259,486 research outputs found
Selecting between two transition states by which water oxidation intermediates on an oxide surface decay
While catalytic mechanisms on electrode surfaces have been proposed for
decades, the pathways by which the product's chemical bonds evolve from the
initial charge-trapping intermediates have not been resolved in time. Here, we
discover a reactive population of charge-trapping intermediates with states in
the middle of a semiconductor's band-gap to reveal the dynamics of two parallel
transition state pathways for their decay. Upon photo-triggering the water
oxidation reaction from the n-SrTiO3 surface with band-gap, pulsed excitation,
the intermediates' microsecond decay reflects transition state theory (TST)
through: (1) two distinct and reaction dependent (pH, T, Ionic Strength, and
H/D exchange) time constants, (2) a primary kinetic salt effect on each
activation barrier and an H/D kinetic isotope effect on one, and (3) realistic
activation barrier heights (0.4 - 0.5 eV) and TST pre-factors (10^11 - 10^12
Hz). A photoluminescence from midgap states in n-SrTiO3 reveals the reaction
dependent decay; the same spectrum was previously assigned by us to
hole-trapping at parallel Ti-O(dot)-Ti (bridge) and perpendicular Ti-O(dot)
(oxyl) O-sites using in situ ultrafast vibrational and optical spectroscopy.
Therefore, the two transition states are naturally associated with the decay of
these respective intermediates. Furthermore, we show that reaction conditions
select between the two pathways, one of which reflects a labile intermediate
facing the electrolyte (the oxyl) and the other a lattice oxygen (the bridge).
Altogether, we experimentally isolate an important activation barrier for water
oxidation, which is necessary for designing water oxidation catalysts with high
O2 turn over. Moreover, in isolating it, we identify competing mechanisms for
O2 evolution at surfaces and show how to use reaction conditions to select
between them
Time delays and energy transport velocities in three dimensional ideal cloaking
We obtained the energy transport velocity distribution for a three
dimensional ideal cloak explicitly. Near the operation frequency, the energy
transport velocity has rather peculiar distribution. The velocity along a line
joining the origin of the cloak is a constant, while the velocity approaches
zero at the inner boundary of the cloak. A ray pointing right into the origin
of the cloak will experience abrupt changes of velocities when it impinges on
the inner surface of the cloak. This peculiar distribution causes infinite time
delays for the ideal cloak within a geometric optics description.Comment: A scaling factor is added to convert the parameter \tau into the
physical tim
Analysis of a Classical Matrix Preconditioning Algorithm
We study a classical iterative algorithm for balancing matrices in the
norm via a scaling transformation. This algorithm, which goes back
to Osborne and Parlett \& Reinsch in the 1960s, is implemented as a standard
preconditioner in many numerical linear algebra packages. Surprisingly, despite
its widespread use over several decades, no bounds were known on its rate of
convergence. In this paper we prove that, for any irreducible (real
or complex) input matrix~, a natural variant of the algorithm converges in
elementary balancing operations, where
measures the initial imbalance of~ and is the target imbalance
of the output matrix. (The imbalance of~ is , where
are the maximum entries in magnitude in the
th row and column respectively.) This bound is tight up to the
factor. A balancing operation scales the th row and column so that their
maximum entries are equal, and requires arithmetic operations on
average, where is the number of non-zero elements in~. Thus the running
time of the iterative algorithm is . This is the first time
bound of any kind on any variant of the Osborne-Parlett-Reinsch algorithm. We
also prove a conjecture of Chen that characterizes those matrices for which the
limit of the balancing process is independent of the order in which balancing
operations are performed.Comment: The previous version (1) (see also STOC'15) handled UB ("unique
balance") input matrices. In this version (2) we extend the work to handle
all input matrice
Applications of diffraction theory to aeroacoustics
A review is given of the fundamentals of diffraction theory and the application of the theory to several problems of aircraft noise generation, propagation, and measurement. The general acoustic diffraction problem is defined and the governing equations set down. Diffraction phenomena are illustrated using the classical problem of the diffraction of a plane wave by a half-plane. Infinite series and geometric acoustic methods for solving diffraction problems are described. Four applications of diffraction theory are discussed: the selection of an appropriate shape for a microphone, the use of aircraft wings to shield the community from engine noise, the reflection of engine noise from an aircraft fuselage and the radiation of trailing edge noise
Two-dimensional arrays of low capacitance tunnel junctions: general properties, phase transitions and Hall effect
We describe transport properties of two-dimensional arrays of low capacitance
tunnel junctions, such as the current voltage characteristic and its dependence
on external magnetic field and temperature. We discuss several experiments in
which the small capacitance of the junctions plays an important role. In arrays
where the junctions have a relatively large charging energy, (i.e. when they
have a low capacitance) and a high normal state resistance, the low bias
resistance increases with decreasing temperature and eventually at very low
temperature the array becomes insulating even though the electrodes in the
array are superconducting. This transition to the insulating state can be
described by thermal activation. In an intermediate region where the junction
resistance is of the order of the quantum resistance and the charging energy is
of the order of the Josephson coupling energy, the arrays can be tuned between
a superconducting and an insulating state with a magnetic field. We describe
measurements of this magnetic-field-tuned superconductor insulator transition,
and we show that the resistance data can be scaled over several orders of
magnitude. Four arrays follow the same universal function. At the transition
the transverse (Hall) resistance is found to be very small in comparison with
the longitudinal resistance. However, for magnetic field values larger than the
critical value.we observe a substantial Hall resistance. The Hall resistance of
these arrays oscillates with the applied magnetic field. features in the
magnetic field dependence of the Hall resistance can qualitatively be
correlated to features in the derivative of the longitudinal resistance,
similar to what is found in the quantum Hall effect.Comment: 29 pages, 16 eps figures, uses aipproc.sty and epsfig.sty,
contribution to Euroschool on "Superconductivity in Networks and Mesoscopic
Systems", held in Siena, Italy (8-20 september 1997
Correction of the definition of mass-flow parameter in dynamic inflow modelling
No abstract available
Transformation media that rotate electromagnetic fields
We suggest a way to manipulate electromagnetic wave by introducing a rotation
mapping of coordinates that can be realized by a specific transformation of
permittivity and permeability of a shell surrounding an enclosed domain. Inside
the enclosed domain, the information from outside will appear as if it comes
from a different angle. Numerical simulations were performed to illustrate
these properties.Comment: 5 pages, 3 figure
Using LIP to Gloss Over Faces in Single-Stage Face Detection Networks
This work shows that it is possible to fool/attack recent state-of-the-art
face detectors which are based on the single-stage networks. Successfully
attacking face detectors could be a serious malware vulnerability when
deploying a smart surveillance system utilizing face detectors. We show that
existing adversarial perturbation methods are not effective to perform such an
attack, especially when there are multiple faces in the input image. This is
because the adversarial perturbation specifically generated for one face may
disrupt the adversarial perturbation for another face. In this paper, we call
this problem the Instance Perturbation Interference (IPI) problem. This IPI
problem is addressed by studying the relationship between the deep neural
network receptive field and the adversarial perturbation. As such, we propose
the Localized Instance Perturbation (LIP) that uses adversarial perturbation
constrained to the Effective Receptive Field (ERF) of a target to perform the
attack. Experiment results show the LIP method massively outperforms existing
adversarial perturbation generation methods -- often by a factor of 2 to 10.Comment: to appear ECCV 2018 (accepted version
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