497 research outputs found
Quantum Walks with Non-Orthogonal Position States
Quantum walks have by now been realized in a large variety of different
physical settings. In some of these, particularly with trapped ions, the walk
is implemented in phase space, where the corresponding position states are not
orthogonal. We develop a general description of such a quantum walk and show
how to map it into a standard one with orthogonal states, thereby making
available all the tools developed for the latter. This enables a variety of
experiments, which can be implemented with smaller step sizes and more steps.
Tuning the non-orthogonality allows for an easy preparation of extended states
such as momentum eigenstates, which travel at a well-defined speed with low
dispersion. We introduce a method to adjust their velocity by momentum shifts,
which allows to investigate intriguing effects such as the analog of Bloch
oscillations.Comment: 5 pages, 4 figure
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
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
The Relationship between Brachycephalic Head Features in Modern Persian Cats and Dysmorphologies of the Skull and Internal Hydrocephalus
Background: Cat breeders observed a frequent occurrence of internal hydrocephalus in Persian cats with extreme brachycephalic head morphology. Objective: To investigate a possible relationship among the grade of brachycephaly, ventricular dilatation, and skull dysmorphologies in Persian cats. Animals: 92 Persian-, 10 Domestic shorthair cats. Methods: The grade of brachycephaly was determined on skull models based on CT datasets. Cranial measurements were examined with regard to a possible correlation with relative ventricular volume, and cranial capacity. Persians with high (peke-face Persians) and lower grades of brachycephaly (doll-face Persians) were investigated for the presence of skull dysmorphologies.
Results: The mean cranial index of the peke-face Persians (0.97 ± 0.14) was significantly higher than the mean cranial index of doll-face Persians (0.66 ± 0.04; P < 0.001). Peke-face Persians had a lower relative nasal bone length (0.15 ± 0.04) compared to doll-face (0.29 ± 0.08; P < 0.001). The endocranial volume was significantly lower in doll-face than peke-face Persians (89.6 ± 1.27% versus 91.76 ± 2.07%; P < 0.001). The cranial index was significantly correlated with this variable (Spearman´s r: 0.7; P < 0.0001).
Mean ventricle: Brain ratio of the peke-face group (0.159 ± 0.14) was significantly higher compared to doll-face Persians (0.015 ± 0.01; P < 0.001). Conclusion and Clinical Relevance: High grades of brachycephaly are also associated with malformations of the calvarial and facial bones as well as dental malformations. As these dysmorphologies can affect animal welfare, the selection for extreme forms of brachycephaly in Persian cats should be reconsidered
Surface sticking and lateral diffusion of lipids in supported bilayers
The diffusion of fluorescently labeled lipids in supported bilayers is studied using two different methods: Z-scan fluorescence correlation spectroscopy (z-scan FCS) and two-focus fluorescence correlation spectroscopy (2f-FCS). It is found that the data can be fitted consistently only when taking into account partial sticking of the labeled lipids to the supporting glass surface. A kinetic reaction-diffusion model is developed and applied to the data. We find a very slow sticking rate which, however, when neglected, leads to strongly varying estimates of the free diffusion coefficient. The study reveals a strong sensitivity of FCS on even slight binding/unbinding kinetics of the labeled molecules, which has significance for related diffusion measurements in cellular lipid membranes
Phase sensitive detection of dipole radiation in a fiber-based high numerical aperture optical system
We theoretically study the problem of detecting dipole radiation in an
optical system of high numerical aperture in which the detector is sensitive to
\textit{field amplitude}. In particular, we model the phase sensitive detector
as a single-mode cylindrical optical fiber. We find that the maximum in
collection efficiency of the dipole radiation does not coincide with the
optimum resolution for the light gathering instrument. The calculated results
are important for analyzing fiber-based confocal microscope performance in
fluorescence and spectroscopic studies of single molecules and/or quantum dots.Comment: 12 pages, 2 figure
photoluminescence of a single quantum emitter in a strongly inhomogeneous chemical environment
A comprehensive photoluminescence study of defect centers in single SiO2 nanoparticles provides new insight into the complex photo-physics of single quantum emitters embedded into a random chemical environment
Absolute quantum yield measurements of fluorescent proteins using a plasmonic nanocavity
One of the key photophysical properties of fluorescent proteins that is most difficult to measure is the quantum yield. It describes how efficiently a fluorophore converts absorbed light into fluorescence. Its measurement using conventional methods become particularly problematic when it is unknown how many of the proposedly fluorescent molecules of a sample are indeed fluorescent (for example due to incomplete maturation, or the presence of photophysical dark states). Here, we use a plasmonic nanocavity-based method to measure absolute quantum yield values of commonly used fluorescent proteins. The method is calibration-free, does not require knowledge about maturation or potential dark states, and works on minute amounts of sample. The insensitivity of the nanocavity-based method to the presence of non-luminescent species allowed us to measure precisely the quantum yield of photo-switchable proteins in their on-state and to analyze the origin of the residual fluorescence of protein ensembles switched to the dark state
Anomaly close to an electronic topological semimetal-insulator transition in elemental fcc-Yb under pressure
The Lifshitz-type semimetal-insulator transition, which is a transition of the
electronic topology, has been considered as the most fundamental metal-
insulator transition. Here, we present resistivity measurements under pressure
in the vicinity of the quantum critical point of fcc Yb. We apply a previously
suggested scaling for this type of transition and identify its universality
class. Moreover, we observe an anomaly in the screening coefficient A of the T
2 term in the resistivity at low temperatures in the metallic phase. We
suggest an interpretation of this phenomenon as an effect of doping by Ca
impurities unintentionally present in the Yb crystals. The observed behavior
may very well be applicable to any doped system in the vicinity of such a
transition
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
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