226 research outputs found
Young people's beliefs about help-seeking for first episode psychosis
The importance of early intervention for positive outcomes following a first episode of psychosis (FEP) is recognised, however, there are still delays in young people seeking help. The aim of this qualitative study was to explore beliefs about help-seeking in young people who have recently experienced a FEP. Twelve participants were interviewed and transcripts were analysed using grounded theory. Five categories of beliefs were identified which contributed to the development of a preliminary model of beliefs about a need for help-seeking. Young people’s beliefs about their experiences as normal or abnormal, and about the process and outcome of help-seeking, influenced beliefs about needing help. Existing beliefs about mental health problems, and beliefs about the impact of experiences on the self were important to the meaning young people ascribed to beliefs about help-seeking. Social networks were important in the development of these beliefs.
The study identified beliefs which promoted and inhibited young people’s help-seeking beliefs for FEP. Self-stigma was present prior to help-seeking. Implications for clinical interventions include education to promote help-seeking for distress and not abnormality, and to dispel myths about the help-seeking process and outcome. Further research is needed to determine which beliefs are most important to young people’s help-seeking for FEP
Soil deformations caused by soft-ground tunnelling
This thesis discusses the interaction between methods of tunnelling in soil and sources of ground loss. Two distinct phases of settlement in cohesive soils are identified. Short-term settlements are caused by loss of ground into the tunnel and long term settlements are caused by consolidation of the ground around the tunnel. A stochastic model of ground movements caused by volume loss into the tunnel is developed in order to explain in-situ observations. Consolidation settlement is estimated with the aid of flow nets developed by finite difference numerical modelling. These nets are also used to estimate the contribution of seepage to tunnel face instability. Field observations of ground movements caused by tunnelling In soft, cohesive ground were made at three sites. These measurements were taken In order not only to add to the store of case history evidence already available, but also in a direct attempt to confirm or disprove the theoretical model. Tunnelling conditions were different in each case. One tunnel was shield-driven in laminated clay, one was shield-driven with the aid of compressed air support In alluvial organic slit, and one was driven without a shield in stiff, stony clay. These case histories confirm that settlement troughs of Gaussian configuration were developed, agreeing with the stochastic model, and that long-term consolidation may develop in clay soils on the removal of compressed air support from the tunnel
Continuous-Time Monte Carlo study of the pseudogap Bose-Fermi Kondo model
We study the pseudogap Bose-Fermi Anderson model with a continuous-time
quantum Monte Carlo (CT-QMC) method. We discuss some delicate aspects of the
transformation from this model to the Bose-Fermi Kondo model. We show that the
CT-QMC method can be used at sufficiently low temperatures to access the
quantum critical properties of these models.Comment: SCES 2010 Proceeding
Local quantum phase transition in the pseudogap Anderson model: scales, scaling and quantum critical dynamics
The pseudogap Anderson impurity model provides a paradigm for understanding
local quantum phase transitions, in this case between generalised fermi liquid
and degenerate local moment phases. Here we develop a non-perturbative local
moment approach to the generic asymmetric model, encompassing all energy scales
and interaction strengths and leading thereby to a rich description of the
problem. We investigate in particular underlying phase boundaries, the critical
behaviour of relevant low-energy scales, and single-particle dynamics embodied
in the local spectrum. Particular attention is given to the resultant universal
scaling behaviour of dynamics close to the transition in both the GFL and LM
phases, the scale-free physics characteristic of the quantum critical point
itself, and the relation between the two.Comment: 39 pages, 19 figure
A Local Moment Approach to magnetic impurities in gapless Fermi systems
A local moment approach is developed for the single-particle excitations of a
symmetric Anderson impurity model (AIM), with a soft-gap hybridization
vanishing at the Fermi level with a power law r > 0. Local moments are
introduced explicitly from the outset, and a two-self-energy description is
employed in which the single-particle excitations are coupled dynamically to
low-energy transverse spin fluctuations. The resultant theory is applicable on
all energy scales, and captures both the spin-fluctuation regime of strong
coupling (large-U), as well as the weak coupling regime. While the primary
emphasis is on single particle dynamics, the quantum phase transition between
strong coupling (SC) and (LM) phases can also be addressed directly; for the
spin-fluctuation regime in particular a number of asymptotically exact results
are thereby obtained. Results for both single-particle spectra and SC/LM phase
boundaries are found to agree well with recent numerical renormalization group
(NRG) studies. A number of further testable predictions are made; in
particular, for r < 1/2, spectra characteristic of the SC state are predicted
to exhibit an r-dependent universal scaling form as the SC/LM phase boundary is
approached and the Kondo scale vanishes. Results for the `normal' r = 0 AIM are
moreover recovered smoothly from the limit r -> 0, where the resultant
description of single-particle dynamics includes recovery of Doniach-Sunjic
tails in the Kondo resonance, as well as characteristic low-energy Fermi liquid
behaviour.Comment: 52 pages, 19 figures, submitted to Journal of Physics: Condensed
Matte
Criticality of the Mean-Field Spin-Boson Model: Boson State Truncation and Its Scaling Analysis
The spin-boson model has nontrivial quantum phase transitions at zero
temperature induced by the spin-boson coupling. The bosonic numerical
renormalization group (BNRG) study of the critical exponents and
of this model is hampered by the effects of boson Hilbert space
truncation. Here we analyze the mean-field spin boson model to figure out the
scaling behavior of magnetization under the cutoff of boson states . We
find that the truncation is a strong relevant operator with respect to the
Gaussian fixed point in and incurs the deviation of the exponents
from the classical values. The magnetization at zero bias near the critical
point is described by a generalized homogeneous function (GHF) of two variables
and . The universal function has a
double-power form and the powers are obtained analytically as well as
numerically. Similarly, is found to be a GHF of
and . In the regime , the truncation produces no effect.
Implications of these findings to the BNRG study are discussed.Comment: 9 pages, 7 figure
Anderson impurities in gapless hosts: comparison of renormalization group and local moment approaches
The symmetric Anderson impurity model, with a soft-gap hybridization
vanishing at the Fermi level with power law r > 0, is studied via the numerical
renormalization group (NRG). Detailed comparison is made with predictions
arising from the local moment approach (LMA), a recently developed many-body
theory which is found to provide a remarkably successful description of the
problem. Results for the `normal' (r = 0) impurity model are obtained as a
specific case. Particular emphasis is given both to single-particle excitation
dynamics, and to the transition between the strong coupling (SC) and local
moment (LM) phases of the model. Scaling characteristics and asymptotic
behaviour of the SC/LM phase boundaries are considered. Single-particle spectra
are investigated in some detail, for the SC phase in particular. Here, the
modified spectral functions are found to contain a generalized Kondo resonance
that is ubiquitously pinned at the Fermi level; and which exhibits a
characteristic low-energy Kondo scale that narrows progressively upon approach
to the SC->LM transition, where it vanishes. Universal scaling of the spectra
as the transition is approached thus results. The scaling spectrum
characteristic of the normal Anderson model is recovered as a particular case,
and is captured quantitatively by the LMA. In all cases the r-dependent scaling
spectra are found to possess characteristic low-energy asymptotics, but to be
dominated by generalized Doniach-Sunjic tails, in agreement with LMA
predictions.Comment: 26 pages, 14 figures, submitted for publicatio
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
A spin-dependent local moment approach to the Anderson impurity model
We present an extension of the local moment approach to the Anderson impurity
model with spin-dependent hybridization. By employing the two-self-energy
description, as originally proposed by Logan and co-workers, we applied the
symmetry restoration condition for the case with spin-dependent hybridization.
Self-consistent ground states were determined through variational minimization
of the ground state energy. The results obtained with our spin-dependent local
moment approach applied to a quantum dot system coupled to ferromagnetic leads
are in good agreement with those obtained from previous work using numerical
renormalization group calculations
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