Photoelectron Spectroscopy for Functional Oxides

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Photoelectron Spectroscopy for Functional Oxides

  1. 1. International Summer School on Surfaces and Interfaces in Correlated Oxiides, Vancouver, 29 Aug – 01 Sep 2011 FOR 1346Photoelectron spectroscopy of functional oxides:Heterostructures and buried interfacesRalph Claessen (U Würzburg, Germany)• Photoelectron spectroscopy (PES)• PES theory in a nutshell• PES with hard x-rays (HAXPES)• HAXPES of oxide heterostructures
  2. 2. Heterostructures of functional oxides3d transition metal oxidesstrong coupling between charge/orbital/spin/latticedegrees of freedom lead to: - metal-insulator transitions - charge and orbital ordering - local magnetism (ferro, antiferro,…) - high-temperature superconductivity - collossal magnetoresistance -…Epitaxial heterostructures by MBE, PLDcontrolled interfaces, additional functionalities: - strain engineering - interfacial 2dim electron gas (2DEG) - electrostatic doping (by polarity or field effect) - artificial multiferroics - spin injection -…
  3. 3. Oxide heterostructures "The interface is the device" (H. Kroemer, Nobel lecture 2000) Want information on: • chemical composition • electronic structure • vertical depth profile photoelectron spectroscopy (PES) with soft and hard x-rays
  4. 4. Photoelectron spectroscopy (PES)
  5. 5. Photoelectron spectroscopy (PES) spectrum hνsample Ekin Ekin = hν – EB - Φ0 measure kinetic energy distribution of photoelectrons
  6. 6. Photoelectron spectroscopy (PES) spectrumsample Chemistry (core levels): → composition → chemical bonding → valencies Electronic structure (valence band): → density of states → band structure → Fermi surface → spectral function A<(k,E)
  7. 7. Core level spectroscopy: ESCAElectron Spectroscopy for Chemical Analysis Bi2Sr2CaCu2O8+δ Bi 4f5/2 Bi 4f7/2 Intensity [a.u.] O 1s Bi 4f Intensity [a.u.] Bi 4d C 1s 1310 1320 1330 1340 Kinetic Energy [eV] Ca 2p •Cu 2p Sr 3d Fermi level Intensity [a.u.] •CuO Bi 5d hν = 1486.6 eV [Al - Kα] 1470 1480 1490 1500 400 600 800 1000 1200 1400 Kinetic Energy [eV] Kinetic Energy [eV] courtesy of A.F. Santander-Syro
  8. 8. Core level spectroscopy: Chemical shift and valencyExample: alkali metal doping of TiOCl Ti2p 3/2 (+ Na) Na1s 3+ Ti 2+ TiNa doping x (%) 37% 32% 23% 15% 10% 4% valence change: Ti3+(3d1)  Ti2+(3d2) 462 460 458 456 454 1080 1075 1070 1065 binding energy (eV) binding energy (eV) PRL 106, 056403 (2011)
  9. 9. Valence band spectroscopy k-integrated spectrum TiOCl Ti 3d O 2p / Cl 3p PRB 72, 125127 (2005)
  10. 10. Valence band spectroscopy: ARPESAngle-Resolved PhotoElectron Spectroscopy band structure and Fermi surface emission angle (i.e. momentum) energy courtesy T. Deveraux/A. Damascelli
  11. 11. PES instrumentation• rare gas discharge lamp (<40.2 eV) • hemispherical anylzer• x-ray tube (1.256 and 1.486 keV) • time of flight (TOF) analyzer)• synchrotron radiation (10 eV … 10 keV) typically 10-10 mbar Wikipedia
  12. 12. PES theory in a nutshell:1) Independent electron approximation
  13. 13. PES theory: Independent electronsTime-dependent perturbation theoryUnperturbed electron system:one-electron states ψ with energy EPerturbation:    i ( k ⋅r −2πνt ) classical radiation field with vector potential A(r , t ) = A0e Fermi´s Golden Rule for the photoinduced transition rate from initial to final states:  ik ⋅r   2 wi → f ∝ ψ f A0e ⋅ p ψ i δ ( E f − Ei − hν ) Hence, the total photoelectron current is: I PES (ε ) ∝ ∑ wi → f δ (ε − E f ) i, f
  14. 14. PES theory: Independent electrons  ik ⋅r   2 wi → f ∝ ψ f A0e ⋅ p ψ i δ ( E f − Ei − hν )final state: energy conservationinverted LEED state initial state:(eigenstate of semi-infinite crystal) Bloch wave or core level
  15. 15. PES theory: Independent electrons  ik ⋅r   2 wi → f ∝ ψ f A0e ⋅ p ψ i δ ( E f − Ei − hν )final state: high-energy Bloch state of infinite crystal,inverted LEED stateand 3 incoherently decoupled steps 2(eigenstate of semi-infinite crystal) One-step model Three-step model courtesy A. Damascelli
  16. 16. PES theory: Independent electrons  ik ⋅r   2wi → f ∝ ψ f A0e ⋅ p ψ i δ ( E f − Ei − hν ) transition matrix elementIf the radiadion field is only weakly modulated on   atomic length scales,(i.e. λ = 2π k >> few Å), the photon momentum k can be neglected inthe transition matrix element:  ik ⋅r       f A0e ⋅ p i ≈ f A0 ⋅ p i ∝ A0 ⋅ f er iExamples: Dipole approximationhν = 20 eV  λ ≈ 600 Åhν = 2000 eV  λ ≈ 6 Å
  17. 17. PES theory: Independent electronsDipole approximation and k-selection rule for Bloch states momentum conservation: only"vertical"     k f = ki + G + k photon transitions  ARPES
  18. 18. Transition metal oxides: electronic correlationsoxides of the 3d transition metals: M = Ti, V, … ,Ni, Cu O2-basic building blocks: MO6 octahedra (or other ligand shells)electronic configuration: O 2s2p6 = [Ne] TMX+ TM 3dn quasi-atomic, strongly localized  strong intraatomic Coulomb interaction and breakdown of independent electron approx.cubic perovskites perovskite-like anatas rutile spinel
  19. 19. PES theory in a nutshell: 2) Many-body picture
  20. 20. Many-body effects in photoemission N interacting electrons: Ekinhν Photoemission process: sudden removal of an electron from N-particle system "loss" of kinetic energy due to interaction-related excitation energy stored in the remaining N-1 electron system !
  21. 21. Reinterpretation of Fermi´s Golden RuleFermi´s Golden Rule for N-particle states: 2I (ε ) ∝ ∑ Ψ f , s ∆ Ψi ,0 δ ( E N , s − E N ,0 − hν ) ˆ swithΨi ,0 = N ,0 N-electron ground state of energy EN, 0 ("initial state") Ψ f ,s = k , N − 1, s N-electron excited state of energy EN, s, ("final state") consisting of N-1 electrons in the solid and  a free photoelectron of momentum k and energy ε    Nˆ = ∑ A(ri ) ⋅ pi = M if c + ci∆ f in second quantization with suitable one- i =1 electron basis one-particle matrix element
  22. 22. Electron removal spectrumFermi´s Golden Rule for N-particle states: 2I (ε ) ∝ ∑ Ψ f , s ∆ Ψi ,0 δ ( E N , s − E N ,0 − hν ) ˆ s a little bit of math and a few plausible assumptions (sudden approximation)The ARPES signal I (ε ) is directly proportional to the < 1single-particle spectral function A (ω ) = − Im G (ω ) × f (ω ) π probability of removing an electron single-particle at energy ω from the system Green´s function
  23. 23. Example: PES of the Mott insulator TiOCl spectral function A<(ω) (DMFT)TiOCl Ti 3d1 O 2p / Cl 3p d1 → d0 d1 → d2 LHB UHB U µ ω
  24. 24. Photoemission probing depth: soft and hard x-ray PES
  25. 25. Inelastic scattering of the photoelectron Step 2: photoelectron transport to the surface  inelastic scattering with other electrons Three-step model (excitation of e-h-pairs, plasmons) • generation of secondary electrons ("inelastic background") intensity intrinsic spectrum incl. background EkincourtesyA. Damascelli
  26. 26. Inelastic scattering of the photoelectron Step 2: photoelectron transport to the surface  inelastic scattering with other electrons Three-step model (excitation of e-h-pairs, plasmons) • generation of secondary electrons ("inelastic background") • loss of unscattered photoelectron current ⇒ inelastic mean free path λcourtesyA. Damascelli
  27. 27. Photoemission probing depth λ(Ekin) "universal curve" hν Ekin λ(Ekin) hard x-ray PES = HAXPES soft x-ray PES (SX-PES)"conventional" VUV/XUV-PES:surface sensitive on  probing depth (3λ) up to >10 nmatomic length scale !  access to bulk, buried nanostructures, and interfaces  depth profiling of thin films
  28. 28. Transition metal oxides: Instability of polar surfaces O2-Transition metal (TM) oxides form lattice of ionic charges TMX+ Classification of surfaces (Tasker): - surface charge Q  - electrical dipole moment µ in repeat unit Q=0 Q≠0 Q≠0  µ =0  µ =0  µ≠0 TMX+ O2- P. W. Tasker, J. Phys. C 12, 4977 (1979)
  29. 29. Transition metal oxides: Instability of polar surfaces O2- type 3 surfaces are energetically unfavorable: TMX+ charge field potential-σ+σ "polarization catastrophe"-σ+σ will be avoided by atomic/ionic/electronic surface reconstruction ⇒ surface ≠ bulk
  30. 30. Transition metal oxides: Instability of polar surfaces different reconstructionsExample: Fe3O4 (magnetite) of the (111) surface (STM) 8.2 Å PRB 76, 075412 (2007)
  31. 31. Transition metal oxides: Instability of polar surfacesExample: Fe3O4 (magnetite) VUV-PES Soft X-ray PES surface-sensitive probing depth 2x larger EPL 70, 789 (2005)
  32. 32. Surface effects in Mott-Hubbard-type oxides U spectral function (DMFT for n=1) t U/t kinetic energy, itinerancyH = −tˆ ∑ σ ci+ c jσ + U ∑ ni ↓ ni ↑ i , j ,σ i local Coulomb energy, localization
  33. 33. Surface effects in Mott-Hubbard-type oxides Example: CaVO3 spectral function (DMFT for n=1) quasiparticle surface peak "bulk" U/t lower Hubbard bandA. Sekiyama et al., PRL 2004
  34. 34. Surface effects in Mott-Hubbard-type oxides Example: CaVO3 quasiparticle surface peak "bulk" reduced atomic coordination @ surface: lower Hubbard band  stronger electron localization  smaller effective bandwidth Wsurf < Wbulk  surface stronger correlated: U / Wsurf >U / WbulkA. Sekiyama et al., PRL 2004
  35. 35. Photoemission probing depth λ(Ekin) "universal curve" hν Ekin λ(Ekin) hard x-ray PES = HAXPES soft x-ray PES (SX-PES)"conventional" VUV/XUV-PES:surface sensitive on  probing depth (3λ) up to >10 nmatomic length scale !  access to bulk, buried nanostructures, and interfaces  depth profiling of thin films
  36. 36. HAXPES: drawbacks and caveatsNon-negligible photon momentum hν = 6 keV  λ ≈ 2 Å, kphot ≈ 3 Å-1
  37. 37. HAXPES: drawbacks and caveatsNon-negligible photon momentum hν = 6 keV  λ ≈ 2 Å, kphot ≈ 3 Å-1• suppression of direct (k-conserving) transitions ( Debye-Waller factor for direct transitions Wdir = exp − αk photT M atom 2 ) ARPES of W(110) @ hν = 870 eV Plucinski et al., PRB 78, 035108 (2008)
  38. 38. HAXPES: drawbacks and caveatsNon-negligible photon momentum hν = 6 keV  λ ≈ 2 Å, kphot ≈ 3 Å-1• suppression of direct (k-conserving) transitions• atomic recoil effect photon-absorbing atom takes up recoil energy Ekin =  2 k phot 2 M at the expense of 2 photoelectron energy, depending on atom mass and lattice stiffness Y. Takata et al., PRB 75, 233404 (2007)
  39. 39. HAXPES: drawbacks and caveatsNon-negligible photon momentum hν = 6 keV  λ ≈ 2 Å, kphot ≈ 3 Å-1• suppression of direct (k-conserving) transitions• atomic recoil effect• quadrupolar contribution to transition matrix element    ( )  ik ⋅r    f A0e ⋅ p i ≈ f A0 1 + ik ⋅ r ⋅ p i
  40. 40. HAXPES: drawbacks and caveatsNon-negligible photon momentum hν = 6 keV  λ ≈ 2 Å, kphot ≈ 3 Å-1• suppression of direct (k-conserving) transitions• atomic recoil effect• quadrupolar contribution to transition matrix elementLow photoemission signal• cross section for photoemission σ ∝ (hν ) −3 −1• electron analyzer transmission t ∝ Ekin need bright x-ray source…
  41. 41. HAXPES set-up @ PETRA III (DESY, Hamburg) X-rays from PETRA III "High-resolution hard x-ray photoemission for materials science" (BMBF) • joint project with C. Felser (U Mainz) and W. Drube (DESY) • photon energy: 2.5…15 keV • energy resolution: 30 meVother HAXPES instruments worldwide: • linearly/circularly polarized x-- Spring-8, Japan (>4)- BESSY, Germany (HIKE) ray radiation- ESRF, France (ID-9) • commissioned in 2010- Soleil, France (under construction) • user operation since 2011- Diamond, UK (under construction)
  42. 42. HAXPES of oxide heterostructures: (1) Fe3O4/GaAs
  43. 43. Epitaxial growth of Fe3O4/GaAs PRB 79, 233101 (2009)surface Datta-Das spin transistorFe3O4 GaAs semiconductor with semimetallic ferromagnet large spin diffusion (100% spin polarization @ EF) length resistively matched to semiconductor  Fe3O4 (magnetite), (RE,Sr)MnO3, CrO2, Heusler compounds, …
  44. 44. Epitaxial growth of Fe3O4/GaAs PRB 79, 233101 (2009)surface MBE growth of thin magnetite film:Fe3O4 • epitaxial Fe deposition @ RT • postoxidation @ 600 - 800K / p(O2) = 10-5 mbar (10-30 min) GaAs  Fe valency?  mixed-valent Fe3O4 vs. (Fe2+ )O and (Fe 3+)2O3 ?  chemical depth profile ?
  45. 45. Valence signatures in Fe 2p spectrum Fe2O3 Fe3O4 Fe3+ FeO Fe2+/Fe3+ charge transfer satellites Fe2+ Fe 2p1/2 2p3/2 Fe0700 705 710 715 720 725 730 735 740 745 750 binding energy (eV)
  46. 46. Depth profiling of Fe3O4/GaAs PRB 79, 233101 (2009) Fe 2p spectrasurfaceFe3O4 GaAs interface surface
  47. 47. Depth profiling of Fe3O4/GaAsTuning the information depth by variation of(1) photon energy, or (2) photoelectron escape angle θ λeff mean free path λeff = λIMFP cos θ energy
  48. 48. Depth profiling of Fe3O4/GaAs PRB 79, 233101 (2009) Fe 2p spectrasurfaceFe3O4 GaAs interface surface film: mixed-valent Fe2+/3+ interface: divalent and metallic Fe (O-deficient)
  49. 49. Depth profiling of Fe3O4/GaAs PRB 79, 233101 (2009) Fe 2p spectra As 2p3/2 spectra surface Fe3O4 interface(Fe, FeOx, GaOx, AsOx) GaAs film: mixed-valent Fe2+/3+ interface: divalent and metallic Fe (O-deficient) oxidized Ga,As
  50. 50. Validation by electron microscopy TEM STEM-EELS surface Fe3O4 interface(Fe, FeOx, GaOx, AsOx) GaAs J. Verbeeck, H. Tian, and G. van Tendeloo, U Antwerp
  51. 51. Fe3O4/ZnO: An all-oxide structure APL 98, 012512 2011 film grown by reactive depositionFe3O4 in O2-atmosphere (∼10-6 mbar)ZnO HAXPES TEM also PLD-grown contacts: R. Gross et al.
  52. 52. HAXPES of oxide heterostructures:(2) Interface 2DEG in LaAlO3/SrTiO3
  53. 53. LAO/STO heterostructures in a nutshell• epitaxial growth by PLD LaAlO3 ∆=5.6eV SrTiO3 ∆=3.2eV A. Ohtomo et al., Nature 419, 378 (2004) S. Thiel et al., Science 313, 1942 (2006) N. Reyren et al., Science 317, 1196 (2007)
  54. 54. LAO/STO heterostructures in a nutshell• epitaxial growth by PLD• both oxides: wide gap insulators• if LaAlO3 film thicker than 3 unit cells (uc) : → formation of a high-mobility 2DEG LaAlO3 at the interface ∆=5.6eV conductivity 2DEG SrTiO3 ∆=3.2eV sheet carrier density (Hall) A. Ohtomo et al., Nature 419, 378 (2004) S. Thiel et al., Science 313, 1942 (2006) N. Reyren et al., Science 317, 1196 (2007)
  55. 55. LAO/STO heterostructures in a nutshellproperties of the 2DEG:• tunable conductivity by electric gate field LaAlO3• superconducting below 200 mK ∆=5.6eV• magnetoresistance 2DEG• coexistence of s.c and magnetism / electronic phase separation SrTiO3 ∆=3.2eV origin of 2DEG, threshold behavior ? A. Ohtomo et al., Nature 419, 378 (2004) S. Thiel et al., Science 313, 1942 (2006) N. Reyren et al., Science 317, 1196 (2007)
  56. 56. Polar catastrophe and how to avoid itcharge: -1 AlO2 +1 LaO electrostatic energy increases -1 AlO2 +1 LaO linearly with thickness of -1 AlO2 polar film +1 LaO 0 TiO2 polar catastrophe 0 SrO 0 TiO2 0 SrO -1/2 AlO2 +1 LaO charge reconstruction -1 AlO2 electronic or ionic∆q = -1/2 +1 LaO -1 AlO2 0.5e- per layer unit cell +1 LaO -1/2  n2D = 3.5×1014 cm-2 TiO2 0 SrO partial Ti 3d occupation 0 TiO2 0  Ti3.5 (d0.5) = Ti3+/Ti4+ SrONakagawa et al., Nature Mat. 5, 204 (2006)
  57. 57. HAXPES of LAO/STO heterostructures Ti 2p spectrum Ti4+ LaAlO3 2DEG Ti3+ SrTiO3 2p1/2 2p3/2undoped SrTiO3: |3d0>  Ti4+doped LAO/STO interface: |3d0> + |3d1>  Ti3+/Ti4+ PRL 102, 176805 (2009)
  58. 58. Dependence on LAO overlayer thickness Ti3+ Ti4+ Ti3+ interface charge density increases with LAO overlayer thickness non-zero Ti d1 signal already for 2uc sample (?) PRL 102, 176805 (2009)
  59. 59. Depth profiling by angle-resolved HAXPES e- θ e- d  2DEG thickness  sheet carrier density PRL 102, 176805 (2009)
  60. 60. Quantitative analysis: 2DEG thickness Sample 2 uc 4 uc 5 uc 6 uc d (uc*) 3±1 1 ± 0.5 6±2 8±2e- θ *lattice constant of STO unit cell (uc) = 3.8 Å e-  interface thickness < 3 nm d consistent with - CT-AFM Basletic et al. (2008) - TEM-EELS Nakagawa et al. (2006) - density functional theory Pentcheva et al. (2009) - 2D superconductivity Reyren et al. (2007) - ellipsometry Dubroka et al. (2010) PRL 102, 176805 (2009)
  61. 61. Quantitative analysis: sheet carrier densitySample 2 uc 4 uc 5 uc 6 uc el. reconstructionn2D (1013 cm-2) 2.1 3.9 8.1 11.1 35  n2D << electronic reconstruction value  n2D >> Hall effect data PRL 102, 176805 (2009)
  62. 62. RIXS on LAO/STO RIXS eg-excitation as fct. of # LAO-overlayers Ti3+ (3d1) eg Ti 3d photon t2g in photon out Ti 2pPRB 82, 241405(R) (2010)
  63. 63. Sheet carrier density: HAXPES, RIXS & Hall effect • n2D much smaller than expected for purely electronic reconstruction (35 x 1013 cm-2) • n2D higher than Hall effect data • photo-generated carriers cannot fully account for observed excess • remaining excess due to additional localized Ti 3d electrons? (cf. DFT - Popovic et al., PRL 2008)PRB 82, 241405(R) (2010)
  64. 64. LAO/STO: Valence band spectroscopy with HAXPES LaAlO3 2DEG ~3 eV SrTiO3 O2p-derived vb states Ti 3d electrons should be here, but HAXPES cross-section too small ! (theor. estimate: 10-4 of O2p emission)
  65. 65. Band situation from density-functional theory STO LAO 2DEGE surface CBM EF VBM core levels Yu Lin et al., arXiv 0904.1636 (2009) Pentcheva and Pickett, PRL 102, 107602 (2009)
  66. 66. Band situation from density-functional theory STO LAO 2DEGE holes surface @ LAO VBM CBM e- e- EF interface VBM electrons @ STO CBM core levels Yu Lin et al., arXiv 0904.1636 (2009) Pentcheva and Pickett, PRL 102, 107602 (2009)
  67. 67. Band situation from density-functional theory STO LAO 2DEGE E holes surface @ LAO VBM CBM e- e- EF VBM electrons @ STO CBM core levels Yu Lin et al., arXiv 0904.1636 (2009) Pentcheva and Pickett, PRL 102, 107602 (2009)
  68. 68. Results from HAXPES valence band Al 1s core level ~3 eV VBM: ~ 3 eV below EF same width for all samples!
  69. 69. band theory versus experiment STO LAO 2DEGE STO LAO surface CBM e- EF VBM core levels also observed by Segal et al., PRB 80, 241107(R) (2009)
  70. 70. Valence band offsets band alignment valence band analysis CB STO LAO VB STO LAO type I type II• VBMLAO above VBMSTO• type II interface (valence band offset: 0.35 ± 0.1eV)• confirmed by core level analysis 0.35eV
  71. 71. Band alignment: A possible scenarioDFT band theory: STO LAO localized hole states induced by surface O-vacanciesPhotoemission: interface states (itinerant and localized)
  72. 72. HAXPES of oxide heterostructures:(3) LaVO3/SrTiO3 – electrostatic doping of a Mott a insulator
  73. 73. Electrostatic doping of a Mott insulator LAO/STO LVO/STO LaAlO3 polar LaVO3band ins. Mott ins. …∆=5.6eV ∆≈1 eV (AlO2)- Idea: (LaO)+ replace Al3+ by q2DEG (TiO2)0 ??? trivalent transition metal (SrO)0  LaVO SrTiO3 3 SrTiO3band ins. … non-polar band ins.∆=3.2eV ∆=3.2eV Ohtomo/Hwang, Nature 427, 423 (2004) Hotta et al., PRL 99, 236805 (2007)
  74. 74. Electrostatic doping of a Mott insulator LVO/STO LaVO3: - valence configuration V3+ (d2) LaVO3 - polar oxideMott ins.∆≈1 eV - Mott insulator (∆LVO << ∆STO)  electronic reconstruction and ??? formation of interface 2DEG ?  extra carriers on which side of interface SrTiO3 (LVO or STO) ?band ins.∆=3.2eV  band-filling controlled Mott transition without chemical doping ?
  75. 75. LVO/STO: Sample growth and characterization RHEED pattern AFM image pulsed laser depositionRHEED oscillations STEM image interface
  76. 76. LVO/STO: metal-insulator transition in transport metal-insulator transition for n-type interface p-type interface insulating critical thickness: ∼ 9 uc LVO (Hotta et al.: 5 uc) high carrier mobility
  77. 77. HAXPES of LVO/STO: V 2p depth profilesinsulating conducting extra electronic homogeneous 10 uc LVO charge on V 6 uc LVO "V3+" profile near interface STO STO
  78. 78. HAXPES of LVO/STO: Ti 2p extra electronic 10 uc LVO charge on V near interface no Ti3+ (d1) signal possibly some bandbending STO on STO side of interface
  79. 79. LVO/STO: electronic reconstruction picture
  80. 80. Electrostatic doping of a Mott insulator LaVO3/SrTiO3: LaVO3 • creation of 2D metal states in aMott ins. correlated electron system∆≈1 eV by interface engeering • purely electrostatic doping"q2DEG" • no disorder by chemical dopants SrTiO3band ins.∆=3.2eV
  81. 81. SummaryPhotoelectron spectroscopy of functional oxides:Heterostructures and buried interfaces• Photoelectron spectroscopy (PES) yields (destruction-free) information on - chemical composition, valencies, local chemistry - electronic structure (band structure, spectral function)• PES with hard x-rays (HAXPES) - enhanced probing depth giving access to bulk and buried interfaces - needs high x-ray intensity ( synchrotron radiation) - caveat: high photon momentum (ARPES difficult, recoil effects)• Future directions: - magnetic information with polarized x-rays (XMCD, XMLD) and/or spin detection - soft x-ray ARPES: band mapping of buried interfaces
  82. 82. ReadingPhotoemission:• S. Hüfner, Photoelectron Spectroscopy – Principles and Applications, 3rd ed. (Berlin, Springer, 2003)• A. Damascelli, Angle-resolved photoemission studies of the cuprate superconductors, Rev. Mod. Phys. 75, 473 (2003)HAXPES:• K. Kobayashi: Hard x-ray photoemission spectroscopy, Nucl. Instr. Meth. Phys. Res. A 601, 32 (2009)• László Kövér: X-ray photoelectron spectroscopy using hard X-rays, J. Electron Spectrosc. Rel. Phen. 178-179, 241 (2010)HAXPES of oxide heterostructures• R. Claessen et al.: Hard x-ray photoelectron specroscopy of oxide hybrid and heterostructures: a new method for the study of buried interfaces, New J. Phys. 11, 125007 (2009)
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