We have established a plot of the anion height dependence of superconducting transition temperature T c for the typical Fe-based superconductors. The plot showed a symmetric curve with a peak around 1.38 Å. Both data at ambient pressure and under high pressure obeyed the unique curve. This plot will be one of the key strategies for both understanding the mechanism of Fe-based superconductivity and search for the new Fe-based superconductors with higher T c .2

We report unusual magnetic and superconducting ͑SC͒ characteristics in multilayered CuO 2 planes in Hgand Cu-based high-T c cuprates through the 63 Cu-NMR measurements. These compounds, in which the number of CuO 2 planes ͑n͒ ranges from 3 to 5 in a unit cell, include crystallographically inequivalent outer ͑OP͒ and inner ͑IP͒ CuO 2 plane that are surrounded by pyramidal and square oxygen, respectively. The Knight shift ( 63 K) at the OP and IP exhibits respective characteristic temperature dependence, consistent with its own doping level. Using an experimental relation between the spin part in 63 K at room temperature and the doping level in a CuO 2 layer N h , we show that N h (OP) at the OP is larger than N h (IP) at the IP for all the systems and its difference ⌬N h ϭN h (OP)ϪN h (IP) increases as either a total carrier content ␦ or n increases. At ⌬N h 's exceeding a critical value, the pseudogap behavior in the normal state is seen alone at the IP, and a bulk SC transition does not set in simultaneously at the IP and OP. A SC nature at the OP becomes consistent with a mean-field behavior only below T c2 that is significantly lower than T c . Reduction in T c with increasing n is associated with an increase in ⌬N h . It is a rather remarkable aspect that a T c is not always reduced even though these multilayered high-T c compounds are heavily overdoped. This arises, we show, because the IP remains underdoped and keeps a high value of T c , while the OP is predominantly overdoped. This may be a microscopic origin for the lowest anisotropic SC characteristics reported to date in Cu-based multilayered high-T c compounds.

We have investigated a gap structure in a newly discovered superconductor, MgB2, through measurement of the (11)B 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 2Delta/k(B)T(c) approximately 5 which suggests it is in a strong-coupling regime.

From Ru- and Cu-NMR studies, we present evidence for coexistence of superconductivity and ferromagnetism in a cuprate superconductor RuSr2YCu2O8 (RuY1212). The observation of a large enhancement of a radio-frequency field for the Ru-NMR signal at zero field reveals the existence of a ferromagnetic (FM) component in the ordered RuO2 plane below a Curie temperature of TM = 150 K. Just below the onset temperature of superconductivity T(onset)c = 45 K, a remarkable decrease of the nuclear spin-lattice relaxation rate 1/T1 was observed within the ordered RuO2 plane as well as the CuO2 plane, revealing that the superconducting gap coexists with the FM component in the RuO2 plane on a microscopic scale. In addition, from the observation of a sharp peak in 101(1/T1) at T(zero)c approximately 23 K where the resistivity becomes zero, we suggest that the motion of self-induced vortices originating from fluctuations of the FM component induces the resistivity between T(onset)c and T(zero)c in RuY1212.

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In conventional metals, modification of electron trajectories under magnetic field gives rise to a magnetoresistance that varies quadratically at low field, followed by a saturation at high field for closed orbits on the Fermi surface. Deviations from the conventional behaviour, for example, the observation of a linear magnetoresistance, or a non-saturating magnetoresistance, have been attributed to exotic electron scattering mechanisms. Recently, linear magnetoresistance has been observed in many Dirac materials, in which the electron–electron correlation is relatively weak. The strongly correlated helimagnet CrAs undergoes a quantum phase transition to a nonmagnetic superconductor under pressure. Here we observe, near the magnetic instability, a large and non-saturating quasilinear magnetoresistance from the upper critical field to 14 T at low temperatures. We show that the quasilinear magnetoresistance may arise from an intricate interplay between a nontrivial band crossing protected by nonsymmorphic crystal symmetry and strong magnetic fluctuations.

We report the electronic and superconducting properties in the Pr-based filled-skutterudite superconductor PrRu4Sb12 with Tc = 1.3 K via the measurements of nuclear-quadrupole-resonance (NQR) frequency νQ and nuclear-spin-lattice-relaxation time T1 of Sb nuclei. The temperature dependence of νQ has revealed the energy scheme of Pr 3+ crystal electric field (CEF) that is consistent with an energy separation ∆CEF ∼ 70K between the ground state and the first-excited state. In the normal state, the Korringa relation of (1/T1T )P r =const. is valid, with [(1/T1T )P r /(1/T1T )La] 1/2 ∼ 1.44 where (1/T1T )La is for LaRu4Sb12. These results are understood in terms of a conventional Fermi liquid picture in which the Pr-4f 2 state derives neither magnetic nor quadrupolar degrees of freedom at low temperatures. In the superconducting state, 1/T1 shows a distinct coherence peak just below Tc, followed by an exponential decrease with a value of 2∆/kB Tc = 3.1. These results demonstrate that PrRu4Sb12 is a typical weak-coupling s-wave superconductor, in strong contrast with the heavy-fermion superconductor PrOs4Sb12 that is in an unconventional strong coupling regime. The present study on PrRu4Sb12 highlights that the Pr-4f 2 derived non-magnetic doublet plays a key role in the unconventional electronic and superconducting properties in PrOs4Sb12. PACS numbers: 71.27.+a, 76.60.-k Filled-skutterudite compounds ReT 4 Pn 12 (Re = rare earth; T = Fe, Ru and Os; Pn = pnictogen) show rich properties.PrRu 4 P 12 and PrFe 4 P 12 show a metal-insulator transition and undergo into an anomalous heavy-fermion (HF) state, respectively, whereas PrRu 4 As 12 , PrRu 4 Sb 12 and PrOs 4 Sb 12 exhibit a superconducting (SC) transition. 1,2,3,4 Bauer et al. reported that PrOs 4 Sb 12 shows HF behavior and superconducts at T c = 1.85 K. It is the first Pr-based HF superconductor. 4 Its HF state was inferred from the jump in the specific heat at T c , the slope of the upper critical field H c2 near T c , and the electronic specific-heat coefficient γ ∼ 350 − 500 mJ/mole K 2 . Magnetic susceptibility, thermodynamic measurements, and inelastic neutron scattering experiments revealed the ground state of the Pr 3+ ions in the cubic crystal electric field (CEF) to be the Γ 3 nonmagnetic doublet 4,5 . In the Pr-based compounds with the Γ 3 ground state, electric quadrupolar interactions play an important role. In analogy with a quadrupolar Kondo model, 6 it was suggested that the HF-like behavior exhibited by PrOs 4 Sb 12 may have something to do with a Pr-4f 2 -derived quadrupolar Kondolattice. An interesting issue to be addressed is what role of Pr-4f 2 -derived quadrupolar fluctuations plays in relevance with the onset of the superconductivity in this compound.Meanwhile, Kotegawa et al. have reported the Sb-NQR results which evidence the HF behavior and the unconventional SC property in PrOs 4 Sb 12 . 7 The temperature (T ) dependencies of nuclear-spin-lattice-relaxation rate, 1/T 1 and nuclear-quadrupole-resonance (NQR) fre-quency unraveled a low-ly...

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