Microscopic observation of superconducting fluctuations in $kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br by $^{13}$C NMR spectroscopy

We performed $^{13}$C-NMR experiment and measured spin-lattice relaxation rate divided by temperature $1/T_{1}T$ near the superconducting (SC) transition temperature $T_{c}$ in $kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br ($kappa$-Br salt), and $kappa$-(BEDT-TTF)$_{2}$Cu(NCS)$_{2}$ ($kappa$-NCS salt). We observed the reduction of $1/T_{1}T$ starting at the temperature higher than $T_c$ in $kappa$-Br salt. Microscopic observation of quasi-particle density of states in the fluctuating SC state revealed the effects of short-range Cooper pairs induced in the normal state to the quasi-particle density of states. We also performed systematic measurements in the fields both parallel and perpendicular to the conduction plane in $kappa$-Br and $kappa$-NCS salts, and confirmed that the reduction of $1/T_{1}T$ above $T_{c}$ is observed only in $kappa$-Br salt regardless of the external field orientation.

Strongly three-dimensional electronic structure and Fermi Surfaces of SrFe$_{2}$(As$_{0.65}$P$_{0.35}$)$_{2}$: Comparison with BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$

The isovalent-substituted iron-pnictide superconductor SrFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ ($x$=0.35) has a slightly higher optimum critical temperature than the similar system BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$, and its parent compound SrFe$_{2}$As$_{2}$ has a much higher Neel temperature than BaFe$_{2}$As$_{2}$. We have studied the band structure and the Fermi surfaces of optimally-doped SrFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ by angle-resolved photoemission spectroscopy (ARPES). Three holelike Fermi surfaces (FSs) around (0,0) and two electronlike FSs around ($pi$,$pi$) have been observed as in the case of BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$. Measurements with different photon energies have revealed that one of the hole FSs is more strongly warped along the $k_{z}$ direction than the corresponding one in BaFe(As$_{1-x}$P$_{x}$)$_{2}$, while the electron FSs are almost cylindrical unlike corrugated ones in BaFe(As$_{1-x}$P$_{x}$)$_{2}$. Comparison of the ARPES data with first-principles band-structure calculation revealed that the quasiparticle mass renormalization factors are different not only between bands of different orbital character but also between the hole and electron FSs of the same orbital character. By examining nesting conditions between the hole and electron FSs, we conclude that magnetic interactions between FeAs layers rather than FS nesting play an important role in stabilizing the antiferromagnetic order. The insensitivity of superconductivity to the FS nesting can be explained if only the $d_{xy}$ and/or $d_{xz/yz}$ orbitals are active in inducing superconductivity or if FS nesting is not important for superconductivity.

Unconventional multiband superconductivity with nodes in single-crystalline SrFe2(As_0.65P_0.35)2 as seen via 31P-NMR and specific heat

We report 31P-NMR and specific heat measurements on an iron (Fe)-based superconductor SrFe2(As0.65P0.35)2 with Tc=26 K, which have revealed the development of antiferromagnetic correlations in the normal state and the unconventional superconductivity(SC) with nodal gap dominated by the gapless low-lying quasiparticle excitations. The results are consistently argued with an unconventional multiband SC state with the gap-size ratio of different bands being significantly large; the large full gaps in spm-wave state keep Tc high, whereas a small gap with a nodal-structure causes gapless feature under magnetic field. The present results will develop an insight into the strong material dependence of SC-gap structure in Fe-based superconductors.