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Shimizu-group Naohiro OKI

Observation of a Superconducting Gap in Boron-Doped Diamond by Laser-Excited Photoemission Spectroscopy. Shimizu-group Naohiro OKI. K. Ishizaka et al . PRL 98 , 047003 (2007). Contents. Introduction My experiment Boron-Doped Diamond Density of states Motivation Experimental method

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Shimizu-group Naohiro OKI

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  1. Observation of a Superconducting Gap in Boron-Doped Diamond by Laser-Excited Photoemission Spectroscopy Shimizu-group Naohiro OKI K. Ishizaka et al. PRL 98, 047003 (2007)

  2. Contents • Introduction • My experiment • Boron-Doped Diamond • Density of states • Motivation • Experimental method • Results and Discussion • Summary

  3. Introduction Superconductivity in Boron-doped Diamond (BDD) BDD Electrical resistivity fell to zero. Tcoffset = 2.3 K(0 GPa) Synthesis method: High pressure high temperature (HPHT) Superconducting transition temperature E.A.Ekimov, et al. Nature (London) 428,542(2004)‏ BDD attracts attention as a superconductor of the diamonds.

  4. 5B 6C About BDD • Diamond • Pure diamond is an electrical insulator. (Eg = 5.5 eV) • Highest Debye temperature. (θD= 2200 K) High Tc ? C http://newton.ex.ac.uk/research/qsystems/people/sque/diamond/structure/ • Images of BDD - - - + more doping + + wide-gap p-type semiconductor Superconductor metal • B concentration • nB<1020 cm-3 B concentration nB>3×1020 cm-3

  5. Previous works • Synthesis method • High pressure high temperature (HPHT) • nB :～4.9 × 1021 /cm3 (~2.8 %) • Tcoffset ～ 2.3 K • Synthesis method • Chemical vapor deposition (CVD) • nB :～8.4×1021/cm3 (~4.7 %) • Tcoffset ～ 7.4 K Tc shows the tendency to rise with increasing boron concentration at (111) film. ? E.A.Ekimov, et al. Nature (London) 428,542(2004)‏ Takano et al. Appl. Phys. Lett., Vol. 85, No. 14, 4 October 2004 H.Umezawa,et al. cond-mat/ 0503303 (2005)‏ Many investigations about superconductivity of BDD are still performed.

  6. EF DOS DOS Density of States (DOS) in Superconductivity In the BCS theory Cooper pair stability Necessary energy to disturb Cooper pairsis equivalent to 2⊿ Density of States at 0K at 0K SC gap (⊿) Bose condensation • SC gap Strength of the electron-phonon interaction • DOS at Fermi energy N(0) One of the factors to decide Tc • Symmetricalness of DOS To characterize the superconductivity Investigating DOS is very important to study superconductivity.

  7. DOS EF SC gap(⊿) Motivation • Studying the superconductivity mechanism in BDD by investigating DOS using photoemission spectroscopy (PES).

  8. Experimental method • Sample • Boron-doped (111) Diamond film • synthesis method : CVD • nB : 8.4×1021cm-3 (B/C～5%) Temperature dependence of electrical resistivity and magnetization • Electrical resistivity • Tcoff : ～7 K (0T) • Electrical conductivity (inset) • T0 : ～11 K • deviating temperature • Meissner response • Tcm : 6.6 K Superconducting transition temperature

  9. photoelectron EK hν Vacuum energy (EV) W Fermi energy (EF) EB electron hν DOS EB : Binding energy EK : Kinetic energy W : Work function Photoemission Spectroscopy (PES) Incident radiation EK = hν- EB- W DOS is observed by measuring intensity according to the EK.

  10. high low Photoemission Spectroscopy (PES) Density of States (DOS) Metal Superconductor QP peak SC gap(⊿) T Intensity Intensity N(0) : Amount of DOS at EF 0(EF) 0(EF) Binding Energy Binding Energy N(0) decreases with T decreasing. SC gap and quasi-particle (QP) peak develop with T decreasing. DOS shows the Fermi-Dirac distribution.

  11. Photoemission Spectroscopy (PES) Ultra-violet Photoelectron Spectroscopy (UPS) (hν: 6eV ～ 40 eV) To study density of state near Fermi energy X-ray Photoelectron Spectroscopy (XPS) (hν : 40 eV ～ 100 keV) To study the energy levels of atomic core electrons

  12. Photoemission Spectroscopy (PES) Measurement Scienta R4000 Ultraviolet laser : hν= 6.994 eV The energy resolution was 0.7 meV http://www.vgscienta.jp/products/r4000.html

  13. Result T dependence of DOS in BDD • N(0) decreased slightly,. • at the lowest T (4.5 K) • Considerable amount of N(0) remains. • Clear QP peak cannot be observed. • ⊿(T=4.5) is 0.78 meV • (By using the function for the curve fitting) • At 0 K (assuming the BCS-like T dependence) • ⊿(T=0)/kBTc is 1.78 • ⊿(T=0) is 1.01 meV • This value is close to the typical value for weakly coupled BCS superconductors.

  14. Result • N(0)starts to decrease and SC gap gradually forms below 9.5K. • We cannot discern a well-defined QP peak and the gap structure is fairly diffusive.

  15. Discussion • ≒T0(11K) • ＞Tcm(6.6K) The onset T of gap evolution : ～11K This is caused by pseudo-gap effect as observed in under-doped high-Tccuprates. 擬ギャップ BDD may consist of the local “high-Tc” phase.

  16. Discussion • Considerable amount of N(0) remains. • Clear QP peak cannot be observed. • BDD may consist of the local “high-Tc” phase. They indicate that the superconductivity in this system is strongly affected by randomness and inhomogeneity introduced by boron doping. 異質性 http://www.kawarada-lab.com/research/boron_doped_diamond/tyodendou.html

  17. Summary • SC gap of BDD is close to the value for weakly coupled BCS superconductor. • Superconductivity in this system is strongly affected by randomness and inhomogeneity introduced by boron doping.

  18. My experiment I am investigating the pressure effect of superconducting transition temperature in BDD. dTc / dP ≒ - 0.02 K/GPa

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