9,144 research outputs found
Adhesion and electronic structure of graphene on hexagonal boron nitride substrates
We investigate the adsorption of graphene sheets on h-BN substrates by means
of first-principles calculations in the framework of adiabatic connection
fluctuation-dissipation theory in the random phase approximation. We obtain
adhesion energies for different crystallographic stacking configurations and
show that the interlayer bonding is due to long-range van der Waals forces. The
interplay of elastic and adhesion energies is shown to lead to stacking
disorder and moir\'e structures. Band structure calculations reveal substrate
induced mass terms in graphene which change their sign with the stacking
configuration. The dispersion, absolute band gaps and the real space shape of
the low energy electronic states in the moir\'e structures are discussed. We
find that the absolute band gaps in the moir\'e structures are at least an
order of magnitude smaller than the maximum local values of the mass term. Our
results are in agreement with recent STM experiments.Comment: 8 pages, 8 figures, revised and extended version, to appear in Phys.
Rev.
HST/FOS spectra of PG 1351+64: An intrinsic absorber at low redshift
A 1 A resolution spectra of the nearby (z = 0.08797) Seyfert galaxy PG 1351+64 taken with the Faint Object Spectrograph (FOS) onboard the Hubble Space Telescope is presented. Spectral coverage runs from 1200-3200 A in the observed frame and includes emission and absorption features due to Ly-alpha, N 5, Si 4, C 4, and Mg 2. Three distinct intrinsic absorption systems in Ly-alpha, N 5, Si 4, and C 4, and tentatively in Mg 2, at velocities of 900 km/s, 1630 km/s, and 2900 km/s (plus or minus 100 km/s) relative to the emission-line redshift of the QSO were detected. The maximum relative velocity of these absorbers is less than 5000 km/s and therefore does not meet Weymann, Carswell, & Smith's criteria for Broad-Absorption-Line (BAL) QSO's at high-z. However, the absorptions are almost certainly intrinsic to the QSO given the low redshift of this object. In addition, PG 1351+64 is marginally radio-quiet, as are all BALQSO's, based on recent estimates of the radio-loud/radio-quiet dividing line. The narrow velocity width, less than 500 km/s, and low outflow velocities of the absorption systems are more similar to so called 'associated absorbers' seen at high-z in radio-loud quasars, but whose absorptions are thought to arise in clouds much farther from the nucleus (greater than 1 kpc) than are BAL clouds (1-10 pc). Despite the qualitative resemblance to the associated absorbers, the absorption systems in PG 1351+64 appear to be the low-luminosity analogs of BALQSO absorption troughs. The lower observed outflow velocities in PG 1351+64 are due to the much lower luminosity of the nuclear source in comparison to the high-z, high-luminosity BALQSO's. In addition, 'satellite' emission lines displaced 4000-5000 km/s blueward and redward of the Mg 2 emission were discovered
Coordinate time and proper time in the GPS
The Global Positioning System (GPS) provides an excellent educational example
as to how the theory of general relativity is put into practice and becomes
part of our everyday life. This paper gives a short and instructive derivation
of an important formula used in the GPS, and is aimed at graduate students and
general physicists.
The theoretical background of the GPS (see \cite{ashby}) uses the
Schwarzschild spacetime to deduce the {\it approximate} formula, ds/dt\approx
1+V-\frac{|\vv|^2}{2}, for the relation between the proper time rate of a
satellite clock and the coordinate time rate . Here is the gravitational
potential at the position of the satellite and \vv is its velocity (with
light-speed being normalized as ). In this note we give a different
derivation of this formula, {\it without using approximations}, to arrive at
ds/dt=\sqrt{1+2V-|\vv|^2 -\frac{2V}{1+2V}(\n\cdot\vv)^2}, where \n is the
normal vector pointing outward from the center of Earth to the satellite. In
particular, if the satellite moves along a circular orbit then the formula
simplifies to ds/dt=\sqrt{1+2V-|\vv|^2}.
We emphasize that this derivation is useful mainly for educational purposes,
as the approximation above is already satisfactory in practice.Comment: 5 pages, revised, over-over-simplified... Does anyone care that the
GPS uses an approximate formula, while a precise one is available in just a
few lines??? Physicists don'
Quantum Mechanics of Extended Objects
We propose a quantum mechanics of extended objects that accounts for the
finite extent of a particle defined via its Compton wavelength. The Hilbert
space representation theory of such a quantum mechanics is presented and this
representation is used to demonstrate the quantization of spacetime. The
quantum mechanics of extended objects is then applied to two paradigm examples,
namely, the fuzzy (extended object) harmonic oscillator and the Yukawa
potential. In the second example, we theoretically predict the phenomenological
coupling constant of the meson, which mediates the short range and
repulsive nucleon force, as well as the repulsive core radius.Comment: RevTex, 24 pages, 1 eps and 5 ps figures, format change
A simulation of the Schwinger model in the overlap formalism
In the continuum, the single flavor massless Schwinger model has an exact
global axial symmetry in the sector of perturbative gauge fields. This
symmetry is explicitly broken by gauge fields with nonzero topological charge
inducing a nonzero expectation value for the bilinear . We show
that a lattice formulation of this model, using the overlap formalism to treat
the massless fermions, explicitly exhibits this phenomenon. A Monte Carlo
simulation of the complete system yields the correct value of the fermion
condensate and shows unambiguously that it originates from the sector of
topological charge equal to unity.Comment: Plain TeX file, 9 pages
Canonical General Relativity on a Null Surface with Coordinate and Gauge Fixing
We use the canonical formalism developed together with David Robinson to st=
udy the Einstein equations on a null surface. Coordinate and gauge conditions =
are introduced to fix the triad and the coordinates on the null surface. Toget=
her with the previously found constraints, these form a sufficient number of
second class constraints so that the phase space is reduced to one pair of
canonically conjugate variables: \Ac_2\and\Sc^2. The formalism is related to
both the Bondi-Sachs and the Newman-Penrose methods of studying the
gravitational field at null infinity. Asymptotic solutions in the vicinity of
null infinity which exclude logarithmic behavior require the connection to fall
off like after the Minkowski limit. This, of course, gives the previous
results of Bondi-Sachs and Newman-Penrose. Introducing terms which fall off
more slowly leads to logarithmic behavior which leaves null infinity intact,
allows for meaningful gravitational radiation, but the peeling theorem does not
extend to in the terminology of Newman-Penrose. The conclusions are in
agreement with those of Chrusciel, MacCallum, and Singleton. This work was
begun as a preliminary study of a reduced phase space for quantization of
general relativity.Comment: magnification set; pagination improved; 20 pages, plain te
Gravitational Waves from a Fissioning White Hole
We present a fully nonlinear calculation of the waveform of the gravitational
radiation emitted in the fission of a vacuum white hole. At early times, the
waveforms agree with close-approximation perturbative calculations but they
reveal dramatic time and angular dependence in the nonlinear regime. The
results pave the way for a subsequent computation of the radiation emitted
after a binary black hole merger.Comment: 11 pages, 6 figures, RevTeX
Gauge Theories in a Bag
We investigate multi-flavour gauge theories confined in d\es 2n-dimensional
Euclidean bags. The boundary conditions for the 'quarks' break the axial
flavour symmetry and depend on a parameter . We determine the
-dependence of the fermionic correlators and determinants and find that
a -breaking -term is generated dynamically. As an application we
calculate the chiral condensate in multi-flavour and the abelian
projection of . In the second model a condensate is generated in the
limit where the number of colours, , tends to infinity. We prove that the
condensate in decreases with increasing bag radius at least as
. Finally we determine the correlators of mesonic currents
in .Comment: 40 pages, LATEX-fil
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