arXiv subjects

Danhong Huang

Publications and source records attributed to Danhong Huang.

At least 37 records · Page 2Linked to original sources

arXiv2021

Influence of electric and magnetic fields and $σ$-edge bands on the electronic and optical spectra of graphene nanoribbons

The unusual electronic and optical properties of armchair and zigzag graphene nanoribbons (GNRs) subject to in-plane transverse electric and perpendicular magnetic fields have been systematically investigated. Our calculations were carried out within the generalized multi-orbital tight-binding model based on a Hamiltonian which takes into account hopping integrals among the (s, $p_x$, $p_y$, $p_z$) atomic orbitals as well as the external electric and magnetic fields. The electronic structure consists of $π$ bands arising from the $p_z$ orbital and $σ$ bands originating from the (s, $p_x$, $p_y$) orbitals. The energy bands and optical spectra are diversified by both the nature of the edge of the nanoribbon and strength of the external fields. Armchair GNRs display a width-dependent energy gap in addition to low-energy $σ$ bands while the zigzag system has the unfilled flat band with $π$ edge states at zero energy and partially filled wide-range $σ$ bands. An applied in-plane electric field leads to the splitting of energy bands and shifted Fermi level, thereby enriching the inter-band and intra-band optical conductivities. The interplay between an external magnetic field and the edge geometry gives rise to extraordinary quantized Landau levels and special optical spectra.

cond-mat.mes-hall↗

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.

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↗

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.

cond-mat.mes-hall↗

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.

cond-mat.mes-hall↗

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.

cond-mat.mes-hall↗

arXiv2019

Valley- and spin-dependent quantum Hall states in bilayer silicene

The Hall conductivity $σ_{xy}$ of many condensed matter systems presents a step structure when a uniform perpendicular magnetic field is applied. We report the quantum Hall effect in buckled AB-bottom-top bilayer silicene and its robust dependence on the electronic valley and spin-orbit coupling. With the unique multi-valley electronic structure and the lack of spin degeneracy, the quantization of the Hall conductivity in this system is unlike the conventional sequence as reported for graphene. Furthermore, the conductivity plateaux take different step values for conduction ($2e^2/h$) and valence ($6e^2/h$) bands since their originating valleys present inequivalent degeneracy. We also report the emergence of fractions under significant effect of a uniform external electric field on the quantum Hall step structure by the separation of orbital distributions and the mixing of Landau levels from distinct valleys. The valley- and spin-dependent quantum Hall conductivity arises from the interplay of lattice geometry, atomic interaction, spin-orbit coupling, and external magnetic and electric fields.

physics.comp-ph↗

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↗

arXiv2019

Magneto-transport properties of doped graphene

The effect due to doping by B, Si, N on the magneto-transport properties of graphene is investigated using the generalized tight-binding model in conjunction with the Kubo formula. The crucial electronic and transport properties are greatly diversified by the type of dopant and doping concentration. The contribution from the guest atoms may open a band gap, thereby giving rise to the rich Landau level energy spectra and consequently the unique quantum Hall conductivity. The Fermi energy-dependent quantum Hall effect appears as a step structure having both integer and half-integer plateaus. Doping leads to the occurrence of zero conductivity, unlike the plateau sequence for pristine graphene. The predicted dopant- and concentration-enriched quantum Hall effect for doped graphene should provide useful information for magneto-transport measurements and possible technological applications as well as metrology.

physics.comp-ph↗

arXiv2019

Diverse quantization phenomena in AA bilayer silicene

The rich magneto-electronic properties of AA-bottom-top (bt) bilayer silicene are investigated using a generalized tight-binding model. The electronic structure exhibits two pairs of oscillatory energy bands in which the lowest conduction and highest valence states of the low-lying pair are away from the K point. The quantized Landau levels (LLs) are classified into various separated groups by the localization behaviors of spatial distributions. The LLs in the vicinity of Fermi energy do not present simple wave function modes which are quite different from other two-dimensional systems. The geometry symmetry, intralayer and interlayer atomic interactions, and effect of a perpendicular magnetic field are responsible for the peculiar LL energy spectra in AA-bt bilayer silicene. This work provides a better understanding of the diverse magnetic quantization phenomena in 2D condensed-matter materials.

physics.comp-ph↗

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.

cond-mat.mes-hall↗

arXiv2019

Self-consistent quantum-kinetic theory for interplay between pulsed-laser excitation and nonlinear carrier transport in a quantum-wire array

We propose a self-consistent many-body theory for coupling the ultrafast dipole-transition and carrier-plasma dynamics in a linear array of quantum wires with the scattering and absorption of ultrashort laser pulses. The quantum-wire non-thermal carrier occupations are further driven by an applied DC electric field along the wires in the presence of resistive forces from intrinsic phonon and Coulomb scattering of photo-excited carriers. The same strong DC field greatly modifies the non-equilibrium properties of the induced electron-hole plasma coupled to the propagating light pulse, while the induced longitudinal polarization fields of each wire significantly alters the nonlocal optical response from neighboring wires. Here, we clarify several fundamental physics issues in this laser-coupled quantum wire system, including laser influence on local transient photo-currents, photoluminescence spectra, and the effect of nonlinear transport in a micro-scale system on laser pulse propagation. Meanwhile, we also anticipate some applications from this work, such as specifying the best combination of pulse sequence through a quantum-wire array to generate a desired THz spectrum and applying ultra-fast optical modulations to nonlinear carrier transport in nanowires.

physics.optics↗

arXiv2018

Thickness dependence of electron-electron interactions in topological p-n junctions

Electron-electron interactions in topological p-n junctions consisting of vertically stacked topological insulators are investigated. n-type Bi2Te3 and p-type Sb2Te3 of varying relative thicknesses are deposited using molecular beam epitaxy and their electronic properties measured using low-temperature transport. The screening factor is observed to decrease with increasing sample thickness, a finding which is corroborated by semi-classical Boltzmann theory. The number of two-dimensional states determined from electron-electron interactions is larger compared to the number obtained from weak-antilocalization, in line with earlier experiments using single layers.

cond-mat.mes-hall↗

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.

cond-mat.mes-hall↗

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.

cond-mat.mes-hall↗

arXiv2018

Decay of semiclassical massless Dirac fermions from integrable and chaotic cavities

Conventional microlasing of electromagnetic waves requires (1) a high $Q$ cavity and (2) a mechanism for directional emission. Previous theoretical and experimental work demonstrated that the two requirements can be met with deformed dielectric cavities that generate chaotic ray dynamics. Is it possible for a massless Dirac spinor wave in graphene or its photonic counterpart to exhibit a similar behavior? Intuitively, because of the absence of backscattering of associated massless spin-1/2 particles and Klein tunneling, confining the wave in a cavity for a long time seems not feasible. Deforming the cavity to generate classical chaos would make confinement even more difficult. Investigating the decay of a spin-1/2 wave from a scalar potential barrier defined cavity characterized by an effective refractive index $n$ that depends on the applied potential and the particle energy, we uncover the striking existence of an interval of the refractive index in which the average lifetime of the massless spin-1/2 wave in the cavity can be as high as that of the electromagnetic wave, for both integrable and chaotic cavities. We also find scaling laws for the ratio between the mean escape time associated with electromagnetic waves and that with massless spin-1/2 particles versus the index outside of this interval. The scaling laws hold regardless of the nature of the classical dynamics. All the results are verified numerically. The findings provide insights into the emergent field of Dirac electron optics and have potential applications in developing unconventional electronics using 2D Dirac materials.

cond-mat.mes-hall↗

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↗

arXiv2018

Diverse Magnetic Quantization in Bilayer Silicene

The generalized tight-binding model is developed to investigate the rich and unique electronic properties of AB-bt (bottom-top) bilayer silicene under uniform perpendicular electric and magnetic fields. The first pair of conduction and valence bands, with an observable energy gap, displays unusual energy dispersions. Each group of conduction/valence Landau levels (LLs) is further classified into four subgroups, that is, there exist the sublattice- and spin-dominated LL subgroups. The magnetic-field-dependent LL energy spectra exhibit irregular behavior corresponding to the critical points of the band structure. Moreover, the electric field can induce many LL anti-crossings. The main features of the LLs are uncovered with many van Hove singularities in the density-of-states and non-uniform delta-function-like peaks in the magneto-absorption spectra. The feature-rich magnetic quantization directly reflects the geometric symmetries, intra-layer and inter-layer atomic interactions, spin-orbital couplings, and the field effects. The results of this work can be applied to novel designs of $Si$-based nano-electronics and nano-devices with enhanced mobilities.

cond-mat.mtrl-sci↗