Physics Colloquia

January, 01/22/2017
Events and times subject to change

January 24, 2017 Tuesday 11:00 AM  +
Meyer 611
Physics Colloquia (colloquia)
Other Physics Department Events (other)


Javad Shabani
City College of New York

Two-dimensional Epitaxial Superconductor-semiconductor Heterostructures: A Platform for Topological Superconducting Networks

Progress in the emergent field of topological superconductivity relies on synthesis of new material combining superconductivity, low density, and spin-orbit coupling. For example, theory indicates that the interface between a one-dimensional semiconductor with strong spin orbit coupling and a superconductor hosts Majorana modes with nontrivial topological properties. Recently, epitaxial growth of Al on InAs nanowires was shown to yield a high quality superconductor-semiconductor (S-Sm) system with uniformly transparent interfaces and a hard induced gap, indicted by strongly suppressed subgap tunneling conductance. Here we report the realization of a two-dimensional (2D) InAs/InGaAs heterostructure with epitaxial Al, yielding a planar S-Sm system with structural and transport characteristics as good as the epitaxial wires. The realization of 2D epitaxial S-Sm systems represent a significant advance over wires, allowing extended networks via top-down processing. Among numerous potential applications, this new material system can serve as a platform for complex networks of topological superconductors with gate-controlled Majorana zero modes. We demonstrate gateable Josephson junctions and a highly transparent 2D S-Sm interface based on the product of excess current and normal state resistance.


January 26, 2017 Thursday 11:00 AM  +
Meyer 611
Physics Colloquia (colloquia)
Other Physics Department Events (other)


Julia Mundy
University of California, Berkeley

Design and Construction of Oxide Heterostructures with Emergent Properties

Materials systems with many strongly interacting degrees of freedom can host some of the most exotic physical ground states known. The subtle interplay of Coulomb interactions, electron-lattice coupling and spin/orbital ordering gives rise to phenomena as diverse as high-temperature superconductivity and topological insulating states as well as ferroelectricity and ferromagnetism. Many of these emergent ground states are found in perovskite oxide crystals where the close lattice matching across the series further permits abrupt heterointerfaces to be formed. Here I will show how advanced thin film deposition, in conjunction with analytical electron microscopy, can be used to engineer novel multifunctional complex oxide materials. In particular, I will discuss the design of the first material shown to be a strong magnetoelectric multiferroic at room-temperature, a promising candidate for low-power electronics in which electric fields control magnetic states.


January 26, 2017 Thursday 4:00 PM  +
Meyer 122
Physics Colloquia (colloquia)


Jane Kondev

TBA



January 31, 2017 Tuesday 11:00 AM  +
Meyer 611
Physics Colloquia (colloquia)
Other Physics Department Events (other)


Liang Wu
University of California, Berkeley

Shining Light on Topological Insulators and Weyl Semimetals

The last decade has witnessed an explosion of research investigating the role of topology in band-structure, as exemplified by the wealth of recent works on topological insulators (TIs) and Weyl semimetals (WSMs). In this talk I hope to convince you that optical probes of solids give unique insight into these topological states of matter. First, I will discuss how we can probe the special low-energy electrodynamics of 3D TI thin films of Bi2Se3 using time-domain THz spectroscopy[1]. I will then discuss our work following the evolution of the response as a function of magnetic field from a semi-classical transport regime [2] to a quantum regime [3]. In the later case, although DC transport is still semi-classical, we find evidence for Faraday and Kerr rotation angles quantized in units of the fine structure constant [3]. This is consistent with the long-sought “axion electrodynamics” and the topological magneto-electric effect of 3D TIs. Among other aspects this give a purely solid-state measure of the fine structure constant based on a topological invariant [3]. I will also discuss how optics can observe quantized Hall conductance without involving the edge states [3]. Finally, I will present our most recent discovery of the largest 2nd harmonic generation in transition monopnictide Weyl semimetals such as TaAs [4] and talk about a new perspective of nonlinear optics in term of probing the Berry connection/curvature in momentum space [4]. (The focus of my talk will be on Refs. [3, 4].)
1. Wu, et al, Nat. Phys. 9, 410-414 (2013).
2. Wu, et al, Phy. Rev. Lett. 115, 217602 (2015).
3. Wu, et al, Science 354, 1124-1127 (2016).
4. Wu, et al, Nat. Phys. (2016). doi:10.1038/nphys3969


February 2, 2017 Thursday 4:00 PM  +
Meyer 122
Physics Colloquia (colloquia)


Mehran Kardar

TBA



February 16, 2017 Thursday 4:00 PM  +
Meyer 122
Physics Colloquia (colloquia)


Eric Siggia
Rockefeller University

TBA



February 23, 2017 Thursday 4:00 PM  +
Meyer 122
Physics Colloquia (colloquia)


David Huse
Princeton University

TBA



March 7, 2017 Tuesday 11:00 AM  +
Meyer 611
Physics Colloquia (colloquia)
Other Physics Department Events (other)


Alex Frano
University of California, Berkeley

Periodically Modulated Electronic Wavefunctions in Transition Metal Oxides Explored by Resonant X-ray Scattering

Familiar x-ray diffraction experiments produce a Fourier transform of the atomic landscape via elastic scattering and are routinely used to study the structure of solids. Resolving the inelastic energy exchange between photons and atomic ions probes the dynamical structure factor, a quantity that reveals collective dynamics of the atomic lattice and their dispersion. These techniques provide unambiguous windows into atomic order in solids. What if the principle could be used analogously to probe electronic order in correlated “quantum materials”?
By tuning the x-ray energy to electronic transitions and judiciously exciting electrons into states near the Fermi level, spatial modulations and temporal dynamics of electronic wavefunctions can be studied in Fourier space. Propelled by technological advances in synchrotron science over the last decades, Resonant (in)elastic X-Ray Scattering (RXS) provides this exciting and powerful tool to investigate the electronic structure of solids. Quintessential cases are transition metal oxides, where strong electronic correlations yield nontrivial, ordered patterns of the spin, charge and orbital character of the d-wavefunctions.
In this talk, let us discuss examples of how these subtle phases can be detected using RXS and how they can be tuned by external conditions like strain, interfacial geometries, and applied magnetic fields. These include the discovery and stabilization of charge density wave order in superconducting cuprates, dimensionality-induced magnetism in perovskite nickelates, and ‘Kitaev frustration’ exposed by external magnetic fields in the honeycomb iridate Li2IrO3. Finally, I will discuss the future of RXS with new light sources such as the nearby NSLS-II.


March 9, 2017 Thursday 4:00 PM  +
Meyer 122
Physics Colloquia (colloquia)


David Holland
NYU

TBA



March 23, 2017 Thursday 4:00 PM  +
Meyer 122
Physics Colloquia (colloquia)


Raman Sundrum
University of Maryland

TBA



April 27, 2017 Thursday 4:00 PM  +
Meyer 122
Physics Colloquia (colloquia)


Daniel Whiteson
University of California, Irvine

TBA



May 4, 2017 Thursday 4:00 PM  +
Meyer 122
Physics Colloquia (colloquia)


Eva Halkiadakis
Rutgers University

TBA