1. The electronic structures of 2D atomically uniform thin film S.- J. Tang, T. Miller, and T.-C. Chiang Department of Physics, University of Illinois at Urbana- Champaign, 1110 West Green Street, Urbana, Illinois 61801- 3080

2. Surface state and Quantum well state Surface Thin Film Vacuum Substrate SS qw

3. Photoemission Excited electron and hole states Excited hole state Excited electron state (photoelectron) Photon (Final) (Initial) Work function E f –E i = E k +e  = E i +

4. One particle picture to many body effect Many body interaction Spectral function Spectra function in the form of a Lorentzian I ~ Photoemission intensity Quasi particle One state -----> One single Lorentzian peak

5. Quantum well states Bohr-Sommerfeld quantization rule n: Quantum number, 1,2,3,..  : interface phase shift Nt: thickness All the information of substrate bands are embedded in  i

6. Quantum size effect Ag bulk band perpendicular to the surface Ag qws bands parallel to the surface, which are normally similar to the corresponding bulk band dispersions

7. Atomically uniform film revealed by photoemission qws peak 1 ML 1.5 ML 1 ML qws peak 1 ML2 ML

8. Atomically uniform + quantum size effect Thickness changes (uniform films) The allowed quantized k  ’s changes The energy positions of the QWS changes Properties of films change Surface energy, work function, thermal stability, adsorption, superconductivity transition temperature, magnetic interlayer coupling………. J. J. Paggel, T. Miller, and T.-C. Chiang, Phys. Rev. Lett. 81, 5632 (1998); Science 283, 1709 (1999)

9. Substrate effect Substrate Thin film

10. My three major findings on substrate effect 1.Many body interaction between the quantum well state and substrate state Kink dispersion S.-J. Tang, L. Basile, T. Miller, and T.-C. Chiang, Phys. Rev. Lett. 93, 216804 (2004) 2.Strong hybridization between the surface state and substrate bands Display of HH,LH and SO major substrate bands S.-J. Tang, T. Miller, and T.-C. Chiang, Phys. Rev. Lett. 96, 036802 (2006). 3.Diffraction from the substrate surface QWS of second kind S.-J. Tang, Y. R. Lee, S. -L. Chang, T. Miller, and T. -C. Chiang, Phys. Rev. Lett. 96, 216803 (2006)

11. Surface Brillouin zone for Ag(111) and Ge(111)

12. The band structures of atomically uniform thin film of Ag/Ge(111) 21 ML ss QW 1 2 3 4 5

13. Band dispersions of Ag/Ge(111) Two types of splitting !! Kink Dramatic linewidth change of QWS across substrate band edge Layer resolution The band structures of QWS in 8.6 ML are equal to the linear combination of 8ML and 9ML. Emission Angle (  ) ss QW n= 1.2 … Type 2 Type 1

14. Atomically uniform film Ag/ Ge(111)

15. Atomically Uniform Film 1. moving discretely from its 8 ML ( 9 ML) position to the 9 ML (10 ML) position. 2. The 8ML (9ML) peak is absent in the 9ML (10ML) spectrum and vice vesra. Quantum well peak

16. Non quasiparticle behavior The two-peak line shape across the substrate band edge is non quasiparticle One dimensional form of the density of the states for the substrate band edge Spectral function Retarded Green function for a single electron state entering into the bulk band continuum.

17. Surface state 2D electron state residing on or near the surface The product of reduced symmetry Nt1/q When 1/q  Nt Decay lengthThickness Substrate film Vacuum Something interesting happens !!! Surface state touches down to the substrate

18. The anomalous lineshape of the surface state at lower thickness Ag/Ge(111), Normal emission Quantum well state ? Substrate state ?

19. Thickness dependence of the surface state dispersion Decay length of Ag surface state  12ML Surface state energy shifts with the thickness New finding !!! Emission Angle (  ) T. C. Hsieh, T. Miller, and T. C. Chiang, Phys. Rev. Lett. 55, 2483 (1985)   11.7

20. Thickness dependence of the surface state dispersion Strong hybridization between the surface state and substrate states makes the photoemission intensity proportional to the density of states for substrate bands Three major Ge bands of HH, LH and SO cross this area Emission Angle (  )

21. Fitting the data with the model (a) (b) (c) (a) (b) (c) Fitting result from the model Image processed

22. Comparison with the calculated Ge bands and other measurements HH LH SO Expt Cyclotron measurements 0.30 0.034 0.21 0.33 0.043 0.095 Effective Mass Spin-orbital Splitting Expt 0.301 eV 0.296 eV Optical measurements

23. Quantum well state of the second kind (Ge)

24. Ag bulk bands projected to (111) direction

25. Surface Brillouin zone for Ag(111) and Ge(111)

26. Vacuum Substrate (k ||,k  )(k ||,-k  )(0,-k  )(0,k  )(-k ||,k  )(-k ||,-k  ) First Kind

27. The Ge substrate working as a diffraction grating Vacuum 1 2 3 4 Second Kind

28. Generalized Bohr-Sommerfeld quantization rule, d= Nt, t=2.36 Å for 1ML of Ag in (111) direction Due to energy conservation

29. Hybridization between qws of the first kind and qws of the second kind

30. Thickness dependence of hybridization between two different kinds of quantum well states 9ML bands and bands anti- cross interactions V (d) = ?

31. Relationship between quantum well state and surface state from 5 ML down to 1ML QWS SS QWS +SS ?

32. Second kind of quantum well from 5ML down 1ML

33. Substrate is the key

34. Doping effect on the quantum well states e- Lightly doped n type substrate Heavily doped n type substrate film Depleting area film Electronic Fringe Structure in Silver Films on Highly Doped Silicon, N. J. Speer, S. -J. Tang, T. Miller, and T.-C. Chiang*, recently accepted by Science.

35. Atomically uniform film+ quantum size effect + substrate effect High Tc Magnetism bulk film Substrate

36. Future research plan Photoemission 1.Investigate the temperature dependence of the surface states, energy shift and electron-phonon coupling, on the metal surfaces like Be, Be, Al(100), Al (110) surfaces. The central idea is to study the interplay between the electron structures and lattice structures on the surfaces. 2.Investigate the structures and behavior of surface states and film states (quantum well states) in an atomically uniform thin filmfor various systems like Ag/Ge(111), Pb/Ge(111), Co/Ge(111) and Ag/Cr(100). 3.Study the electronic structure of quantum wire, quantum dots, nanotubes and complex materials.

37. Future research plan Low energy electron microscopy (LEEM) 1.Study the surface morphology of metal surface or metal cluster. (Time dependence and Temperature dependence)  example Cr(100) Sublimation of Atomic Layers from a Chromium Surface, S. -J. Tang, S. Kodambaka, W. Swiech, I. Petrov, T. -C. Chiang, Phys. Rev. Lett., in press. 2. Study the quantum size effect of metal thin film ( the intensity of the islands oscillates with electron energy)  example Ag/Cr(100)

38. 445 C 467 C 1 HOUR LATER Ag/Cr(100)

39. 7.5 eV 16.7 eV 38.5 eV 51.5 eV Ag/Cr(100)

40. I V curve qws

41. Surface Superconductivity on Bulk Be =0.24, T c = 0.024 K =0.646, T c = 17 K Localized S1 state Small coherence length  HTSC Dominant e-p coupling with 50-80 meV optical phonon MgB 2 1.Light atoms 2.covalent like sp B-B bonding with DOS at Fermi level 3. Dominant coupling with in-plane B-B vibration phonon about 64 meV

42. Back bonding model  rehybridzation of broken surface bonds Hybridization between S2 and SR states SRS2 Charge density distribution of SR and S2 on (0010) cut plane 1 4 5 8 9 12 S and Pz type Py type (J-H Cho )

43. Back bonding model  rehybridzation of broken surface bonds Hybridization between S2 and SR states SRS2 Charge density distribution of SR and S2 on (0010) cut plane 1 4 5 8 9 12 S and Pz type Py type (J-H Cho )

44. Inversion symmetry