326 research outputs found
Dynamics of capacitively coupled double quantum dots
We consider a double dot system of equivalent, capacitively coupled
semiconducting quantum dots, each coupled to its own lead, in a regime where
there are two electrons on the double dot. Employing the numerical
renormalization group, we focus here on single-particle dynamics and the
zero-bias conductance, considering in particular the rich range of behaviour
arising as the interdot coupling is progressively increased through the strong
coupling (SC) phase, from the spin-Kondo regime, across the SU(4) point to the
charge-Kondo regime; and then towards and through the quantum phase transition
to a charge-ordered (CO) phase. We first consider the two-self-energy
description required to describe the broken symmetry CO phase, and implications
thereof for the non-Fermi liquid nature of this phase. Numerical results for
single-particle dynamics on all frequency scales are then considered, with
particular emphasis on universality and scaling of low-energy dynamics
throughout the SC phase. The role of symmetry breaking perturbations is also
briefly discussed.Comment: 14 pages, 6 figure
Renormalization group study of capacitively coupled double quantum dots
The numerical renormalization group is employed to study a double quantum
(DQD) dot system consisting of two equivalent single-level dots, each coupled
to its own lead and with a mutual capacitive coupling embodied in an interdot
interaction U', in addition to the intradot Coulomb interaction U. We focus on
the regime with two electrons on the DQD, and the evolution of the system on
increasing U'/U. The spin-Kondo effect arising for U'=0 (SU(2) x SU(2)) is
found to persist robustly with increasing U'/U, before a rapid but continuous
crossover to (a) the SU(4) point U'=U where charge and spin degrees of freedom
are entangled and the Kondo scale strongly enhanced; and then (b) a
charge-Kondo state, in which a charge-pseudospin is quenched on coupling to the
leads/conduction channels. A quantum phase transition of Kosterlitz-Thouless
type then occurs from this Fermi liquid, strong coupling (SC) phase, to a
broken symmetry, non-Fermi liquid charge ordered (CO) phase at a critical U'_c.
Our emphasis in this paper is on the structure, stability and flows between the
underlying RG fixed points, on the overall phase diagram in the (U,U')-plane
and evolution of the characteristic low-energy Kondo scale inherent to the SC
phase; and on static physical properties such as spin- and
charge-susceptibilities (staggered and uniform), including universality and
scaling behaviour in the strongly correlated regime. Some exact results for
associated Wilson ratios are also obtained.Comment: 27 pages, 12 figure
Single-particle dynamics of the Anderson model: a two-self-energy description within the numerical renormalization group approach
Single-particle dynamics of the Anderson impurity model are studied using
both the numerical renormalization group (NRG) method and the local moment
approach (LMA). It is shown that a 'two-self-energy' description of dynamics
inherent to the LMA, as well as a conventional 'single-self-energy'
description, arise within NRG; each yielding correctly the same local
single-particle spectrum. Explicit NRG results are obtained for the broken
symmetry spectral constituents arising in a two-self-energy description, and
the total spectrum. These are also compared to analytical results obtained from
the LMA as implemented in practice. Very good agreement between the two is
found, essentially on all relevant energy scales from the high-energy Hubbard
satellites to the low-energy Kondo resonance.Comment: 12 pages, 6 figure
Universal conductance enhancement and reduction of the two-orbital Kondo effect
We investigate theoretically the linear and nonlinear conductance through a
nanostructure with two-fold degenerate single levels, corresponding to the
transport through nanostructures such as a carbon nanotube, or double dot
systems with capacitive interaction. It is shown that the presence of the
interaction asymmetry between orbits/dots affects significantly the profile of
the linear conductance at finite temperature, and, of the nonlinear
conductance, particularly around half-filling, where the two-particle Kondo
effect occurs. Within the range of experimentally feasible parameters, the
SU(4) universal behavior is suggested, and comparison with relevant experiments
is made.Comment: 10 pages, 16 figure
Robust Henderson III estimators of variance components in the nested error model
Common methods for estimating variance components in Linear Mixed Models include Maximum Likelihood (ML) and Restricted Maximum Likelihood (REML). These methods are based on the strong assumption of multivariate normal distribution and it is well know that they are very sensitive to outlying observations with respect to any of the random components. Several robust altematives of these methods have been proposed (e.g. Fellner 1986, Richardson and Welsh 1995). In this work we present several robust alternatives based on the Henderson method III which do not rely on the normality assumption and provide explicit solutions for the variance components estimators. These estimators can later be used to derive robust estimators of regression coefficients. Finally, we describe an application of this procedure to small area estimation, in which the main target is the estimation of the means of areas or domains when the within-area sample sizes are small
Evaluating the impact of post-qualifying social work education.
Post-qualifying awards in social work are well established within the continuing professional development agenda for qualified social workers in the UK. The evaluation of education and training should be an integral part of this agenda because it is important to ensure that programmes continue to meet standards of delivery, are successful in meeting their aims and objectives and are making an impact on practice. However, there is limited amount of published work on the evaluation of post-qualifying social work education, with studies often focusing on programme delivery rather than on their impact on practice.
This paper explores evaluative work within the current post-qualifying social work framework, and discusses the results of an evaluation of the Vulnerable Adults and Community Care Practice programme, a specialist post-qualifying social work education programme run by a UK university, as an example of an evaluation of the impact on practice. The results indicate positive evidence of impact on practice and demonstrate examples of how the programme has had a direct effect on individuals, teams, organisations and on people who use services
Thermal Symmetry Crossover and Universal Behaviors in Carbon Nanotube Dots
Motivated by recent experiments on electronic transport through a carbon
nanotube, we investigate the role of the intra- and inter-orbital Coulomb
interactions on the temperature evolution of the conductance. It is shown that
small amount (~10%) of asymmetry between these Coulomb repulsions substantially
deforms the conductance profile at finite temperature, particularly around
half-filling. The nature of such thermal symmetry crossover is elucidated.Comment: published version; 11pages, 4 figure
The numerical renormalization group method for quantum impurity systems
In the beginning of the 1970's, Wilson developed the concept of a fully
non-perturbative renormalization group transformation. Applied to the Kondo
problem, this numerical renormalization group method (NRG) gave for the first
time the full crossover from the high-temperature phase of a free spin to the
low-temperature phase of a completely screened spin. The NRG has been later
generalized to a variety of quantum impurity problems. The purpose of this
review is to give a brief introduction to the NRG method including some
guidelines of how to calculate physical quantities, and to survey the
development of the NRG method and its various applications over the last 30
years. These applications include variants of the original Kondo problem such
as the non-Fermi liquid behavior in the two-channel Kondo model, dissipative
quantum systems such as the spin-boson model, and lattice systems in the
framework of the dynamical mean field theory.Comment: 55 pages, 27 figures, submitted to Rev. Mod. Phy
Values‐led curriculum co‐creation : a curriculum re‐innovation case study
Personal values influence goals and motivate actions. The case study reported in this paper explored whether an understanding of values would provide a useful framework to guide the co-creation of the undergraduate cognitive psychology curriculum at a UK University. A design team composed of staff and students ran two co-creation workshops to explore underlying values. These values were translated into curriculum ideas which were then shared via an online survey to students and staff for feedback. The activities revealed a set of values that were salient when imagining future curriculum designs: feeling stimulated, choice and autonomy, developing competence, feeling safe and secure, community and fairness. In addition, a deeper value layer was visible which reflected participants' orientations to learning and education. We describe our process for eliciting values and the intertwined and iterative relationship between value elicitation and a co-created curriculum. We also reflect on the position of co-creation within the value landscape of higher education and the social dynamics of staff-student partnerships. We argue that whilst using values to frame co-creation allows for deeper insight into how to embed curriculum re-innovation, it is important to attend to the value system of co-creation and those who do not participate
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