1,331 research outputs found
Far-Infrared Excitations below the Kohn Mode: Internal Motion in a Quantum Dot
We have investigated the far-infrared response of quantum dots in modulation
doped GaAs heterostructures. We observe novel modes at frequencies below the
center-of-mass Kohn mode. Comparison with Hartree-RPA calculations show that
these modes arise from the flattened potential in our field-effect confined
quantum dots. They reflect pronounced relative motion of the charge density
with respect to the center-of-mass.Comment: 8 pages, LaTeX with integrated 6 PostScript figure
Oscillation modes of two-dimensional nanostructures within the time-dependent local-spin-density approximation
We apply the time-dependent local-spin-density approximation as general
theory to describe ground states and spin-density oscillations in the linear
response regime of two-dimensional nanostructures of arbitrary shape. For this
purpose, a frequency analysis of the simulated real-time evolution is
performed. The effect on the response of the recently proposed spin-density
waves in the ground state of certain parabolic quantum dots is considered. They
lead to the prediction of a new class of excitations, soft spin-twist modes,
with energies well below that of the spin dipole oscillation.Comment: 4 RevTex pages and 4 GIF figures, accepted in PR
Gateway vectors for efficient artificial gene assembly in vitro and expression in yeast Saccharomyces cerevisiae
Peer reviewedPublisher PD
Smy2p Participates in COPII Vesicle Formation Through the Interaction with Sec23p/Sec24p Subcomplex
The coat protein complex II (COPII) is essential for vesicle formation from the endoplasmic reticulum (ER) and is composed of two heterodimeric subcomplexes, Sec23p/Sec24p and Sec13p/Sec31p, and the small guanosine triphosphatase Sar1p. In an effort to identify novel factors that may participate in COPII vesicle formation, we isolated SMY2, a yeast gene encoding a protein of unknown function, as a multicopy suppressor of the temperature-sensitive sec24-20 mutant. We found that even a low-copy expression of SMY2 was sufficient for the suppression of the sec24-20 phenotypes, and the chromosomal deletion of SMY2 led to a severe growth defect in the sec24-20 background. In addition, SMY2 exhibited genetic interactions with several other genes involved in the ER-to-Golgi transport. Subcellular fractionation analysis showed that Smy2p was a peripheral membrane protein fractionating together with COPII components. However, Smy2p was not loaded onto COPII vesicles generated in vitro. Interestingly, coimmunoprecipitation between Smy2p and the Sec23p/Sec24p subcomplex was specifically observed in sec23-1 and sec24-20 backgrounds, suggesting that this interaction was a prerequisite for the suppression of the sec24-20 phenotypes by overexpression of SMY2. We propose that Smy2p is located on the surface of the ER and facilitates COPII vesicle formation through the interaction with Sec23p/Sec24p subcomplex
3D atomic structure from a single XFEL pulse
X-ray Free Electron Lasers (XFEL) are the most advanced pulsed x-ray sources.
Their extraordinary pulse parameters promise unique applications. Indeed,
several new methods have been developed at XFEL-s. However, no methods are
known, which would allow ab initio atomic level structure determination using
only a single XFEL pulse. Here, we present experimental results, demonstrating
the determination of the 3D atomic structure from data obtained during a single
25 fs XFEL pulse. Parallel measurement of hundreds of Bragg reflections was
done by collecting Kossel line patterns of GaAs and GaP. With these
measurements, we reached the ultimate temporal limit of the x-ray structure
solution possible today. These measurements open the way for studying
non-repeatable fast processes and structural transformations in crystals for
example measuring the atomic structure of matter at extremely non-ambient
conditions or transient structures formed in irreversible physical, chemical,
or biological processes. It would also facilitate time resolved pump-probe
structural studies making them significantly shorter than traditional serial
crystallography.Comment: 16 pages of manuscript followed by 15 pages of supplementary
informatio
Expected resolution limits of x-ray free-electron laser single-particle imaging for realistic source and detector properties
The unprecedented intensity of x-ray free-electron laser sources has enabled single-particle x-ray diffraction imaging (SPI) of various biological specimens in both two-dimensional projection and three dimensions (3D). The potential of studying protein dynamics in their native conditions, without crystallization or chemical staining, has encouraged researchers to aim for increasingly higher resolutions with this technique. The currently achievable resolution of SPI is limited to the sub-10 nanometer range, mainly due to background effects, such as instrumental noise and parasitic scattering from the carrier gas used for sample delivery. Recent theoretical studies have quantified the effects of x-ray pulse parameters, as well as the required number of diffraction patterns to achieve a certain resolution, in a 3D reconstruction, although the effects of detector noise and the random particle orientation in each diffraction snapshot were not taken into account. In this work, we show these shortcomings and address limitations on achievable image resolution imposed by the adaptive gain integrating pixel detector noise
AC-conductance of a quantum wire with electron-electron interaction
The complex ac-response of a quasi-one dimensional electron system in the
one-band approximation with an interaction potential of finite range is
investigated. It is shown that linear response is exact for this model. The
influence of the screening of the electric field is discussed. The complex
absorptive conductance is analyzed in terms of resistive, capacitive and
inductive behaviors.Comment: 13 pages, REVTeX, 7 eps figures, to appear in Phys. Rev.
Voltage-tunable singlet-triplet transition in lateral quantum dots
Results of calculations and high source-drain transport measurements are
presented which demonstrate voltage-tunable entanglement of electron pairs in
lateral quantum dots. At a fixed magnetic field, the application of a
judiciously-chosen gate voltage alters the ground-state of an electron pair
from an entagled spin singlet to a spin triplet.Comment: 8.2 double-column pages, 10 eps figure
What do general practitioners know about ADHD? Attitudes and knowledge among first-contact gatekeepers: systematic narrative review
Background: Attention Deficit Hyperactivity Disorder (ADHD) is a common childhood disorder with international prevalence estimates of 5 % in childhood, yet significant evidence exists that far fewer children receive ADHD services. In many countries, ADHD is assessed and diagnosed in specialist mental health or neuro-developmental paediatric clinics, to which referral by General (Family) Practitioners (GPs) is required. In such ‘gatekeeper’ settings, where GPs act as a filter to diagnosis and treatment, GPs may either not recognise potential ADHD cases, or may be reluctant to refer. This study systematically reviews the literature regarding GPs’ views of ADHD in such settings.
Methods: A search of nine major databases was conducted, with wide search parameters; 3776 records were initially retrieved. Studies were included if they were from settings where GPs are typically gatekeepers to ADHD services; if they addressed GPs’ ADHD attitudes and knowledge; if methods were clearly described; and if results for GPs were reported separately from those of other health professionals.
Results: Few studies specifically addressed GP attitudes to ADHD. Only 11 papers (10 studies), spanning 2000–2010, met inclusion criteria, predominantly from the UK, Europe and Australia. As studies varied methodologically, findings are reported as a thematic narrative, under the following themes: Recognition rate; ADHD controversy (medicalisation, stigma, labelling); Causes of ADHD; GPs and ADHD diagnosis; GPs and ADHD treatment; GP ADHD training and sources of information; and Age, sex differences in knowledge and attitudes.
Conclusions: Across times and settings, GPs practising in first-contact gatekeeper settings had mixed and often unhelpful attitudes regarding the validity of ADHD as a construct, the role of medication and how parenting contributed to presentation. A paucity of training was identified, alongside a reluctance of GPs to become involved in shared care practice. If access to services is to be improved for possible ADHD cases, there needs to be a focused and collaborative approach to training
Far-infrared edge modes in quantum dots
We have investigated edge modes of different multipolarity sustained by
quantum dots submitted to external magnetic fields. We present a microscopic
description based on a variational solution of the equation of motion for any
axially symmetric confining potential and multipole mode. Numerical results for
dots with different number of electrons whose ground-state is described within
a local Current Density Functional Theory are discussed. Two sum rules, which
are exact within this theory, are derived. In the limit of a large neutral dot
at B=0, we have shown that the classical hydrodynamic dispersion law for edge
waves \omega(q) \sim \sqrt{q \ln (q_0/q)} holds when quantum and finite size
effects are taken into account.Comment: We have changed some figures as well as a part of the tex
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