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
02/03/2021 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. Zoom Link:
02/10/2021 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. Join Zoom Meeting:  
02/17/2021 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. Join Zoom Meeting:
02/24/2021 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). Join Zoom Meeting:
03/03/2021 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.  Join Zoom Meeting:
04/28/2021 Adrian Del Maestro, University of Tennessee
An Effective Bose-Hubbard Model for Helium on Graphene
Fall 2020
Date Speaker/Title/Abstract
09/09/2020 Li Yang, Washington University St. Louis (virtual)
Light-matter interactions and magnetic topological defects in two-dimensional structures
In this talk, I will start from a general picture of light-matter interactions, such as quasiparticles and excitons, in solids and how to calculate them by first-principles many-body perturbation approaches. Then I will focus on light-matter interactions of emerging two-dimensional (2D) magnetic materials. By calculating electron-electron and electron-hole interactions, we can understand and explain important measurements. Moreover, beyond linear light-matter interactions, we propose to realize giant photocurrent and photo spin current in magnetic topological insulators and semiconductors, giving hope to realizing light-driven pure spin current. Finally, in addition to light-matter interactions, I will show how to combine different levels of first-principles tools and models to predict fundamental properties of 2D magnetic materials and novel topological spin defects, meron pairs, in monolayer CrCl3.
09/16/2020 Dr. Jiaxin Yin, Princeton University
Scanning tunneling microscopy of emergent topological matter
The search for topological matter is evolving towards strongly interacting systems including topological magnets and superconductors, where novel effects and unusual phases emerge from the quantum level interplay between geometry, correlation, and topology. Equipped with unprecedented spatial resolution, electronic detection, and magnetic tunability, scanning tunneling microscopy has become an advanced tool to probe and discover the emergent topological matter. In this talk, I will review the proof-of-principle methodology to study the elusive quantum topology in this discipline, with particular attention on the studies under a vector magnetic field as the new direction, and project future perspectives in tunneling into other hitherto unknown topological matter. References: Jia-Xin Yin et al. Observation of a robust zero-energy bound state in iron-based superconductor Fe(Te,Se). Nature Physics 11, 543 (2015). Jia-Xin Yin et al. Giant and anisotropic many-body spin–orbit tunability in a strongly correlated kagome magnet. Nature 562, 91-95 (2018). S. S. Zhang, Jia-Xin Yin*, et al. Vector field controlled vortex lattice symmetry in LiFeAs using scanning tunneling microscopy. Phys. Rev. B 99, 161103(R) (2019). Jia-Xin Yin et al. Negative flatband magnetism in a spin-orbit coupled kagome magnet. Nature Physics 15, 443–448 (2019). Jia-Xin Yin* et al. Quantum phase transition of iron-based superconductivity in Li(Fe,Co)As. Phys. Rev. Lett. 123, 217004 (2019). S. S. Zhang, Jia-Xin Yin* et al. Field-free platform for Majorana-like zero mode in superconductors with a topological surface state. Phys. Rev. B 101, 100507 (R) (2020). S. S. Zhang, Jia-Xin Yin* et al. Many-body resonance in a correlated topological kagome antiferromagnet. Phys. Rev. Lett. 125, 046401 (2020). Jia-Xin Yin* et al. Quantum-limit Chern topological magnetism in TbMn6Sn6. Nature 583, 533-536 (2020). Jia-Xin Yin* et al. Fermion-boson many-body interplay in a frustrated kagome paramagnet. Nature Communications 11, 4003 (2020).
09/23/2020 Giovanni Vignale, MU Physics
Hall diffusion anomaly and transverse Einstein relation
It is commonly believed that the current response of an electron fluid to a mechanical force (such as an electric field) or to a “statistical force” (e.g., a gradient of chemical potential) are governed by a single linear transport coefficient - the electric conductivity. We argue that this is not the case in anomalous Hall materials. In particular, we find that transverse (Hall) currents manifest two distinct Hall responses governed by an unconventional transverse Einstein relation that captures an anomalous relation between the Hall conductivity and the Hall diffusion constant. We give examples of when the Hall diffusion anomaly is prominent, resulting in situations where the transverse diffusion process overwhelms the Hall conductivity and vice versa.
09/30/2020 Chao Zhou, Department of Biomedical Engineering, Washington University in St. Louis
Ultrahigh-Speed Optical Coherence Tomography and its Biomedical Applications
Optical coherence tomography (OCT) is a powerful tool for assessing tissue architectural morphology. It enables three-dimensional (3D) imaging with micron-scale resolutions, and can be performed in vivo and in real-time without the need to remove and process specimens. OCT has gradually become the standard- of-care to non-invasively evaluate retinal pathology in ophthalmology clinics. Improving imaging speed has been a major driving force for OCT development. In this talk, recent technical advances to achieve ultrahigh imaging speed for OCT (e.g., space-division multiplexing OCT) will be presented. Furthermore, applications of OCT have started to grow in other clinical and research areas. Novel applications using OCT for label-free cancer diagnosis, high-throughput drug screening with tumor spheroids, and studying the heart development in Drosophila will be discussed.
10/07/2020 Zubin Jacob, Purdue College of Engineering
Topological Electromagnetic Phases of Matter
10/14/2020 Carsten Ullrich, MU Physics
First-principles calculations of excitonic effects in solids: linear response versus real time
This talk gives an overview of our recent work using time-dependent density-functional theory (TDDFT) to calculate optical spectra of periodic insulators and semiconductors, with an emphasis on excitonic effects. I will show that excitons can be described from the point of view of frequency-dependent linear response, as well as using real-time propagation following an ultrafast excitation. Applications and examples include one-dimensional toy models as well as a variety of real materials, including several types of perovskites. A recently developed screened hybrid approach is shown to be as accurate as the Bethe-Salpeter equation, but computationally much more efficient.
10/21/2020 Giovanna Guidoboni, MU Mathematics, Engineering
Multiscale/multiphysics modeling of ocular physiology: the eye as a window on the body
The eye is the only place in the human body where vascular and hemodynamic features can be observed and measured easily and non-invasively down to the capillary level. Numerous clinical studies have shown correlations between alterations in ocular blood flow and ocular diseases (e.g. glaucoma, age-related macular degeneration, diabetic retinopathy), neurodegenerative diseases (e.g. Alzheimer’s disease, Parkinson’s disease) and other systemic diseases (e.g. hypertension, diabetes). Thus, deciphering the mechanisms governing ocular blood flow could be the key to the use of eye examinations as a non-invasive approach to the diagnosis and continuous monitoring for many patients. However, many factors influence ocular hemodynamics, including arterial blood pressure, intraocular pressure, cerebrospinal fluid pressure and blood flow regulation, and it is extremely challenging to single out their individual contributions during clinical and animal studies. In the recent years, we have been developing mathematical models and computational methods to aid the interpretation of clinical data and provide new insights in ocular physiology in health and disease. In this talk, we will review how these mathematical models have helped elucidate the mechanisms governing the interaction between ocular biomechanics, hemodynamics, solute transport and delivery in health and disease. We will also present a web-based interface that allows the user to run and utilize these models independently, without the need of advanced software expertise.
11/04/2020 Wouter Montfrooij, MU Physics
Spontaneous cluster formation in strongly correlated electron systems
We discuss how macroscopically uniform systems housing magnetic ions, the so-called Kondo lattices, can spontaneously fragment into lattices populated with magnetic clusters upon cooling. We show evidence for this behavior in a chemically substituted compound and demonstrate how these magnetic clusters dominate the low temperature response of the system, both in transport measurements as well as to microscopic probes such as neutron scattering. We argue that this spontaneous fragmentation should not be limited to chemically substituted systems but should be prevalent in stoichiometric systems as well, especially in systems that are close to a quantum critical point. As such, this fragmentation could well provide the explanation behind the puzzling critical behavior observed in quantum critical systems and might even be directly relevant to high-Tc sduperconductors. Zoom Meeting:
11/11/2020 Suchi Guha, MU Physics
Tuning the Structural and Optical Properties of Halide Perovskites via Pressure: Implications in Optoelectronic Devices
Halide perovskites offer a rich landscape of structural and optical properties, which can be explored and possibly controlled by applying high pressure. These materials share the general formula: ABX3, in which A, B, and X are occupied by monovalent cations (either organic or inorganic), bivalent metal cations, and halide anions, respectively. Organic-inorganic halide perovskites (OIHPs) in solar cells have achieved greater than 25% power conversion efficiencies and all-inorganic halide perovskites (IHPs) have shown over 20% external quantum efficiency in light-emitting diodes. Despite this progress, challenges of environmental and operational stability of electronic devices remain. At a fundamental level, questions pertaining to structure-property relationships, the bulk/surface Rashba effect, and the nature of ferroelectric phenomena are not completely understood in this class of materials.           In this talk, I will show two examples of pressure-induced phase transformations in lead bromide perovskites which impact their electronic and optical properties. In methylammonium OIHP, an isostructural phase transformation at 2 GPa is accompanied by a coupling of the organic cation and the inorganic lattice. A second example of cesium IHP nanocrystals with planar defects shows enhanced photoluminescence at 1 GPa, suggesting that high pressure could serve as a tuning parameter for engineering the defects to improve the overall optical properties for optoelectronic applications. Join Zoom Meeting:
11/18/2020 Shulei Zhang, Case Western Reserve University
Spin and charge transport in magnetic Weyl semimetals
Weyl semimetals (WSMs) are a newly discovered class of topological materials, which possess unique electronic properties such as the Fermi arc surface states and chiral anomaly that are protected by their topological band structures. Recently, there has been increasing interest in the pursuit of magnetic WSMs [1-3] where band topology meets magnetism. I will discuss two spin-charge transport effects in magnetic WSMs that we predicted recently.  One is a spin-to-charge conversion effect, namely, a charge current induced by injecting a pure spin current into a magnetic WSM layer from an adjacent nonmagnetic metal layer [4]. I will show that this effect is closely related to the Fermi arc states and hence exhibits remarkable features that are distinctly different from that arising from either the Rashba or Dirac surface state [5,6]. The other one is a nonlinear Hall effect which emanates from the concerted actions of the chiral anomaly and anomalous velocity in the presence of non-perpendicular external electrical and magnetic fields [7]. The corresponding Hall resistance is linear in the applied electric and magnetic fields, at variance with the notable negative magnetoresistance which is independent of the electric field and quadratic in the magnetic field. I will also discuss experimental and materials considerations for measuring the two effects.     [1] I. Belopolski et al., Science 365, 1278 (2019). [2] N. Morali et al., Science 365, 1286 (2019). [3] D. F. Liu et al., Science 365, 1282 (2019). [4] S. S.-L. Zhang, A. A. Burkov, I. Martin, and O. G. Heinonen, Phys. Rev. Lett. 123, 187201 (2019). [5] J.-C. Rojas-Sánchez et al., Nat. Commun. 4, 2944 (2013).   Join Zoom Meeting:
12/02/2020 Sean Fayfar, Mu Physics
A new universality class pertinent to quantum phase transitions
Percolation theory is the study of the phase transitions that occur in systems in which connections are lost or made, and whose critical behavior falls into known universality classes. Site percolation models these systems as lattices with sites that are either occupied or empty. As sites are removed from a lattice, groups of sites will form clusters when all the surrounding sites become empty. When enough sites are removed from the system, the percolation threshold – the point when the system spanning cluster is fractured – is reached.  When the system approaches this threshold, universal behavior is manifested that is characterized by power laws with critical exponents that only depend on the dimensionality of the structure.  In this talk, I will introduce percolation theory and our new percolation method called protected percolation; it has the added restriction that only sites from the lattice spanning connection can be removed. Protected percolation was inspired by the observations made when studying the heavily-doped quantum critical compound Ce_2(Fe_{1-x}Ru_x)_2Ge_2  in which a magnetic Kondo lattice follows our protected percolation model upon cooling. The Harris criterion tests whether a transition from a disordered state to an ordered one will be stable against impurities. We have proven that protected percolation violates the Harris criterion and that there will be a qualitatively different phase transition dependent on the unique impurities of each system.  Zoom Link:
Spring 2020
Date Speaker/Title/Abstract
02/12/2020 Prof. Chiswili Chabu, MU Division of Biological ciences
02/19/2020 Shi-Jie Chen, MU Physics
03/11/2020 Giovanni Vignale, MU Physics
Nonlinear magnetoresistance from spin-momentum locking
Surface states of topological insulators exhibit the phenomenon of spin-momentum locking, whereby the orientation of an electron spin is determined by its momentum. We have discovered a close link between the spin texture of these states and a new type of nonlinear magnetoresistance, which depends on the relative orientation of the current with respect to the magnetic field as well as the crystallographic axes, and scales linearly with both the applied electric and magnetic fields. The nonlinear magnetoresistance originates from the conversion of a non-equilibrium spin current into a charge current under the application of an external magnetic field. Additionally, we find that the nonlinear planar Hall effect, manifested as a transverse component of the nonlinear current, exhibits a pi/2 phase shift with respect to the nonlinear longitudinal current, in marked contrast to the usual pi/4 phase difference that exists between the linear planar Hall current and the linear longitudinal current in typical topological insulators and transition metal ferromagnets.  The agreement between the theory and experiments done on the surface of topological insulator Bi2Se3 films is excellent.


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