Introduction to Scanning Tunneling Microscopy for download mail to - culprit1234@gmail.com

1. Introduction to Scanning Tunneling Microscopy:
An atomic perspective on condensed matter physics
Dr. Yan Pennec.
Department of Physics and Astronomy, University of British Columbia, Vancouver BC CA.
Contact: ypennec@physics.ubc.ca
Lecture outline
 General introduction
Principles of operation
Basic theoretical framework
Capabilities
 Instrumentation
STM
UBC LAIR
 Application to Oxides
Surface characterization of Sr2RuO4
Probing High Tc Cuprates
YBCO vs. BSCCO
UBC-MPI Quantum Materials Institute Summer School August 2011

2. The gallery of atomic resolution
Cu (111) Si (100) Graphite HOPG
Metal Semiconductor Bi-dimensional crystal
SrRuO2 Co-TPP on Cu(111) H20 on Au (111)
Oxide Functional molecules Snowflake :)

3. STM Principle of operation
Bring a tip at within atomic reach to a surface
Measure a tunneling current (It) with a high gain amplifier
Stabilize the tip with a feedback loop on It
Track the tip height variation as the tip is raster in the XY plane
Process the signal to form a 3D rendering of the tip trajectory

4. 3D rendering of Bismuth 111

5. I: Tunneling 101
It decays exponentially with an increasing barrier width
Characteristic decay length equal 10-10m ,
the size of an atom!
Plane wave travelling trough an energy barrier
defined by the work function of the sample from 40*40nm STM image of the Silver 111 surface showing
the sample to the tip separated by an external four distinct atomic terraces.
bias eV

6. II:Tunneling 201
Tunneling current by first-order perturbation
theory:
Tunneling matrix depends on the sample and
tip wave function overlap
Sample and tip wavefunction can be expanded
into a two dimensional Fourier transform
The original Bardeen’s theory is applied to evaluate the overlap integral

7. II: Lateral resolution
Tunneling matrix element is proportional
to the sample wavefunction at tip center:
The charge density of the sample at
the tip center can be estimated using
atom charge superposition:
charge density:

8. II: Lateral resolution
Correction factors:
s-s 1
s-d 1.66
d-d 2.77
Tip change during a single scan
of CO on Cu(111)
Convolution of a localized tip state with the sample
with an unstable WAu tip The shape of the tip is critical for atomic resolution

9. III: Spectroscopy
STM images are
contour plot of electron density
STS leads to the DOS

10. III: Spectroscopy
Lock-In Amplifier detection
Local spectroscopy
STS map
A lock-in amplifier is sensitive to the modulation
of It induced by the an added modulation on Vb.
It improves the signal to noise ratio dramatically
by reducing the measurement bandwidth

11. III: Spectroscopy “Textbook” example.
The 2DEG Shockley surface state.
1D model of the surface symmetry breaking
Electrons are influenced by the
oscillatory potential of the
crystal which stop brutally at the
surface
=> Requires the resolution of Schrödinger equation

12. III: Spectroscopy “Textbook” example.
The 2DEG Shockley surface state.
Electron density  surface
Photoemission
N. Memmel, Surf. Sci. Rep. 32, 91 (1998)

13. III: Spectroscopy “Textbook” example.
The 2DEG Shockley surface state.
0.3nm
4nm
STM of Ag(111) surface overlaid with a dI/dV STS map in binding energy: -63 meV
order to enhance the standing wave pattern of the surface effective mass m*: 0.40 me
state electrons reflected at the step edges Fermi wave length: 7.6 nm
population: 0.011 e/10Å2

14. III:

15. IV: Momentum space Spectroscopy.
Quasi Particle Interference in an 1D channel
h 2k 2
E  E0
2me
STM of an Half closed 1D resonator
STS conductance map 1D FFT

16. V: Local Spectroscopy.
DFT DFT
LUMO 0 +1 LUMO +2 +3
1007Mv 2130Mv
STS STS

17. VI: Breakdown of the STS=>LDOS equivalence
An example (among many...).
Calculated band structure.
Note the Dirac cone @ K point
STS shows a wide +/- 60meV
STM on Graphene unexpected gap at “Ef ”

18. VI: Breakdown of the STS=>LDOS equivalence
An example (among many...).
Out of plane decay length of the  Phonon assisted tunneling trough a
centered band exceed widely decay virtual state at  point enhance
length of K point centered band dramatically the conductance

19. VII: Spin Polarized STM
≠ current for parallel or anti-parallel magnetic
configuration between the tip and the sample.
Contrast enhanced trough
M. Bode Rep. Prog. Phys. 66 (2003) 523–582 a direct measurement of the SP LDOS

20. VII: Spin Polarized STM of Fe3O4

21. VII: Spin Polarized AFM of NiO

22. Atomic manipulation
-4.4
Tip heigth (A)
-4.8
-5.2
0 5 10 15 20 25
Current (nA)
50
40
0 5 10 15 20 25 30
X (A)

24. XtremE manipulation.
Molecule Cascades
A. J. Heinrich, C. P. Lutz, J. A. Gupta and D. M. Eigler
Science Vol. 298 no. 5597 pp. 1381-1387

25. VIII: Time resolution with an STM.
Time lapse microscopy. resolution ~s

26. VIII: Time resolution with an STM.
Real time tracking of
tunneling current.
resolution ~100us
Tip induced switching of a Si dimer
Kinetic Monte Carlo modelling of phason motion
A spin chain becomes instable due to the presence of a phase defect
Motion activated by inelastic tunneling of hot electrons in the empty state of Si 100

27. Pulsed STS.
VIII: Time resolution with an STM. resolution ~25ps
Bias induced Spin Tracking spin relaxation
Pump probe scheme excitation above Magnon with time dependent spin
threshold polarized tunneling
Ultrafast Spectroscopy with a STM.
Ian Moult, Marie Herve and Yan Pennec..
Applied Physics Letters

28. Instrumentation: from a few pennies to M$++

29. Instrumentation:
One of UBC STM head
High resonant frequency. RF= 4775Hz
Tube dimensions OD 6.35, ID 5.35 L 16.933
Small scanning range Dx=267nm @ 200V
60mm
52mm
3D Inchworm 10*5*5mm travel In situ
Body 3 S-SMA 40 GHz connectors Tip exchange

30. Instrumentation:
UBC home built in air GHz STM (6k$ STM + 60K$ controls)
Acoustic enclosure
Air damping legs
Home-built (including stick slip
piezoelectric nano-motors)
Ultra low noise electronics
Air/Liquid/optical access
Atomic resolution
High resolution spectroscopy
3 GHz Bandwidth

31. Instrumentation:
UBC semi-commercial STM (700k$)
UHV < 10-10 mbar
Full MBE/LEED /Cleaving
Low noise sub 1pm RMS
Sub 10pm/hr drift
(constant height mapping
available)
Full STS capability
1meV resolution
Point local, map, full grid
Low consumption cryostat
7L N2, 1L He /day
External Air damping legs +
Internal spring/eddy dampers
5K Base temperature
createc.de, lt-stm.com.

32. Instrumentation: Under construction UBC
A microscope and spectrometer for novel quantum materials. 1.5M$
home built UHV-GHz-2DT-ULT-STM
Surface characterisation
Local electronic density of states
Fermi Surface and band dispersion
Magnetism
Electronic dynamics
50fm/√Hz low noise environment
30 mK temperature
2 SPM head
MBE preparation chamber
LEED/FIM/PES Analysis chamber
4*40 GHz bandwidth + 8 DC.
7z/2x Tesla SC Magnet

33. Instrumentation: Under construction UBC
Laboratory for Atomic Imaging research: LAIR. 2M$
Three “pods” will house:
Createc UHV 4K STM, Omicron UHV 4K STM/AFM, Home built UHV 50mK STM
Double 30cm thick concrete wall for acoustic isolation, no ventilation
Floating inertia block of 70T, 40T and 20T
5.5 m true floor to ceiling height, 3 m effective

35. STM on “Quantum Materials”
What help can STM provide?
Crystallography
Step height
Surface termination
Surface periodicity
Surface reconstruction
Monitoring defect
Electronic properties
Spectroscopy
Fermi surface
Case studies
Sr2RuO4
Cuprates: BiSCCO vs YBCO

36. Case study I: Sr2RuO4
Our starting point for this study:
Why the surface electronic structure of Sr2RuO4 depends on the cleaving temperature?
High T cleave Low T cleave

37. Sr2RuO4: Step edges
High T cleave Low T cleave
Both low and high T cleaves presented the same step height.
The 6.4A step height corresponds to a full unit cell of the crystal

38. Sr2RuO4: Atomic resolution and spectroscopy
0.5
STS dI/dV a.u.
5
High T
I (nA)
0.0
-0.5 0
-400 -200 0 200 400
Bias (mV)
10000
8000
dI/dV a.u.
6000
Low T
4000
2000 Atome 1
Atome 2
Black hole
0
-150 -100 -50 0 50 100 150
Bias mV
For both cleaving temperatures:
metallic-like spectra
“Atomic” Periodicity: 3.8 A. (full lattice unit) corrugation ~ 10pm
Superimposed modulation ~ 4 pm => signature of a reconstructed surface!

39. Sr2RuO4: Understanding atomic resolution in Sr2 RuO4
Charge density isolines from High Resolution STM showing
DFT at a height of 2.13 A °. the √2x √2 reconstruction
DFT tells us that “atomic” resolution arise from extended Sr centered wavefunctions
STM shows an added modulation.
=> structural rotation of the O tetrahedrons arising from surface FM

40. Sr2RuO4: Cleaving induced defects
Large scale STM topograph of sample cleaved at high
temperature shows dramatic increase in surface
defect density.
a) 10 10 nm2 STM images obtained from a 200 K cleave.
Two types of characteristic defects are show
with false color maps in (b), the protrusion, and (c), the hole.
Blue dots are the Sr locations on the SrO terminated surface,
(d) Fully relaxed DFT for a charge neutral SrO molecule
missing from the surface.
While not affecting the surface reconstruction nor the LDOS a small density of scattering
center appears sufficient to remove any signature of the surface LDOS in ARPES.
=> Could be potentially use as lever to differentiate surface from bulk state in ARPES

41. polar neutral
YBCO BISCCO
Probing superconductivity in Cuprates: YBCO vs. BISCCO
BSCCO cleaves between two charge neutral BiO planes: surface  bulk
YBCO cleaves between a charge neutral BaO plane and a +1 CuO chains
polar surface requires electronic reconstruction ≠ bulk

42. STM-STS on BSCCO
O
Bi
“Atomic” resolution showing the Bi
atoms at the surface.
1.0
0.8
STS (a.u.)
0.6
0.4
0.2
Large scale topograph showing a ~5b 0.0
-200 0 200
incommensurate supermodulation Bias (mV)
Typical STS showing an asymmetric behaviour
and a clear d-wave gap with sharp quasi particle
coherence peaks.

43. Quasi Particle Interference on BSCCO
Quasi Particle Interference.
From real state LDOS modulation to momentum space “Fermi” surface

44. Phase diagram 56x56nm map of the
of bulk cuprates gap widthof UD Bi2212
Spatial Gap inhomogeneity in BSSCO
RVB modelling STS of Dy-Bi2212

45. STM-STS on YBCO6.5: CuO termination
FFT
10.63A
7.7A
3.85A
No d-wave Gap, no “atomic” resolution
Instead => Charge density wave!

46. STM-STS on YBCO6.5: CuO termination
CDW phase inversion upon bias inversion
-800mV +800mV
0.4 B
B
0.2
Z (A)
0.0
-0.2
-0.4
0 2 4 6 8
X (nm)

47. STM-STS on YBCO6.5: BaO termination
Ef ?
BaO plane presents a square lattice, but slightly reconstructed!
STS shows an asymetric behavior similar to BSCCO link to Mott physics.

48. BaO surface reconstruction
possible driver of the CuO electronic structure
T081115.081541.dat
V090408.173225.dat
Biasvoltage: 0.50000V Current: 5.0E-10A Temperature: 999.98999 [K]
Biasvoltage: 0.05000V Current: 1.0E-09A Temperature: 999.98999 [K]
0 100 200 300 400
0 100 200 0 300 400
0
0.8
0.7 2
100
0.6
100
0.5
1.5
200
0.4
0.3
200
300
0.2 1
0.1
V090408.153835.dat
Biasvoltage: 0.50000V Current: 3.0E-10A Temperature: 999.98999 [K]
YBCO 7 YBCO 6.5 YBCO 6.5
400 0
300
0.5
0 10 20 30 40 50
0
1.6
10 1.4
400 0
1.2
20
1
0.8
30
0.6
0.4
40
0.2
50 0

49. Conclusion
 It allows real space characterization of surface
morphology with “atomic resolution”
 It can be used as an atomic trowel
 It is one of the most surface sensitive probes
 It can performs electronic spectroscopy with sub mV
resolution both for the occupied and empty states
 It can probe time dependent phenomena from hours
down to few picoseconds
 It will play a key role in the understanding and
engineering of complex oxide

50. Some additional reading
 In touch with atoms G. Binnig
 Feynman lectures on physics, Vol. III
 Ziman Theory of Solids
 Guntherodt & Wiesendanger Scanning Tunneling Microscopy
 C. Julian Chen, Theory of spin-polarized STM and AFM
 Introduction to STM. P. Gambardella
http://www.icn.cat/~ams/lectures/UABmaster_PG_9_10.pdf
 Spin Polarized STM
M. Bode Rep. Prog. Phys. 66 (2003) 523–582
 Magnetism in ultrathin film structures. C A F Vaz1, J A C Bland and G Lauhoff
Rep. Prog. Phys. 71 (2008) 056501 (78pp) doi:10.1088/0034-4885/71/5/056501
 Theories of scanning probe microscopes at the atomic scale
REVIEWS OF MODERN PHYSICS, VOLUME 75, OCTOBER 2003
Werner A. Hofer, Adam S. Foster, Alexander L. Shluger
 A 10 mK scanning probe microscopy facility
Young Jae Song, Alexander F. Otte, Vladimir Shvarts, Zuyu Zhao, Young Kuk, Steven R. Blankenship, Alan
Band, Frank M. Hess, and Joseph A. Stroscio
Rev. Sci. Instrum. 81, 121101 (2010)
 Scanning tunneling spectroscopy of high-temperature superconductors
Øystein Fischer,* Martin Kugler, Ivan Maggio-Aprile, and Christophe Berthod, Christoph Renner
REVIEWS OF MODERN PHYSICS, VOLUME 79, JANUARY–MARCH 2007