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High Pressure Study on MgB2

B.Lorenz, et al. Phys. Rev.B 64,012507(2001)

Shimizu-group

Naohiro Oki

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Contents Introduction

・ BCS theory

・ About MgB2

・ Motivation Experiment Results & Discussions My experiment Summary

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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）

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Boron

On this time, I will talk about MgB2.

Superconductor of elements

http://www.phys.aoyama.ac.jp/~w3-jun/achievements/study_sc_type_element.html

5Bβ-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 )

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MgB2 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.

•MgB2 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

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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.

TcThe highest Tc ever recorded

164K （ at 31GPa ）L.Gao, et al Phys. Rev. B 50 (1994) 4260

For example…

MgB2

Pressure dependence of Tc

(not BCS superconductor)

pressure increase

decreaseor

These are rare example.

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•Sample : MgB2•Cell : Teflon•High pressure cramp : BeCu•Pressure medium : Fluorinert

Experiment

•Measurement of Electrical resistanceFour 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

0GPaImage of

hydrostatic pressure

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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

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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

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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 MgB2 decreased with pressure applying.

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Diamond Boron-Doped Diamond (BDD)

C

C C

C

C C

C

B

+

- C

C

C

C

C

C

C

C

Boron Doping

Insulator

Superconductor

Semiconductor Metal

Tc ～ 4KE.A.Ekimov, et al. Nature (London) 428,542(2004)

My experiment

Tcpressure increase

decreaseor

BDD

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Summary

Pressure plays very important role in the study of superconductivity.

Unfortunately, Tc of MgB2 decreased with pressure applying.

My experimental purpose is to observe the pressure effect on Tc in Boron-Doped Diamond.