Topic Name: Angle-Resolved Photoelectron Spectroscopy
Professor Zhi-Xun Shen
Professor of Physics and Applied Physics - Stanford University and SSRL
Location: McCullough Building 342
476 Lomita MallStanford University,Stanford, CA 94305-4045,, United States
Photoelectron spectroscopy is a general term which refers to all techniques based on the photoelectric effect originally observed by Hertz. This was later explained as a manifestation of the quantum nature of light by Einstein, who recognized that when light is incident on a sample, an electron can absorb a photon and escape from the material with a maximum kinetic energy, hυ-φ, where hυ is the light energy and φ is the work-function of the material. The energy of an electron inside a solid can be obtained using photoelectron spectroscopy—the core electrons will have a lower kinetic energy than the valence electrons when absorbing the same photon energy. In principle, the momentum of the electrons can also be obtained—different momentum electrons will escape at different angles from the surface of a material. However, since the electrons are being projected through the surface, the momentum perpendicular to the surface is not conserved. Therefore, angle-resolved photoemission is ideal for 2D materials where the principle momentum directions of interest are parallel to the surface. One of the primary materials of interest are the 2D copper-oxide perovskite family in which the undoped parent compounds are anti-ferromagnetic insulators and the doped compounds have the highest known superconducting transition temperatures, yet to be described by theory. In practice, the electrons ejected from the material are collected using a hemispherical detector in which lens voltages direct the electron onto a two-dimensional (energy, momentum) multi-channel plate. The sample and the detector are kept in an ultra high vacuum (UHV) chamber in order to minimize surface contamination. Light sources are either synchrotron radiation at ~20-100eV, plasma Helium discharge at ~20eV, or more recently, modern-day lasers ~ 6 eV.Since ARPES measures the energy and momentum of electrons inside a solid directly, it has a very natural theoretical description. The initial state of the electrons evolves, upon interaction with light, to a final state with a transition probability given by Fermi’s golden rule:see In Eqn:1 In picturewhere the interaction Hamiltonian is given by:see In Eqn:2The initial state of this dipole-interaction Hamiltonian is a neutral solid, while the final state is a dipole—the hole in the material left by the ejected electron and the photoelectron itself, which is a combination of the light and the ejected electron. If one assumes that the solid does not relax in the time it takes to measure the photoelectron (the so-called “sudden approximation”), then the intensity (I) of the spectral weight can be thought of as:See in eqn:3 inpicturewhere I0 describes a matrix element dependent on incident photon energy, and f(w) is the Fermi function, signifying that ARPES measures the occupied density of states.
A (k, ω) is the single-particle spectral function that describes, theoretically, the energy and momentum of an electron in a solid. The “bare-band” electron dispersion is denoted by εk, while the “self-energy”, Σ, describes all the complex interactions that an electron has with other electrons and the lattice in a solid. This description is based on Landau’s Fermi liquid theory in which electrons, though part of a solid, remain independent “quasi-particles” near the Fermi level. They can therefore be understood as relatively free electrons, but with a renormalized dispersion due to interactions. Differences in how electrons interact lead solids to form metals, insulators, magnets, or superconductors. ARPES allows direct access to the electron spectral function through which these interactions take place.
About Professor Zhi-Xun Shen :
Experience, Service and Awards
Dr. Shen is the Paul Pigott Professor in Physical Sciences. Dr. Shen has been a Professor of Physics, Applied Physics, and SSRL since 2000, an Associate Professor (1996-2000), and Assistant Professor (1992-1996). He is also a Professor of Electrical Engineering by courtesy. Dr. Shen is currently the Director of the Geballe Laboratory for Advanced Materials. He is also the Director of X-Ray Laboratory for Advanced Materials at SLAC Campus. He is also the Co-Director of the Stanford-Chevron Program for Diamondoid Nanoscience.
His awards include: Sloan Research Fellow (1993); Materials Science Research Award for Outstanding Scientific Accomplishment in Solid State Physics, Office of Basic Energy Science, Department of Energy (1994); American Physical Society Centennial Lecture (1999); Kammerlingh Onnes Prize (2000); The Takeda Foundation Techno-Entrepreneurship Award (2002), American Physical Society Fellow (2002); Paul Pigott Professorship in Physical Sciences (2006).
Physics of Quantum Matter: including superconducting, magnetic, ferroelectric and dielectric materials, organic conductors and superconductors, low-dimensional compounds, quantum phase transitions, elementary excitations and collective modes, Kondo and mixed valence problem, magneto-resistive materials, metal-insulator transition.
Interaction between Light and Matter, and Advanced Spectroscopy, Scattering and Imaging Techniques: synchrotron radiation and free electron laser, high-resolution photoelectron spectroscopy with angle, spin and time resolution, inelastic x-ray scattering, laser based photoelectron spectroscopy and microcopy, soft x-ray emission, and Raman spectroscopy.
Physics of the Ultra-Small and Ultra-Fast: nanostructured materials, scanning microwave microscopy, time resolved photoemission spectroscopy, pump probe experiments.
Surface and interface properties of materials - metals, semiconductors, insulators, superconductors, thin film growth and characterization.
Education, Teaching and Publication
Dr. Shen received his Ph.D. in Applied Physics from Stanford University in 1989, M.S. from Rutgers University in 1985, and B.S from Fudan University in 1983.
He has mentored about thirty five graduate students and postdoctoral fellows; fifteen of them work in industry, business and government, while about twenty of them joined the faculty of major universities in North America, Asia and Europe
He has more than 200 publications, including 55 in three important journals of his field: Nature, Physical Review Letters, and Science. Six of his papers have been identified by the citation tracking algorithm of the Institute for Scientific Information (ISI) as among the most cited recent papers in its periodic surveys.