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Superconductivity in CuxBi2Se3

Wei-Feng Tsai

Joint Group Meeting

Feb 10, 2010

Ref: (1) Y.S. Hor et al, PRL 104, 057001 (2010) (2) N. Bray-Ali and S. Haas, Physics 3, 11 (2010) (3) L. Wray et al., arXiv:0912.3341

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Outline

• Motivation• Crystal and band structures• Effect of doping in the normal state• Superconductivity • Discussion on the pairing mechanism

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Nutshell of the TRI Topological Insulators

• Bulk gap (mainly from SOI, band inversion)

• Gapless surface/edge states (topology protected)

Physics: Nontrivial GS without spontaneous symmetry breaking

Application: Spintronic devices and creating excitations with non-Abelian statistics necessary in making fault-tolerant quantum computers (junction requested!)

CB

VB

SS

How to turn a TI into a SC?

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Crystal structure of CuxBi2Se3

Compare with Bi2-xCuxSe3: non-superconducting!

a=b~4Åc~29Å

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Band structure at T>Tc for Cu0.12Bi2Se3

Rhombohedral: R¹3m spacegroupRhombohedral: R¹3m spacegroup

~0.4eV

Massive Dirac-like spectrum for CB; SS are still “well-defined”

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Effect of Doping (T>Tc)

• Though c increases, the band gap remains the same• Surface-state dispersion is renormalized (vF is reduced

by 30%)• Fermi surface gets anisotropy• Nonlinear doping

CuxBi2Se3 (electron doped)

Bi2-xCuxSe3 (weakly hole doped)→CB is above EF!

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Superconductivity (resistivity)

CuxBi2Se3 superconducts when 0.1<x<0.15; Bi2-xCuxSe3 doesn’t!

Fraction ~ 20% at Tc

X=0.12ne~2x1020cm-3

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Superconductivity (magnetization)

• Anisotropic SC: ξab = 140Å, ξc = 52Å as T→0•GL κ ~ 50: strong type II

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STM result (topography) near Tc

Intercalated Cu on the surface for 1, subsurface for 2; open question for 3?

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Mechanism for Cooper pairing Electron-phonon?

Bend in bulk dispersion near 90meV binding energy

Two possible SC states

Parity even (non-Abelian)

Parity odd (topological SC)

PΔ(k)P = ± Δ(-k)

Fu and Berg, arXiv: 0912.3294