1. Search for high temperature superconductivity of Sr 2 VO 4 under high pressure Shimizu Lab Kaide Naohiro

2. Contents Introduction : Mott insulator La 2 CuO 4 Sr 2 VO 4 Research Summary 2

3. Introduction

4. Metal, semiconductor, insulator ρ (Ω cm) “metal” “semiconductor” “insulator” Aluminium Gold Silicon Germanium Quartz

5. Mott insulator Mott insulator are a class of materials that should conduct electricity under conventional band theories, but are insulators when measured. This effect is due to electron–electron interactions, which are not considered in conventional band theory. ・ What is the difference between metal and insulator ? → Band theory, R v.s. T… ・ What is electron–electron interactions ? → On-site coulomb interaction U The definition :

6. Band theories “insulator” “metal” : electron ε ε D(ε)D(ε) D(ε)D(ε) D(ε) : density of states εFεF εFεF

7. “metal” T (K) “insulator” T (K) The temperature dependence of resistivity positive negative

8. Correlated electron effects t : kinetic energy of electron U : On-site coulomb interaction U http://www.kh.phys.waseda.ac.jp/thesis/m_s_adachihara.pdf In case of U >> t, Electron is localized.

9. Mott insulator Mott insulator are a class of materials that should conduct electricity under conventional band theories, but are insulators when measured. This effect is due to electron–electron interactions, which are not considered in conventional band theory. The definition : T (K) ε D(ε)D(ε)

10. （ ex ） La 2 CuO 4 ・ Layered perovskite ・ d 9 Mott insulator La Cu O J. G. Bednorz et al., Science. 236 (4797), 73 -75 (1987). Sr Ba Ca

11. Cu O Crystal field Cu 3d 9 egeg t 2g 3d3d d zx d yz d xy d 3z^2 - r^2 d x^2 – y^2 Crystal field (ex)La 2 CuO 4

12. egeg Jahn-Teller effect d x^2 – y^2 d 3z^2 - r^2 Cu O Jahn-Teller effect stable

13. The strong on-site Coulomb repulsion U Crystal structure of La 2 CuO 4 C. Chaillout et al., Physica C 158, 183 (1989) Sr O V Crystal field Cu 3d 9 egeg t 2g 3d3d （ ex ） La 2 CuO 4 U Jahn-Teller effect ・ Half-filled state ・ The strong on-site Coulomb repulsion U →Mott insulator （ usually antiferromagnet ） E EFEF U E EFEF large On-site coulomb interaction U

14. Sr 2 VO 4

15. Sr O V 空間群 I4/mmm(No.139) ・ Layered perovskite (as La 2 CuO 4 ) ・ d 1 Mott insulator (t 2g state)

16. Electric resistance (at ambient pressure) V. Giannakopoulou et al, Solid State Communications, Vol. Y3, No. 7, pp. 57Y-5X3, lYY5 (1995) T (K) insulator

17. Motivation Uniaxial pressure V 3d 1 egeg t 2g 3d3d d xy d yz,, d zx U Pressure increasing High T c superconductivity?? Crystal field Cu 3d 9 egeg t 2g 3d3d （ ex ） La 2 CuO 4 U

18. Research

19. How to make high pressure Pt Ruby :Sr 2 VO 4 (single crystal) ・ sample: Sr 2 VO 4 ・ culet : 200μm ・ gasket : Re ・ using a film ・ insulator : c-BN 4-terminal method film gasket diamond Cu On the film insulator

20. Result ・ The slope of resistivity is suppressed. ・ It is expected that resistivity measurements reveals M-I transition at about 50 GPa. ・ Superconductivity was not observed. Fig. Temperature dependence of electrical resistance (normalized at 250 K) at pressure of 15- 41 GPa.

21. Summary

22. Applying pressure on Sr 2 VO 4, the slope of resistivity is suppressed. It is expected that resistivity measurements reveals M-I transition at about 50 GPa in Sr 2 VO 4. Superconductivity was not observed.

23. ρ v.s. P