8,752 research outputs found
Electron Interactions and Scaling Relations for Optical Excitations in Carbon Nanotubes
Recent fluorescence spectroscopy experiments on single wall carbon nanotubes
reveal substantial deviations of observed absorption and emission energies from
predictions of noninteracting models of the electronic structure. Nonetheless,
the data for nearly armchair nanotubes obey a nonlinear scaling relation as a
function the tube radius . We show that these effects can be understood in a
theory of large radius tubes, derived from the theory of two dimensional
graphene where the coulomb interaction leads to a logarithmic correction to the
electronic self energy and marginal Fermi liquid behavior. Interactions on
length scales larger than the tube circumference lead to strong self energy and
excitonic effects that compete and nearly cancel so that the observed optical
transitions are dominated by the graphene self energy effects.Comment: 4 page
Interference of an Array of Independent Bose-Einstein Condensates
Interference of an array of independent Bose-Einstein condensates, whose
experiment has been performed recently, is theoretically studied in detail.
Even if the number of the atoms in each gas is kept finite and the phases of
the gases are not well defined, interference fringes are observed on each
snapshot. The statistics of the snapshot interference patterns, i.e., the
average fringe amplitudes and their fluctuations (covariance), are computed
analytically, and concise formulas for their asymptotic values for long time of
flight are derived. Processes contributing to these quantities are clarified
and the relationship with the description on the basis of the symmetry-breaking
scenario is revealed.Comment: 13 pages, 3 figure
Cyclotron radiation and emission in graphene
Peculiarity in the cyclotron radiation and emission in graphene is
theoretically examined in terms of the optical conductivity and relaxation
rates to propose that graphene in magnetic fields can be a candidate to realize
the Landau level laser, proposed decades ago [H. Aoki, Appl. Phys. Lett. {\bf
48}, 559 (1986)].Comment: 4 pages, 3 figure
Interband Light Absorption at a Rough Interface
Light absorption at the boundary of indirect-band-gap and direct-forbidden
gap semiconductors is analyzed. It is found that the possibility of the
electron momentum nonconservation at the interface leads to essential
enhancement of absorption in porous and microcrystalline semiconductors. The
effect is more pronounced at a rough boundary due to enlargement of the share
of the interface atoms.Comment: LATEX, 19 pages, 4 PostScript figure
Anharmonicity and asymmetry of Landau levels for a two-dimensional electron gas
We calculate the density of states of a two dimensional electron gas located
at the interface of a GaAlAs/GaAs heterojunction. The disorder potential which
is generally created by a single doping layer behind a spacer, is here enhanced
by the presence of a second delta doped layer of scatterers which can be
repulsive or attractive impurities. We have calculated the density of states by
means of the Klauder's approximation, in the presence of a magnetic field of
arbitrary strength. At low field either band tails or impurity bands are
observed for attractive potentials, depending on the impurity concentration. At
higher field, impurity bands are observed for both repulsive and attractive
potentials. We discuss the effect of such an asymmetrical density of states on
the transport properties in the quantum Hall effect regime.Comment: 22 pages, 12 figures. submitted to Phys. Rev.
Self-Consistent Electron Subbands of Gaas/Algaas Heterostructure in Magnetic Fields Parallel to the Interface
The effect of strong magnetic fields parallel to GaAs/AlGaAs interface on the
subband structure of a 2D electron layer is ivestigated theoretically. The
system with two levels occupied in zero magnetic field is considered and the
magnetic field induced depletion of the second subband is studied. The
confining potential and the electron dispersion relations are calculated
self-consistently, the electron- electron interaction is taken into account in
the Hartree approximation.Comment: written in LaTeX, 8 pages, 4 figs. available on request from
[email protected]
Ising-like Spin Anisotropy and Competing Antiferromagnetic - Ferromagnetic Orders in GdBaCo_{2}O_{5.5} Single Crystals
In RBaCo_{2}O_{5+x} compounds (R is rare earth), a
ferromagnetic-antiferromagnetic competition is accompanied by a giant
magnetoresistance. We study the magnetization of detwinned GdBaCo_{2}O_{5.5}
single crystals, and find a remarkable uniaxial anisotropy of Co^{3+} spins
which is tightly linked with the chain oxygen ordering in GdO_{0.5} planes.
Reflecting the underlying oxygen order, CoO_2 planes also develop a spin-state
order consisting of Co^{3+} ions in alternating rows of S=1 and S=0 states. The
magnetic structure appears to be composed of weakly coupled ferromagnetic
ladders with Ising-like moments, which gives a simple picture for
magnetotransport phenomena.Comment: 5 pages, 4 figures, accepted to Phys.Rev.Let
A theoretical approach to thermal noise caused by an inhomogeneously distributed loss -- Physical insight by the advanced modal expansion
We modified the modal expansion, which is the traditional method used to
calculate thermal noise. This advanced modal expansion provides physical
insight about the discrepancy between the actual thermal noise caused by
inhomogeneously distributed loss and the traditional modal expansion. This
discrepancy comes from correlations between the thermal fluctuations of the
resonant modes. The thermal noise spectra estimated by the advanced modal
expansion are consistent with the results of measurements of thermal
fluctuations caused by inhomogeneous losses.Comment: 10 pages, 4 figure
Contact resistance and shot noise in graphene transistors
Potential steps naturally develop in graphene near metallic contacts. We
investigate the influence of these steps on the transport in graphene Field
Effect Transistors. We give simple expressions to estimate the
voltage-dependent contribution of the contacts to the total resistance and
noise in the diffusive and ballistic regimes.Comment: 6 pages, 4 figures; Figs 3 and 4 completed and appendix adde
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