411 research outputs found
Self-assembly of the simple cubic lattice with an isotropic potential
Conventional wisdom presumes that low-coordinated crystal ground states
require directional interactions. Using our recently introduced optimization
procedure to achieve self-assembly of targeted structures (Phys. Rev. Lett. 95,
228301 (2005), Phys. Rev. E 73, 011406 (2006)), we present an isotropic pair
potential for a three-dimensional many-particle system whose classical
ground state is the low-coordinated simple cubic (SC) lattice. This result is
part of an ongoing pursuit by the authors to develop analytical and
computational tools to solve statistical-mechanical inverse problems for the
purpose of achieving targeted self-assembly. The purpose of these methods is to
design interparticle interactions that cause self-assembly of technologically
important target structures for applications in photonics, catalysis,
separation, sensors and electronics. We also show that standard approximate
integral-equation theories of the liquid state that utilize pair correlation
function information cannot be used in the reverse mode to predict the correct
simple cubic potential. We report in passing optimized isotropic potentials
that yield the body-centered cubic and simple hexagonal lattices, which provide
other examples of non-close-packed structures that can be assembled using
isotropic pair interactions.Comment: 16 pages, 12 figures. Accepted for publication in Physical Review
The formation of an energy gap in graphene on ruthenium by controlling the interface
In this work, we have investigated the spectral function of graphene on a monolayer of intercalated gold on Ru(0001) using angle-resolved photoemission spectroscopy (ARPES). The intercalation leads to a decoupling of the graphene film, as documented by emergence of the characteristic linear π-bands near the Fermi level. However, a band gap at the band crossing is observed. We relate this gap opening to the broken symmetry of the two carbon sublattices, induced by the special lattice mismatch of the graphene and the intercalated gold monolayer
Influence of static electric fields on an optical ion trap
We recently reported on a proof-of-principle experiment demonstrating optical
trapping of an ion in a single-beam dipole trap superimposed by a static
electric potential [Nat. Photonics 4, 772--775 (2010)]. Here, we first discuss
the experimental procedures focussing on the influence and consequences of the
static electric potential. These potentials can easily prevent successful
optical trapping, if their configuration is not chosen carefully. Afterwards,
we analyse the dipole trap experiments with different analytic models, in which
different approximations are applied. According to these models the
experimental results agree with recoil heating as the relevant heating effect.
In addition, a Monte-Carlo simulation has been developed to refine the
analysis. It reveals a large impact of the static electric potential on the
dipole trap experiments in general. While it supports the results of the
analytic models for the parameters used in the experiments, the analytic models
cease their validity for significantly different parameters. Finally, we
propose technical improvements for future realizations of experiments with
optically trapped ions.Comment: 16 pages, 16 figure
Charge and spin distributions in GaMnAs/GaAs Ferromagnetic Multilayers
A self-consistent electronic structure calculation based on the
Luttinger-Kohn model is performed on GaMnAs/GaAs multilayers. The Diluted
Magnetic Semiconductor layers are assumed to be metallic and ferromagnetic. The
high Mn concentration (considered as 5% in our calculation) makes it possible
to assume the density of magnetic moments as a continuous distribution, when
treating the magnetic interaction between holes and the localized moment on the
Mn(++) sites. Our calculation shows the distribution of heavy holes and light
holes in the structure. A strong spin-polarization is observed, and the charge
is concentrated mostly on the GaMnAs layers, due to heavy and light holes with
their total angular momentum aligned anti-parallel to the average
magnetization. The charge and spin distributions are analyzed in terms of their
dependence on the number of multilayers, the widths of the GaMnAs and GaAs
layers, and the width of lateral GaAs layers at the borders of the structure.Comment: 12 pages,7 figure
Gap modification of atomically thin boron nitride by phonon mediated interactions
A theory is presented for the modification of bandgaps in atomically thin
boron nitride (BN) by attractive interactions mediated through phonons in a
polarizable substrate, or in the BN plane. Gap equations are solved, and gap
enhancements are found to range up to 70% for dimensionless electron-phonon
coupling \lambda=1, indicating that a proportion of the measured BN bandgap may
have a phonon origin
Electron-Phonon Coupling in Highly-Screened Graphene
Photoemission studies of graphene have resulted in a long-standing
controversy concerning the strength of the experimental electron-phonon
interaction in comparison with theoretical calculations. Using high-resolution
angle-resolved photoemission spectroscopy we study graphene grown on a copper
substrate, where the metallic screening of the substrate substantially reduces
the electron-electron interaction, simplifying the comparison of the
electron-phonon interaction between theory and experiment. By taking the
nonlinear bare bandstructure into account, we are able to show that the
strength of the electron-phonon interaction does indeed agree with theoretical
calculations. In addition, we observe a significant bandgap at the Dirac point
of graphene.Comment: Submitted to Phys. Rev. Lett. on July 20, 201
On the Application of a Monolithic Array for Detecting Intensity-Correlated Photons Emitted by Different Source Types
It is not widely appreciated that many subtleties are involved in the
accurate measurement of intensity-correlated photons; even for the original
experiments of Hanbury Brown and Twiss (HBT). Using a monolithic 4x4 array of
single-photon avalanche diodes (SPADs), together with an off-chip algorithm for
processing streaming data, we investigate the difficulties of measuring
second-order photon correlations g2 in a wide variety of light fields that
exhibit dramatically different correlation statistics: a multimode He-Ne laser,
an incoherent intensity-modulated lamp-light source and a thermal light source.
Our off-chip algorithm treats multiple photon-arrivals at pixel-array pairs, in
any observation interval, with photon fluxes limited by detector saturation, in
such a way that a correctly normalized g2 function is guaranteed. The impact of
detector background correlations between SPAD pixels and afterpulsing effects
on second-order coherence measurements is discussed. These results demonstrate
that our monolithic SPAD array enables access to effects that are otherwise
impossible to measure with stand-alone detectors.Comment: 17 pages, 6 figure
Optical Trapping of an Ion
For several decades, ions have been trapped by radio frequency (RF) and
neutral particles by optical fields. We implement the experimental
proof-of-principle for trapping an ion in an optical dipole trap. While
loading, initialization and final detection are performed in a RF trap, in
between, this RF trap is completely disabled and substituted by the optical
trap. The measured lifetime of milliseconds allows for hundreds of oscillations
within the optical potential. It is mainly limited by heating due to photon
scattering. In future experiments the lifetime may be increased by further
detuning the laser and cooling the ion. We demonstrate the prerequisite to
merge both trapping techniques in hybrid setups to the point of trapping ions
and atoms in the same optical potential.Comment: 5 pages, 3 figure
Quantum dots and spin qubits in graphene
This is a review on graphene quantum dots and their use as a host for spin
qubits. We discuss the advantages but also the challenges to use graphene
quantum dots for spin qubits as compared to the more standard materials like
GaAs. We start with an overview of this young and fascinating field and will
then discuss gate-tunable quantum dots in detail. We calculate the bound states
for three different quantum dot architectures where a bulk gap allows for
confinement via electrostatic fields: (i) graphene nanoribbons with armchair
boundary, (ii) a disc in single-layer graphene, and (iii) a disc in bilayer
graphene. In order for graphene quantum dots to be useful in the context of
spin qubits, one needs to find reliable ways to break the valley-degeneracy.
This is achieved here, either by a specific termination of graphene in (i) or
in (ii) and (iii) by a magnetic field, without the need of a specific boundary.
We further discuss how to manipulate spin in these quantum dots and explain the
mechanism of spin decoherence and relaxation caused by spin-orbit interaction
in combination with electron-phonon coupling, and by hyperfine interaction with
the nuclear spin system.Comment: 23 pages, 10 figures, topical review prepared for Nanotechnolog
Multi-phonon Raman scattering in semiconductor nanocrystals: importance of non-adiabatic transitions
Multi-phonon Raman scattering in semiconductor nanocrystals is treated taking
into account both adiabatic and non-adiabatic phonon-assisted optical
transitions. Because phonons of various symmetries are involved in scattering
processes, there is a considerable enhancement of intensities of multi-phonon
peaks in nanocrystal Raman spectra. Cases of strong and weak band mixing are
considered in detail. In the first case, fundamental scattering takes place via
internal electron-hole states and is participated by s- and d-phonons, while in
the second case, when the intensity of the one-phonon Raman peak is strongly
influenced by the interaction of an electron and of a hole with interface
imperfections (e. g., with trapped charge), p-phonons are most active.
Calculations of Raman scattering spectra for CdSe and PbS nanocrystals give a
good quantitative agreement with recent experimental results.Comment: 16 pages, 2 figures, E-mail addresses: [email protected],
[email protected], [email protected], accepted for publication in
Physical Review
- …