7,801 research outputs found
Frequency-Weighted Model Reduction with Applications to Structured Models
In this paper, a frequency-weighted extension of a
recently proposed model reduction method for linear systems
is presented. The method uses convex optimization and can be
used both with sample data and exact models. We also obtain
bounds on the frequency-weighted error. The method is combined
with a rank-minimization heuristic to approximate multiinputâ
multi-output systems.We also present two applicationsâ
environment compensation and simplification of interconnected
models â where we argue the proposed methods are useful
Fast readout of a single Cooper-pair box using its quantum capacitance
We have fabricated a single Cooper-pair box (SCB) together with an on-chip
lumped element resonator. By utilizing the quantum capacitance of the SCB, its
state can be read out by detecting the phase of a radio-frequency (rf) signal
reflected off the resonator. The resonator was optimized for fast readout. By
studying quasiparticle tunneling events in the SCB, we have characterized the
performance of the readout and found that we can perform a single shot parity
measurement in approximately 50 ns. This is an order of magnitude faster than
previously reported measurements.Comment: 7 pages, 5 figure
Trident: A three-pronged galaxy survey. I. Lyman alpha emitting galaxies at z~2 in GOODS North
Context. Lyman alpha emitting galaxies (LAEs) are used to probe the distant
universe and are therefore important for galaxy evolution studies and for
providing clues to the nature of the epoch of reionization, but the exact
circumstances under which Lyman alpha escapes a galaxy are still not fully
understood.
Aims. The Trident project is designed to simultaneously examine Lyman alpha,
H-alpha and Lyman Continuum emission from galaxies at redshift z~2, thus
linking together these three aspects of ionising radiation in galaxies. In this
paper, we outline the strategy of this project and examine the properties of
LAEs in the GOODS North field.
Methods. We performed a narrowband LAE survey in GOODS North using existing
and two custom made filters at the Nordic Optical Telescope with MOSCA. We use
complementary broad band archival data in the field to make a careful candidate
selection and perform optical to near-IR SED fitting. We also estimate
far-infrared luminosities by matching our candidates to detections in
Spitzer/MIPS 24{\mu}m and Herschel/PACS catalogs.
Results. We find a total of 25 LAE candidates, probing mainly the bright end
of the LAE luminosity function with L_Ly {\alpha} ~ 1-15e42 erg/s. They display
a range of masses of ~0.5-50e9 M_solar, and average ages from a few tens of Myr
to 1 Gyr when assuming a constant star formation history. The majority of our
candidates also show signs of recent elevated star formation. Three candidates
have counterparts in the GOODS-Herschel far-IR catalogue, with luminosities
consistent with ultra-luminous infrared galaxies (ULIRGs).
Conclusions. The wide range of parameters derived from our SED fitting, as
well as part of our sample being detected as ULIRGs, seems to indicate that at
these Lyman alpha luminosities, LAEs do not necessarily have to be young
dwarfs, and that a lack of dust is not required for Lyman alpha to escape.Comment: 16 pages, 12 figures. Accepted version for publication in A&
Photon generation in an electromagnetic cavity with a time-dependent boundary
We report the observation of photon generation in a microwave cavity with a
time-dependent boundary condition. Our system is a microfabricated quarter-wave
coplanar waveguide cavity. The electrical length of the cavity is varied using
the tunable inductance of a superconducting quantum interference device. It is
measured in the quantum regime, where the temperature is significantly less
than the resonance frequency (~ 5 GHz). When the length is modulated at
approximately twice the static resonance frequency, spontaneous oscillations of
the cavity field are observed. Time-resolved measurements of the dynamical
state of the cavity show multiple stable states. The behavior is well described
by theory. Connections to the dynamical Casimir effect are discussed.Comment: 5 pages, 3 Figure
The pumpistor: a linearized model of a flux-pumped SQUID for use as a negative-resistance parametric amplifier
We describe a circuit model for a flux-driven SQUID. This is useful for
developing insight into how these devices perform as active elements in
parametric amplifiers. The key concept is that frequency mixing in a
flux-pumped SQUID allows for the appearance of an effective negative
resistance. In the three-wave, degenerate case treated here, a negative
resistance appears only over a certain range of allowed input signal phase.
This model readily lends itself to testable predictions of more complicated
circuits.Comment: 4 pages, 3 figure
Efficient Computations of a Security Index for False Data Attacks in Power Networks
The resilience of Supervisory Control and Data Acquisition (SCADA) systems
for electric power networks for certain cyber-attacks is considered. We analyze
the vulnerability of the measurement system to false data attack on
communicated measurements. The vulnerability analysis problem is shown to be
NP-hard, meaning that unless there is no polynomial time algorithm to
analyze the vulnerability of the system. Nevertheless, we identify situations,
such as the full measurement case, where it can be solved efficiently. In such
cases, we show indeed that the problem can be cast as a generalization of the
minimum cut problem involving costly nodes. We further show that it can be
reformulated as a standard minimum cut problem (without costly nodes) on a
modified graph of proportional size. An important consequence of this result is
that our approach provides the first exact efficient algorithm for the
vulnerability analysis problem under the full measurement assumption.
Furthermore, our approach also provides an efficient heuristic algorithm for
the general NP-hard problem. Our results are illustrated by numerical studies
on benchmark systems including the IEEE 118-bus system
"Je ne regret rien?" : the value of adding anticipated regret to the theory of planned behaviour
Regret is fundamentally motivational, where there is a desire to be able to undo what has or has not been done; anticipating regret, therefore, seems a useful way of avoiding this negative experience and has been found to be a powerful mechanism for changing behaviour in areas such as economics and litigation (e.g. Larrick & Boles, 1995). Recent research has also been conducted into incorporating this explicitly affective component into the utilitarian-bound Theory of Planned Behaviour (Ajzen, 1985, 1988, 1991). A meta-analysis conducted in this thesis on all previous relevant research established that anticipated affective reactions, including regret, did make a significant contribution to the model in terms of increasing the variance in intentions. The remit of this PhD was to specifically consider anticipated regretâs predictive value to a broader variety of behaviours in a completely novel way. Study 1 differentiated between two particular behaviour types, Immediate Hedonic and Distal Benefit Behaviours, which mapped on to action and inaction regret respectively. A selection of these behaviours was used in Studies 2, 3 and 4, with Study 4 using an intervention designed to explicitly increase regret salience. Studies 5 and 6 set out to replicate the results from previous research into exercise behaviour by Abraham and Sheeran (2003; 2004), whilst Study 7 used an objective measure of exercise behaviour. The studies were either traditional pen-and-paper or web studies. The results provide overwhelming evidence that anticipated regret (unambiguously defined) adds significantly and independently to the predictive validity of the TPB in terms of intentions over and above the traditional TPB variables, even when controlling for past behaviour, experiential and instrumental attitudes. Replication of Abraham and Sheeranâs (2003; 2004) results (i.e. moderation by anticipated regret on the intention-behaviour relationship, mediated by intention stability) failed in pen-and-paper Study 5, although success was evident in web Study 6. Finally, although Study 7 explicitly established the disparate value of anticipated regret to the predictive validity of intention, there was a failure to detect any moderation or mediation effects: rather the most important predictor of exercise behaviour by Sports Centre members was past behaviour. Issues regarding multi-item versus single-item scales, web versus pen-and-paper designs, student versus âotherâ populations, and observational designs are discussed with recommendations for further research
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