120 research outputs found
Magnetically controlled current flow in coupled-dot arrays
Quantum transport through an open periodic array of up to five dots is
investigated in the presence of a magnetic field. The device spectrum exhibits
clear features of the band structure of the corresponding one-dimensional
artificial crystal which evolves with varying field. A significant magnetically
controlled current flow is induced with changes up to many orders of magnitude
depending on temperature and material parameters. Our results put forward a
simple design for measuring with current technology the magnetic subband
formation of quasi one-dimensional Bloch electrons.Comment: 9 pages, 5 figure
Magnetic Miniband Structure and Quantum Oscillations in Lateral Semiconductor Superlattices
We present fully quantum-mechanical magnetotransport calculations for
short-period lateral superlattices with one-dimensional electrostatic
modulation. A non-perturbative treatment of both magnetic field and modulation
potential proves to be necessary to reproduce novel quantum oscillations in the
magnetoresistance found in recent experiments in the resistance component
parallel to the modulation potential. In addition, we predict oscillations of
opposite phase in the component perpendicular to the modulation not yet
observed experimentally. We show that the new oscillations originate from the
magnetic miniband structure in the regime of overlapping minibands.Comment: 6 pages with 4 figure
Magnetotransport in Two-Dimensional Electron Systems with Spin-Orbit Interaction
We present magnetotransport calculations for homogeneous two-dimensional
electron systems including the Rashba spin-orbit interaction, which mixes the
spin-eigenstates and leads to a modified fan-chart with crossing Landau levels.
The quantum mechanical Kubo formula is evaluated by taking into account
spin-conserving scatterers in an extension of the self-consistent Born
approximation that considers the spin degree of freedom. The calculated
conductivity exhibits besides the well-known beating in the Shubnikov-de Haas
(SdH) oscillations a modulation which is due to a suppression of scattering
away from the crossing points of Landau levels and does not show up in the
density of states. This modulation, surviving even at elevated temperatures
when the SdH oscillations are damped out, could serve to identify spin-orbit
coupling in magnetotransport experiments. Our magnetotransport calculations are
extended also to lateral superlattices and predictions are made with respect to
1/B periodic oscillations in dependence on carrier density and strength of the
spin-orbit coupling.Comment: 8 pages including 8 figures; submitted to PR
Inverse flux quantum periodicity of magnetoresistance oscillations in two-dimensional short-period surface superlattices
Transport properties of the two-dimensional electron gas (2DEG) are
considered in the presence of a perpendicular magnetic field and of a {\it
weak} two-dimensional (2D) periodic potential modulation in the 2DEG plane. The
symmetry of the latter is rectangular or hexagonal. The well-known solution of
the corresponding tight-binding equation shows that each Landau level splits
into several subbands when a rational number of flux quanta pierces the
unit cell and that the corresponding gaps are exponentially small. Assuming the
latter are closed due to disorder gives analytical wave functions and
simplifies considerably the evaluation of the magnetoresistivity tensor
. The relative phase of the oscillations in and
depends on the modulation periods involved. For a 2D modulation
with a {\bf short} period nm, in addition to the Weiss oscillations
the collisional contribution to the conductivity and consequently the tensor
show {\it prominent peaks when one flux quantum passes
through an integral number of unit cells} in good agreement with recent
experiments. For periods nm long used in early experiments, these
peaks occur at fields 10-25 times smaller than those of the Weiss oscillations
and are not resolved
First Evidence of Immunomodulation in Bivalves under Seawater Acidification and Increased Temperature
Water acidification, temperature increases and changes in seawater salinity are predicted to occur in the near future. In such a global climate change (GCC) scenario, there is growing concern for the health status of both wild and farmed organisms. Bivalve molluscs, an important component of coastal marine ecosystems, are at risk. At the immunological level, the ability of an organism to maintain its immunosurveillance unaltered under adverse environmental conditions may enhance its survival capability. To our knowledge, only a few studies have investigated the effects of changing environmental parameters (as predicted in a GCC scenario) on the immune responses of bivalves. In the present study, the effects of both decreased pH values and increased temperature on the important immune parameters of two bivalve species were evaluated for the first time. The clam Chamelea gallina and the mussel Mytilus galloprovincialis, widespread along the coast of the Northwestern Adriatic Sea, were chosen as model organisms. Bivalves were exposed for 7 days to three pH values (8.1, 7.7 and 7.4) at two temperatures (22 and 28°C). Three independent experiments were carried out at salinities of 28, 34 and 40 PSU. The total haemocyte count, Neutral Red uptake, haemolymph lysozyme activity and total protein levels were measured. The results obtained demonstrated that tested experimental conditions affected significantly most of the immune parameters measured in bivalves, even if the variation pattern of haemocyte responses was not always linear. Between the two species, C. gallina appeared more vulnerable to changing pH and temperature than M. galloprovincialis. Overall, this study demonstrated that climate changes can strongly affect haemocyte functionality in bivalves. However, further studies are needed to clarify better the mechanisms of action of changing environmental parameters, both individually and in combination, on bivalve haemocytes
Helical spin-density wave in doped V2O3
Recent neutron scattering and nuclear magnetic resonance experiments have
revealed that the low temperature phase of doped V_{2-y}O_3 is an itinerant
antiferromagnet with a helical spin structure. We use a band structure
calculation as the point of departure to show that these experiments are in
agreement with mean field results for an Overhauser spin-density wave state.
The influences of a finite life-time and of dilute magnetic impurities are
discussed.Comment: 6 pages RevTex incl. 7 postscript figures, to be published by PR
Long-term and trans-life-cycle effects of exposure to ocean acidification in the green sea urchin Strongylocentrotus droebachiensis
Anthropogenic CO2 emissions are acidifying the world’s oceans. A growing body of evidence demonstrates that ocean acidification can impact survival, growth, development and physiology of marine invertebrates. Here, we tested the impact of long-term (up to 16 months) and trans-life-cycle (adult, embryo/larvae and juvenile) exposure to elevated pCO2 (1,200 μatm, compared to control 400 μatm) on the green sea urchin Strongylocentrotus droebachiensis. Female fecundity was decreased 4.5-fold when acclimated to elevated pCO2 for 4 months during reproductive conditioning, while no difference was observed in females acclimated for 16 months. Moreover, adult pre-exposure for 4 months to elevated pCO2 had a direct negative impact on subsequent larval settlement success. Five to nine times fewer offspring reached the juvenile stage in cultures using gametes collected from adults previously acclimated to high pCO2 for 4 months. However, no difference in larval survival was observed when adults were pre-exposed for 16 months to elevated pCO2. pCO2 had no direct negative impact on juvenile survival except when both larvae and juveniles were raised in elevated pCO2. These negative effects on settlement success and juvenile survival can be attributed to carry-over effects from adults to larvae and from larvae to juveniles. Our results support the contention that adult sea urchins can acclimate to moderately elevated pCO2 in a matter of a few months and that carry-over effects can exacerbate the negative impact of ocean acidification on larvae and juveniles
Cost-minimization analysis in a blind randomized trial on small-incision versus laparoscopic cholecystectomy from a societal perspective: sick leave outweighs efforts in hospital savings
Background: After its introduction, laparoscopic cholecystectomy rapidly expanded around the world and was accepted the procedure of choice by consensus. However, analysis of evidence shows no difference regarding primary outcome measures between laparoscopic and small-incision cholecystectomy. In absence of clear clinical benefit it may be interesting to focus on the resource use associated with the available techniques, a secondary outcome measure. This study focuses on a difference in costs between laparoscopic and small-incision cholecystectomy from a societal perspective with emphasis on internal validity and generalisability Methods: A blinded randomized single-centre trial was conducted in a general teaching hospital in The Netherlands. Patients with reasonable to good health diagnosed with symptomatic cholecystolithiasis scheduled for cholecystectomy were included. Patients were randomized between laparoscopic and small-incision cholecystectomy. Total costs were analyzed from a societal perspective. Results: Operative costs were higher in the laparoscopic group using reusable laparoscopic instruments (difference 203 euro; 95% confidence interval 147 to 259 euro). There were no significant differences in the other direct cost categories (outpatient clinic and admittance related costs), indirect costs, and total costs. More than 60% of costs in employed patients were caused by sick leave. Conclusion: Based on differences in costs, small-incision cholecystectomy seems to be the preferred operative technique over the laparoscopic technique both from a hospital and societal cost perspective. Sick leave associated with convalescence after cholecystectomy in employed patients results in considerable costs to society
Effects of elevated seawater pCO2 on gene expression patterns in the gills of the green crab, Carcinus maenas
Background: The green crab Carcinus maenas is known for its high acclimation potential to varying environmental
abiotic conditions. A high ability for ion and acid-base regulation is mainly based on an efficient regulation
apparatus located in gill epithelia. However, at present it is neither known which ion transport proteins play a key
role in the acid-base compensation response nor how gill epithelia respond to elevated seawater pCO2 as
predicted for the future. In order to promote our understanding of the responses of green crab acid-base
regulatory epithelia to high pCO2, Baltic Sea green crabs were exposed to a pCO2 of 400 Pa. Gills were screened
for differentially expressed gene transcripts using a 4,462-feature microarray and quantitative real-time PCR.
Results: Crabs responded mainly through fine scale adjustment of gene expression to elevated pCO2. However, 2%
of all investigated transcripts were significantly regulated 1.3 to 2.2-fold upon one-week exposure to CO2 stress.
Most of the genes known to code for proteins involved in osmo- and acid-base regulation, as well as cellular stress
response, were were not impacted by elevated pCO2. However, after one week of exposure, significant changes
were detected in a calcium-activated chloride channel, a hyperpolarization activated nucleotide-gated potassium
channel, a tetraspanin, and an integrin. Furthermore, a putative syntaxin-binding protein, a protein of the
transmembrane 9 superfamily, and a Cl-/HCO3
- exchanger of the SLC 4 family were differentially regulated. These
genes were also affected in a previously published hypoosmotic acclimation response study.
Conclusions: The moderate, but specific response of C. maenas gill gene expression indicates that (1) seawater
acidification does not act as a strong stressor on the cellular level in gill epithelia; (2) the response to hypercapnia
is to some degree comparable to a hypoosmotic acclimation response; (3) the specialization of each of the
posterior gill arches might go beyond what has been demonstrated up to date; and (4) a re-configuration of gill
epithelia might occur in response to hypercapnia
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