545 research outputs found
Gauge approach to the specific heat in the normal state of cuprates
Many experimental features of the electronic specific heat and entropy of
high Tc cuprates in the normal state, including the nontrivial temperature
dependence of the specific heat coefficient and negative intercept of the
extrapolated entropy to T=0 for underdoped cuprates, are reproduced using the
spin-charge gauge approach to the t-J model. The entropy turns out to be
basically due to fermionic excitations, but with a temperature dependence of
the specific heat coefficient controlled by fluctuations of a gauge field
coupling them to gapful bosonic excitations. In particular the negative
intercept of the extrapolated entropy at T=0 in the pseudogap ``phase'' is
attributed to the scalar component of the gauge field, which implements the
local no-double occupancy constraint.Comment: 5 pages, 5 figure
What can we learn from comparison between cuprates and He films ? : phase separation and fluctuating superfluidity
In the underdoped, overdoped, Zn-doped or stripe-forming regions of
high- cuprate superconductors (HTSC), the superfluid density
at shows universal correlations with . Similar
strong correlations exist between 2-dimensional superfluid density and
superfluid transition temperature in thin films of He in non-porous or
porous media, and He/He film adsorbed on porous media. Based on
analogy between HTSC and He film systems, we propose a model for cuprates
where: (1) the overdoped region is characterized by a phase separation similar
to He/He; and (2) pair (boson) formation and fluctuating
superconductivity occur at separate temperatures above in the
underdoped region.Comment: 8 pages, 5 figures. Invited paper presented at the third
international conference on stripes and high-Tc superconductivity
(STRIPE-2000), Sept. 25-30th, 2000, Rome, Italy. To be published in the
International Journal of Modern Physics
Peculiarities of electronic heat capacity of thulium cuprates in pseudogap state
Precise calorimetric measurements have been carried out in the 7 - 300 K
temperature range on two ceramic samples of thulium 123 cuprates TmBa2Cu3O6.92
and TmBa2Cu3O6.70. The temperature dependence of the heat capacity was analyzed
in the region where the pseudogap state (PGS) takes place. The lattice
contribution was subtracted from the experimental data. The PGS component has
been obtained by comparing electronic heat capacities of two investigated
samples because the PGS contribution for the 6.92 sample is negligible. The
anomalous behavior of the electronic heat capacity near the temperature
boundary of PGS was found. It is supposed that this anomaly is due to
peculiarities in N(E) function where N is the density of electronic states and
E is the energy of carriers of charge.Comment: 12 pages, 3 Postscript figure
On the Relationship Between the Pseudo- and Superconducting Gaps: Effects of Residual Pairing Correlations Below Tc
The existence of a normal state spectral gap in underdoped cuprates raises
important questions about the associated superconducting phase. For example,
how does this pseudogap evolve into its below Tc counterpart? In this paper we
characterize this unusual superconductor by investigating the nature of the
``residual'' pseudogap below Tc and, find that it leads to an important
distinction between the superconducting excitation gap and order parameter. Our
approach is based on a conserving diagrammatic BCS Bose-Einstein crossover
theory which yields the precise BCS result in weak coupling at any T<Tc and
reproduces Leggett's results in the T=0 limit. We explore the resulting
experimental implications.Comment: REVTeX, 4 pages, 1 EPS figure (included
Condensation Energy and High Tc Superconductivity
From an analysis of the specific heat of one of the cuprate superconductors
it is shown, that even if a large part of the experimental specific heat
associated with the superconducting phase transition is due to fluctuations,
this part must be counted when one tries to extract the condensation energy
from the data. Previous work by Chakravarty, Kee and Abrahams, where the
fluctuation part was subtracted, has resulted in an incorrect estimation of the
condensation energy.Comment: 4 pages, 5 encapsulated Postscript figures, uses ReVTeX.st
A possible scenario for the mechanism of high-Tc superconductivity based on experimental data
The issue of the mechanism of high-Tc superconductivity remains open. In this
contribution, we propose a new scenario for the mechanism of superconductivity
in cuprates based on analysis of experimental data, mainly tunneling, neutron
scattering and muon-spin-relaxation data, made earlier (see e.g. Mod. Phys.
Lett. B 19 (2005) 743). A specific feature of this scenario is the mechanism of
the establishment of long-range phase coherence among Cooper pairs, based on
recent experimental data obtained in nonsuperconducting materials.Comment: 2 pages with 2 figures (Dresden conference
Kinetic energy change with doping upon superfluid condensation in high temperature superconductors
In conventional BCS superconductors, the electronic kinetic energy increases
upon superfluid condensation (the change DEkin is positive). Here we show that
in the high critical temperature superconductor Bi-2212, DEkin crosses over
from a fully compatible conventional BCS behavior (DEkin>0) to an
unconventional behavior (DEkin<0) as the free carrier density decreases. If a
single mechanism is responsible for superconductivity across the whole phase
diagram of high critical temperature superconductors, this mechanism should
allow for a smooth transition between such two regimes around optimal doping.Comment: 3 pages, 2 figure
Thermodynamic transitions in inhomogeneous d-wave superconductors
We study the spectral and thermodynamic properties of inhomogeneous d-wave
superconductors within a model where the inhomogeneity originates from atomic
scale pair disorder. This assumption has been shown to be consistent with the
small charge and large gap modulations observed by scanning tunnelling
spectroscopy (STS) on BSCCO. Here we calculate the specific heat within the
same model, and show that it gives a semi-quantitative description of the
transition width in this material. This model therefore provides a consistent
picture of both surface sensitive spectroscopy and bulk thermodynamic
properties.Comment: 4 pages, 4 figure
A national scale inventory of resource provision for biodiversity within domestic gardens
The human population is increasingly disconnected from nature due to urbanisation. To counteract this phenomenon, the UK government has been actively promoting wildlife gardening. However, the extent to which such activities are conducted and the level of resource provision for biodiversity (e.g., food and nesting sites) within domestic gardens remains poorly documented. Here we generate estimates for a selection of key resources provided within gardens at a national scale, using 12 survey datasets gathered across the UK. We estimate that 22.7 million households (87% of homes) have access to a garden. Average garden SiZe is 190 m(2), extrapolating to a total area of 432,924 ha. Although substantial, this coverage is still an order of magnitude less than that of statutory protected areas. Approximately 12.6 million (48%) households provide supplementary food for birds, 7.4 million of which specifically use bird feeders. Similarly, there are a minimum of 4.7 million nest boxes within gardens. These figures equate to one bird feeder for every nine potentially feeder-using birds in the UK, and at least one nest box for every six breeding pairs of cavity nesting birds. Gardens also contain 2.5-3.5 million ponds and 28.7 million trees, which is just under a quarter of all trees occurring outside woodlands. Ongoing urbanisation, characterised by increased housing densities, is inevitable throughout the UK and elsewhere. The important contribution domestic gardens make to the green space infrastructure in residential areas must be acknowledged, as their reduction will impact biodiversity conservation, ecosystem services, and the well-being of the human population
Condensation energy in Eliashberg theory -- from weak to strong coupling
We consider two issues related to the condensation energy in superconductors
described by the Eliashberg theory for various forms of the pairing
interaction, associated either with phonon or electronic mechanisms of
superconductivity. First, we derive a leading correction to the BCS formula for
the condensation energy to first order in the coupling . Second, we
show that at a given , the value of the condensation energy strongly
depends on the functional form of the effective pairing interaction .Comment: 6 pages, 5 figures, published in PRB; missing reference has been
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