2,525 research outputs found
Endohedral Impurities in Carbon Nanotubes
A generalization of the Anderson model that includes pseudo-Jahn-Teller
impurity coupling is proposed to describe distortions of an endohedral impurity
in a carbon nanotube. Treating the distortion within mean-field theory,
spontaneous axial symmetry breaking is found when the vibronic coupling
strength g exceeds a critical value g. The effective potential in the
symmetry-broken state is found to have O(2) symmetry, in agreement with
numerical calculations. For metallic zigzag nanotubes endohedrally-doped with
transition metals in the dilute limit, the low-energy properties of the system
may display two-channel Kondo behavior; however, strong vibronic coupling is
seen to exponentially suppress the Kondo energy scale.Comment: 4 pages, 2 figure
Superconductivity and local non-centrosymmetricity in crystal lattices
Symmetry of the crystal lattice can be a determining factor for the structure
of Cooper pairs in unconventional superconductors. In this study we extend the
discussion of superconductivity in non-centrosymmetric materials to the case
when inversion symmetry is missing locally, but is present on a global level.
Concretely, we investigate the staggered non-centrosymmetricity within a
regular sublattice structure, in some analogy to the discussion of
superconductivity in antiferromagnetic systems. Three crystal structures are
analyzed in detail as illustrative examples for the extended classification of
Cooper-pairing channels. One of the cases may be relevant for the class of
iron-pnictide superconductors
Phonon self-energy corrections to non-zero wavevector phonon modes in single-layer graphene
Phonon self-energy corrections have mostly been studied theoretically and
experimentally for phonon modes with zone-center (q = 0) wave-vectors. Here,
gate-modulated Raman scattering is used to study phonons of a single layer of
graphene (1LG) in the frequency range from 2350 to 2750 cm-1, which shows the
G* and the G'-band features originating from a double-resonant Raman process
with q \not= 0. The observed phonon renormalization effects are different from
what is observed for the zone-center q = 0 case. To explain our experimental
findings, we explored the phonon self-energy for the phonons with non-zero
wave-vectors (q \not= 0) in 1LG in which the frequencies and decay widths are
expected to behave oppositely to the behavior observed in the corresponding
zone-center q = 0 processes. Within this framework, we resolve the
identification of the phonon modes contributing to the G* Raman feature at 2450
cm-1 to include the iTO+LA combination modes with q \not= 0 and the 2iTO
overtone modes with q = 0, showing both to be associated with wave-vectors near
the high symmetry point K in the Brillouin zone
Energy Band Gap Engineering of Graphene Nanoribbons
We investigate electronic transport in lithographically patterned graphene
ribbon structures where the lateral confinement of charge carriers creates an
energy gap near the charge neutrality point. Individual graphene layers are
contacted with metal electrodes and patterned into ribbons of varying widths
and different crystallographic orientations. The temperature dependent
conductance measurements show larger energy gaps opening for narrower ribbons.
The sizes of these energy gaps are investigated by measuring the conductance in
the non-linear response regime at low temperatures. We find that the energy gap
scales inversely with the ribbon width, thus demonstrating the ability to
engineer the band gap of graphene nanostructures by lithographic processes.Comment: 7 pages including 4 figure
Influence of molecular temperature on the coherence of fullerenes in a near-field interferometer
We study C70 fullerene matter waves in a Talbot-Lau interferometer as a
function of their temperature. While the ideal fringe visibility is observed at
moderate molecular temperatures, we find a gradual degradation of the
interference contrast if the molecules are heated before entering the
interferometer. A method is developed to assess the distribution of the
micro-canonical temperatures of the molecules in free flight. This way the
heating-dependent reduction of interference contrast can be compared with the
predictions of quantum theory. We find that the observed loss of coherence
agrees quantitatively with the expected decoherence rate due to the thermal
radiation emitted by the hot molecules.Comment: 11 pages, 9 figure
Stacking Faults, Bound States, and Quantum Hall Plateaus in Crystalline Graphite
We analyze the electronic properties of a simple stacking defect in Bernal
graphite. We show that a bound state forms, which disperses as |\bfk-\bfK|^3
in the vicinity of either of the two inequivalent zone corners \bfK. In the
presence of a strong c-axis magnetic field, this bound state develops a Landau
level structure which for low energies behaves as E\nd_n\propto |n B|^{3/2}.
We show that buried stacking faults have observable consequences for surface
spectroscopy, and we discuss the implications for the three-dimensional quantum
Hall effect (3DQHE). We also analyze the Landau level structure and chiral
surface states of rhombohedral graphite, and show that, when doped, it should
exhibit multiple 3DQHE plateaus at modest fields.Comment: 19 page
Weak antilocalization in a strained InGaAs/InP quantum well structure
Weak antilocalization (WAL) effect due to the interference corrections to the
conductivity has been studied experimentally in a strained InGaAs/InP quantum
well structure. From measurements in tilted magnetic filed, it was shown that
both weak localization and WAL features depend only on the normal component of
the magnetic field for tilt angles less than 84 degrees. Weak antilocalization
effect showed non-monotonous dependence on the gate voltage which could not be
explained by either Rashba or Dresselhouse mechanisms of the spin-orbit
coupling. To describe magnetic field dependence of the conductivity, it was
necessary to assume that spin-orbit scattering time depends on the external
magnetic field which quenches the spin precession around effective, spin-orbit
related, magnetic fields.Comment: Presented at EP2DS 2003 (Nara), to be published in Physica
Variational discrete variable representation for excitons on a lattice
We construct numerical basis function sets on a lattice, whose spatial
extension is scalable from single lattice sites to the continuum limit. They
allow us to compute small and large bound states with comparable, moderate
effort. Adopting concepts of discrete variable representations, a diagonal form
of the potential term is achieved through a unitary transformation to Gaussian
quadrature points. Thereby the computational effort in three dimensions scales
as the fourth instead of the sixth power of the number of basis functions along
each axis, such that it is reduced by two orders of magnitude in realistic
examples. As an improvement over standard discrete variable representations,
our construction preserves the variational principle. It allows for the
calculation of binding energies, wave functions, and excitation spectra. We use
this technique to study central-cell corrections for excitons beyond the
continuum approximation. A discussion of the mass and spectrum of the yellow
exciton series in the cuprous oxide, which does not follow the hydrogenic
Rydberg series of Mott-Wannier excitons, is given on the basis of a simple
lattice model.Comment: 12 pages, 7 figures. Final version as publishe
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