99,172 research outputs found
Evidence for very strong electron-phonon coupling in YBa_{2}Cu_{3}O_{6}
From the observed oxygen-isotope shift of the mid-infrared two-magnon
absorption peak of YBaCuO, we evaluate the oxygen-isotope
effect on the in-plane antiferromagnetic exchange energy . The exchange
energy in YBaCuO is found to decrease by about 0.9% upon
replacing O by O, which is slightly larger than that (0.6%) in
LaCuO. From the oxygen-isotope effects, we determine the lower
limit of the polaron binding energy, which is about 1.7 eV for
YBaCuO and 1.5 eV for LaCuO, in quantitative
agreement with angle-resolved photoemission data, optical conductivity data,
and the parameter-free theoretical estimate. The large polaron binding energies
in the insulating parent compounds suggest that electron-phonon coupling should
also be strong in doped superconducting cuprates and may play an essential role
in high-temperature superconductivity.Comment: 4 pages, 1 figur
Renormalization Scheme Ambiguities in the Models with More than One Coupling
The process of renormalization to eliminate divergences arising in quantum
field theory is not uniquely defined; one can always perform a finite
renormalization, rendering finite perturbative results ambiguous. The
consequences of making such finite renormalizations have been examined in the
case of there being one or two couplings. In this paper we consider how finite
renormalizations can affect more general models in which there are more than
two couplings. In particular, we consider the Standard Model in which there are
essentially five couplings. We show that in this model (when neglecting all
mass parameters) if we use mass independent renormalization, then the
renormalization group beta-functions are not unique beyond one loop order, that
it is not in general possible to eliminate all terms beyond certain order for
all these beta-functions, but that for a physical process all contributions
beyond one loop order can be subsumed into the beta-functions
Multiple Couplings and Renormalization Scheme Ambiguities
The ambiguities inherent in renormalization are considered when using
mass-independent renormalization in massless theories that involve two coupling
coupling constants. We review how there is no renormalization scheme in which
the beta-functions can be chosen to vanish beyond a certain order in
perturbation theory, but that the beta-functions always contain ambiguities
beyond first order. We examine how the coupling constants depend on the
coefficients of the beta-function beyond one loop order. A way of
characterizing renormalization schemes that doesn't use coefficients of the
beta-function is considered for models with either one or two couplings. The
renormalization scheme ambiguities of physical quantities computed to finite
order in perturbation theory are also examined. We demonstrate how summation of
the logarithms that have explicit dependence on the renormalization scale
parameter mu in a physical quantity R leads to a cancellation with the implicit
dependence of R on mu through the running couplings. It is also shown that
there exists a renormalization scheme in which all radiative effects beyond
lowest order are incorporated into the behaviour of the running couplings
Light Front Quantization with the Light Cone Gauge
The Dirac procedure for dealing with constraints is applied to the
quantization of gauge theories on the light front. The light cone gauge is used
in conjunction with the first class constraints that arise and the resulting
Dirac brackets are found. These gauge conditions are not used to eliminate
degrees of freedom from the action prior to applying the Dirac constraint
procedure. This approach is illustrated by considering Yang-Mills theory and
the superparticle in a 2 + 1 dimensional target space
Buried heterostructure vertical-cavity surface-emitting laser with semiconductor mirrors
We report a buried heterostructure vertical-cavity surface-emitting laser
fabricated by epitaxial regrowth over an InGaAs quantum well gain medium. The
regrowth technique enables microscale lateral confinement that preserves a high
cavity quality factor (loaded 4000) and eliminates parasitic
charging effects found in existing approaches. Under optimal spectral overlap
between gain medium and cavity mode (achieved here at = 40 K) lasing was
obtained with an incident optical power as low as = 10 mW
( = 808 nm). The laser linewidth was found to be 3
GHz at 5
Correction of the definition of mass-flow parameter in dynamic inflow modelling
No abstract available
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