259 research outputs found
Vibrations of micro-eV energies in nanocrystalline microstructures
The phonon density of states of nanocrystalline bcc Fe and nanocrystalline fcc Ni3Fe were measured by inelastic neutron scattering in two different ranges of energy. As has been reported previously, the nanocrystalline materials showed enhancements in their phonon density of states at energies from 2 to 15 meV, compared to control samples composed of large crystals. The present measurements were extended to energies in the micro-eV range, and showed significant, but smaller, enhancements in the number of modes in the energy range from 5 to 18 mueV. These modes of micro-eV energies provide a long-wavelength limit that bounds the fraction of modes at milli-eV energies originating with the cooperative dynamics of the nanocrystalline microstructure
Layer- and bulk roton excitations of 4He in porous media
We examine the energetics of bulk and layer-roton excitations of 4He in
various porous medial such as aerogel, Geltech, or Vycor, in order to find out
what conclusions can be drawn from experiments on the energetics about the
physisorption mechanism. The energy of the layer-roton minimum depends
sensitively on the substrate strength, thus providing a mechanism for a direct
measurement of this quantity. On the other hand, bulk-like roton excitations
are largely independent of the interaction between the medium and the helium
atoms, but the dependence of their energy on the degree of filling reflects the
internal structure of the matrix and can reveal features of 4He at negative
pressures. While bulk-like rotons are very similar to their true bulk
counterparts, the layer modes are not in close relation to two-dimensional
rotons and should be regarded as a third, completely independent kind of
excitation
Excitations in confined helium
We design models for helium in matrices like aerogel, Vycor or Geltech from a
manifestly microscopic point of view. For that purpose, we calculate the
dynamic structure function of 4He on Si substrates and between two Si walls as
a function of energy, momentum transfer, and the scattering angle. The
angle--averaged results are in good agreement with the neutron scattering data;
the remaining differences can be attributed to the simplified model used here
for the complex pore structure of the materials. A focus of the present work is
the detailed identification of coexisting layer modes and bulk--like
excitations, and, in the case of thick films, ripplon excitations. Involving
essentially two--dimensional motion of atoms, the layer modes are sensitive to
the scattering angle.Comment: Phys. Rev. B (2003, in press
Dynamics of liquid 4He in Vycor
We have measured the dynamic structure factor of liquid 4He in Vycor using
neutron inelastic scattering. Well-defined phonon-roton (p-r) excitations are
observed in the superfluid phase for all wave vectors 0.3 < Q < 2.15. The p-r
energies and lifetimes at low temperature (T = 0.5 K) and their temperature
dependence are the same as in bulk liquid 4He. However, the weight of the
single p-r component does not scale with the superfluid fraction (SF) as it
does in the bulk. In particular, we observe a p-r excitation between T_c =
1.952 K, where SF = 0, and T_(lambda)=2.172 K of the bulk. This suggests, if
the p-r excitation intensity scales with the Bose condensate, that there is a
separation of the Bose-Einstein condensation temperature and the superfluid
transition temperature T_c of 4He in Vycor. We also observe a two-dimensional
layer mode near the roton wave vector. Its dispersion is consistent with
specific heat and SF measurements and with layer modes observed on graphite
surfaces.Comment: 3 pages, 4 figure
The High-Flux Backscattering Spectrometer at the NIST Center for Neutron Research
We describe the design and current performance of the high-flux
backscattering spectrometer located at the NIST Center for Neutron Research.
The design incorporates several state-of-the-art neutron optical devices to
achieve the highest flux on sample possible while maintaining an energy
resolution of less than 1mueV. Foremost among these is a novel phase-space
transformation chopper that significantly reduces the mismatch between the beam
divergences of the primary and secondary parts of the instrument. This resolves
a long-standing problem of backscattering spectrometers, and produces a
relative gain in neutron flux of 4.2. A high-speed Doppler-driven monochromator
system has been built that is capable of achieving energy transfers of up to
+-50mueV, thereby extending the dynamic range of this type of spectrometer by
more than a factor of two over that of other reactor-based backscattering
instruments
Questions of fairness and anti-doping in US cycling: The contrasting experiences of professionals and amateurs
The focus of researchers, media and policy on doping in cycling is often limited to the professional level of the sport. However, anti-doping test results since 2001 demonstrate that banned substances are also used by US cyclists at lower levels of the sport, necessitating a broader view of the patterns and motivations of substance use within the sport. In this article, we describe and explain the doping culture that has emerged in domestic US cycling among amateur and semi-professionals. Through analysis of records from sports governing bodies and journalistic reports, we assess the range of violation types and discuss the detection and punishing of riders who were not proven to have intended to cheat but became "collateral damage" in the war on doping. We argue that the phenomenon of doping is more complex than what has been shown to occur in elite sport, as it includes a wider variety of behaviours, situations and motivations. We develop fresh insights by examining cases where doping has been accidental, intrinsically motivated, non-performance enhancing or the result of prescribed medical treatments banned by anti-doping authorities. Such trends call into question the fairness of anti-doping measures, and we discuss the possibility of developing localised solutions to testing and sanctioning amateur athletes
The excited-state structure, vibrations, lifetimes, and nonradiative dynamics of jet-cooled 1-methylcytosine
We have investigated the S0 → S1 UV vibronic spectrum and time-resolved S1 state dynamics of
jet-cooled amino-keto 1-methylcytosine (1MCyt) using two-color resonant two-photon ionization,
UV/UV holeburning and depletion spectroscopies, as well as nanosecond and picosecond timeresolved
pump/delayed ionization measurements. The experimental study is complemented with
spin-component-scaled second-order coupled-cluster and multistate complete active space second
order perturbation ab initio calculations. Above the weak electronic origin of 1MCyt at 31 852 cm−1
about 20 intense vibronic bands are observed. These are interpreted as methyl group torsional
transitions coupled to out-of-plane ring vibrations, in agreement with the methyl group rotation
and out-of-plane distortions upon 1ππ∗ excitation predicted by the calculations. The methyl torsion
and ν′1 (butterfly) vibrations are strongly coupled, in the S1 state. The S0 → S1 vibronic spectrum
breaks off at a vibrational excess energy Eexc ∼ 500 cm−1, indicating that a barrier in front of the
ethylene-type S1 S0 conical intersection is exceeded, which is calculated to lie at Eexc = 366 cm−1.
The S1 S0 internal conversion rate constant increases from kIC = 2 · 109 s−1 near the S1(v = 0)
level to 1 · 1011 s−1 at Eexc = 516 cm−1. The 1ππ∗ state of 1MCyt also relaxes into the lower-lying
triplet T1 (3ππ∗) state by intersystem crossing (ISC); the calculated spin-orbit coupling (SOC) value
is 2.4 cm−1. The ISC rate constant is 10–100 times lower than kIC; it increases from kISC = 2 · 108 s−1
near S1(v = 0) to kISC = 2 · 109 s−1 at Eexc = 516 cm−1. The T1 state energy is determined from the
onset of the time-delayed photoionization efficiency curve as 25 600 ± 500 cm−1. The T2 (3nπ∗)
state lies >1500 cm−1 above S1(v = 0), so S1 T2 ISC cannot occur, despite the large SOC
parameter of 10.6 cm−1. An upper limit to the adiabatic ionization energy of 1MCyt is determined
as 8.41 ± 0.02 eV. Compared to cytosine, methyl substitution at N1 lowers the adiabatic ionization
energy by ≥0.32 eV and leads to a much higher density of vibronic bands in the S0 → S1 spectrum.
The effect of methylation on the radiationless decay to S0 and ISC to T1 is small, as shown by
the similar break-off of the spectrum and the similar computed mechanismsThis research has been supported by the Schweiz. Nationalfonds (Grant Nos. 121993 and 132540), the Agència de Gestió d’Ajuts Universitaris i de Recerca (AGAUR) from Catalonia (Spain) (Grant No. 2014SGR1202), the Ministerio de EconomÃa y Competividad (MINECO) from Spain (Grant No. CTQ2015-69363-P), and the National Natural Science Foundation of China (Grant No. 21303007
Phase phonon spectrum and melting in a quantum rotor model with diagonal disorder
We study the zero-temperature () quantum rotor model with on-site
disorder in the charging energy. Such a model may serve as an idealized
Hamiltonian for an array of Josephson-coupled small superconducting grains, or
superfluid He in a disordered environment. In the approximation of
small-amplitude phase fluctuations, the Hamiltonian maps onto a system of
coupled harmonic oscillators with on-site disorder. We study the effects of
disorder in this harmonic regime, using the coherent potential approximation
(CPA), obtaining the density of states and the lifetimes of the spin-wave-like
excitations for several choices of the parameters which characterize the
disorder. Finally, we estimate the parameters characterizing the
quantum melting of the phase order, using a suitable Lindemann criterion.Comment: 8 pages, 5 figures. To be published in Phys. Rev. B. Minor change
Effect of an exercise training intervention with resistance bands on blood cell counts during chemotherapy for lung cancer: a pilot randomized controlled trial
PURPOSE: Chemotherapy for lung cancer can have a detrimental effect on white blood cell (WBC) and red blood cell (RBC) counts. Physical exercise may have a role in improving WBCs and RBCs, although few studies have examined cancer patients receiving adjuvant therapies. The purpose of this pilot trial was to examine the effects of an exercise intervention utilizing resistance bands on WBCs and RBCs in lung cancer patients receiving curative intent chemotherapy. METHODS: A sample of lung cancer patients scheduled for curative intent chemotherapy was randomly assigned to the exercise intervention (EX) condition or usual care (UC) condition. The EX condition participated in a three times weekly exercise program using resistance bands for the duration of chemotherapy. RESULTS: A total of 14 lung cancer patients completed the trial. EX condition participants completed 79% of planned exercise sessions. The EX condition was able to maintain WBCs over the course of the intervention compared to declines in the UC condition (p = .008; d = 1.68). There were no significant differences in change scores in RBCs. CONCLUSIONS: Exercise with resistance bands may help attenuate declines in WBCs in lung cancer patients receiving curative intent chemotherapy. Larger trials are warranted to validate these findings. Ultimately these findings could be informative for the development of supportive care strategies for lung cancer patients receiving chemotherapy. TRIAL REGISTRATION: Clinical Trials Registration #: NCT01130714
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