CM/BIO/ECE Seminars

The combined Condensed Matter/Biological Physics/Electrical and Computer Engineering Seminars take place every Wednesday at 4 PM in the Physics Library (rm 223A, Physics Bldg.)

Spring 2021
Date Speaker/Title/Abstract
3/24/21 Prof. Peng Li, Auburn University
Control of Magnetization in Topological Insulator/Magnetic Insulator Heterostructures

Spintronics-based technology, which uses spins to represent and propagate information, holds promise to realize devices that surpass the current CMOS transistor technology in power, density and speed. For example, magnetic random-access memory (MRAM) based on magnetic tunnel junctions were identified as promising non-volatile memory but its use has been limited. A second generation MRAM-based on spin transfer torque has reduced currents. However, next generation MRAM based on pure spin currents may provide even more energy efficiency. My research is focused on developing power-efficient ways to generate, propagate and manipulate spins via pure spin currents. In order to develop such pure spin current technologies, the development of new materials such as topological insulators must come hand in hand with the development of new devices. In this talk, I will discuss (i) low damping ferromagnetic insulating thin films for achieving efficient spin current generation in spintronic devices, (ii) spin current generation in these films and large spin-charge interconversion in neighboring layers, (iii) spin interactions in ferromagnetic insulator/topological insulator heterostructures. Together these results lay the foundation for new energy-efficient pure spin current-based electronics.

Reference:

1. Li, P. et al. Topological Hall Effect in a Topological Insulator Interfaced with a Magnetic Insulator. Nano Lett. 21, 1, 84 (2021).

2. Li, P. et al. Switching magnetization utilizing topological surface state. Science Advances 5, eaaw3415 (2019).

3/3/21 Smitha Vishveshwara, UIUC
Hunting for topological phases amidst Hofstadter butterflies and disordered landscapes

In this talk, I will discuss rich topological behavior in two related models – the Majorana wire and a Su-Schrieffer-Heeger ladder- in the presence of potential energy landscapes. An introduction of the two models and of techniques that directly provide information on edge-state properties will form the starting point for obtaining topological phase diagrams in these models. In the case of these systems subject to a quasiperiodic potential, a beautiful topological phase diagram emerges mimicking Hofstadter’s butterfly patterns. In the case of disordered potential landscapes, Anderson localization physics informs the behavior of the disordered topological phase diagram.  Finally, I will discuss the possible implementation of this physics in a variety of experimental systems, including solid state, cold atomic and electro-mechanical settings. 

2/17/21 Julia Medvedeva, University of Missouri S&T
Fundamentals of Amorphous Oxide Semiconductors

Amorphous oxide semiconductors (AOS)—ternary or quaternary oxides of post-transition metals—have attracted a lot of attention due to high carrier mobility which is an order of magnitude larger than that of amorphous silicon (a-Si:H). Unlike Si-based semiconductors, AOS exhibit optical, electrical, thermal, and mechanical properties that are comparable or even superior to those possessed by their crystalline counterparts. However, the properties of AOS are extremely sensitive to deposition conditions, oxygen stoichiometry, and metal composition, rendering the available research data inconsistent or hard to reproduce, thus, hampering further progress. Moreover, owing to the weak metal-oxygen bonding as well as many degrees of freedom in disordered materials, defects in AOS have the structural, thermal, and electronic characteristics that differ fundamentally from those in the crystalline transparent conducting oxides.

To navigate the large parameter space for AOS materials, computationally-intensive ab-initio Molecular Dynamics simulations followed by comprehensive structural analysis and accurate Density-Functional calculations, are performed for several AOS families. Integrated with systematic experimental measurements, the results provide microscopic understanding of complex relationships between the morphology, carrier generation, and electron transport across the crystalline-amorphous transition and help derive versatile design principles for next-generation transparent amorphous semiconductors with a combination of properties not achievable in Si-based architectures.

2/10/21 Maria Mills, MU Physics
Combined Magnetic Tweezers-TIRF microscopy for studying DNA-protein interactions

Magnetic tweezers allow the user to apply force and torque to magnetic beads attached to single DNA molecules, and to observe the resulting changes in DNA extension. This technique, however, is limited to measuring a single degree of freedom: the distance between the magnetic bead and the microscope slide surface. Total internal reflection fluorescence microscopy enables visualization of single molecules that have been tagged with fluorescent dyes. By combining TIRF microscopy and magnetic tweezers, we can simultaneously manipulate DNA molecules and use fluorescence to detect additional parameters, such as the presence of a protein or orthogonal changes in the DNA structure. We have recently installed a custom MT-TIRF instrument. In this talk I will discuss the instrument design, the physics underlying the two techniques, and how we plan to utilize them together to extract more information from our systems of interest.

2/3/21 Dmytro Pesin, University of Virginia
Manifestations of band geometry in linear and nonlinear transport

I will describe how the geometry of the band structure of metals manifests itself in their optical and transport properties. I particular, I will show that the natural optical activity of metals, equivalent to the so-called dynamic chiral magnetic effect, stems from the intrinsic magnetic moments of quasiparticles, and demonstrate that these magnetic moments can be of both intrinsic and extrinsic origin. I will then discuss optical Hall response of chiral crystals in the presence of a DC transport current – the gyrotropic Hall effect – and show that it is related to the Berry curvature dipole. The latter fact makes the gyrotropic Hall effect a diagnostic tool for topological properties of three-dimensional chiral metals. If time permits, I will discuss how to observe the chiral magnetic effect in Weyl semimetals using the heating effect of a transport electric field.

Spring Semester, 2022
Date Speaker/Title/Abstract
4/13/22 Dr. Daniel Hill, University of Missouri
Chiral magnetism: a geometric perspective
,
Abstract

Chiral ferromagnets have spatially modulated magnetic order exemplified by helices, spirals, and more complex patterns such as skyrmion crystals. The theoretical understanding of these states is based on a competition of a strong Heisenberg exchange interaction favoring uniform magnetization and a weaker Dzyaloshinskii-Moriya (DM) interaction promoting twists in magnetization. In this seminar, we consider a geometric approach, in which chiral forces are treated as a manifestation of curvature in spin parallel transport. The resulting theory is a gauged version of the Heisenberg model, with the DM vectors serving as background SO(3) gauge fields. This geometrization of chiral magnetism is akin to the treatment of gravity in general relativity, where gravitational interactions are reduced to a curvature of spacetime. The geometric perspective provides a simple way to define a conserved spin current in the presence of spin-orbit interaction. We show that the gauged Heisenberg model in d=2 is partly solvable when an applied magnetic field matches the gauge curvature. This model is shown to have a skyrmion-crystal ground state in the presence of a sufficiently small magnetic field.

Reference: Daniel Hill, Valeriy Slastikov, Oleg Tchernyshyov, SciPost Phys. 10, 078 (2021) 

3/2/22 Prof. Suchi Guha, University of Missouri
Ultrafast laser system in MU: new tools for nonlinear optics and time-resolved spectroscopy

Abstract:

 

As part of an NSF- MRI award, we have established an amplified ultrafast laser laboratory. The ultrafast laser system is applicable to a broad range of multidisciplinary problems, including transport of carriers in organic electronics and quantum materials, quasiparticle dynamics in multifunctional materials, second and higher harmonic generation, laser pulse shaping, and basic research on the spectroscopy of all materials - from hard to soft condensed matter materials.

 

In this presentation, I will briefly discuss the capabilities of the laser system and show our preliminary results in the area of nonlinear optics, in particular, a time-resolved optical method for monitoring transport in devices, and time-resolved spectroscopic studies from halide perovskites.