University of Wyoming Physics & Astronomy Colloquium Series

Fridays -- 4:10 PM -- Prowse Room 234

Pre-Colloquium tea served at 3:45 in the Prowse Room

Fall 2017 & Spring 2018 Schedule

First Friday in Fall Semester is September 1
September 22 Magnetoelectric effects in noncollinear antiferromagnets
Hua Chen, Colorado State University

Despite the significant academic interest in them and their richness in nature, antiferromagnets have always been overshadowed by ferromagnets in real-life applications based on magnetism or spintronics. This is primarily due to the fact that antiferromagnet order parameters, in contrast to the ferromagnetic magnetization, are only weakly coupled to magnetic fields, and are hence difficult, in conventional view, to be manipulated. In this talk I will discuss a number of recent theoretical and experimental developments that counter this conventional wisdom, in a class of antiferromagnets that have stable noncollinear magnetic orders. As an introduction I will talk about the discovery of the anomalous Hall effect (AHE), which is the voltage perpendicular to the current or electric field direction in the absence of external magnetic fields and has been conventionally assumed to exist only in ferromagnets, in noncollinear antiferromagnets. AHE can thus be used as an efficient probe to determine the global orientation of the noncollinear antiferromagnetic order. I will then explain how conventional spin Hall effect (SHE) and inverse spin Hall effect (ISHE) can be understood as local response functions involving spin density and charge current in a quantum kinetic theory. Using this approach we are able to explain the recent experimental discovery of time-reversal-symmetry-breaking counterparts of the conventional SHE and ISHE in the noncollinear antiferromagnet Mn3Sn, which we name as the magnetic spin Hall effect (MSHE) and the magnetic inverse spin Hall effect (MISHE), respectively. Finally I will discuss how to use a bulk response function to describe the MSHE in a magnetic insulator, without using the concept of spin currents.

September 28
Honyung Lee, Oak Ridge National Laboratory

September 29 Asymmetric Terahertz Metasurfaces for Optical & Sensing Applications
Thomas Searles, Howard University

Symmetry plays an important role in many facets of modern society including art, architecture, fashion and mathematics. In a series of Lectures on Physics, Richard Feynman highlighted the "symmetry of the physical laws" and presented symmetry with respect to space, time, quantum phase and matter. Similarly, breaking symmetry can also lead to important implications with respect to the physical properties of materials. Recently, physicists have highlighted the importance of asymmetry in the defects of crystals, in the transport properties of strained topological insulators and the electromagnetic response from the unit cells of metamaterials. In this talk, we will present how asymmetry in metasurfaces results in additional modes with extremely high quality factor for applications in communications and sensing. Furthermore, we will show how the physical properties of nanomaterials, specifically graphene, can be manipulated to add new functionality to future devices such as terahertz modulators.

October 10 Chalcogenide Compounds as Emerging Semiconducting Materials
Hao Zeng, University of Buffalo

Recent topics of interest in condensed matter physics, ranging from topological materials, iron-based superconductors to two-dimensional semiconductors, share one common trait: they often involve chalcogenide compounds. In this talk, I will discuss two material systems: 2D transition-metal dichalcogenides (TMDCs) and chalcogenide perovskites. A monolayer TMDC with broken inversion symmetry possesses two degenerate valleys that can be selectively excited by circularly polarized light. The valley degeneracy can be broken by an external magnetic field, leading to valley Zeeman splitting. We demonstrate that the valley splitting can be enhanced by more than an order of magnitude, by exploiting the interfacial exchange field from a ferromagnetic substrate. This approach opens up new possibility for valley control for valleytronics applications. The rapid progress in halide perovskites for photovoltaics has inspired us to search for novel semiconductor materials that can inherit the excellent optoelectronic properties of halides, while avoiding their toxicity and instability problems. I will present results on the synthesis and characterization of chalcogenide perovskites-an emerging class of ionic semiconductors. These earth abundant, stable inorganic materials with tunable band gap and strong light absorption are promising candidates for solar absorbers and optoelectronic applications.

October 13 Topological Hall effect and multiple magnetic states in the A-phase of Fe doped MnSi
Minyhea Lee, University of Colorado at Boulder

The discovery of a crystalline structure of non-collinear and non-coplanar spin arrangements --- the so-called Skyrmion lattice --- in the A-phase of MnSi provides a new route to manipulate electrical properties via spin degrees of freedom. The topological Hall effect in the A-phase is directly connected to the gauge field emerging from this non-trivial spin texture. In this talk, I will present our electrical transport studies in Fe-doped MnSi, particularly focusing on understanding the role of Fe impurities and the magnetic anisotropy of the A-phase.

October 17
Mark Coffey, Colorado School of Mines

The knapsack problem is one of the standard and important difficult challenges for computer science, having broad applications in financial transactions and packing and stock-cutting contexts. As an optimization problem, the main idea is to maximize benefit (profit) subject to weight (cost) constraints. Moreover, solutions of this problem may serve as a facilitator for resolving more complicated problems, including scheduling. This presentation reviews this problem, and briefly its applications, before discussing how a version of quantum computing could be implemented for its solution. This alternative methodology of quantum computing uses systems which have been widely used to model magnetic and other phenomena. It is mentioned that a commercial claimed quantum computing architecture for such problems is available.The aspect of easy- versus difficult-problem instances for NP-hard problems such as knapsack and its subproblem subset sum may also be touched upon.

October 20
Barry Zink, Denver University

KIC 9832227 is a contact binary star system with an 11-hour orbital period (see Figure). In 2014, we found its orbital period to be decreasing at an ever-increasing rate and suggested that by 2022 the system will spiral together, merge and consequently erupt as a luminous red nova. Our suggestion was based on an analogy with the period changes of the system V1309 Sco prior to its 2008 outburst (found in data that were analyzed after the outburst). In the three years since, we have performed rigorous tests of this unique prediction with new timing and spectroscopic measurements. Each test result to date has been affirmative. The physical mechanism that drives the period change in the final years before merger is not known. We are undertaking a broad suite of observations to make the most of this unique opportunity to study this evolutionary stage, hoping thereby to shed light on the mechanism as well as to provide initial conditions for models of the red nova outburst itself. I will conclude with a new idea for a possible mechanism along with two ways to test the idea.

October 27 KIC 9832227: Precursor to a Luminous Red Nova
Larry Molnar (Calvin College)

November 03
Pu Du (Institute of High Energy Physics)

November 10
Mario Flock

November 17
Kirstin Alberi, NREL

Final Friday in Spring Semester is May 4

Previous colloquia series: Fall 2002 Spring 2003 Fall 2003 Spring 2004 Fall 2004 Spring 2005 Fall 2005 Spring 2006 Fall 2006 Spring 2007 Fall 2007 Spring 2008
2008-09 2009-10 2010-11 2011-12 2012-13 2013-14 2014-15 2015-16