Coexistence of superconductivity and antiferromagnetism in self-doped bilayer t-t'-J model

A self-doped bilayer t-t'-J model of an electron- and a hole-doped planes is studied by the slave-boson mean-field theory. A hopping integral between the differently doped planes, which are generated by a site potential, are renormalized by the electron-electron correlation. We find coexistent phases of antiferromagnetic (AFM) and superconducting orders, although the magnitudes of order parameters become more dissimilar in the bilayer away from half-filling. Fermi surfaces (FS's) with the AFM order show two pockets around the nodal and the anti-nodal regions. These results look like a composite of electron- and hole-doped FS's. In the nodal direction, the FS splitting is absent even in the bilayer system, since one band is flat due to the AFM order.Comment: 6 pages, 4 figure

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

Inelastic neutron scattering study on the resonance mode in an optimally doped superconductor LaFeAsO0.92_{0.92}F0.08_{0.08}

An optimally doped iron-based superconductor LaFeAsO$_{0.92}$F$_{0.08}$ with $T_c = 29$ K has been studied by inelastic powder neutron scattering. The magnetic excitation at $Q=1.15$ \AA$^{-1}$ is enhanced below $T_c$, leading to a peak at $E_{res}\sim13$ meV as the resonance mode, in addition to the formation of a gap at low energy below the crossover energy $\Delta_{c}\sim10 meV$. The peak energy at $Q=1.15$ \AA$^{-1}$ corresponds to $5.2 k_B T_c$ in good agreement with the other values of resonance mode observed in the various iron-based superconductors, even in the high-$T_c$ cuprates. Although the phonon density of states has a peak at the same energy as the resonance mode in the present superconductor, the $Q$-dependence is consistent with the resonance being of predominately magnetic origin.Comment: 4 pages, 5 Postscript figure

Effect of Co doping on the in-plane anisotropy in the optical spectrum of underdoped Ba(Fe1-xCox)2As2

We investigated the anisotropy in the in-plane optical spectra of detwinned Ba(Fe1-xCox)2As2. The optical conductivity spectrum of BaFe2As2 shows appreciable anisotropy in the magnetostructural ordered phase, whereas the dc resistivity is almost isotropic at low temperatures. Upon Co doping, the resistivity becomes highly anisotropic, while the finite-energy intrinsic anisotropy is suppressed. It is found that anisotropy in resistivity arises from anisotropic impurity scattering from doped Co atoms, extrinsic in origin. Intensity of a specific optical phonon mode is also found to show striking anisotropy in the ordered phase. The anisotropy induced by Co impurity and that observed in the optical phonon mode are hallmarks of the highly polarizable electronic state in the ordered phase.Comment: 5 pages, 4 figure

No anomalous spin fluctuation in optimally doped and overdoped iron-based oxypnictide superconductors LaFeAsO1−x_{1-x}Fx_x probed by muon spin relaxation

Zero field and longitudinal field muon spin relaxation measurements have been performed in optimally doped and overdoped superconductors LaFeAsO$_{1-x}$F$_x$ in order to investigate the magnetic fluctuation over a wide range of temperature and longitudinal field. We have observed no sign of magnetic fluctuation against temperature in the muons' time window (10$^{-5}\sim$ 10$^{-9}$s). Considering the current results and other results, i.e., spin fluctuation observed by neutron scattering, pseudogap-like behaviors by NMR and photoemission spectroscopy, it is suggested that not only the spin fluctuation but also the multiband character with several different orbital contributions at the Fermi surface may play an important role in the superconducting pairing mechanism of LaFeAsO$_{1-x}$F$_x$.Comment: 5 page

Microwave Penetration Depth and Quasiparticle Conductivity in PrFeAsO_1-y Single Crystals : Evidence for a Full-Gap Superconductor

In-plane microwave penetration depth $\lambda_{ab}$ and quaiparticle conductivity at 28 GHz are measured in underdoped single crystals of the Fe-based superconductor PrFeAsO$_{1-y}$ ($T_c\approx 35$ K) by using a sensitive superconducting cavity resonator. $\lambda_{ab}(T)$ shows flat dependence at low temperatures, which is incompatible with the presence of nodes in the superconducting gap $\Delta({\bf k})$. The temperature dependence of the superfluid density demonstrates that the gap is non-zero ($\Delta/k_BT_c\gtrsim 1.6$) all over the Fermi surface. The microwave conductivity below $T_c$ exhibits an enhancement larger than the coherence peak, reminiscent of high-$T_c$ cuprate superconductors.Comment: 4 pages, 3 figures. Version accepted for publication in Phys. Rev. Lett. For related results of hole-doped 122 system, see arXiv:0810.350