1. 1 High Pressure Study on MgB 2 B.Lorenz, et al. Phys. Rev.B 64,012507(2001) Shimizu-group Naohiro Oki

2. 2 Contents Introduction ・ BCS theory ・ About MgB 2 ・ Motivation Experiment Results & Discussions My experiment Summary

3. 3 cooper pair Materials containing light or hard elements are predicted to have High Tc. (But ～ 30K) Introduction BCS theory 超伝導転移温度 http://www.phys.aoyama.ac.jp/~w3-jun/ achievements/study_sc_chara.html Phonon Electron (cooper pair) Superconductivity 格子 電子 In the BCS theory Interaction ( 相互作用）

4. 4 Boron On this time, I will talk about MgB 2. Superconductor of elements http://www.phys.aoyama.ac.jp/~w3-jun/ achievements/study_sc_type_element.html 5B5B β-Boron very light very hard High Tc? Materials containing boron Tc ～ 6K (at 175GPa) Mikhail I. Eremets, et al. Science 293, 272 (2001) not high (1Gpa ～ 1 万気圧 )

5. 5 MgB 2 structure http://www.phys.aoyama.ac.jp/~w3-jun/ achievements/study_sc_history_mgb2.html B : Honeycomb network layer Mg :Triangular lattice layer 蜂の巣構造 三角格子 Previous work Zero resistivity Tc ～ 39K Higher than a limit of Tc of 30K in the BCS theory. MgB 2 attracted great attention J. Nagamatsu et al., Nature (London) 410, 63 (2001) Graphite Temperature dependence of electrical resistivity. http://www.f-denshi.com/000okite/ 500metal/nanotube01.html Superconductive transition

6. 6 Motivation β-Boron Superconductor (at 175GPa) dTc/dP>0 Mikhail I. Eremets, et al. Science 293, 272 (2001) Hg-1223 In the study of superconductivity, pressure application is a very important method. Tc The highest Tc ever recorded 164K （ at 31GPa ） L.Gao, et al Phys. Rev. B 50 (1994) 4260 For example… MgB 2 Pressure dependence of Tc (not BCS superconductor) pressure increase decrease or These are rare example.

7. 7 Sample : MgB2 Cell : Teflon High pressure cramp : BeCu Pressure medium : Fluorinert Experiment Measurement of Electrical resistance Four lead technique 四端子法 圧力媒体 Hydrostatic pressure 静水圧 Measurement of magnetic susceptibility ac : inductance method with LR-700 Bridge dc : Quantum Design SQUID magnetometer 磁化率 ～ 1.84GPa 圧力計 Manometer : Pb Thermometer : chromel-alumel thermocouple (300 ～ 45K) germanium (45K ～ ) 温度計 超伝導量子干渉装置 0GPa Image of hydrostatic pressure

8. 8 Result & Discussion.1 300K ～ 40K : The ρ decreased like a metal on cooling 39K : The ρ started to drop rapidly Temperature dependence of electrical resistivity. Zero resistivity 0GPa Superconductivity transition 電気抵抗率

9. 9 Result & discussion.2 Pressure effect on Tc is determined ac magnetic susceptibility.( ～ 1.84GPa) Superconducting transition is shifted toward low temperature. Temperature dependence of ac magnetic susceptibility at various pressure. 交流磁化率 38.5K37.4K Tc: midpoint of transition. pressure

10. 10 Result & Discussion.3 Pressure dependence of Tc dTc/dP = -1.6K/Gpa 1 ～ 8 : Run number This result is good agreement with the calculated value in BCS framework. (dTc/dP = -1.4K/GPa) Tc of MgB 2 decreased with pressure applying.

11. 11 Diamond Boron-Doped Diamond (BDD) C CC C CC C B + - C C C C C C C C Boron Doping Insulator Superconductor SemiconductorMetal Tc ～ 4K E.A.Ekimov, et al. Nature (London) 428,542(2004) My experiment Tc pressure increase decrease or BDD

12. 12 Summary Pressure plays very important role in the study of superconductivity. Unfortunately, Tc of MgB 2 decreased with pressure applying. My experimental purpose is to observe the pressure effect on Tc in Boron-Doped Diamond.