cylindrical nanotubes.
Physical Review B, 2014
A theory for the collective plasma excitations in a linear periodic array of spherical twodimensi... more A theory for the collective plasma excitations in a linear periodic array of spherical twodimensional electron gases (S2DEGs) is presented. This is a simple model for an ultra thin and narrow microribbon of fullerenes or metallic shells. Coulomb coupling between electrons located on the same sphere and on different spheres is included in the random-phase approximation (RPA). Electron hopping between spheres is neglected in these calculations. The resulting plasmon-dispersion equation is solved numerically. Results are presented for a superlattice of single-wall S2DEGs as a function of the wave vector. The plasmon dispersions are obtained for different spherical separations. We show that the one-dimensional translational symmetry of the lattice is maintained in the plasmon spectrum. Additionally, we compare the plasmon dispersion when the superlatice direction is parallel or perpendicular to the axis of quantization. However, because of anisotropy in the Coulomb matrix elements, there is anticrossing in the plasmon dispersion only in the direction perpendicular to the quantization axis. The S2DEG may serve as a simple model for fullerenes, when their energy bands are far apart.
Solid State Communications, 2009
An experimental and theoretical study of electronic excitations in MgB 2 covering the domain of l... more An experimental and theoretical study of electronic excitations in MgB 2 covering the domain of large energy and momentum transfers is reported. Energy-loss spectra for several values of momentum transfers were measured in a polycrystalline sample by means of inelastic X-ray scattering spectroscopy. Ab initio calculations of the dielectric function as well as the energy-loss function were performed in the frame of the time-dependent local density approximation with inclusion of crystal local-field effects. We obtained very good agreement between the experimental and the theoretical energy dispersion of the peak maximum of the loss function. We found that crystal local-field effects are responsible for this agreement at large momenta. Fine structure observed in the measured spectra was interpreted in terms of strong interband transitions predicted by the calculations in the Γ A and Γ K directions. The theoretical dispersion of these features is in good accordance with the experimental data. Further spectral features in the measured spectra due to Mg 2s and 2p core electron excitations are also discussed.
Physics Letters A, 2008
We present a formalism for calculating the absorption coefficient of a pair of coaxial tubules. A... more We present a formalism for calculating the absorption coefficient of a pair of coaxial tubules. A spatially nonlocal, dynamical self-consistent field theory is obtained by calculating the electrostatic potential produced by the charge density fluctuations as well as the external electric field. There are peaks in the absorption spectrum arising from plasma excitations corresponding either to plasmon or particle-hole modes. In this paper, we numerically calculate the plasmon contribution to the absorption spectrum when an external electric field is applied. The number of peaks depends on the radius of the inner as well as outer tubule. The height of each peak is determined by the plasmon wavelength and energy. For a chosen wave number, the most energetic plasmon has the highest peak corresponding to the largest oscillator strength of the excited modes. Some of the low-frequency plasmon modes have such weak coupling to an external electric field that they are not seen on the same scale as the modes with larger energy of excitation. We plot the peak positions of the plasmon excitations for a pair of coaxial tubules. The coupled modes on the two tubules are split by the Coulomb interaction. The energies of the two highest plasmon branches increase with the radius of the outer tubule. On the contrary, the lowest modes decrease in energy as this radius is increased. No effects due to inter-tubule hopping are included in these calculations.
Physical Review B, 2009
We present ab-initio time-dependent density-functional theory calculation results for low-energy ... more We present ab-initio time-dependent density-functional theory calculation results for low-energy collective electron excitations in MgB 2. The existence of a long-lived collective excitation corresponding to coherent charge density fluctuations between the boron σ-and π-bands (σπ mode) is demonstrated. This mode has a sine-like oscillating dispersion for energies below 0.5 eV. At even lower energy we find another collective mode (σσ mode). We show the strong impact of local-field effects on dielectric functions in MgB2. These effects account for the long q-range behavior of the modes. We discuss the physics that these collective excitations bring to the energy region typical for lattice vibrations.
Physical Review B, 2006
We present a formalism for the rate of transfer of energy from a current of charged particles to ... more We present a formalism for the rate of transfer of energy from a current of charged particles to multiwall and a linear array of nanotubes. The method is based on a self-consistent field theory involving Laplace's equation for the total screened potential and the density fluctuations on the nanotubes. It is demonstrated that one can identify the plasmon excitations from the spectrum of energy transfer. For a group of the excited plasmons, there is an instability associated with their decay. This occurrence is determined by the relationship between the phase velocity of the plasmon and the drift velocity of the current. A "dip" in the energy transfer spectrum corresponds to a plasmon mode instability. This is confirmed by solving the plasma dispersion equation in the complex frequency plane.
Journal of Physics: Condensed Matter, 2014
We have discovered a novel feature in the plasmon excitations for a pair of Coulomb-coupled nonco... more We have discovered a novel feature in the plasmon excitations for a pair of Coulomb-coupled nonconcentric spherical two-dimensional electron gases (S2DEGs). Our results show that the plasmon excitations for such pairs depend on the orientation with respect to the external electromagnetic probe field. The origin of this anisotropy of the inter-sphere Coulomb interaction is due to the directional asymmetry of the electrostatic coupling of electrons in excited states which depend on both the angular momentum quantum number L and its projection M on the axis of quantization taken as the probe E-field direction. We demonstrate the anisotropic inter-sphere Coulomb coupling in space and present semi-analytic results in the random-phase approximation both perpendicular and parallel to the axis of quantization. For the incidence of light with a finite orbital or spin angular momentum, the magnetic field generated from an induced oscillating electric dipole on one sphere can couple to an induced magnetic dipole on another sphere in a way depending on the direction parallel or perpendicular to the probe E field. Such an effect from the plasmon spatial correlation is expected to be experimentally observable by employing circularly-polarized light or a helical light beam for incidence. The S2DEG serves as a simple model for fullerenes as well as metallic dimers, when the energy bands are far apart.
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Papers by Antonios Balassis