1,725 research outputs found
Reply to the Comment on the 'Hole-digging' in ensembles of tunneling molecular magnets
Reply to the Comment of J.J. Alonso and J.F. Fernandez on the paper
"'Hole-digging' in ensembles of tunneling molecular magnets" of I.S. Tupitsyn,
P.C.E. Stamp and N.V. Prokof'ev (Phys. Rev. B 69, 132406, (2004)).Comment: 1 LaTeX page, 1 PS figure; submitted to PR
Effective Hamiltonian in the Problem of a "Central Spin" Coupled to a Spin Environment
We consider here the problem of a "giant spin", with spin quantum number
S>>1, interacting with a set of microscopic spins. Interactions between the
microscopic spins are ignored. This model describes the low-energy properties
of magnetic grains or magnetic macromolecules interacting with a surrounding
spin environment, such as nuclear spins. We describe a general method for
truncating the model to another one, valid at low energies, in which a
two-level system interacts with the environmental spins, and higher energy
terms are absorbed into a new set of couplings. This is done using an instanton
technique. We then verify the accuracy of this technique, by comparing the
results for the low energy effective Hamiltonian, with results derived for the
original giant spin, coupled to a microscopic spin, using exact diagonalisation
techniques.Comment: 15 pages, Latex, with 9 ps figure
Quantum spin glass in anisotropic dipolar systems
The spin-glass phase in the \LHx compound is considered. At zero transverse
field this system is well described by the classical Ising model. At finite
transverse field deviations from the transverse field quantum Ising model are
significant, and one must take properly into account the hyperfine
interactions, the off-diagonal terms in the dipolar interactions, and details
of the full J=8 spin Hamiltonian to obtain the correct physical picture. In
particular, the system is not a spin glass at finite transverse fields and does
not show quantum criticality.Comment: 6 pages, 2 figures, to appear in J. Phys. Condens. Matter
(proceedings of the HFM2006 conference
Development of food photographs for use with children aged 18 months to 16 years:comparison against weighed food diaries – The Young Person’s Food Atlas (UK)
Traditional dietary assessment methods, used in the UK, such as weighed food diaries impose a large participant burden, often resulting in difficulty recruiting representative samples and underreporting of energy intakes. One approach to reducing the burden placed on the participant is to use portion size assessment tools to obtain an estimate of the amount of food consumed, removing the need to weigh all foods. An age range specific food atlas was developed for use in assessing children’s dietary intakes. The foods selected and portion sizes depicted were derived from intakes recorded during the UK National Diet and Nutrition Surveys of children aged 1.5 to 16 years. Estimates of food portion sizes using the food atlas were compared against 4-day weighed intakes along with in-school / nursery observations, by the research team. Interviews were conducted with parents the day after completion of the diary, and for children aged 4 to 16 years, also with the child. Mean estimates of portion size consumed were within 7% of the weight of food recorded in the weighed food diary. The limits of agreement were wide indicating high variability of estimates at the individual level but the precision increased with increasing age. For children 11 years and over, agreement with weighed food diaries, was as good as that of their parents in terms of total weight of food consumed and of intake of energy and key nutrients. The age appropriate food photographs offer an alternative to weighed intakes for dietary assessment with children
Decoherence and Quantum Walks: anomalous diffusion and ballistic tails
The common perception is that strong coupling to the environment will always
render the evolution of the system density matrix quasi-classical (in fact,
diffusive) in the long time limit. We present here a counter-example, in which
a particle makes quantum transitions between the sites of a d-dimensional
hypercubic lattice whilst strongly coupled to a bath of two-level systems which
'record' the transitions. The long-time evolution of an initial wave packet
is found to be most unusual: the mean square displacement of the particle
density matrix shows long-range ballitic behaviour, but simultaneously a kind
of weakly-localised behaviour near the origin. This result may have important
implications for the design of quantum computing algorithms, since it describes
a class of quantum walks.Comment: 4 pages, 1 figur
Sharp transition for single polarons in the one-dimensional Su-Schrieffer-Heeger model
We study a single polaron in the Su-Schrieffer-Heeger (SSH) model using four
different techniques (three numerical and one analytical). Polarons show a
smooth crossover from weak to strong coupling, as a function of the
electron-phonon coupling strength , in all models where this coupling
depends only on phonon momentum . In the SSH model the coupling also depends
on the electron momentum ; we find it has a sharp transition, at a critical
coupling strength , between states with zero and nonzero momentum of
the ground state. All other properties of the polaron are also singular at
, except the average number of phonons in the polaronic
cloud. This result is representative of all polarons with coupling depending on
and , and will have important experimental consequences (eg., in ARPES
and conductivity experiments)
What are the interactions in quantum glasses?
The form of the low-temperature interactions between defects in neutral
glasses is reconsidered. We analyse the case where the defects can be modelled
either as simple 2-level tunneling systems, or tunneling rotational impurities.
The coupling to strain fields is determined up to 2nd order in the displacement
field. It is shown that the linear coupling generates not only the usual
Ising-like interaction between the rotational tunneling defect modes,
which cause them to freeze around a temperature , but also a random field
term. At lower temperatures the inversion symmetric tunneling modes are still
active - however the coupling of these to the frozen rotational modes, now via
the 2nd-order coupling to phonons, generates another random field term acting
on the inversion symmetric modes (as well as shorter-range interactions
between them). Detailed expressions for all these couplings are given.Comment: 12 pages, 2 figures. Minor modifications, published versio
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