836 research outputs found
Second order coupling between excited atoms and surface polaritons
Casimir-Polder interactions between an atom and a macroscopic body are
typically regarded as due to the exchange of virtual photons. This is strictly
true only at zero temperature. At finite temperature, real-photon exchange can
provide a significant contribution to the overall dispersion interaction. Here
we describe a new resonant two-photon process between an atom and a planar
interface. We derive a second order effective Hamiltonian to explain how atoms
can couple resonantly to the surface polariton modes of the dielectric medium.
This leads to second-order energy exchanges which we compare with the standard
nonresonant Casimir-Polder energy.Comment: 7 pages, 2 figure
Ground-state van der Waals forces in planar multilayer magnetodielectrics
Within the frame of lowest-order perturbation theory, the van der Waals
potential of a ground-state atom placed within an arbitrary dispersing and
absorbing magnetodielectric multilayer system is given. Examples of an atom
situated in front of a magnetodielectric plate or between two such plates are
studied in detail. Special emphasis is placed on the competing attractive and
repulsive force components associated with the electric and magnetic matter
properties, respectively, and conditions for the formation of repulsive
potential walls are given. Both numerical and analytical results are presented.Comment: 16 pages, 8 figures, minor correction
On kernel engineering via PaleyâWiener
A radial basis function approximation takes the form
where the coefficients a 1,âŠ,a n are real numbers, the centres b 1,âŠ,b n are distinct points in â d , and the function Ï:â d ââ is radially symmetric. Such functions are highly useful in practice and enjoy many beautiful theoretical properties. In particular, much work has been devoted to the polyharmonic radial basis functions, for which Ï is the fundamental solution of some iterate of the Laplacian. In this note, we consider the construction of a rotation-invariant signed (Borel) measure ÎŒ for which the convolution Ï=ÎŒ Ï is a function of compact support, and when Ï is polyharmonic. The novelty of this construction is its use of the PaleyâWiener theorem to identify compact support via analysis of the Fourier transform of the new kernel Ï, so providing a new form of kernel engineering
Residual strain in free-standing CdTe nanowires overgrown with HgTe
We investigate the crystal properties of CdTe nanowires overgrown with HgTe.
Scanning electron microscopy (SEM) and scanning transmission electron
microscopy (STEM) confirm, that the growth results in a high ensemble
uniformity and that the individual heterostructures are single-crystalline,
respectively. We use high-resolution X-ray diffraction (HRXRD) to investigate
strain, caused by the small lattice mismatch between the two materials. We find
that both CdTe and HgTe show changes in lattice constant compared to the
respective bulk lattice constants. The measurements reveal a complex strain
pattern with signatures of both uniaxial and shear strains present in the
overgrown nanowires
Quantum tunneling through planar p-n junctions in HgTe quantum wells
We demonstrate that a p-n junction created electrically in HgTe quantum wells
with inverted band-structure exhibits interesting intraband and interband
tunneling processes. We find a perfect intraband transmission for electrons
injected perpendicularly to the interface of the p-n junction. The opacity and
transparency of electrons through the p-n junction can be tuned by changing the
incidence angle, the Fermi energy and the strength of the Rashba spin-orbit
interaction. The occurrence of a conductance plateau due to the formation of
topological edge states in a quasi-one-dimensional p-n junction can be switched
on and off by tuning the gate voltage. The spin orientation can be
substantially rotated when the samples exhibit a moderately strong Rashba
spin-orbit interaction.Comment: 4 pages, 4 figure
Fine structure of "zero-mode" Landau levels in HgTe/HgCdTe quantum wells
HgTe/HgCdTe quantum wells with the inverted band structure have been probed
using far infrared magneto-spectroscopy. Realistic calculations of Landau level
diagrams have been performed to identify the observed transitions.
Investigations have been greatly focused on the magnetic field dependence of
the peculiar pair of "zero-mode" Landau levels which characteristically split
from the upper conduction and bottom valence bands, and merge under the applied
magnetic field. The observed avoided crossing of these levels is tentatively
attributed to the bulk inversion asymmetry of zinc blend compounds.Comment: 5 pages, 4 figure
Nonequilibrium thermal Casimir-Polder forces
We study the nonequilibrium Casimir-Polder force on an atom prepared in an
incoherent superposition of internal energy-eigenstates, which is placed in a
magnetoelectric environment of nonuniform temperature. After solving the
coupled atom--field dynamics within the framework of macroscopic quantum
electrodynamics, we derive a general expression for the thermal Casimir-Polder
force.Comment: 5 page
Casimir force on amplifying bodies
Based on a unified approach to macroscopic QED that allows for the inclusion
of amplification in a limited space and frequency range, we study the Casimir
force as a Lorentz force on an arbitrary partially amplifying system of
linearly locally responding (isotropic) magnetoelectric bodies. We demonstrate
that the force on a weakly polarisable/magnetisable amplifying object in the
presence of a purely absorbing environment can be expressed as a sum over the
Casimir--Polder forces on the excited atoms inside the body. As an example, the
resonant force between a plate consisting of a dilute gas of excited atoms and
a perfect mirror is calculated
Molecular beam epitaxy of high structural quality Bi2Se3 on lattice matched InP(111) substrates
Epitaxial layers of the topological insulator Bi2Se3 have been grown by
molecular beam epitaxy on laterally lattice-matched InP(111)B substrates. High
resolution X-ray diffraction shows a significant improvement of Bi2Se3 crystal
quality compared to layers deposited on other substrates. The measured full
width at half maximum of the rocking curve is Delta omega=13 arcsec, and the
(omega-2theta) scans exhibit clear layer thickness fringes. Atomic force
microscope images show triangular twin domains with sizes increasing with layer
thickness. The structural quality of the domains is confirmed on the
microscopic level by transmission electron microscopy.Comment: 4 pages, 4 figure
Non-Perturbative Theory of Dispersion Interactions
Some open questions exist with fluctuation-induced forces between extended
dipoles. Conventional intuition derives from large-separation perturbative
approximations to dispersion force theory. Here we present a full
non-perturbative theory. In addition we discuss how one can take into account
finite dipole size corrections. It is of fundamental value to investigate the
limits of validity of the perturbative dispersion force theory.Comment: 9 pages, no figure
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