arXiv subjects

Andrii Iurov

Publications and source records attributed to Andrii Iurov.

At least 37 records · Page 2Linked to original sources

arXiv2021

Generalized WKB theory for electron tunneling in gapped $α-\mathcal{T}_3$ lattices

We generalize Wentzel-Kramers-Brillouin (WKB) semi-classical equations for pseudospin-1 $α-\mathcal{T}_3$ materials with arbitrary hopping parameter $0 < α< 1$, which includes the dice lattice and graphene as two limiting cases. In conjunction with a series-expansion method in powers of Planck constant $\hbar$, we acquired and solved a system of recurrent differential equations for semi-classical electron wave functions in $α-\mathcal{T}_3$. Making use of these obtained wave functions, we analyzed the physics-related mechanism and quantified the transmission of pseudospin-1 Dirac electrons across non-rectangular potential barriers in $α-\mathcal{T}_3$ materials with both zero and finite band gaps. Our studies reveal several unique features, including the way in which the electron transmission depends on the energy gap, the slope of the potential barrier profile and the transverse momentum of incoming electrons. Specifically, we have found a strong dependence of the obtained transmission amplitude on the geometry-phase $ϕ= \tan^{-1} α$ of $α-\mathcal{T}_3$ lattices. We believe our current findings can be applied to Dirac cone-based tunneling transistors in ultrafast analog RF devices, as well as to tunneling-current control by a potential barrier through a one-dimensional array of scatters.

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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.

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arXiv2020

Anomalous Klein paradox due to misalignment of optically-tunable elliptical dispersion for Dirac-cone dressed states and direction of incoming particles

After having derived boundary conditions for dressed-state electrons in a dice lattice, we investigate the electron tunneling through a square electrostatic potential barrier in both dice lattices and graphene under a linearly-polarized off-resonance and high-frequency dressing field, and demonstrate the anomalous Klein paradox for a nonzero incident angle, resulted from the misalignment of optically-controllable elliptical dispersion for Dirac-cone dressed states and the direction of incoming kinetic particles in our system. This finite incident angle is found depending on the type of light polarization, the light-induced anisotropy in energy dispersion and the strength of electron-light coupling. Meanwhile, we also observe much larger off-peak transmission amplitudes in dice lattices in comparison with graphene. We expect the theoretical results in this paper could be used for wide range of Dirac materials and applied to controlling both coherent tunneling and ballistic transport of electrons for constructing novel optical and electronic nano-scale switching devices.

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arXiv2020

Many-Body Effects and Optical Properties of Single- and Double Layer $α$-$\mathcal{T}_3$ Lattices

An extensive analytical and numerical investigation has been carried out to examine the role played by many-body effects on various $α$-$\mathcal{T}_3$ materials under an off-resonance optical dressing field. Additionally, we explore its dependence on the hopping parameter $α$ as well as the electron-light coupling strength $λ_0$. The obtained dressed states due to mutual interaction between Dirac electrons and incident light are shown to demonstrate rather different electronic and optical properties in comparison with those in the absence of incident light. Specifically, various collective transport and optical properties of these electron dressed states are discussed in detail and compared for both single- and double layer $α$-$\mathcal{T}_3$ lattices. All of these novel properties are due to the presence of a middle flat band and the interband transitions between it and an upper conduction band. Also, coupled plasmon dispersions for interacting double layer $α$-$\mathcal{T}_3$ lattices are calculated, revealing a lower acoustic-like plasmon branch with tunable group velocity determined by both the layer separation and Fermi energy of each layer. Finally, a many-body theory is presented within the random-phase approximation for calculating the optical absorbance of doped multi-layered $α$-$\mathcal{T}_3$ lattices in a linearly-polarized light field. We anticipate that the discoveries reported here could impact the design of the next-generation nano-optical and nano-plasmonic devices.

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arXiv2019

Nonlinear transport of ballistic Dirac electrons tunneling through a tunable potential barrier in graphene

Dirac-electronic tunneling and nonlinear transport properties with both finite and zero energy bandgap are investigated for graphene with a tilted potential barrier under a bias. For validation, results from a finite-difference based numerical approach, which is developed for calculating transmission and reflection coefficients with a dynamically-tunable (time-dependent bias field) barrier-potential profile, are compared with those of both an analytical model for a static square-potential barrier and a perturbation theory using Wentzel-Kramers-Brillouin (WKB) approximation. For a biased barrier, both transmission coefficient and tunneling resistance are computed and analyzed, indicating a full control of the peak in tunneling resistance by bias field for a tilted barrier, gate voltage for barrier height, and energy for incoming electrons. Moreover, a finite energy gap in graphene is found to suppress head-on transmission as well as skew transmission with a large transverse momentum. For a gapless graphene, on the other hand, filtering of Dirac electrons outside of normal incidence is found and can be used for designing electronic lenses. All these predicted attractive transport properties are expected extremely useful for the development of novel electronic and optical graphene-based devices.

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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.

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arXiv2019

Interplay of Lorentz-Berry forces in position-momentum spaces for valley-dependent impurity scattering in alpha-T3 lattices

The Berry-phase mediated valley-selected skew scattering in alpha-T3 lattices is demonstrated. The interplay of Lorentz and Berry forces in position and momentum spaces is revealed and analyzed. Many-body screening of the electron-impurity interaction is taken into account to avoid overestimation of back- and skew-scattering of electrons in the system. Triplet peak from skew interactions at two valleys is found in near-vertical and near-horizontal forward- and backward-scattering directions for small Berry phases and low magnetic fields. Magnetic-field dependence in both non-equilibrium and thermal-equilibrium currents is also presented for valley-dependent longitudinal and transverse transports mediated by a Berry phase. Mathematically, two Boltzmann moment equations are employed for computing scattering-angle distributions of non-equilibrium skew currents by using microscopic inverse energy- and momentum-relaxation times. Meanwhile, a valley-dependent unbalanced thermal-equilibrium anomalous Hall current induced by the Berry force in momentum space, due to different mobilities for two valleys, is also computed for comparisons.

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arXiv2018

Effects of site asymmetry and valley mixing on Hofstadter-type spectra of bilayer graphene in a square-scatter array potential

Under a magnetic field perpendicular to an monolayer graphene, the existence of a two-dimensional periodic scatter array can not only mix Landau levels of the same valley for displaying split electron-hole Hofstadter-type energy spectra, but also couple two sets of Landau subbands from different valleys in a bilayer graphene. Such a valley mixing effect with a strong scattering strength has been found observable and studied thoroughly in this paper by using a Bloch-wave expansion approach and a projected $2\times 2$ effective Hamiltonian including interlayer effective mass, interlayer coupling and asymmetrical on-site energies due to a vertically-applied electric field. For bilayer graphene, we find two important characteristics, i.e., mixing and interference of intervalley scatterings in the presence of a scatter array, as well as a perpendicular-field induced site-energy asymmetry which deforms severely or even destroy completely the Hofstadter-type band structures due to the dependence of Bloch-wave expansion coefficients on the applied electric field.

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arXiv2018

Peculiar electronic states, symmetries and Berry phases in irradiated $α$-$T_3$ materials

We have laid out the results of a rigorous theoretical investigation into the response of electron dressed states, i.e., interacting Floquet states arising from the off-resonant coupling of Dirac spin-1 electrons in the $α$-$T_3$ model, to external radiation with various polarizations. Specifically, we have examined the role played by the parameter $α$ that is a measure of the coupling strength with the additional atom at the center of the honeycomb graphene lattice and which, when varied, continuously gives a different Berry phase. We have found that the electronic properties of the $α$ -$T_3$ model (consisting of a flat band and two cones) could be modified depending on the polarization of the imposed irradiation. We have demonstrated that under elliptically-polarized light the low-energy band structure of such lattice directly depends on the valley index $τ$. We have obtained and analyzed the corresponding wave functions, their symmetries and the corresponding Berry phases, and revealed that such phases could be finite even for a dice lattice, which has not been observed in the absence of the dressing field. This results lead to possible radiation-generated band structure engineering, as well as experimental and technological realization of such optoelectronic devices and photonic crystals.

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arXiv2017

Role of electron back action on photons in hybridizing double-layer graphene plasmons with localized photons

Induced polarization by Dirac electrons in double-layer graphene can affect hybridization of radiative and evanescent fields. Electron back action appears as a localized optical field to modify an incident surface-plasmon-polariton (SPP) evanescent field. This leads to high sensitivity (beyond the diffraction limit) to local environments and provides a scrutiny tool for molecules or protein selectively bounded with carbon. A scattering matrix with frequencies around the surface-plasmon (SP) resonance supports this scrutiny tool and exhibits sensibly the increase, decrease and even a full suppression of the polarization field in the vicinity of a conducting surface for longer SPP wavelengthes. Moreover, triply-hybridized absorption peaks associated with SP, acoustic- and optical-like graphene plasmons become significant only at high SP frequencies, but are overshadowed by a round SPP peak for low SP frequencies. These resonant features (different from 3D photonic lattices) facilitate the polariton-only excitations, giving rise to possible polariton condensation for a threshold-free laser. The current graphene-plasmon hybridization formalism can be easily generalized to other two-dimensional materials, such as silicene, germanene, molybdenum disulfide, etc.

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arXiv2017

Optical and Boltzmann conductivities for extrinsic buckled honeycomb lattices at finite temperature

The optical and Boltzmann conductivities have been calculated for doped buckled honeycomb lattice structures such as silicene and germanene, as functions of temperature. By making use of previous results for the temperature-dependent chemical potential for gapped Dirac systems, we have calculated the dynamical polarization function and investigated the way in which initial doping affects its behavior at arbitrary temperature, frequency and wave number. We have calculated the optical and Boltzmann conductivities in the relaxation time approximation. Both these quantities are directly related to the polarizability, with the former being proportional to its long-wavelength limit, whereas the latter depends on static screening and the corresponding dielectric function. We demonstrated that initial doping substantially increases each type of conductivity at intermediate temperatures and we have introduced a formalism for calculating the inverse relaxation time and transition rates for the two inequivalent subbands in silicene.

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arXiv2017

Many-body theory for proton-induced point-defect effects on losses of electron energy and photons in quantum wells

The effects of point defects on the loss of either energies of ballistic electron beams or incident photons are studied by using a many-body theory in a multi-quantum-well system. This includes the defect-induced vertex correction to a bare polarization function of electrons within the ladder approximation as well as the intralayer and interlayer screening of defect-electron interactions are also taken into account in the random-phase approximation. The numerical results of defect effects on both energy-loss and optical-absorption spectra are presented and analyzed for various defect densities, number of quantum wells, and wave vectors. The diffusion-reaction equation is employed for calculating distributions of point defects in a layered structure. For completeness, the production rate for Frenkel-pair defects and their initial concentration are obtained based on atomic-level molecular-dynamics simulations. By combining defect-effect, diffusion-reaction and molecular-dynamics models proposed in this paper with a space-weather forecast model for the first time, it will be possible to enable specific designing for electronic and optoelectronic quantum devices that will be operated in space with radiation-hardening protection, and therefore, will effectively extend the lifetime of these satellite onboard electronic and optoelectronic devices.

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arXiv2017

Finite-temperature Coulomb Excitations in Extrinsic Dirac Structures

We have derived algebraic, analytic expressions for the chemical potential without any restriction on temperature for all types of doped, or extrinsic, gapped Dirac cone materials including gapped graphene, silicene, germanene and single-layer transition metal dichalcogenides. As an important intermediate step of our derivations, we have established a reliable piecewise-linear model for cal- culating the density-of-states in molybdenum disulfide, showing good agreement with previously obtained numerical results. For the spin- and valley-resolved band structure, we obtain an additional decrease of the chemical potential due to thermally induced doping of the upper subband at finite temperature. It has been demonstrated that since the symmetry between the electron and hole states in $MoS_2$ is broken, the chemical potential could cross the zero-energy level at sufficiently high temperature. These results allow us to investigate the collective properties, polarizability, plasmons and their damping. Emphasis is placed on low temperatures, when initial electron doping plays a crucial role. We clearly demonstrated the contribution of the initial doping to the finite-temperature collective properties of the considered materials.

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arXiv2017

Quantum-Electron Back Action on Hybridization of Radiative and Evanescent Field Modes

A back action from Dirac electrons in graphene on the hybridization of radiative and evanescent fields is found as an analogy to Newton's third law. Here, the back action appears as a localized polarization field which greatly modifies an incident surface-plasmon-polariton (SPP) field. This yields a high sensitivity to local dielectric environments and provides a scrutiny tool for molecules or proteins selectively bounded with carbons. A scattering matrix is shown with varied frequencies nearby the surface-plasmon (SP) resonance for the increase, decrease and even a full suppression of the polarization field, which enables accurate effective-medium theories to be constructed for Maxwell-equation finite-difference time-domain methods. Moreover, double peaks in the absorption spectra for hybrid SP and graphene-plasmon modes are significant only with a large conductor plasma frequency, but are overshadowed by a round SPP peak at a small plasma frequency as the graphene is placed close to conductor surface. These resonant absorptions facilitate the polariton-only excitations, leading to polariton condensation for a threshold-free laser.

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arXiv2017

Exploring the Optical States for Black Phosphorus: Anisotropy and Bandgap Tuning

The dressed states arising from the interaction between electrons and holes, and off-resonant electromagnetic radiation have been investigated for recently fabricated gapped and anisotropic black phosphorus. Our calculations were carried out for the low-energy electronic subbands near the $Γ$ point. States for both linear and circular polarizations of the incoming radiation have been computed. However, our principal emphasis is on linearly polarized light with arbitrary polarization since this case has not been given much attention for dressing fields imposed on anisotropic structures. We have considered various cases for one- and few-layer phosphorus, including massless Dirac fermions with tunable in-plane anisotropy. Initial Hamiltonian parameters are renormalized in a largely different way compared to those for previously reported for gapped Dirac structures and, most importantly, existing anisotropy which could be modified in every direction.

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arXiv2017

Controlling plasmon modes and damping in buckled two-dimensional material open systems

Full ranges of both hybrid plasmon-mode dispersions and their damping are studied systematically by our recently developed mean-field theory in open systems involving a conducting substrate and a two-dimensional (2D) material with a buckled honeycomb lattice, such as silicene, germanene, and a group \rom{4} dichalcogenide as well. In this hybrid system, the single plasmon mode for a free-standing 2D layer is split into one acoustic-like and one optical-like mode, leading to a dramatic change in the damping of plasmon modes. In comparison with gapped graphene, critical features associated with plasmon modes and damping in silicene and molybdenum disulfide are found with various spin-orbit and lattice asymmetry energy bandgaps, doping types and levels, and coupling strengths between 2D materials and the conducting substrate. The obtained damping dependence on both spin and valley degrees of freedom is expected to facilitate measuring the open-system dielectric property and the spin-orbit coupling strength of individual 2D materials. The unique linear dispersion of the acoustic-like plasmon mode introduces additional damping from the intraband particle-hole modes which is absent for a free-standing 2D material layer, and the use of molybdenum disulfide with a large bandgap simultaneously suppresses the strong damping from the interband particle-hole modes.

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arXiv2016

Temperature-dependent collective effects for silicene, germanene and monolayer black phosphorus

We have calculated numerically electron exchange, correlation energies and dynamical polarization function for newly discovered silicene, germanene and black phosphorus (BP), consisting of puckered layers of elemental phosphorus atoms, broadening the range of two-dimensional (2D) materials at various temperatures. As a matter of fact, monolayer BP, produced by mechanical and liquid exfoliation techniques, has been predicted to be an insulator with a large energy splitting 1.6 eV for the quasiparticle band structure. We compare the dependence of these energies on the chemical potential, field-induced gap and the temperature and concluded that in many cases they behave qualitatively similarly, i.e., increasing with the doping, decreasing significantly at elevated temperatures, and displaying different dependence on the asymmetry gap at various temperatures. Furthermore, we used the dynamical polarizability to investigate the new "split" plasmon branches in the puckered lattices and predict a unique splitting, different from that in gapped graphene, for various energy gaps. Our results are crucial for stimulating electronic, transport and collective studies of these novel materials, as well as for enhancing silicene-based fabrication and technologies for photovoltaics and transistor devices.

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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.

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