593 research outputs found
Spin relaxation and anticrossing in quantum dots: Rashba versus Dresselhaus spin-orbit coupling
The spin-orbit splitting of the electron levels in a two-dimensional quantum
dot in a perpendicular magnetic field is studied. It is shown that at the point
of an accidental degeneracy of the two lowest levels above the ground state the
Rashba spin-orbit coupling leads to a level anticrossing and to mixing of
spin-up and spin-down states, whereas there is no mixing of these levels due to
the Dresselhaus term. We calculate the relaxation and decoherence times of the
three lowest levels due to phonons. We find that the spin relaxation rate as a
function of a magnetic field exhibits a cusp-like structure for Rashba but not
for Dresselhaus spin-orbit interaction.Comment: 6 pages, 1 figur
On paraquaternionic submersions between paraquaternionic K\"ahler manifolds
In this paper we deal with some properties of a class of semi-Riemannian
submersions between manifolds endowed with paraquaternionic structures, proving
a result of non-existence of paraquaternionic submersions between
paraquaternionic K\"ahler non locally hyper paraK\"ahler manifolds. Then we
examine, as an example, the canonical projection of the tangent bundle, endowed
with the Sasaki metric, of an almost paraquaternionic Hermitian manifold.Comment: 13 pages, no figure
Feasibility of Data-Driven, Model-Free Quantitative MRI Protocol Design: Application to Brain and Prostate Diffusion-Relaxation Imaging
Brain; Protocol design; Quantitative MRI (qMRI)Cerebro; Diseño de protocolo; Resonancia magnética cuantitativa (qMRI)Cervell; Disseny del protocol; Ressonància magnètica quantitativa (qMRI)Purpose: We investigate the feasibility of data-driven, model-free quantitative MRI (qMRI) protocol design on in vivo brain and prostate diffusion-relaxation imaging (DRI).
Methods: We select subsets of measurements within lengthy pilot scans, without identifying tissue parameters for which to optimise for. We use the “select and retrieve via direct upsampling” (SARDU-Net) algorithm, made of a selector, identifying measurement subsets, and a predictor, estimating fully-sampled signals from the subsets. We implement both using artificial neural networks, which are trained jointly end-to-end. We deploy the algorithm on brain (32 diffusion-/T1-weightings) and prostate (16 diffusion-/T2-weightings) DRI scans acquired on three healthy volunteers on two separate 3T Philips systems each. We used SARDU-Net to identify sub-protocols of fixed size, assessing reproducibility and testing sub-protocols for their potential to inform multi-contrast analyses via the T1-weighted spherical mean diffusion tensor (T1-SMDT, brain) and hybrid multi-dimensional MRI (HM-MRI, prostate) models, for which sub-protocol selection was not optimised explicitly.
Results: In both brain and prostate, SARDU-Net identifies sub-protocols that maximise information content in a reproducible manner across training instantiations using a small number of pilot scans. The sub-protocols support T1-SMDT and HM-MRI multi-contrast modelling for which they were not optimised explicitly, providing signal quality-of-fit in the top 5% against extensive sub-protocol comparisons.
Conclusions: Identifying economical but informative qMRI protocols from subsets of rich pilot scans is feasible and potentially useful in acquisition-time-sensitive applications in which there is not a qMRI model of choice. SARDU-Net is demonstrated to be a robust algorithm for data-driven, model-free protocol design.This project was funded by the Engineering and Physical Sciences Research Council (EPSRC EP/R006032/1, M020533/1, G007748, I027084, N018702). This project has received funding under the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 634541 and 666992, and from: Rosetrees Trust (United Kingdom, funding FG); Prostate Cancer United Kingdom Targeted Call 2014 (Translational Research St.2, project reference PG14-018-TR2); Cancer Research United Kingdom grant ref. A21099; Spinal Research (United Kingdom), Wings for Life (Austria), Craig H. Neilsen Foundation (United States) for jointly funding the INSPIRED study; Wings for Life (#169111); United Kingdom Multiple Sclerosis Society (grants 892/08 and 77/2017); the Department of Health’s National Institute for Health Research (NIHR) Biomedical Research Centres and UCLH NIHR Biomedical Research Centre; Champalimaud Centre for the Unknown, Lisbon (Portugal); European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 101003390. FG is currently supported by the investigator-initiated PREdICT study at the Vall d’Hebron Institute of Oncology (Barcelona), funded by AstraZeneca and CRIS Cancer Foundation
Transferencia de Políticas activas de empleo para el Desarrollo Local de Pequeños Productores Ganaderos de Catuna, Provincia de La Rioja
El presente proyecto se encuentra en ejecución en la Localidad de Catuna, Dpto. Gral. Ocampo, Provincia de La Rioja. Los destinatarios directos son los beneficiarios del Seguro de Capacitación y Empleo, dependientes del Ministerio de Trabajo de la Nación, identificados como Pequeños Productores Ganaderos locales. El objetivo general del proyecto consiste en “Posibilitar la organización de PPG, mediante la transferencia de políticas activas de empleo como medio que favorezca el Desarrollo Local”. Este responde a las condiciones desfavorables con que los PPG en la zona de los llanos de La Rioja realizan su actividad. La Falta de asesoramiento técnico, capacitación permanente, de recursos económicos y de organización provoca en la ganadería un sistema de baja eficiencia económica que, sumada a la degradación del recurso forrajero y forestal, lo torna una actividad poco sustentable. Además, uno de los principales problemas de los PPG de esta zona, es la falta de organización en el sector, lo que los aleja del trabajo mancomunado y organizado que posibilite una óptima producción e identificación de potenciales fuentes financieras. A pesar de los problemas detectados, la actividad ganadera ocupa un lugar importante en la economía familiar; por ello, en la búsqueda de mejorar las condiciones de vida de las personas, como sujetos portadores de derechos, el Desarrollo Local cumple un rol fundamental. En un proceso de este tipo, es necesaria la colaboración de todos los actores, por ello, en el mismo participan la Extensión Académica de Catuna de la UNLaR, el Ministerio de Trabajo de la Nación y Comunidad local. Para ello se siguen dos líneas de acción:1)- Organizativa: donde se realizan acuerdos sobre las líneas generales de implementación del proyecto y encuentros con los PPG, para la planificación y realización de las actividades planteadas en el proyecto (reuniones informativas, jornadas de capacitación, talleres, encuentros de análisis y reflexión). Asimismo, se tienen pensadas instancias de evaluación y seguimiento del proyecto que tendrá como ejes de análisis el proceso de grupo, así como instancias sistemáticas de evaluaciones individuales y grupales que permitirán afianzar los conocimientos que se vayan construyendo entre todo el grupo. Por otra parte, existirá una instancia de coordinación y monitoreo a cargo del equipo responsable del proyecto. Otra de las líneas consiste en la “Innovación en la intervención” para promover la inserción de la Carrera de Trabajo Social en nuevos ámbitos de práctica, acorde a las estrategias e intervenciones que el medio demanda
Rashba spin-orbit coupling and spin relaxation in silicon quantum wells
Silicon is a leading candidate material for spin-based devices, and
two-dimensional electron gases (2DEGs) formed in silicon heterostructures have
been proposed for both spin transport and quantum dot quantum computing
applications. The key parameter for these applications is the spin relaxation
time. Here we apply the theory of D'yakonov and Perel' (DP) to calculate the
electron spin resonance linewidth of a silicon 2DEG due to structural inversion
asymmetry for arbitrary static magnetic field direction at low temperatures. We
estimate the Rashba spin-orbit coupling coefficient in silicon quantum wells
and find the and times of the spins from this mechanism as a
function of momentum scattering time, magnetic field, and device-specific
parameters. We obtain agreement with existing data for the angular dependence
of the relaxation times and show that the magnitudes are consistent with the DP
mechanism. We suggest how to increase the relaxation times by appropriate
device design.Comment: Extended derivations and info, fixed typos and refs, updated figs and
data. Worth a re-downloa
Understanding the Heteroatom Effect on the Ullmann Copper-Catalyzed Cross-Coupling of X-Arylation (X = NH, O, S) Mechanism
Density Functional Theory (DFT) calculations have been carried out in order to unravel the governing reaction mechanism in copper-catalyzed cross-coupling Ullmann type reactions between iodobenzene (1, PhI) and aniline (2-NH, PhNH2), phenol (2-O, PhOH) and thiophenol (2-S, PhSH) with phenanthroline (phen) as the ancillary ligand. Four different pathways for the mechanism were considered namely Oxidative Addition–Reductive Elimination (OA-RE), σ-bond Metathesis (MET), Single Electron Transfer (SET), and Halogen Atom Transfer (HAT). Our results suggest that the OA-RE route, involving CuIII intermediates, is the energetically most favorable pathway for all the systems considered. Interestingly, the rate-determining step is the oxidative addition of the phenyl iodide to the metal center regardless of the nature of the heteroatom. The computed energy barriers in OA increase in the order O < S < NH. Using the Activation Strain Model (ASM) of chemical reactivity, it was found that the strain energy associated with the bending of the copper(I) complex controls the observed reactivity
Shot noise and spin-orbit coherent control of entangled and spin polarized electrons
We extend our previous work on shot noise for entangled and spin polarized
electrons in a beam-splitter geometry with spin-orbit (\textit{s-o})
interaction in one of the incoming leads (lead 1). Besides accounting for both
the Dresselhaus and the Rashba spin-orbit terms, we present general formulas
for the shot noise of singlet and triplets states derived within the scattering
approach. We determine the full scattering matrix of the system for the case of
leads with \textit{two} orbital channels coupled via weak \textit{s-o}
interactions inducing channel anticrossings. We show that this interband
coupling coherently transfers electrons between the channels and gives rise to
an additional modulation angle -- dependent on both the Rashba and Dresselhaus
interaction strengths -- which allows for further independent coherent control
of the electrons traversing the incoming leads. We derive explicit shot noise
formulas for a variety of correlated pairs (e.g., Bell states) and lead spin
polarizations. Interestingly, the singlet and \textit{each} of the triplets
defined along the quantization axis perpendicular to lead 1 (with the local
\textit{s-o} interaction) and in the plane of the beam splitter display
distinctive shot noise for injection energies near the channel anticrossings;
hence, one can tell apart all the triplets, in addition to the singlet, through
noise measurements. We also find that spin-orbit induced backscattering within
lead 1 reduces the visibility of the noise oscillations, due to the additional
partition noise in this lead. Finally, we consider injection of two-particle
wavepackets into leads with multiple discrete states and find that two-particle
entanglement can still be observed via noise bunching and antibunching.Comment: 30 two-column pages and 7 figure
Rashba spin precession in quantum Hall edge channels
Quasi--one dimensional edge channels are formed at the boundary of a
two-dimensional electron system subject to a strong perpendicular magnetic
field. We consider the effect of Rashba spin--orbit coupling, induced by
structural inversion asymmetry, on their electronic and transport properties.
Both our analytical and numerical results show that spin--split quantum--Hall
edge channels exhibit properties analogous to that of Rashba--split quantum
wires. Suppressed backscattering and a long spin life time render these edge
channels an ideal system for observing voltage--controlled spin precession.
Based on the latter, we propose a magnet--less spin--dependent electron
interferometer.Comment: 7 pages, 6 figure
Anisotropic splitting of intersubband spin plasmons in quantum wells with bulk and structural inversion asymmetry
In semiconductor heterostructures, bulk and structural inversion asymmetry
and spin-orbit coupling induce a k-dependent spin splitting of valence and
conduction subbands, which can be viewed as being caused by momentum-dependent
crystal magnetic fields. This paper studies the influence of these effective
magnetic fields on the intersubband spin dynamics in an asymmetric n-type
GaAs/AlGaAs quantum well. We calculate the dispersions of intersubband spin
plasmons using linear response theory. The so-called D'yakonov-Perel'
decoherence mechanism is inactive for collective intersubband excitations,
i.e., crystal magnetic fields do not lead to decoherence of spin plasmons.
Instead, we predict that the main signature of bulk and structural inversion
asymmetry in intersubband spin dynamics is a three-fold, anisotropic splitting
of the spin plasmon dispersion. The importance of many-body effects is pointed
out, and conditions for experimental observation with inelastic light
scattering are discussed.Comment: 8 pages, 6 figure
Anisotropic transport in the two-dimensional electron gas in the presence of spin-orbit coupling
In a two-dimensional electron gas as realized by a semiconductor quantum
well, the presence of spin-orbit coupling of both the Rashba and Dresselhaus
type leads to anisotropic dispersion relations and Fermi contours. We study the
effect of this anisotropy on the electrical conductivity in the presence of
fixed impurity scatterers. The conductivity also shows in general an anisotropy
which can be tuned by varying the Rashba coefficient. This effect provides a
method of detecting and investigating spin-orbit coupling by measuring
spin-unpolarized electrical currents in the diffusive regime. Our approach is
based on an exact solution of the two-dimensional Boltzmann equation and
provides also a natural framework for investigating other transport effects
including the anomalous Hall effect.Comment: 10 pages, 1 figure included. Discussion of experimental impact
enlarged; error in calculation of conductivity contribution corrected (cf.
Eq. (A14)), no changes in qualitative results and physical consequence
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