Loeb Lectures in Physics: Zhi-Xun Shen

November 4, 2019
Zhi-Xun Shen portrait

Paul Pigott professor of Physical Sciences, Professor of Physics and Applied Physics, Stanford University
Professor of Photon Science, SLAC National Accelerator Laboratory 

Monday, November 4, 2019, @4:30pm
Jefferson Lab 250 

Colloquium: "Angle-Resolved Photoemission – a Many-Body Spectroscopy for Quantum Materials"

Complex phenomenon in quantum materials is a major theme of physics today.  As better controlled model systems, a sophisticated understanding on the universality and diversity of these solids may lead to revelations well beyond themselves. Angle-resolved photoemission spectroscopy (ARPES), formulated after Einstein’s photoelectric effect, has been a key tool to uncover the microscopic processes of the electrons that give raise to the rich physics in these solids. Over the last three decades, the improved resolution and carefully matched experiments have been the keys to turn this technique from a band mapping tool to a sophisticated many-body spectroscopy. In recent years, the availability of modern ultrafast UV laser and spin polarimetry makes photoemission capable of measuring all the important microscopic quantities of electrons – energy, momentum, spin and time dynamics – and thus great insights from such rich and high precision information. 

In the first Loeb lecture, I will describe the general concept, key milestone and ingredients of modern ARPES.  After the introduction on the technique and its evolution, I will showcase four examples that illustrate the power of this many-body spectroscopy: i) the anisotropic superconducting gap in unconventional superconductors; ii) enhanced superconductivity of mono-layer FeSe grown on SrTiO3; iii) correlation enhanced electron-phonon interaction; iv) towards spin-orbit decomposition of electron wave-function.

In the Loeb lectures II and III, I will discuss two science problems where ARPES has made the biggest impact – cuprate high temperature superconductors (Lecture II) and topological materials (lecture III).

Tuesday, November 5, 2019, @3:00pm
Jefferson Lab 250

"Electronic Phase Diagram of Cuprate Superconductors – a Balancing Act"

High-temperature superconductivity in cupper based materials, with critical temperature well above what was anticipated by the BCS theory, remains a major unsolved physics problem more than 30 years after its discovery. The problem is fascinating because it is simultaneously simple - being a single band and ½ spin system, yet extremely rich - boasting d-wave superconductivity, pseudogap, spin and charge orders, and strange metal phenomenology. For this reason, cuprates emerge as the most important model system for correlated electrons – stimulating conversations on the physics of Hubbard model, quantum critical point, Planckian metal and others.

Heart of this challenge is the complex electronic phase diagram consisting of intertwined states with unusual properties. Angle-resolved photoemission spectroscopy has emerged as the leading experimental tool to understand the electronic structure of these states and their relationships. In this talk, I will describe our results on band structures and Fermi surfaces; d-wave superconducting state; the birth of a metal from a Mott insulator; the two energy scales of the pseudogap; the temperature, doping and symmetry properties of the low energy pseudogap and its competition with superconductivity; the missing quasiparticle and the chemical potential puzzle, the interplay of electron-electron and electron-phonon interactions and the enhanced superconductivity; the incoherent metal sharply bounded by a critical doping and quantum critical point. The rich phenomenology suggests that a delicate balance between local Coulomb interaction and electron-phonon interaction holds the key to cuprate physics. 

Wednesday, November 6, 2019, @3:00pm
Jefferson Lab 250 

"Electronic Structure of Topological Materials"

Instead of classifying matters by broken symmetries, a new paradigm of topological classification emerged in the 1980s with the discovery Quantum Hall insulator. In such a system, there exists a novel surface state at the interface where the topological invariant changes, due to so-called bulk-surface correspondence. It is then recognized that in spin-orbit coupled system, a situation can occur where the spin-orbit interaction plays an analogous role as the external magnetic field in QHI. Extending to 3D system, this leads to topological insulators. Here, topological invariant can be defined using information encoded in the wave-function. In practice, however, it is easier to look for energy band inversion in systems where the spin-orbit interaction opens an energy gap and a non-trivial surface state appears. If the band gap closes to zero, the material becomes a Dirac semimetal. Symmetry breakings in topological insulators and Dirac semimetals lead to interesting physics such as topological superconductor, Majorana fermions, massive Dirac cone, quantum anomalous Hall, Weyl semimetal and surface Fermi arcs.

Due to its surface sensitivity, ability to directly map the band structure with high momentum resolution, as well as spin-texture determination capability, ARPES has played a crucial role in the field of topological materials. It identified key early topological insulators, the massive Dirac-cone, the Dirac semimetals, and more recently Weyl semimetals and topological superconductors. I will describe these experimental progresses, the challenges, for example the complexity in interpreting spin texture data in spin-orbit coupled systems, as well as the practical utility in guiding the preparation of thin films that led to the first demonstration of quantum anomalous Hall effect. 

17 Oxford Street, 
Cambridge, MA

Dr. Zhi-Xun Shen is the Paul Pigott professor of Physical Sciences, Stanford University; Professor of Physics and Applied Physics of Stanford University; and Professor of Photon Science, SLAC National Accelerator Laboratory.  He is a member of the US National Academy of Science, the American Academy of Arts and Sciences and a Foreign Member of the Chinese Academy of Sciences.  His primary interest is the novel quantum phenomena in materials. His contributions have been recognized by the DOE E.O. Lawrence Award, the APS Oliver E. Buckley Prize, the International H. Kamerlingh Onnes Prize, and the Einstein Professorship Award of Chinese Academy of Sciences. He served as the Chief Scientist of SLAC National Accelerator Laboratory, the Director of the Stanford Institute for Material and Energy Sciences and the Geballe Laboratory for Advanced Materials of Stanford University.