• Invention of STM
• Field emission from metal surface (Fowler & Nordheim)
Quantum Mechanical Tunneling
p Ek
L Esaki
I Giaever
Quantum Mechanical Tunneling
To be able to measure a tunneling current the two metals must be spaced no more than about 100Å apart, and we decided early in the game not to attempt to use air or vacuum between the two metals because of problems with vibration. ...... After a few months we hit on the correct idea: to use evaporated metal films and to separate them by a naturally grown oxide layer.
Nobel lecture, I Giaever, 1973
Quantum Mechanical Tunneling between Metals
Insulator
Density of States (DOS):
energy level
DOS
Density of States (DOS):
energy level
DOS
Density of States (DOS):
energy level
DOS
No tunneling current
Tunneling between two metals
Quantum Mechanical Tunneling between Metals
Tunneling spectroscopy
Tunneling spectroscopy
V
V
Tunneling: an approach to measure DOS
Example: Coulomb interaction + disorder ⇒ DOS ~ E1/2 at EF Al’tshuler & Aronov (1979)
Al
Al2O3
Ge1-xAux
Pb
MgO
Mg
Inelastic excitations in barrier
dI(V ) dV
V
∝
dI /d
V V
Elastic tunneling
E0
Development of Microscopy
Leeuwenhoek Bacteria
Leeuwenhoek: father of microscopy
1933
Erwin W. Müller
Albert Crewe
Nobel Prize in Physics, 1986
Gerd Binnig: superconductivity Heinrich Rohrer: superconductivity, Kondo, phase transition Christoph Gerber: joined IBM in 1966, worked with HR, craftsmanly, inventor of AFM
“...... gave us the courage and lightheartedness to start something which should not have worked in principle”
None in microscopy or surface science
Motivation: Local study of growth and electrical properties of thin insulating layers as tunneling junctions
Goal: not to build a microscope, but to perform spectroscopy locally on an area less than 100 Å in diameter
Contact over insulating film
Instead of scanning tip in contact over a surface, a small gap of a few angstroms was maintained and controlled by the
tunneling current
Not only a local spectroscopic probe But spectroscopic and topographic imaging
Profilometer
1978
Si(111) 7x7 (1982) 1998
How to avoid mechanical vibrations that move tip and sample
against each other?
How to move a tip on such a fine scale?
How to move the sample on a fine scale over long distances?
How to avoid strong thermally excited length fluctuations of
sample and tip?
2st generation spring
2st generation spring
Tip movement The continuous deformation of piezomaterial in the angstrom and sub- angstrom range was established only later by the tunneling experiments themselves.
Piezoelectric Materials
EFM, electrostatic force microscope
FMM, force modulation microscopy
MRFM, magnetic resonance force microscopy
NSOM, near-field scanning optical microscopy
PFM, piezo force microscopy
SCM, scanning capacitance microscopy
SECM, scanning electrochemical microscopy
SGM, scanning gate microscopy
SVM, scanning voltage microscopy
SSM, scanning SQUID microscope
4STM Instrumentation
Pre-amplifier
dI/dV: Lock-in amplifier
dI(V )
= I(V ) cos (!t + ) +
EBL#2:
t
t
L
y
-Vy
Smaller scanning range
Scanning range ~ micron
shear piezo
Hexagonal Prism
Acoustic-isolation room
1
1
2
Vibration noise
UHV STM-Clean Surcace
High vacuum: 10-7 to 10-9 torr Ultra high vacuum: < 10-9 torr
Mean free path • 10-9 torr: 105 m • 10-10 torr: 106 m • 10-11 torr: 107 m
Monolayer formation time • 10-9 torr: 103 s • 10-10 torr: 104 s • 10-11 torr: 105 s ~ days
Cryogenic STM
• Higher energy resolution
• Low thermal drift
• Slow down dynamics
10-5K fluctuation @ 300K
10-2K fluctuation @ 4K
Al
Cu
SS
Ti
Cryogenic STM-Continuous Flow
• Variable T • Rapid cooling down • Compact • High LHe consumption • No magnet
Cryogenic STM-Bath Cryostat
Cryogenic STM-Bath Cryostat
1.0 m 10 mK 15 T @NIST
Tsinghua
Ultra low temperature STM
1.0 m 10 mK 15 T @NIST 1.52 m 1.52 m
1.41 m
• WO3 soluble in strong base • Oxide reduced to metal
Etching tungsten tip • AC etching: blunt but less oxide • DC etching: sharp but more oxide • Optimal procedure: AC+DC+Strong acid
Tip etcher
DC etching
STM Tip
Ar+
Ar+
~ 1kV
12uA
Repeated cycles of heating and self-sputtering in vacuum to remove oxide and sharpen tip
STM Tip
e-A
STM Tip
LV field emission in vacuum to fix blunt, multiple tip
5 ~ 10V 1 sec
Sample
180V
9V
Sample
Sample
Silicon
I
I
II
Bi2Te3
KFe2Se2
Looking for structures (peak, dip, step) in dI/dV Spectroscopic imaging
-13.6 eV
-3.4 eV
-1.5 eVd
50 meV
100 meV
150 meV
300 meV
200 meV
x y
EnergySpectroscopic imaging
En = ED + sgn(n)vF
p 2eB~|n|
Massless Dirac fermion
Graphene
Electronic States-Landau Quantization
TI: Bi2Se3
-300 -250 -200 -150 -100 -50 0 50 Sample Bias (mV)
0 T
d I/d
20ML
22ML
Electronic States-Quantum Confinement
Electronic States-Quantum Confinement
9Å off center
Circle’s center
dI /d
Peaks at circle’s center Extra peaks 9Å off center
0.6
0.4
0.2
0.0
-0.2
-0.4
Ni Al
Atomic chain
-8 -4 0 4 8 Wave Vector (10 m )9 -1
m = 0.5 meff e 0.0
0.5
1.0
1.5
2.0
2.5
3.0
9
HOMO split LUMO LUMO+1
Theory
Exp
Crommie et al, PRL 2003
Electronic States-Kondo
F1 F2
Cu(111)
1
3
5
7
9
11
13
K
M
q only in Γ-M directions
Backscattering is forbidden in topological insulator
Electronic States-Standing Wave
Γ-K
Γ-M
K
M
Γ-K
Γ-M
K
M
E0
Ho, et al, Science 280, 1732 (1998)
Cu(100)
358 meV: stretching mode of C2H2
Inelastic Electron Tunneling Spectroscopy (IETS)
358 mV
Cu(111)
CO
dimer
trimer
-40
-20
0
20
40
Eigler, et al, Science 298, 1381 (1998)
STM Topography of array of CO dI/dV image at 35.5 mV
Inelastic Electron Tunneling Spectroscopy (IETS)
[001]
[110]
Ho, et al, Science 286, 1719 (1999)
20
0
-20
Δ=gµBS·B
Spin flip spectroscopy
Inelastic Electron Tunneling Spectroscopy (IETS)
H=JS1·S2
Spin flip spectroscopy
Spin flip spectroscopy
4 5 6 7 0.4
0.6
0.8
0.8
0.9
1.0
B=0 B=2.8T B=4.2T B=5.6T B=7T
S ca
le d
d l/d
Pb
dI /d
V
Sample Bias (mV) -40 -20 0 20 40-40 -20 0 20 40
Spin flip IETS of spin chains
2 spins 3 spins 4 spins
0.66J
J S = 1/2 J = 18 meV
H = J S1 . S2+J S2 . S3H = J S1 . S2 H = J S1 . S2+J S2 . S3+J S3 . S4
11T
5T
1.5T
Sample Bias (mV) 17 18 19 20 21 22 23
dI /d
V (a
Inelastic Electron Tunneling Spectroscopy (IETS)
S = 1
S = 0
B=0 B=0
Singlet to triplet transition
5Å
????
????
????
???? ???? ????
???? ????
-10 -8 -6 -4 -2 0 2 4 6 8 10
0.05
0.10
0.15
0.20
???? ????
???? ????
????????
-10 -8 -6 -4 -2 0 2 4 6 8 10
0.05
0.10
0.15
0.20
0T
1T
3T
5T
????
????
????
???? ???? ????
???? ????
-10 -8 -6 -4 -2 0 2 4 6 8 10
0.05
0.10
0.15
0.20
???? ????
???? ????
????????
-10 -8 -6 -4 -2 0 2 4 6 8 10
0.05
0.10
0.15
0.20
0T
1T
3T
5T
D =-1.55 meV E = 0.31 meV
Spin Polarized STM
Tip Sample E E
[001]
[011]-
Spin Polarized STM
W tip
[001]
[011]-
Readout Output
Bbias Bpulse
Co CoFe
1 nm
Spin Polarized STM
`_
T(K) 0 0.5 1.0 1.5 2.0TC
c (m
ill ijo
ul es
/m ol
e- K
Superconductivity
Pb
Hg
Nb
NbN
Nb3Sn
V3Si
Nb3Ge
CNT diamond YbC6
0
10
20
30
40
dI/dV map at -0.21 mV
Superconducting gap
Sm
U
0
2
4
6
Mn
Si(111)
Pb
Mn Cr
dI /d
0.5
1.0
1.5
2.0
2.5
-1.5 mV
kx (2!/a)
1/2
1
q1
q2
q3
Superconductivity
0 10 20 30 40 50 60 70 80 90 0
5
10
15
20
25
30
35
40
!k "( !)
( m
eV )
1
2
3
4
5
0
1
2
3
4
5
0
• J. A. Stroscio & W. J. Kaiser, Scanning tunneling microscopy
• R. Wiesendanger, Scanning probe microscopy and spectroscopy