633 research outputs found
Similar ultrafast dynamics of several dissimilar Dirac and Weyl semimetals
Recent years have seen the rapid discovery of solids whose low-energy
electrons have a massless, linear dispersion, such as Weyl, line-node, and
Dirac semimetals. The remarkable optical properties predicted in these
materials show their versatile potential for optoelectronic uses. However,
little is known of their response in the picoseconds after absorbing a photon.
Here we measure the ultrafast dynamics of four materials that share non-trivial
band structure topology but that differ chemically, structurally, and in their
low-energy band structures: ZrSiS, which hosts a Dirac line node and Dirac
points; TaAs and NbP, which are Weyl semimetals; and
SrMnSb, in which Dirac fermions coexist with broken
time-reversal symmetry. After photoexcitation by a short pulse, all four relax
in two stages, first sub-picosecond, and then few-picosecond. Their rapid
relaxation suggests that these and related materials may be suited for optical
switches and fast infrared detectors. The complex change of refractive index
shows that photoexcited carrier populations persist for a few picoseconds
Atom lithography using MRI-type feature placement
We demonstrate the use of frequency-encoded light masks in neutral atom
lithography. We demonstrate that multiple features can be patterned across a
monotonic potential gradient. Features as narrow as 0.9 microns are fabricated
on silicon substrates with a metastable argon beam. Internal state manipulation
with such a mask enables continuously adjustable feature positions and feature
densities not limited by the optical wavelength, unlike previous light masks.Comment: 4 pages, 4 figure
Restricted and unrestricted Hartree-Fock calculations of conductance for a quantum point contact
Very short quantum wires (quantum contacts) exhibit a conductance structure
at a value of conductance close to . It is believed that the
structure arises due to the electron-electron interaction, and it is also
related to electron spin. However details of the mechanism of the structure are
not quite clear. Previously we approached the problem within the restricted
Hartree-Fock approximation. This calculation demonstrated a structure similar
to that observed experimentally. In the present work we perform restricted and
unrestricted Hartree-Fock calculations to analyze the validity of the
approximations. We also consider dependence of the effect on the electron
density in leads. The unrestricted Hartree-Fock method allows us to analyze
trapping of the single electron within the contact. Such trapping would result
in the Kondo model for the ``0.7 structure''. The present calculation confirms
the spin-dependent bound state picture and does not confirm the Kondo model
scenario.Comment: 6 pages, 9 figure
Mitochondrial Control Region and microsatellite analyses on harbour porpoise (Phocoena phocoena) unravel population differentiation in the Baltic Sea and adjacent waters
The population status of the harbour porpoise (Phocoena phocoena) in the Baltic area has been a continuous matter of debate. Here we present the by far most comprehensive genetic population structure assessment to date for this region, both with regard to geographic coverage and sample size: 497 porpoise samples from North Sea, Skagerrak, Kattegat, Belt Sea, and Inner Baltic Sea were sequenced at the mitochondrial Control Region and 305 of these specimens were typed at 15 polymorphic microsatellite loci. Samples were stratified according to sample type (stranding vs. by-caught), sex, and season (breeding vs. non-breeding season). Our data provide ample evidence for a population split between the Skagerrak and the Belt Sea, with a transition zone in the Kattegat area. Among other measures, this was particularly visible in significant frequency shifts of the most abundant mitochondrial haplotypes. A particular haplotype almost absent in the North Sea was the most abundant in Belt Sea and Inner Baltic Sea. Microsatellites yielded a similar pattern (i.e., turnover in occurrence of clusters identified by STRUCTURE). Moreover, a highly significant association between microsatellite assignment and unlinked mitochondrial haplotypes further indicates a split between North Sea and Baltic porpoises. For the Inner Baltic Sea, we consistently recovered a small, but significant separation from the Belt Sea population. Despite recent arguments that separation should exceed a predefined threshold before populations shall be managed separately, we argue in favour of precautionary acknowledging the Inner Baltic porpoises as a separate management unit, which should receive particular attention, as it is threatened by various factors, in particular local fishery measures. © Springer Science+Business Media B.V. 2009
Extreme sensitivity of the spin-splitting and 0.7 anomaly to confining potential in one-dimensional nanoelectronic devices
Quantum point contacts (QPCs) have shown promise as nanoscale spin-selective
components for spintronic applications and are of fundamental interest in the
study of electron many-body effects such as the 0.7 x 2e^2/h anomaly. We report
on the dependence of the 1D Lande g-factor g* and 0.7 anomaly on electron
density and confinement in QPCs with two different top-gate architectures. We
obtain g* values up to 2.8 for the lowest 1D subband, significantly exceeding
previous in-plane g-factor values in AlGaAs/GaAs QPCs, and approaching that in
InGaAs/InP QPCs. We show that g* is highly sensitive to confinement potential,
particularly for the lowest 1D subband. This suggests careful management of the
QPC's confinement potential may enable the high g* desirable for spintronic
applications without resorting to narrow-gap materials such as InAs or InSb.
The 0.7 anomaly and zero-bias peak are also highly sensitive to confining
potential, explaining the conflicting density dependencies of the 0.7 anomaly
in the literature.Comment: 23 pages, 7 figure
Using coherent phonons for ultrafast control of the Dirac node of SrMnSb\u3csub\u3e2\u3c/sub\u3e
SrMnSb2 is a candidate Dirac semimetal whose electrons near the Y point have the linear dispersion and low mass of a Dirac cone. Here we demonstrate that ultrafast, 800-nm optical pulses can launch coherent phonon oscillations in Sr0.94Mn0.92Sb2, particularly an Ag mode at 4.4 THz. Through first-principles calculations of the electronic and phononic structure of SrMnSb2, we show that high-amplitude oscillations of this mode would displace the atoms in a way that transiently opens and closes a gap at the node of the Dirac cone. The ability to control the nodal gap on a subpicosecond timescale could create opportunities for the design and manipulation of Dirac fermions
Interaction Effects in a One-Dimensional Constriction
We have investigated the transport properties of one-dimensional (1D)
constrictions defined by split-gates in high quality GaAs/AlGaAs
heterostructures. In addition to the usual quantized conductance plateaus, the
equilibrium conductance shows a structure close to , and in
consolidating our previous work [K.~J. Thomas et al., Phys. Rev. Lett. 77, 135
(1996)] this 0.7 structure has been investigated in a wide range of samples as
a function of temperature, carrier density, in-plane magnetic field
and source-drain voltage . We show that the 0.7
structure is not due to transmission or resonance effects, nor does it arise
from the asymmetry of the heterojunction in the growth direction. All the 1D
subbands show Zeeman splitting at high , and in the wide channel
limit the -factor is , close to that of bulk GaAs.
As the channel is progressively narrowed we measure an exchange-enhanced
-factor. The measurements establish that the 0.7 structure is related to
spin, and that electron-electron interactions become important for the last few
conducting 1D subbands.Comment: 8 pages, 7 figures (accepted in Phys. Rev. B
Gamow Shell Model Description of Weakly Bound Nuclei and Unbound Nuclear States
We present the study of weakly bound, neutron-rich nuclei using the nuclear
shell model employing the complex Berggren ensemble representing the bound
single-particle states, unbound Gamow states, and the non-resonant continuum.
In the proposed Gamow Shell Model, the Hamiltonian consists of a one-body
finite depth (Woods-Saxon) potential and a residual two-body interaction. We
discuss the basic ingredients of the Gamow Shell Model. The formalism is
illustrated by calculations involving {\it several} valence neutrons outside
the double-magic core: He and O.Comment: 19 pages, 20 encapsulated PostScript figure
Theoretical description of deformed proton emitters: nonadiabatic coupled-channel method
The newly developed nonadiabatic method based on the coupled-channel
Schroedinger equation with Gamow states is used to study the phenomenon of
proton radioactivity. The new method, adopting the weak coupling regime of the
particle-plus-rotor model, allows for the inclusion of excitations in the
daughter nucleus. This can lead to rather different predictions for lifetimes
and branching ratios as compared to the standard adiabatic approximation
corresponding to the strong coupling scheme. Calculations are performed for
several experimentally seen, non-spherical nuclei beyond the proton dripline.
By comparing theory and experiment, we are able to characterize the angular
momentum content of the observed narrow resonance.Comment: 12 pages including 10 figure
The impact of HIV and antiretroviral therapy on TB risk in children: a systematic review and meta-analysis.
BACKGROUND: Children (<15â
years) are vulnerable to TB disease following infection, but no systematic review or meta-analysis has quantified the effects of HIV-related immunosuppression or antiretroviral therapy (ART) on their TB incidence. OBJECTIVES: Determine the impact of HIV infection and ART on risk of incident TB disease in children. METHODS: We searched MEDLINE and Embase for studies measuring HIV prevalence in paediatric TB cases ('TB cohorts') and paediatric HIV cohorts reporting TB incidence ('HIV cohorts'). Study quality was assessed using the Newcastle-Ottawa tool. TB cohorts with controls were meta-analysed to determine the incidence rate ratio (IRR) for TB given HIV. HIV cohort data were meta-analysed to estimate the trend in log-IRR versus CD4%, relative incidence by immunological stage and ART-associated protection from TB. RESULTS: 42 TB cohorts and 22 HIV cohorts were included. In the eight TB cohorts with controls, the IRR for TB was 7.9 (95% CI 4.5 to 13.7). HIV-infected children exhibited a reduction in IRR of 0.94 (95% credible interval: 0.83-1.07) per percentage point increase in CD4%. TB incidence was 5.0 (95% CI 4.0 to 6.0) times higher in children with severe compared with non-significant immunosuppression. TB incidence was lower in HIV-infected children on ART (HR: 0.30; 95% CI 0.21 to 0.39). Following initiation of ART, TB incidence declined rapidly over 12â
months towards a HR of 0.10 (95% CI 0.04 to 0.25). CONCLUSIONS: HIV is a potent risk factor for paediatric TB, and ART is strongly protective. In HIV-infected children, early diagnosis and ART initiation reduces TB risk. TRIAL REGISTRATION NUMBER: CRD42014014276
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