7,952 research outputs found
Digital Alchemy for Materials Design: Colloids and Beyond
Starting with the early alchemists, a holy grail of science has been to make
desired materials by modifying the attributes of basic building blocks.
Building blocks that show promise for assembling new complex materials can be
synthesized at the nanoscale with attributes that would astonish the ancient
alchemists in their versatility. However, this versatility means that making
direct connection between building block attributes and bulk behavior is both
necessary for rationally engineering materials, and difficult because building
block attributes can be altered in many ways. Here we show how to exploit the
malleability of the valence of colloidal nanoparticle "elements" to directly
and quantitatively link building block attributes to bulk behavior through a
statistical thermodynamic framework we term "digital alchemy". We use this
framework to optimize building blocks for a given target structure, and to
determine which building block attributes are most important to control for
self assembly, through a set of novel thermodynamic response functions, moduli
and susceptibilities. We thereby establish direct links between the attributes
of colloidal building blocks and the bulk structures they form. Moreover, our
results give concrete solutions to the more general conceptual challenge of
optimizing emergent behaviors in nature, and can be applied to other types of
matter. As examples, we apply digital alchemy to systems of truncated
tetrahedra, rhombic dodecahedra, and isotropically interacting spheres that
self assemble diamond, FCC, and icosahedral quasicrystal structures,
respectively.Comment: 17 REVTeX pages, title fixed to match journal versio
Distributed Quantum Computation Based-on Small Quantum Registers
We describe and analyze an efficient register-based hybrid quantum
computation scheme. Our scheme is based on probabilistic, heralded optical
connection among local five-qubit quantum registers. We assume high fidelity
local unitary operations within each register, but the error probability for
initialization, measurement, and entanglement generation can be very high
(~5%). We demonstrate that with a reasonable time overhead our scheme can
achieve deterministic non-local coupling gates between arbitrary two registers
with very high fidelity, limited only by the imperfections from the local
unitary operation. We estimate the clock cycle and the effective error
probability for implementation of quantum registers with ion-traps or
nitrogen-vacancy (NV) centers. Our new scheme capitalizes on a new efficient
two-level pumping scheme that in principle can create Bell pairs with
arbitrarily high fidelity. We introduce a Markov chain model to study the
stochastic process of entanglement pumping and map it to a deterministic
process. Finally we discuss requirements for achieving fault-tolerant operation
with our register-based hybrid scheme, and also present an alternative approach
to fault-tolerant preparation of GHZ states.Comment: 22 Pages, 23 Figures and 1 Table (updated references
Weak continuous monitoring of a flux qubit using coplanar waveguide resonator
We study a flux qubit in a coplanar waveguide resonator by measuring
transmission through the system. In our system with the flux qubit decoupled
galvanically from the resonator, the intermediate coupling regime is achieved.
In this regime dispersive readout is possible with weak backaction on the
qubit. The detailed theoretical analysis and simulations give a good agreement
with the experimental data and allow to make the qubit characterization.Comment: 4 pages, 3 figures, to be published in Phys. Rev.
Measurement Based Quantum Computation on Fractal Lattices
In this article we extend on work which establishes an analology between
one-way quantum computation and thermodynamics to see how the former can be
performed on fractal lattices. We find fractals lattices of arbitrary dimension
greater than one which do all act as good resources for one-way quantum
computation, and sets of fractal lattices with dimension greater than one all
of which do not. The difference is put down to other topological factors such
as ramification and connectivity. This work adds confidence to the analogy and
highlights new features to what we require for universal resources for one-way
quantum computation
First Results from Lattice Simulation of the PWMM
We present results of lattice simulations of the Plane Wave Matrix Model
(PWMM). The PWMM is a theory of supersymmetric quantum mechanics that has a
well-defined canonical ensemble. We simulate this theory by applying rational
hybrid Monte Carlo techniques to a naive lattice action. We examine the strong
coupling behaviour of the model focussing on the deconfinement transition.Comment: v3 20 pages, 8 figures, comment adde
XMM-Newton X-ray Observations of the Wolf-Rayet Binary System WR 147
We present results of a 20 ksec X-ray observation of the Wolf-Rayet (WR)
binary system WR 147 obtained with XMM-Newton. Previous studies have shown that
this system consists of a nitrogen-type WN8 star plus an OB companion whose
winds are interacting to produce a colliding wind shock. X-ray spectra from the
pn and MOS detectors confirm the high extinction reported from IR studies and
reveal hot plasma including the first detection of the Fe K-alpha line complex
at 6.67 keV. Spectral fits with a constant-temperature plane-parallel shock
model give a shock temperature kT(shock) = 2.7 keV [T(shock) ~ 31 MK], close to
but slightly hotter than the maximum temperature predicted for a colliding wind
shock. Optically thin plasma models suggest even higher temperatures, which are
not yet ruled out. The X-ray spectra are harder than can be accounted for using
2D numerical colliding wind shock models based on nominal mass-loss parameters.
Possible explanations include: (i) underestimates of the terminal wind speeds
or wind abundances, (ii) overly simplistic colliding wind models, or (iii) the
presence of other X-ray emission mechanisms besides colliding wind shocks.
Further improvement of the numerical models to include potentially important
physics such as non-equilibrium ionization will be needed to rigorously test
the colliding wind interpretation.Comment: 8 pages, 7 figure
Supercooling of the disordered vortex lattice in Bi_2Sr_2CaCu_2O_8+d
Time-resolved local induction measurements near to the vortex lattice
order-disorder transition in optimally doped
BiSrCaCuO single crystals shows that the
high-field, disordered phase can be quenched to fields as low as half the
transition field. Over an important range of fields, the electrodynamical
behavior of the vortex system is governed by the co-existence of the two phases
in the sample. We interpret the results in terms of supercooling of the
high-field phase and the possible first order nature of the order-disorder
transition at the ``second peak''.Comment: 4 pages, 3 figures. Submitted to Nature, July 10th, 1999; Rejected
August 8th for lack of broad interest Submitted to Physical Review Letters
September 10th, 199
The Soft X-ray Spectrum from NGC 1068 Observed with LETGS on Chandra
Using the combined spectral and spatial resolving power of the Low Energy
Transmission Grating (LETGS) on board Chandra, we obtain separate spectra from
the bright central source of NGC 1068 (Primary region), and from a fainter
bright spot 4" to the NE (Secondary region). Both spectra are dominated by line
emission from H- and He-like ions of C through S, and from Fe L-shell ions, but
also include narrow radiative recombination continua, indicating that most of
the soft X-ray emission arises in low-temperature (kT few eV) photoionized
plasma. We confirm the conclusions of Kinkhabwala et al. (2002), based on
XMM-Newton RGS observations, that the entire nuclear spectrum can be explained
by recombination/radiative cascade following photoionization, and radiative
decay following photoexcitation, with no evidence for hot, collisionally
ionized plasma. In addition, this model also provides an excellent fit to the
spectrum of the Secondary region, albeit with radial column densities a factor
of three lower, as would be expected given its distance from the source of the
ionizing continuum. The remarkable overlap and kinematical agreement of the
optical and X-ray line emission, coupled with the need for a distribution of
ionization parameter to explain the X-ray spectra, collectively imply the
presence of a distribution of densities (over a few orders of magnitude) at
each radius in the ionization cone. Relative abundances of all elements are
consistent with Solar abundance, except for N, which is 2-3 times Solar. The
long wavelength spectrum beyond 30 A is rich of L-shell transitions of Mg, Si,
S, and Ar, and M-shell transitions of Fe. The velocity dispersion decreases
with increasing ionization parameter, as deduced from these long wavelength
lines and the Fe-L shell lines.Comment: 12 pages, 11 figures, accepted for publication in Astronomy and
Astrophysic
The Chemical Evolution of the Milky Way
The field of chemical evolution modeling of the Galaxy is experiencing in the
last years a phase of high activity and important achievements. There are,
however, several open questions which still need to be answered. In this review
I summarize what have been the most important achievements and what are some of
the most urgent questions to be answered.Comment: 10 pages including 3 figs, to appear in "The Chemical Evolution of
the Milky Way. Stars vs Clusters", Proceedings of the Sept.1999 Vulcano
Workshop, F.Giovannelli and F.Matteucci eds (Kluwer, Dordrecht) in pres
Incommensuration Effects and Dynamics in Vortex Chains
We examine the motion of one-dimensional (1D) vortex matter embedded in a 2D
vortex system with weak pinning using numerical simulations. We confirm the
conjecture of Matsuda et al. [Science 294, 2136 (2001)] that the onset of the
temperature induced motion of the chain is due to an incommensuration effect of
the chain with the periodic potential created by the bulk vortices. In
addition, under an applied driving force we find a two stage depinning
transition, where the initial depinning of the vortex chain occurs through
soliton like pulses. When an ac drive is added to the dc drive, we observe
phase locking of the moving vortex chain.Comment: 4 pages, 4 postscript figure
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