9,427 research outputs found
Generalized Stacking Fault Energy Surfaces and Dislocation Properties of Silicon: A First-Principles Theoretical Study
The generalized stacking fault (GSF) energy surfaces have received
considerable attention due to their close relation to the mechanical properties
of solids. We present a detailed study of the GSF energy surfaces of silicon
within the framework of density functional theory. We have calculated the GSF
energy surfaces for the shuffle and glide set of the (111) plane, and that of
the (100) plane of silicon, paying particular attention to the effects of the
relaxation of atomic coordinates. Based on the calculated GSF energy surfaces
and the Peierls-Nabarro model, we obtain estimates for the dislocation
profiles, core energies, Peierls energies, and the corresponding stresses for
various planar dislocations of silicon.Comment: 9 figures (not included; send requests to [email protected]
Fully quantum mechanical dynamic analysis of single-photon transport in a single-mode waveguide coupled to a traveling-wave resonator
We analyze the dynamics of single photon transport in a single-mode waveguide
coupled to a micro-optical resonator using a fully quantum mechanical model. We
examine the propagation of a single-photon Gaussian packet through the system
under various coupling conditions. We review the theory of single photon
transport phenomena as applied to the system and we develop a discussion on the
numerical technique we used to solve for dynamical behavior of the quantized
field. To demonstrate our method and to establish robust single photon results,
we study the process of adiabatically lowering or raising the energy of a
single photon trapped in an optical resonator under active tuning of the
resonator. We show that our fully quantum mechanical approach reproduces the
semi-classical result in the appropriate limit and that the adiabatic invariant
has the same form in each case. Finally, we explore the trapping of a single
photon in a system of dynamically tuned, coupled optical cavities.Comment: 24 pages, 10 figure
Interacting damage models mapped onto Ising and percolation models
We introduce a class of damage models on regular lattices with isotropic
interactions, as e.g. quasistatic fiber bundles. The system starts intact with
a surface-energy threshold required to break any cell sampled from an
uncorrelated quenched-disorder distribution. The evolution of this
heterogeneous system is ruled by Griffith's principle which states that a cell
breaks when the release in elastic energy in the system exceeds the
surface-energy barrier necessary to break the cell. By direct integration over
all possible realizations of the quenched disorder, we obtain the probability
distribution of each damage configuration at any level of the imposed external
deformation. We demonstrate an isomorphism between the distributions so
obtained and standard generalized Ising models, in which the coupling constants
and effective temperature in the Ising model are functions of the nature of the
quenched-disorder distribution and the extent of accumulated damage. In
particular, we show that damage models with global load sharing are isomorphic
to standard percolation theory, that damage models with local load sharing rule
are isomorphic to the standard Ising model, and draw consequences thereof for
the universality class and behavior of the autocorrelation length of the
breakdown transitions corresponding to these models. We also treat damage
models having more general power-law interactions, and classify the breakdown
process as a function of the power-law interaction exponent. Last, we also show
that the probability distribution over configurations is a maximum of Shannon's
entropy under some specific constraints related to the energetic balance of the
fracture process, which firmly relates this type of quenched-disorder based
damage model to standard statistical mechanics.Comment: 16 pages, 3 figure
Rapid state purification protocols for a Cooper pair box
We propose techniques for implementing two different rapid state purification
schemes, within the constraints present in a superconducting charge qubit
system. Both schemes use a continuous measurement of charge (z) measurements,
and seek to minimize the time required to purify the conditional state. Our
methods are designed to make the purification process relatively insensitive to
rotations about the x-axis, due to the Josephson tunnelling Hamiltonian. The
first proposed method, based on the scheme of Jacobs [Phys. Rev. A 67,
030301(R) (2003)] uses the measurement results to control bias (z) pulses so as
to rotate the Bloch vector onto the x-axis of the Bloch sphere. The second
proposed method, based on the scheme of Wiseman and Ralph [New J. Phys. 8, 90
(2006)] uses a simple feedback protocol which tightly rotates the Bloch vector
about an axis almost parallel with the measurement axis. We compare the
performance of these and other techniques by a number of different measures.Comment: 14 pages, 14 figures. v2: Revised version after referee comments.
Accepted for publication by Physical Review
Scattering in an environment
The cross section of elastic electron-proton scattering taking place in an
electron gas is calculated within the Closed Time Path method. It is found to
be the sum of two terms, one being the expression in the vacuum except that it
involves dressing due to the electron gas. The other term is due to the
scattering particles-electron gas entanglement. This term dominates the usual
one when the exchange energy is in the vicinity of the Fermi energy.
Furthermore it makes the trajectories of the colliding particles more
consistent and the collision more irreversible, rendering the scattering more
classical in this regime.Comment: final version to appear in Phys. Rev.
Project Fire Flight 1 vibration data
Spectral density analysis on Fire Project flight vibration dat
Ames collaborative study of cosmic ray neutrons
The results of a collaborative study to define both the neutron flux and the spectrum more precisely and to develop a dosimetry package that can be flown quickly to altitude for solar flare events are described. Instrumentation and analysis techniques were used which were developed to measure accelerator-produced radiation. The instruments were flown in the Ames Research Center high altitude aircraft. Neutron instrumentation consisted of Bonner spheres with both active and passive detector elements, threshold detectors of both prompt-counter and activation-element types, a liquid scintillation spectrometer based on pulse-shape discrimination, and a moderated BF3 counter neutron monitor. In addition, charged particles were measured with a Reuter-Stokes ionization chamber system and dose equivalent with another instrument. Preliminary results from the first series of flights at 12.5 km (41,000 ft) are presented, including estimates of total neutron flux intensity and spectral shape and of the variation of intensity with altitude and geomagnetic latitude
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