150 research outputs found
Transmission Phase of an Isolated Coulomb-Blockade Resonance
In two recent papers, O. Entin-Wohlman et al. studied the question: ``Which
physical information is carried by the transmission phase through a quantum
dot?'' In the present paper, this question is answered for an islolated
Coulomb-blockade resonance and within a theoretical model which is more closely
patterned after the geometry of the actual experiment by Schuster et al. than
is the model of O. Entin-Wohlman et al. We conclude that whenever the number of
leads coupled to the Aharanov-Bohm interferometer is larger than two, and the
total number of channels is sufficiently large, the transmission phase does
reflect the Breit-Wigner behavior of the resonance phase shift.Comment: 6 pages and one figur
Kinetic energy density functionals from the Airy gas, with an application to the atomization kinetic energies of molecules
We construct and study several semilocal density functional approximations
for the positive Kohn-Sham kinetic energy density. These functionals fit the
kinetic energy density of the Airy gas and they can be accurate for integrated
kinetic energies of atoms, molecules, jellium clusters and jellium surfaces. We
find that these functionals are the most accurate ones for atomization kinetic
energies of molecules and for fragmentation of jellium clusters. We also report
that local and semilocal kinetic energy functionals can show "binding" when the
density of a spin unrestricted Kohn-Sham calculation is used.Comment: 7 pages, 7 figure
Scattering phases in quantum dots: an analysis based on lattice models
The properties of scattering phases in quantum dots are analyzed with the
help of lattice models. We first derive the expressions relating the different
scattering phases and the dot Green functions. We analyze in detail the Friedel
sum rule and discuss the deviation of the phase of the transmission amplitude
from the Friedel phase at the zeroes of the transmission. The occurrence of
such zeroes is related to the parity of the isolated dot levels. A statistical
analysis of the isolated dot wave-functions reveals the absence of significant
correlations in the parity for large disorder and the appearance, for weak
disorder, of certain dot states which are strongly coupled to the leads. It is
shown that large differences in the coupling to the leads give rise to an
anomalous charging of the dot levels. A mechanism for the phase lapse observed
experimentally based on this property is discussed and illustrated with model
calculations.Comment: 18 pages, 9 figures. to appear in Physical Review
Towards an Explanation of the Mesoscopic Double-Slit Experiment: a new model for charging of a Quantum Dot
For a quantum dot (QD) in the intermediate regime between integrable and
fully chaotic, the widths of single-particle levels naturally differ by orders
of magnitude. In particular, the width of one strongly coupled level may be
larger than the spacing between other, very narrow, levels. In this case many
consecutive Coulomb blockade peaks are due to occupation of the same broad
level. Between the peaks the electron jumps from this level to one of the
narrow levels and the transmission through the dot at the next resonance
essentially repeats that at the previous one. This offers a natural explanation
to the recently observed behavior of the transmission phase in an
interferometer with a QD.Comment: 4 pages, 2 figures, Journal versio
Spin Effects and Transport in Quantum Dots with overlapping Resonances
The role of spin is investigated in the transport through a quantum dot with
two overlapping resonances (one having a width larger than the level separation
and the other very narrow, cf. Silvestrov and Imry, Phys. Rev. Lett. {\bf 85},
2565 (2000)). For a series of consecutive charging resonances, one electron
from the leads populates one and the same broad level in the dot. Moreover,
there is the tendency to occupy the same level also by the second electron
within the same resonance. This second electron is taken from the narrow levels
in the dot. The narrow levels are populated (and broad level is depopulated)
via sharp rearrangements of the electronic configuration in the Coulomb
blockade valleys. Possible experimental manifestations of this scenario are
considered. Among these there are sharp features in the valleys and in the
Mixed Valence regime and an unusual Kondo effect.Comment: 7 pages, 3 figures, just a published versio
Quantum characterization of superconducting photon counters
We address the quantum characterization of photon counters based on
transition-edge sensors (TESs) and present the first experimental tomography of
the positive operator-valued measure (POVM) of a TES. We provide the reliable
tomographic reconstruction of the POVM elements up to 11 detected photons and
M=100 incoming photons, demonstrating that it is a linear detector.Comment: 3 figures, NJP (to appear
The Flat Transmission Spectrum of the Super-Earth GJ1214b from Wide Field Camera 3 on the Hubble Space Telescope
Capitalizing on the observational advantage offered by its tiny M dwarf host,
we present HST/WFC3 grism measurements of the transmission spectrum of the
super-Earth exoplanet GJ1214b. These are the first published WFC3 observations
of a transiting exoplanet atmosphere. After correcting for a ramp-like
instrumental systematic, we achieve nearly photon-limited precision in these
observations, finding the transmission spectrum of GJ1214b to be flat between
1.1 and 1.7 microns. Inconsistent with a cloud-free solar composition
atmosphere at 8.2 sigma, the measured achromatic transit depth most likely
implies a large mean molecular weight for GJ1214b's outer envelope. A dense
atmosphere rules out bulk compositions for GJ1214b that explain its large
radius by the presence of a very low density gas layer surrounding the planet.
High-altitude clouds can alternatively explain the flat transmission spectrum,
but they would need to be optically thick up to 10 mbar or consist of particles
with a range of sizes approaching 1 micron in diameter.Comment: 17 pages, 12 figures, accepted for publication in Ap
Quantum universal detectors
We address the problem of estimating the expectation value of an
arbitrary operator O via a universal measuring apparatus that is independent of
O, and for which the expectation values for different operators are obtained by
changing only the data-processing. The ``universal detector'' performs a joint
measurement on the system and on a suitably prepared ancilla. We characterize
such universal detectors, and show how they can be obtained either via Bell
measurements or via local measurements and classical communication between
system and ancilla.Comment: 4 pages, no figure
Laplacian-level density functionals for the kinetic energy density and exchange-correlation energy
We construct a Laplacian-level meta-generalized gradient approximation
(meta-GGA) for the non-interacting (Kohn-Sham orbital) positive kinetic energy
density of an electronic ground state of density . This meta-GGA is
designed to recover the fourth-order gradient expansion in the
appropiate slowly-varying limit and the von Weizs\"{a}cker expression
in the rapidly-varying limit. It is constrained to
satisfy the rigorous lower bound .
Our meta-GGA is typically a strong improvement over the gradient expansion of
for atoms, spherical jellium clusters, jellium surfaces, the Airy gas,
Hooke's atom, one-electron Gaussian density, quasi-two dimensional electron
gas, and nonuniformly-scaled hydrogen atom. We also construct a Laplacian-level
meta-GGA for exchange and correlation by employing our approximate in
the Tao, Perdew, Staroverov and Scuseria (TPSS) meta-GGA density functional.
The Laplacian-level TPSS gives almost the same exchange-correlation enhancement
factors and energies as the full TPSS, suggesting that and
carry about the same information beyond that carried by and . Our
kinetic energy density integrates to an orbital-free kinetic energy functional
that is about as accurate as the fourth-order gradient expansion for many real
densities (with noticeable improvement in molecular atomization energies), but
considerably more accurate for rapidly-varying ones.Comment: 9 pages, 16 figure
Aharonov-Bohm Interferometry with Interacting Quantum Dots: Spin Configurations, Asymmetric Interference Patterns, Bias-Voltage-Induced Aharonov-Bohm Oscillations, and Symmetries of Transport Coefficients
We study electron transport through multiply-connected mesoscopic geometries
containing interacting quantum dots. Our formulation covers both equilibrium
and non-equilibrium physics. We discuss the relation of coherent transport
channels through the quantum dot to flux-sensitive Aharonov-Bohm oscillations
in the total conductance of the device. Contributions to transport in first and
second order in the intrinsic line width of the dot levels are addressed in
detail. We predict an interaction-induced asymmetry in the amplitude of the
interference signal around resonance peaks as a consequence of incoherence
associated with spin-flip processes. This asymmetry can be used to probe the
total spin of the quantum dot. Such a probe requires less stringent
experimental conditions than the Kondo effect, which provides the same
information. We show that first-order contributions can be partially or even
fully coherent. This contrasts with the sequential-tunneling picture, which
describes first-order transport as a sequence of incoherent tunneling
processes. We predict bias-voltage induced Aharonov-Bohm oscillations of
physical quantities which are independent of flux in the linear-response
regime. Going beyond the Onsager relations we analyze the relations between the
space symmetry group of the setup and the flux-dependent non-linear
conductance.Comment: 22 pages, 11 figure
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