264 research outputs found
Acoustic Attenuation in High- Superconductors
We analyze the acoustic attenuation rate in high- superconductors, and
find that this method offers an additional way to examine the anisotropy of the
superconducting order parameter in these materials. We argue that it should be
possible to distinguish the electronic contribution to the acoustic
attenuation, which has a strong temperature dependence near , from the
lattice contribution, which does not show a strong temperature dependence near
. We propose that this can be utilized to measure the anisotropy of the
order parameter by measuring the attenuation rate near in different
directions.Comment: 9 pages, latex, 2 postscript figures, in press Physica C, (uuencoded
file consisting of paper and 2 figures, please contact J.C. Swihart
([email protected]) for a printed copy
Evidence for Strong-coupling S-wave Superconductivity in MgB2 :11B NMR Study
We have investigated a gap structure in a newly-discovered superconductor,
MgB2 through the measurement of 11B nuclear spin-lattice relaxation rate,
^{11}(1/T_1). ^{11}(1/T_1) is proportional to the temperature (T) in the normal
state, and decreases exponentially in the superconducting (SC) state, revealing
a tiny coherence peak just below T_c. The T dependence of 1/T_1 in the SC state
can be accounted for by an s-wave SC model with a large gap size of 2\Delta
/k_BT_c \sim 5 which suggests to be in a strong-coupling regime.Comment: 2 pages with 1 figur
Fermi-liquid ground state in n-type copper-oxide superconductor Pr0.91Ce0.09LaCuO4-y
We report nuclear magnetic resonance studies on the low-doped n-type
copper-oxide Pr_{0.91}LaCe_{0.09}CuO_{4-y} (T_c=24 K) in the superconducting
state and in the normal state uncovered by the application of a strong magnetic
field. We find that when the superconductivity is removed, the underlying
ground state is the Fermi liquid state. This result is at variance with that
inferred from previous thermal conductivity measurement and contrast with that
in p-type copper-oxides with a similar doping level where high-T_c
superconductivity sets in within the pseudogap phase. The data in the
superconducting state are consistent with the line-nodes gap model.Comment: version to appear in Phys. Rev. Let
Possible Multiple Gap Superconductivity with Line Nodes in Heavily Hole-Doped Superconductor KFe2As2 Studied by 75As-NQR and Specific Heat
We report the 75As nuclear quadrupole resonance (NQR) and specific heat
measurements of the heavily hole-doped superconductor KFe2As2 (Tc = 3.5 K). The
spin-lattice relaxation rate 1/T1 in the superconducting state exhibits quite
gradual temperature dependence with no coherence peak below Tc. The
quasi-particle specific heat C_QP/T shows small specific heat jump which is
about 30% of electronic specific heat coefficient just below Tc. In addition,
it suggests the existence of low-energy quasi-particle excitation at the lowest
measurement temperature T = 0.4 K \simeq Tc/10. These temperature dependence of
1/T1 and C_QP/T can be explained by multiple nodal superconducting gap scenario
rather than multiple fully-gapped s_\pm-wave one within simple gap analysis.Comment: 5 pages, 5 figures, to be published in J. Phys. Soc. Jpn. No.8 issue
(2009
Coherence effect in a two-band superconductor: Application to iron pnictides
From a theoretical point of view, we propose an experimental method to
determine the pairing symmetry of iron pnictides. We focus on two kinds of
pairing symmetries, and , which are strong candidates for the
pairing symmetry of iron pnictides. For each of these two symmetries, we
calculate both the density and spin response functions by using the two-band
BCS model within the one-loop approximation. As a result, a clear difference is
found between the - and -wave states in the temperature
dependence of the response functions at nesting vector , which connects
the hole and electron Fermi surfaces. We point out that this difference comes
from the coherence effect in the two-band superconductor. We suggest that the
pairing symmetry could be clarified by observing the temperature dependence of
both the density and spin structure factors at the nesting vector in
neutron scattering measurements.Comment: 15 pages, 7 figures, 1 tabl
New magnetic coherence effect in superconducting La_{2-x}Sr_{x}CuO_{4}
We have used inelastic neutron scattering to examine the magnetic
fluctuations at intermediate frequencies in the simplest high temperature
superconductor, La_{2-x}Sr_{x}Cu_{4}. The suppression of the low energy
magnetic response in the superconducting state is accompanied by an increase in
the response at higher energies. Just above a threshold energy of ~7 meV there
is additional scattering present below T_{c} which is characterised by an
extraordinarily long coherence length, in excess of 50 \AA.Comment: 11 pages, RevTeX, 4 postscript figure
Superconductivity and Spin Fluctuations in the Electron-Doped Infinitely-Layered High Tc Superconductor SrLaCuO (Tc=42K)
This paper describes the first 63-Cu NMR study of an electron-doped
infinitely-layered high Tc superconductor SrLaCuO (Tc=42K). The
spin dynamics in the normal state above Tc exhibits qualitatively the same
behavior as some hole-doped materials with significantly enhanced spin
fluctuations. Below Tc, we observed no signature of a Hebel-Slichter coherence
peak, suggesting an unconventional nature of the symmetry of the
superconducting order parameter.Comment: Invited Paper to SNS-95 Conference (Spectroscopies on Novel
Superconductors 1995 at Stanford). Also presented at Aspen Winter Conference
on Superconductivity and Grenoble M^2S-HTSC in 199
Multiple superconducting gap and anisotropic spin fluctuations in iron arsenides: Comparison with nickel analog
We present extensive 75As NMR and NQR data on the superconducting arsenides
PrFeAs0.89F0.11 (Tc=45 K), LaFeAsO0.92F0.08 (Tc=27 K), LiFeAs (Tc = 17 K) and
Ba0.72K0.28Fe2As2 (Tc = 31.5 K) single crystal, and compare with the nickel
analog LaNiAsO0.9F0.1 (Tc=4.0 K) . In contrast to LaNiAsO0.9F0.1 where the
superconducting gap is shown to be isotropic, the spin lattice relaxation rate
1/T1 in the Fe-arsenides decreases below Tc with no coherence peak and shows a
step-wise variation at low temperatures. The Knight shift decreases below Tc
and shows a step-wise T variation as well. These results indicate spinsinglet
superconductivity with multiple gaps in the Fe-arsenides. The Fe
antiferromagnetic spin fluctuations are anisotropic and weaker compared to
underdoped copper-oxides or cobalt-oxide superconductors, while there is no
significant electron correlations in LaNiAsO0.9F0.1. We will discuss the
implications of these results and highlight the importance of the Fermi surface
topology.Comment: 6 pages, 11 figure
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