1,363 research outputs found
Optical study of the anisotropic erbium spin flip-flop dynamics
We investigate the erbium flip-flop dynamics as a limiting factor of the
electron spin lifetime and more generally as an indirect source of decoherence
in rare-earth doped insulators. Despite the random isotropic arrangement of
dopants in the host crystal, the dipolar interaction strongly depends on the
magnetic field orientation following the strong anisotropy of the -factor.
In Er:YSiO, we observe by transient optical spectroscopy a three
orders of magnitude variation of the erbium flip-flop rate (10ppm dopant
concentration). The measurements in two different samples, with 10ppm and 50ppm
concentrations, are well-supported by our analytic modeling of the dipolar
coupling between identical spins with an anisotropic -tensor. The model can
be applied to other rare-earth doped materials. We extrapolate the calculation
to Er:CaWO, Er:LiNbO and Nd:YSiO at
different concentrations
Superhyperfine induced photon-echo collapse of erbium in YSiO
We investigate the decoherence of Er in YSiO at low magnetic
fields using the photon-echo technique. We reproduce accurately a variety of
the decay curves with a unique coherence time by considering the so-called
superhyperfine modulation induced by a large number of neighbouring spins.
There is no need to invoke any characteristic time of the spin fluctuations to
reproduce very different decay curves. The number of involved nuclei increases
when the magnetic is lowered. The experiment is compared with a model
associating 100 surrounding ions with their exact positions in the crystal
frame. We also derive an approximate spherical model (angular averaging) to
interpret the main feature the observed decay curves close to zero-field
Public perceptions of diabetes, healthy living and conversational agents in Singapore: a needs assessment
Background: The incidence of chronic diseases such as type 2 diabetes is on the rise in countries worldwide, including Singapore. Health professional-delivered healthy lifestyle interventions have been shown to prevent type 2 diabetes. Yet ongoing personalised guidance from health professionals is not feasible or affordable at the population level. Novel digital interventions delivered using mobile technology such as conversational agents are a potential alternative for delivery of healthy lifestyle change behavioural interventions to the public. Objective: We explored Singaporeans’ perceptions on and experience of healthy living, diabetes and mobile health interventions (apps and conversational agents). This survey was done to help inform the design and development of a conversational agent focusing on healthy lifestyle change. Methods: This qualitative study was conducted over Aug and Sept 2019. 20 participants were recruited from relevant healthy living Facebook pages and groups. Semi-structured interviews were conducted in person or over the telephone using an interview guide. Interviews were transcribed and analysed in parallel by two researchers using Burnard’s method, a structured approach for thematic content analysis. Results: The collected data was organised into four main themes: (1) use of conversational agents, (2) ubiquity of smartphone applications, (3) understanding of diabetes and (4) barriers and facilitators to a healthy living in Singapore. Most participants used health-related mobile applications as well as conversational agents unrelated to healthcare. They provided diverse suggestions for future conversational agent-delivered interventions. Participants also highlighted several knowledge gaps in relation to diabetes and healthy living. In terms of barriers to healthy living, frequent dining out, high stress levels, lack of work-life balance and dearth of free time to engage in physical activity were mentioned. In contrast, discipline, pre-planning and sticking to a routine were important for enabling a healthy lifestyle. Conclusions: Participants in our study commonly used mobile health interventions and provided important insights into their knowledge gaps and needs in relation to healthy lifestyle behaviour change. Future digital interventions like conversational agents focusing on healthy lifestyle and diabetes prevention should aim to address the barriers highlighted in our study and motivate individuals to adopt habits for healthy living
Local Simulation Algorithms for Coulomb Interaction
Long ranged electrostatic interactions are time consuming to calculate in
molecular dynamics and Monte-Carlo simulations. We introduce an algorithmic
framework for simulating charged particles which modifies the dynamics so as to
allow equilibration using a local Hamiltonian. The method introduces an
auxiliary field with constrained dynamics so that the equilibrium distribution
is determined by the Coulomb interaction. We demonstrate the efficiency of the
method by simulating a simple, charged lattice gas.Comment: Last figure changed to improve demonstration of numerical efficienc
Diffusion of hydrogen in crystalline silicon
The coefficient of diffusion of hydrogen in crystalline silicon is calculated
using tight-binding molecular dynamics. Our results are in good quantitative
agreement with an earlier study by Panzarini and Colombo [Phys. Rev. Lett. 73,
1636 (1994)]. However, while our calculations indicate that long jumps dominate
over single hops at high temperatures, no abrupt change in the diffusion
coefficient can be observed with decreasing temperature. The (classical)
Arrhenius diffusion parameters, as a consequence, should extrapolate to low
temperatures.Comment: 4 pages, including 5 postscript figures; submitted to Phys. Rev. B
Brief Repor
Theoretical Study of Cubic Structures Based on Fullerene Carbon Clusters: CC and (C
We study a new hypothetical form of solid carbon \csc, with a unit cell which
is composed of the \cs \ fullerene cluster and an additional single carbon atom
arranged in the zincblende structure. Using {\it ab initio} calculations, we
show that this new form of solid carbon has lower energy than hyperdiamond, the
recently proposed form composed of \cs \ units in the diamond structure. To
understand the bonding character of of these cluster-based solids, we analyze
the electronic structure of \csc \ and of hyperdiamond and compare them to the
electronic states of crystalline cubic diamond.Comment: 15 pages, latex, no figure
Ab-initio Molecular Dynamics study of electronic and optical properties of silicon quantum wires: Orientational Effects
We analyze the influence of spatial orientation on the optical response of
hydrogenated silicon quantum wires. The results are relevant for the
interpretation of the optical properties of light emitting porous silicon. We
study (111)-oriented wires and compare the present results with those
previously obtained within the same theoretical framework for (001)-oriented
wires [F. Buda {\it et al.}, {\it Phys. Rev. Lett.} {\bf 69}, 1272, (1992)]. In
analogy with the (001)-oriented wires and at variance with crystalline bulk
silicon, we find that the (111)-oriented wires exhibit a direct gap at whose value is largely enhanced with respect to that found in bulk
silicon because of quantum confinement effects. The imaginary part of the
dielectric function, for the external field polarized in the direction of the
axis of the wires, shows features that, while being qualitatively similar to
those observed for the (001) wires, are not present in the bulk. The main
conclusion which emerges from the present study is that, if wires a few
nanometers large are present in the porous material, they are
optically active independently of their specific orientation.Comment: 14 pages (plus 6 figures), Revte
Event-based relaxation of continuous disordered systems
A computational approach is presented to obtain energy-minimized structures
in glassy materials. This approach, the activation-relaxation technique (ART),
achieves its efficiency by focusing on significant changes in the microscopic
structure (events). The application of ART is illustrated with two examples:
the structure of amorphous silicon, and the structure of Ni80P20, a metallic
glass.Comment: 4 pages, revtex, epsf.sty, 3 figure
Generation of Intrinsic Vibrational Gap Modes in Three-Dimensional Ionic Crystals
The existence of anharmonic localization of lattice vibrations in a perfect
3-D diatomic ionic crystal is established for the rigid-ion model by molecular
dynamics simulations. For a realistic set of NaI potential parameters, an
intrinsic localized gap mode vibrating in the [111] direction is observed for
fcc and zinc blende lattices. An axial elastic distortion is an integral
feature of this mode which forms more readily for the zinc blende than for the
fcc structure. Molecular dynamics simulations verify that in each structure
this localized mode may be stable for at least 200 cycles.Comment: 5 pages, 4 figures, RevTeX, using epsf.sty. To be published in Phys.
Rev. B. Also available at http://www.msc.cornell.edu/~kiselev
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