854 research outputs found
Phase Separation of the Two-Dimensional t-J model
The boundary of phase separation of the two-dimensional t-J model is
investigated by the power-Lanczos method and Maxwell construction. The method
is similar to a variational approach and it determines the lower bound of the
phase separation boundary with in the limit . In
the physical interesting regime of high T_c superconductors where
there is no phase separation.Comment: LaTex 5 pages, 4 figure
Self-field effects upon the critical current density of flat superconducting strips
We develop a general theory to account self-consistently for self-field
effects upon the average transport critical current density Jc of a flat
type-II superconducting strip in the mixed state when the bulk pinning is
characterized by a field-dependent depinning critical current density Jp(B),
where B is the local magnetic flux density. We first consider the possibility
of both bulk and edge-pinning contributions but conclude that bulk pinning
dominates over geometrical edge-barrier effects in state-of-the-art YBCO films
and prototype second-generation coated conductors. We apply our theory using
the Kim model, JpK(B) = JpK(0)/(1+|B|/B0), as an example. We calculate Jc(Ba)
as a function of a perpendicular applied magnetic induction Ba and show how
Jc(Ba) is related to JpK(B). We find that Jc(Ba) is very nearly equal to
JpK(Ba) when Ba > Ba*, where Ba* is the value of Ba that makes the net flux
density zero at the strip's edge. However, Jc(Ba) is suppressed relative to
JpK(Ba) at low fields when Ba < Ba*, with the largest suppression occurring
when Ba*/B0 is of order unity or larger.Comment: 9 pages, 4 figures, minor revisions to add four reference
Nonlinear Meissner Effect in CuO Superconductors
Recent theories of the NMR in the CuO superconductors are based on a
spin-singlet order parameter. Since this state has nodal lines on
the Fermi surface, nonlinear effects associated with low-energy quasiparticles
become important, particularly at low temperatures. We show that the
field-dependence of the supercurrent, below the nucleation field for vortices,
can be used to locate the positions of the nodal lines of an unconventional gap
in momentum space, and hence test the proposed state.Comment: 5 pages (RevTex), 1 figure (postscript file incl.
Measurement of the Transverse Beam Spin Asymmetry in Elastic Electron Proton Scattering and the Inelastic Contribution to the Imaginary Part of the Two-Photon Exchange Amplitude
We report on a measurement of the asymmetry in the scattering of transversely
polarized electrons off unpolarized protons, A, at two Q values of
\qsquaredaveragedlow (GeV/c) and \qsquaredaveragedhighII (GeV/c) and a
scattering angle of . The measured transverse
asymmetries are A(Q = \qsquaredaveragedlow (GeV/c)) =
(\experimentalasymmetry alulowcorr \statisticalerrorlow
\combinedsyspolerrorlowalucor) 10 and
A(Q = \qsquaredaveragedhighII (GeV/c)) = (\experimentalasymme
tryaluhighcorr \statisticalerrorhigh
\combinedsyspolerrorhighalucor) 10. The first
errors denotes the statistical error and the second the systematic
uncertainties. A arises from the imaginary part of the two-photon
exchange amplitude and is zero in the one-photon exchange approximation. From
comparison with theoretical estimates of A we conclude that
N-intermediate states give a substantial contribution to the imaginary
part of the two-photon amplitude. The contribution from the ground state proton
to the imaginary part of the two-photon exchange can be neglected. There is no
obvious reason why this should be different for the real part of the two-photon
amplitude, which enters into the radiative corrections for the Rosenbluth
separation measurements of the electric form factor of the proton.Comment: 4 figures, submitted to PRL on Oct.
Evidence for Strange Quark Contributions to the Nucleon's Form Factors at = 0.108 (GeV/c)
We report on a measurement of the parity violating asymmetry in the elastic
scattering of polarized electrons off unpolarized protons with the A4 apparatus
at MAMI in Mainz at a four momentum transfer value of = \Qsquare
(GeV/c) and at a forward electron scattering angle of 30. The measured asymmetry is = (\Aphys
\Deltastat \Deltasyst) 10. The
expectation from the Standard Model assuming no strangeness contribution to the
vector current is A = (\Azero \DeltaAzero) 10. We
have improved the statistical accuracy by a factor of 3 as compared to our
previous measurements at a higher . We have extracted the strangeness
contribution to the electromagnetic form factors from our data to be +
\FakGMs = \GEsGMs \DeltaGEsGMs at = \Qsquare (GeV/c).
As in our previous measurement at higher momentum transfer for + 0.230
, we again find the value for + \FakGMs to be positive,
this time at an improved significance level of 2 .Comment: 4 pages, 3 figure
Mesoscopic phase separation in La2CuO4.02 - a 139La NQR study
In crystals of La2CuO4.02 oxygen diffusion can be limited to such small
length scales, that the resulting phase separation is invisible for neutrons.
Decomposition of the 139La NQR spectra shows the existence of three different
regions, of which one orders antiferromagnetically below 17K concomitantly with
the onset of a weak superconductivity in the crystal. These regions are
compared to the macroscopic phases seen previously in the title compound and
the cluster-glass and striped phases reported for the underdoped Sr-doped
cuprates.Comment: 4 pages, RevTeX, 5 figures, to be published in PR
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