Scientific Opportunities

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• Scientific Opportunities
• with Photoemission Spectroscopy
• Zahid Hussain
• Scientific Support Group
• Advanced Light Source
• Lawrence Berkeley National Laboratory

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OUTLINE

• What are the grand challenges?
• What are techniques of choice for
  understanding electronic properties of complex
  systems?
• Photoemission Spectroscopy
• From Einsteins Photoemission to Present and
  Future?

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The Bigger PictureGrand Challenges
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Grand Challenges!!
Energy problem search for 20 TWatts of
energy, solar energy, hydrogen fuel, nuclear
energy ??
Membrane Proteins from 3D structure of
Macromolecules to understanding
functions-dynamics
Understanding Emergent Phenomena - Phenomena
which are not the properties of the individual
elementary components BUT of the assembly of
such components Strongly correlated
electron systems - high Tc superconductor
The ultra-small - imaging of single atom in a
host of others, manipulation of single spin
The ultra-fast science in the ps-fs-as
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Why X-Rays ?Not neutrons or electrons
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Why X-Rays ( not neutrons or electrons ) ?
Tunable x-rays offer variable interaction cross
section
Electrons
Optical
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Science with Light Sources
Spectroscopy
Structure
Valence Electrons
Core Electrons
Photon Energy Wave- length
Protein Crystallo- graphy
Lithography
Nanostructures
Proteomics
Adopted from Franz Himpsel, CMMP 07
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WHAT DOES SR BRIGHTNESS BUY YOU?
Tomographic reconstruction of Saccharomyces
cerevisiae (yeast).
20 mm
Very High Energy Resolution
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IMPORTANCE FOR MATERIALS AND CHEMICAL SCIENCES
Ultimately, the
50 ?m
electronic,
magnetic,
chemical,
288.5 eV
mechanical,
3 mm
optical, thermal, and structural properties of
matter depend on the behavior of electrons.
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Techniques of SR

• The fundamental parameters necessary for
  perception of physical world
• Energy (spectroscopy, state of matter)
• Momentum (scattering)
• Position (imaging, spatial distribution)
• Time (dynamics)
• numerous techniques of SR

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Emergent PhenomenaStrongly Correlated electron
Systems
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Strongly Correlated Electronic Systems Condensed
matter physics
Rich and Novel Electronic Phenomena
Unconventional Superconductivity
Electronic Phase separation and Charge order
Phenomena
Magnetism
Ferromagnetism
Non-S-wave
Charge density waves
Spin liquid
Broken T-Reversal Symmetry
Stripes/Checkerboard order
Antiferromagnetism
Role of Bosonic Excitations
Negative - U centers
Kondo effects
Coupled charge/Spin order
Superconductivity and magnetism
Superconductivity and charge/spin ordered states
Gap Inhomogeneities
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Highly Correlated Electron System
correlated system
uncorrelated system
ground state
with external perturbation
The responses are different due to correlation
effect!
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Colossal Magnetoresistance (CMR) Effect
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Manganites Exhibit Interplay of Charge, Spin,
lattice and Orbital degrees of freedom
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Energy Scale of Important Excitations

• Superconducting gap 1 100meV
• Optical Phonons 40 - 70 meV
• Magnons 10 meV - 40 meV
• Pseudogap 30-300 meV
• Multiphonons and multimagnons 50-500 meV
• Orbital fluctuations (originated from optically
  forbidden d-d excitations) 100 meV - 1.5 eV

Requirement High Energy Resolution with High
Intensity
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What are techniques of choice?with both energy
and momentum resolution
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Fundamental Spectroscopies of Condensed Matter

• Spectral functions (One-particle properties)
• Correlation functions (two-particle properties)
• 1-particle response
• Angle resolved photoemission (ARPES)
• Single-particle spectrum A(k,w)
• 2-particle responses
• Spin Inelastic Neutron Scattering (INS)
• (neutrons carry magnetic moment)
• Spin fluctuation spectrum S(q,w)
• Charge Inelastic x-ray scattering (IXS)
• Coupled excitation in the
• Charge Channel N(q,w)
• (MERLIN/QERLIN (ALS) FEL)

?
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X-ray Spectroscopy of Condensed Matter

• Quantum Number Selectivity
• Absorption
• Angle-integrated photoemission
• Angle-resolved photoemission (also inelastic
  scattering)
• !!! Spin-polarized photoemission

w e2 Þ DE E- Ei
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Story of Modern Photoemission Spectroscopy -
100 Years Since Einsteins Photon

• Electron Spectroscopy for Chemical Analaysis
  (ESCA)
• X-Ray Photoelectron Spectroscopy (XPS)
• Photoemission Spectroscopy (PES, or ARPES)
• (study of core and/or valence electrons)
• Ultraviolet Photoemission Spectroscopy (UPS)
• (study of core valence electrons)

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100 Years of Photoemission
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Light as a Wave Thomas Young 1803
Thomas Young Light as a wave, 1803
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Maxwells Eqs Describe the propagation of
Electromagnetic radiation
James Clerk Maxwell Electromagnetism, 1865
Electromagnetic wave c turned out to be the speed
of light! 1865
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100 Years of Photoemission
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In his 1913 letter nominating Einstein for the
membership of Prussian Academy, Max Planck wrote

• In sum, one can say there is hardly one among
  the great problems in which modern physics is so
  rich to which Einstein has not made a remarkable
  contribution. That he may sometimes has missed
  the target in his speculations, as, for example,
  in his hypothesis of light quanta, cannot really
  be held too much against him, for it is not
  possible to introduce really new ideas even in
  the most exact sciences without sometimes taking
  a risk.

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Einsteins equation
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Photoemission Core Level Spectroscopy
Chemical State simple analysis
Sensitivity to Hydrogen Bonding?
(Siegbahn et al) Lab x-ray source Resolution
0.5 eV (mono)
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High resolution C1s Photoelectron Spectra of
hydrocarbon
C 1s photoelectron spectra of propyne and two
model compounds ethyne and ethane
measured. Unambiguous assignment of peaks in
propyne spectrum is made possible by
characteristic vibrational structure and ab
initio theory. Shift of the methyl (CH3) peak in
propyne relative to ethane is due to the
electronegativity of the ethyne (HCºC)
group. Previous C 1s spectrum of propyne measured
with a lab source is indicated by the dashed line
From BL 10.0 (AMO, ALS) Thomas et al, PRL
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• Identify
• Core levels
• (photoelectrons)
• Valence level
• Plasmons
• Auger electrons
• How could we separate
• Auger from Photoelectrons??

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Photoelectron probing Depth
The powerful spectroscopic tools such as XPS and
UPS might be limited in in-situ chemical analysis
because of the short penetration depth of
electrons.
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Electron Escape Depth Surface Sensitivity
Why are electrons so useful as probes of
surfaces? Or Not so useful for studying bulk
properties !!
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What are new opportunities with core level
photoemission ?In-Situ dynamical studies of
chemical reactions at surfaces
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Ambient pressure (in-situ) soft x-ray
spectroscopy

• Goal Photoemission at higher pressures
• Most surface-sensitive spectroscopic probes
  require high vacuum
• An important tool for surface chemistry
• atomic composition core-level XPS/ESCA
• chemical bonding core-level shifts
• chemical binding valence states UPS
• Synchrotron source adds power
• Variable photon energy and polarization, high
  resolution
• Resonant excitation of unoccupied states
  NEXAFS/XANES
• structure photoelectron diffraction

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Ambient pressure soft x-ray spectroscopy
Concept
controlled gas atmosphere
Photoelectrons from sample surface AND
near-surface gas
differentially pumped electron transport
synchrotron beam enters through window
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Ambient pressure soft x-ray spectroscopy
Basic Concept
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Prototype Ambient Pressure Photo-Emission System
Modify conventional surface science vacuum system
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Ambient Pressure Photoemission 10 torr
Four Pressure Zones (differential pumping)
Experimental Cell with temperature-controlled
sample, gas flow control and variable distance to
nozzle
Hemispherical Analyzer Lens
Four Electrostatic Lenses
X-rays enter through a silicon nitride window
D.F. Ogletree, H. Bluhm, Ch. Fadley, Z. Hussain,
M. Salmeron, Materials Sciences Division and
Advanced Light Source, LBNL.
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Oxidation of Silicon - Dynamics
(Himpsel et al,1985, NSLS)
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Oxidation dynamics

• Strong temperature dependence

250C
450C
Si(100) oxidized by water vapour _at_ .1 torr
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Surface/Interface Structure DeterminationPhotoe
lectron Diffraction
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Photoelectron Diffraction
Direct wave
Scattered wave
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Structure Analysis of SiO2/Si(100) by PhD
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Why momentum resolution ?Anisotropic
delocalized valence electrons
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Condensed Matter Physics
Real vs. Reciprocal Space
(Real) x-Space
(Momentum) k-Space
Localized core electrons
Constant-energy surface
Delocalized valence band electrons
Eli Rotenberg Lecture Angle-Resolved
Photoemission (ARPES)
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Angle-resolved Photoemssion Spectroscopy
Data processing
Instrumentation
Materials
Scientific issues
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Momentum Distribution of Spectral Weight
Anisotropic (d-wave) superconducting gap
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What kind of information could be extracted ?
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ARPES A Tool for Many-Body Effect
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ultra-high energy resolution ARPES- Capabilities

• Key physical properties measurable in ARPES
• Energy vs. wave vector(k) (Band Structure)
• Fermi Surface
• Mean Free Paths
• Effective carrier masses
• Scattering rates
• Electron Self energies
• Scattering or nesting vectors
• Pseudogaps and superconducting gaps
• Number of carriers

• ARPES is the technique of choice to measure these
  properties with momentum ( k ) resolution.
• The only technique that can measure all in a self
  consistent way.

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Low-Temperature Goniometer with Six Degrees of
Freedom
(1). Six degrees of freedom 3 rotational and 3
translational (2). Samp temperature 10 K (no
radiation shield) (3). Stability of sample
against temperature change
Designed and fabricated by John Pepper (BL 10.0)
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ARPES Endstation on BL10.0.1, ALS
Beam in
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Any limitations with photoemission ?gt1012
photons /sec ? space charge !Surface sensitive
(depth1nm) technique !
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Space Charge Effect in Photoemission Caution
5X1011
X. J. Zhou, Z. Hussain, Z.-X. Shen, J. Electron
Spectroscopy and Related Phenomena (2005).
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meV Resolution Spectroscopy Beamline

• Specifications
• Resolving power E/?E100,000 with 5?m slits
• i.e. better than 1 meV when photon energy is
  below 100eV
• Photon energy range 15eV to 100eV, fully
  optimized
• maximum achievable photon energy 140eV
• Elliptically Polarized Undulador (EPU) full
  polarization selection (linear and/or circular)
• Photon Flux 5 ?1011 photons/s/meV

Optical Layout (SGM)
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What next for angle-resolved photoemission? Spi
n-resolved studies Spatial_Resolved Studies
(NanoARPES, Eli Rotenberg) Time-dependent
studies
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Spin detection (two schemes)
Mott Detector
Exchange scattering interaction
Spin-orbit interaction
Reflectivity contains a term a P
M
Hint L S
e- counter
e- counter
e-
e-
M
e-
20-100 KeV
e-
Gold target (heavy nuclei)
Magnetized thin film (e.g.,Co/W(110))
2- 5 eV
x 100
FOM 10-4
FOM 10-2
R. Bertacco et al. 2001 Hillebrecht et al.
2002 R. Zdyb and E. Bauer 2003
D.T. Pierce et al. 1988 .
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Spin-Resolved Photoemission (TOF Project)

• Time-of-Flight energy analysis
• (multichannel detection in time (energy)
• highly suitable for spin ARPES as spin
  detector are single channel detection.
• 10-100 times more efficient than single
  channel dispersive analyzer)
• Exchange Scattering based spin analysis
• (100 times more efficient than Mott
  Detector)
• The main goal of project is the development of
  much improved efficiency ( 1000 times vs.
  existing (Mott dispersive analyzer) system) and
  with high energy (10meV) resolution
• TOF is inherently low noise detection as detector
  counts for short time only the time window when
  electrons of interest arrive

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Spin Resolved Examples
Cu
O
F.C. Zhang, PRB (1988)
N.B. Brookes, et al. PRL (2001)
J.-H. Park, et al. Nature (1998)
?E 0.2 eV Angle Integrated Spin Resolved
?E 0.75 eV Angle Integrated Spin Resolved
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Photoemission with circularly polarized light
and spin detection
Selective excitations (use of elliptically
polarizing undulator)
Courtesy Yulin Chen
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TOF Resolutions
?t2
?t1
E
e-
?L

• Good literature
• R.Z. Bachrach, J. Vac. Sci Tech. 12, 309 (1975)
• M.G. White, Rev. Sci. Instrum. 50, 1268 (1979)
• U. Becker, Phys. Scr. 41, 127 (1992)
• O. Hemmers, Rev. Sci. Instrum. 69, 3809 (1998)

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TOF Spin-Resolved Photoemission
Electrostatic Lenses. Long flight path at low
energy high energy resolution!
6-axis sample manipulator and cryostat
Two layers of magnetic shielding
photons
e-
Electron Flight Paths
Scattering Target
In-situ target magnetization coils
Band-Pass filter
High Speed MCP
Bent path option for higher resolution, narrow
spectra acquisition
High speed MCP electron detector for spin
integrated spectra
Straight path option for fast, wide spectra
acquisition
Target positioning manipulator
Thin film target preparation chamber
(courtesy Chris Jozwiac)
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Alessandra Lanzara
Andreas Schmid
Zahid Hussain
Nord Andresen
Chris Jozwiak
Jeff Graff
Gennadi Lebedev
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Comparison of the Hemispherical Analyzerand the
TOF Analyzer (under development)
(Bi2212 Bi-layer splitting)
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Acknowledgement

• o ZX Shen (Stanford University)
• Yi-De Chuang, Simon Muns Jonathan
  Denlinger,Eli Rotenberg, Miguel Salmeron (LBNL,
  ALS)
• Zahid Hasan (Princeton University)
• Franz Himpsel (univ of Wisconsin)
• Chris Jozwiac, Alessandra Lanzara, Gennadi
  Lebedev (UCB, LBNL)
• Chuck Fadley (LBNL, UCD)

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Conclusions
We may need to look harder but a lot of new
physics still to come!!
Thank You