8 research outputs found
An assessment of Evans' unified field theory I
Evans developed a classical unified field theory of gravitation and
electromagnetism on the background of a spacetime obeying a Riemann-Cartan
geometry. This geometry can be characterized by an orthonormal coframe theta
and a (metric compatible) Lorentz connection Gamma. These two potentials yield
the field strengths torsion T and curvature R. Evans tried to infuse
electromagnetic properties into this geometrical framework by putting the
coframe theta to be proportional to four extended electromagnetic potentials A;
these are assumed to encompass the conventional Maxwellian potential in a
suitable limit. The viable Einstein-Cartan(-Sciama-Kibble) theory of gravity
was adopted by Evans to describe the gravitational sector of his theory.
Including also the results of an accompanying paper by Obukhov and the author,
we show that Evans' ansatz for electromagnetism is untenable beyond repair both
from a geometrical as well as from a physical point of view. As a consequence,
his unified theory is obsolete.Comment: 39 pages of latex, modified because of referee report, mistakes and
typos removed, partly reformulated, taken care of M.W.Evans' rebutta
Pinned Balseiro-Falicov Model of Tunneling and Photoemission in the Cuprates
The smooth evolution of the tunneling gap of Bi_2Sr_2CaCu_2O_8 with doping
from a pseudogap state in the underdoped cuprates to a superconducting state at
optimal and overdoping, has been interpreted as evidence that the pseudogap
must be due to precursor pairing. We suggest an alternative explanation, that
the smoothness reflects a hidden SO(N) symmetry near the (pi,0) points of the
Brillouin zone (with N = 3, 4, 5, or 6). Because of this symmetry, the
pseudogap could actually be due to any of a number of nesting instabilities,
including charge or spin density waves or more exotic phases. We present a
detailed analysis of this competition for one particular model: the pinned
Balseiro-Falicov model of competing charge density wave and (s-wave)
superconductivity. We show that most of the anomalous features of both
tunneling and photoemission follow naturally from the model, including the
smooth crossover, the general shape of the pseudogap phase diagram, the
shrinking Fermi surface of the pseudogap phase, and the asymmetry of the
tunneling gap away from optimal doping. Below T_c, the sharp peak at Delta_1
and the dip seen in the tunneling and photoemission near 2Delta_1 cannot be
described in detail by this model, but we suggest a simple generalization to
account for inhomogeneity, which does provide an adequate description. We show
that it should be possible, with a combination of photoemission and tunneling,
to demonstrate the extent of pinning of the Fermi level to the Van Hove
singularity. A preliminary analysis of the data suggests pinning in the
underdoped, but not in the overdoped regime.Comment: 18 pages LaTeX, 26 ps. figure
In Situ ATR-SEIRAS of Carbon Dioxide Reduction at a Plasmonic Silver Cathode
Illumination of a voltage-biased plasmonic Ag cathode during CO2 reduction results in a suppression of the H2 evolution reaction while enhancing CO2 reduction. This effect has been shown to be photonic rather than thermal, but the exact plasmonic mechanism is unknown. Here, we conduct an in situ ATR-SEIRAS (attenuated total reflectance-surface-enhanced infrared absorption spectroscopy) study of a sputtered thin film Ag cathode on a Ge ATR crystal in CO2-saturated 0.1 M KHCO3 over a range of potentials under both dark and illuminated (365 nm, 125 mW cm-2) conditions to elucidate the nature of this plasmonic enhancement. We find that the onset potential of CO2 reduction to adsorbed CO on the Ag surface is -0.25 VRHE and is identical in the light and the dark. As the production of gaseous CO is detected in the light near this onset potential but is not observed in the dark until -0.5 VRHE, we conclude that the light must be assisting the desorption of CO from the surface. Furthermore, the HCO3- wavenumber and peak area increase immediately upon illumination, precluding a thermal effect. We propose that the enhanced local electric field that results from the localized surface plasmon resonance (LSPR) is strengthening the HCO3- bond, further increasing the local pH. This would account for the decrease in H2 formation and increase the CO2 reduction products in the light.Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.ChemE/Materials for Energy Conversion & StorageLarge Scale Energy Storag