11,208 research outputs found
Interfaces Within Graphene Nanoribbons
We study the conductance through two types of graphene nanostructures:
nanoribbon junctions in which the width changes from wide to narrow, and curved
nanoribbons. In the wide-narrow structures, substantial reflection occurs from
the wide-narrow interface, in contrast to the behavior of the much studied
electron gas waveguides. In the curved nanoribbons, the conductance is very
sensitive to details such as whether regions of a semiconducting armchair
nanoribbon are included in the curved structure -- such regions strongly
suppress the conductance. Surprisingly, this suppression is not due to the band
gap of the semiconducting nanoribbon, but is linked to the valley degree of
freedom. Though we study these effects in the simplest contexts, they can be
expected to occur for more complicated structures, and we show results for
rings as well. We conclude that experience from electron gas waveguides does
not carry over to graphene nanostructures. The interior interfaces causing
extra scattering result from the extra effective degrees of freedom of the
graphene structure, namely the valley and sublattice pseudospins.Comment: 19 pages, published version, several references added, small changes
to conclusion
Multiple ionization of neon by soft X-rays at ultrahigh intensity
At the free-electron laser FLASH, multiple ionization of neon atoms was
quantitatively investigated at 93.0 eV and 90.5 eV photon energy. For ion
charge states up to 6+, we compare the respective absolute photoionization
yields with results from a minimal model and an elaborate description. Both
approaches are based on rate equations and take into acccout a Gaussian spatial
intensity distribution of the laser beam. From the comparison we conclude, that
photoionization up to a charge of 5+ can be described by the minimal model. For
higher charges, the experimental ionization yields systematically exceed the
elaborate rate based prediction.Comment: 10 pages, 3 figure
Sampling functions for multimode homodyne tomography with a single local oscillator
We derive various sampling functions for multimode homodyne tomography with a
single local oscillator. These functions allow us to sample multimode
s-parametrized quasidistributions, density matrix elements in Fock basis, and
s-ordered moments of arbitrary order directly from the measured quadrature
statistics. The inevitable experimental losses can be compensated by proper
modification of the sampling functions. Results of Monte Carlo simulations for
squeezed three-mode state are reported and the feasibility of reconstruction of
the three-mode Q-function and s-ordered moments from 10^7 sampled data is
demonstrated.Comment: 12 pages, 8 figures, REVTeX, submitted Phys. Rev.
Graphene Rings in Magnetic Fields: Aharonov-Bohm Effect and Valley Splitting
We study the conductance of mesoscopic graphene rings in the presence of a
perpendicular magnetic field by means of numerical calculations based on a
tight-binding model. First, we consider the magnetoconductance of such rings
and observe the Aharonov-Bohm effect. We investigate different regimes of the
magnetic flux up to the quantum Hall regime, where the Aharonov-Bohm
oscillations are suppressed. Results for both clean (ballistic) and disordered
(diffusive) rings are presented. Second, we study rings with smooth mass
boundary that are weakly coupled to leads. We show that the valley degeneracy
of the eigenstates in closed graphene rings can be lifted by a small magnetic
flux, and that this lifting can be observed in the transport properties of the
system.Comment: 12 pages, 9 figure
Finite-temperature order-disorder phase transition in a frustrated bilayer quantum Heisenberg antiferromagnet in strong magnetic fields
We investigate the thermodynamic properties of the frustrated bilayer quantum
Heisenberg antiferromagnet at low temperatures in the vicinity of the
saturation magnetic field. The low-energy degrees of freedom of the spin model
are mapped onto a hard-square gas on a square lattice. We use exact
diagonalization data for finite spin systems to check the validity of such a
description. Using a classical Monte Carlo method we give a quantitative
description of the thermodynamics of the spin model at low temperatures around
the saturation field. The main peculiarity of the considered two-dimensional
Heisenberg antiferromagnet is related to a phase transition of the hard-square
model on the square lattice, which belongs to the two-dimensional Ising model
universality class. It manifests itself in a logarithmic (low-)temperature
singularity of the specific heat of the spin system observed for magnetic
fields just below the saturation field
Experimental Test of a Two-dimensional Approximation for Dielectric Microcavities
Open dielectric resonators of different shapes are widely used for the
manufacture of microlasers. A precise determination of their resonance
frequencies and widths is crucial for their design. Most microlasers have a
flat cylindrical geometry, and a two-dimensional approximation, the so-called
method of the effective index of refraction, is commonly employed for numerical
calculations. Our aim has been an experimental test of the precision and
applicability of a model based on this approximation. We performed very
thorough and accurate measurements of the resonance frequencies and widths of
two passive circular dielectric microwave resonators and found significant
deviations from the model predictions. From this we conclude that the model
generally fails in the quantitative description of three-dimensional dielectric
resonators.Comment: 10 pages, 13 figure
Symmetry Classes in Graphene Quantum Dots: Universal Spectral Statistics, Weak Localization, and Conductance Fluctuations
We study the symmetry classes of graphene quantum dots, both open and closed,
through the conductance and energy level statistics. For abrupt termination of
the lattice, these properties are well described by the standard orthogonal and
unitary ensembles. However, for smooth mass confinement, special time-reversal
symmetries associated with the sublattice and valley degrees of freedom are
critical: they lead to block diagonal Hamiltonians and scattering matrices with
blocks belonging to the unitary symmetry class even at zero magnetic field.
While the effect of this structure is clearly seen in the conductance of open
dots, it is suppressed in the spectral statistics of closed dots, because the
intervalley scattering time is shorter than the time required to resolve a
level spacing in the closed systems but longer than the escape time of the open
systems.Comment: 4 pages, 4 figures, RevTex, submitted to Phys. Rev. Let
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