90 research outputs found
Electronic excitations and the tunneling spectra of metallic nanograins
Tunneling-induced electronic excitations in a metallic nanograin are
classified in terms of {\em generations}: subspaces of excitations containing a
specific number of electron-hole pairs. This yields a hierarchy of populated
excited states of the nanograin that strongly depends on (a) the available
electronic energy levels; and (b) the ratio between the electronic relaxation
rate within the nano-grain and the bottleneck rate for tunneling transitions.
To study the response of the electronic energy level structure of the nanograin
to the excitations, and its signature in the tunneling spectrum, we propose a
microscopic mean-field theory. Two main features emerge when considering an Al
nanograin coated with Al oxide: (i) The electronic energy response fluctuates
strongly in the presence of disorder, from level to level and excitation to
excitation. Such fluctuations produce a dramatic sample dependence of the
tunneling spectra. On the other hand, for excitations that are energetically
accessible at low applied bias voltages, the magnitude of the response,
reflected in the renormalization of the single-electron energy levels, is
smaller than the average spacing between energy levels. (ii) If the tunneling
and electronic relaxation time scales are such as to admit a significant
non-equilibrium population of the excited nanoparticle states, it should be
possible to realize much higher spectral densities of resonances than have been
observed to date in such devices. These resonances arise from tunneling into
ground-state and excited electronic energy levels, as well as from charge
fluctuations present during tunneling.Comment: Submitted to the Physical Review
Tunneling Via Individual Electronic States in Ferromagnetic Nanoparticles
We measure electron tunneling via discrete energy levels in ferromagnetic
cobalt particles less than 4 nm in diameter, using non-magnetic electrodes. Due
to magnetic anisotropy, the energy of each tunneling resonance shifts as an
applied magnetic field rotates the particle's magnetic moment. We see both
spin-increasing and decreasing tunneling transitions, but we do not observe the
spin degeneracy at small magnetic fields seen previously in non-magnetic
materials. The tunneling spectrum is denser than predicted for independent
electrons, possibly due to spin-wave excitations.Comment: 4 pages, 4 figures. Improved by comments from referees, to appear in
Phys. Rev. Let
Spontaneous Magnetization and Electron Momentum Density in 3D Quantum Dots
We discuss an exactly solvable model Hamiltonian for describing the
interacting electron gas in a quantum dot. Results for a spherical square well
confining potential are presented. The ground state is found to exhibit
striking oscillations in spin polarization with dot radius at a fixed electron
density. These oscillations are shown to induce characteristic signatures in
the momentum density of the electron gas, providing a novel route for direct
experimental observation of the dot magnetization via spectroscopies sensitive
to the electron momentum density.Comment: 5 pages (Revtex4), 4 (eps) figure
Suppression of Ground-State Magnetization in Finite-Sized Systems Due to Off-Diagonal Interaction Fluctuations
We study a generic model of interacting fermions in a finite-sized disordered
system. We show that the off-diagonal interaction matrix elements induce
density of states fluctuations which generically favor a minimum spin ground
state at large interaction amplitude, . This effect competes with the
exchange effect which favors large magnetization at large , and it
suppresses this exchange magnetization in a large parameter range. When
off-diagonal fluctuations dominate, the model predicts a spin gap which is
larger for odd-spin ground states as for even-spin, suggesting a simple
experimental signature of this off-diagonal effect in Coulomb blockade
transport measurements.Comment: Final, substantially modified version of the article. Accepted for
publication in Physical Review Letter
T-duality in the weakly curved background
We consider the closed string propagating in the weakly curved background
which consists of constant metric and Kalb-Ramond field with infinitesimally
small coordinate dependent part. We propose the procedure for constructing the
T-dual theory, performing T-duality transformations along coordinates on which
the Kalb-Ramond field depends. The obtained theory is defined in the
non-geometric double space, described by the Lagrange multiplier and
its -dual . We apply the proposed T-duality procedure to the
T-dual theory and obtain the initial one. We discuss the standard relations
between T-dual theories that the equations of motion and momenta modes of one
theory are the Bianchi identities and the winding modes of the other
Protocol for the development and validation procedure of the managing the link and strengthening transition from child to adult mental health care (MILESTONE) suite of measures
Background:
Mental health disorders in the child and adolescent population are a pressing public health concern. Despite the high prevalence of psychopathology in this vulnerable population, the transition from Child and Adolescent Mental Health Services (CAMHS) to Adult Mental Health Services (AMHS) has many obstacles such as deficiencies in planning, organisational readiness and policy gaps. All these factors contribute to an inadequate and suboptimal transition process. A suite of measures is required that would allow young people to be assessed in a structured and standardised way to determine the on-going need for care and to improve communication across clinicians at CAMHS and AMHS. This will have the potential to reduce the overall health economic burden and could also improve the quality of life for patients travelling across the transition boundary. The MILESTONE (Managing the Link and Strengthening Transition from Child to Adult Mental Health Care) project aims to address the significant socioeconomic and societal challenge related to the transition process. This protocol paper describes the development of two MILESTONE transition-related measures: The Transition Readiness and Appropriateness Measure (TRAM), designed to be a decision-making aide for clinicians, and the Transition Related Outcome Measure (TROM), for examining the outcome of transition.
Methods:
The TRAM and TROM have been developed and were validated following the US FDA Guidance for Patient-reported Outcome Measures which follows an incremental stepwise framework. The study gathers information from service users, parents, families and mental health care professionals who have experience working with young people undergoing the transition process from eight European countries.
Discussion:
There is an urgent need for comprehensive measures that can assess transition across the CAMHS/AMHS boundary. This study protocol describes the process of development of two new transition measures: the TRAM and TROM. The TRAM has the potential to nurture better transitions as the findings can be summarised and provided to clinicians as a clinician-decision making support tool for identifying cases who need to transition and the TROM can be used to examine the outcomes of the transition process.
Trial registration:
MILESTONE study registration: ISRCTN83240263 Registered 23-July-2015 - ClinicalTrials.gov NCT03013595 Registered 6 January 2017
Localization and Capacitance Fluctuations in Disordered Au Nano-junctions
Nano-junctions, containing atomic-scale gold contacts between strongly
disordered leads, exhibit different transport properties at room temperature
and at low temperature. At room temperature, the nano-junctions exhibit
conductance quantization effects. At low temperatures, the contacts exhibit
Coulomb-Blockade. We show that the differences between the room-temperature and
low temperature properties arise from the localization of electronic states in
the leads. The charging energy and capacitance of the nano-junctions exhibit
strong fluctuations with applied magnetic field at low temperature, as
predicted theoretically.Comment: 20 pages 8 figure
Decoherence in Nearly-Isolated Quantum Dots
Decoherence in nearly-isolated GaAs quantum dots is investigated using the
change in average Coulomb blockade peak height upon breaking time-reversal
symmetry. The normalized change in average peak height approaches the predicted
universal value of 1/4 at temperatures well below the single-particle level
spacing, but is greatly suppressed for temperature greater than the level
spacing, suggesting that inelastic scattering or other dephasing mechanisms
dominate in this regime.Comment: Significant revisions to include comparison to theory. Related papers
available at http://marcuslab.harvard.ed
Generalized Husimi Functions: Analyticity and Information Content
The analytic properties of a class of generalized Husimi functions are
discussed, with particular reference to the problem of state reconstruction.
The class consists of the subset of Wodkiewicz's operational probability
distributions for which the filter reference state is a squeezed vacuum state.
The fact that the function is analytic means that perfectly precise knowledge
of its values over any small region of phase space provides enough information
to reconstruct the density matrix. If, however, one only has imprecise
knowledge of its values, then the amplification of statistical errors which
occurs when one attempts to carry out the continuation seriously limits the
amount of information which can be extracted. To take account of this fact a
distinction is made between explicate, or experimentally accessible
information, and information which is only present in implicate, experimentally
inaccessible form. It is shown that an explicate description of various aspects
of the system can be found localised on various 2 real dimensional surfaces in
complexified phase space. In particular, the continuation of the function to
the purely imaginary part of complexified phase space provides an explicate
description of the Wigner function.Comment: 16 pages, 2 figures, AMS-latex. Replaced with published versio
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