Kazumi Igawa

Superconductivity at 43 K in an iron-based layered compound LaO1-xFxFeAs

The iron- and nickel-based layered compounds LaOFeP (refs 1, 2) and LaONiP (ref. 3) have recently been reported to exhibit low-temperature superconducting phases with transition temperatures Tc of 3 and 5 K, respectively. Furthermore, a large increase in the midpoint Tc of up to ∼26 K has been realized in the isocrystalline compound LaOFeAs on doping of fluoride ions at the O2- sites (LaO1-xFxFeAs). Experimental observations and theoretical studies suggest that these transitions are related to a magnetic instability, as is the case for most superconductors based on transition metals. In the copper-based high-temperature superconductors, as well as in LaOFeAs, an increase in Tc is often observed as a result of carrier doping in the two-dimensional electronic structure through ion substitution in the surrounding insulating layers, suggesting that the application of external pressure should further increase Tc by enhancing charge transfer between the insulating and conducting layers. The effects of pressure on these iron oxypnictide superconductors may be more prominent than those in the copper-based systems, because the As ion has a greater electronic polarizability, owing to the covalency of the Fe–As chemical bond, and, thus, is more compressible than the divalent O2- ion. Here we report that increasing the pressure causes a steep increase in the onset Tc of F-doped LaOFeAs, to a maximum of ∼43 K at ∼4 GPa. With the exception of the copper-based high-Tc superconductors, this is the highest Tc reported to date. The present result, together with the great freedom available in selecting the constituents of isocrystalline materials with the general formula LnOTMPn (Ln, Y or rare-earth metal; TM, transition metal; Pn, group-V, ‘pnicogen’, element), indicates that the layered iron oxypnictides are promising as a new material platform for further exploration of high-temperature superconductivity.

Superconductivity under high pressure in LaFeAsO

Electrical resistivity measurements under high pressures up to 29 GPa were performed for oxypnictide compound LaFeAsO. We found a pressure-induced superconductivity in LaFeAsO. The maximum value of Tc is 21 K at ~12 GPa. The pressure dependence of the Tc is similar to those of LaFeAsO1-xFx series reported previously.

Pressure-Induced Superconductivity in Iron Pnictide Compound SrFe2As2

Electrical resistivity under high pressure have been measured on nominally pure SrFe2As2 up to 14 GPa. The resistivity drop appeared with increasing pressure, and we clearly observed zero resistivity. The maximum of superconducting transition temperature (Tc) is 38 K. The value is corresponding to the one of optimally doping AFe2As2 (A=Sr, Ba) system with K+ ions at the A2+ site.

High-pressure Studies on Superconducting iron-based LaFeAsO1-xFx , LaFePO and SrFe2As2

Electrical resistivity and magnetic susceptibility measurements under high pressure were performed on iron-based superconductor LaFePO and LaFeAsO 1- x F x system. A steep increase in superconducting transition temperature ( T c ) was observed for LaFePO and “optimum-doped” and “over-doped” LaFeAsO 1- x F x . Pressure-induced superconductivity was confirmed in undoped LaFeAsO and SrFe 2 As 2 by electrical resistivity measurements under high pressure. X-ray diffraction measurements were also performed under high pressure up to 10 GPa for LaFePO and LaFeAsO 1- x F x system, where the anisotropic decrease of the lattice constants was observed with applying pressure.

Pressure effects on superconducting and structural properties for nickel-based superconductors LaNiXO (X = P and As)

We have performed electrical resistivity and powder X-ray diffraction measurements under high pressures for nickel-based superconducors LaNiXO (X = P and As). Superconducting transition temperatures T c of X = P and As increase to 3.78 K at 1.67 GPa and 3.14 K at 0.75 GPa, respectively. A compressibility of the lattice parameter c of LaNiPO is 1.5 times larger than that of the lattice parameter a , resulting in a linear decrease of an axial ratio c / a .

Pressure Study of Superconducting Oxypnictide LaFePO

Electrical resistivity and magnetic susceptibility measurements under high pressure were performed on an iron-based superconductor LaFePO. A steep increase in superconducting transition temperature (Tc) of LaFePO with dTc/dP>4 K/GPa to a maximum of 8.8 K for P = 0.8 GPa was observed. These results are similar to isocrystalline LaFeAsO1-xFx system reported previously. X-ray diffraction measurements were also performed under high pressure up to 10 GPa, where linear compressibility ka and kc are presented.Electrical resistivity and magnetic susceptibility measurements under high pressure were performed on an iron-based superconductor LaFePO. A steep increase in superconducting transition temperature ( T c ) of LaFePO with d T c /d P >4 K/GPa below P = 0.8 GPa was observed. These results are similar to isocrystalline LaFeAsO 1- x F x system reported previously. X-ray diffraction measurements were also performed under high pressure up to 10 GPa, where linear compressibility κ a and κ c are presented.

Pressure enhancement of superconductivity in an iron-based layered compound LaFeAsO1-xFx

Electrical resistivity measurements under high pressure were performed on iron-based superconductor LaFeAsO1-xFx. A steep increase in superconducting transition temperature (Tc) was observed for optimum-doped LaFeAsO0.89F0.11, whereas under-doped LaFeAsO0.95F0.05 shows a moderate increase in Tc with applying pressure. X-ray diffraction measurements were also performed under high pressure up to 10 GPa. A ratio c/a was observed to decrease with applying pressure, which is similar to the pressure effect on cuprate high-Tc superconductors.

Pressure effects on Tc of iron-based layered superconductor LaTMPO (TM = Fe, Ni)

We report electrical and magnetic properties of LaTMPO (TM = Fe, Ni) under high pressures. From resistivity measurements, superconducting transition temperature Tc for LaFePO and LaNiPO increases steeply up to about 0.8 GPa, and the dTc/dP values are more than 4 K/GPa and 1 K/GPa, respectively. From the results of high-pressure x-ray diffraction measurements, the oxypnictide compounds show an anisotropic compression along a-axis and c-axis.

Pressure studies in filled skutterudite La0.8Rh4P12 and LaRh4As12

The general chemical formula of ternary filled skutterudites is MT4X12 (where M, T, and X are rare-earth or alkaline-earth metals, transition metals, and pnictides, respectively). These substance are known to show various physical properties. La0.8Rh4P12 has a superconducting transition temperature (Tc) of 13.6 K and LaRh4As12 is a new filled skutterudite compound. In this study, we performed the electrical resistivity and the x-ray diffraction measurements for La0.8Rh4P12 and LaRh4As12 under high pressures. From these measurements, we discuss the condition to rise the Tc in filled skutterudite compounds.

Pressure studies on the electrical properties in R2-xSrxNi1-yCuyO4+δ (R=La, Nd) and La3Ni2O7+δ

The Ruddlesden-Popper series of compounds Lan+1NinO3n+1 are one of the strongly correlated electron systems. Among these compounds, La2NiO4 (n=1) is a well-known two-dimensional antiferromagnetic insulator. Electrical resistivity under high pressure have been measured on R2-xSrxNi1-yCuyO4+δ (R=La, Nd) up to 2 GPa and La3Ni2O7+δ up to ~20 GPa. Elecrical resistivty of La1.92Sr1.08NiO4 under high pressure reveals an enhancement of the electronic localization under pressure. On the other hand, in case of Nd1.6Sr0.4Ni0.5Cu0.5O4+δ, a metal-insulator transition is observed at ~240 K and a metallic state is favorable under pressure, in contrast to La1.92Sr1.08NiO4. In La3Ni2O7+δ pressure also enhances a metallic state up to 2 GPa.