Superconductivity in HgBa 2 Ca m-1 Cu m O 2m+2+δ (m=1,2, and 3) under quasihydrostatic pressures L. Gao et al., Phys. Rev. B 50, 4260 (1994) C. Ambrosch-Draxl et al., Phys. Rev. Lett. 92, (2004) Shimizu-group Hanzawa Akinori

Contents Introduction Cuprates series HgBa 2 Ca m-1 Cu m O 2m+2+δ CuO 2 layer hole concentration Motivation Experiments Results & Discussions Summary

Cuprates series La-Ba-Cu-O T c = 32 K T c = 40 K at 1.4 GPa Hg-Ba-Ca-Cu-O T c =134 K Most of high-temperature superconductors belong to cuprates series. Y-Ba-Cu-O T c = 90 K T c = 91 K at 1.8 GPa Tl-Ba-Ca-Cu-O T c = 116 K T c = 131 K at 7.5 GPa

HgBa 2 Ca m-1 Cu m O 2m+2+δ CuO 2 layer m=1m=2m=3 For High Temperature cuprate Superconductor (HTS), CuO 2 Layer is very important factor.

CuO 2 layer Mott Insulator ： Spins are alternately up and down. When coulomb repulsion for two electrons in a Cu is big, Electrons cannot move freely. antiferromagnetism Cu Carriers are doped valence : +2 → +3 metalization The origin of occurring superconductivity. (Insulator)

hole concentration T c vs n, where n is the number of carriers per CuO 2 layer: △ Hg-1201, ○ Hg-1212, □ Hg-1223; solid symbols: samples used in this study. △ ○□ under dopeover dope super- conductor metal Carrier concentration Temperature Anti Ferro Solid Symbols(●, ▲, and ■) are optimally doped. T c is the highest on optimally doped at ambient pressure. optimal dope

Motivation High pressure is employed in the study of HTS. ・ The cause for pressure-induced T c increase in these optimally doped compounds ? ・ Ceiling of T c under high pressure ?

Experiment Electrical resistance measurement 300μm 0.3μm diamond pressure medium : MgO insulator : Al 2 O 3 + epoxy gasket : stainless sample ( ～ 300×300×25 ) Diamond Anvil Cell (DAC) ruby pseude four-lead resistance measurement tungsten leads

Results T c reached ・・・ Hg-1223 : 164 K at 31 GPa Hg-1212 : 154 K at 29 GPa Hg-1201 : 118 K at 24 GPa T c ＝ 164 K for Hg-1223 is the highest T c in the world !

Discussion 1 Si (semiconductor) “modulation doping” ① impurity is doped ② electrons move into another semiconductor gap is wide gap is narrow ③ electrical current flow Based on the results of ・・・ ① amount of oxygen change ② carriers are doped ③ enhance more T c effectively

Discussion 2 n vs P (theory) T c vs P (experiment) Pressure perpendicular to the CuO 2 layer moves a CuO 2 layer close to another CuO 2 layer. This effect induces a charge transfer. outer plane inner plane

Discussion 3 Each phase purity by X-ray data. ～ 95% ～ 90% ～ 80% These samples were the highest purity at that time. Now the purity is more improved ･･･ T c can raise much more !

Summary ・ Universal large T c enhancement was observed in HgBa 2 Ca m-1 Cu m O 2m+2+δ. ・ A record high temperature of 164 K was reached in Hg-1223 at 31 GPa. ・ If we use high purity sample, T c will make a record high.