25th International Conference on Low Temperature Physics High-pressure studies on superconducting iron-based compound LaFeAsO1-xFx Title 25th International Conference on Low Temperature Physics August 8, 2008

Authors H.Takahashi, H.Okada, K.Igawa, K.Arii, Y.Takahashi Nihon University (High pressure studies; DAC, Piston-cylinder) Y.Kamihara, S.Matsuishi, M.Hirano, H.Hosono JST, Tokyo Institute of Technology (Sample preparations) K.Matsubayashi, Y.Uwatoko ISSP, The University of Tokyo (High-pressure studies; cubic anvil press) High pressure work was done by our group. cubic anvil press are uesed at Uwatoko Group at ISSP, Univ of Tokyo. Samples we measured are prepared by Hosono’s group.

LaFeAsO1-xFx (x=0,0.05,0.11,0.14) e- Fe As La La-O layer Fe-As O F x=0 : undoped x=0.05: under-doped x=0.11: optimum-doped x=0.14: over-doped Fe As La e- La-O layer Fe-As O F :This study : Previous study Firstly I report the F doped LaFeAsO We measured the pressure effect for these 3 compound. Undope and So-called under-doped and optimally doped Schematic crystal structure of LaFeAsO1-xFx Tc(x) in LaFeAsO1-xFx Y.Kamihara et al., J.Am.Chem.Soc. 130 (2008)

Optimum doped x=0.11 ・Maximum Tc = 43 K ・Tc decreases above 4 GPa ・Initial increase of Tc is very steep ・Maximum Tc = 43 K ・Tc decreases above 4 GPa LaFeAsO1-xFx Fig. 2d summarizes the onset TC values in the P–T phase diagram of La(O0.89F0.11)FeAs. Note that TC increases steeply from 1 to 2 GPa, where dln TC/dP is more than 20%/GPa and then decreases sharply with a further increase in P above ~4 GPa: the maximum TC of 43 K is observed at ~4 GPa. It is noteworthy that the small resistance loss caused by superconductivity was observed at around 50 K in 4 GPa data Pressure dependence of Tc for x=0.11 (Run1: Piston cylinder, Run2: Diamond anvil cell ) H.Takahashi et al., Nature 453 (2008)

Pressure dependence of TC Under-doped x = 0.05 LaFeAsO1-xFx Pressure dependence of TC for x=0.11 and x=0.05 R(T) curve for x=0.05(under-doped) at several pressures up to 20 GPa The initial increase in optimally doped one is much steeper compared with those of high-TC superconductors, for which the dln TC/dP values are less than 10% / GPa. Under-doped LaFeAsO1-xFx (x=0.05) shows similar behavior to x=0.11, but pressure dependence of Tc is small. Maximum Tc is also smaller.

Pressure dependence of Tc Over-doped x=0.14 Pressure dependence of Tc for all samples R(T) curve for x=0.14 using piston cylinder device ・Initial increase of Tc is very steep for over-doped x=0.14 ・Maximum Tc = 43 K for over-deped x=0.14 , same as x=0.11 ・Tc decreases above 6 GPa for over-doped x=0.14 ・Pressure-induced superconductivity was observed for undoped LaFeAsO(x=0)

Undoped LaFeAsO Cubic anvil press ( up to 12 GPa) Pressure transmitting medium: Fluorinert Gasket: MgO Zero resistivity

Pressure dependence of electrical resistivity for SrFe2As2 Maximum Tc～35K enlarge Zero resistivity Cubic anvil press ( up to 8 GPa) 　　Medium: Fluorinert 　　Gasket: Pyrophylite

Summary Pressure dependence of TC in LaFeAsO1-xFx (x=0.0, 0.05, 0.11,0.14) are obtained. TC’s initially increase with applying pressure. Tc (P=0) dTc/dP (K/GPa-1) Tc max undoped - 3 22 under-doped 25 2 29 optimum-doped 28 8 43 over-doped 19 12 ・Pressure-induced superconductivity is observed in undoped LaFeAsO and undoped SrFe2As2