331 research outputs found
Quantum Effects in the Mechanical Properties of Suspended Nanomechanical Systems
We explore the quantum aspects of an elastic bar supported at both ends and
subject to compression. If strain rather than stress is held fixed, the system
remains stable beyond the buckling instability, supporting two potential
minima. The classical equilibrium transverse displacement is analogous to a
Ginsburg-Landau order parameter, with strain playing the role of temperature.
We calculate the quantum fluctuations about the classical value as a function
of strain. Excitation energies and quantum fluctuation amplitudes are compared
for silicon beams and carbon nanotubes.Comment: RevTeX4. 5 pages, 3 eps figures. Submitted to Physical Review Letter
Intrinsic thermal vibrations of suspended doubly clamped single-wall carbon nanotubes
We report the observation of thermally driven mechanical vibrations of
suspended doubly clamped carbon nanotubes, grown by chemical vapor deposition
(CVD). Several experimental procedures are used to suspend carbon nanotubes.
The vibration is observed as a blurring in images taken with a scanning
electron microscope. The measured vibration amplitudes are compared with a
model based on linear continuum mechanics.Comment: pdf including figures, see:
http://www.unibas.ch/phys-meso/Research/Papers/2003/NT-Thermal-Vibrations.pd
Quasielastic 12C(e,e'p) Reaction at High Momentum Transfer
We measured the 12C(e,e'p) cross section as a function of missing energy in
parallel kinematics for (q,w) = (970 MeV/c, 330 MeV) and (990 MeV/c, 475 MeV).
At w=475 MeV, at the maximum of the quasielastic peak, there is a large
continuum (E_m > 50 MeV) cross section extending out to the deepest missing
energy measured, amounting to almost 50% of the measured cross section. The
ratio of data to DWIA calculation is 0.4 for both the p- and s-shells. At w=330
MeV, well below the maximum of the quasielastic peak, the continuum cross
section is much smaller and the ratio of data to DWIA calculation is 0.85 for
the p-shell and 1.0 for the s-shell. We infer that one or more mechanisms that
increase with transform some of the single-nucleon-knockout into
multinucleon knockout, decreasing the valence knockout cross section and
increasing the continuum cross section.Comment: 14 pages, 7 figures, Revtex (multicol, prc and aps styles), to appear
in Phys Rev
Nuclear Transparency to Intermediate-Energy Protons
Nuclear transparency in the (e,e'p) reaction for 135 < Tp < 800 MeV is
investigated using the distorted wave approximation. Calculations using
density-dependent effective interactions are compared with phenomenological
optical potentials. Nuclear transparency is well correlated with proton
absorption and neutron total cross sections. For Tp < 300 MeV there is
considerable sensitivity to the choice of optical model, with the empirical
effective interaction providing the best agreement with transparency data. For
Tp > 300 MeV there is much less difference between optical models, but the
calculations substantially underpredict transparency data and the discrepancy
increases with A. The differences between Glauber and optical model
calculations are related to their respective definitions of the semi-inclusive
cross section. By using a more inclusive summation over final states the
Glauber model emphasizes nucleon-nucleon inelasticity, whereas with a more
restrictive summation the optical model emphasizes nucleon-nucleus
inelasticity; experimental definitions of the semi-inclusive cross section lie
between these extremes.Comment: uuencoded gz-compressed tar file containing revtex and bbl files and
5 postscript figures, totalling 31 pages. Uses psfi
A Measurement of the Interference Structure Function, R_LT, for the 12C(e,e'p) reaction in the Quasielastic Region
The coincidence cross-section and the interference structure function, R_LT,
were measured for the 12C(e,e'p) 11B reaction at quasielastic kinematics and
central momentum transfer of q=400 MeV/c. The measurement was at an opening
angle of theta_pq=11 degrees, covering a range in missing energy of E_m = 0 to
65 MeV. The R_LT structure function is found to be consistent with zero for E_m
> 50 MeV, confirming an earlier study which indicated that R_L vanishes in this
region. The integrated strengths of the p- and s-shell are compared with a
Distorted Wave Impulse Approximation calculation. The s-shell strength and
shape are compared with a Hartree Fock-Random Phase Approximation calculation.
The DWIA calculation overestimates the cross sections for p- and s-shell proton
knockout as expected, but surprisingly agrees with the extracted R_LT value for
both shells. The HF-RPA calculation describes the data more consistently, which
may be due to the inclusion of 2-body currents in this calculation.Comment: 8 Pages LaTex, 5 postscript figures. Submitted to Phys. Rev.
The Impact of Major Events on the Lives of Family Caregivers of Children with Disabilities
Copyright 1996 Families International, Inc.The authors examine the family caregiving experience among families with children with severe emotional
disabilities from a perspective that recognizes the importance of the family's views and feelings. This viewpoint anticipates
the occurrence of both positive and negative experiences and seeks to illuminate the caregiving process from the perspective
of outcomes achieved. Family caregivers of 164 children with serious emotional disorders were asked to identify
major pleasant and stressful events that had occurred in the past 12 months. The most frequently described pleasant
events related to children's behavior, school activities, and interactions with professionals and friends. Frequently described
problem areas included children's behavior, professionals/services, and difficulty with school. The impact of these
pleasant and stressful events was examined with respect to caregivers' perceived well-being: (I) overall stress, (2) the
ability to fulfill responsibilities, and (3) pleasure experienced in various life domains. Implications of the study findings
for supporting family caregivers in their roles are discussed
Recoil Polarization Measurements for Neutral Pion Electroproduction at Q^2=1 (GeV/c)^2 Near the Delta Resonance
We measured angular distributions of differential cross section, beam
analyzing power, and recoil polarization for neutral pion electroproduction at
Q^2 = 1.0 (GeV/c)^2 in 10 bins of W across the Delta resonance. A total of 16
independent response functions were extracted, of which 12 were observed for
the first time. Comparisons with recent model calculations show that response
functions governed by real parts of interference products are determined
relatively well near 1.232 GeV, but variations among models is large for
response functions governed by imaginary parts and for both increases rapidly
with W. We performed a nearly model-independent multipole analysis that adjusts
complex multipoles with high partial waves constrained by baseline models.
Parabolic fits to the W dependence of the multipole analysis around the Delta
mass gives values for SMR = (-6.61 +/- 0.18)% and EMR = (-2.87 +/- 0.19)% that
are distinctly larger than those from Legendre analysis of the same data.
Similarly, the multipole analysis gives Re(S0+/M1+) = (+7.1 +/- 0.8)% at
W=1.232 GeV, consistent with recent models, while the traditional Legendre
analysis gives the opposite sign because its truncation errors are quite
severe. Finally, using a unitary isobar model (UIM), we find that excitation of
the Roper resonance is dominantly longitudinal with S1/2 = (0.05 +/- 0.01)
GeV^(-1/2) at Q^2=1. The ReS0+ and ReE0+ multipoles favor pseudovector coupling
over pseudoscalar coupling or a recently proposed mixed-coupling scheme, but
the UIM does not reproduce the imaginary parts of 0+ multipoles well.Comment: 60 pages, 54 figure
Recoil Polarization for Delta Excitation in Pion Electroproduction
We measured angular distributions of recoil-polarization response functions
for neutral pion electroproduction for W=1.23 GeV at Q^2=1.0 (GeV/c)^2,
obtaining 14 separated response functions plus 2 Rosenbluth combinations; of
these, 12 have been observed for the first time. Dynamical models do not
describe quantities governed by imaginary parts of interference products well,
indicating the need for adjusting magnitudes and phases for nonresonant
amplitudes. We performed a nearly model-independent multipole analysis and
obtained values for Re(S1+/M1+)=-(6.84+/-0.15)% and Re(E1+/M1+)=-(2.91+/-0.19)%
that are distinctly different from those from the traditional Legendre analysis
based upon M1+ dominance and sp truncation.Comment: 5 pages, 2 figures, for PR
The Quasielastic 3He(e,e'p)d Reaction at Q^2 = 1.5 GeV^2 for Recoil Momenta up to 1 GeV/c
We have studied the quasielastic 3He(e,e'p)d reaction in perpendicular
coplanar kinematics, with the energy and momentum transferred by the electron
fixed at 840 MeV and 1502 MeV/c, respectively. The 3He(e,e'p)d cross section
was measured for missing momenta up to 1000 MeV/c, while the A_TL asymmetry was
extracted for missing momenta up to 660 MeV/c. For missing momenta up to 150
MeV/c, the measured cross section is described well by calculations that use a
variational ground-state wave function of the 3He nucleus derived from a
potential that includes three-body forces. For missing momenta from 150 to 750
MeV/c, strong final-state interaction effects are observed. Near 1000 MeV/c,
the experimental cross section is more than an order of magnitude larger than
predicted by available theories. The A_TL asymmetry displays characteristic
features of broken factorization, and is described reasonably well by available
models.Comment: 5 pages, 3 figures, submitted to Physical Review Letters, v3: changed
conten
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