3,385 research outputs found
Nonadiabatic generation of coherent phonons
The time-dependent density functional theory (TDDFT) is the leading
computationally feasible theory to treat excitations by strong electromagnetic
fields. Here the theory is applied to coherent optical phonon generation
produced by intense laser pulses. We examine the process in the crystalline
semimetal antimony (Sb), where nonadiabatic coupling is very important. This
material is of particular interest because it exhibits strong phonon coupling
and optical phonons of different symmetries can be observed. The TDDFT is able
to account for a number of qualitative features of the observed coherent
phonons, despite its unsatisfactory performance on reproducing the observed
dielectric functions of Sb. A simple dielectric model for nonadiabatic coherent
phonon generation is also examined and compared with the TDDFT calculations.Comment: 19 pages, 11 figures. This is prepared for a special issue of Journal
of Chemical Physics on the topic of nonadiabatic processe
Time-dependent density functional theory for strong electromagnetic fields in crystalline solids
We apply the coupled dynamics of time-dependent density functional theory and
Maxwell equations to the interaction of intense laser pulses with crystalline
silicon. As a function of electromagnetic field intensity, we see several
regions in the response. At the lowest intensities, the pulse is reflected and
transmitted in accord with the dielectric response, and the characteristics of
the energy deposition is consistent with two-photon absorption. The absorption
process begins to deviate from that at laser intensities ~ 10^13 W/cm^2, where
the energy deposited is of the order of 1 eV per atom. Changes in the
reflectivity are seen as a function of intensity. When it passes a threshold of
about 3 \times 1012 W/cm2, there is a small decrease. At higher intensities,
above 2 \times 10^13 W/cm^2, the reflectivity increases strongly. This behavior
can be understood qualitatively in a model treating the excited electron-hole
pairs as a plasma.Comment: 27 pages; 11 figure
Global Optical Control of a Quantum Spin Chain
Quantum processors which combine the long decoherence times of spin qubits
together with fast optical manipulation of excitons have recently been the
subject of several proposals. I show here that arbitrary single- and entangling
two-qubit gates can be performed in a chain of perpetually coupled spin qubits
solely by using laser pulses to excite higher lying states. It is also
demonstrated that universal quantum computing is possible even if these pulses
are applied {\it globally} to a chain; by employing a repeating pattern of four
distinct qubit units the need for individual qubit addressing is removed. Some
current experimental qubit systems would lend themselves to implementing this
idea.Comment: 5 pages, 3 figure
A BROAD SYMMETRY CRITERION FOR NONPARAMETRIC VALIDITY OF PARAMETRICALLY-BASED TESTS IN RANDOMIZED TRIALS
Summary. Pilot phases of a randomized clinical trial often suggest that a parametric model may be an accurate description of the trial\u27s longitudinal trajectories. However, parametric models are often not used for fear that they may invalidate tests of null hypotheses of equality between the experimental groups. Existing work has shown that when, for some types of data, certain parametric models are used, the validity for testing the null is preserved even if the parametric models are incorrect. Here, we provide a broader and easier to check characterization of parametric models that can be used to (a) preserve nonparametric validity of testing the null hypothesis, i.e., even when the models are incorrect, and (b) increase power compared to the non- or semiparametric bounds when the models are close to correct. We demonstrate our results in a clinical trial of depression in Alzheimer\u27s patients
Configuration mixing calculation for complete low-lying spectra with the mean-field Hamiltonian
We propose a new theoretical approach to ground and low-energy excited states
of nuclei extending the nuclear mean-field theory. It consists of three steps:
stochastic preparation of many Slater determinants, the parity and angular
momentum projection, and diagonalization of the generalized eigenvalue
problems. The Slater determinants are constructed in the three-dimensional
Cartesian coordinate representation capable of describing arbitrary shape of
nuclei. We examine feasibility and usefulness of the method by applying the
method with the BKN interaction to light 4N-nuclei, 12C, 16O, and 20Ne. We
discuss difficulties of keeping linear independence for basis states projected
on good parity and angular momentum and present a possible prescription.Comment: 12 pages, revtex
THE EFFECT OF COMPRESSION TIGHTS AND DURATION OF TESTING ON CONTINUOUS JUMPING MECHANICAL POWER
INTRODUCTION: In order to improve their performance, athletes seek advancements in technology, such as clothing. Manufacturers of compression tights, advertise that their product adds support to lower extremity musculature, thus may slow the onset of fatigue. Few scientific studies, however, have been conducted to identify how advancements in apparel influence an athleteâs performance (Kraemer et al., 1996). The purpose of the present study was to identify the effects of compression tights on mechanical power for continuous jumping. Secondly, the effect of duration on the mechanical power output by using Boscoâs method (1983) over 15, 30, 45, and 60 sec time-frames, as well as the interaction between apparel and duration
Hydrogen in the gas plume of an open-vent volcano, Mount Etna, Italy
We report here on the first hydrogen determinations in the volcanic gas plume of
Mount Etna, in Italy, which we obtained during periodic field surveys on the volcanoâs summit area with an upgraded MultiGAS. Using a specific (EZT3HYT) electrochemical sensor, we resolved H2 concentrations in the plume of 1â3 ppm above ambient (background) atmosphere and derived H2âSO2 and H2âH2O plume molar ratios of
0.002â0.044 (mean 0.013) and 0.0001â0.0042 (mean 0.0018), respectively. Taking the
above H2âSO2 ratios in combination with a timeâaveraged SO2 flux of 1600 Gg yrâ1,
we evaluate that Etna contributes a timeâaveraged H2 flux of âŒ0.65 Gg yrâ1,
suggesting that the volcanogenic contribution to the global atmospheric H2 budget (70,000â100,000 Gg yrâ1) is marginal. We also use our observed H2âH2O ratios to
propose that Etnaâs passive plume composition is (at least partially) representative of a quenched (temperatures between 750°C and 950°C) equilibrium in the gasâmagma system, at redox conditions close to the nickelânickel oxide (NNO) mineral buffer. The positive dependence between H2âSO2, H2âH2O, and CO2âSO2 ratios suggests that H2 is likely supplied (at least in part) by deeply rising CO2ârich gas bubbles, fluxing through a CO2âdepleted shallow conduit magma.PublishedB102041.2. TTC - Sorveglianza geochimica delle aree vulcaniche attive2.4. TTC - Laboratori di geochimica dei fluidiJCR Journalrestricte
Electron spin coherence in metallofullerenes: Y, Sc and La@C82
Endohedral fullerenes encapsulating a spin-active atom or ion within a carbon
cage offer a route to self-assembled arrays such as spin chains. In the case of
metallofullerenes the charge transfer between the atom and the fullerene cage
has been thought to limit the electron spin phase coherence time (T2) to the
order of a few microseconds. We study electron spin relaxation in several
species of metallofullerene as a function of temperature and solvent
environment, yielding a maximum T2 in deuterated o-terphenyl greater than 200
microseconds for Y, Sc and La@C82. The mechanisms governing relaxation (T1, T2)
arise from metal-cage vibrational modes, spin-orbit coupling and the nuclear
spin environment. The T2 times are over 2 orders of magnitude longer than
previously reported and consequently make metallofullerenes of interest in
areas such as spin-labelling, spintronics and quantum computing.Comment: 5 pages, 4 figure
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