65 research outputs found
High-Tc Nodeless s_\pm-wave Superconductivity in (Y,La)FeAsO_{1-y} with Tc=50 K: 75As-NMR Study
We report 75As-NMR study on the Fe-pnictide high-Tc superconductor
Y0.95La0.05FeAsO_{1-y} (Y0.95La0.051111) with Tc=50 K that includes no magnetic
rare-earth elements. The measurement of the nuclear-spin lattice-relaxation
rate 75(1/T1) has revealed that the nodeless bulk superconductivity takes place
at Tc=50 K while antiferromagnetic spin fluctuations (AFSFs) develop moderately
in the normal state. These features are consistently described by the multiple
fully-gapped s_\pm-wave model based on the Fermi-surface (FS) nesting.
Incorporating the theory based on band calculations, we propose that the reason
that Tc=50 K in Y0.95La0.051111 is larger than Tc=28 K in La1111 is that the FS
multiplicity is maximized, and hence the FS nesting condition is better than
that in La1111.Comment: 4 pages, 3 figures, accepted for publication in Phys Rev. Let
Emergent Phases of Nodeless and Nodal Superconductivity Separated by Antiferromagnetic Order in Iron-based Superconductor (Ca4Al2O6)Fe2(As1-xPx)2: 75As- and 31P-NMR Studies
We report P- and As-NMR studies on
(CaAlO)Fe(AsP) with an isovalent substitution
of P for As. We present the novel evolution of emergent phases that the
nodeless superconductivity (SC) in 00.4 and the nodal one around
=1 are intimately separated by the onset of a commensurate stripe-type
antiferromagnetic (AFM) order in 0.5 0.95, as an isovalent
substitution of P for As decreases a pnictogen height measured from
the Fe plane. It is demonstrated that the AFM order takes place under a
condition of 1.32\AA1.42\AA, which is also the case for other
Fe-pnictides with the Fe state in (Fe) layers. This novel
phase evolution with the variation in points to the importance of
electron correlation for the emergence of SC as well as AFM order.Comment: 5pages, 4figures; accepted for publication as a Rapid Communication
in Phys. Rev.
Comment on "Isotope effect in multi-band and multi-channel attractive systems and inverse isotope effect in iron-based superconductors" by T. Yanagisawa, et al
In a recent paper Yanagisawa et al. [1] claim from a theoretical analysis of
a multi-channel multi-band superconductor model that an inverse isotope
exponent on the superconducting transition temperature Tc can be realized in
iron-based superconductors. Simultaneously, a subgroup of the authors of Ref. 1
performed the corresponding isotope effect experiment on (Ba, K)Fe2As2 by
investigating the iron isotope exchange effect on Tc [2]. In accordance with
their theoretical analysis they indeed report an unusually large sign reversed
isotope exponent of {\alpha} \simeq -0.18(3) which is in strong contrast to
previous experiments on the nominally same system with the same composition in
Ba, K content, namely Ba0.6K0.4Fe2As2 [3], where the exponent was determined to
be {\alpha} \simeq 0.37(3). This conflict remains unsolved until now with the
exception of Ref. 4 where the iron isotope exponent has been determined for
FeSe. In accordance with the results of Ref. 3 a large positive isotope
exponent has been seen thus questioning the outcome of Ref. 1 and implicitly
the findings of Ref. 2. Here, we do not comment on the controversial
experimental situation but address the theoretical analysis of Ref. 1, where a
variety of misleading assumptions have led to the conclusion that a sign
reversed isotope exponent can be realized in a multi-band and multi-channel
attractive model for iron based superconductors.Comment: 4 page
Antiferromagnetic Spin Fluctuations and Unconventional Nodeless Superconductivity in an Iron-based New Superconductor (Ca_4Al_2O_{6-y})(Fe_2As_2):75As-NQR Study
We report 75As-nuclear quadrupole resonance (NQR) studies on
(Ca_4Al_2O_{6-y})(Fe_2As_2) with Tc=27K, which unravel unique normal-state
properties and point to unconventional nodeless superconductivity (SC).
Measurement of nuclear-spin-relaxation rate 1/T_1 has revealed a significant
development of two dimensional (2D) antiferromagnetic (AFM) spin fluctuations
down to Tc, in association with the fact that FeAs layers with the smallest
As-Fe-As bond angle are well separated by thick perovskite-type blocking layer.
Below Tc, the temperature dependence of 1/T_1 without any trace of the
coherence peak is well accounted for by an s(+-)-wave multiple gaps model. From
the fact that Tc=27K in this compound is comparable to Tc=28K in the
optimally-doped LaFeAsO_{1-y} in which AFM spin fluctuations are not dominant,
we remark that AFM spin fluctuations are not a unique factor for enhancing Tc
among existing Fe-based superconductors, but a condition for optimizing SC
should be addressed from the lattice structure point of view.Comment: 4pages, 4figures, accepted for publication in Phys. Rev. Let
Inverse Iron Isotope Effect on the transition temperature of the (Ba,K)Fe2As2 superconductor
We report that (Ba,K)Fe2As2 superconductor (a transition temperature, Tc = 38
K) shows inverse Iron isotope effect (-0.18) (the sample including the larger
atomic weight of Fe depicts higher Tc). Measurements of both temperature
dependent magnetization and resistivity reveal a clear inverse shift by
systematic studies on Tc using three types of Fe-isotopes (Fe-54, natural Fe
and Fe-57). This indicates the first evidence of the inverse isotope effect in
high-Tc superconductors. This atomic mass dependence on Tc implies the exotic
coupling mechanism.Comment: 12 pages, 6 figure
Superconductivity and Magnetism in REFeAsO1-xFx (RE=Rare Earth Elements)
Fluoride-doped iron-based oxypnictides containing rare-earth gadolinium
(GdFeAsO0.8F0.2) and co-doping with yttrium (Gd0.8Y0.2FeAsO0.8F0.2) have been
prepared via conventional solid state reaction at ambient pressure. The
non-yttrium substituted oxypnictide show superconducting transition as high as
43.9 K from temperature dependent resistance measurements with the Meissner
effect observed at a lower temperature of 40.8 K from temperature dependent
magnetization measurements. By replacing a small amount of gadolinium with
yttrium Tc was observed to be lowered by 10 K which might be caused by a change
in the electronic or magnetic structures since the crystal structure was not
altered.Comment: 4 pages, 4 figures, Journal of Physics: Conference Series
(Proceedings in the LT25 Low Temperature Physics Conference) Submitte
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