243 research outputs found
Hybrid Quantum Dot-2D Electron Gas Devices for Coherent Optoelectronics
We present an inverted GaAs 2D electron gas with self-assembled InAs quantum
dots in close proximity, with the goal of combining quantum transport with
quantum optics experiments. We have grown and characterized several wafers --
using transport, AFM and optics -- finding narrow-linewidth optical dots and
high-mobility, single subband 2D gases. Despite being buried 500 nm below the
surface, the dots are clearly visible on AFM scans, allowing precise
localization and paving the way towards a hybrid quantum system integrating
optical dots with surface gate-defined nanostructures in the 2D gas.Comment: 4 pages, 5 figures (color
A scalable optical detection scheme for matter wave interferometry
Imaging of surface adsorbed molecules is investigated as a novel detection
method for matter wave interferometry with fluorescent particles. Mechanically
magnified fluorescence imaging turns out to be an excellent tool for recording
quantum interference patterns. It has a good sensitivity and yields patterns of
high visibility. The spatial resolution of this technique is only determined by
the Talbot gratings and can exceed the optical resolution limit by an order of
magnitude. A unique advantage of this approach is its scalability: for certain
classes of nano-sized objects, the detection sensitivity will even increase
significantly with increasing size of the particle.Comment: 10 pages, 4 figure
Adaption eines elektronischen Laborbuchs (Elab) an ein objektorientiertes Datenbank-Management-System
Das Elektronische Laborbuch Elab ist ein Softwarewerkzeug zur Unterstützung der Entwicklung von Herstellungsprozessen von Mikrosystemen. Es dient der Darstellung und dem Modifizieren von Prozessen und Prozeßreihen. Dabei wird auch die Variantenbildung unterstützt.
Elab ist ein objektorientiert aufgebautes System. Mehrbenutzerbetrieb, Zugriffsberechtigungen und Versionsverwaltung machen die Anbindung an ein objektorientiertes Datenbank-Management-System erforderlich. Aus diesem Grund wurde das Datenbanksystem ObjectStore mit Sprachanbindung zu Smalltalk (OSST) ausgewählt.
Der parallele Zugriff von verschiedenen Applikationen auf den persistenten Datenbestand erfordert spezielle Sperrmechanismen auf der Grundlage einfacherer Mechanismen der ObjectStore Datenbanken. Durch die Implementierung von semantischen Sperren kann objektweises Sperren erreicht werden.
Im Mehrbenutzerbetrieb gehören die Benutzer bestimmten Projektgruppen an, die nur Zugriff auf die Daten ihres Projekts haben. Ein Superuser übernimmt die Aufgaben der Benutzerverwaltung
Structural and magnetic properties of Fe/ZnSe(001) interfaces
We have performed first principles electronic structure calculations to
investigate the structural and magnetic properties of Fe/ZnSe(001) interfaces.
Calculations involving full geometry optimizations have been carried out for a
broad range of thickness of Fe layers(0.5 monolayer to 10 monolayers) on top of
a ZnSe(001) substrate. Both Zn and Se terminated interfaces have been explored.
Total energy calculations show that Se segregates at the surface which is in
agreement with recent experiments.
For both Zn and Se terminations, the interface Fe magnetic moments are higher
than the bulk bcc Fe moment.
We have also investigated the effect of adding Fe atoms on top of a
reconstructed ZnSe surface to explore the role of reconstruction of
semiconductor surfaces in determining properties of metal-semiconductor
interfaces. Fe breaks the Se dimer bond formed for a Se-rich (2x1)
reconstructed surface. Finally, we looked at the reverse growth i.e. growth of
Zn and Se atoms on a bcc Fe(001) substrate to investigate the properties of the
second interface of a magnetotunnel junction. The results are in good agreement
with the theoretical and experimental results, wherever available.Comment: 7 pages, 8 figures, accepted for publication in PR
Impedance model for the polarization-dependent optical absorption of superconducting single-photon detectors
We measured the single-photon detection efficiency of NbN superconducting
single photon detectors as a function of the polarization state of the incident
light for different wavelengths in the range from 488 nm to 1550 nm. The
polarization contrast varies from ~5% at 488 nm to ~30% at 1550 nm, in good
agreement with numerical calculations. We use an optical-impedance model to
describe the absorption for polarization parallel to the wires of the detector.
For lossy NbN films, the absorption can be kept constant by keeping the product
of layer thickness and filling factor constant. As a consequence, we find that
the maximum possible absorption is independent of filling factor. By
illuminating the detector through the substrate, an absorption efficiency of
~70% can be reached for a detector on Si or GaAs, without the need for an
optical cavity.Comment: 15 pages, 5 figures, submitted to Journal of Applied Physic
Comparison of the scintillation noise above different observatories measured with MASS instruments
Scintillation noise is a major limitation of ground base photometric
precision. An extensive dataset of stellar scintillation collected at 11
astronomical sites world-wide with MASS instruments was used to estimate the
scintillation noise of large telescopes in the case of fast photometry and
traditional long-exposure regime. Statistical distributions of the
corresponding parameters are given. The scintillation noise is mostly
determined by turbulence and wind in the upper atmosphere and comparable at all
sites, with slightly smaller values at Mauna Kea and largest noise at Tolonchar
in Chile. We show that the classical Young's formula under-estimates the
scintillation noise.The temporal variations of the scintillation noise are also
similar at all sites, showing short-term variability at time scales of 1 -- 2
hours and slower variations, including marked seasonal trends (stronger
scintillation and less clear sky during local winter). Some correlation was
found between nearby observatories.Comment: Accepted for publication in Astronomy and Astrophysics, 14 pages, 11
figure
Autoinducers act as biological timers in Vibrio harveyi
Quorum sensing regulates cell density-dependent phenotypes and involves the synthesis, excretion and detection of so-called autoinducers. Vibrio harveyi strain ATCC BAA-1116 (recently reclassified as Vibrio campbellii), one of the best-characterized model organisms for the study of quorum sensing, produces and responds to three autoinducers. HAI-1, AI-2 and CAI-1 are recognized by different receptors, but all information is channeled into the same signaling cascade, which controls a specific set of genes. Here we examine temporal variations of availability and concentration of the three autoinducers in V. harveyi, and monitor the phenotypes they regulate, from the early exponential to the stationary growth phase in liquid culture. Specifically, the exponential growth phase is characterized by an increase in AI-2 and the induction of bioluminescence, while HAI-1 and CAI-1 are undetectable prior to the late exponential growth phase. CAI-1 activity reaches its maximum upon entry into stationary phase, while molar concentrations of AI-2 and HAI-1 become approximately equal. Similarly, autoinducer-dependent exoproteolytic activity increases at the transition into stationary phase. These findings are reflected in temporal alterations in expression of the luxR gene that encodes the master regulator LuxR, and of four autoinducer-regulated genes during growth. Moreover, in vitro phosphorylation assays reveal a tight correlation between the HAI-1/AI-2 ratio as input and levels of receptor-mediated phosphorylation of LuxU as output. Our study supports a model in which the combinations of autoinducers available, rather than cell density per se, determine the timing of various processes in V. harveyi populations
Numerical study of the strongly screened vortex glass model in an external field
The vortex glass model for a disordered high-T_c superconductor in an
external magnetic field is studied in the strong screening limit. With exact
ground state (i.e. T=0) calculations we show that 1) the ground state of the
vortex configuration varies drastically with infinitesimal variations of the
strength of the external field, 2) the minimum energy of global excitation
loops of length scale L do not depend on the strength of the external field,
however 3) the excitation loops themself depend sensibly on the field. From 2)
we infer the absence of a true superconducting state at any finite temperature
independent of the external field.Comment: 6 pages RevTeX, 5 eps-figures include
Quantum physics meets biology
Quantum physics and biology have long been regarded as unrelated disciplines,
describing nature at the inanimate microlevel on the one hand and living
species on the other hand. Over the last decades the life sciences have
succeeded in providing ever more and refined explanations of macroscopic
phenomena that were based on an improved understanding of molecular structures
and mechanisms. Simultaneously, quantum physics, originally rooted in a world
view of quantum coherences, entanglement and other non-classical effects, has
been heading towards systems of increasing complexity. The present perspective
article shall serve as a pedestrian guide to the growing interconnections
between the two fields. We recapitulate the generic and sometimes unintuitive
characteristics of quantum physics and point to a number of applications in the
life sciences. We discuss our criteria for a future quantum biology, its
current status, recent experimental progress and also the restrictions that
nature imposes on bold extrapolations of quantum theory to macroscopic
phenomena.Comment: 26 pages, 4 figures, Perspective article for the HFSP Journa
Home birth and barriers to referring women with obstetric complications to hospitals: a mixed-methods study in Zahedan, southeastern Iran
<p>Abstract</p> <p>Background</p> <p>One factor that contributes to high maternal mortality in developing countries is the delayed use of Emergency Obstetric-Care (EmOC) facilities. The objective of this study was to determine the factors that hinder midwives and parturient women from using hospitals when complications occur during home birth in Sistan and Baluchestan province, Iran, where 23% of all deliveries take place in non- hospital settings.</p> <p>Methods</p> <p>In the study and data management, a mixed-methods approach was used. In the quantitative phase, we compared the existing health-sector data with World Health Organization (WHO) standards for the availability and use of EmOC services. The qualitative phase included collection and analysis of interviews with midwives and traditional birth attendants and twenty-one in-depth interviews with mothers. The data collected in this phase were managed according to the principles of qualitative data analysis.</p> <p>Results</p> <p>The findings demonstrate that three distinct factors lead to indecisiveness and delay in the use of EmOC by the midwives and mothers studied. Socio-cultural and familial reasons compel some women to choose to give birth at home and to hesitate seeking professional emergency care for delivery complications. Apprehension about being insulted by physicians, the necessity of protecting their professional integrity in front of patients and an inability to persuade their patients lead to an over-insistence by midwives on completing deliveries at the mothers' homes and a reluctance to refer their patients to hospitals. The low quality and expense of EmOC and the mothers' lack of health insurance also contribute to delays in referral.</p> <p>Conclusions</p> <p>Women who choose to give birth at home accept the risk that complications may arise. Training midwives and persuading mothers and significant others who make decisions about the value of referring women to hospitals at the onset of life-threatening complications are central factors to increasing the use of available hospitals. The hospitals must be safe, comfortable and attractive environments for parturition and should give appropriate consideration to the ethical and cultural concerns of the women. Appropriate management of financial and insurance-related issues can help midwives and mothers make a rational decision when complications arise.</p
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