We report the ac magnetic susceptibility ac and resistivity measurements of EuFe 2 As 2 under high pressure P. By observing nearly 100% superconducting shielding and zero resistivity at P ¼ 28 kbar, we establish that P-induced superconductivity occurs at T c $ 30 K in EuFe 2 As 2 . shows an anomalous nearly linear temperature dependence from room temperature down to T c at the same P. ac indicates that an antiferromagnetic order of Eu 2þ moments with T N $ 20 K persists in the superconducting phase. The temperature dependence of the upper critical field is also determined.KEYWORDS: iron pnictides, pressure-induced superconductivity, susceptibility, upper critical field DOI: 10.1143/JPSJ.78.083701The discovery of superconductivity (SC) at a transition temperature T c ¼ 26 K in LaFeAsO 1Àx F x by Kamihara et al.1) has triggered extensive studies of SC in layered iron pnictides and related compounds. Rotter et al. found that BaFe 2 As 2 with a simpler structure can be made superconducting by doping:2) Perhaps more importantly, it is reported that 122 compounds of the form AFe 2 As 2 (A ¼ Ca, Sr, Ba, and Eu) can be tuned to SC by the application of high pressure P. 3-10)P tuning can provide opportunities to determine the nature of the iron-pnictide high-temperature SC without being adversely affected by disorder due to doping. However, most of these reports are based only on resistivity measurements and hence cannot establish the bulk nature of P-induced SC.11) Even when magnetic measurements are reported, results are not conclusive: In ref. 5, magnetic measurements were performed on SrFe 2 As 2 and BaFe 2 As 2 , but the observed volume fraction was expressed in arbitrary units. In ref. 9, the volume fraction of the P-induced superconducting phase of CaFe 2 As 2 was estimated to be at least 50%, while in ref. 12 CaFe 2 As 2 was reported not to exhibit SC under hydrostatic P produced by the use of helium as a pressure-transmitting medium.EuFe 2 As 2 exhibits two phase transitions, at T o $ 190 K and T N $ 19 K, at ambient P.13-16) The transition at T o is a combined structural and magnetic transition, similar to those in the other 122 compounds: the crystal structure changes from tetragonal to orthorhombic and the Fe 2þ moments order antiferromagnetically. The transition at T N is due to the antiferromagnetic (AFM) ordering of the Eu 2þ moments. The AFM coupling of the Eu 2þ moments is rather weak: the field-induced paramagnetic state with a saturated moment of $7 B /Eu is easily reached by the application of $1 or 2 T in the ab-plane or along the c-axis, respectively. 17) A temperature (T)-P phase diagram has been determined from measurements: 10) while T o decreases with P and is not detected above P ¼ 23 kbar, T N is nearly P-independent up to 26 kbar (the highest P in ref. 10). The authors of ref. 10 state that P-induced SC at T c $ 30 K occurs above 20 kbar. However, their data (at P ¼ 21:6 kbar) shows only a partial drop and approximately half of the normal-state appears to remain as T ! 0. Obviously, further e...

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.

A pressure transmitting medium named Daphne 7474, which solidifies at P(s)=3.7 GPa at room temperature, is presented. The value of P(s) increases almost linearly with temperature up to 6.7 GPa at 100 degrees C. The high pressure realized by a medium at the liquid state allows a higher limit of pressurization, which assures an ideal hydrostatic pressure. We show a volume change against pressure, pressure reduction from room to liquid helium temperature in a clamped piston cylinder cell, pressure distribution and its standard deviation in a diamond anvil cell, and infrared properties, which might be useful for experimental applications.

Calibration of a Pt resistance thermometer (Netsushin) in magnetic fields and under pressure in the range of 1.5–300 K and below 1.5 GPa is presented. With the pressure medium, olefin olygomers, Daphne 7373, the pressure is continuous at its solidification and the pressure drop from 300 to 4.2 K by 0.15–0.17 GPa is constant, irrespective of the initial clamped pressure at 300 K. The applicability of the thermometer and the medium for precise study in field and pressure at low temperature is discussed.

The solidification pressure of Daphne 7373, which is widely used as a pressure medium in high pressure studies, was examined at room temperature. Using a new generation clamp-type pressure cell, we found that Daphne 7373 solidifies at 2.2 GPa at room temperature. This is exactly on the natural extrapolation of the melting curve obtained at lower pressures and temperatures in our previous report. The solidification pressure of Daphne 7373 is twice as high as that of another well-known medium Fluorinert 77/70 (0.9 GPa). This allows us to hold hydrostatic pressure even in the newly developed BeCu–NiCrAl clamp-type pressure cell, which exceeds the limit of 1.5 GPa generated by a conventional BeCu cell.