910 research outputs found
Quantum inequalities for the free Rarita-Schwinger fields in flat spacetime
Using the methods developed by Fewster and colleagues, we derive a quantum
inequality for the free massive spin- Rarita-Schwinger fields in
the four dimensional Minkowski spacetime. Our quantum inequality bound for the
Rarita-Schwinger fields is weaker, by a factor of 2, than that for the
spin- Dirac fields. This fact along with other quantum inequalities
obtained by various other authors for the fields of integer spin (bosonic
fields) using similar methods lead us to conjecture that, in the flat
spacetime, separately for bosonic and fermionic fields, the quantum inequality
bound gets weaker as the the number of degrees of freedom of the field
increases. A plausible physical reason might be that the more the number of
field degrees of freedom, the more freedom one has to create negative energy,
therefore, the weaker the quantum inequality bound.Comment: Revtex, 11 pages, to appear in PR
Quantum Weak Energy Inequalities for the Dirac field in Flat Spacetime
Quantum Weak Energy Inequalities (QWEIs) have been established for a variety
of quantum field theories in both flat and curved spacetimes. Dirac fields are
known (by a result of Fewster and Verch) to satisfy QWEIs under very general
circumstances. However this result does not provide an explicit formula for the
QWEI bound, so its magnitude has not previously been determined. In this paper
we present a new and explicit QWEI bound for Dirac fields of arbitrary mass in
four-dimensional Minkowski space. We follow the methods employed by Fewster and
Eveson for the scalar field, modified to take account of anticommutation
relations. A key ingredient is an identity for Fourier transforms established
by Fewster and Verch. We also compare our QWEI with those previously obtained
for scalar and spin-1 fields.Comment: 8 pages, REVTeX4, version to appear in Phys Rev
Lightcone fluctuations in quantum gravity and extra dimensions
We discuss how compactified extra dimensions may have potentially observable
effects which grow as the compactification scale decreases. This arises because
of lightcone fluctuations in the uncompactified dimensions which can result in
the broadening of the spectral lines from distant sources. We analyze this
effect in a five dimensional model, and argue that data from gamma ray burst
sources require the compactification length to be greater than about cm
in this model.Comment: Two additional references adde
Femtosecond free-electron laser by chirped pulse amplification
In this work we combine elements of chirped pulse amplification techniques, now familiar in solid-state lasers, with an amplifier based upon a seeded free-electron laser (FEL). The resulting device would produce amplified pulses of unprecedented brevity at wavelengths shorter than can be currently obtained by any tunable laser system. We use a subharmonically seeded FEL to illustrate the concept. Radiation from a Ti:sapphire laser is frequency tripled and stretched optically to provide a coherent seed pulse for the FEL. When coupled to an electron beam inside a magnetic wiggler, the seed radiation introduces an additional energy modulation on the electron bunch, which has been prepared with an energy chirp to match the chirp in the optical pulse. The energy modulated electrons are then spatially bunched in a dispersion magnet and introduced to a wiggler configured to be resonant to a harmonic of the seed laser, providing additional frequency multiplication. The coherent radiation produced by these electrons is amplified as it traverses the wiggler and is recompressed optically. The preservation of phase coherence provided by this scheme results in a device which can yield 4-fs pulses with 0.3 mJ at a central wavelength of ca. 8 nm, easily the shortest duration of amplified pulses produced by any laser. In this paper we discuss various aspects of the concept, including the generation of short pulses, temporal stretching and compression, and potential applications of the device. The phase distortion during the wide bandwidth FEL amplification is discussed in detail, and is shown to be within the bounds required to produce a 4-fs pulse upon compression
Femtosecond free-electron laser by chirped pulse amplification
In this work we combine elements of chirped pulse amplification techniques, now familiar in solid-state lasers, with an amplifier based upon a seeded free-electron laser (FEL). The resulting device would produce amplified pulses of unprecedented brevity at wavelengths shorter than can be currently obtained by any tunable laser system. We use a subharmonically seeded FEL to illustrate the concept. Radiation from a Ti:sapphire laser is frequency tripled and stretched optically to provide a coherent seed pulse for the FEL. When coupled to an electron beam inside a magnetic wiggler, the seed radiation introduces an additional energy modulation on the electron bunch, which has been prepared with an energy chirp to match the chirp in the optical pulse. The energy modulated electrons are then spatially bunched in a dispersion magnet and introduced to a wiggler configured to be resonant to a harmonic of the seed laser, providing additional frequency multiplication. The coherent radiation produced by these electrons is amplified as it traverses the wiggler and is recompressed optically. The preservation of phase coherence provided by this scheme results in a device which can yield 4-fs pulses with 0.3 mJ at a central wavelength of ca. 8 nm, easily the shortest duration of amplified pulses produced by any laser. In this paper we discuss various aspects of the concept, including the generation of short pulses, temporal stretching and compression, and potential applications of the device. The phase distortion during the wide bandwidth FEL amplification is discussed in detail, and is shown to be within the bounds required to produce a 4-fs pulse upon compression
An Analog Model for Quantum Lightcone Fluctuations in Nonlinear Optics
We propose an analog model for quantum gravity effects using nonlinear
dielectrics. Fluctuations of the spacetime lightcone are expected in quantum
gravity, leading to variations in the flight times of pulses. This effect can
also arise in a nonlinear material. We propose a model in which fluctuations of
a background electric field, such as that produced by a squeezed photon state,
can cause fluctuations in the effective lightcone for probe pulses. This leads
to a variation in flight times analogous to that in quantum gravity. We make
some numerical estimates which suggest that the effect might be large enough to
be observable.Comment: 15 pages, no figure
Kv11.1 (hERG)-induced cardiotoxicity: a molecular insight from a binding kinetics study of prototypical Kv11.1 (hERG) inhibitors
BACKGROUND AND PURPOSE\nDrug-induced arrhythmia due to blockade of the Kv 11.1 channel (also known as the hERG K(+) channel) is a frequent side effect. Previous studies have primarily focused on equilibrium parameters, i.e. affinity or potency, of drug candidates at the channel. The aim of this study was to determine the kinetics of the interaction with the channel for a number of known Kv 11.1 blockers and to explore a possible correlation with the affinity or physicochemical properties of these compounds.\nEXPERIMENTAL APPROACH\nThe affinity and kinetic parameters of 15 prototypical Kv 11.1 inhibitors were evaluated in a number of [(3) H]-dofetilide binding assays. The lipophilicity (logKW - C8 ) and membrane partitioning (logKW - IAM ) of these compounds were determined by means of HPLC analysis.\nKEY RESULTS\nA novel [(3) H]-dofetilide competition association assay was set up and validated, which allowed us to determine the binding kinetics of the Kv 11.1 blockers used in this study. Interestingly, the compounds' affinities (Ki values) were correlated to their association rates rather than dissociation rates. Overall lipophilicity or membrane partitioning of the compounds were not correlated to their affinity or rate constants for the channel.\nCONCLUSIONS AND IMPLICATIONS\nA compound's affinity for the Kv 11.1 channel is determined by its rate of association with the channel, while overall lipophilicity and membrane affinity are not. In more general terms, our findings provide novel insights into the mechanism of action for a compound's activity at the Kv 11.1 channel. This may help to elucidate how Kv 11.1-induced cardiotoxicity is governed and how it can be circumvented in the future.Medicinal Chemistr
Self-driven electron enrichment of ultrafine PdAu nanoparticles for electrochemical CO<sub>2</sub> reduction:High applicability of work function as an activity descriptor
Highly coupled metal/dopant-incorporated carbon dyads provide a possibility to modulate the electron density of metallic materials by forming a rectifying interface, thus showing an enhanced activity in electrochemical CO2 reduction reaction (ECRR). However, understanding the promotion effects of dopants for ECRR is limited to the prediction by theoretical interpretation and case-by-case studies. Herein, we report the direct experimental evidence that the work function, regulated by single structural factor-dopant contents, is significantly correlated to the ECRR reaction activity and kinetics. We prepared a series of PdAu/NxC electrocatalysts composed of ultrafine (∼5.7 nm) PdAu bimetallic nanoparticles and tailorable N-doped carbon supports. The wide range of the amount of N dopants allowed the modification of the band gap of the carbon easily. Using ultraviolet photoelectron spectroscopy (UPS) measurements, we demonstrate that the reactivity and kinetics trends of the PdAu/NxC in the ECRR can be intrinsically correlated with the work function of the catalysts. PdAu/N7.50C electrocatalyst with the highest N contents displays a 100% CO2-to-CO conversion and high conversion efficiency over a wide potential window, superior over other reported PdAu catalysts. This work provides a novel way to boost ECRR performance by deliberately lowering the work function of the metal/carbon electrocatalysts through the enhancement by dopants.</p
Density of states and magnetoconductance of disordered Au point contacts
We report the first low temperature magnetotransport measurements on
electrochemically fabricated atomic scale gold nanojunctions. As , the
junctions exhibit nonperturbatively large zero bias anomalies (ZBAs) in their
differential conductance. We consider several explanations and find that the
ZBAs are consistent with a reduced local density of states (LDOS) in the
disordered metal. We suggest that this is a result of Coulomb interactions in a
granular metal with moderate intergrain coupling. Magnetoconductance of atomic
scale junctions also differs significantly from that of less geometrically
constrained devices, and supports this explanation.Comment: 5 pages, 5 figures. Accepted to PRB as Brief Repor
Allosteric modulators of the hERG K+ channel Radioligand binding assays reveal allosteric characteristics of dofetilide analogs
Medicinal Chemistr
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