PreColloquium tea served at 3:45 in the Prowse Room


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 reallife 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 timereversalsymmetrybreaking 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, ironbased superconductors to twodimensional semiconductors, share one common trait: they often involve chalcogenide compounds. In this talk, I will discuss two material systems: 2D transitionmetal 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 perovskitesan 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 Aphase of Fe doped MnSi Minyhea Lee, University of Colorado at Boulder The discovery of a crystalline structure of noncollinear and noncoplanar spin arrangements  the socalled Skyrmion lattice  in the Aphase of MnSi provides a new route to manipulate electrical properties via spin degrees of freedom. The topological Hall effect in the Aphase is directly connected to the gauge field emerging from this nontrivial spin texture. In this talk, I will present our electrical transport studies in Fedoped MnSi, particularly focusing on understanding the role of Fe impurities and the magnetic anisotropy of the Aphase. 
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 stockcutting 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 difficultproblem instances for NPhard 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 11hour orbital period (see Figure). In 2014, we found its orbital period to be decreasing at an everincreasing 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 