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Yawar Mohammadi

Publications and source records attributed to Yawar Mohammadi.

16 recordsLinked to original sources
arXiv2026

Strain- and doping-tunable optical resonance in Kekulé-Y graphene

We investigate the optical response of Kekulé-Y graphene under uniaxial strain and carrier doping. Using a low-energy effective Hamiltonian, we show that strain reshapes the low-energy electronic structure of the Kekulé-Y phase and induces Van Hove singularities at energies well below those of pristine graphene. Within the Kubo formalism, we calculate the optical conductivity and identify multiple anisotropic interband features, with a pronounced resonance arising from strain-induced Van Hove singularities. The pronounced resonance is strongly anisotropic and robust against moderate thermal broadening and disorder, providing a clear optical signature of Kekulé-Y ordering. We further derive analytical expressions for the low-energy optical conductivity and the Drude weight, providing a detailed characterization of the strain- and doping-dependent optical response. Our results establish strain engineering as an effective route for controlling valley-dependent optical properties in Kekulé-Y graphene, originating from the Kekulé-induced coupling of the Dirac valleys, and suggest feasible optical probes for the experimental identification of the Kekulé-Y phase.

cond-mat.mes-hall↗
arXiv2026

Optical fingerprints across the strain-driven semi-Dirac transition in Kekulé-O graphene

We show that the strain-driven semi-Dirac transition in Kekulé-O graphene gives rise to a sequence of anisotropic optical fingerprints associated with band structure reconstruction. Across the transition, optical spectral weight is continuously redistributed among the dominant interband transitions, leading to a pronounced enhancement of the optical anisotropy. Combining numerical four-band calculations with analytical low-energy results, we identify three low-energy fingerprints that emerge with increasing strain: gapped absorption peaks, semi-Dirac critical scaling, and a pronounced van Hove optical resonance. At the semi-Dirac critical point, where the Kekulé gap closes at the $Γ$ point, the low-energy optical conductivity is characterized by $σ_{xx}(Ω)\proptoΩ^{1/2}$ and $σ_{yy}(Ω)\proptoΩ^{-1/2}$. Beyond the transition, the semi-Dirac point splits into two anisotropic Dirac cones, accompanied by the emergence of saddle points near the $Γ$ point. The resulting saddle-point excitations produce a pronounced van Hove optical resonance at energies well below those of graphene, while the split Dirac cones give rise to an anisotropic constant optical conductivity. We further show that the low-energy optical fingerprints can be traced to the continuous evolution of a dominant optical transition channel driven by strain-induced band reconstruction. Moreover, the fingerprints remain identifiable in the presence of moderate disorder broadening and finite-temperature effects, indicating their potential observability under experimentally realistic conditions.

cond-mat.mes-hall↗
arXiv2022

Electronic spectrum and optical properties of Y-shaped Kekule-patterned graphene: Band nesting resonance as an optical signature

Employing tight-binding model we investigate the effects of a uniform Y-shaped Kekule lattice distortion on the electronic spectrum and optical conductivity of graphene. We derive a low-energy effective Hamiltonian which is found to be in excellent agreement with one calculated from a diagonalization of the full tight-binding Hamiltonian. Then using the low-energy Hamiltonian and Kubo formula we obtain an analytical expression for the real part of the optical conductivity used to explore the effects of chemical potential, temperature and on-site and hopping energy deviations in details. In particular we find that Y-shaped Kekue-patterned graphene at finite chemical potential displays a large optical response called band nesting resonance. This effect is shown to be robust against increasing temperature, facilitating its detection as an optical signature for the Y-shaped Kekule distortion even at room temperature.

cond-mat.mes-hall↗
arXiv2020

Tunable plasmon modes in doped AA-stacked bilayer graphene

We study plasmon modes in doped AA-stacked bilayer graphene (BLG) within the nearest-neighbor tight-binding and the random phase approximation. We obtain closed analytical expressions for the polarizability function which are used to obtain the low-energy dispersion relations of and the numerical results for both acoustic and optical plasmon modes. Our result reveal the potential of AA-stacked BLG to be used as a tunable plasmonic device. In particular we find that the long-wavelength acoustic plasmon disperse as $ω_{+}\approx\sqrt{max(|μ|,t_{1})q}$ with a phase space which shrinks and vanishes as the chemical potential approaches the interlayer hopping energy, preventing the existence of long-lived acoustic plasmon. Furthermore, we show that AA-stacked BLG support coherent optical plasmon only when the condition $(1+\frac{g_σg_{v}e^{2}t_{1}d}{κv_{F}^{2}}\frac{|μ|}{t_{1}})^{1/2}<\frac{|μ|}{t_{1}}$ is satisfied, specially indicating Landau damping of the optical plasmon in undoped AA-staked BLG even at long-wavelength limit. We also find that the optical plasmon mode disperses as $ω_{-}\approx Δ+Cq^{2}$ with constants that can be tuned by tuning the chemical potential.

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arXiv2019

Integer quantum Hall effect in AAA-stacked trilayer graphene

We obtain an analytical expression for the Hall conductivity in AAA-stacked trilayer graphene (TLG) within the linear response Kubo formalism. Interestingly, we find that the quantization of the Hall conductivity is severely dependent on the value of the chemical potential and the magnetic field. The origin of this unusual quantization, attributed to the Landau level spectrum of AAA-stacked TLG, is explained in detail. We show that is a characteristic of the multilayer graphene structures with AA stacking order.

cond-mat.mes-hall↗
arXiv2017

Photo-induced spin and valley-dependent Seebeck effect in the low-buckled Dirac materials

Employing the Landauer-Buttiker formula we investigate the spin and valley dependence of Seebeck effect in low-buckled Dirac materials (LBDMs), whose band structure are modulated by local application of a gate voltage and off-resonant circularly polarized light. We calculate the charge, spin and valley Seebeck coefficients of an irradiated LBDM as functions of electronic doping, light intensity and the amount of the electric field in the linear regime. Our calculation reveal that all Seebeck coefficients always shows an odd features with respect to the chemical potential. Moreover, we show that, due to the strong spin-orbit coupling in the LBDMs, the induced thermovoltage in the irradiated LBDMs is spin polarized, and can also become valley polarized if the gate voltage is applied too. It is also found that the valley (spin) polarization of the induced thermovoltage could be inverted by reversing the circular polarization of light or reversing the direction the electric field (only by reversing the circular polarization of light).

cond-mat.mes-hall↗
arXiv2015

Controllable photo-induced spin and valley filtering in silicene

We study ballistic transport of Dirac electrons through a strip in silicene, when the strip is exposed to off-resonant circularly polarized light and an electric field applied perpendicular to the silicene plane. We show that the conductance through the strip is spin- or/and valley-polarized. This can be explained by spin-valley coupling in silicene, and modification of its band structure through virtual absorption/emission processes and also by the perpendicular electric field. The spin- (valley-) polarization can be enhanced by tuning the light intensity and the value of the perpendicular electric field, leading to perfect spin (valley) filtering for certain of their values. Further, the spin (valley) polarization can be inverted by reversing the perpendicular electric field (by reversing the perpendicular electric field or reversing the circular polarization of the light irradiation). The conditions necessary for the fully valley polarization is determined.

cond-mat.mes-hall↗
arXiv2015

Strain engineering the charged-impurity-limited carrier mobility in phosphorene

We investigate, based on the tight-binding model and in the linear deformation regime, the strain dependence of the electronic band structure of phosphorene, exposed to a uniaxial strain in one of its principle directions, the normal, the armchair and the zigzag directions. We show that the electronic band structure of strained phosphorene, for experimentally accessible carrier densities and uniaxial strains, is well described by a strain-dependent decoupled electron-hole Hamiltonian. Then, employing the decoupled Hamiltonian, we consider the strain dependence of the charged-impurity-limited carrier mobility in phosphorene, for both types of carrier, arbitrary carrier density and in both armchair and zigzag directions. We show that a uniaxial tensile (compressive) strain in the normal direction enhances (weakens) the anisotropy of the carrier mobility, while a uniaxial strain in the zigzag direction acts inversely. Moreover applying a uniaxial strain in the armchair direction is shown to be ineffective on the anisotropy of the carrier mobility. These will be explained based on the effect of the strain on the carrier effective mass.

cond-mat.mes-hall↗
arXiv2015

Controllable intrinsic DC spin/valley Hall conductivity in ferromagnetic silicene: Exploring a fully spin/valley polarized transport

We study intrinsic DC spin and valley Hall conductivity in doped ferromagnetic silicene in the presence of an electric filed applied perpendicular to silicene sheet. By calculating its energy spectrum and wavefunction and by making use of Kubo formalism, we obtain a general relation for the transverse Hall conductivity which can be used to obtain spin- and valley-Hall conductivity. Our results, in the zero limit of the exchange field, reduces to the previous results. Furthermore we discuss electrically tunable spin and valley polarized transport in ferromagnetic silicene and obtain the necessary conditions for observing a fully spin or valley polarized transport.

cond-mat.mes-hall↗
arXiv2014

Charge screening and carrier transport in AA-stacked bilayer graphene: tuning via a perpendicular electric field

The static dielectric function in AA-stacked bilayer graphene (BLG), subjected to an electric field applied perpendicular to layers, is calculated analytically within the random phase approximation (RPA). This result is used to calculate the screened Coulomb interaction and the electrical conductivity. The screened Coulomb interaction, which here can be tuned by the perpendicular electric field, shows a power-law decay as $1/(γ^{2}+V^2)$ at long-distance limit where $V$ and $γ$ are the electrical potential and the inter-layer hopping energy respectively, indicating that the Coulomb interaction is suppressed at high perpendicular electric fields. Furthermore, Our results for the effect of the short-range and the long-range (Coulomb) scattering on the electrical conductivity show that the shot-range scattering yields a constant electrical conductivity which is not affected by the perpendicular electric filed. While the electrical conductivity limited by the Coulomb scattering is enhanced by the perpendicular electric field and increases linearly in $V^2$ at small $V$ with a finite value at $V=0$, indicating that we can tune the electrical conductivity in AA-stacked BLG by applying a perpendicular electric field.

cond-mat.mes-hall↗
arXiv2014

RKKY interaction in bilayer graphene

We study the RKKY interaction between two magnetic impurities located on same layer (intralayer case) or on different layers (interlayer case) in undoped bilayer graphene in the four-bands model, by directly calculating the Green functions in the eigenvalues and eigenvectors representation. Our results show that both intra- and interlayer RKKY interactions between two magnetic impurities located on same (opposite) sublattice are always ferromagnetic (antiferromagnetic). Furthermore we find unusual long-distance decay of the RKKY interaction in BLG. The intralyer RKKY interactions between two magnetic impurities located on same sublattice, $J^{A_{n}A_{n}}(\mathbf{R})$ and $J^{B_{n}B_{n}}(\mathbf{R)}$, decay closely as $1/R^{6}$ and $1/R^{2}$ at large impurity distances respectively, but when they are located on opposite sublattices the RKKY interactions exhibit $1/R^{4}$ decays approximately. In the interlayer case, the RKKY interactions between two magnetic impurities located on same sublattice show a decay close to $1/R^{4}$ at large impurity distances, but if two magnetic impurities be on opposite sublattices the RKKY interactions, $J^{A_{1}B_{2}}(\mathbf{R})$ and $J^{B_{1}A_{2}}(\mathbf{R)}$, decay closely as $1/R^{6}$ and $1/R^{2}$ respectively. Both intra- and interlayer RKKY interactions have anisotropic oscillatory factors which for intralayer case is equal to that for single layer graphene. Our results at weak and strong interlayer coupling limits reduce to the RKKY interaction of SLG and that of BLG in the two-bands approximation respectively.

cond-mat.str-el↗
arXiv2014

Electronic properties of armchair AA-stacked bilayer graphene nanoribbons

We study analytically, based on the tight-binding model, the electronic band structure of armchair AA-stacked bilayer graphene nanoribbons (BLGNRs) in several regimes. We apply hard-wall boundary conditions to determine the discretion dominating on the Bloch wavefunctions in the confined direction. First we consider an ideal case, perfect nanoribbons without any edge deformation, and show that their electronic properties are strongly size-dependent. We find that the narrow armchair AA-stacked BLGNRs (similar to single-layer graphene nanoribbons) may be metallic or semiconducting depending on their width determined by the number of dimer lines across the ribbon width, while the wide ribbons are metallic. Then we show that, when the edge deformation effects are taken into account, all narrow armchair AA-stacked BLGNRs become semiconducting while the wide ribbons remain metallic. We also investigate effects of an electric filed applied perpendicular to the nanoribbon layers and show it can be used to tune the electronic properties of these nanoribbons leading to a semiconducting-to-metallic phase transition at a critical value of the electric field which depends on the nanoribbon width. Furthermore, in all regimes, we calculate the corresponding wavefunctions which can be used to investigate and predict various properties in these nanoribbons.

cond-mat.mes-hall↗
arXiv2014

Magnetism of an adatom on biased AA-stacked bilayer graphene

We study magnetism of an adatom adsorbed on AA-stacked bilayer graphene (BLG) in both unbiased and biased cases, using the Anderson impurity model. We find different magnetic phase diagrams for the adatom, depending on the energy level of the adatom, which varies from the magnetic phase diagram of adatom in normal metals to that in graphene. This is due to the individual energy dependence of the density of states (DOS) of AA-stacked BLG and anomalous broadening of the adatom energy level. Furthermore we investigate the effect of a bias voltage on DOS of AA-stacked and show that the magnetization of the adatom can be controlled by applying the bias voltage. This allows for possibility of using AA-stacked BLG in spintronic devices.

cond-mat.mes-hall↗
arXiv2014

Local moment formation in bilayer graphene

The local properties of bilayer graphene (BLG) due to the spatial inhomogeneity of its sublattices are of interest. We apply Anderson impurity model to consider the local moment formation on a magnetic impurity which could be adsorbed on different sublattices of BLG. We find different features for the impurity magnetization when it is adsorbed A and B sublattices. The impurity adsorbed on A sublattice can magnetize even when the impurity level is above the Fermi level and the on-site coulomb energy is very small. But when the impurity is adsorbed on B sublattice the magnetization is possible for limited values of the impurity level and the on-site coulomb energy. This is due to different local density of the low energy states at A and B sublattices which originates from their spatial inhomogeneity. Also we show that electrical controlling the magnetization of adatoms besides it's inhomogeneity in BLG allow for possibility of using BLG in spintronic devices with higher potential than graphene.

cond-mat.mes-hall↗
arXiv2014

Effects of doping and bias voltage on the screening in AAA-stacked trilayer graphene

We calculate the static polarization of AAA-stacked trilayer graphene (TLG) and study its screening properties within the random phase approximation (RPA) in all undoped, doped and biased regimes. We find that the static polarization of undoped AAA-stacked TLG is a combination of the doped and undoped single layer graphene static polarization. This leads to an enhancement of the dielectric background constant along a Thomas-Fermi screening with the Thomas-Fermi wave vector which is independent of carrier concentrations and a 1/r^3 power law decay for the long-distance behavior of the screened coulomb potential. We show that effects of a bias voltage can be taken into account by a renormalization of the interlayer hopping energy to a new bias-voltage-dependent value, indicating screening properties of biased AAA-stacked TLG can be tuned electrically. We also find that screening properties of doped AAA-stacked TLG, when $μ$ exceeds $\sqrt{2}γ$, are similar to that of doped SLG only depending on doping. While for $μ<\sqrt{2}γ$, its screening properties are a combination of SLG and AA-stacked screening properties and they are determined by doping and the interlayer hopping energy.

cond-mat.mes-hall↗