26 research outputs found
Spin and orbital fluctuations in non-equilibrium transport through quantum dots: A renormalisation-group analysis
We study non-equilibrium current and occupation probabilities of a
two-orbital quantum dot. The couplings to the leads are allowed to be
asymmetric and orbital dependent as it is generically the case in transport
experiments on molecules and nanowires. Starting from a two-orbital Anderson
model, we perform a generalised Schrieffer-Wolff transformation to derive an
effective Kondo model. This generates an orbital potential scattering
contribution which is of the same order as the spin exchange interaction. In a
first perturbative analysis we identify a regime of negative differential
conductance and a cascade resonance in the presence of an external magnetic
field, which both originate from the non-equilibrium occupation of the
orbitals. We then study the logarithmic enhancement of these signatures by
means of a renormalisation-group treatment. We find that the orbital potential
scattering qualitatively changes the renormalisation of the spin exchange
couplings and strongly affects the differential conductance for asymmetric
couplings.Comment: 6 pages, 4 figures, revised version as publishe
Transconductance of a double quantum dot system in the Kondo regime
We consider a lateral double-dot system in the Coulomb blockade regime with a
single spin-1/2 on each dot, mutually coupled by an anti-ferromagnetic exchange
interaction. Each of the two dots is contacted by two leads. We demonstrate
that the voltage across one of the dots will have a profound influence on the
current passing through the other dot. Using Poor Man's scaling, we find that
the Kondo-effect can lead to a strong enhancement of this {\it
transconductance}.Comment: updated to published versio
Nonequilibrium Cotunneling through a Three-Level Quantum Dot
We calculate the nonlinear cotunneling conductance through a quantum dot with
3 electrons occupying the three highest lying energy levels. Starting from a
3-orbital Anderson model, we apply a generalized Schrieffer-Wolff
transformation to derive an effective Kondo model for the system. Within this
model we calculate the nonequilibrium occupation numbers and the corresponding
cotunneling current to leading order in the exchange couplings. We identify the
inelastic cotunneling thresholds and their splittings with applied magnetic
field, and make a qualitative comparison to recent experimental data on carbon
nanotube and InAs quantum-wire quantum dots. Further predictions of the model
like cascade resonances and a magnetic-field dependence of the orbital level
splitting are not yet observed but within reach of recent experimental work on
carbon nanotube and InAs nanowire quantum dots.Comment: 12 pages, 13 figure
Transport properties of a superconducting single-electron transistor coupled to a nanomechanical oscillator
We investigate a superconducting single-electron transistor capacitively
coupled to a nanomechanical oscillator and focus on the double Josephson
quasiparticle resonance. The existence of two coherent Cooper pair tunneling
events is shown to lead to pronounced backaction effects. Measuring the current
and the shot noise provides a direct way of gaining information on the state of
the oscillator. In addition to an analytical discussion of the linear-response
regime, we discuss and compare results of higher-order approximation schemes
and a fully numerical solution. We find that cooling of the mechanical
resonator is possible, and that there are driven and bistable oscillator states
at low couplings. Finally, we also discuss the frequency dependence of the
charge noise and the current noise of the superconducting single electron
transistor.Comment: 19 pages, 11 figures, published in PR
Nonequilibrium transport through a spinful quantum dot with superconducting leads
We study the nonlinear cotunneling current through a spinful quantum dot
contacted by two superconducting leads. Applying a general nonequilibrium Green
function formalism to an effective Kondo model, we study the rich variation in
the IV-characteristics with varying asymmetry in the tunnel coupling to source
and drain electrodes. The current is found to be carried respectively by
multiple Andreev reflections in the symmetric limit, and by spin-induced
Yu-Shiba-Russinov bound states in the strongly asymmetric limit. The interplay
between these two mechanisms leads to qualitatively different
IV-characteristics in the cross-over regime of intermediate symmetry,
consistent with recent experimental observations of negative differential
conductance and re-positioned conductance peaks in sub-gap cotunneling
spectroscopy.Comment: 5 pages, 3 fig
Nonequilibrium transport via spin-induced sub-gap states in superconductor/quantum dot/normal metal cotunnel junctions
We study low-temperature transport through a Coulomb blockaded quantum dot
(QD) contacted by a normal (N), and a superconducting (S) electrode. Within an
effective cotunneling model the conduction electron self energy is calculated
to leading order in the cotunneling amplitudes and subsequently resummed to
obtain the nonequilibrium T-matrix, from which we obtain the nonlinear
cotunneling conductance. For even occupied dots the system can be conceived as
an effective S/N-cotunnel junction with subgap transport mediated by Andreev
reflections. The net spin of an odd occupied dot, however, leads to the
formation of sub-gap resonances inside the superconducting gap which gives rise
to a characteristic peak-dip structure in the differential conductance, as
observed in recent experiments.Comment: 13 pages, 13 figures (new version contains reformulations and
corrections of typos etc
Electronic charge and orbital reconstruction at cuprate-titanate interfaces
In complex transition metal oxide heterostructures of physically dissimilar
perovskite compounds, interface phenomena can lead to novel physical properties
not observed in either of their constituents. This remarkable feature opens new
prospects for technological applications in oxide electronic devices based on
nm-thin oxide films. Here we report on a significant electronic charge and
orbital reconstruction at interfaces between YBa2Cu3O6 and SrTiO3 studied using
local spin density approximation (LSDA) with intra-atomic Coulomb repulsion
(LSDA+U). We show that the interface polarity results in the metallicity of
cuprate-titanate superlattices with the hole carriers concentrated
predominantly in the CuO2 and BaO layers and in the first interface TiO2 and
SrO planes. We also find that the interface structural relaxation causes a
strong change of orbital occupation of Cu 3d orbitals in the CuO2 layers. The
concomitant change of Cu valency from +2 to +3 is related to the partial
occupation of the Cu orbitals at the interface with SrO planes
terminating SrTiO3. Interface-induced predoping and orbital reconstruction in
CuO2 layers are key mechanisms which control the superconducting properties of
field-effect devices developed on the basis of cuprate-titanate
heterostructures.Comment: 11 pages, 8 figures, to appear in the "Proceedings of Third Joint
HLRB and KONWIHR Result and Reviewing Workshop", Springer 200
Participant engagement with a UK community-based preschool childhood obesity prevention programme: : a focused ethnography study
Background
Children’s centres in the UK provide a setting for public health programmes; offering support to families living in the most disadvantaged areas where obesity prevalence is at its highest. Health, Exercise and Nutrition in the Really Young (HENRY) is an eight-week obesity prevention programme currently delivered in children’s centres across the UK. However, low participant engagement in some local authorities threatens its potential reach and impact. This study aimed to explore the factors influencing participant engagement with HENRY to describe where local intervention may support engagement efforts.
Method
A focused ethnography study was undertaken in five children’s centres delivering HENRY across the UK. One hundred and ninety hours of field observations, 22 interviews with staff (commissioners, HENRY co-ordinators, managers and facilitators) and six focus groups (36 parents), took place over five consecutive days in each centre. The Consolidated Framework for Implementation Research (CFIR) was used to guide the observations and analysis of the data.
Results
Three overarching themes described the factors influencing participant engagement with HENRY: local authority decision making around children’s centre programmes; children’s centre implementation of HENRY; and the participant experience of HENRY. The results indicate that factors influencing participant engagement with public health programmes begin at the commissioning body level, influencing children’s centre implementation and subsequently the experience of participants. Local authority funding priorities and constraints influence availability of places and who these places are offered to, with funding often targeted towards those deemed most at need. This was perceived to have a detrimental effect on participant experience of the programme.
Conclusion
In summary, participant engagement is affected by multiple factors, working at different levels of the children’s centre and local authority hierarchy, most of which are at play even before participants decide whether or not they choose to enrol and maintain attendance. For programmes to achieve their optimal reach and impact, factors at the commissioning and local implementation level need to be addressed prior to addressing participant facing issues
Seasonality of nitrogen sources, cycling, and loading in a New England river discerned from nitrate isotope ratios
Coastal waters globally are increasingly impacted due to the anthropogenic
loading of nitrogen (N) from the watershed. To assess dominant sources
contributing to the eutrophication of the Little Narragansett Bay estuary in
New England, we carried out an annual study of N loading from the Pawcatuck
River. We conducted weekly monitoring of nutrients and nitrate
(NO3-) isotope ratios (15N / 14N, 18O / 16O, and
17O / 16O) at the mouth of the river and from the larger of two
wastewater treatment facilities (WWTFs) along the estuary, as well as
seasonal along-river surveys. Our observations reveal a direct relationship
between N loading and the magnitude of river discharge and a consequent
seasonality to N loading into the estuary – rendering loading from the
WWTFs and from an industrial site more important at lower river flows during
warmer months, comprising ∼ 23 % and ∼ 18 % of N loading,
respectively. Riverine nutrients derived predominantly from deeper
groundwater and the industrial point source upriver in summer and from
shallower groundwater and surface flow during colder months – wherein
NO3- associated with deeper groundwater had higher
15N / 14N ratios than shallower groundwater. Corresponding
NO3- 18O / 16O ratios were lower during the warm season,
due to increased biological cycling in-river. Uncycled atmospheric
NO3-, detected from its unique mass-independent NO3-
17O / 16O vs. 18O / 16O fractionation, accounted for
< 3 % of riverine NO3-, even at elevated discharge.
Along-river, NO3- 15N / 14N ratios showed a correspondence
to regional land use, increasing from agricultural and forested catchments
to the more urbanized watershed downriver. The evolution of
18O / 16O isotope ratios along-river conformed to the notion of
nutrient spiraling, reflecting the input of NO3- from the
catchment and from in-river nitrification and its coincident removal by
biological consumption. These findings stress the importance of considering
seasonality of riverine N sources and loading to mitigate eutrophication in
receiving estuaries. Our study further advances a conceptual framework that
reconciles with the current theory of riverine nutrient cycling, from which
to robustly interpret NO3- isotope ratios to constrain cycling and
source partitioning in river systems.</p