Andrea M. Fischer

Exciton storage in a nanoscale Aharonov-Bohm ring with electric field tuning

We study analytically the optical properties of a simple model for an electron-hole pair on a ring subjected to perpendicular magnetic flux and in-plane electric field. We show how to tune this excitonic system from optically active to optically dark as a function of these external fields. Our results offer a simple mechanism for exciton storage and readout.

Critical parameters for the disorder-induced metal-insulator transition in FCC and BCC lattices

We use a transfer-matrix method to study the disorder-induced metal-insulator transition. We take isotropic nearest-neighbor hopping and an onsite potential with uniformly distributed disorder. Following the previous work done on the simple-cubic lattice, we perform numerical calculations for the body-centered cubic and face-centered cubic lattices, which are more common in nature. We obtain the localization length from calculated Lyapunov exponents for different system sizes. This data is analyzed using finite-size scaling to find the critical parameters. We create an energy-disorder phase diagram for both lattice types, noting that it is symmetric about the band center for the body-centered cubic lattice but not for the face-centered cubic lattice. We find a critical exponent of approximately 1.5-1.6 for both lattice types for transitions occurring either at fixed energy or at fixed disorder, agreeing with results previously obtained for other systems belonging to the same orthogonal universality class. We notice an increase in critical disorder with the number of nearest neighbors, which agrees with intuition.

Magnetoplasmons and SU(4) symmetry in graphene

We study magnetoplasmons or neutral collective excitations of graphene in a strong perpendicular magnetic field, which can be modelled as bound electron-hole pairs. The SU(4) symmetry of graphene arising from spin and valley pseudospin degrees of freedom is explored using Young diagrams to correctly predict the degeneracies of these excitations. The multiplet structure of the states is identical to that of mesons composed of first and second generation quarks.

Symmetry content and spectral properties of charged collective excitations for graphene in strong magnetic fields

We show that graphene in a strong magnetic field with partially filled Landau levels sustains charged collective excitations—bound states of a neutral magnetoplasmon and free particles. In the limit of low density of excess charges, these are bound three-particle complexes. Some of these states are optically bright and may be detected in spectroscopy experiments, providing a direct probe of electron-electron interactions in graphene. The charged excitations can be classified using the geometrical symmetries—non-commutative magnetic translations and generalized rotations—in addition to the dynamical SU4 symmetry in graphene. From the SU4 symmetry point of view, such excitations are analogous to bound states of two quarks and one antiquark with four flavors. We establish a flavor optical selection rule to identify the bright states for experimental studies.

Localized collective excitations in doped graphene in strong magnetic fields

We consider collective excitations in graphene with filled Landau levels (LLs) in the presence of an external potential due to a single charged donor D{sup +} or acceptor A{sup -} impurity. We show that localized collective modes split off the magnetoplasmon continuum and, in addition, quasibound states are formed within the continuum. A study of the evolution of the strengths and energies of magneto-optical transitions is performed for integer filling factors {nu}=1,2,3,4 of the lowest LL. We predict impurity absorption peaks above as well as below the cyclotron resonance. We find that the single-particle electron-hole symmetry of graphene leads to a duality between the spectra of collective modes for the D{sup +} and A{sup -}. The duality shows up as a set of the D{sup +} and A{sup -} magnetoabsorption peaks having the same energies but active in different circular polarizations.

Magnetoplasmons bound to short-range impurities in graphene: Symmetries and optics

We consider a graphene sheet in the presence of a strong perpendicular magnetic field with a single short-range delta impurity situated at one of the carbon sites. We study the neutral inter-Landau level collective excitations, magnetoplasmons, which become localized on the impurity. Some of these excitations involve a pseudospin flip (intervalley transitions), since the impurity can scatter electrons between the two valleys. We propose a classification of states of the excitations in graphene and introduce the appropriate quantum numbers. The energies and optical strengths of collective excitations are calculated for a range of integer filling factors and impurity strengths. We establish a set of symmetries matching the energies and absorption strengths of collective excitations for different sublattice locations of the impurity, filling factors, circular light polarizations, and signs of the impurity potential.

Localised magneto-optical collective excitations of impure graphene

We study optically-induced collective excitations of graphene in the presence of a strong perpendicular magnetic field and a single impurity. We determine the energies and absorption strengths of these excitations, which become localised on the impurity. Two different types of impurity are considered i. the long-range Coulomb impurity, ii. a delta-function impurity located at either an A or B graphene sublattice site. Both impurity types result in some bound states appearing both above and below the magnetoplasmon continuum, although the effect of the short-range impurity is less pronounced. The dependence of the energies and oscillator strengths of the bound states on the filling factor is investigated