Pressure effects on Tc of LaTMPO (TM = Fe, Ni) Thank you, chairman. O.K, I would like to talk about pressure effects on Tc of these compounds. International Conference on Low Temperature Physics August 8, 2008

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

LaFePO Pressure dependence of electrical resistivity for LaFePO NIHON UNIV. At first, the increase in Tc with pressure up to 0.8 GPa is very steep, and the initial increasing rate of Tc at ambient pressure is about 70 %/GPa. And Tc decreases slowly with further increasing pressure up to 12 GPa. Inset right figure, these results are consistent with the results of magnetic susceptivility measurements under high pressure. LaFePO Tc = 5.80 K at 0 GPa [K. Igawa et al., submitted] Tc = 8.87 K at 0.8GPa d(lnTc)/dP ~ 0.706 GPa-1 (at 0 GPa)

LaNiPO Pressure dependence of electrical resistivity for LaNiPO Next, for LaNiPO, the Tc increases rapidly with pressure up to 0.9 GPa. And the initial increasing rate of Tc at ambient pressure is about 35 %/GPa. The maximum of Tc for LaNiPO is 3.8 K at 1.7 GPa. [H. Okada et al., submitted] LaNiPO Tc = 2.68 K at 0 GPa Tc = 3.78 K at 1.7GPa d(lnTc)/dP ~ 0.369 GPa-1 (at 0 GPa)

･ The lattice along c-axis is more compressible than along a-axis. High pressure x-ray diffraction measurements for LaTMPO NIHON UNIV. KEK BL18C a b c La O TM P TMP layer LaO ･ There is no difference for the compressibilities of unit cell volume V between LaFePO and LaNiPO. ･ The lattice along c-axis is more compressible than along a-axis. This is the result of high pressure x-ray diffraction measurements. The normalized volume V/V0 decreases with increasing pressure. There is no difference for the compressibilities of unit cell volume between LaFePO and LaNiPO. However, note that the lattice along c-axis is more compressible than along a-axis. Then, LaFePO has a larger anisotropic compression compared with LaNiPO.

Summary ･ The increase in superconducting transition temperature Tc with pressure up to ~ 1.0 GPa is very steep, and the d(lnTc)/dP values at 0 GPa are about 70 %/GPa for LaFePO and 35 %/GPa for LaNiPO. ・ The lattice along c-axis is more compressible than along a-axis, and LaFePO has a larger anisotropic compression compared with LaNiPO. This is Summary, there is a possibility that an anisotropic compression is greatly related to the pressure effect on Tc. Thank you, for your attention.