4,505 research outputs found
Antiferromagnetic Alignment and Relaxation Rate of Gd Spins in the High Temperature Superconductor GdBa_2Cu_3O_(7-delta)
The complex surface impedance of a number of GdBaCuO
single crystals has been measured at 10, 15 and 21 GHz using a cavity
perturbation technique. At low temperatures a marked increase in the effective
penetration depth and surface resistance is observed associated with the
paramagnetic and antiferromagnetic alignment of the Gd spins. The effective
penetration depth has a sharp change in slope at the N\'eel temperature, ,
and the surface resistance peaks at a frequency dependent temperature below 3K.
The observed temperature and frequency dependence can be described by a model
which assumes a negligibly small interaction between the Gd spins and the
electrons in the superconducting state, with a frequency dependent magnetic
susceptibility and a Gd spin relaxation time being a strong function
of temperature. Above , has a component varying as , while below it increases .Comment: 4 Pages, 4 Figures. Submitted to Phys. Rev.
A hybrid memory kernel approach for condensed phase non-adiabatic dynamics
The spin-boson model is a simplified Hamiltonian often used to study
non-adiabatic dynamics in large condensed phase systems, even though it has not
been solved in a fully analytic fashion. Herein, we present an exact analytic
expression for the dynamics of the spin-boson model in the infinitely slow bath
limit and generalize it to approximate dynamics for faster baths. We achieve
the latter by developing a hybrid approach that combines the exact slow-bath
result with the popular NIBA method to generate a memory kernel that is
formally exact to second order in the diabatic coupling but also contains
higher-order contributions approximated from the second order term alone. This
kernel has the same computational complexity as NIBA, but is found to yield
dramatically superior dynamics in regimes where NIBA breaks down---such as
systems with large diabatic coupling or energy bias. This indicates that this
hybrid approach could be used to cheaply incorporate higher order effects into
second order methods, and could potentially be generalized to develop alternate
kernel resummation schemes
Rotation profiles of solar-like stars with magnetic fields
The aim of this work is to investigate rotation profile of solar-like stars
with magnetic fields. A diffusion coefficient of magnetic angular momentum
transport is deduced. Rotating stellar models with different mass are computed
under the effect of the coefficient. Then rotation profiles are obtained from
the theoretical stellar models. The total angular momentum of solar model with
only hydrodynamic instabilities is about 13 times larger than that of the Sun
at the age of the Sun, and this model can not reproduce quasi-solid rotation in
the radiative region. However, not only can the solar model with magnetic
fields reproduce an almost uniform rotation in the radiative region, but its
total angular momentum is consistent with helioseismic result at the level of 3
at the age of the Sun. The rotation of solar-like stars with magnetic
fields is almost uniform in the radiative region. But there is an obvious
transition region of angular velocity between the convective core and the
radiative region of models with 1.2 - 1.5 , where angular velocity
has a sharp radial change, which is different from the rotation profile of the
Sun and massive stars with magnetic fields. Moreover the changes of the angular
velocity in the transition region increase with the increasing in the age and
mass.Comment: Accepted for publication in ChjA
Oscillations of alpha UMa and other red giants
There is growing observational evidence that the variability of red giants
could be caused by excitation of global modes of oscillation. The most recent
evidence of such oscillations was reported for alpha UMa by Buzasi et
al.(2000). We address the problem of radial and nonradial mode excitation in
red giants from the theoretical point of view. In particular, we present the
results of numerical computations of oscillation properties of a model of alpha
UMa and of several models of a 2M_sun star in the red-giant phase. In the red
giant stars by far most of the nonradial modes are confined to to the deep
interior, where they have the g-mode character. Only modes at resonant
frequencies of the p-mode cavity have substantial amplitudes in the outer
layers. We have shown that such modes can be unstable with the linear growth
rates similar to those of corresponding to radial modes.
We have been unable to explain the observed oscillation properties of alpha
UMa, either in terms of mode instability or in terms of stochastic excitation
by turbulent convection. Modes at the lowest frequencies, which exhibit the
largest amplitudes and may correspond to the first three radial modes, are
found stable if all effects of convection are taken into account. The observed
frequency dependence of amplitudes does not agree with what one expects from
stochastic excitation. The predicted fundamental mode amplitude is by about two
orders of magnitude smaller than those of high frequency modes, which is in
stark disagreement with the observations.Comment: MNRAS in pres
Noether’s theorem for dissipative quantum dynamical semi-groups
Noether's Theorem on constants of the motion of dynamical systems has
recently been extended to classical dissipative systems (Markovian semi-groups)
by Baez and Fong. We show how to extend these results to the fully quantum
setting of quantum Markov dynamics. For finite-dimensional Hilbert spaces, we
construct a mapping from observables to CP maps that leads to the natural
analogue of their criterion of commutativity with the infinitesimal generator
of the Markov dynamics. Using standard results on the relaxation of states to
equilibrium under quantum dynamical semi-groups, we are able to characterise
the constants of the motion under quantum Markov evolutions in the
infinite-dimensional setting under the usual assumption of existence of a
stationary strictly positive density matrix. In particular, the Noether
constants are identified with the fixed point of the Heisenberg picture
semigroup.Comment: 8 pages, no figure
Stellar turbulence and mode physics
An overview of selected topical problems on modelling oscillation properties
in solar-like stars is presented. High-quality oscillation data from both
space-borne intensity observations and ground-based spectroscopic measurements
provide first tests of the still-ill-understood, superficial layers in distant
stars. Emphasis will be given to modelling the pulsation dynamics of the
stellar surface layers, the stochastic excitation processes and the associated
dynamics of the turbulent fluxes of heat and momentum.Comment: Proc. HELAS Workshop on 'Synergies between solar and stellar
modelling', eds M. Marconi, D. Cardini, M. P. Di Mauro, Astrophys. Space
Sci., in the pres
Promoting Mental Health and Wellbeing with Men and Boys: What Works?
In May 2014 the Movember Foundation commissioned the Centre for Men’s Health, Leeds Beckett University, in collaboration with the Men’s Health Forum (England & Wales), to gather the current research evidence and practical (‘tacit’) knowledge about the core elements that make for successful work with boys and men around mental health promotion, early intervention and stigma reduction
Successful mental health promotion with men: evidence from 'tacit knowledge'
There remains significant concern about men’s mental health, particularly in terms of personal and societal barriers to help-seeking, negative coping mechanisms and high suicide rates. This paper presents findings from a multi-phase study looking at ‘what works’ in mental health promotion for men. Work here reports the collection and analysis of the tacit knowledge of those working within mental health promotion interventions for men. A ‘multiple hub and spoke’ approach was used to assist data collection. Thirteen key players, active in the men’s mental health field, half from the UK and half beyond, formed an Investigative Network collecting data, mainly through interviews, from wider geographical and professional community contexts where they had networks. The focus of data collection was on ‘what works’ in mental health promotion for men. Data was analysed using thematic analysis techniques. Findings suggest that settings which created safe male spaces acted to promote trust, reduce stigma and normalise men’s engagement in interventions. Embedding interventions within the communities of men being engaged, fully involving these men, and holding ‘male-positive’ values engendered familiarity and consolidated trust. Using ‘male-sensitive’ language and activity-based approaches allowed for positive expressions of emotions, facilitated social engagement, and provided a base for open communication. Appropriate partnerships were also seen as a necessary requirement for success and as crucial for maximising intervention impact. The importance of gender and ‘masculinity’ was apparent throughout these findings and taking time to understand gender could facilitate positive ways of working alongside men, increasing levels of engagement and successful outcomes
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