294 research outputs found
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
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
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
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
Transfer matrix method for interface optical-phonon modes in multiple-interface heterostructure systems
Interactions of carriers with interface optical phonons dominate over other carrier–phonon scatterings in narrow quantum-well structures. Herein, a transfer matrix method is used to establish a formalism for determining the dispersion relations, electrostatic potentials, and Fröhlich interaction Hamiltonians of the interface optical phonons for multiple-interface heterostructure systems within the framework of the macroscopic dielectric continuum model. This method facilitates systematic calculations for complex structures where the conventional method is very difficult to implement. Several specific cases are treated to illustrate the advantages of the general formalism. © 1997 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70863/2/JAPIAU-82-7-3363-1.pd
Epitaxial Growth Kinetics with Interacting Coherent Islands
The Stranski-Krastanov growth kinetics of undislocated (coherent)
3-dimensional islands is studied with a self-consistent mean field rate theory
that takes account of elastic interactions between the islands. The latter are
presumed to facilitate the detachment of atoms from the islands with a
consequent decrease in their average size. Semi-quantitative agreement with
experiment is found for the time evolution of the total island density and the
mean island size. When combined with scaling ideas, these results provide a
natural way to understand the often-observed initial increase and subsequent
decrease in the width of the coherent island size distribution.Comment: 4 pages, 4 figure
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