10,466 research outputs found
Observation of magnetism in Au thin films
Direct magnetization measurements of thin gold films are presented. These
measurements integrate the signal from the thin film under study and the
magnetic contribution of the film's interface with the substrate. The
diamagnetic contribution to the signal from the bulk substrate is of the same
order as the noise level. we find that thin gold films can exhibit positive
magnetization. The character of their magnetic behavior is strongly substrate
dependent.Comment: 9 pages, 4 figure
Bounded Quantifier Instantiation for Checking Inductive Invariants
We consider the problem of checking whether a proposed invariant
expressed in first-order logic with quantifier alternation is inductive, i.e.
preserved by a piece of code. While the problem is undecidable, modern SMT
solvers can sometimes solve it automatically. However, they employ powerful
quantifier instantiation methods that may diverge, especially when is
not preserved. A notable difficulty arises due to counterexamples of infinite
size.
This paper studies Bounded-Horizon instantiation, a natural method for
guaranteeing the termination of SMT solvers. The method bounds the depth of
terms used in the quantifier instantiation process. We show that this method is
surprisingly powerful for checking quantified invariants in uninterpreted
domains. Furthermore, by producing partial models it can help the user diagnose
the case when is not inductive, especially when the underlying reason
is the existence of infinite counterexamples.
Our main technical result is that Bounded-Horizon is at least as powerful as
instrumentation, which is a manual method to guarantee convergence of the
solver by modifying the program so that it admits a purely universal invariant.
We show that with a bound of 1 we can simulate a natural class of
instrumentations, without the need to modify the code and in a fully automatic
way. We also report on a prototype implementation on top of Z3, which we used
to verify several examples by Bounded-Horizon of bound 1
Remote sensing of tropical tropopause layer radiation balance using A-train measurements
Determining the level of zero net radiative heating (LZH) is critical to understanding parcel trajectory in the Tropical Tropopause Layer (TTL) and associated stratospheric hydration processes. Previous studies of the TTL radiative balance have focused on using radiosonde data, but remote sensing measurements from polar-orbiting satellites may provide the relevant horizontal and vertical information for assessing TTL solar heating and infrared cooling rates, especially across the Pacific Ocean. CloudSat provides a considerable amount of vertical information about the distribution of cloud properties relevant to heating rate analysis. The ability of CloudSat measurements and ancillary information to constrain LZH is explored. We employ formal error propagation analysis for derived heating rate uncertainty given the CloudSat cloud property retrieval algorithms. Estimation of the LZH to within approximately 0.5 to 1 km is achievable with CloudSat, but it has a low-altitude bias because the radar is unable to detect thin cirrus. This can be remedied with the proper utilization of Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) lidar backscatter information. By utilizing an orbital simulation with the GISS data set, we explore the representativeness of non-cross-track scanning active sounders in terms of describing the LZH distribution. In order to supplement CloudSat, we explore the ability of Atmospheric Infrared Sounder (AIRS) and Advanced Microwave Scanning Radiometer-EOS (AMSR-E) to constrain LZH and find that these passive sounders are useful where the cloud top height does not exceed 7 km. The spatiotemporal distributions of LZH derived from CloudSat and CALIPSO measurements are presented which suggest that thin cirrus have a limited effect on LZH mean values but affect LZH variability
Secondary Radiation Influence on LSF Shapes in Radiography
A radiographic image is generated by both primary and secondary radiations. In a previous study [1, 2] the influence of secondary radiation on the generated image was shown for square based blocks of finite dimensions (few mean free paths). The response to a step change in the block’s thickness, varies drastically with the step location relative to the block’s edges [3]. However, even when the radiographed object is large and the step response is studied far off the object’s limits, the effect of the secondary radiation is still significant. This radiation distorts the “ideal” step response shape expected from the primary radiation
Quantum matchgate computations and linear threshold gates
The theory of matchgates is of interest in various areas in physics and
computer science. Matchgates occur in e.g. the study of fermions and spin
chains, in the theory of holographic algorithms and in several recent works in
quantum computation. In this paper we completely characterize the class of
boolean functions computable by unitary two-qubit matchgate circuits with some
probability of success. We show that this class precisely coincides with that
of the linear threshold gates. The latter is a fundamental family which appears
in several fields, such as the study of neural networks. Using the above
characterization, we further show that the power of matchgate circuits is
surprisingly trivial in those cases where the computation is to succeed with
high probability. In particular, the only functions that are
matchgate-computable with success probability greater than 3/4 are functions
depending on only a single bit of the input
Interplay between structure and magnetism in nanowires
We investigate the equilibrium geometry and electronic structure of
MoSI nanowires using ab initio Density Functional
calculations. The skeleton of these unusually stable nanowires consists of
rigid, functionalized Mo octahedra, connected by flexible, bi-stable sulphur
bridges. This structural flexibility translates into a capability to stretch up
to approximate 20% at almost no energy cost. The nanowires change from
conductors to narrow-gap magnetic semiconductors in one of their structural
isomers.Comment: 4 pages with PRL standards and 3 figure
Quadratic Scaling Bosonic Path Integral Molecular Dynamics
We present an algorithm for bosonic path integral molecular dynamics
simulations, which reduces the computational complexity with the number of
particles from cubic to quadratic. Path integral molecular dynamics simulations
of large bosonic systems are challenging, since a straightforward
implementation scales exponentially with the number of particles. We recently
developed a recursive algorithm that reduced the computational complexity from
exponential to cubic. It allowed performing the first path integral molecular
dynamics simulations of ~100 bosons, but the cubic scaling hindered
applications to much larger systems. Here, we report an improved algorithm that
scales only quadratically with system size. Simulations with our new method are
orders of magnitude faster, with a speedup that scales as , where and
are the number of beads (imaginary time slices) and particles,
respectively. In practice, this eliminates most of the cost of including
bosonic exchange effects in path integral molecular dynamics simulations. We
use the algorithm to simulate thousands of interacting bosons using path
integral molecular dynamics for the first time, spending days of computation on
simulations that would have otherwise taken decades to complete
Long-range order in the A-like phase of superfluid 3He in aerogel
A mutual action of the random anisotropy brought in the superfluid 3He by
aerogel and of the global anisotropy caused by its deformation is considered.
Strong global anisotropy tends to suppress fluctuations of orientation of the
order parameter and stabilizes ABM order parameter. In a limit of vanishing
anisotropy these fluctuations are getting critical. It is argued that still in
a region of small fluctuations the average order parameter can acquire "robust"
component. This component maintains a long-range order even in a limit of
vanishing global anisotropy.Comment: A contribution to QFS 2007 in Kazan, revised for publication in the
Proceeding
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