Iron-based superconductor --- an overview. Hideo Aoki

Transcription

1 Thermal Quantum Field Theory Workshop, Yukawa Institute, Kyoto, 31 August 2010 Iron-based superconductor --- an overview Hideo Aoki Dept Physics, Univ Tokyo My talk today A brief introduction for the iron-based SC Mechanism for s± pairing Material-dependence (s± d) Collective (phase) modes in multiband SC Keyword: Multiband SC

2 Kazuhiko Kuroki, Hidetomo Usui Univ Electro-Commun Ryotaro Arita Univ Tokyo Yukihiro Ota, Masahiko Machida Japan Atomic Energy Agency Tomio Koyama Tohoku Univ

3 room T Why multiband SC? liq N Tc (K) Multiband SC (theory) (Suhl et al, Kondo 1950's-60's) multiband SC LaOFFeAs picene liq 4 He year

4 isotropic pairing * phonon mechanism attraction * electron mechanism phonon el-el repulsion (spin /charge) Tc ~ 0.1ω D Tc ~ 0.01t 10K 100K 100K 10000K anisotropic pairing

5 Phase diagrams for various classes of materials Iron compound cuprate AF AF SC K2 Chris Warner fullerene heavy fermion 4 He Uemura, nature mat, news&views 2009

6 Hubbard model (a generic model) U t

7 Tc Tc ~ T F /100 is VERY low! (1)Pairing int action from el-el repulsion = weak Cf. Laser-cooled Fermi gas(2004) Tc 0.1 T F attractive int action Feshbach resonance (2) Self-energy correction quasi-particles short-lived (Uemura 2004) T F (or n for bosons) (3) Pairing from el-el repulsion = anisotropic (i.e., nodes in D BCS )

8 SC from repulsion: nothing strange Attraction isotropic SC Repulsion anisotropic SC spin-fluctuation mediated pairing interaction V(k,k ) attraction as in d wave pairing in cuprates Scalapino et al, Moriya & Ueda, QMC: Kuroki & Aoki, DCA: Jarrell, VMC: Yokoyama et al,...

9 2D or 3D? ω D k y k z > k k y z k x k (Arita et al, JPSJ; PRB1999; x Monthoux & Lonzarich PRB 1999)

10 My talk today A brief introduction for the iron-based SC Mechanism for s± pairing [Kuroki et al, PRL 101, (2008)] Material-dependence (s± d) Collective (phase) modes in multiband SC Keyword: Multiband SC

11 Disconnected Fermi surfaces + + TMTSF (Kuroki & Arita, 2001) (Kuroki et al 2001; 2004)

12 Disconnected Fermi surfaces in real materials Co compound Hf nitrides Fe compound Kuroki et al, JPSJ(06); PRL(07); JPSJ(07) Kuroki, to appear in Proc LT 25; arxiv Kuroki et al, PRL(08)

13 Suhl-Kondo mechanism (Suhl et al, PRL 1959; Kondo 1963) s Tc enhanced ~ O(V sd2 ) d

14 Fe-compound Kamihara et al, JACS 130, 3296 (2008) Kuroki et al, PRL 101, (2008) (cited 301 times) + -

15 single-band vs multi-band Fe compound: multi-band Cuprate: nearly half-filled one-band

16 Band dispersion / Fermi surface band filling (=# electrons / # sites) n = % doping (Kuroki et al, PRL08) dispersion band 4 Fermi surface band 3 band 2

17 Phase diagram High-Tc Iron-based SDW SC AF SC SC hole concentration (Luetkens et al, 2008)

18 Susceptibilities: 5-band RPA result 5-band RPA (Kuroki et al, PRL 2008) s± pairing Inelastic neutron scattering (Lumsden et al, nature phys 2010) colinear SDW

19 Dirac cones in the iron-based SC SDW (Luetkens et al, 2008) Calculation: Ran et al, PRB 2009 ARPES: Richard et al, PRL 2010

20 Experimental evidences for multiband nature Quasiparticle excitations (penetration depth, thermal conductivity) ARPES NMR ARPES (BaK)Fe 2 As 2 : Ding et al, EPL 83, (2008)

21 JPS 2010 March meeting Symposium "Pairing symmetry in the iron-based SC" Shibauchi: Quasiparticle excitation probes incl. penetration depth, thermal cond consistent with full gap, while P-compound has a node Shimojima: ARPES 122 has full gap, but factors other than spin fluctuation may be relevant Hanaguri: Fourier transform STM s± Mukuda: NMR various powers in T observed for T 1 belowt C Sato: neutron scattering magnetic excitations, resonance peak Iketa: 1st principles calc for multiorbits, impurity potential s± impurity (Nagai et al, in prep) Onari: : Theory for impurity effect, resonance peak s++, orbital fluctuation mechanism

22 Phase-sensitive measurement (1) (Chen et al, nature physics 2010) Nb AB flux (Wollman et al, PRL 1993)

23 Phase-sensitive measurement (2) --- Fourier-transform STM spectroscopy (Hanaguri et al, Science 2010)

24 My talk today A brief introduction for the iron-based SC Mechanism for s± pairing Material-dependence (s± d) [Kuroki et al, PRB 79, (2009)] Collective (phase) modes in multiband SC Keyword: Multiband SC

25 NdFeAsO h = 1.38 A LaFeAsO h = 1.32 A LaFePO h = 1.14 A dx 2 -y 2 dz 2

26 Multibands modified pnictogen height (Kuroki et al, PRB 2009) LaFePO LaFeAsO, NdFeAsO b-g nesting b 1 -b 2 nesting

27 Multiple nesting vectors compete Kuroki et al, PRB 79, (2009) high Tc s± NdFeAsO h = 1.38 A LaFeAsO h = 1.32 A Pnictogen height as a switch Height matters Frustration in momentum space low Tc nodal s LaFePO h = 1.14 A low Tc nodal d

28 Multiband systems various pairings when materials / p are tuned s± Degeneracy point s+id? (S.C. Zhang's group, PRL 2009) d (Kuroki et al, PRB 2009)

29 When T-broken pairing can occur? When the space group has a two-dimensional rep: as in p + ip for triplet SC d + id for singlet SC (Onari et al, PRB 2002) p + ip for graphene (Uchoa et al, PRL 2007) d 1 d 2 + i (space group: G 6 + )

30 My talk today A brief introduction for the iron-based SC Mechanism for s± pairing Material-dependence (s± d) Collective (phase) modes in multiband SC [Ota et al, arxiv: ] Keyword: Multiband SC

31 Collective excitation modes in SC Collective mode in one-band SC Phase modes (Bogoliubov 1959, Anderson 1958, Nambu 1960) = massless Nambu-Goldstone mode for neutral SC massive for real (charged) SC (Anderson-Higgs mechanism 1963) as observed in NbSe 2 (Raman: Sooryakumar & Klein 1980, Littlewood & Varma 1981) Collective modes in two-band SC Out-of-phase (countersuperflow) mode (massive, Leggett 1966)

32 Two-band SC in phase as observed in MgB 2 (Blumberg et al 2007) out of phase = Leggett mode

33 Question here: 3-band = 2-band in terms of the collective modes? Yes, more than one collective Leggett modes emerge in 3-band SC large difference in mass when multiple (internal) Josephson currents subtract (Ota et al, arxiv: )

34 Question here: 3-band = 2-band in terms of the collective modes? (Ota et al, 2010) D (1) g 12 D (2) g -1 = g 31 D (3) g 23 [class "even"] internal Josephson c's add even / odd (g -1 ) ij [class "odd"] Josephson c's subtract

35 w/d When the pairing interactions g ij are varied multiple Leggett modes (Ota et al, arxiv ) mass difference w L+ repulsive g ij 's always class odd large mass difference g 31 w L- D (1) g 12 D (2) q z g 31 g 23 D (3) g 12 g 23 =-0.07

36 Im "Frustration" in the three-band SC complex (T-reversal broken) D (Stanev & Tesanovic, PRB 2010) when g 12 ~ g 23 ~ g 31 D 1 Re D 3 D 2

37 Summary and outlook Iron-based SC --- a prototype multiband SC various possibilities incl. various pairings Collective phase mode in 3-band Multiband SC rich prospects Other multiband SC s? Even the cuprate has to be viewed as multiband

38 Cuprates: single-band Iron-based: multi-band

39 Cuprates: still a lot of puzzles Various classes of cuprates single-layered (simplest possible) La 2 CuO 4 : Tc ~ 40K Why such a huge difference? Wgy Why so much different T c 's? HgBa 2 CuO 4 : Tc ~ 90K Cu O

40 La Th: Tc(La) > Tc(Hg) Ex: 40 K < 90 K Hg contradiction! (Sakakibara et al, PRL 105, (2010)) d x 2 -y 2 d x 2 -y 2 O 2.42 A Cu 2.78A smaller xtal field d z 2 Γ d x2-y2 Γ Γ 0.91 ev d z ev d z2 d x2-y2 d x2-y2 d Γ

41 Outlook (1): diverse multiband SC's 1.Even typical "low-t " SC's are often multiband Pb (Gross et al, 2009) Metallic H at 414 GPa 2. Molecular solids tend to be multiband (Gross et al, 2009) 3. Cold atoms (different hyperfine states on optical lattices)

42 SC in elemental Fe under p [Shimizu et al, Nature 412, 316 (2001)] bcc(magnetic) hcp(nonmagnetic, SC) Theories: el-ph cannot explain Tc [Suzuki et al, 2002; Mazin et al, PRB 2002], spin fluctuations have to be considered [Schafer et al, PRB (2005)]

43 To actually look at the interactions attraction phonon el-el repulsion J-PARC

44 Outlook (2): Colour SC in hadron physics T ~ 170 MeV Quark-gluon plasma SC hole concentration Spin-fluct mediated, Multi-band SC,... Vacuum Hadronic fluid ~ 1 GeV Colour superconductor μ B Gluon, various SC phases,...

45 Differences in SC in solid-state vs hadron phys Energy scale ~ 0.01 ev ~ 100 MeV Length scale 1 ~ 10 3 nm 1 ~ 10 fm Particles involved e's with anisotropic FS relativistic quarks with isotr FS Interaction e-e, e-phonon gluon-mediated long-range Tc Tc ~ 0.01 e F Tc ~ 0.1 e F Internal deg spin, orbit colour, flavour, spin Order of phase tr weakly 1st fl of EM 1st thermal fl of gluons

46 IPMU Focus Week Feb 2010 "Condensed Matter Physics Meets High Energy Physics"