8,133 research outputs found
Phase separation and pairing in coupled chains and planes
A generalization of the model in a system of two coupled chains or
planes is studied by numerical diagonalization of small clusters. In
particular, the effect of density fluctuations between these one- or
two-dimensional coupled layerson intralayer phase separation and pairing is
analyzed. The most robust signals of superconductivity are found at quarter
filling for couplings just before the fully interlayer phase separated regime.
The possibility of an enhancement of the intralayer superconducting pairing
correlations by the interlayer couplings is investigated.Comment: 13 pages + 3 figures, available upon request, LATEX, preprint
ORNL/CCIP/93/1
A note on density correlations in the half-filled Hubbard model
We consider density-density correlations in the one-dimensional Hubbard model
at half filling. On intuitive grounds one might expect them to exhibit an
exponential decay. However, as has been noted recently, this is not obvious
from the Bethe Ansatz/conformal field theory (BA/CFT) approach. We show that by
supplementing the BA/CFT analysis with simple symmetry arguments one can easily
prove that correlations of the lattice density operators decay exponentially.Comment: 3 pages, RevTe
Superconductivity in the Cuo Hubbard Model with Long-Range Coulomb Repulsion
A multiband CuO Hubbard model is studied which incorporates long-range (LR)
repulsive Coulomb interactions. In the atomic limit, it is shown that a
charge-transfer from copper to oxygen ions occurs as the strength of the LR
interaction is increased. The regime of phase separation becomes unstable, and
is replaced by a uniform state with doubly occupied oxygens. As the holes
become mobile a superfluid condensate is formed, as suggested by a numerical
analysis of pairing correlation functions and flux quantization. Although most
of the calculations are carried out on one dimensional chains, it isComment: LATEX, 14 pages, 4 figures available as postcript files or hard copy,
preprint ORNL-CCIP/93/1
Effects of Umklapp Scattering on Electronic States in One Dimension
The effects of Umklapp scattering on electronic states are studied in one
spatial dimension at absolute zero. The model is basically the Hubbard model,
where parameters characterizing the normal () and Umklapp () scattering
are treated independently. The density of states is calculated in the t-matrix
approximation by taking only the forward and Umklapp scattering into account.
It is found that the Umklapp scattering causes the global splitting of the
density of states. In the presence of sufficiently strong Umklapp scattering, a
pole in the t-matrix appears in the upper half plane, signalling an instability
towards the 'pairing' ordered state ( is the reciprocal lattice
vector), whose consequences are studied in the mean field approximation. It
turns out that this ordered state coexists with spin-density-wave state and
also brings about Cooper-pairs. A phase diagram is determined in the plane of
and electron filling .Comment: 22 pages, LaTeX, 17 figures included, uses jpsj.st
Experimental evidence of s-wave superconductivity in bulk CaC
The temperature dependence of the in-plane magnetic penetration depth,
, has been measured in a c-axis oriented polycrystalline
CaC bulk sample using a high-resolution mutual inductance technique. A
clear exponential behavior of has been observed at low
temperatures, strongly suggesting isotropic s-wave pairing. Data fit using the
standard BCS theory yields Angstroem and
meV. The ratio
gives indication for a conventional weakly coupled superconductor.Comment: To appear in Phys. Rev. Let
Unusual metallic phase in a chain of strongly interacting particles
We consider a one-dimensional lattice model with the nearest-neighbor
interaction and the next-nearest neighbor interaction with filling
factor 1/2 at zero temperature. The particles are assumed to be spinless
fermions or hard-core bosons. Using very simple assumptions we are able to
predict the basic structure of the insulator-metal phase diagram for this
model. Computations of the flux sensitivity support the main features of the
proposed diagram and show that the system maintains metallic properties at
arbitrarily large values of and along the line ,
where is the hopping amplitude, and . We think that close
to this line the system is a ``weak'' metal in a sense that the flux
sensitivity decreases with the size of the system not exponentially but as
with .Comment: To appear in J. Phys. C; 9 revtex preprint pages + 4 ps figures,
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Colossal Magnetoresistance in the Mn2+ Oxypnictides NdMnAsO1-xFx
Colossal magnetoresistance (CMR) is a rare phenomenon in which the electronic
resistivity of a material can be decreased by orders of magnitude upon
application of a magnetic field. Such an effect could be the basis of the next
generation of magnetic memory devices. Here we report CMR in the
antiferromagnetic oxypnictide NdMnAsO1-xFx as a result of competition between
an antiferromagnetic insulating phase with strong electron correlations and a
paramagnetic semiconductor upon application of a magnetic field. The discovery
of CMR in antiferromagnetic Mn2+ oxypnictide materials could open up an array
of materials for further investigation and optimisation for technological
applications
Evidence of Strong-Coupled Superconductivity in CaC6 from Tunneling Spectroscopy
Point-contact tunneling on CaC crystals reproducibly reveals
superconducting gaps, , of 2.30.2 meV which are ~40% larger
than earlier reports. That puts CaC into the class of very strong-coupled
superconductors since 2/kT~4.6. Thus soft Ca phonons will be
primarily involved in the superconductivity, a conclusion that explains the
large Ca isotope effect found recently for CaC. Consistency among
superconductor-insulator-normal metal (SIN), SIS and Andreev reflection (SN)
junctions reinforces the intrinsic nature of this result.Comment: 2nd version, 4 pages, 4 figures, re-submitted to Physical Review
Letter
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