847 research outputs found
Kinetic energy driven superconductivity in doped cuprates
Within the t-J model, the mechanism of superconductivity in doped cuprates is
studied based on the partial charge-spin separation fermion-spin theory. It is
shown that dressed holons interact occurring directly through the kinetic
energy by exchanging dressed spinon excitations, leading to a net attractive
force between dressed holons, then the electron Cooper pairs originating from
the dressed holon pairing state are due to the charge-spin recombination, and
their condensation reveals the superconducting ground-state. The electron
superconducting transition temperature is determined by the dressed holon pair
transition temperature, and is proportional to the concentration of doped holes
in the underdoped regime. With the common form of the electron Cooper pair, we
also show that there is a coexistence of the electron Cooper pair and
antiferromagnetic short-range correlation, and hence the antiferromagnetic
short-range fluctuation can persist into the superconducting state. Our results
are qualitatively consistent with experiments.Comment: 6 pages, Revtex, two figures are included, corrected typo
Heat transport of electron-doped Cobaltates
Within the t-J model, the heat transport of electron-doped cobaltates is
studied based on the fermion-spin theory. It is shown that the temperature
dependent thermal conductivity is characterized by the low temperature peak
located at a finite temperature. The thermal conductivity increases
monotonously with increasing temperature at low temperatures T 0.1, and
then decreases with increasing temperature for higher temperatures T
0.1, in qualitative agreement with experimental result observed from
NaCoO .Comment: 4 pages, 1 fig, corrected typos, accepted for publication in Commun.
Theor. Phy
Unitarized Diffractive Scattering in QCD and Application to Virtual Photon Total Cross Sections
The problem of restoring Froissart bound to the BFKL-Pomeron is studied in an
extended leading-log approximation of QCD. We consider parton-parton scattering
amplitude and show that the sum of all Feynman-diagram contributions can be
written in an eikonal form. In this form dynamics is determined by the phase
shift, and subleading-logs of all orders needed to restore the Froissart bound
are automatically provided. The main technical difficulty is to find a way to
extract these subleading contributions without having to compute each Feynman
diagram beyond the leading order. We solve that problem by using nonabelian cut
diagrams introduced elsewhere. They can be considered as colour filters used to
isolate the multi-Reggeon contributions that supply these subleading-log terms.
Illustration of the formalism is given for amplitudes and phase shifts up to
three loops. For diffractive scattering, only phase shifts governed by one and
two Reggeon exchanges are needed. They can be computed from the
leading-log-Reggeon and the BFKL-Pomeron amplitudes. In applications, we argue
that the dependence of the energy-growth exponent on virtuality for
total cross section observed at HERA can be interpreted as the
first sign of a slowdown of energy growth towards satisfying the Froissart
bound. An attempt to understand these exponents with the present formalism is
discussed.Comment: 41 pages in revtex preprint format, with 10 figure
Kinetic energy driven superconductivity in the electron doped cobaltate NaCoOHO
Within the charge-spin separation fermion-spin theory, we have shown that the
mechanism of superconductivity in the electron doped cobaltate
NaCoOHO is ascribed to its kinetic energy. The dressed
fermions interact occurring directly through the kinetic energy by exchanging
magnetic excitations. This interaction leads to a net attractive force between
dressed fermions, then the electron Cooper pairs originating from the dressed
fermion pairing state are due to the charge-spin recombination, and their
condensation reveals the superconducting ground state. The superconducting
transition temperature is identical to the dressed fermion pair transition
temperature, and is suppressed to a lower temperature due to the strong
magnetic frustration. The optimal superconducting transition temperature occurs
in the electron doping concentration , and then decreases
for both underdoped and overdoped regimes, in qualitative agreement with the
experimental results.Comment: 6 pages, 2 figs, corrected typos, accepted for publication in Commun.
Theor. Phy
Microscopic phase separation in the overdoped region of high-Tc cuprate superconductors
We propose a phenomenological model for high- superconductors (HTSC)
assuming: (1) a microscopic phase separation between superconducting and
normal-metal areas in the overdoped region; and (2) existence of a homogeneous
superconducting phase only below the pseudo-gap line, which shows a
sharp reduction towards at a mildly overdoped critical
concentration . This model explains anomalous doping and temperature
dependences of (superconducting carrier density / effective mass)
observed in several overdoped HTSC systems. We point out an analogy to
superfluid He/He films, and discuss an energetic origin of
microscopic phase separation.Comment: 5 pages, 4 figure
Density measurements of endothermic hydrocarbon fuels at temperatures from 235 k to 353 k and pressures up to 4.09 mpa
The densities of endothermic hydrocarbon fuels were
measured covering the temperature from (235.0 to 353.0) K at
pressures of (0.68,1.47, 3.06 and 4.09) MPa. The densitometer
is based on the attenuation theory of gamma ray with a count
rate mode. When the intensity of a gamma beam passes through
fuels, it decreases exponentially. According to Beer−Lambert’s
law, densities of fuel were calculated through the different
count rates and densities of referenced fluid. Pure hexane and a
binary mixture of n-heptane and n-octane were adopted
respectively to validate the reliability and accuracy of the
densitometer. Results showed that the average absolute
deviation(AAD) was lower than 0.32 % and the maximum
absolute deviation(MAD) was within 0.67 %.Papers presented at the 13th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Portoroz, Slovenia on 17-19 July 2017 .International centre for heat and mass transfer.American society of thermal and fluids engineers
The Pinch Technique to All Orders
The generalization of the pinch technique to all orders in perturbation
theory is presented. The effective Green's functions constructed with this
procedure are singled out in a unique way through the full exploitation of the
underlying Becchi-Rouet-Stora-Tyutin symmetry. A simple all-order
correspondence between the pinch technique and the background field method in
the Feynman gauge is established.Comment: 10 pages, 4 figures; one reference added, typos corrected; final
version to match the pubblished on
Fermi liquid interactions and the superfluid density in d-wave superconductors
We construct a phenomenological superfluid Fermi liquid theory for a
two-dimensional d-wave superconductor on a square lattice, and study the effect
of quasiparticle interactions on the superfluid density. Using simple models
for the dispersion and the Landau interaction function, we illustrate the
deviation of these results from those for the isotropic superfluid. This allows
us to reconcile the value and doping dependence of the superfluid density slope
at low temperature obtained from penetration depth measurements, with
photoemission data on nodal quasiparticles.Comment: 5 latex pages, 1 eps-figure. submitted to PR
Raman study of carrier-overdoping effects on the gap in high-Tc superconducting cuprates
Raman scattering in the heavily overdoped (Y,Ca)Ba_2Cu_3O_{7-d} (T_c = 65 K)
and Bi_2Sr_2CaCu_2O_{8+d} (T_c = 55 K) crystals has been investigated. For the
both crystals, the electronic pair-breaking peaks in the A_{1g} and B_{1g}
polarizations were largely shifted to the low energies close to a half of
2Delta_0, Delta_0 being the maximum gap. It strongly suggests s-wave mixing
into the d-wave superconducting order parameter and the consequent
manifestation of the Coulomb screening effect in the B_{1g}-channel. Gradual
mixing of s-wave component with overdoping is not due to the change of crystal
structure symmetry but a generic feature in all high-T_c superconducting
cuprates.Comment: 5 pages, 4 figures, to be published in Phys. Rev. B, Rapid
communicaito
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