389 research outputs found
Universality of Quantum Gravity Corrections
We show that the existence of a minimum measurable length and the related
Generalized Uncertainty Principle (GUP), predicted by theories of Quantum
Gravity, influence all quantum Hamiltonians. Thus, they predict quantum gravity
corrections to various quantum phenomena. We compute such corrections to the
Lamb Shift, the Landau levels and the tunnelling current in a Scanning
Tunnelling Microscope (STM). We show that these corrections can be interpreted
in two ways: (a) either that they are exceedingly small, beyond the reach of
current experiments, or (b) that they predict upper bounds on the quantum
gravity parameter in the GUP, compatible with experiments at the electroweak
scale. Thus, more accurate measurements in the future should either be able to
test these predictions, or further tighten the above bounds and predict an
intermediate length scale, between the electroweak and the Planck scale.Comment: v1: 4 pages, LaTeX; v2: typos corrected, references updated, version
to match published version in Physical Review Letter
Transport and Boundary Scattering in Confined Geometries: Analytical Results
We utilize a geometric argument to determine the effects of boundary
scattering on the carrier mean-free path in samples of various cross sections.
Analytic expressions for samples with rectangular and circular cross sections
are obtained. We also outline a method for incorporating these results into
calculations of the thermal conductivity.Comment: 35 pages, Late
Quantum theory of intersubband polarons
We present a microscopic quantum theory of intersubband polarons,
quasiparticles originated from the coupling between intersubband transitions
and longitudinal optical phonons. To this aim we develop a second quantized
theory taking into account both the Fr\"ohlich interaction between phonons and
intersubband transitions and the Coulomb interaction between the intersubband
transitions themselves. Our results show that the coupling between the phonons
and the intersubband transitions is extremely intense, thanks both to the
collective nature of the intersubband excitations and to the natural tight
confinement of optical phonons. Not only the coupling is strong enough to
spectroscopically resolve the resonant splitting between the modes (strong
coupling regime), but it can become comparable to the bare frequency of the
excitations (ultrastrong coupling regime). We thus predict the possibility to
exploit intersubband polarons both for applied optoelectronic research, where a
precise control of the phonon resonances is needed, and also to observe
fundamental quantum vacuum physics, typical of the ultrastrong coupling regime
A proposal for testing Quantum Gravity in the lab
Attempts to formulate a quantum theory of gravitation are collectively known
as {\it quantum gravity}. Various approaches to quantum gravity such as string
theory and loop quantum gravity, as well as black hole physics and doubly
special relativity theories predict a minimum measurable length, or a maximum
observable momentum, and related modifications of the Heisenberg Uncertainty
Principle to a so-called generalized uncertainty principle (GUP). We have
proposed a GUP consistent with string theory, black hole physics and doubly
special relativity theories and have showed that this modifies all quantum
mechanical Hamiltonians. When applied to an elementary particle, it suggests
that the space that confines it must be quantized, and in fact that all
measurable lengths are quantized in units of a fundamental length (which can be
the Planck length). On the one hand, this may signal the breakdown of the
spacetime continuum picture near that scale, and on the other hand, it can
predict an upper bound on the quantum gravity parameter in the GUP, from
current observations. Furthermore, such fundamental discreteness of space may
have observable consequences at length scales much larger than the Planck
scale. Because this influences all the quantum Hamiltonians in an universal
way, it predicts quantum gravity corrections to various quantum phenomena.
Therefore, in the present work we compute these corrections to the Lamb shift,
simple harmonic oscillator, Landau levels, and the tunneling current in a
scanning tunneling microscope.Comment: v1: 10 pages, REVTeX 4, no figures; v2: minor typos corrected and a
reference added. arXiv admin note: has substantial overlap with
arXiv:0906.5396 , published in a different journa
Determination of step--edge barriers to interlayer transport from surface morphology during the initial stages of homoepitaxial growth
We use analytic formulae obtained from a simple model of crystal growth by
molecular--beam epitaxy to determine step--edge barriers to interlayer
transport. The method is based on information about the surface morphology at
the onset of nucleation on top of first--layer islands in the submonolayer
coverage regime of homoepitaxial growth. The formulae are tested using kinetic
Monte Carlo simulations of a solid--on--solid model and applied to estimate
step--edge barriers from scanning--tunneling microscopy data on initial stages
of Fe(001), Pt(111), and Ag(111) homoepitaxy.Comment: 4 pages, a Postscript file, uuencoded and compressed. Physical Review
B, Rapid Communications, in press
Variational Derivation of Relativistic Fermion-Antifermion Wave Equations in QED
We present a variational method for deriving relativistic two-fermion wave
equations in a Hamiltonian formulation of QED. A reformulation of QED is
performed, in which covariant Green functions are used to solve for the
electromagnetic field in terms of the fermion fields. The resulting modified
Hamiltonian contains the photon propagator directly. The reformulation permits
one to use a simple Fock-space variational trial state to derive relativistic
fermion-antifermion wave equations from the corresponding quantum field theory.
We verify that the energy eigenvalues obtained from the wave equation agree
with known results for positronium.Comment: 25 pages, accepted in Journal of Mathematical Physics (2004
The Density of States of hole-doped Manganites: A Scanning Tunneling Microscopy/Spectroscopy study
Variable temperature scanning tunneling microscopy/spectroscopy studies on
single crystals and epitaxial thin films of hole-doped manganites, which show
colossal magnetoresistance, have been done. We have investigated the variation
of the density of states, at and near the Fermi energy (), as a function
of temperature. Simple calculations have been carried out, to find out the
effect of temperature on the tunneling spectra and extract the variation of
density of states with temperature, from the observed data. We also report
here, atomic resolution images, on the single crystals and larger range images
showing the growth patterns on thin films. Our investigation shows
unambiguously that there is a rapid variation in density of states for
temperatures near the Curie temperature (). While for temperatures below
, a finite DOS is observed at , for temperatures near a hard
gap opens up in the density of states near . For temperatures much higher
than , this gap most likely gives way to a soft gap. The observed hard gap
for temperatures near , is somewhat higher than the transport gap for all
the materials. For different materials, we find that the magnitude of the hard
gap decreases as the of the material increases and eventually, for
materials with a close to 400 K, the value of the gap approaches zero.Comment: 9 pages RevTeX, 12 postscript figures, 1 table included in text,
submitted to Physical Review
The process of irreversible nucleation in multilayer growth. I. Failure of the mean-field approach
The formation of stable dimers on top of terraces during epitaxial growth is
investigated in detail. In this paper we focus on mean-field theory, the
standard approach to study nucleation. Such theory is shown to be unsuitable
for the present problem, because it is equivalent to considering adatoms as
independent diffusing particles. This leads to an overestimate of the correct
nucleation rate by a factor N, which has a direct physical meaning: in average,
a visited lattice site is visited N times by a diffusing adatom. The dependence
of N on the size of the terrace and on the strength of step-edge barriers is
derived from well known results for random walks. The spatial distribution of
nucleation events is shown to be different from the mean-field prediction, for
the same physical reason. In the following paper we develop an exact treatment
of the problem.Comment: 19 pages, 3 figures. To appear in Phys. Rev.
Irreversible nucleation in molecular beam epitaxy: From theory to experiments
Recently, the nucleation rate on top of a terrace during the irreversible
growth of a crystal surface by MBE has been determined exactly. In this paper
we go beyond the standard model usually employed to study the nucleation
process, and we analyze the qualitative and quantitative consequences of two
important additional physical ingredients: the nonuniformity of the
Ehrlich-Schwoebel barrier at the step-edge, because of the existence of kinks,
and the steering effects, due to the interaction between the atoms of the flux
and the substrate. We apply our results to typical experiments of second layer
nucleation.Comment: 11 pages. Table I corrected and one appendix added. To be published
in Phys. Rev. B (scheduled issue: 15 February 2003
Temporal stimulated intersubband emission of photoexcited electrons
We have studied the transient evolution of electrons distributed over two
levels in a wide quantum well, with the two levels below the optical phonon
energy, after an ultrafast interband excitation and cascade emission of optical
phonons. If electrons are distributed near the top of the passive region, a
temporal negative absorption appears to be dominant in the intersubband
response. This is due to the effective broadening of the upper level state
under the optical phonon emission. We have then considered the amplification of
the ground mode in a THz waveguide with a multiquantum well placed at the
center of the cavity. A huge increase of the probe signal is obtained, which
permits the temporal stimulated emission regime of the photoexcited electrons
in the THz spectral region.Comment: 5 pages, 5 figures, brief repor
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