1,333 research outputs found
Microscopic theories for cubic and tetrahedral superconductors: application to PrOs_4Sb_{12}
We examine weak-coupling theory for unconventional superconducting states of
cubic or tetrahedral symmetry for arbitrary order parameters and Fermi surfaces
and identify the stable states in zero applied field. We further examine the
possibility of having multiple superconducting transitions arising from the
weak breaking of a higher symmetry group to cubic or tetrahedral symmetry.
Specifically, we consider two higher symmetry groups. The first is a weak
crystal field theory in which the spin-singlet Cooper pairs have an approximate
spherical symmetry. The second is a weak spin orbit coupling theory for which
spin-triplet Cooper pairs have a cubic orbital symmetry and an approximate
spherical spin rotational symmetry. In hexagonal UPt_3, these theories easily
give rise to multiple transitions. However, we find that for cubic materials,
there is only one case in which two superconducting transitions occur within
weak coupling theory. This sequence of transitions does not agree with the
observed properties of PrOs_4Sb_{12}. Consequently, we find that to explain two
transitions in PrOs_4Sb_{12} using approximate higher symmetry groups requires
a strong coupling theory. In view of this, we finally consider a weak coupling
theory for which two singlet representations have accidentally nearly
degenerate transition temperatures (not due to any approximate symmetries). We
provide an example of such a theory that agrees with the observed properties of
PrOs_4Sb_{12}.Comment: 11 pages,1 figur
Imaging Three Dimensional Two-particle Correlations for Heavy-Ion Reaction Studies
We report an extension of the source imaging method for analyzing
three-dimensional sources from three-dimensional correlations. Our technique
consists of expanding the correlation data and the underlying source function
in spherical harmonics and inverting the resulting system of one-dimensional
integral equations. With this strategy, we can image the source function
quickly, even with the finely binned data sets common in three-dimensional
analyses.Comment: 13 pages, 11 figures, submitted to Physical Review
Identification of Non-unitary triplet pairing in a heavy Fermion superconductor UPt_3
A NMR experiment recently done by Tou et al. on a heavy Fermion
superconductor UPt is interpreted in terms of a non-unitary spin-triplet
pairing state which we have been advocating. The proposed state successfully
explains various aspects of the seemingly complicated Knight shift behaviors
probed for major orientations, including a remarkable d-vector rotation under
weak fields. This entitles UPt as the first example that a charged many
body system forms a spin-triplet odd-par ity pairing at low temperatures and
demonstrates unambiguously that the putative spin-orbit coupling in UPt is
weak.Comment: 4 pages, 2 eps figures, to be published in J. Phys. Soc. Jpn. 67
(1998) No.
Evidence for a two component magnetic response in UPt3
The magnetic response of the heavy fermion superconductor UPt_3 has been
investigated on a microscopic scale by muon Knight shift studies. Two distinct
and isotropic Knight shifts have been found for the field in the basal plane.
While the volume fractions associated with the two Knight shifts are
approximately equal at low and high temperatures, they show a dramatic and
opposite temperature dependence around T_N. Our results are independent on the
precise muon localization site. We conclude that UPt_3 is characterized by a
two component magnetic response.Comment: 5 pages, 4 figure
Direct observation of the formation of polar nanoregions in Pb(MgNb)O using neutron pair distribution function analysis
Using neutron pair distribution function (PDF) analysis over the temperature
range from 1000 K to 15 K, we demonstrate the existence of local polarization
and the formation of medium-range, polar nanoregions (PNRs) with local
rhombohedral order in a prototypical relaxor ferroelectric
Pb(MgNb)O. We estimate the volume fraction of the PNRs as a
function of temperature and show that this fraction steadily increases from 0 %
to a maximum of 30% as the temperature decreases from 650 K to 15 K.
Below T200 K the PNRs start to overlap as their volume fraction reaches
the percolation threshold. We propose that percolating PNRs and their
concomitant overlap play a significant role in the relaxor behavior of
Pb(MgNb)O.Comment: 4 pages, 3 figure
Chiral Correction to the Spin Fluctuation Feedback in two-dimensional p-wave Superconductors
We consider the stability of the superconducting phase for spin-triplet
p-wave pairing in a quasi-two-dimensional system. We show that in the absence
of spin-orbit coupling there is a chiral contribution to spin fluctuation
feedback which is related to spin quantum Hall effect in a chiral
superconducting phase. We show that this mechanism supports the stability of a
chiral p-wave state.Comment: 8 pages. The final version is accepted for publication in Europhys
Let
Observation of two time scales in the ferromagnetic manganite La(1-x)Ca(x)MnO(3), x = 0.3
We report new zero-field muon spin relaxation and neutron spin echo
measurements in ferromagnetic (FM) (La,Ca)MnO3 which taken together suggest two
spatially separated regions in close proximity possessing very different Mn-ion
spin dynamics. One region corresponds to an extended cluster which displays
'critical slowing down' near Tc and an increasing volume fraction below Tc. The
second region possesses more slowly fluctuating spins and a decreasing volume
fraction below Tc. These data are discussed in terms of the growth of small
polarons into overlapping regions of correlated spins below Tc, resulting in a
microscopically inhomogeneous FM transition.Comment: 10 pages, 3 figure
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