31,626 research outputs found
Acoustic Attenuation by Two-dimensional Arrays of Rigid Cylinders
In this Letter, we present a theoretical analysis of the acoustic
transmission through two-dimensional arrays of straight rigid cylinders placed
parallelly in the air. Both periodic and completely random arrangements of the
cylinders are considered. The results for the sound attenuation through the
periodic arrays are shown to be in a remarkable agreement with the reported
experimental data. As the arrangement of the cylinders is randomized, the
transmission is significantly reduced for a wider range of frequencies. For the
periodic arrays, the acoustic band structures are computed by the plane-wave
expansion method and are also shown to agree with previous results.Comment: 4 pages, 3 figure
How do top construction companies diversify in the international construction market?
This article won the Best Paper Award in the symposiumGlobalization has created an international market that allows construction companies transcend
traditional national boundaries and conduct business overseas. With new opportunities being brought
to contractors, competition also grows exponentially in this market. Diversification is frequently
adopted by these contractors as a strategy by the contractors for either growth, or risk management, or
both in this competitive environment. However, the pattern of diversification has not been well
measured, mapped, and analyzed. The aim of this research is to develop a Diversify Index (DI) and
examine international contractors’ diversification pattern. The data is from the Top 225 International
Contractors’ reports ranked by ENR (Engineering News-Record) from 1995 to 2014. Distributions of
the DI were explored first and then case studies were used to investigate the specific diversification
strategies adopted by the top international contractors. It is discovered that a downward trend of DI
with subsequent changes in rankings expresses that larger contractors increasingly adopt various
diversification strategies in international competition. The results provide valuable sights on the
relationship between the competitive success and their diversification strategies as well as the
tendency of diversification strategies adopted by top international contractors in different regions.postprin
Planar cyclotron motion in unidirectional superlattices defined by strong magnetic and electric fields: Traces of classical orbits in the energy spectrum
We compare the quantum and the classical description of the two-dimensional
motion of electrons subjected to a perpendicular magnetic field and a
one-dimensional lateral superlattice defined by spatially periodic magnetic and
electric fields of large amplitudes. We explain in detail the complicated
energy spectra, consisting of superimposed branches of strong and of weak
dispersion, by the correspondence between the respective eigenstates and the
``channeled'' and ``drifting'' orbits of the classical description.Comment: 11 pages, 11 figures, to appear in Physical Review
Diffuse Neutron Scattering Study of Relaxor Ferroelectric (1-x)Pb(Zn1/3Nb2/3)O3-xPbTiO3(PZN-xPT)
Diffuse neutron scattering is a valuable tool to obtain information about the
size and orientation of the polar nanoregions that are a characteristic feature
of relaxor ferroelectrics. In this paper, we present new diffuse scattering
results obtained on Pb(Zn1/3Nb2/3)O3 (PZN for short) and
(1-x)Pb(Zn1/3Nb2/3)O3-xPbTiO3(PZN-xPT)single crystals (with x=4.5 and 9%),
around various Bragg reflections and along three symmetry directions in the
[100]-[011] zone. Diffuse scattering is observed around reflections with mixed
indices, (100), (011) and (300), and along transverse and diagonal directions
only. No diffuse scattering is found in longitudinal scans. The diffuse
scattering peaks can be fitted well with a Lorentzian function, from which a
correlation length is extracted. The correlation length increases with
decreasing temperatures down to the transition at Tc, first following a
Curie-Weiss law, then departing from it and becoming flat at very low
temperatures. These results are interpreted in terms of three temperature
regions: 1) dynamic polarization fluctuations (i.e. with a finite lifetime) at
high temperatures, 2) static polarization reorientations (condensation of polar
nanoregions) that can still reorient as a unit (relaxor behavior) at
intermediate temperatures and 3) orientational freezing of the polar
nanoregions with random strain fields in pure PZN or a structural phase
transition in PZN-xPT at low temperatures. The addition of PT leads to a
broadening of the diffuse scattering along the diagonal ([111]) relative to the
transverse ([100]) direction, indicating a change in the orientation of the
polar regions. Also, with the addition of PT, the polar nanoregions condense at
a higher temperature above Tc.Comment: AIP 6x9 style files, 9 pages, 5 figures, Conference-Fundamental
Physics of Ferroelectrics 200
Systematic study of Optical Feshbach Resonances in an ideal gas
Using a narrow intercombination line in alkaline earth atoms to mitigate
large inelastic losses, we explore the Optical Feshbach Resonance (OFR) effect
in an ultracold gas of bosonic Sr. A systematic measurement of three
resonances allows precise determinations of the OFR strength and scaling law,
in agreement with coupled-channels theory. Resonant enhancement of the complex
scattering length leads to thermalization mediated by elastic and inelastic
collisions in an otherwise ideal gas. OFR could be used to control atomic
interactions with high spatial and temporal resolution.Comment: Significant changes to text and figure presentation to improve
clarity. Extended supplementary material. 4 pages, 4 figures; includes
supplementary material 8 pages, 4 figures. Submitted to Physical Review
Letter
Non-linear Spectroscopy of Sr Atoms in an Optical Cavity for Laser Stabilization
We study the non-linear interaction of a cold sample of strontium-88 atoms
coupled to a single mode of a low finesse optical cavity in the so-called bad
cavity limit and investigate the implications for applications to laser
stabilization. The atoms are probed on the weak inter-combination line \lvert
5s^{2} \, ^1 \textrm{S}_0 \rangle \,-\, \lvert 5s5p \, ^3 \textrm{P}_1 \rangle
at 689 nm in a strongly saturated regime. Our measured observables include the
atomic induced phase shift and absorption of the light field transmitted
through the cavity represented by the complex cavity transmission coefficient.
We demonstrate high signal-to-noise-ratio measurements of both quadratures -
the cavity transmitted phase and absorption - by employing FM spectroscopy
(NICE-OHMS). We also show that when FM spectroscopy is employed in connection
with a cavity locked to the probe light, observables are substantially modified
compared to the free space situation where no cavity is present. Furthermore,
the non-linear dynamics of the phase dispersion slope is experimentally
investigated and the optimal conditions for laser stabilization are
established. Our experimental results are compared to state-of-the-art cavity
QED theoretical calculations.Comment: 7 pages, 4 figure
Hofstadter-type energy spectra in lateral superlattices defined by periodic magnetic and electrostatic fields
We calculate the energy spectrum of an electron moving in a two-dimensional
lattice which is defined by an electric potential and an applied perpendicular
magnetic field modulated by a periodic surface magnetization. The spatial
direction of this magnetization introduces complex phases into the Fourier
coefficients of the magnetic field. We investigate the effect of the relative
phases between electric and magnetic modulation on band width and internal
structure of the Landau levels.Comment: 5 LaTeX pages with one gif figure to appear in Phys. Rev.
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