Kyle Shen Stanford University

1. SSRL 2002 : X-ray Imaging and Spectro-microscopy Workshop High-Resolution Photoemission Studies of Many-Body Effects in the Solid State :"The story from Einstein's electrons" Kyle Shen Stanford University

2. SSRL 2002 : X-ray Imaging and Spectro-microscopy Workshop Angle-Resolved Photoemission at Stanford Professor Zhi-Xun Shen (Stanford & SSRL) Dr. Donghui Lu (SSRL) Other group members (past & present) : Changyoung Kim, Andrea Damascelli, N. Peter Armitage, Filip Ronning, Donglai Feng, Nik Ingle, Hiroshi Eisaki, Weisheng Lee

3. SSRL 2002 : X-ray Imaging and Spectro-microscopy Workshop The Photoelectric Effect History of Photoemission • First experimental work performed by H. Hertz (1886), W. Hallwachs (1888), von Lenard (1900) • Theoretical explanation by Einstein (1905) • FIRST EXPERIMENTAL EVIDENCE FOR QUANTIZATION OF LIGHT! Is there anything else we can learn from the photoelectric effect? Insights into the solid-state!

4. SSRL 2002 : X-ray Imaging and Spectro-microscopy Workshop Electrons in Reciprocal Space Understanding the Solid State 1) Crystal Structure? X-ray diffraction 2) Electronic Structure? Photoemission

5. SSRL 2002 : X-ray Imaging and Spectro-microscopy Workshop The interactions between the electrons and the lattice potential “One-Electron” Picture : Band Structure and Fermi Surfaces • Ignore interactions between electrons (correlations) • Consider a single electron travelling through a periodic potential • Fermi surfaces determined primarily by size and shape of Brillouin zone and number of electrons • Basis for modern calculations of electronic structure of solids Fermi Surfaces of Metals

6. SSRL 2002 : X-ray Imaging and Spectro-microscopy Workshop Energy Conservation EB= hn - Ekin - F Momentum Conservation K||= k||+ G|| Photoemission as a Probe of the Solid State Measured Quantities Ekin, q, f Desired QuantitiesEB, k||

7. SSRL 2002 : X-ray Imaging and Spectro-microscopy Workshop Copper Silver F. Reinert et al., PRB 63, 115415 (2001) F. Baumberger et al., PRB 64, 195411 (2001) A “Simple” Example : Metal Surfaces (Cu and Ag)

8. SSRL 2002 : X-ray Imaging and Spectro-microscopy Workshop The interactions between the electrons and each other, or with excitations inside the crystal : 1) A “many-body” problem : intrinsically hard to calculate and understand 2) Responsible for many surprising phenomena : Superconductivity, Magnetism, Density Waves, .... Interaction effects between electrons : “Many-body Physics” Non-Interacting Interacting

9. SSRL 2002 : X-ray Imaging and Spectro-microscopy Workshop Single-particle spectral function Observing “Many Body” Effects by Photoemission Photoemission intensity: I(k,w)=I0 |M(k,w)|2f(w) A(k,w) S(k,w) : the “self-energy” - captures the effects of interactions

10. SSRL 2002 : X-ray Imaging and Spectro-microscopy Workshop DE (meV) Dq past 20-40 2° now 2-10 0.2° Parallel multi-angle recording • Improved energy resolution • Improved momentum resolution • Improved data-acquisition efficiency State-of-the-art Photoemission F. Reinert et al., PRB 63 (2001)

11. SSRL 2002 : X-ray Imaging and Spectro-microscopy Workshop NIM/SCIENTA System SSRL Beamline 5-4 : NIM / Scienta System

12. SSRL 2002 : X-ray Imaging and Spectro-microscopy Workshop SSRL Beamline 5-4 : NIM / Scienta System • Low base temperature (~ 10 K) • Ultra-high vacuum (~ 10-11 torr) • High angular precision (+/- 0.1o) • Wide temperature range (10 - 350 K) • Variable photon energies (12-30 eV) • Multiple light sources (Plasma discharge) • Sample surface preparation & cleaning • Single crystal cleaving • Low-Energy Electron Diffraction (LEED)

13. SSRL 2002 : X-ray Imaging and Spectro-microscopy Workshop Superconductivity

14. SSRL 2002 : X-ray Imaging and Spectro-microscopy Workshop k -k Cooper Pair Bardeen, Cooper, and Schrieffer (1957) The BCS Theory of Superconductivity

15. SSRL 2002 : X-ray Imaging and Spectro-microscopy Workshop Metallic Density of States Superconducting Density of States Pb “Classic Low-temperature” Superconductors • Conventional “low” temperature superconductors • Superconductivity can only be seen on low energy scales and needs high resolution! V3Si Nb F. Reinert et al., PRL 85 (2000), A. Chainani et al., PRL 85 (2000)

16. SSRL 2002 : X-ray Imaging and Spectro-microscopy Workshop 1. Sr2RuO4 : A “Spin-Triplet” Superconductor Layered perovskite compounds “Exotic” Superconductors : Insights from Photoemission • Discovered by Y. Maeno in 1994 • Highly unconventional, low-Tc ( ~ 1 K) superconductor • Electrons “pair” together with PARALLEL spins • Strange interplay between superconductivity and magnetism 2. The High-Tc Cuprate Superconductors • Discovered by J.G. Bednorz and K.A. Muller in 1986 • Very high maximum Tc’s (current record is 167 K) • Many potential applications • Strong electronic correlations cause the cuprates to insulate at low doping levels RuO2, CuO2

17. SSRL 2002 : X-ray Imaging and Spectro-microscopy Workshop D.J. Singh, PRB 52, 1358 (1995) ARPES : present day A. Damascelli et al., PRL 85, 5194 (2000) K.M. Shen et al., PRB 64, 180502R (2001) ARPES : circa 1996 Fermi Surface of Sr2RuO4

18. SSRL 2002 : X-ray Imaging and Spectro-microscopy Workshop Mott Insulator Half-Filled Metal High-Temperature Superconductors Increase inter-electron Coulomb repulsion (U)

19. SSRL 2002 : X-ray Imaging and Spectro-microscopy Workshop ARPES Spectra of Insulating Ca2CuO2Cl2 along (0,0)-(p,p) High-Temperature Superconductors

20. SSRL 2002 : X-ray Imaging and Spectro-microscopy Workshop High-Temperature Superconductors “s-wave” “d-wave”

21. SSRL 2002 : X-ray Imaging and Spectro-microscopy Workshop Bi2Sr2CaCu2O8+d YBa2Cu3O7-d Nd2-xCexCuO4 High-Temperature Superconductors : Fermi Surfaces Ca2-xNaxCuO2Cl2 Bi2Sr2Ca2Cu3O10+d

22. SSRL 2002 : X-ray Imaging and Spectro-microscopy Workshop Advantages Limitations Advantages and Limitations of ARPES • Direct information about electronic states! • Straightforward comparison with theory - little or no modelling. • High-resolution information about BOTH energy and momentum • Surface-sensitive probe • Sensitive to “many-body” effects • Can be applied to small samples (100 mm x 100 mm x 10 nm) • Not bulk sensitive • Requires clean, atomically flat surfaces in ultra-high vacuum • Cannot be studied as a function of pressure or magnetic field

23. SSRL 2002 : X-ray Imaging and Spectro-microscopy Workshop 1. Higher Brightness = Smaller Single Crystals! Advancing the State-of-the-Art • On the materials end, appears to be fundamental issues on the achievable maximum single crystal size. Current “optimal” size on SSRL BL5-4 is ~ 1 mm x 1 mm • Electronic states of fabricated nanostructures? 2. New Insertion Devices • Circularly polarized light (EPU) should allow for novel ARPES studies of the solid state, especially in systems exhibiting dichroism (magnetism) • May be combined with spin-resolved photoemission to gain new insight into spin / orbital physics in the solid state 3. Ultrafast Pulses • Time-resolved Photoemission has been demonstrated using femtosecond lasers. An ultrafast light source (SPPS / LCLS) would provide unprecedented information into the dynamics of electrons in the solid state!