Photoelectron Spectroscopy for Functional Oxides

1. International Summer School on Surfaces and Interfaces in Correlated Oxiides, Vancouver, 29 Aug – 01 Sep 2011
FOR
1346
Photoelectron spectroscopy of functional oxides:
Heterostructures and buried interfaces
Ralph Claessen (U Würzburg, Germany)
• Photoelectron spectroscopy (PES)
• PES theory in a nutshell
• PES with hard x-rays (HAXPES)
• HAXPES of oxide heterostructures

2. Heterostructures of functional oxides
3d transition metal oxides
strong coupling between charge/orbital/spin/lattice
degrees of freedom lead to:
- metal-insulator transitions
- charge and orbital ordering
- local magnetism (ferro, antiferro,…)
- high-temperature superconductivity
- collossal magnetoresistance
-…
Epitaxial heterostructures by MBE, PLD
controlled interfaces, additional functionalities:
- strain engineering
- interfacial 2dim electron gas (2DEG)
- electrostatic doping (by polarity or field effect)
- artificial multiferroics
- spin injection
-…

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. Photoelectron spectroscopy (PES)

5. Photoelectron spectroscopy (PES)
spectrum
hν
sample
Ekin
Ekin = hν – EB - Φ0
measure kinetic energy
distribution of photoelectrons

6. Photoelectron spectroscopy (PES)
spectrum
sample Chemistry (core levels):
→ composition
→ chemical bonding
→ valencies
Electronic structure (valence band):
→ density of states
→ band structure
→ Fermi surface
→ spectral function A<(k,E)

7. Core level spectroscopy: ESCA
Electron 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. Core level spectroscopy: Chemical shift and valency
Example: alkali metal doping of TiOCl
Ti2p 3/2 (+ Na) Na1s
3+
Ti 2+
Ti
Na
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. Valence band spectroscopy
k-integrated spectrum
TiOCl
Ti 3d
O 2p / Cl 3p
PRB 72, 125127 (2005)

10. Valence band spectroscopy: ARPES
Angle-Resolved PhotoElectron Spectroscopy
 band structure and Fermi surface
emission angle (i.e. momentum)
energy
courtesy T. Deveraux/A. Damascelli

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. PES theory in a nutshell:
1) Independent electron approximation

13. PES theory: Independent electrons
Time-dependent perturbation theory
Unperturbed electron system:
one-electron states ψ with energy E
Perturbation:    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. PES theory: Independent electrons
 ik ⋅r 
 2
wi → f ∝ ψ f A0e ⋅ p ψ i δ ( E f − Ei − hν )
final state: energy conservation
inverted LEED state initial state:
(eigenstate of semi-infinite crystal) Bloch wave or core level

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. PES theory: Independent electrons
 ik ⋅r 
 2
wi → f ∝ ψ f A0e ⋅ p ψ i δ ( E f − Ei − hν )
transition matrix element
If 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 in
the transition matrix element:
 ik ⋅r 
    
f A0e ⋅ p i ≈ f A0 ⋅ p i ∝ A0 ⋅ f er i
Examples:
Dipole approximation
hν = 20 eV  λ ≈ 600 Å
hν = 2000 eV  λ ≈ 6 Å

17. PES theory: Independent electrons
Dipole approximation and k-selection rule for Bloch states
momentum conservation: only"vertical"
   
k f = ki + G + k photon transitions
 ARPES

18. Transition metal oxides: electronic correlations
oxides 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. PES theory in a nutshell:
2) Many-body picture

20. Many-body effects in photoemission
N interacting electrons:
Ekin
hν
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. Reinterpretation of Fermi´s Golden Rule
Fermi´s Golden Rule for N-particle states:
2
I (ε ) ∝ ∑ Ψ f , s ∆ Ψi ,0 δ ( E N , s − E N ,0 − hν )
ˆ
s
with
Ψi ,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
  
N
ˆ = ∑ A(ri ) ⋅ pi = M if c + ci
∆ f
in second quantization with suitable one-
i =1 electron basis
one-particle matrix element

22. Electron removal spectrum
Fermi´s Golden Rule for N-particle states:
2
I (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
< 1
single-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. 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. Photoemission probing depth:
soft and hard x-ray PES

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
Ekin
courtesy
A. Damascelli

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 λ
courtesy
A. Damascelli

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 nm
atomic length scale !  access to bulk, buried nanostructures, and
interfaces
 depth profiling of thin films

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. 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. Transition metal oxides: Instability of polar surfaces
different reconstructions
Example: Fe3O4 (magnetite)
of the (111) surface (STM)
8.2 Å
PRB 76, 075412 (2007)

31. Transition metal oxides: Instability of polar surfaces
Example: Fe3O4 (magnetite)
VUV-PES Soft X-ray PES
surface-sensitive probing depth 2x larger
EPL 70, 789 (2005)

32. Surface effects in Mott-Hubbard-type oxides
U
spectral function (DMFT for n=1)
t
U/t
kinetic energy,
itinerancy
H = −t
ˆ ∑ σ
ci+ c jσ + U ∑ ni ↓ ni ↑
i , j ,σ i
local Coulomb energy,
localization

33. Surface effects in Mott-Hubbard-type oxides
Example: CaVO3
spectral function (DMFT for n=1)
quasiparticle
surface peak
"bulk"
U/t
lower
Hubbard band
A. Sekiyama et al., PRL 2004

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 / Wbulk
A. Sekiyama et al., PRL 2004

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 nm
atomic length scale !  access to bulk, buried nanostructures, and
interfaces
 depth profiling of thin films

36. HAXPES: drawbacks and caveats
Non-negligible photon momentum hν = 6 keV  λ ≈ 2 Å, kphot ≈ 3 Å-1

37. HAXPES: drawbacks and caveats
Non-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. HAXPES: drawbacks and caveats
Non-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. HAXPES: drawbacks and caveats
Non-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. HAXPES: drawbacks and caveats
Non-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
Low photoemission signal
• cross section for photoemission σ ∝ (hν )
−3
−1
• electron analyzer transmission t ∝ Ekin
 need bright x-ray source…

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 meV
other 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. HAXPES of oxide heterostructures:
(1) Fe3O4/GaAs

43. Epitaxial growth of Fe3O4/GaAs
PRB 79, 233101 (2009)
surface Datta-Das spin transistor
Fe3O4
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. 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. Valence signatures in Fe 2p spectrum
Fe2O3
Fe3O4
Fe3+
FeO Fe2+/Fe3+
charge transfer satellites
Fe2+
Fe
2p1/2 2p3/2
Fe0
700 705 710 715 720 725 730 735 740 745 750
binding energy (eV)

46. Depth profiling of Fe3O4/GaAs
PRB 79, 233101 (2009)
Fe 2p spectra
surface
Fe3O4
GaAs interface
surface

47. Depth profiling of Fe3O4/GaAs
Tuning 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. Depth profiling of Fe3O4/GaAs
PRB 79, 233101 (2009)
Fe 2p spectra
surface
Fe3O4
GaAs interface
surface
film: mixed-valent Fe2+/3+
interface: divalent and metallic Fe (O-deficient)

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. 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. Fe3O4/ZnO: An all-oxide structure
APL 98, 012512 2011
film grown by reactive deposition
Fe3O4
in O2-atmosphere (∼10-6 mbar)
ZnO
HAXPES TEM
also PLD-grown contacts: R. Gross et al.

52. HAXPES of oxide heterostructures:
(2) Interface 2DEG in LaAlO3/SrTiO3

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. 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. LAO/STO heterostructures in a nutshell
properties 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. Polar catastrophe and how to avoid it
charge: -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+
SrO
Nakagawa et al., Nature Mat. 5, 204 (2006)

57. HAXPES of LAO/STO heterostructures
Ti 2p spectrum
Ti4+
LaAlO3
2DEG
Ti3+ SrTiO3
2p1/2 2p3/2
undoped SrTiO3: |3d0>  Ti4+
doped LAO/STO interface: |3d0> + |3d1>  Ti3+/Ti4+
PRL 102, 176805 (2009)

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. Depth profiling by angle-resolved HAXPES
e- θ
e-
d
 2DEG thickness
 sheet carrier density
PRL 102, 176805 (2009)

60. Quantitative analysis: 2DEG thickness
Sample 2 uc 4 uc 5 uc 6 uc
d (uc*) 3±1 1 ± 0.5 6±2 8±2
e-
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. Quantitative analysis: sheet carrier density
Sample 2 uc 4 uc 5 uc 6 uc el. reconstruction
n2D (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. RIXS on LAO/STO
RIXS eg-excitation as fct. of # LAO-overlayers
Ti3+ (3d1)
eg
Ti 3d
photon t2g
in photon
out
Ti 2p
PRB 82, 241405(R) (2010)

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. 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. Band situation from density-functional theory
STO LAO
2DEG
E
surface
CBM
EF
VBM
core levels
Yu Lin et al., arXiv 0904.1636 (2009)
Pentcheva and Pickett, PRL 102, 107602 (2009)

66. Band situation from density-functional theory
STO LAO
2DEG
E 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. Band situation from density-functional theory
STO LAO
2DEG
E 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. Results from HAXPES
valence band Al 1s core level
~3 eV
VBM: ~ 3 eV below EF same width for all samples!

69. band theory versus experiment
STO LAO
2DEG
E
STO LAO
surface
CBM e-
EF
VBM
core levels
also observed by Segal et al.,
PRB 80, 241107(R) (2009)

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. Band alignment: A possible scenario
DFT band theory:
STO LAO localized hole states
induced by surface
O-vacancies
Photoemission:
interface states (itinerant and localized)

72. HAXPES of oxide heterostructures:
(3) LaVO3/SrTiO3 – electrostatic doping of a
Mott a insulator

73. Electrostatic doping of a Mott insulator
LAO/STO LVO/STO
LaAlO3 polar LaVO3
band ins. Mott ins.
…
∆=5.6eV ∆≈1 eV
(AlO2)- Idea:
(LaO)+ replace Al3+ by
q2DEG (TiO2)0
???
trivalent transition metal
(SrO)0
 LaVO
SrTiO3 3
SrTiO3
band ins. …
non-polar
band ins.
∆=3.2eV ∆=3.2eV
Ohtomo/Hwang, Nature 427, 423 (2004) Hotta et al., PRL 99, 236805 (2007)

74. Electrostatic doping of a Mott insulator
LVO/STO
LaVO3: - valence configuration V3+ (d2)
LaVO3 - polar oxide
Mott 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. LVO/STO: Sample growth and characterization
RHEED pattern AFM image
pulsed laser deposition
RHEED oscillations
STEM image
interface

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. HAXPES of LVO/STO: V 2p depth profiles
insulating conducting
extra electronic
homogeneous 10 uc LVO
charge on V
6 uc LVO "V3+" profile near interface
STO STO

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. LVO/STO: electronic reconstruction picture

80. Electrostatic doping of a Mott insulator
LaVO3/SrTiO3:
LaVO3 • creation of 2D metal states in a
Mott ins. correlated electron system
∆≈1 eV by interface engeering
• purely electrostatic doping
"q2DEG"
• no disorder by chemical dopants
SrTiO3
band ins.
∆=3.2eV

81. Summary
Photoelectron 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. Reading
Photoemission:
• 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)