arXiv subjects

Godfrey Gumbs

Publications and source records attributed to Godfrey Gumbs.

At least 19 recordsLinked to original sources

arXiv2026

Suppressed plasmon excitations, enhanced damping and static screening in Kek-Y strained $\alpha-\mathcal{T}_3$ model

We performed a rigorous theoretical and numerical investigation into the polarization function, plasmon excitations, and plasmon damping in the Kek-$\alpha$ model, a two-dimensional material combining the key features of the $\alpha-\mathcal{T}_3$ lattice and Kekule-distorted graphene. Unlike conventional Kek-Y graphene, the Kekule modulation in the Kek-$\alpha$ model affects only one of the two sublattices, giving rise to a fundamentally new model with unusual electronic properties. The low-energy spectrum consists of two degenerate flat bands and two inequivalent Dirac cones with different Fermi velocities, referred to as the fast and slow cones. The particle-hole continuum responsible for Landau damping exhibits two distinct branches associated with transitions involving these Dirac cones. An additional particle-hole mode originates from electron transitions associated with the fast Dirac cone, appearing above the main diagonal. As the parameter $\alpha$ increases, the contribution from the fast Dirac cone becomes dominant. The additional transitions involving the flat bands and the fast Dirac cone substantially reduce the region where undamped plasmons can exist, similarly to the conventional $\alpha-\mathcal{T}_3$. Consequently, stable plasmons are observed only for relatively small values of $\alpha$ or at very small wave vectors. These unusual electronic and collective properties make the Kek-$\alpha$ model a promising platform for future plasmonic and nanoscale electronic applications.

cond-mat.mes-hall↗

arXiv2026

Surface Response, Plasma Modes of coated Multi-Layered anisotropic Semi-Dirac Heterostructures

We derived closed-form analytical expressions for the surface response functions (SRFs) for heterostructure. We investigate structures consisting of up to three layered, coated heterostructure of two-dimensional (2D) materials with a dielectric medium or vacuum interface. The dielectric media serves to inhibit charge transfer between layers for the case when a pair of 2D layers serve as coatings for a dielectric film. Our results revise the established picture for the dispersion equation for two layers of reduced dimensionality surrounded by dielectric media. An impinging electromagnetic field incident on the surface leads to Coulomb coupled plasma excitations in the structure which are yielded by the SRF. This is achieved by employing Maxwell's equations and linear response theory. We use these results to investigate the plasmonic properties of tilted semi-Dirac materials both analytically and numerically. Closed-form analytical expressions are derived for the plasmon dispersions in the long wavelength limit for single and double layers. We numerically obtain density plots of the loss functions and observe anisotropic behavior in different momentum directions. For the cases when there are two or three layers, we observe two plasmon branches corresponding to in-phase and out-of-phase charge density oscillations, where the in-phase optical modes have higher intensity than the out-of-phase acoustic modes. We calculated the optical absorption spectra for plasma modes in layered semi-Dirac materials produced by an external electromagnetic field carrying an electric polarization and frequency. Possible applications include durable protection coatings providing UV resistance, chemical protection and improving upon traditional ceramic coatings.

cond-mat.mes-hall↗

arXiv2026

Exploring stable long-lifetime plasmon excitations in the Lieb lattice

The subject of the present paper is a thorough numerical investigation of plasmon expectations, their dispersions and damping within a Lieb lattice. The Lieb lattice is known for its unique low-energy band structure which consists of a bandgap as well as a flat band intersecting the conduction band at its lowest point. In contrast to previously studied dice lattice, the location of the current flat band exhibits reduced and broken symmetries, which give rise to interesting electronic and optical properties of this new material. In this work, we have investigated the conditions for observing a well-defined and stable plasmon mode within a wide frequency range. Specifically, we have considered a free-standing layer with various doping levels, as well as different types of monolayers of the Lieb lattice interacting with a surface-plasmon mode localized on top of a semi-infinite conductor. In particular, we have observed and described fully long-living plasmon modes with unusual energy dispersions. Additionally, we have carried out a detailed investigation on the static screening associated with the Lieb lattice. Our study has further revealed that these predicted features seem to be quite different from those of pseudospin-1 materials but resemble those of graphene instead.

cond-mat.mtrl-sci↗

arXiv2026

Floquet Control of Electron and Exciton Transport in Kekul\'e-Distorted Graphene

This work investigates the Floquet dynamics of electrons and excitons (particle-hole pairs) in a Dirac material referred to as Kekul\'e-distorted graphene. Specifically, we examine the role played by a high frequency driving electromagnetic field on the tunneling and blocking by a potential barrier on both the charged single particles as well as the neutral composite particles. We demonstrate that the small effective masses of the electron and hole for the energy spectrum of this Kekul\'e distorted graphene leads to practically almost perfect transmission across a symmetric potential barrier for any angle of incidence of impinging excitons. However, this unexpected Klein paradox for excitons does not hold for the single-particle electrons. The reduced total transmission of electron due to Kekul\'e distortion is more suppressed due to irradiation. Additionally, we calculate and investigate the exciton binding energy since the quantum tunneling of a bound electron-hole pair across a potential barrier is governed by its mass measured in the center of mass and binding energy of the composite pair. Thus, irradiation with circularly polarized light fundamentally modifies exciton formation, coherence and transport properties, thereby producing unusual topological behaviors. These behaviors are unlike conventional Dirac materials. Possible technical applications of the results arising from our investigation include valleytronics due to the folding of the valleys, thereby making intervalley coupling feasible. Other practical applications include optoelectronics due to Floquet tuning of energy spectrum and transport properties.

cond-mat.mes-hall↗

arXiv2025

Quantum Hall Effect and Chern Phases in the 1/5-Depleted Square Lattice

We investigate the fractional energy spectrum and quantum Hall response of a two-dimensional 1/5-depleted square lattice subjected to a perpendicular magnetic field. Using a tight-binding model that includes both nearest-neighbor (t_1) and next-nearest-neighbor (t_2) hopping, we compute the Hofstadter butterfly and extract quantized Hall conductivities via Chern number calculations. In the absence of diagonal hopping (t_2 =0), the spectrum exhibits exact particle-hole and flux-inversion symmetries, and the total Chern number across all bands vanishes. When t_2 is introduced, these symmetries are broken, the butterfly becomes deformed, new gaps open, and -remarkably-a nonzero total Chern sum can emerge, signaling unconventional topological phases. By systematically varying t_1 and t_2, we identify regimes with large individual Chern indices and parameter windows where gap stability and Hall plateaus are optimized. Our results demonstrated that lattice depletion combined with diagonal hopping provides a tunable route to engineer robust Chern insulators in both artificial and oxide-based square-lattice systems.

cond-mat.mtrl-sci↗

arXiv2025

Tuning excitons and superfluidity of dipolar excitons in the double layers of kagome lattice by applying circularly polarized irradiation

We present detailed calculations for several significant properties of the kagome lattice. We employ the Floquet-Magnus perturbation expansion to obtain the energy bands and the corresponding wave functions near the Dirac points for the kagome lattice in the presence of circularly or linearly polarized irradiation. In contrast with linearly polarized irradiation, a band gap is opened up near the Dirac points, between the valence and conduction bands in the presence of circularly polarized irradiation. We calculated the exciton binding energy, and the exciton energy for gapped kagome lattice as a function of the frequency and intensity of the irradiation. We compare the exciton binding energy and exciton energy in a monolayer with those in a double layer separated by an insulator to inhibit recombination. We predict that a phase transition in the kagome lattice from the semiconducting phase to the excitonic insulating phase can be induced by applying irradiation. We also examined the conditions for such a phase transition. We explore opportunities to tune exciton binding energy, the energy spectrum of collective excitations, the sound velocity and the critical temperature of the superfluidity by applying circularly polarized irradiation. We propose observation of Bose-Einstein condensation and superfluidity of quasi-two-dimensional dipolar excitons in two-layer kagome lattices in the presence of pumping by circularly polarized light. We have also analyzed the dependence of superfluid density $n_s$ and the temperature of the Kosterlitz-Thouless phase transition temperature on excitonic density n, the interlayer separation D and the parameters for circularly polarized light.

cond-mat.mes-hall↗

arXiv2025

Theory of two-component superfluidity of microcavity polaritons

We develop a microscopic mean-field theory describing the coexistence of Bose-Einstein condensates of upper and lower polaritons (UP/LP) in a semiconductor microcavity. Incorporating interbranch scattering within a modified polariton Hamiltonian, we introduce a phenomenological population-split parameter $\alpha$ that quantifies the relative LP/UP occupations. At zero detuning, the critical temperature becomes independent of $\alpha$, converging to a single value that marks the balanced, resonant regime. Away from resonance, variations in $\alpha$ lead to distinctive and experimentally resolvable changes in both the sound velocity $c_s$ and critical temperature $T_c$, relative to the single-component (LP-only) condensate limit. The system under study consists of excitons confined in a transition metal dichalcogenide (TMDC) monolayer, particularly WSe$_2$ embedded within a planar optical microcavity of GaAs where they strongly couple to cavity photons. Our analysis focuses on monolayer WSe$_2$ embdedded in a GaAs microcavity. We present results for GaAs/AlGaAs quantum wells embedded in a GaAs microcavity in the Appendix. While mean-field in scope, the framework provides analytic benchmarks and physical insight for future treatments that include dissipation and fluctuations in nonequilibrium polariton superfluids.

cond-mat.quant-gas↗

arXiv2025

Long-living superfluidity of dark excitons in a strip of strained transition metal dichalcogenides double layer

We have proposed the superfluidity of dipolar excitons in a strip of double-layer transition metal dichalcogenides (TMDCs) heterostructures. We have shown that strain causes a shift in k-space between the minimum of the conduction band and the maximum of the valence band. Therefore, we expect that applying strain to this system can cause dark excitons to be created. We have numerically calculated the energy spectrum of dark dipolar excitons in strained MoSe$_2$, and we have calculated their binding energies and effective masses. We have shown that the dark dipolar excitons in strained TMDC heterostructures form superfluids, and we have calculated the sound velocity in the energy spectrum of collective excitations, as well as the mean-field critical temperature for superfluidity. We have shown that two separate superfluid flows moving in opposite directions will appear in the system, one on each edge of the strip, forming the double layer. We have seen that the critical temperature for superfluidity increases with the concentration of dark excitons, as well as with the inter-layer separation. The fact that dark excitons cannot decay by the simple emission of photons, makes it so that the superfluids and condensates formed by them have a much longer lifetime than that formed by bright excitons. We propose a way to experimentally verify the predicted phenomena.

cond-mat.mes-hall↗

arXiv2024

Flat bands of TaS$_2$ under superlattice potential modulation: A Wannier tight-binding model study

In this work, we construct a Wannier tight-binding model for TaS$_2$ under a superlattice potential modulation, based on the Joint Automated Repository for Various Integrated Simulations database established by the U.S. National Institute of Standards and Technology, so as to study the electronic properties of the structure. Our computational method enables direct calculation of the energy bands from the Hamiltonian without any additional assumptions. We observed a pair of dispersionless flat bands, significant interactions between energy bands, and nontrivial modification of band dispersion at low modulated electric potentials. This work provides a valuable reference for researchers investigating two-dimensional condensed matter materials under superlattice potential modulation.

cond-mat.mtrl-sci↗

arXiv2024

Polarizability and plasmons in pseudospin-1 gapped materials with a flat band

The collective electronic properties of various types of pseudospin-$1$ Dirac-cone materials with a flat band and finite bangaps in their energy spectra are the subject of our reported investigation. Specifically, we have calculated the dynamical polarization, plasmon dispersions as well as their decay rates due to Landau damping. Additionally, we present closed-form analytical expressions for the wave function overlaps for both the gapped dice lattice and the Lieb lattice. The gapped dice lattice is a special case of the more general $\alpha$-${\cal T}_3$ model since its band structure is symmetric and the flat band remains dispersionless. On the other hand, the Lieb lattice has a flat band which appears at the lowest point of its conduction band. Our results for these two cases exhibit unique features in the plasmon spectra and their damping regions, which have never been reported in previous studies. For example, the particle-hole modes of a Lieb lattice appear as finite-size regions, while the plasmon modes exist only in a region with small wave numbers but an extended range of frequencies.

cond-mat.mes-hall↗

arXiv2024

Superfluidity of dipolar excitons in a double layer of $\alpha-T_3$ with a mass term

We predict Bose-Einstein condensation and superfluidity of dipolar excitons, formed by electron-hole pairs in spatially separated gapped hexagonal $\alpha-T_{3}$ (GHAT3) layers. In the $\alpha-T_{3}$ model, the AB-honeycomb lattice structure is supplemented with C atoms located at the centers of the hexagons in the lattice. We considered the $\alpha-T_{3}$ model in the presence of a mass term which opens a gap in the energy dispersive spectrum. The gap opening mass term, caused by a weak magnetic field, plays the role of Zeeman splitting at low magnetic fields for this pseudospin-1 system. The band structure of GHAT3 monolayers leads to the formation of two distinct types of excitons in the GHAT3 double layer. We consider two types of dipolar excitons in double-layer GHAT3: (a) ``A excitons'', which are bound states of electrons in the conduction band (CB) and holes in the intermediate band (IB) and (b) ``B excitons'', which are bound states of electrons in the CB and holes in the valence band (VB). The binding energy of A and B dipolar excitons is calculated. For a two-component weakly interacting Bose gas of dipolar excitons in a GHAT3 double layer, we obtain the energy dispersion of collective excitations, the sound velocity, the superfluid density, and the mean-field critical temperature $T_{c}$ for superfluidity.

cond-mat.mtrl-sci↗

arXiv2024

Quantum entanglement of multiple excitons in strained graphene

We studied the effects arising from a coherent source of photons on the entanglement between excitons in a strained graphene monolayer. The graphene layer was considered to be embedded in an imperfect optical microcavity. In our investigation, we have studied the entanglement dynamics of systems consisting of up to five excitons, which are treated as atomic-like qubits. Entangled states of multiple qubits are useful in quantum error correction codes. We have monitored the time evolution of the concurrence, three-$\pi$, mutual information, and especially the negativity. We have demonstrated that coherent pumping can create lasting entanglement between the excitons. However, the entanglement only persists when the rate at which photons are pumped is smaller than the decay rate of the cavity. Our results show that the degree in entanglement between the excitons is increased with the intensity of the strain-induced pseudomagnetic field in the graphene sheet. Additionally, we have shown that a maximum amount of entanglement occurs at a finite number of excitons in the system which depends on the parameters describing the structure.

cond-mat.mes-hall↗

arXiv2024

Unique electronic and optical properties of stacking-modulated bilayer graphene under external magnetic fields

This study delves into the magneto-electronic and magneto-optical properties of stacking-modulated bilayer graphene. By manipulating domain walls (DWs) across AB-BA domains periodically, we unveil oscillatory Landau subbands and the associated optical excitations. The DWs act as periodic potentials, yielding fascinating 1D spectral features. Our exploration reveals 1D phenomena localized to Bernal stacking, DW regions, and stacking boundaries, highlighting the intriguing formation of Landau state quantization influenced by the commensuration between the magnetic length and the system. The stable quantized localization within different regions leads to the emergence of unconventional quantized subbands. This study provides valuable insights into the essential properties of stacking-modulated bilayer graphene.

cond-mat.mes-hall↗

arXiv2024

Superfluidity of indirect momentum space dark dipolar excitons in a double layer with massive anisotropic tilted semi-Dirac bands

We have theoretically investigated the spin- and valley-dependent superfluidity properties of indirect momentum space dark dipolar excitons in double layers with massive anisotropic tilted semi-Dirac bands in the presence of circularly polarized irradiation. An external vertical electric field is also applied to the structure and is responsible for tilting and gap opening for the band structure. For our calculations we used the parameters of a double layer of 1T$^\prime$-MoS$_2$. Closed form analytical expressions are presented for the energy spectrum for excitons, their associated wave functions and binding energies. Additionally, we examine the effects which the intensity and frequency of circularly polarized irradiation has for 1T$^\prime$-MoS$_2$ on the effective mass of the excitons since it has been demonstrated that the application of an external high-frequency dressing field tailors the crucial electronic including the exciton binding energy, as well as the critical temperature for superfluidity. We also calculate the sound velocity in the anisotropic weakly-interacting Bose gas of two-component indirect momentum space dark excitons for a double layer of 1T$^\prime$-MoS$_2$. We show that the critical velocity of superfluidity, the spectrum of collective excitations, concentrations of the superfluid and normal component, and mean field critical temperature for superfluidity are anisotropic and formed by a two-component system. The critical temperature for superfluidity is increased when the exciton concentration and interlayer separation are increased. We propose the use of phonon-assisted photoluminescence to experimentally confirm directional superfluidity of indirect momentum space dark excitons in a double layer with massive anisotropic tilted semi-Dirac bands.

cond-mat.mes-hall↗

arXiv2023

Influence of Dynamical Floquet Spectrum on the Plasmon Excitations and Exchange Energy of tilted monolayer 1T$^\prime$MoS$_2$

It is now well established that a high-frequency electromagnetic dressing field within the off-resonance regime significantly modifies the electronic transport and optical properties on Dirac materials. Here, using light with circular polarization, we investigate its effect on the energy spectrum of tilted monolayer 1T$^\prime$MoS$_2$ which acquires two energy gaps associated with up- and down- pseudospin. We can adjust its electronic properties over a wider range by varying these two band gaps in contrast with graphene. With the use of the Lindhard approach for the frequency-dependent polarizability propagator, we have developed a rigorous theoretical formalism for employing the Floquet energy spectrum for investigating the many-body effects on the plasmon excitations, their lifetimes due to Landau damping and the exchange energy of tilted monolayer 1T$^\prime$MoS$_2$ under normal incidence of electromagnetic radiation at arbitrary temperature. The dressed states at very low temperature corresponding to circular polarization suppress the response of the system to the external probe. This gives rise to the weak but long lived plasmon excitations at small wavenumber $q$ when compared to the plasmon spectrum in this regime in the absence of irradiation. However, $\sqrt{qT}$-dependent plasmons are restored at high temperatures. Our calculations have shown that the tilting, anisotropy, direct and indirect band gaps lead to a reduced exchange energy, which has some potential applications such as, tunability of exciton polariton and plasmon excitations.

cond-mat.mes-hall↗

arXiv2023

Dynamical polarization function, plasmons, their damping and collective effects in semi-Dirac bands

We have calculated the dynamical polarization, plasmons and damping rates in semi-Dirac bands (SDB's) with zero band gap and half-linear, half-parabolic low-energy spectrum. The obtained plasmon dispersions are strongly anisotropic and demonstrate some crucial features of both two-dimensional electron gas and graphene. Such gapless energy dispersions lead to a localized area of undamped and low-damped plasmons in a limited range of the frequencies and wave vectors. The calculated plasmon branches demonstrate an increase of their energies for a finite tilting of the band structure and a fixed Fermi level which could be used as a signature of a specific tilted spectrum in a semi-Dirac band.

cond-mat.mes-hall↗

arXiv2023

Strain Engineering of Magneto-optical Properties in C$_3$B/C$_3$N van der Waals Heterostructure

Carbon-based bilayer van der Waals (vdW) materials are attracting much attention due to their predicted interesting physical properties. Here, we theoretically investigate electronic and optical properties of C$_3$B/C$_3$N vdW heterostructure (HTS) under external magnetic field and mechanical strain. The tight-binding model of the system is constructed to include the strain-induced modification of the hopping interactions. The influence of a uniform perpendicular magnetic field is included by using the Peierls substitution method. We observe the intriguing electronic and optical characteristics of the HTS under mechanical strain, covering the band inversion, alteration of band gap and optical gap, distortion of band-edge states, as well as significant enhancement of optical absorption. Furthermore, the interplay between external magnetic field and biaxial strain leads to exotic features of quantization and optical spectra. This work provides important information for the comprehension of the engineering of materials by external effects. Our study suggests that C$_3$B/C$_3$N vdW HTS is a promising candidate for next-generation electronic and optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2023

Quantized Hall current in topological nodal-line semimetal

Photocurrent acts as one of measurable responses of material to light, which has proved itself to be crucial for sensing and energy harvesting. Topological semimetals with gapless energy dispersion and abundant topological surface and bulk states exhibit exotic photocurrent responses, such as novel quantized circular photogalvanic effect observed in Weyl semimetals. Here we find that for a topological nodal-line semimetal (NLSM) with nodal ring bulk states and drumhead surface states (DSS), a significant photocurrent can be produced by an electromagnetic (EM) wave by means of the quantum Hall effect. The Hall current is enabled by electron transfer between Landau levels (LLs) and triggered by both the electric field and magnetic field components of an EM wave. This Hall current is physically connected to an unusually large quantum-Hall conductivity of the zeroth LLs resulting from quantized DSS. These LLs are found to be highly degenerate due to the unique band-folding effect associated with magnetic-field-induced expansion of a unit cell. Furthermore, we observe that the Hall current induced solely by an in-plane linearly-polarized EM wave becomes a quantized entity which allows for possible direct measurement of the DSS density in a topological NLSM. This work paves a way toward designing high-magnetic-field-sensitivity detection devices for industrial and space applications, such as the development of self-detection of current-surge-induced overheating in electronic devices and accurate Earth's magnetic-anomaly maps for guiding a self-navigating drone or an aircraft.

cond-mat.mes-hall↗