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

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We report resistivity measurements of the helimagnet CrAs under pressures. The helimagnetic transition with TN ∼ 265 K at ambient pressure is completely suppressed above a critical pressure of Pc ∼ 0.7 GPa, and superconductivity is observed at ∼ 2.2 K for zero resistance, which exists in a wide pressure range extending beyond 3 GPa. Both the upper critical field Hc2 and the coefficient A in the resistivity increase toward Pc, suggesting that the superconductivity of CrAs is mediated by electronic correlations enhanced in the vicinity of the helimagnetic phase.KEYWORDS: CrAs, superconductivity, helimagnet, pressure 3d electron systems can offer stages that induce intriguing superconductivity such as that realized in cuprates, Fe pnictides, and cobalt oxyhydrate. Superconducting (SC) mechanisms and symmetries are diverse depending on the material, and thus the discovery of superconductivity in a new system is crucial for the development of the field of research on superconductivity. In this paper, we report the discovery of a pressure-induced superconductivity of ∼ 2.2 K in the helimagnet CrAs through resistivity measurements. This is the first example of superconductivity in Cr-based magnetic systems. Very recently, W. Wu and coworkers have independently obtained similar results on the occurrence of superconductivity under high pressure in CrAs through resistivity and susceptibility measurements. 1CrAs has an orthorhombic MnP-type crystal structure with the space group of P nma and shows a firstorder magnetic transition at T N ∼ 265 K. Most research studies of the magnetic property of CrAs have been performed in the 1970s. The magnetic structure of CrAs is a double-helical one represented by a propagation vector of 0.354 · 2πc * and a magnetic moment of ∼ 1.7µ B /Cr that lies in the ab plane.2, 3 The magnetic transition is accompanied by a large magnetostriction of ∆b/b = +5.5%, ∆a/a = −0.3%, and ∆c/c = −0.9% below T N .4, 5 The crystal structure in the helimagnetic phase has been reported to belong to the same space group as the paramagnetic (PM) phase, 4, 5 although the magnetic structure is incommensurate.Single crystals of CrAs were prepared by the Sn-flux method similar to that described in Ref. 6. The resistivity measurements under pressure were performed using samples #1 and #2, which are from different batches. Sample #1 was obtained accidentally from a mixture of Sr:Cr:As:Sn=1:2:2:20, which was prepared with the aim of producing SrCr 2 As 2 ; however, CrAs was obtained as the main product. For sample #2, a mixture of Cr:As:Sn=1:1:10 was prepared. For both samples, the mixture was placed in an alumina crucible and sealed in an evacuated quartz ampoule. The ampoule was heated slowly up to 1050• C, and held there for 2 h, and then cooled to 600

The temperature dependence of the 195 Pt Knight shift, K, for the high quality single crystal UPt 3 has been measured down to T 28 mK in applied magnetic fields parallel and perpendicular to the hexagonal c axis. No change of K's has been found across the superconducting transition temperature T c down to 28 mK regardless of the crystal directions and independent of the superconducting multiphases. It is demonstrated that UPt 3 is the odd-parity superconductor with parallel spin pairing following the direction of the magnetic field in a range of 4.4 -15.6 kOe without an appreciable pinning of the order parameter to the lattice. [S0031-9007(96)

195 Pt Knight shift (KS) measurements covering the superconducting multiple phases for major field (H) orientations have been carried out on the high-quality single crystal UPt 3 . For H . 5 kOe, the KS does not change below the superconducting transition temperature T c down to 28 mK, regardless of major crystal orientations, which provides evidence that the odd-parity superconductivity with the parallel spin pairing is realized. By contrast, the KS decreases below T c for H b k b axis and H b , 5 kOe and for H c k c axis and H c , 2.3 kOe, whereas the KS for H a k a axis is T independent across T c down to H a ϳ 1.764 kOe. These novel findings entitle UPt 3 as the first spin-triplet oddparity superconductor including a nonunitary pairing characterized by the two-component d vector like d b 1 id c at low T and low H. [S0031-9007(98)

We report the precise pressure dependence of FeSe from a resistivity measurement up to 4.15 GPa. Superconducting transition temperature (Tc) increases sensitively under pressure, but shows a plateau between 0.5 − 1.5 GPa. The maximum Tc, which is determined by zero resistance, is 21 K at approximately 3.5 GPa. The onset value reaches ∼ 37 K at 4.15 GPa. We also measure the nuclear spin-lattice relaxation rate 1/T1 under pressure using 77 Se-NMR measurement. 1/T1 shows that bulk superconductivity is realized in the zero-resistance state. The pressure dependence of 1/T1T just above Tc shows a plateau as well as the pressure dependence of Tc, which gives clear evidence of the close relationship between 1/T1T and Tc. Spin fluctuations are suggested to contribute to the mechanism of superconductivity.

We report resistivity measurement under pressure in single crystals of SrFe2As2, which is one of the parent materials of Fe-based superconductors. The structural and antiferromagnetic (AFM) transition of T0 = 198 K at ambient pressure is suppressed under pressure, and the ordered phase disappears above Pc ∼ 3.6 − 3.7 GPa. Superconductivity with a sharp transition appears accompanied by the suppression of the AFM state. Tc exhibits a maximum of 34.1 K, which is realized close to the phase boundary at Pc. This Tc is the highest among those of the stoichiometric Fe-based superconductors.

Thermal expansion coefficient in single-walled carbon nanotube bundles was determined as (Ϫ0.15 Ϯ0.20)ϫ10 Ϫ5 (1/K) for the tube diameter and (0.75Ϯ0.25)ϫ10 Ϫ5 (1/K) for the triangular lattice constant by means of x-ray scattering between 300 K to 950 K. The value for the intertube gap was (4.2Ϯ1.4) ϫ10 Ϫ5 (1/K), which is larger than 2.6ϫ10 Ϫ5 (1/K) for the c-axis thermal expansion in graphite. The results reveal the presence of a remarkably larger lattice anharmonicity in nanotube bundles than that of graphite. The small value for the tube diameter is consistent with the seamless tube structure formed by a strong covalent bond between carbon atoms comparable to that in graphite.