11,833 research outputs found
Phase transition in protocols minimizing work fluctuations
For two canonical examples of driven mesoscopic systems - a
harmonically-trapped Brownian particle and a quantum dot - we numerically
determine the finite-time protocols that optimize the compromise between the
standard deviation and the mean of the dissipated work. In the case of the
oscillator, we observe a collection of protocols that smoothly trade-off
between average work and its fluctuations. However, for the quantum dot, we
find that as we shift the weight of our optimization objective from average
work to work standard deviation, there is an analog of a first-order phase
transition in protocol space: two distinct protocols exchange global optimality
with mixed protocols akin to phase coexistence. As a result, the two types of
protocols possess qualitatively different properties and remain distinct even
in the infinite duration limit: optimal-work-fluctuation protocols never
coalesce with the minimal work protocols, which therefore never become
quasistatic.Comment: 6 pages, 6 figures + SI as ancillary fil
Black Strings and Classical Hair
We examine the geometry near the event horizon of a family of black string
solutions with traveling waves. It has previously been shown that the metric is
continuous there. Contrary to expectations, we find that the geometry is not
smooth, and the horizon becomes singular whenever a wave is present. Both five
dimensional and six dimensional black strings are considered with similar
results.Comment: 14 pages, harvma
Higher-dimensional resolution of dilatonic black hole singularities
We show that the four-dimensional extreme dilaton black hole with dilaton
coupling constant can be interpreted as a {\it completely
non-singular}, non-dilatonic, black -brane in dimensions provided
that is {\it odd}. Similar results are obtained for multi-black holes and
dilatonic extended objects in higher spacetime dimensions. The non-singular
black -brane solutions include the self-dual three brane of ten-dimensional
N=2B supergravity and a multi-fivebrane solution of eleven-dimensional
supergravity. In the case of a supersymmetric non-dilatonic -brane solution
of a supergravity theory, we show that it saturates a bound on the energy per
unit -volume.Comment: 27 pages, R/94/28, UCSBTH-94-35 (Comments added to the discussion
section
Photocatalytic production of organic compounds from CO and H2O in a simulated Martian atmosphere
[14C]CO2 and [14C]organic compounds are formed when a mixture of [14C]CO and water vapor diluted in [12C]CO2 or N2 is irradiated with ultraviolet light in the presence of soil or pulverized vycor substratum. The [14C]CO2 is recoverable from the gas phase, the [14C]organic products from the substratum. Three organic products have been tentatively identified as formaldehyde, acetaldehyde, and glycolic acid. The relative yields of [14C]CO2 and [14C]organics are wavelength- and surface-dependent. Conversion of CO to CO2 occurs primarily at wavelengths shorter than 2000 angstrom, apparently involves the photolysis of water, and is inhibited by increasing amounts of vycor substratum. Organic formation occurs over a broad spectral range below 3000 angstrom and increases with increasing amounts of substratum. It is suggested that organic synthesis results from adsorption of CO and H2O on surfaces, with excitation of one or both molecules occurring at wavelengths longer than those absorbed by the free gases. This process may occur on Mars and may have been important on the primitive earth
Fast high--voltage amplifiers for driving electro-optic modulators
We describe five high-voltage (60 to 550V peak to peak), high-speed (1-300ns
rise time; 1.3-300MHz bandwidth) linear amplifiers for driving capacitive or
resistive loads such as electro-optic modulators. The amplifiers use bipolar
transistors in various topologies. Two use electron tubes to overcome the speed
limitations of high-voltage semiconductors. All amplifiers have been built.
Measured performance data is given for each.Comment: 9pages, 6figures, 6tables, to appear in Review of Scientific
Instrument
Wide-band current preamplifier for conductance measurements with large input capacitance
A wide-band current preamplifier based on a composite operational amplifier
is proposed. It has been shown that the bandwidth of the preamplifier can be
significantly increased by enhancing the effective open-loop gain of the
composite preamplifier. The described preamplifier with current gain 10 V/A
showed the bandwidth of about 100 kHz with 1 nF input shunt capacitance. The
current noise of the amplifier was measured to be about 46 fA/
at 1 kHz, close to the design noise minimum. The voltage noise was found to be
about 2.9 nV/ at 1 kHz, which is in a good agreement with the
value expected for the operational amplifier used in the input stage. By
analysing the total noise produced by the preamplifier we found the optimal
frequency range suitable for the fast lock-in measurements to be from 1 kHz to
2 kHz. To get the same signal-to-noise ratio, the reported preamplifier
requires roughly 10% of the integration time used in measurements made with a
conventional preamplifier.Comment: 5 pages, 4 figure
A method for measuring the nonlinear response in dielectric spectroscopy through third harmonics detection
We present a high sensitivity method allowing the measurement of the non
linear dielectric susceptibility of an insulating material at finite frequency.
It has been developped for the study of dynamic heterogeneities in supercooled
liquids using dielectric spectroscopy at frequencies 0.05 Hz < f < 30000 Hz .
It relies on the measurement of the third harmonics component of the current
flowing out of a capacitor. We first show that standard laboratory electronics
(amplifiers and voltage sources) nonlinearities lead to limits on the third
harmonics measurements that preclude reaching the level needed by our physical
goal, a ratio of the third harmonics to the fundamental signal about 7 orders
of magnitude lower than 1. We show that reaching such a sensitivity needs a
method able to get rid of the nonlinear contributions both of the measuring
device (lock-in amplifier) and of the excitation voltage source. A bridge using
two sources fulfills only the first of these two requirements, but allows to
measure the nonlinearities of the sources. Our final method is based on a
bridge with two plane capacitors characterized by different dielectric layer
thicknesses. It gets rid of the source and amplifier nonlinearities because in
spite of a strong frequency dependence of the capacitors impedance, it is
equilibrated at any frequency. We present the first measurements of the
physical nonlinear response using our method. Two extensions of the method are
suggested.Comment: 25 pages, 8 figure
Large-Area, Low-Noise, High Speed, Photodiode-Based Fluorescence Detectors with Fast Overdrive Recovery
Two large-area, low noise, high speed fluorescence detectors have been built.
One detector consists of a photodiode with an area of 28 mm x 28 mm and a low
noise transimpedance amplifier. This detector has a input light-equivalent
spectral noise density of less than 3 pW/Hz^1/2, can recover from a large
scattered light pulse within 10 us, and has a bandwidth of at least 900 kHz.
The second detector consists of a 16 mm diameter avalanche photodiode and a
low-noise transimpedance amplifier. This detector has an input light-equivalent
spectral noise density of 0.08 pW/Hz^1/2, also can recover from a large
scattered light pulse within 10 us, and has a bandwidth of 1 MHz.Comment: Submitted to Review of Scientific Instrument
On Pair Creation of Extremal Black Holes and Kaluza-Klein Monopoles
Classical solutions describing charged dilaton black holes accelerating in a
background magnetic field have recently been found. They include the Ernst
metric of the Einstein-Maxwell theory as a special case. We study the extremal
limit of these solutions in detail, both at the classical and quantum levels.
It is shown that near the event horizon, the extremal solutions reduce
precisely to the static extremal black hole solutions. For a particular value
of the dilaton coupling, these extremal black holes are five dimensional
Kaluza-Klein monopoles. The euclidean sections of these solutions can be
interpreted as instantons describing the pair creation of extremal black
holes/Kaluza-Klein monopoles in a magnetic field. The action of these
instantons is calculated and found to agree with the Schwinger result in the
weak field limit. For the euclidean Ernst solution, the action for the extremal
solution differs from that of the previously discussed wormhole instanton by
the Bekenstein-Hawking entropy. However, in many cases quantum corrections
become large in the vicinity of the black hole, and the precise description of
the creation process is unknown.Comment: 45 pages, 5 figures, EFI-93-74, UCSBTH-93-38. (Omitted
acknowledgements added, typos fixed
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