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Register a new userFeature-rich electronic excitations in external fields of 2D silicene
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Electronic Coulomb excitations in monolayer silicene are investigated by using the Lindhard dielectric function and a newly developed generalized tight-binding model (G-TBM). G-TBM simultaneously contains the atomic interactions, the spin-orbit coupling, the Coulomb interactions, and the various external fields at an arbitrary chemical potential. We exhibit the calculation results of the electrically tunable magnetoplasmons and the strong magnetic field modulation of plasmon behaviors. The two intriguing phenomena are well explained by determining the dominant transition channels in the dielectric function and through understanding the electron behavior under the multiple interactions (intrinsic and external). A further tunability of the plasmon features is demonstrated with the momentum transfer and the Fermi energy. The methodological strategy could be extended to several other 2D materials like germanene and stanene, and might open a pathway to search a better system in nanoplasmonic applications.
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We develop a comprehensive theory for the effective dynamics of Bloch electrons based on symmetry. We begin with a scheme to systematically derive the irreducible representations (IRs) characterizing the Bloch functions. Starting from a tight-binding (TB) approach, we decompose the TB basis functions into localized symmetry-adapted atomic orbitals and crystal-periodic symmetry-adapted plane waves. Each of these subproblems is independent of the details of a particular crystal structure and it is largely independent of the other subproblem, hence permitting for each subproblem an independent universal solution. Taking monolayer MoS$_2$ and few-layer graphene as examples, we tabulate the symmetrized $p$ and $d$ orbitals as well as the symmetrized plane wave spinors for these systems. The symmetry-adapted basis functions block-diagonalize the TB Hamiltonian such that each block yields eigenstates transforming according to one of the IRs of the group of the wave vector $G_k$. For many crystal structures, it is possible to define multiple distinct coordinate systems such that for wave vectors $k$ at the border of the Brillouin zone the IRs characterizing the Bloch states depend on the coordinate system, i.e., these IRs of $G_k$ are not uniquely determined by the symmetry of a crystal structure. The different coordinate systems are related by a coordinate shift that results in a rearrangement of the IRs of $G_k$ characterizing the Bloch states. We illustrate this rearrangement with three coordinate systems for MoS$_2$ and tri-layer graphene. Using monolayer MoS$_2$ as an example, we combine the symmetry analysis of its bulk Bloch states with the theory of invariants to construct a generic multiband Hamiltonian for electrons near the $K$ point of the Brillouin zone. The Hamiltonian includes the effect of spin-orbit coupling, strain and external electric and magnetic fields.
We study the geometric and electronic structures of silicene monolayer using density functional theory based calculations. The electronic structures of silicene show that it is a semi-metal and the charge carriers in silicene behave like massless Dirac-Fermions since it possesses linear dispersion around Dirac point. Our results show that the band gap in silicene monolayer can be opened up at Fermi level due to an external electric field by breaking the inversion symmetry. The presence of buckling in geometric structure of silicene plays an important role in breaking the inversion symmetry. We also show that the band gap varies linearly with the strength of external electric field. Further, the value of band gap can be tuned over a wide range.
We have found out that the band inversion in a silicene quantum dot (QD), in perpendicular magnetic $B$ and electric $Delta_z$ fields, drastically depends on the strength of the magnetic field. We study the energy spectrum of the silicene QD where the electric field provides a tunable band gap $Delta$. Boundary conditions introduce chirality, so that negative and positive angular momentum $m$ zero Landau level (ZLL) edge states show a quite different behavior regarding the band-inversion mechanism underlying the topological insulator transition. We show that, whereas some ZLLs suffer band inversion at $Delta=0$ for any $B>0$, other ZLLs only suffer band inversion above critical values of the magnetic field at nonzero values of the gap.
We present first-principles calculations of silicene/graphene and germanene/graphene bilayers. Various supercell models are constructed in the calculations in order to reduce the strain of the lattice-mismatched bilayer systems. Our energetics analysis and electronic structure results suggest that graphene can be used as a substrate to synthesize monolayer silicene and germanene. Multiple phases of single crystalline silicene and germanene with different orientations relative to the substrate could coexist at room temperature. The weak interaction between the overlayer and the substrate preserves the low-buckled structure of silicene and germanene, as well as their linear energy bands. The gap induced by breaking the sublattice symmetry in silicene on graphene can be up to 57 meV.
The authors report that anisotropic confining potentials in laterally-coupled semiconductor quantum dots (QDs) have large impacts in optical transitions and energies of inter-shell collective electronic excitations. The observed anisotropies are revealed by inelastic light scattering as a function of the in-plane direction of light polarization and can be finely controlled by modifying the geometrical shape of the QDs. These experiments show that the tuning of the QD confinement potential offers a powerful method to manipulate electronic states and far-infrared inter-shell optical transitions in quantum dots.
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