Mele, Eugene J.
Email Address
ORCID
Disciplines
relationships.isProjectOf
relationships.isOrgUnitOf
Position
Introduction
Research Interests
Collection
24 results
Search Results
Now showing 1 - 10 of 24
Publication Surface State Magnetization and Chiral Edge States on Topological Insulators(2013-01-25) Kane, Charles L; Zhang, Fan; Mele, Eugene J.We study the interaction between a ferromagnetically ordered medium and the surface states of a topological insulator with a general surface termination that were identified recently [F. Zhang et al.Phys. Rev. B 86 081303(R) (2012)]. This interaction is strongly crystal face dependent and can generate chiral states along edges between crystal facets even for a uniform magnetization. While magnetization parallel to quintuple layers shifts the momentum of the Dirac point, perpendicular magnetization lifts the Kramers degeneracy at any Dirac points except on the side face, where the spectrum remains gapless and the Hall conductivity switches sign. Chiral states can be found at any edge that reverses the projection of the surface normal to the stacking direction of quintuple layers. Magnetization also weakly hybridizes noncleavage surfaces.Publication Mean-Field Theory for Interchain Orientational Ordering of Conjugated Polymers(1989-08-15) Harris, A. Brooks; Choi, Han-Yong; Mele, Eugene J.We consider a generalized anisotropic planar-rotor model on a triangular lattice for interchain orientational ordering of undoped and doped polyacetylene, and investigate the effects of various terms on the symmetry and the range of stability of the observed herringbone (HB) phases. Dipole, quadrupole, and octopole interactions are included in the model with sixfold crystal-field anisotropy and are analyzed within the mean-field theory. The relative strength of these interactions can be estimated from the observed setting angle of the HB phase with the help of the smallness of crystal-field anisotropy. A model where the polymer chain is represented by a ‘‘quadrupolar’’ mass density only has various phases as the temperature and the interaction parameters are varied. Among them, the HB phase is found below a critical temperature Tc for some range of the parameter space, and the setting angle of the HB phase is 45° and independent of temperature. Competition between quadrupole and other interactions such as dipole or octopole, parametrized by the ratio of interaction strengths λ, results in an additional phase transition at T’c(λ) and makes the setting angle vary with the temperature below T’c(λ). For a model with quadrupole and octopole terms, there are two degenerate states of the setting angle related by θ’=π/2-θ. This degeneracy does not reflect a symmetry of the system and is lifted by the dipole terms. For a model with quadrupole and dipole interactions, the setting angle increases as the temperature is reduced below T’c(λ). From these results, we conclude that quadrupole and dipole interactions are important terms to explain experimental observations. Effects of crystal-field anisotropy resolve the twofold degeneracy, destroy the critical behavior associated with T’c(λ), and make the setting angle temperature dependent over the entire range of temperature below Tc. Crucial information on the interaction parameters of the model can be obtained through the temperature dependence of the setting angle of the HB phase.Publication Dirac Semimetal in Three Dimensions(2012-04-06) Young, Steve M; Kane, Charles L; Zaheer, Saad; Mele, Eugene J.; Teo, Jeffrey C; Rappe, A MWe show that the pseudorelativistic physics of graphene near the Fermi level can be extended to three dimensional (3D) materials. Unlike in phase transitions from inversion symmetric topological to normal insulators, we show that particular space groups also allow 3D Dirac points as symmetry protected degeneracies. We provide criteria necessary to identify these groups and, as an example, present ab initio calculations of β-cristobalite BiO2 which exhibits three Dirac points at the Fermi level. We find that β-cristobalite BiO2 is metastable, so it can be physically realized as a 3D analog to graphene.Publication Low-Energy Coherent Transport in Metallic Carbon Nanotube Junctions(2011-01-03) Maarouf, Ahmed A.; Mele, Eugene JWe study the low-energy electronic properties of a junction made of two crossed metallic carbon nanotubes of general chiralities. We derive a tight-binding tunneling matrix element that couples low-energy states on the two tubes, which allows us to calculate the contact conductance of the junction. We find that the intrinsic asymmetries of the junction cause the forward- and backward-hopping probabilities from one tube to another to be different. This defines a zero-field Hall conductance for the junction, which we find to scale inversely with the junction contact conductance. Through a systematic study of the dependence of the junction conductance on different junction parameters, we find that the crossing angle is the dominant factor that determines the magnitude of the conductance.Publication Photoluminescence and Band Gap Modulation in Graphene Oxide(2009-03-19) Luo, Zhengtang; Vora, Patrick; Mele, Eugene J; Johnson, A.T. Charlie; Kikkawa, James MWe report broadband visible photoluminescence from solid graphene oxide, and modifications of the emission spectrum by progressive chemical reduction. The data suggest a gapping of the two-dimensional electronic system by removal of π-electrons. We discuss possible gapping mechanisms, and propose that a Kekule pattern of bond distortions may account for the observed behavior.Publication Chirality Dependence of the K-momentum Dark Excitons in Carbon Nanotubes(2010-04-27) Vora, Patrick M; Mele, Eugene J; Tu, X; Kikkawa, James M; Zheng, MUsing a collection of 12 semiconducting carbon-nanotube samples, each highly enriched in a single chirality, we study the chirality dependence of the K-momentum dark singlet exciton using phonon sideband optical spectroscopy. Measurements of bright absorptive and emissive sidebands of this finite momentum exciton identify its energy as 20–38 meV above the bright singlet exciton, a separation that exhibits systematic dependencies on tube diameter, 2n+m family, and chiral index. We present calculations that explain how chiral angle dependence in this energy separation relates to the Coulomb exchange interaction and elaborate the dominance of the KAA; phonon sidebands over the zone-center phonon sidebands over a wide range of chiralities. The Kataura plot arising from these data is qualitatively well described by theory but the energy separation between the sidebands shows a larger chiral dependence than predicted. This latter observation may indicate a larger dispersion for the associated phonon near the K point than expected from finite distance force modeling.Publication Carbon Nanotubes in Helically Modulated Potentials(2008-02-27) Michalski, P. J.; Mele, Eugene J.We calculate effects of an applied helically symmetric potential on the low energy electronic spectrum of a carbon nanotube in the continuum approximation. The spectrum depends on the strength of this potential and on a dimensionless geometrical parameter, P, which is the ratio of the circumference of the nanotube to the pitch of the helix. We find that the minimum band gap of a semiconducting nanotube is reduced by an arbitrarily weak helical potential, and for a given field strength there is an optimal P which produces the biggest change in the band gap. For metallic nanotubes the Fermi velocity is reduced by this potential and for strong fields two small gaps appear at the Fermi surface in addition to the gapless Dirac point. A simple model is developed to estimate the magnitude of the field strength and its effect on DNA-carbon nanotube complexes in an aqueous solution. We find that under typical experimental conditions the predicted effects of a helical potential are likely to be small and we discuss several methods for increasing the size of these effects.Publication Casimir Interactions Between Scatterers in Metallic Carbon Nanotubes(2009-10-02) Zhabinskaya, Dina; Mele, Eugene J.We study interactions between localized scatterers on metallic carbon nanotubes by a mapping onto a one-dimensional Casimir problem. Backscattering of electrons between localized scattering potentials mediates long-range forces between them. We model spatially localized scatterers by local and nonlocal potentials and treat simultaneously the effects of intravalley and intervalley backscattering. We find that the long-range forces between scatterers exhibit the universal power-law decay of the Casimir force in one dimension, with prefactors that control the sign and strength of the interaction. These prefactors are nonuniversal and depend on the symmetry and degree of localization of the scattering potentials. We find that local potentials inevitably lead to a coupled valley scattering problem, though by contrast nonlocal potentials lead to two decoupled single-valley problems in a physically realized regime. The Casimir effect due to two-valley scattering potentials is characterized by the appearance of spatially periodic modulations of the force.Publication Band Symmetries and Singularities in Twisted Multilayer Graphene(2011-12-27) Mele, Eugene J.The electronic spectra of rotationally faulted graphene bilayers are calculated using a continuum formulation for small fault angles that identifies two distinct electronic states of the coupled system. The low-energy spectra of one state features a Fermi velocity reduction, which ultimately leads to pairwise annihilation and regeneration of its low-energy Dirac nodes. The physics in the complementary state is controlled by pseudospin selection rules that prevent a Fermi velocity renormalization and produce second generation symmetry-protected Dirac singularities in the spectrum. These results are compared with previous theoretical analyses and with experimental data.Publication Theoretical Investigation of the Evolution of the Topological Phase of Bi2Se3 under Mechanical Strain(2011-08-19) Young, Steve M.; Chowdhury, Sugata; Mele, Eugene J.; Kane, Charles L; Rappe, Andrew M; Walter, Eric J.The topological insulating phase results from inversion of the band gap due to spin-orbit coupling at an odd number of time-reversal symmetric points. In Bi2Se3, this inversion occurs at the Γ point. For bulk Bi2Se3, we have analyzed the effect of arbitrary strain on the Γ point band gap using density functional theory. By computing the band structure both with and without spin-orbit interactions, we consider the effects of strain on the gap via Coulombic interaction and spin-orbit interaction separately. While compressive strain acts to decrease the Coulombic gap, it also increases the strength of the spin-orbit interaction, increasing the inverted gap. Comparison with Bi2Te3 supports the conclusion that effects on both Coulombic and spin-orbit interactions are critical to understanding the behavior of topological insulators under strain, and we propose that the topological insulating phase can be effectively manipulated by inducing strain through chemical substitution.