Publications
Recent Publications (2025-2021)
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Lauren I. Massaro, Connor Meese, Nancy P. Sandler, and Mahmoud M. Asmar “Photo induced multiply quantized vortex states in Dirac-like materials”
Physical Review B 111, 085402 17 (2025)
Abstract: Subjecting a massive two-dimensional Dirac material to a vortex light beam provides a mechanism for the photo-induction of multiply quantized vortices. Using Floquet theory, we show that electronic vortices, characterized by their total angular momentum, are exclusive to circularly polarized vortex beams. The equations for the driven system at the one photon-resonance are mapped to the Bogoliubov–de Gennes equations of 𝑠-wave superconductors with multiply quantized vortices. This mapping provides valuable analytical tools for the analysis of the system's spectral properties.
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Modi Ke, Mahmoud M. Asmar, and Wang-Kong Tse, “Theory of Tunneling between Two-Dimensional Electron Layers Driven by Spin Pumping: Adiabatic Regime and Beyond”
arXiv:2501.03593 (2025)
Abstract: Tunneling spectroscopy between parallel two-dimensional (2D) electronic systems provides a powerful method to probe the underlying electronic properties by measuring tunneling conductance. In this work, we present a theoretical framework for spin transport in 2D-to-2D tunneling systems, driven by spin pumping. This theory applies to a vertical heterostructure where two layers of metallic 2D electron systems are separated by an insulating barrier, with one layer exchange-coupled to a magnetic layer driven at resonance. Utilizing a non-perturbative Floquet-Keldysh formalism, we derive general expressions for the tunneling spin and charge currents across a broad range of driving frequencies, extending beyond the traditional adiabatic pumping regime. At low frequencies, we obtain analytical results that recover the known behaviors in the adiabatic regime. However, at higher frequencies, our numerical findings reveal significant deviations in the dependence of spin and charge currents on both frequency and precession angle. This work offers fresh insights into the role of magnetization dynamics in tunneling transport, opening up new avenues for exploring non-adiabatic spin pumping phenomena.
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Mahmoud M. Asmar and Wang-Kong Tse, “Photo-induced non-collinear interlayer RKKY coupling in bulk Rashba semiconductors”
New Journal of Physics 26, 083016 (2024)
Abstract: The interplay between light-matter, spin-orbit, and magnetic interactions allows the investigation of light-induced magnetic phenomena that are otherwise absent without irradiation. We present our analysis of light-driven effects on the interlayer exchange coupling mediated by a bulk Rashba semiconductor in a magnetic multilayer. The collinear magnetic exchange coupling mediated by the photon-dressed spin-orbit coupled electrons of BiTeI develops light-induced oscillation periods and displays new decay power laws, both of which are enhanced with an increasing light-matter coupling. For magnetic layers with non-collinear magnetization, we find a non-collinear magnetic exchange coupling uniquely generated by light-driving of the multilayer. As the non-collinear magnetic exchange coupling mediated by the photon-dressed electrons of BiTeI is unique to the irradiated system and it is enhanced with increasing light-matter coupling, this effect offers a promising platform of investigation of light-driven effects on magnetic phenomena in spin-orbit coupled systems. In this platform, light properties, such as its intensity, can serve as external knobs for inducing non-collinear couplings of the interlayer exchange and for modulating the collinear couplings. Both of these effects signify the photo-generated modification in the spin textures of spin-orbit coupled electrons in BiTeI.
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David J. Alspaugh, D. N. Sheng, and Mahmoud M. Asmar, “Local density of state oscillations in laterally heterostructured topological insulator-semiconductor systems”
Physical Review B 110, 085301 (2024)
Abstract: We study local density of state (LDOS) oscillations arising from the scattering of electrons at atomic edge defects in topological insulator (TI) surfaces. To create edge scattering on the surface of a TI, we assume that half of its surface is covered with a semiconductor. In addition to modifying the TI states in the covered half, the presence of the semiconductor leads to a localized edge potential at the vacuum-semiconductor boundary. We study the induced LDOS by imposing time-reversal (TR) invariance and current conservation across the boundary. Additionally, we explore how the scattering of TI junctions with dissimilar spin textures and anisotropic Fermi velocities affect the modulations of the LDOS away from the junction edge. In all cases, for energies close to the Dirac point, we find that the decay envelope of the LDOS oscillations is insensitive to the scattering at the atomic edge defect, with a decay power given by 𝑥−3/2. Quantitative differences in the amplitude of these oscillations depend on the details of the interface and the spin textures, while the period of the oscillations is defined by the size of the Fermi surface.
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Modi Ke, Mahmoud M. Asmar, and Wang-Kong Tse, “Floquet-driven indirect exchange interactionmediated by topological insulator surface states”
Physical Review B 110, 035307 (2024)
Abstract: Light drives offer a potential tool for the dynamical control of magnetic interactions in matter. We theoretically investigate the indirect exchange coupling between two parallel chains of magnetic impurities on the surface of a topological insulator, driven by a time-periodic circularly polarized light field in the high-frequency, off-resonant regime. We derive a closed-form analytic expression for the spin susceptibility of the photon-dressed topological insulator surface states and obtain the irradiation dependence of the Ising, Heisenberg, and Dzyaloshinskii-Moriya exchange couplings between the impurity chains. Our results show a two-pronged modification of these exchange couplings by periodic drives. First, the Ruderman-Kittel-Kasuya-Yosida (RKKY) oscillation period of the exchange couplings can be extended by enhancing the driving strength. Second, increasing driving strength enhances the envelope of RKKY oscillations of the Ising type while suppressing those of the Heisenberg type and Dzyaloshinskii-Moriya type. Our work provides useful insights for realizing Floquet engineering of collinear and noncollinear indirect exchange interactions in topological insulating systems.
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C. Dhital, G. Pokharel, B. Wilson, I. Kendrick, Mahmoud M. Asmar, et al., “Fermi surface of the magnetic kagome compound GdV6Sn6 investigated using de Haas–van Alphen oscillations”
Physical Review B 109, 235145 (2024)
Abstract: The shape of the Fermi surface, and the cyclotron effective mass of the kagome magnet GdV6Sn6 charge carriers are investigated using de Haas–van Alphen (dHvA) oscillation measurements and electronic band structure calculations. The temperature- and angle-dependent torque magnetometry measurements revealed at least nine different frequencies ranging from ∼10 T up to ∼9000 T. These frequencies correspond to extremal areas of the Fermi surface ranging from ∼0.2% up to 50% of the first Brillouin zone, qualitatively consistent with the electronic band structure calculations. The angle-dependent dHvA oscillation frequencies indicate that the smaller pockets of the Fermi surface have an almost three-dimensional character whereas the bigger pockets of the Fermi surface are mostly two dimensional. We also find evidence of the presence of light [0.28(1)𝑚0] as well as heavy [2.37(18)𝑚0] charge carriers through the analysis of the temperature dependence of dominant frequencies. The comparison of the observed frequencies with the electronic band structure calculations indicates that the heavy masses correspond to saddle-point-like features of electronic band structure at the 𝑀 point. The observation of the multiple low frequencies and the calculated contributions from various bands to such low frequencies prevent the estimation of the topological nature of bands containing lighter fermions. In conclusion, our work reveals the features of a Fermi surface containing enhanced mass fermions originating from saddle points in the electronic band structure at the 𝑀 point, which is inherent to kagome lattices.
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Elias Andrade, Ramon Carrillo-Bastos, Mahmoud M. Asmar, and Gerardo G. Naumis, “Kekulé-induced valley birefringence and skew scattering in graphene”
Physical Review B 106, 195413 (2022)
Abstract: In graphene, a Kekulé-Y bond texture modifies the electronic band structure, generating two concentric Dirac cones with different Fermi velocities lying in the Γ point in reciprocal space. The energy dispersion results in different group velocities for each isospin component at a given energy. This energy spectrum combined with the negative refraction index in 𝑝−𝑛 junctions allows the emergence of an electronic analog of optical birefringence in graphene. We characterize the valley birefringence produced by a circularly symmetric Kekulé patterned and gated region using the scattering approach. We found caustics with two cusps separated in space by a distance dependent on the Kekulé interaction and that provides a measure of its strength. Then at low carrier concentration, we find a nonvanishing skew cross-section, showing the asymmetry in the scattering of electrons around the axis of the incoming flux. This effect is associated with the appearance of the valley Hall effect as electrons with opposite valley polarization are deflected toward opposite directions.
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Mahmoud M. Asmar and Wang-Kong Tse, “Impurity screening and Friedel oscillations in Floquet-driven two-dimensional metals”
Journal of Physics: Condensed Matter 34, 315602 (2022)
Abstract: The interplay between light-matter, spin-orbit, and magnetic interactions allows the investigation of light-induced magnetic phenomena that are otherwise absent without irradiation. We present our analysis of light-driven effects on the interlayer exchange coupling mediated by a bulk Rashba semiconductor in a magnetic multilayer. The collinear magnetic exchange coupling mediated by the photon-dressed spin-orbit coupled electrons of BiTeI develops light-induced oscillation periods and displays new decay power laws, both of which are enhanced with an increasing light-matter coupling. For magnetic layers with non-collinear magnetization, we find a non-collinear magnetic exchange coupling uniquely generated by light-driving of the multilayer. As the non-collinear magnetic exchange coupling mediated by the photon-dressed electrons of BiTeI is unique to the irradiated system and it is enhanced with increasing light-matter coupling, this effect offers a promising platform of investigation of light-driven effects on magnetic phenomena in spin-orbit coupled systems. In this platform, light properties, such as its intensity, can serve as external knobs for inducing non-collinear couplings of the interlayer exchange and for modulating the collinear couplings. Both of these effects signify the photo-generated modification in the spin textures of spin-orbit coupled electrons in BiTeI.
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Mahmoud M. Asmar, Gaurav Gupta, and Wang-Kong Tse, “Particle–hole asymmetry and quantum confinement effects on the magneto-optical response of topological insulator thin-films”
Journal of Applied Physics 131, 164305 (2022)
Abstract: Intrinsically broken symmetries in the bulk of topological insulators (TIs) are manifested in their surface states. Despite particle–hole asymmetry in TIs, it has often been assumed that their surface states are characterized by a particle–hole symmetric Dirac energy dispersion. In this work, we demonstrate that the effect of particle–hole asymmetry is essential to correctly describe the energy spectrum and the magneto-optical response in TIs thin-films. In thin-films of TIs with a substantial degree of particle–hole symmetry breaking, such as Sb2Te3, the longitudinal optical conductivity displays absorption peaks arising from optical transitions between bulk and surface Landau levels for low photon energies. The transition energies between the bulk and surface Landau levels exhibit clearly discernable signatures from those between surface Landau levels due to their distinct magnetic field dependence. Bulk contributions to the magneto-optical conductivity in a TI thin-film are enhanced via one type of doping while being suppressed by the other. This asymmetric dependence on the type of doping aids in revealing the particle–hole asymmetry in TI thin-films.
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David J. Alspaugh, Mahmoud M. Asmar, Daniel E. Sheehy, and Ilya Vekhter,“Andreev spectroscopy of nonhelical spin textures in topological insulators”
Physical Review B 105, 054502 (2022)
Abstract: We study how nonhelical spin textures affect the proximity-induced superconductivity of topological insulator (TI)-superconductor (SC) interface states. In particular we calculate the conductance of lateral heterojunctions which comprise a TI surface that is only partially covered by a superconducting material. Interface potentials at the TI-SC interface may lead to a Fermi velocity and spin texture mismatch between the two regions of the lateral heterojunction. By enforcing the conservation of current across the interface, we derive the boundary conditions and calculate the conductance in both the normal and superconducting state. The total Andreev conductance is calculated for both 𝑠-wave and spin-triplet parent SCs, and for several examples of nonhelical spin textures which lead to different Fermi surface mismatches between the two planar regions of the heterojunction. We find that for spin-triplet SCs, nonzero conductance signatures only appear for certain combinations of nonhelical spin textures and parent superconducting material.
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Mahmoud M. Asmar and Wang-Kong Tse, “Floquet control of indirect exchange interaction inperiodically driven two-dimensional electron systems”
New Journal Physics 23, 123031 (2021)
Abstract: We present a theory for the Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction mediated by a two-dimensional (2D) electron system subjected to periodic driving. This is demonstrated for a 2D metal with two ferromagnetic chains deposited in parallel. Our calculations reveal new non-analytic features in the time-averaged spin susceptibility. For weak light–matter coupling, the RKKY interaction shows oscillations with a period tunable by the light amplitude and frequency. For stronger light–matter coupling, the interaction becomes non-oscillatory and remains purely ferromagnetic. Our findings open a path forward for realizing dynamic control of the indirect exchange interaction in 2D magnetic structures.
