Introduction to Scanning Tunneling Microscopy:An atomic perspective on condensed matter physicsDr. Yan Pennec.Department of Physics and Astronomy, University of British Columbia, Vancouver BC CA.Contact: firstname.lastname@example.org 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
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 :)
STM Principle of operationBring a tip at within atomic reach to a surfaceMeasure a tunneling current (It) with a high gain amplifierStabilize the tip with a feedback loop on ItTrack the tip height variation as the tip is raster in the XY planeProcess the signal to form a 3D rendering of the tip trajectory
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 barrierdefined by the work function of the sample from 40*40nm STM image of the Silver 111 surface showingthe sample to the tip separated by an external four distinct atomic terraces.bias eV
II:Tunneling 201Tunneling current by first-order perturbationtheory:Tunneling matrix depends on the sample andtip wave function overlapSample and tip wavefunction can be expandedinto a two dimensional Fourier transformThe original Bardeen’s theory is applied to evaluate the overlap integral
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:
II: Lateral resolution Correction factors: s-s 1 s-d 1.66 d-d 2.77Tip 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
III: Spectroscopy STM images are contour plot of electron density STS leads to the DOS
III: Spectroscopy Lock-In Amplifier detection Local spectroscopy STS mapA lock-in amplifier is sensitive to the modulationof It induced by the an added modulation on Vb.It improves the signal to noise ratio dramatically by reducing the measurement bandwidth
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
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 meVorder 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
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 ”
VI: Breakdown of the STS=>LDOS equivalence An example (among many...).Out of plane decay length of the Phonon assisted tunneling trough acentered band exceed widely decay virtual state at point enhancelength of K point centered band dramatically the conductance
VII: Spin Polarized STM ≠ current for parallel or anti-parallel magnetic configuration between the tip and the sample. Contrast enhanced troughM. Bode Rep. Prog. Phys. 66 (2003) 523–582 a direct measurement of the SP LDOS
XtremE manipulation.Molecule CascadesA. J. Heinrich, C. P. Lutz, J. A. Gupta and D. M. EiglerScience Vol. 298 no. 5597 pp. 1381-1387
VIII: Time resolution with an STM. Time lapse microscopy. resolution ~s
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
Pulsed STS.VIII: Time resolution with an STM. resolution ~25ps Bias induced Spin Tracking spin relaxationPump 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
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 @ 200V60mm 52mm 3D Inchworm 10*5*5mm travel In situ Body 3 S-SMA 40 GHz connectors Tip exchange
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
Instrumentation: UBC semi-commercial STM (700k$) UHV < 10-10 mbar Full MBE/LEED /Cleaving Low noise sub 1pm RMS Sub 10pm/hr drift (constant height mapping available) Full STS capability 1meV resolution Point local, map, full grid Low consumption cryostat 7L N2, 1L He /day External Air damping legs + Internal spring/eddy dampers 5K Base temperaturecreatec.de, lt-stm.com.
Instrumentation: Under construction UBCA microscope and spectrometer for novel quantum materials. 1.5M$home built UHV-GHz-2DT-ULT-STMSurface characterisationLocal electronic density of statesFermi Surface and band dispersionMagnetismElectronic dynamics50fm/√Hz low noise environment30 mK temperature2 SPM headMBE preparation chamberLEED/FIM/PES Analysis chamber4*40 GHz bandwidth + 8 DC.7z/2x Tesla SC Magnet
Instrumentation: Under construction UBCLaboratory 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
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
Case study I: Sr2RuO4Our starting point for this study:Why the surface electronic structure of Sr2RuO4 depends on the cleaving temperature? High T cleave Low T cleave
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
Sr2RuO4: Atomic resolution and spectroscopy 0.5 STS dI/dV a.u. 5High T I (nA) 0.0 -0.5 0 -400 -200 0 200 400 Bias (mV) 10000 8000 dI/dV a.u. 6000Low 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!
Sr2RuO4: Understanding atomic resolution in Sr2 RuO4Charge 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
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 scatteringcenter 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
polar neutral YBCO BISCCOProbing 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
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 0.2Large 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.
Quasi Particle Interference on BSCCO Quasi Particle Interference. From real state LDOS modulation to momentum space “Fermi” surface
Phase diagram 56x56nm map of the of bulk cuprates gap widthof UD Bi2212 Spatial Gap inhomogeneity in BSSCORVB modelling STS of Dy-Bi2212
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!
STM-STS on YBCO6.5: CuO terminationCDW 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)
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.
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
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