8,838 research outputs found
Incoherent superconductivity well above in high- cuprates - harmonizing the spectroscopic and thermodynamic data
Cuprate superconductors have long been known to exhibit an energy gap that
persists high above the superconducting transition temperature (). Debate
has continued now for decades as to whether it is a precursor superconducting
gap or a pseudogap arising from some competing correlation. Failure to resolve
this has arguably delayed explaining the origins of superconductivity in these
highly complex materials. Here we effectively settle the question by
calculating a variety of thermodynamic and spectroscopic properties, exploring
the effect of a temperature-dependent pair-breaking term in the self-energy in
the presence of pairing interactions that persist well above . We start by
fitting the detailed temperature-dependence of the electronic specific heat and
immediately can explain its hitherto puzzling field dependence. Taking this
same combination of pairing temperature and pair-breaking scattering we are
then able to simultaneously describe in detail the unusual temperature and
field dependence of the superfluid density, tunneling, Raman and optical
spectra, which otherwise defy explanation in terms a superconducting gap that
closes conventionally at . These findings demonstrate that the gap above
in the overdoped regime likely originates from incoherent superconducting
correlations, and is distinct from the competing-order "pseudogap" that appears
at lower doping.Comment: 15 pages, 6 figure
Closing the pseudogap quietly
The physical properties of hole-doped cuprate high-temperature
superconductors are heavily influenced by an energy gap known as the pseudogap
whose origin remains a mystery second only to that of superconductivity itself.
A key question is whether the pseudogap closes at a temperature T*. The absence
of a specific heat anomaly, together with persistent entropy losses up to 300K,
have long suggested that the pseudogap does not vanish at T*. However, amid a
growing body of evidence from other techniques pointing to the contrary we
revisit this question. Here we investigate if, by adding a temperature
dependence to the pseudogap energy and quasiparticle lifetime in the
resonating-valence-bond spin-liquid model of Yang Rice and Zhang, we can close
the pseudogap quietly in the specific heat.Comment: 5 pages, 4 figure
Hall effect and Fermi surface reconstruction via electron pockets in the high- cuprates
The mechanism by which the Fermi surface of high- cuprates undergoes a
dramatic change from a large hole-like barrel to small arcs or pockets on
entering the pseudogap phase remains a question of fundamental importance. Here
we calculate the normal-state Hall coefficient from the resonating-valence-bond
spin-liquid model developed by Yang, Rice and Zhang. In this model,
reconstruction of the Fermi surface occurs via an intermediate regime where the
Fermi surface consists of both hole- and electron-like pockets. We find that
the doping dependence of the Hall number transitions from to
over this narrow doping range. At low temperatures, a switch from a downturn to
an upturn in the Hall coefficient signals the departure of the electron-like
pockets from the Fermi surface.Comment: 5 pages, 5 figure
Electron pockets and pseudogap asymmetry observed in the thermopower of underdoped cuprates
We calculate the diffusion thermoelectric power of high-Tc cuprates using the
resonating-valence-bond spin-liquid model developed by Yang, Rice and Zhang
(YRZ). In this model, reconstruction of the energy-momentum dispersion results
in a pseudogap in the density of states that is heavily asymmetric about the
Fermi level. The subsequent asymmetry in the spectral conductivity is found to
account for the large magnitude and temperature dependence of the thermopower
observed in underdoped cuprates. In addition we find evidence in experimental
data for electron pockets in the Fermi surface, arising from a YRZ-like
reconstruction, near the onset of the pseudogap in the slightly overdoped
regime.Comment: 6 pages, 7 figures, accepted for publication in EP
Saddle-point van Hove singularity and the phase diagram of high-Tc cuprates
We examine the generic phase behavior of high-Tc cuprate superconductors in
terms a universal van Hove singularity in the strongly overdoped region. Using
a rigid ARPES-derived dispersion we solve the BCS gap equation and show that
the pairing interaction or pairing energy cutoff must be a rapidly declining
function of doping. This result is prejudicial to a phonon-based pairing
interaction and more consistent with a magnetic or magnetically enhanced
interaction.Comment: 5 pages, 2 figures, submitted to Physical Review
Two-component electron fluid in underdoped high- cuprate superconductors
Evidence from NMR of a two-component spin system in cuprate high-
superconductors is shown to be paralleled by similar evidence from the
electronic entropy so that a two-component quasiparticle fluid is implicated.
We propose that this two-component scenario is restricted to the optimal and
underdoped regimes and arises from the upper and lower branches of the
reconstructed energy-momentum dispersion proposed by Yang, Rice and Zhang (YRZ)
to describe the pseudogap. We calculate the spin susceptibility within the YRZ
formalism and show that the doping and temperature dependence reproduces the
experimental data for the cuprates.Comment: 5 pages, 2 figures, accepted for publication in European Physics
Letter
Enterprise profiles in deprived areas: Are they distinctive?
This paper examines the extent to which segmenting business activity on the basis of the relative deprivation of a given area provides additional understanding (in terms of analysis and policy) that is not obtained by alternative divisions, e.g., by sector, size, etc. The paper is primarily motivated by the explicit inclusion of a deprived area dimension to many UK small business/enterprise policies introduced since 1997. We use two datasets drawn from the customer records of Barclays Bank PLC to obtain an initial analysis of the business stocks and dynamics in deprived and non-deprived areas of England. The data indicate that the deprived areas of England vary systematically from the wider economy in terms of several business stock characteristics and associated dynamics. These differences include a lower proportion of business service firms, lower female involvement in the owner-manager base and a poorer risk profile. The analysis supports the view that there are likely to be benefits from the tailoring of small business/ enterprise policies to sub-national levels
Improved He I Emissivities in the Case B Approximation
We update our prior work on the case B collisional-recombination spectrum of
He I to incorporate \textit{ab initio} photoionisation cross-sections. This
large set of accurate, self-consistent cross-sections represents a significant
improvement in He I emissivity calculations because it largely obviates the
piecemeal nature that has marked all modern works. A second, more recent set of
\textit{ab initio} cross-sections is also available, but we show that those are
less consistent with bound-bound transition probabilities than our adopted set.
We compare our new effective recombination coefficients with our prior work and
our new emissivities with those by other researchers, and we conclude with
brief remarks on the effects of the present work on the He I error budget. Our
calculations cover temperatures K and densities cm. Full results are available online.Comment: Accepted to MNRAS Letters; 4 pages, 4 figures, 2 tables, 1
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