Fermiology of bilayer colossal magnetoresistant manganites

Transcription

1 Fermiology of bilayer colossal magnetoresistant manganites Mark S. Golden Van der Waals-Zeeman Institute Universiteit van Amsterdam FOM-A-11 Outline ARPES as k-space k microscopy Intro. to colossal magneto-resistant maganites bilayer systems: La 2-2x 2xSr 1+2x Mn 3 O 7 Angle-resolved photoemission data: status quo Fermi surfaces quasiparticles coupling to boson mode(s) surprises in the temperature dependence Summary, conclusions and outlook 1

2 Amsterdam Universiteit van Amsterdam 375 years old students, 2100 academic staff 'general' university Science Faculty Van der Waals-Zeeman Institute for Experimental Physics 2

3 Condensed matter physics at the WZI Anne de Visser quantum electron matter Mark Golden Yingkai Huang Jeroen Goedkoop optoelectronic materials. Tom Gregorkiewicz CMP new magnetic materials and probes Ekkes Brück Electronic, magnetic and optoelectronic properties of novel materials Quantum matters unconventional quantum matter is undergoing a world-wide wide boom: quantum phase transitions quantum criticality macroscopic 'laboratories' for quantum microscopics novel, multifunctional materials ambitious intellectual joint venture: experiment & theory electron quantum matter atom 3

4 What's emerging? simple building blocks well-known rules (QM) "at each new level of complexity, entirely new properties appear, and the understanding of these behaviours requires research which I think is as fundamental in its nature as any other." P.W. Anderson, Science, 1972 wholly unexpected properties VU MIT 'Axis of complexity' Piers Coleman: Ann. Henri Poincaré 4 (2003) 1 not if new phenomena will be discovered but when where should one start looking first...? 4

5 Complexity emergence (almost by definition) experiment led creating unconventional quantum matter: atoms + lasers: e.g. optical lattice matter atoms + chem. bonding: e.g. no synthesis = no progress imaging unconventional quantum matter: in k-k or q-space q (charge and/or spin) in real space in time control understanding Advert: real space microscopy (STM / STS) Alex Groot, Freek Massee, Marco Gobbi HOPG Si (7x7) VT-STM UHV, Createc + J.B. Goedkoop Bi-2212 (Pb,Bi)

6 Outline ARPES as k-space k microscopy Intro. to colossal magneto-resistant maganites bilayer systems: La 2-2x 2xSr 1+2x Mn 3 O 7 Angle-resolved photoemission data: status quo Fermi surfaces quasiparticles coupling to boson mode(s) surprises in the temperature dependence Summary, conclusions and outlook Photoemission: a 'high' energy probe photocurrent vs. E kin and angles θ,φ high energy! High energy means! 'sudden' simple to interpret 6

7 I r r f p A i f, i ARPES : physical quantity measured matrix element hν absolute k k polarisation 2 A( k, ε ) f ( ε ) Fermi function temperature spectral function 1 A( k, ε ) = π Σ"( k, ε ) [ ε ε Σ' ( k, ε )] 2 + [ Σ"( k, ε ) ] 2 k self energy: imaginary part self energy: real part Our goal: the spectral function QP non-interacting system interactions on pic: after Meinders 7

8 ARPES: shopping list Damascelli UvA / PSI SLS pic: SPring8 Photoemission data: structure Intensity EDC E-E F E F 0 EDM Damascelli ReΣ MDC width in k 2 ImΣ 8

9 Outline ARPES as k-space k microscopy Intro. to colossal magneto-resistant maganites bilayer systems: La 2-2x 2xSr 1+2x Mn 3 O 7 Angle-resolved photoemission data: status quo Fermi surfaces quasiparticles coupling to boson mode(s) surprises in the temperature dependence Summary, conclusions and outlook Potted history: colossal magnetoresistant manganites 1969: Searle & Wang, Can. J. Phys. 47, 2701 (1969) La 1-x Pb x MnO 3 - giant (negative) magnetoresistance near T Curie Essential theory: Zener (1951), Anderson & Hasegawa (1955), de Gennes (1960) double exchange + particular filling of the d-orbitals d of the Mn ion 9

10 Ligand fields e g σ-bonding to ligands t 2g π-bonding to ligands pics: orbitron e.g. La x La x Sr 2+ Mn valency: : between 3 and Sr 0.33 MnO MnO 3 3 x O 2-1 x Mn high spin 3.67 Mn3d electrons

11 Double exchange σ i localised t 2g spin operators t hopping of e g electrons from one Mn site to next J H on-site FM coupling between e g spin and the t 2g local spin (σ)( e g electron operator, c iσ Double exchange if Hund's rule energy, J (exp( t. : 2eV) > bandwidth, W t eff = t cos (θ /2) - e g spin always parallel to the t 2g spins and - θ: : angle between the t 2g spins of the neighbouring Mn's 11

12 Ruddelson-Popper series (variations on perovskites) pic: Matt Rosseinsky Colossal (negative) magnetoresistance: : CMR double exchange means: ferromagnetic (FM) situation favours hopping CMR out of plane CMR effects of 4000% quasi 2D structure anisotropy in c and ab transport: in plane T. Kimura and Y. Tokura, Annu. Rev. Mat. Sci.,

13 Bilayer managanites reduced dimensionality greater role for fluctuations connection to the high Tc cuprates? strong anisotropy (even) larger CMR effect cleavage surfaces suitable for surface sensitive probes: ARPES STM / STS cleavage here La,Sr,O rock salt blocks double MnO 2 planes pic: Matt Rosseinsky Picture from: Ling et al. PRB (2000) La 2-2x Sr 1+2x Mn 2xSr T,x phase diagramme Mn 2 O 7 why so complex? Hole doping, x La 2 Sr 1 Mn 2 O 7 as 'parent insulator', x gives no. of additional holes 13

14 Spins charges orbitals (lattice) Jahn-Teller distortions select the e g orbital occupancy Spins charges orbitals (lattice) orbital ordering charge ordering strain reduction Mn 3+ - Mn 4+ - Mn 3+ - Mn Mn 4+ magnetic order: within MnO 2 plane, within bilayer,, inter-bilayer 14

15 T,x phase diagramme Picture from: Ling et al. PRB (2000) La 2-2x Sr 1+2x Mn 2xSr Mn 2 O 7? Focus here: FM metallic ground state CMR effect max. T c Hole doping, x Polarons,, CO, OO above T c double exchange not sufficient for explaining PI phase polarons (polaronic correlations) above Tc OO and/or CO correlations stripeyness complex interplay of charge, spin, orbital degrees of freedom still not understood Campbell, PRB

16 Outline ARPES as k-space k microscopy Intro. to colossal magneto-resistant maganites bilayer systems: La 2-2x 2xSr 1+2x Mn 3 O 7 Angle-resolved photoemission data: status quo Fermi surfaces quasiparticles coupling to boson mode(s) surprises in the temperature dependence Summary, conclusions and outlook Outline ARPES as k-space k microscopy Intro. to colossal magneto-resistant maganites bilayer systems: La 2-2x 2xSr 1+2x Mn 3 O 7 Angle-resolved photoemission data: status quo Fermi surfaces quasiparticles coupling to boson mode(s) surprises in the temperature dependence Summary, conclusions and outlook 16

17 Expectations from band structure calculations DFT says: half-metallic ferromagnet 2001 Majority band: quasi-2d Fermi surface 3d x 2 -y 2 BB 3d z 2- r 2 3d x 2 -y 2 AB M Γ X e g bandwidth: both 3d x 2 -y 2 and 3d z 2 -r2 are occupied Pics from: Huang et al. PRB (2000) Colorado group: ARPES of 2L manganites: history ghost Fermi surface, pseudogap Dessau et al.,, PRL1998 data: x=0.4 Fermi surface nesting (no QP's anywhere) Chuang et al.,, Science 2001 data: x=0.4 hν=50ev integrated ±200meV of E F 17

18 general statement 2L manganites: nodal metal (Fermi arc) data: x=0.4 ARPES of 2L manganites: status quo QP's at antinode no QP's for x=0.4 data: x=0.36,, 0.38, 0.4 Stanford Manella et al. Nature (2005) Colorado Z. Sun et al. PRL (2006) ARPES of 2L manganites: status quo metallicity above T c (phase separation) data: x=0.38, AB band Colorado Z. Sun et al. Nature Physics (2007) T c 18

19 Enter Amsterdam. Crystal characterisation: x=0.36 T c = 130K FM-M PM-I a sharp transitions, excellent cleavage surfaces Step 1: QP at antinode (π/a,0),( or not? S. de Jong et al. cond-mat/ Nov 2006 & W.K. Siu, MSc thesis

20 Step 2: And between (π/a,0) and BZ diagonal? FS map (E F ± 15 mev) T= 30K, hv= 56eV. AB band only. clear AB FS: resembles LDA, moderately nested S. de Jong et al Hunting down the QP's low energy spectral weight all round the AB Fermi surface X Γ hν=56ev S. de Jong et al

21 note: 'QPs' at all k F 's for LSMO Energy distribution curves hν=56ev peaks not yet resolution limited S. de Jong et al µ - ARPES: AB band (at the SLS) hν=56ev at (π/a,0)( antibonding band: sharp QP, resolution limited width S. de Jong et al

22 µ - ARPES: BB band (at the SLS) hν=73ev at (π/a,0)( bonding band: sharp QP? extra feature bonding band: E F MDC width (0.07 π/a) is double that of the AB band S. de Jong et al What about the BZ diagonal? FWHM 0.07 π/a FWHM 0.1 π/a S. de Jong et al

23 k F EDC's for x=0.36: (π/a,0) (π/a,0) QP's visible for: AB FS at (π/a,0)( and zone diagonal and for BB FS at (π/a,0)( BZ diagonal S. de Jong et al Renormalisation effects at (π/a,0)( E-E F (ev) T=30K Position (π/a) Width (π/a) coupling to collective (bosonic) mode(s): clear deviation from non-interacting dispersion between 60 and 110meV also seen in MDC width ( ( QP inverse life-time) S. de Jong et al

24 Outline Intro. to colossal magneto-resistant maganites bilayer systems: La 2-2x 2xSr 1+2x Mn 3 O 7 Angle-resolved photoemission data: history, status quo Fermi surfaces quasiparticles coupling to boson mode(s) surprises in the temperature dependence Summary, conclusions and outlook Temperature dependence at (π/a,0)( T C =130K S. de Jong et al

25 T-dependence: renormalisation effects (π/a,0)( hν=56ev E-E F (ev) at (π/a,0)( T=30 T=95 T=145 peak position (k y ) MDC FWHM (π/a)( (offset) S. de Jong et al T-dependence: MDC width at (π/a,0)( E-E F (ev) strong T-T dependence for low energies change in form within FM-M M phase (30 95K) hν=56ev at (π/a,0)( T=30 T=95 T=145 MDC FWHM (π/a)( S. de Jong et al

26 T-dependence: at (π/a,0)( and BZ diagonal at (π/a,0)( BZ diagonal hν=56ev T c S. de Jong et al T-dependence: k F EDC's at (π/a,0)( and BZ diagonal giant temperature dependence. k-dependent. k QP peak more robust an antinode spectral weight shift even larger at node S. de Jong et al

27 T-dependence: metallic phase above T c? T30-T95 T95 T95-T145 T145 two phase model doesn't work here - QP peak only for T<Tc - different high E behaviour for the two k-points k orbital fluctuations? Some (founded) speculations should be strong near 'FS approach' along diagonal k-dependence of T-dep? relative supression of spectral weight at node? M Γ X 27

28 Some (founded) speculations phase separation (also above Tc)? Aarts, Mydosh LSMO, 113 Rønnow et al., LSMO 327, x=0.3 Some (founded) speculations phase separation and photoemission data? Fermi step also well above Tc k-dependence of spectra vs. temperature 28

29 Conclusions quasiparticles are to be found on both the AB (at zone face and diagonal) and BB Fermi surfaces for x = 0.36 no such thing as a (general) nodal metal strong renormalisation effects clearly identified: - phonon-orbitons orbitons - strongly T-dep.: T magnetism anomalous temperature dependence: - huge shifts of spectral weight - can't fit a 2-phase 2 model analysis underway Glimpse of something hot off the beamline... hν=56ev M Γ X S. de Jong et al

30 Glimpse 2 hν=73ev M Γ X S. de Jong et al ? possible breakdown of 100% spin polarisation? minority (LSDA) majority (LSDA) 30

31 Glimpse 2 hν=73ev S. de Jong et al Quantum Electron Matter Group,, University of Amsterdam Sanne de Jong, Yingkai Huang, Wing Kiu Siu, Iman Santoso, Wim Koops, Freek Massee, Ton Gortenmulder SLS Vladimir Strocov,, Luc Patthey,, Ming Shi Support at BESSY Rolf Follath, Patrick Bressler Olaf Schwarzkopf The Credits Yingkai Huang Funding FOM, EU, UvA Sanne de Jong 31