1 / 37

Wei-Feng Tsai Xiao-Ting Zhou, Chen Fang, Kangjun Seo, Yan-Yang Zhang, Dao-Xin Yao, JiangPing Hu

“Possible probes for detecting s ± -wave pairing symmetry in Iron-Pnictides: Novel Josephson junctions and impurity effects ”. Wei-Feng Tsai Xiao-Ting Zhou, Chen Fang, Kangjun Seo, Yan-Yang Zhang, Dao-Xin Yao, JiangPing Hu (Purdue University) and B. Andrei Bernevig (Princeton University).

masako
Télécharger la présentation

Wei-Feng Tsai Xiao-Ting Zhou, Chen Fang, Kangjun Seo, Yan-Yang Zhang, Dao-Xin Yao, JiangPing Hu

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. “Possible probes for detecting s±-wave pairing symmetry in Iron-Pnictides: Novel Josephson junctions and impurity effects” Wei-Feng Tsai Xiao-Ting Zhou, Chen Fang, Kangjun Seo, Yan-Yang Zhang, Dao-Xin Yao, JiangPing Hu (Purdue University) and B. Andrei Bernevig (Princeton University) Paper ref: arXiv:0812.0661, 0903.1694, 0905.0734

  2. Outline • Introduction • Direct phase-sensitive probe: • Novel π-junction • Indirect probes: • S/N/S± Josephson junction • Impurity-induced bound states • Quasiparticle interference patterns

  3. It is critical to determine pairing symmetry in superconducting Iron Pnictides • Many aspects analogous to high-Tc cuprates: • Parent compound is antiferromagnetic albeit metallic • (possibly proximate to a Mott insulator) • (2) Quasi-2D nature (superconductivity related to the FeAs layer) New features: multi-orbital nature and complex Fermi surfaces J. Zhao et al., Nature Materials 7 (2008) Many theoretical proposals for pairing symmetry: For instance, triplet s-wave, nodal s-wave, d-wave, p-wave, extended s-wave (s±)…etc. X. Dai et al., PRL 101 (2008); K. Kuroki et al., PRL 101 (2008); M. Daghofer et al., PRL 101 (2008); Q. Si and E. Abarahams, PRL 101 (2008); P.A. Lee and X.G. Wen, PRB 78 (2008); I. Mazin et al., PRL (2008)…

  4. Pairing symmetry in two band-{t}-J1-J2 model s-wave pairing coskx+cosky + + + + J1 - + d-wave pairing coskx-cosky + - s-wave pairing coskxcosky J2 d wave pairing sinkxsinky + + + + + - Function peaks at Fermi surfaces Symmetry factors + - K. Seo, B. A. Bernevig, and J.P. Hu PRL 101, 206404 (2008)

  5. Properties of s-wave coskxcosky Pairing Symmetry • Order parameters have different signs at • electron and hole pockets • If magnetic exchanges are symmetric for all orbits, gaps should be determined by single energy scale • Superconducting gaps are larger in smaller • pockets. • Fermi surfaces are generally gapped unless heavy doping crosses gapless line. Gapless lines

  6. Alas, most experiments are only sensitive to SC gap magnitudes Question: How to detect sign-changed s-wave pairing symmetry? D. Parker and I. Mazin, arXiv: 0812.4416 J. Wu and P. Phillips, PRB 79 (2009) X.-Y. Feng and T.-K. Ng, PRB 79 (2009) P. Ghaemi et al., PRL 102 (2009) S. Onari and Y. Tanaka, PRB 79 (2009) J. Linder et al., arXiv: 0901.1895 …

  7. Ic/I0 Φ/Φ0 Ic/I0 Φ/Φ0 Novel π-Junction (I): why usual corner-junctions cannot work for s±? Y.-R. Zhou et al., arXiv:0812.3295 for Co-doped 122 material. s±: non-trivial phase structure of SC order parameter in k-space! D. J. Van Harlingen, RMP 67 (1995)

  8. + - + top s-SC θt p Iron pnictide, s± θm - + ky θb p €€ € € 2 - bottom s-SC 0 Φ= θt -θb - + + p - €€ € € 2 kx - p p p 0 - p p - €€ € € €€ € € 2 2 Φ/π Novel π-Junction (II) – our proposal Key assumption: momentum conserved after tunneling between layers – high-quality interfaces may be required *Suggested s-SC with (1) large FS: MgB2 (a~0.3nm), Be thin film (a~0.23nm); (2) small FS: 2H-NbSe2 (a~0.345nm). Or possibly metallic thin film with large or small FS due to SC proximity effect.

  9. ∆L ∆R (x<0) (x>0) Within WKJB approximation, the junction can be described by a continuum BdG eq. where Andreev bound state solutions ~ e -γ|x| ∆L = ∆R = ∆ ∆L = -∆R = ∆ εbs = ± ∆ εbs = 0 T.K.Ng and N.Nagaosa, arXiv:0809.3343 For the junction with unconventional pairing symmetries, see e.g. S. Kashiwaya and Y. Tanaka, Rep. Prog. Phys. 72 (2000) S-N-S± Junction (I) – basic idea ∆s > 0 s-SC ∆1 > 0, ∆2 < 0 Iron pnictide [ ∆λ(x), s-SC order parameter; λcould be a band index ]

  10. (at x=0within ‘N’ region) (~ ∆FeAs) (in units of |t1|) S-N-S± Junction (II) – QP-LDOS for various pairing symmetries *A two-orbital exchange coupling model on the lattice is used for Iron pnictides

  11. Self-consistent BdG (on 32x32 lattice) T-matrix Approximation + Detection of the (phase) sign change through impurity effects • Questions for s±-SC: • Any non-trivial in-gap bound-states? • (E < ∆coh)[See also T. Zhou et al., 0904.4273; D. Zhang, 0904.3708] • 2)What does the quasi-particle interference pattern look like?[Also suggested by Fa Wang et al. in EPL 85 (2009)] A. V. Balatsky et al, RMP (2006) J. E. Hoffman et al, Science 297 (2002) Q.H. Wang and D.H. Lee, PRB (2003) Strategy: “Hamiltonian” =2-orbital model + a localized single impurity (non-magnetic/magnetic, intra-orbital/inter-orbital)

  12. LDOS near the non-magnetic impurity site BdG calculations with VI=4|t1| and ne~2.1 per site on a 32x32 lattice

  13. Bound state energy vs. impurity scattering strength (non-magnetic, intra-orbital) s±-SC, ∆coh=0.4|t1| [For many impurities, see for instance, Y. Bang et al., PRB 79 (2009)]

  14. impurity site: (16,16) LDOS near the magnetic impurity site JIsz/2=2 The peaks decay quickly after ~3 lattice constants

  15. Quantum phase transition (level-crossing) and subtle features (1) In-gap bound states are more robust (2) No πphase shift at the impurity site [For strong “inter-band” magnetic scattering, see Jian Li and Y. Wang, 0905.3883]

  16. Quasi-particle interference (QPI): some parameters Pairing symmetry: ∆0 coskx cosky (∆0 / W ~ 0.01) DOS for a clean s±-SC ∆coh ~ 0.08 (in units of |t1|) Vimp = 4 ∆0 such that N0Vimp < 1, i.e., in the weak scattering (perturbative) regime

  17. ω=-0.09 ω=-0.09 QPI: induced LDOS(q,ω) for coskx cosky s-SC qy qy non-magnetic magnetic qx qx peaks around(±π,0)/ (0,±π) large peaks around(0,0)

  18. QPI: induced DOS(q,ω) for |coskx cosky| s-SC non-magnetic magnetic • In sign-changed s-wave pairing states: • The peaks around (π,0)/(0,π) show up for the case of non-magnetic • impurity • Anti-correlation between the intensities around (0,0) and (π,0)/(0,π) Y.Y. Zhang et al., arXiv:0903.1694 F Wang et al., EPL 85, 37005 (2009)

  19. Summary Due to the special feature of coskx cosky s-wave pairing symmetry, which changes sign between electron and hole Fermi pockets, we have shown: • A novel tri-layer π-junction. • The presence of non-trivial in-gap bound states in the • S-N-S± Josephson junction, sharply in contrast to other singlet pairing states. • 3. A non-magnetic impurity in s±-SC can induce in-gap bound states in sharp contrast to conventional s-wave SC. • 4. The presence (absence) of (0,π) / (π,0) peaks in QPI for s±-SC with non-magnetic (magnetic) impurities is a distinguishable feature compared with conventional s-SC.

  20. Thank you very much for your attention!

  21. Supplement

  22. sign-changed s-wave s-wave PRL 102 (2009) s-wave arXiv:0812.3295 s-wave Nature 453 (2008)

  23. Small FS Large FS

  24. With finite width d of the N region, the bound state energy appears at With unequal magnitudes of pairing potentials, provided Formula in SNS junction

  25. QP spectrum in SNS± junction

  26. Model Hamiltonian in Iron Pnictides

  27. T-matrix for impurity-induced bound states

  28. Non-magnetic magnetic Sx2y2 X S

  29. SC gap: non-magnetic impurity Sx2y2 S

  30. SC gap: magnetic impurity Sx2y2 S

  31. Spatial distribution of Spin-resolved LDOS at positive bound state energy

  32. T-Matrix approximation for induced LDOS The single-impurity induced Green’s function is The standard perturbation theory gives Therefore the Fourier transform of the induced LDOS is

  33. Intra-orbital scattering dominates QPI along special directions

  34. Two-Orbital: d wave NON-magnetic magnetic ω= 0 ω= 0.03 ω= 0.07 within the gap

  35. Five-Orbital: QPI NON-magnetic magnetic

  36. Five-Orbital: Profiles NON-magnetic magnetic

  37. Five-Orbital: without sign change NON-magnetic magnetic

More Related