CMP Kid's journal club at Stanford

Eric Spanton, Moler Group

Imaging transport current using scanning SQUID microscopy

We use a scanning superconducting quantum interference device (SQUID) to investigate magnetic phenomena on a micron scale. By locally imaging magnetic fields due to current flowing through a sample, we are able to reconstruct a local 2D current density. I will highlight applications of this technique to two different physical systems. First, I will present images of current through two different quantum spin hall systems (HgTe and InAs/GaSb quantum wells), showing edge currents when the devices are tuned into their respective gaps using a top gate. Secondly, I will show images of current through LAO/STO which show an unexpected high conduction along twin boundaries.

Katherine Luna, KGB group
Point-Contact Spectroscopy and the effect of disorder in barium lead bismuth oxide.

BaPb1-xBixO3 (BPBO) was the first oxide superconductor discovered in 1975 by Art Sleight, but its transition temperature Tc is much lower than a similar compound BaKxBi1-xO3 (BKBO). We present point-contact spectroscopy results in the tunneling regime on this material for x=0, 0.20, 0.25, and 0.28. We discuss our results in the context of a scaling theory for a metal-insulator transition in a disordered material. In addition, we estimate the suppression of Tc due to disorder in both BPBO and BKBO.

Michael Sentef, Devereaux group
Theoretical description of nonequilibrium pump-probe spectroscopies in solids

Using modern ultrafast laser techniques, it is now possible to study the interplay of electrons and the crystal lattice on their intrinsic (femtosecond) time scales. In a pump-probe experiment, the system is pumped by a short laser pulse, which excites the electrons and phonons. A subsequent probe pulse with a well-defined pump-probe time delay probes the excited system as it relaxes back to its initial equilibrium state, e.g., using angle-resolved photoemission spectroscopy as a probe. In this talk, I will show how to model a pump-probe experiment in theory and what we can learn from the results of pump-probe spectroscopies.

Scott Riggs, Fisher group
Pyrochlore Iridates: A Materials Playground for Correlated Electrons

Strong electron correlations can lead to a variety of exotic, and often poorly understood phenomena. Typically one expects the Coulomb energy to decrease on going from 3d to 4d to 5d transition metals, due to the increased extent of the d-electron wavefunction, but this is not always the case. One system that has recently attracted widespread interest as a candidate strongly correlated material is the 5d pyrochlore RE2Ir2O7 (RE=Rare Earth). The interplay between electron correlation, strong spin-orbit coupling, and geometric frustration make this a particularly interesting material to study. Here we talk about our recent findings in both the magnetic structure of Y2Ir2O7 and (perhaps quantum oscillation studies) in Bi2Ir2O7.

Satoshi Harashima, Hwang group
Magnetic quantum-oscillations and 'Fermiology'

Magnetic quantum-oscillations, such as Shubnikov-de Haas oscillations, have been playing a crucial role for characterizing electronic structures of metals or doped-semiconductors. Following relaxing(/dull/useless) introduction, I will talk about 'Fermiology', namely how one determines Fermi surface shapes from magnetic quantum oscillations. As well as their early history, a few cutting-edge examples, including materials such as High-Tc superconductors or topological insulators, will be also introduced. If time allows, I will talk about my own experiments, which studies Shubnikov-de Haas oscillations in high-mobility perovskite oxide-heterostructures.

Menyoung Lee, DGG group

Advances in electric field-effect control over electron systems in solids

I will bring up and discuss various experimental strategies that have been developed for the purpose of altering the properties of low-dimensional electron systems via the electric field effect, i.e., electrostatic charging which in turn leads to a change in the number of available electrons in the system. (commonly referred to as gating). In particular these include deposition of high-k dielectrics or ferroelectrics, surface adsorption of polar molecules or charge-transfer dopants, and use of electric double-layers in electrolytes. This will not be a comprehensive review; I will merely highlight a few examples that I am aware of and think are interesting.

Albert Feng, Shen group
Ambient pressure X-ray photoelectron spectroscopy study of electrochemical double layer on an operating ceria-based model electrode

Electrochemical reactions between gas molecules and mixed ionic and electronic conductors (MIECs) control the efficiency of many elevated temperature devices such as solid oxide fuel cells, electrolyzers, and permeation membranes. Understanding the relationship between the reaction kinetics and the microscopic driving forces is an important step toward unraveling the surface electrochemistry.

In this Journal Club presentation, I will present an ambient pressure X-ray photoelectron spectroscopy study of electrochemical double layer on an operating ceria-based model electrode. I will first give a brief introduction to the background and motivation in the context of clean energy conversion and storage. Then I will show you the powerful technique that is more and more widely used in energy and environmental researches. Lastly, I will show you some recent results that shine light on the kinetics of water splitting and hydrogen oxidation reactions.

Maissam Barkeshli, Kivelson group
Synthetic non-Abelian statistics in bilayer fractional quantum Hall systems

An intense focus in the condensed matter community currently is the search for Majorana fermions in solid state systems. Defects which localize Majorana zero modes obey the simplest kind of non-Abelian statistics, and are of interest partially for the goal of achieving topological quantum computing. In this talk I will discuss our new experimental proposal to realize vast generalizations of Majorana fermions using only simple double layer FQH systems.

François Amet
Is graphene insulating at the Dirac point?

The conductivity of graphene close to the Dirac point is strongly influenced by disorder. Charged impurities in the vicinity of the flake induce density fluctuations that dominate transport at low energy, as carriers have to percolate across a landscape of electron and hole-doped puddles. As a result, the conductivity in graphene always remains finite at low temperature, and on the order of a few e²/h.

In this talk, I will present transport measurements in dual-gated graphene devices, where a metallic top-gate is suspended above the flake. The top-gate effectively screens the potential fluctuations and profoundly modifies the electronic behavior. As the temperature is lowered, the peak resistivity diverges with a power-law behavior and becomes as high as several Megaohms per square at the lowest temperature, in contrast with the commonly observed saturation of the conductivity. I will also discuss the quantum Hall regime, where broken-symmetry states appear at extremely low fields.