Showing papers by "Eugene J. Mele published in 2022"
TL;DR: In this paper , it was shown that plasmons in two-dimensional materials with closely located electron and hole Fermi pockets can be amplified when an electrical current bias is applied along the displaced electron-hole pockets, without the need for an external gain media.
Abstract: Surface plasmons, which allow tight confinement of light, suffer from high intrinsic electronic losses. It has been shown that stimulated emission from excited electrons can transfer energy to plasmons and compensate for the high intrinsic losses. To-date, these realizations have relied on introducing an external gain media coupled to the surface plasmon. Here, we propose that plasmons in two-dimensional materials with closely located electron and hole Fermi pockets can be amplified, when an electrical current bias is applied along the displaced electron-hole pockets, without the need for an external gain media. As a prototypical example, we consider WTe2 from the family of 1T[Formula: see text]-MX2 materials, whose electronic structure can be described within a type-II tilted massive Dirac model. We find that the nonlocal plasmonic response experiences prominent gain for experimentally accessible currents on the order of mAμm-1. Furthermore, the group velocity of the plasmon found from the isofrequency curves imply that the amplified plasmons are highly collimated along a direction perpendicular to the Dirac node tilt when the electrical current is applied along it.
6 citations
DOI•
TL;DR: In this paper , the authors propose a solution to solve the problem of the problem: this paper ] of "uniformity" and "uncertainty" of the solution.
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3 citations
TL;DR: In this article , it was shown that polaritonic Dirac points, which are markers for topological phase transition points, can be constructed from the collective coupling between valley excitons and Dirac cones in the presence of both time-reversal and inversion symmetry.
Abstract: Systems with strong light-matter interaction open up new avenues for studying topological phases of matter. Examples include exciton polaritons, mixed light-matter quasiparticles, where the topology of the polaritonic band structure arises from the collective coupling between matter wave and optical fields strongly confined in periodic dielectric structures. Distinct from light-matter interaction in a uniform environment, the spatially varying nature of the optical fields leads to a fundamental modification of the well-known optical selection rules, which were derived under the plane wave approximation. Here we identify polaritonic Chern insulators by coupling valley excitons in transition metal dichalcogenides to photonic Bloch modes in a dielectric photonic crystal slab. We show that polaritonic Dirac points, which are markers for topological phase transition points, can be constructed from the collective coupling between valley excitons and photonic Dirac cones in the presence of both time-reversal and inversion symmetries. Lifting exciton valley degeneracy by breaking time-reversal symmetry leads to gapped polaritonic bands with nonzero Chern numbers. Through numerical simulations, we predict polaritonic chiral edge states residing inside the topological gaps. Our Letter paves the way for the further study of strong exciton-photon interaction in nanophotonic structures and for exploring polaritonic topological phases and their practical applications in polaritonic devices.
3 citations
TL;DR: In this article , two-dimensional van der Waals heterostructures can be engineered into artificial superlattices that host flat bands with significant Berry curvature and provide a favorable environment for the emergence of novel electron dynamics.
Abstract: Two-dimensional van der Waals heterostructures can be engineered into artificial superlattices that host flat bands with significant Berry curvature and provide a favorable environment for the emergence of novel electron dynamics. In particular, the Berry curvature can induce an oscillating trajectory of an electron wave packet transverse to an applied static electric field. Though analogous to Bloch oscillations, this novel oscillatory behavior is driven entirely by quantum geometry in momentum space instead of band dispersion. While the orbits of Bloch oscillations can be localized by increasing field strength, the size of the geometric orbits saturates to a nonzero plateau in the strong-field limit. In non-magnetic materials, the geometric oscillations are even under inversion of the applied field, whereas the Bloch oscillations are odd, a property that can be used to distinguish these two co-existing effects.
2 citations
DOI•
TL;DR: In this paper , the authors show that the sublattice odd and even forms of C-TBG are inflated versions of Bernal and AA stacked bilayer graphene respectively with energy scales reduced by a factor of 110 for the 21 . 79 ◦ commensurate unit cell.
Abstract: Bernal bilayer graphene exhibits a band gap that is tunable through the infrared with an electric field. We show that sublattice odd commensurate twisted bilayer graphene (C-TBG) exhibits a band gap that is tunable through the terahertz with an electric field. We show that from the perspective of terahertz optics the sublattice odd and even forms of C-TBG are “inflated” versions of Bernal and AA stacked bilayer graphene respectively with energy scales reduced by a factor of 110 for the 21 . 79 ◦ commensurate unit cell. This lower energy scale is accompanied by a correspondingly smaller gate voltage, which means that the strong-field regime is more easily accessible than in the Bernal case. Finally, we show that the interlayer coherence energy is a directly accessible experimental quantity through the position of a power-law divergence in the optical conductivity.
1 citations
TL;DR: In this paper , the quadrupole circular photogalvanic effect (QCPGE) was used to study the symmetries of Ta2NiSe5 in low-electron density materials.
Abstract: In low–electron density materials, interactions can lead to highly correlated quantum states of matter. Ta2NiSe5, an excitonic insulator (EI) candidate, exists in a novel broken-symmetry phase below 327 K, characterized by robust exchange interaction and electron-lattice coupling. We study this phase of Ta2NiSe5 using the quadrupole circular photogalvanic effect (QCPGE). Light-matter interaction in Ta2NiSe5 mediated by electric quadrupole/magnetic dipole coupling produces helicity-dependent DC response even with centrosymmetry, making it particularly sensitive to certain other broken symmetries. We show that the exchange interaction in Ta2NiSe5 can lead to a triclinic structure with a broken C2 symmetry. Our results provide an incisive probe of the symmetries of the low-temperature phase of Ta2NiSe5 and add new symmetry constraints to the identification of a strongly correlated EI phase. The high sensitivity of QCPGE to subtle symmetry breaking in centrosymmetric systems will enable its use in studying other complex crystalline systems.
1 citations
01 May 2022
TL;DR: In this article , a space-time screw symmetry can be preserved in periodically driven optical nonlinear materials, which can protect a high-order topological phase, which is demonstrated with a photonic crystal.
Abstract: We present that a space-time screw symmetry can be preserved in periodically driven optical nonlinear materials. Such symmetry can protect a high-order topological phase, which is demonstrated with a photonic crystal.
TL;DR: In this article , the authors combine ab initio , tight-binding methods and analytical theory to study piezoelectric effect of boron nitride nanotubes, and demonstrate that coupling between the uniaxial and shear deformation are only allowed in the Nanotubes with lower chiral symmetry.
Abstract: We combine ab initio , tight-binding methods and analytical theory to study piezoelectric effect of boron nitride nanotubes. We find that piezoelectricity of a heteropolar nanotube depends on its chirality and diameter and can be understood starting from the piezoelectric response of an isolated planar sheet, along with a structure specific mapping from the sheet onto the tube surface. We demonstrate that coupling between the uniaxial and shear deformation are only allowed in the nanotubes with lower chiral symmetry. Our study shows that piezoelectricity of nanotubes is fundamentally different from its counterpart in three dimensional (3D) bulk materials.