4,371 research outputs found
Beats of the Magnetocapacitance Oscillations in Lateral Semiconductor Superlattices
We present calculations on the magnetocapacitance of the two-dimensional
electron gas in a lateral semiconductor superlattice under two-dimensional weak
periodic potential modulation in the presence of a perpendicular magnetic
field. Adopting a Gaussian broadening of magnetic-field-dependent width in the
density of states, we present explicit and simple expressions for the
magnetocapacitance, valid for the relevant weak magnetic fields and modulation
strengths. As the modulation strength in both directions increase, beats of the
magnetocapacitance oscillations are observed, in the low magnetic field range
(Weiss-oscillations regime), which are absent in the one-dimensional weak
modulation case.Comment: 11 pages, 7 figures, accepted by Mod. Phys. Lett. B (March 2007
Thermal properties of fluorinated graphene
Large scale atomistic simulations using the reactive force field approach
(ReaxFF) are implemented to investigate the thermomechanical properties of
fluorinated graphene (FG). A new set of parameters for the reactive force field
potential (ReaxFF) optimized to reproduce key quantum mechanical properties of
relevant carbon-fluor cluster systems are presented. Molecular dynamics (MD)
simulations are used to investigate the thermal rippling behavior of FG and its
mechanical properties and compare them with graphene (GE), graphane (GA) and a
sheet of BN. The mean square value of the height fluctuations and the
height-height correlation function for different system sizes and
temperatures show that FG is an un-rippled system in contrast to the thermal
rippling behavior of graphene (GE). The effective Young's modulus of a flake of
fluorinated graphene is obtained to be 273 N/m and 250 N/m for a flake of FG
under uniaxial strain along arm-chair and zig-zag direction, respectively.Comment: To appear in Phys. Rev.
A perturbative approach to Dirac observables and their space-time algebra
We introduce a general approximation scheme in order to calculate gauge
invariant observables in the canonical formulation of general relativity. Using
this scheme we will show how the observables and the dynamics of field theories
on a fixed background or equivalently the observables of the linearized theory
can be understood as an approximation to the observables in full general
relativity. Gauge invariant corrections can be calculated up to an arbitrary
high order and we will explicitly calculate the first non--trivial correction.
Furthermore we will make a first investigation into the Poisson algebra between
observables corresponding to fields at different space--time points and
consider the locality properties of the observables.Comment: 23 page
Instability due to long range Coulomb interaction in a liquid of polarizable particles (polarons, etc.)
The interaction Hamiltonian for a system of polarons a la Feynman in the
presence of long range Coulomb interaction is derived and the dielectric
function is computed in mean field. For large enough concentration a liquid of
such particles becomes unstable. The onset of the instability is signaled by
the softening of a collective optical mode in which all electrons oscillate in
phase in their respective self-trapping potential. We associate the instability
with a metallization of the system. Optical experiments in slightly doped
cuprates and doped nickelates are analyzed within this theory.
We discuss why doped cuprates matallize whereas nickelates do not.Comment: 5 pages,1 figur
Polaron effects in electron channels on a helium film
Using the Feynman path-integral formalism we study the polaron effects in
quantum wires above a liquid helium film. The electron interacts with
two-dimensional (2D) surface phonons, i.e. ripplons, and is confined in one
dimension (1D) by an harmonic potential. The obtained results are valid for
arbitrary temperature (), electron-phonon coupling strength (), and
lateral confinement (). Analytical and numerical results are
obtained for limiting cases of , , and . We found the
surprising result that reducing the electron motion from 2D to quasi-1D makes
the self-trapping transition more continuous.Comment: 6 pages, 7 figures, submitted to Phys. Rev.
Reply to the comment by D. Kreimer and E. Mielke
We respond to the comment by Kreimer et. al. about the torsional contribution
to the chiral anomaly in curved spacetimes. We discuss their claims and refute
its main conclusion.Comment: 9 pages, revte
Side-channel based intrusion detection for industrial control systems
Industrial Control Systems are under increased scrutiny. Their security is
historically sub-par, and although measures are being taken by the
manufacturers to remedy this, the large installed base of legacy systems cannot
easily be updated with state-of-the-art security measures. We propose a system
that uses electromagnetic side-channel measurements to detect behavioural
changes of the software running on industrial control systems. To demonstrate
the feasibility of this method, we show it is possible to profile and
distinguish between even small changes in programs on Siemens S7-317 PLCs,
using methods from cryptographic side-channel analysis.Comment: 12 pages, 7 figures. For associated code, see
https://polvanaubel.com/research/em-ics/code
Off center centers in a quantum well in the presence of a perpendicular magnetic field: angular momentum transition and magnetic evaporation
We investigate the effect of the position of the donor in the quantum well on
the energy spectrum and the oscillator strength of the D- system in the
presence of a perpendicular magnetic field. As a function of the magnetic field
we find that when the D- centers are placed sufficiently off-center they
undergo singlet-triplet transitions which are similar to those found in
many-electron parabolic quantum dots. The main difference is that the number of
such transitions depends on the position of the donor and only a finite number
of such singlet-triplet transitions are found as function of the strength of
the magnetic field. For sufficiently large magnetic fields the two electron
system becomes unbound. For the near center D- system no singlet-triplet and no
unbinding of the D- is found with increasing magnetic field. A magnetic field
vs. donor position phase diagram is presented that depends on the width of the
quantum well.Comment: 16 pages, 17 figures. Accepted for publication in Phys. Rev.
Spitzer detections of new dust components in the outflow of the Red Rectangle
We present Spitzer high spectral resolution IRS spectroscopy of three
positions in the carbon-rich outflow of post-AGB star HD 44179, better known as
the Red Rectangle. Surprisingly, the spectra show some strong unknown
mid-infrared resonances, in the 13-20 micron range. The shape and position of
these resonances varies with position in the nebula, and are not correlated
with the PAH features. We conclude these features are due to oxygen-rich
minerals, located in a region which is believed to be predominantly
carbon-rich. We provide possible explanations for the presence of oxygen-rich
dust in the carbon-rich outflows. Simple Mg-Fe-oxides are suggested as carriers
of these unidentified features.Comment: accepted by ApJL; 5 pages; 4 figure
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