Photoemission from a surface scientist's view
Shu-Jungg Tangg
Department of Physics and Astronomy, National Tsing Hua University,
Hsinchu 30013 ,Taiwan
National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan

Heinrich Rudolf Hertz

Albert EinsteinElectromagnetic
What is Light ?
(EM) waves• Hertz helped establish the photoelectriceffect ((which was later explainedpbyy Albert
Einstein) when he noticed that a charged
object loses its charge more readily whenilluminated by ultraviolet light. In 1887, hemade observations of the photoelectriceffect and of the production and receptionof electromagnetic (EM) waves.

Original photoelectric experiment

New Journal of Physics 7 (2005) 97

Photoelectric Effect Apparatus• When the tube is kept in thedark, the ammeter reads
zero• When plate E is illuminatedby light having anappropriate wavelength,
wavelength acurrent is detected by theammeter• The current arises fromphotoelectrons emitted fromthe negative plate andcollected at the positiveplate

K1  U1  K 2  U 2
K1  0  0  (e)(V )K max  eVs

Adopted from the book: Physics for Scientists and Engineers

Photoelectric Effect, Work
Function• Electrons ejected from the surface of the metal
and not making collisions with other metal atoms
before escaping possess the maximum kinetic
energy Kmax
• Kmax = hƒ –φ
Einstein’s idea
– φ is called the work function– The work function represents the minimum energy
with which an electron is bound in the metal

Adopted from the book: Physics for Scientists and Engineers

Free particle motion
Free-particle
For a free particle,

1 2 p
k
E  mv 

2
2m 2m
2

2

2

Dispersion relationship

E(k) Parabolic curve
If the particle is an electron, then the E(k) relationshipdefines the electronic structures of the corresponding
crystal material.
material We also call E(k) as energy band
dispersion

The electron shells of an atom

2S
1S

Adopted from the book: Physics for Scientists and EngineersSplitting of Energy
Levels, final• When this splitting is
extended to the large
number of atoms present
in a solid, there is a large
number of levels ofvarying energy• These levels are so
closely spaced they canbe thought of as a band
off energy levelsllAdopted from the book: Physics for Scientists and Engineers

Metals – Energy Bands• At T = 0, the Fermi energy
lies in the middle of the band
– All levels below EF are filledfand those above are empty• If a ppotential difference isapplied to the metal,
electrons having energies
near EF require only a small
amount of additional energy
from the applied field toreach nearby higher energy
levelsE(k)Adopted from the book: Physics for Scientists and EngineersSemiconductors –Energy Bands• Appreciable numbersof electrons arethermally excited into
the conduction band
• A small applied
potential difference caneasily raise the energyof the electrons into the
conduction bandE(k)Adopted from the book: Physics for Scientists and Engineers

Insulator – Energy Bands• Th
The valencelband
b d isi fillfilleddand the conduction bandis empty
p y at T = 0• The Fermi energy lies
somewhere in the energy
gap• At room temperature,very few electrons would
be thermally excited into
the conduction band

Adopted from the book: Physics for Scientists and Engineers

The prototype Photoemission
system back in1916

Modern

Adopted fromhome pagePhotoemission
1 X
1.
X-ray
ray Photoemission spectroscopy (XPS)
Photon energy > 100 eV
Mainly looking at the localized Core level state
2 UV Photoemission spectroscopy (UPS)
2.
Photon energy
gy < 100 eV
Mainly looking at the delocalized anddidispersive
i electronicl t i statet t ini th
the conductiond ti
and valence bands

Energy range of the electrons
sensitive to the surface

The Principle of Photoemission energy
conservation h

Emission angleθEkin      EBe-

Ek    e  Eb

Light source
UV lamp (He I = 21.2 eV, He II =40.8 eV )Synchrotron radiation



XPS

Adoptedpfrom lecture materials on lineAdopted from lecture materials on lineAdopted from lecture materials on lineAdopted from lecture materials on lineAdopted from lecture materials on lineAdopted from lecture materials on lineAdopted from lecture materials on lineAdopted from lecture materials on line

UPS

ARPES

Angle resol ed Photoemission Spectroscop
Angle-resolved
Spectroscopy
Mainly measuring the energy band dispersions of theelectrons in valence level of crystalline solids

Energy versus momentum

E(k)Direction is important !!!

Adopted from lecture materials on line

Energy band to energy band
dispersions

Adopted from lecture materials on line (Eli Rotenberg)e

Th
Three-stept modeld l ffor photoemission
h t
i iConservation of
momentum andenergyMomentum conservation and nonconservation through the inetrfacekout outk

2mE kin
sin 

2


out


k

2mE kincos 
2Surface umklapp effectini//Fi l stateFinal
t t effect
ff t

 k  k Ginf

kout  k inf   kiin  GThroughTh
h a ffree particle
ti l fifinall bband
dapproximation

k

in
f

2m
 ( 2 ( Ekin cos 2   U 0 ))1/ 2
In a crystal, the lattice undergoes a translational
symmetry
Translational vector in a real
spaceElectron number density
1D caseIn 1D case, real lattice areciprocal lattice 2/a

3D caseThe reciprocal (k) lattice
construction of the reciprocal latticeTranslational vector in a real
space

T

Translational vector in a reciprocal(k)space

a useful relation iswith this it is easyy to see whyy

T

Adopted from lecture materials on line

In two dimensions

T

Real lattice

Reciprocal lattice|a1|=a||a2||=b|b1|=2π/a|b2|=2π/bReal lattice

BCC

reciprocal lattice

FCC

The fcc lattice is the reciprocal of the bcc lattice andvice versa.

Condition for diffraction in a crystal
ylattice

Laue condition

Kittel: Chapter 2

Define vector d connecting the planes

1D reciprocal lattice in this direction:
Bragg’diffractionAdopted from lecture materials on line

Effect of periodic potential is mostdramatic on states of k = Gn/2
(the boundaries of Brillouinzones): It opens energy
bandgaps at k = Gn/2 separatingallowedlld energy bands
b d

1st Brillouin zone

Only need to consider states in 1st Brillouin zone
2D surface Brillouin zone
constructed from
LEED patterns of Ag films

Adapted from lecture materials on line

1st Brillouin
zone

fcc

Energygy bands
in Si crystal
Ei(k) along someaxes in 1st
Brillouin zonebccAdapted from lecture materials on line

Projection of bulk states in surface Brillouin zone

Zone center and zone boundaries are
Brillouin
the symmetry2-D
pointsin the reciprocal
zones
space so the energy band dispersions
3-D should be symmetry about these points.

Brillouin
zonesAdopted from lecture materials on line

True Surface States & Surface Resonances

36 5809 , 1987
PRB 36,

Adopted from lecture materials on line

E (k, k )Bulk states (3D)vs.
vsSurface states (2D)E (k )

E. W. PLUMMER, W. EBERHARDT, Advances in Chemical Physics,
Volume 49kk inf   kiin  G

k inf   kiin  G

Final state effect only for bulk state

Why not for 2D states?

Photon energy dependence of theb lk andbulk
d surface
f
state
t t
final

stateBeffectSSk
T. -C.Chiang, Surf. Sci. Rep. 39, 181 (2000) k f   ki   G Ekin      EBinin

Photoemission intensity
I ( Ekin , k ,  ) ext|| M (k , k )   ( E  E   )   (k  k  G )
2

fi

f

i

i , f ,G ,g ||Selection rulefiEnergyconservation ( Ekin    E f )   (k ||ext  k ||int  g|| )Detection byenergyanalyzer

Momentumconservation
parallel tosurfacefiMomentumconservation

Photoemission and many body interactions
Photoemission intensity

  2



IV (E k )  
M fi (k f , ki )  ( A  Ah )(E f  Ei   )  L(k f   ki  G) 
f ,i ,G , g 






  (k f   ki   G )  DE (Ekin  E f  )  Dk (kV   k f   g )0kin, V 

Surface Science Reports 30,153
30,153-206
206 (1998)

Spectra function for the
initial hole stateA(k ,  ) Im (k ,  )   (k )  Re (k , )  Im (k , )
20Spectra function is the form of aLorentzian
Quasiparticle picture
Sum rule A(k , )d  1
2

For a 2D system, we can makea lot of simplification
Peak intensity, peak position

Photoemission intensity
I ~Many bodyinteractions
Line width , Line shape
Spectra function
for photohole
Quasi particleA(k ,  ) Im (k ,  )   (k )  Re (k , )  Im (k , )
0

2Spectra function in the form of a Lorentzian

2

M
Many
b
body
d effects
ff t

excitedit d systemt

Quasiparticle and quasihole are referred to as excited states
induced by the many-body interactions
One particle groundstate energy

T t l excitedTotal
it d energy E

Self-energy0kk   ( k , Ek )

  k0  Re ( k , Ek )  i Im ( k , Ek )screening

decay or scattering process

Adapted from lecture materials on line (Eli Rotenerg)

Symmetry selection rule

Adopted from lecture materials on line

The way to determine the symmetry of initial
electronic state

Final state(even)P polarized
light (even)AA
S polarized
light (odd)



p

s




Initial state( even or odd))

Adopted from lecture materials on line (Eli Roternberg)

We focus on 2D material sytems
Supposedly there should be no final
Supposedly,
final-state
state effect!
I will give examples to introduce five major aspects ofphotoemission spectra
1. Peak position
2. Peak intensity
3 Peak line width3.
4. Peak line shape
5. Energy band dispersions

The electronic structures of 2D systems,y, metalsurface and thin film
Ward Plummer

Surface States on Tungsten
Phys. Rev. Lett. 25, 1493 (1970).

T
T.-C.
C Chiang

Quantum-Well States in a Metallic System:
Ag on Au(111,Phys. Rev. Lett. 61, 1404 (1988)
Observation of film states and surface-state precursors
for Ag films on Si(111), Phys. Rev. B 33, 1460 (1986)

Surface state and Quantum well
state
t t
VacuumSS

SR

Surface
Broken symmetry
Vacuum
QWR
QWS Q
Q

SS

Reduced symmetryThin Film
Substrate

Interface
state
t t

2D electron state residing on or near thesurface

Surface state
The product of reduced symmetry
Vacuumk  p  iq

 (z)  e cos(pz   )qz

1/q
Decay length

film

Nt
Thickness

When 1/qNt

Surface state touches down to the substrate
Substrate

Something interesting happens !!!

Quantum well states
Bohr-Sommerfeld
quantization rule2n s (E) i (E)
k 
2Nt

En  E ( k  )n: Quantum number, 1,2,3,..
N: the number of layers.
t : theh thicknesshi k
off one layerli+ s :interface and surface phaseshift
All the information ofsubstrate bands areembedded in iSurface state (SS)

Quantum well state (QWS)

B

C

d

2k ( E )d  B  C  2n

N. V. Smith. PRB. 32, 3549 (1985)

Iwao. Matsuda et al. E-J. Surf. Sci.
Nanotech. Vol. 2 (2004) 169-177

Quantum well state and surface states within the energy gap
Ag
ss
QWS

Au bulk band edge

ss

QWR

Cu bulk band edge

PRB 36, 5809 , 1987

Metal surfaces and metal thin films
Ag/Ge(111)– 6ML
Be(1010)

Pb/Ge(111)-( 3  3) R30

How many physics stories
have been generated
from these three pictures?

Peak positionp(Thickness dependence of thin films)
Surface states: Due to the interaction
with the substrate when the decayy
length of the surface state is longer
than thickness. The decay length canbe extracted from the thicknessdependent energy shift .
Phys. Rev. B 65, 125319 (2002)
Phys. Rev. Lett. 55, 2483 (1985)
Quantum well states: Due toquantum size effects governed by
Bohr Sommerfeld quantization rule.
Bohr-Sommerfeld
rule
The phase shift at the interface canbe extracted from the thicknessdependent Energy shift.
shiftPeak position(Tempearure dependence)

PRB 65,65 235428 (2002)(2002), PRB77
PRB77, 045405 (2008)
How about QWS?
Temperature dependence of surface state peak positionindicates the surface thermal expansion behavior

Peak position(Adsorption dependence)Phys. Rev. B 70, 205426 (2004).

Adsorption dependence of SS peak position determines the type ofadsorption chemisorptions (charge transfer) or physisorption (van der waals
adsorption,
forces) .

Peak position(Adsorption dependence)Phys. Rev. B 86, 155453

dEd
d  dE  1
 dE  
2 Nt
2t  dk   N
 dE 
 dk 
 

We can even extract the dielectric
constantt t off theth adsorbeddb d materialst i l
via the shift of QWS peak position

3.4eV/  2
3.4eV
d2i 2i(TTC/ Ag) 2i(VAC/ Ag)  ( ) ( )
EV(TTC/ Ag) E(TTC/ Ag)
EV(VAC/ Ag) E(VAC/ Ag)Peak intensity(photon-energy dependence))

PRB 36, 5809 , 1987

PhotonPht energy dependencedd
off theth surface
fstate
t t peakk intensity
i titdetermines the surface state decay lengthPeak intensity(TemperatureTtdependence)dd)

PRB 60,60 12 , 1999

Temperature dependence of SS and QWS peak intensity
determines the Debye temperature via Debye- Waller factor

Peak intensity(Temperature dependence of thin films)
Pb films on Ge(111)

QWS as the indicator of Thermal stability of thin films

Peak linewidthA(k ,  ) Spectral functionIm (k ,  )   (k )  Re (k , )  Im (k , )
2

0

2

(
(Temperaturepdependentp)Im (,T)  peak width  surface state hole life time electron-phonon interaction
electron-electron interaction
surface defect scatteringg
I
Imaginaryipartt off self
lf energy

Im  ep ( , T ) 





 max

 2   2 F ( ' )[1  f (   ' )  2n( ' )  f (   ' )]d 'Eliashberg coupling function
to represent phonon densitiesof state0   if   2

 F ( )   
2

D

when
when

 D
T 
kB

T  DkB

 2 F ( ) 

E
2

D

Debye model (general )Im  ep ( , T )  T

3



Im  ep ( , T )  2k BT (   E )Einstein model (special)Temperature dependence of S1 and S2 surface states at A

Extracting EP coupling strength
Peak wwidth(eV)

0.35

(b)

Debye modelS1 att A

0.30

Einstein model fitting results

Einstein modelE  64meV

0.25

S 1  0.646  0.02

0.20

(bulk  0.24  0.05)

0.15
0

100

200

300

400

Temperature(K)

500

600

700

Debye model fitting results

Peak Widdth (eV)

0.60
0.55

S2 at A

 D  60 meV

0.50

S 2  0.491  0.04

0.45
0.40

(bulk  0.24  0.05)

0.35
100

200

300
400
500
Temperature (K)

600

700

Peak lineshape
player
y resolution
Normally, the sharp and intense lineshapes of SS and QWS
indicate the smooth surfaces and uniform thin films.
Th can furtherTheyf h reveall theh llayer reolutison
l i
1.5 ML

1 ML

qws peak1 ML

qws peak
1 ML

2 ML

Atomically uniform + quantum
size effectThickness changes (uniform films)
The allowed quantized k’s changes
The energy positions of the QWS changes
P
Propertiesti
off films
filchangehSurface energy, work function, thermal stability,
adsorption superconductivity transition
adsorption,
temperature, magnetic interlayer coupling……….

J J.
J.
J Paggel,
Paggel T
T. Miller
Miller, and T
T.-C.
C ChiangChiang, Phys.
Phys Rev.
Rev Lett.
Lett
81, 5632 (1998); Science 283, 1709 (1999

Peak lineshape layer resolution
Layer resolution revealed by thelineshape of surface state of Ag films
on Au(111)APPLIED PHYSICS LETTERS 92, 163102
2008

Peak lineshape many bodyinteractions
A(k ,  ) Quasi-particle
Quasi
particleIm (k ,  )   (k )  Re (k , )  Im (k , )
0

2

2(typical): One state  Lorenzian shape

Non Quasi-particle behavior (special): One state Non-Lorenzian shape
The lineshape for the layer resolution shown before arepof two states of two Lorenzian shapes.
pcomposedElectronic phonon coupling of
surfacefstatest t
Re  ep(,T) The shift of the hole state energy from the groundstate
t t energy due
d to
t the
th many bodyb d interaction
i tti
 Relevant only near Fermi surface within thephonon energy
gy rangegmaxmium p
 Ionic screening causing renormalized hole state velocity oreffective mass m   (1   ) m
Surface state band distortion on crossing phonon energies
e p coupling strength
e–p

most
relevant  Re  ep ( ) 0Peak lineshape many body interactions
electron phonon interaction
SS of Be(0001)()

At T= 0 K

U i D
Using
Debye
b M
Model
d l

Kramer-Kronig relation
Between imaginary part andreal part of self energy
PRB 60, 10796 (1999)

PRB 61, 2371 (2000)

PRL 84, 2925 (2000)
Be(0001)
Be(1010)I n t e n s it y ( a r b . u n it )720x10

3

700

680

660

-0.6

-0.4

-0.2
Initail energy (eV)0.0

Different lineshapes to reveal
coupling to different phonon densityof states
Different lineshapes to reveal
coupling to different opticalHphonon energies   161
meV,  D  117 meV.

Debye model ( D  70meV ) forBe(0001) and Einstein model (E  64meV)f Be(1010))for
B (1010)

Peak lineshape
pmany
y bodyyinteractions electron electron interaction
The two-peak line shape across thesubstrate band edge
is nonquasiparticle
Retarded Green function for asinglei l electron
l t
state
t t enteringt iinto the bulk band continuum.

g   

2

E0

V



E    i s

G  E   E  Eq  i  q  
1g  dOne dimensional form ofA
E0   
E0  
the densityy of the states

for the substrate band
Spectral function edge

1
1 
1



E



Im

G



Im
A(E) 
  E  E  i  AV 2  E  E  i 1/ 2 


q

q

0

Phys. Rev. Lett. 93, 216804 (2004)sAg films on Ge(111)
HH

Ge band edges

LHsoBand dispersionsp Layery resolution
Ag films on Ge(111)Band dispersions electron-phonon
interactions
Be(1010) S1 surface state

From momentum dispersion curves (MDCs)
Be(1010)0.00

dispersion of S1 surface state(through MDC'S)

I n it ia l E n e r g y ( e V )-0.05
optical phonon ~ 64meV
-0.10

-0.15

-0.20
0.70

For more information regarding using MDCs to
extract self –energies, please check

0.72
0.74
-1
K parallel (A )0.76

Be(10 1 0) :Real part of self-energy
Real part

20x10

-3

Be(1010)
S1 surface stateRe (eV)R

15

Extrapolate F () ?
2 Re  ep ( )

 0


10

5

maximumi
entropytmethodth d

0

Optical Phonon ~64 meV

-5
5

-0.30

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

Energy (eV)Distortion band subtracted byy free electron bandPhys.
y Rev. Lett. 92,, 186401 (2004)()
Ncomms 7559 (2015).

Online material: https://web2.ph.utexas.edu/~jrshi/VAS11.pdf

Band dispersions electron-electron interactions
Strong e-e interaction between Pb QWS subbands and Ge band edges

Phys. Rev. Lett. 107, 066802 (2011)

The electron-electron interaction
model for film 2ML
The density of states of QWS is dominated by the Ge bulk band edges
so we can simply use the density of state model to do the fitting

S
Spectrat functionfti

I  E   QWS V bandedgesA(E)density of states (1D form)3g E  i 1

Ai
Ei  E  Ei  E 2g E  M g E

At each K point2g ( E ) ~  dk ( f   i )g ( E ) ~  dkA(k ,  i )for HH, LH and SO band edgesof bulk Ge

The electron-electron interaction
model for film > 2ML
By using Anderson's model for the electron
electron-electron
electron interaction
between the discrete states and continuous states, we do the fitting.
retarded Green’s function
3

Vi



E    iG  E   E  E q  i q   
1

i 1

density of statesg i   Ai
E 0i  

E 0i   

2

E 0i

i
s

g i  d

For HH, LH and SO band edgesof bulk Ge

spectral function





1
1
1

A  E    Im
I  G    Im
I 
3

1/
2
2

 

i
i








E
E
i
A
V
E
E
i

q
q
i
i
s
0


i 1





Vi  QWS Vi bandedgesElectron-electron interaction
becomes the driving force forthe growth of Pb films in (1x1)
configuartionfiti

1/N decaying

Electronic match versus Lattice match
Phys. Rev. Lett. 107, 066802 (2011)

Band dispersions electron-electron interactions

N
Newns-Anderson
A d
model
d l tto explainl i ththe adsorptiondtiiinduceddd Gap
G statet t
Nature Communications 4, 2925 (2013)

Band dispersions Surface
umklapp effectK

Ge(111)

Pb/ Ge(111)  3  3R30



M


 3
G  G out  in  in k  k f   ki  G

45eV

Bi 2Å on Pb(@LT

45 V
45eV

Energgy(eV)Pb thin film 4ML@LT

K//(Å-1)

M

Energy(eeV)



 Bi
G
 Pb
G


K//(Å-11)

 Bi
G

M

 out  in  in   Bik  k f   ki  G , G  2G

Band dispersions effects from theunklampp scattering at the interface(k1  k2  k3  k4 ) * d    2n, n  1,2,3,4(k1 (E1)  k2 (E2 )  k3 (E3 )  k4 (E4 ))* d   2n
Due to energy conservation(k1(E )k2(E )k3(E )k4(E ))*d  2n

KNEW ( E )d    2 n,d= Nt ,t=2.36 Å for 1ML of Ag in (111)
direction

Phys. Rev. Lett. 96, 216803 (2006)

First Kind(k||,k)(k||,-k)

Vacuum(0,-k)(0,k)

Substrate(-k||,-k)(-k||,k)SecondKind

Vacuum1( k  M , k  )2

3(k M , k  )(k M ,kk  )4(k  M , k  )

Th Ge
The
G substrateb t t working
ki as a diffdiffraction
ti grating
ti




K M  K M  GG  2K M

Substrate is the keyyBand dispersions hybridization
 anti-crossingtii  E1 ( k )
V 
H 

V
E
(
k
)
2


E1 (k),E(k) E2(k)  The
Th twotoriginal
i i l energy
band dispersions before
hybridization interaction
V  The hybridization interaction
matrix elements
A particles
Au
ti l adsorbedd b d on A
Ag/Si(111)/Si(111) 3  3R 30 surfacef
PRL 96, 036803 (2006)

Varied hybridizations from metal surfaces and metal thin films
Be(1010)

SS-SS band hybridization
PRB 77,77 045405 (2008)

Ag/Ge(111)– 6ML

Pb/Ge(111)-( 3  3) R30QWS-QWS bandyhybridization
PRB 78, 035443 (2008)Large
g inward relaxation
Negative thermal expansion

Surface state-bulk statehybridization
PRL 96,
96 036802 (2006)
PRB 81, 245706 (2010)
New J. Phys. 16, 045003 (2014)

Ag films on Ge(111)Beautiful
Film

So So
Film

Film qualities revealed by the QWS-QWS band hybridization

Band dispersions Rashba effect
1. Strong spin‐orbital coupling
2. Breaking of the inversion symmetry with time‐reversal symmetry preserved out‐of‐plane potential gradientE(k,) = E(k, )
3. Heavy‐metal crystal surfaces, Au(111), Bi(111), Bi(100), W(110)…
4. Heavy‐metal ultra thin films on semiconductors, Bi/Si(111), Tl/Si(111),
Pb/Ge(111)- 3  3 R30, Bi/Ge(111)- 3  3 R30, and Au/Ge(111)3  3 R30……
5 Heavy-and-light-element
5.
Hd li h lalloysll
on metall surfacesf
BiAg,
BiA PbAg,
PbA SnAg
S A alloys
llon Ag(111) In‐plane potential gradient
6 ?Heavy‐and‐heavy‐element
6.
dhlt alloyll on semiconductor
i d t surfacesf, PbAuPbA alloy.
llSpin-orbital coupling for 2D free electron gas
Assume no crystal potential in kll direction

2

k


Hˆ 
  R (k  zˆ )2mk  k xiˆ  k y ˆj

The corresponding
eigenvalues are： 0  ( R m /  2 )
k
ER 
2m
2m

2 2
0

2 ER
R 
k0

One way to determine
the Rashba parameterAnother way to determine th
Rashba p
parameter 
The third way to determine
the Rashba parameter Au(111)Spin‐ and angle‐resolved PES

M. Hoesch et al., Phys. Rev. B 69, 241401(R) (2004)

Large Rashba splitting for
Pb/Si(111)-3 R30

Nature Commun. 1, 17 (2010).

Isotopic and anisotropic Rashba splittinggBiAg alloyBiAg = 3.05 eVÅ
Rashba splitting at 
Bi/Si(111) trimers

 = 1.37 eVÅ

PRL 98.186807 (2007)
 K

PRL 103.046803
103 046803 (2009)K

Rashba splitting at M
In‐plane Inversion symmetry breaking along horizontal mirror planeRashba +HybridizationPhys.
y Rev. Lett. 101,, 107604 ((2008))

Rashba +Hybridyzation

New J. Phys. 16, 045003 (2014)

Electronic structures of PbAu alloy
with one‐layer
y cappingpp g Au ( T = RT))

Matrix element effect for the coupling of the orbital
symmetryt withith bbeam polarizationl i ti• PbAu alloy film

0

New J. Phys. 17, 0830153 (2015)

Above all, observation and imagination withan excessively optimistic and curious mind is
the most important key!!!
2
2.

1
1.

SS

QWS
A fiimsAg
fii
on G
Ge(111)(111)
3
3.
accidental overnight annealing

Pb fiim on Au/Ge(111)-Root3

PbAu alolly /Pb fiim/Ge(111)-Root3

Conclusion
What I introduced are only based on my past
researches on various surface and thin-film
systems.
t
Th researchTheh fields
fi ld andd materials
t i lsystems that photoemission have been
employed to study are too large and too
many. Nevertheless, I hope some ideas I
pass to you for extracting physics from thephotoemission data can be helpful for yourfuture researches.