2,144 research outputs found
Gauge-invariant response functions of fermions coupled to a gauge field
We study a model of fermions interacting with a gauge field and calculate
gauge-invariant two-particle Green's functions or response functions. The
leading singular contributions from the self-energy correction are found to be
cancelled by those from the vertex correction for small and . As a
result, the remaining contributions are not singular enough to change the
leading order results of the random phase approximation. It is also shown that
the gauge field propagator is not renormalized up to two-loop order. We examine
the resulting gauge-invariant two-particle Green's functions for small and
, but for all ratios of and we conclude that they can
be described by Fermi liquid forms without a diverging effective mass.Comment: Plain Tex, 35 pages, 5 figures available upon request, Revised
Version (Expanded discussion), To be published in Physical Review B 50,
(1994) (December 15 issue
Comments on the d-wave pairing mechanism for cuprate high superconductors: Higher is different?
The question of pairing glue for the cuprate superconductors (SC)is revisited
and its determination through the angle resolved photo-emission spectroscopy
(ARPES) is discussed in detail. There are two schools of thoughts about the
pairing glue question: One argues that superconductivity in the cuprates
emerges out of doping the spin singlet resonating valence bond (RVB) state.
Since singlet pairs are already formed in the RVB state there is no need for
additional boson glue to pair the electrons. The other instead suggests that
the d-wave pairs are mediated by the collective bosons like the conventional
low SC with the alteration that the phonons are replaced by another kind
of bosons ranging from the antiferromagnetic (AF) to loop current fluctuations.
An approach to resolve this dispute is to determine the frequency and momentum
dependences of the diagonal and off-diagonal self-energies directly from
experiments like the McMillan-Rowell procedure for the conventional SC. In that
a simple d-wave BCS theory describes superconducting properties of the cuprates
well, the Eliashberg analysis of well designed high resolution experimental
data will yield the crucial frequency and momentum dependences of the
self-energies. This line of approach using ARPES are discussed in more detail
in this review, and some remaining problems are commented.Comment: Invited review article published in the Journal of Korean Physical
Society; several typos corrected and a few comments and references adde
Non-Fermi liquid behavior and scaling of low frequency suppression in optical conductivity spectra of CaRuO
Optical conductivity spectra of paramagnetic CaRuO are
investigated at various temperatures. At T=10 K, it shows a non-Fermi liquid
behavior of , similar to the case
of a ferromagnet SrRuO. As the temperature () is increased, on the other
hand, in the low frequency region is progressively
suppressed, deviating from the 1/{\omega}^{\frac 12%}-dependence.
Interestingly, the suppression of is found to scale with
at all temperatures. The origin of the scaling
behavior coupled with the non-Fermi liquid behavior is discussed.Comment: 4 pages, 3 figure
Distorted magnetic orders and electronic structures of tetragonal FeSe from first-principles
We use the state-of-the-arts density-functional-theory method to study
various magnetic orders and their effects on the electronic structures of the
FeSe. Our calculated results show that, for the spins of the single Fe layer,
the striped antiferromagnetic orders with distortion are more favorable in
total energy than the checkerboard antiferromagnetic orders with tetragonal
symmetry, which is consistent with known experimental data, and the inter-layer
magnetic interaction is very weak. We investigate the electronic structures and
magnetic property of the distorted phases. We also present our calculated spin
coupling constants and discuss the reduction of the Fe magnetic moment by
quantum many-body effects. These results are useful to understand the
structural, magnetic, and electronic properties of FeSe, and may have some
helpful implications to other FeAs-based materials
using HYP-smeared staggered fermions in unquenched QCD
We present results for kaon mixing parameter calculated using
HYP-smeared improved staggered fermions on the MILC asqtad lattices. We use
three lattice spacings (, and fm), ten different
valence quark masses (), and several light sea-quark
masses in order to control the continuum and chiral extrapolations. We derive
the next-to-leading order staggered chiral perturbation theory (SChPT) results
necessary to fit our data, and use these results to do extrapolations based
both on SU(2) and SU(3) SChPT. The SU(2) fitting is particularly
straightforward because parameters related to taste-breaking and matching
errors appear only at next-to-next-to-leading order. We match to the continuum
renormalization scheme (NDR) using one-loop perturbation theory. Our final
result is from the SU(2) analysis, with the SU(3) result providing a (less
accurate) cross check. We find and ,
where the first error is statistical and the second systematic. The error is
dominated by the truncation error in the matching factor. Our results are
consistent with those obtained using valence domain-wall fermions on lattices
generated with asqtad or domain-wall sea quarks.Comment: 37 pages, 31 figures, most updated versio
A limit on the detectability of the energy scale of inflation
We show that the polarization of the cosmic microwave background can be used
to detect gravity waves from inflation if the energy scale of inflation is
above 3.2 times 10^15 GeV. These gravity waves generate polarization patterns
with a curl, whereas (to first order in perturbation theory) density
perturbations do not. The limiting ``noise'' arises from the second--order
generation of curl from density perturbations, or rather residuals from its
subtraction. We calculate optimal sky coverage and detectability limits as a
function of detector sensitivity and observing time.Comment: 4 pages, 3 figures, submitted to PR
Thermal Conductivity of superconducting (TMTSF)_2ClO_4: evidence for a nodeless gap
We report on the first measurements of thermal conductivity in the
superconducting state of (TMTSF)_2ClO_4. The electronic contribution to heat
transport is found to decrease rapidly below T_c, indicating the absence of
low-energy electronic excitations. We argue that this result provides strong
evidence for a nodeless superconducting gap function but does not exclude a
possible unconventional order parameter.Comment: 4 pages including 4 figures, submitted to Phys. Rev. Let
Phosphor-Free Apple-White LEDs with Embedded Indium-Rich Nanostructures Grown on Strain Relaxed Nano-epitaxy GaN
Phosphor-free apple-white light emitting diodes have been fabricated using a dual stacked InGaN/GaN multiple quantum wells comprising of a lower set of long wavelength emitting indium-rich nanostructures incorporated in multiple quantum wells with an upper set of cyan-green emitting multiple quantum wells. The light-emitting diodes were grown on nano-epitaxially lateral overgrown GaN template formed by regrowth of GaN over SiO2 film patterned with an anodic aluminum oxide mask with holes of 125 nm diameter and a period of 250 nm. The growth of InGaN/GaN multiple quantum wells on these stress relaxed low defect density templates improves the internal quantum efficiency by 15% for the cyan-green multiple quantum wells. Higher emission intensity with redshift in the PL peak emission wavelength is obtained for the indium-rich nanostructures incorporated in multiple quantum wells. The quantum wells grown on the nano-epitaxially lateral overgrown GaN has a weaker piezoelectric field and hence shows a minimal peak shift with application of higher injection current. An enhancement of external quantum efficiency is achieved for the apple-white light emitting diodes grown on the nano-epitaxially lateral overgrown GaN template based on the light -output power measurement. The improvement in light extraction efficiency, ηextraction, was found to be 34% for the cyan-green emission peak and 15% from the broad long wavelength emission with optimized lattice period
Atmospheric neutrino flux from 3-dimensional simulation
The atmospheric muon and neutrino flux have been simulated using the same
approach which successfully accounted for the recent secondary proton, electron
and positron flux measurements in orbit by the AMS experiment. For the muon
flux, a good agreement is obtained with the CAPRICE and HEAT data for altitudes
ranging from sea level up to about 38 km. The general features of the
calculated atmospheric neutrino flux are reported and discussed. The flux
obtained at the Super-Kamiokande experiment location are reported and compared
with other calculations. For low neutrino energies the flux obtained is
significantly smaller than that used in the data analysis of underground
experiment. The simulation results for the SOUDAN experiment site are also
reported.Comment: 33 pages, 27 figures, 12 tables, final version for Phys. Rev.
Density-functional Study of Small Molecules within the Krieger-Li-Iafrate Approximation
We report density-functional studies of several small molecules (, and ) within the Krieger-Li-Iafrate (KLI)
approximation to the exact Kohn-Sham local exchange potential, using a
three-dimensional real-space finite-difference pseudopotential method. It is
found that exchange-only KLI leads to markedly improved eigenvalue spectra
compared to those obtained within the standard local-density approximation
(LDA), the generalized gradient approximation (GGA), and the Hartree-Fock (HF)
method. For structural properties, exchange-only KLI results are close to the
corresponding HF values. We find that the addition of LDA or GGA correlation
energy functionals to the KLI exact exchange energy functional does not lead to
systematic improvements.Comment: 16 pages including 1 fugure, to be published in Phys. Rev. A Nov. 1
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