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.)
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.
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).
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.
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.
Zubin Jacob, Purdue College of Engineering
Topological Electromagnetic Phases of Matter
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.
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.
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: https://umsystem.zoom.us/j/96711514784?pwd=TnQzY0F3RGtzSVRzOWVpQUZMOTY1Zz09
Suchi Guha, MU Physics
Tuning the Structural and Optical Properties of Halide Perovskites via Pressure: Implications in Optoelectronic Devices
Shulei Zhang, Case Western
Sean Fayfar, Mu Physics
A new universality class pertinent to quantum phase transitions
Prof. Chiswili Chabu, MU Division of Biological ciences
Shi-Jie Chen, MU Physics
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.
Dr. Guang Bian, University of Missouri
Cloning of Dirac Electrons in Graphene Heterostructure
Tuning interaction between Dirac states in graphene has attracted great interest because it modifies the spectra of the two-dimensional electron system and, consequently, gives rise to novel condensed-matter phases such as superconductors, Mott insulators, Wigner crystals and quantum anomalous Hall insulators. For example, emergent superconductivity occurs in twisted bilayer graphene at magic angles due to the enhanced correlation between Dirac fermions in the two graphene layers. However, it proved technically difficult forming the delicate structure with two separate atomic monolayers. In this work, we report a new way to engineer the band structure of graphene, namely, cloning Dirac states by the perturbation of substrate potential. We grow the graphene epitaxially on 6H-SiC substrate. The graphene layer and the SiC substrate form a natural epitaxial relation. Meanwhile, the SiC surface potential exerts an incommensurate perturbation to the graphene Dirac bands, resulting in the duplication of Dirac states at different locations in the momentum space. The clone of the Dirac states has been observed in our angle-resolved photoemission spectroscopy (ARPES) experiments. We perform theoretical modeling to illuminate this cloning mechanism and show the possibility of controllably modifying the electronic spectra of two-dimensional atomic crystals by varying the substrate parameters.
Sang Wook Han, Chonbuk National University
Local Structural and Electrical Properties of Metal-to-Insulator-Transition VO2 studied using in-situ XAFS and Resistance Measurements.
Vanadium dioxide (VO2) is a well-known metal-to-insulator-transition (MIT) material. There is a question whether a structural phase transition of VO2 induces the VO2 MIT because the MIT of VO2 is accompanied by a structural phase transition. Although this problem has been discussed for many years, it is still an ongoing controversy. We examined the local structural and electrical properties around V atoms of VO2 films using in-situ x-ray absorption fine structure (XAFS) measurements at V K edge during heating and cooling processes. In order to direct comparison, the XAFS measurements were simultaneously performed with DC-resistance measurements. Many researchers believe that the distance change of V-V pairs along the c-axis in a rutile VO2 is critical in the MIT because a V-V dimerization model can explain the origin of band gap of an insulator-phase VO2. Our XAFS measurements also show a temperature-dependent behavior of V-V pairs. However, the change of V-V distance is contrary to an expected behavior. I will briefly introduce XAFS techniques and discuss the MIT of VO2, comparing with the local structural properties determined by XAFS measurements.
Giovanni Vignale, Department of Physics, MU
Electron hydrodynamics and thermal transport in graphene-based materials
Electric and thermal transport in electronic systems has long been described in terms of a single-particle picture, which emphasizes the role of collisions between electrons and impurities or phonons, while electron-electron collisions play a secondary role. It is only in the past two decades that advances in the fabrication of ultra clean samples have refocused the interest on collective hydrodynamic transport - a transport regime which is controlled by the nearly conserved quantities: number, momentum, and energy, and by electron-electron interactions. In this talk I review some of the recent theoretical and experimental progress in our understanding of electronic hydrodynamics in graphene-based materials. I focus on thermal transport and its relation to electric transport, epitomized by the Wiedemann-Franz law which, in its conventional form, predicts a universal ratio between electric and thermal resistivities. Significant deviations from this prediction are found in single layer and double layer graphene, both in the doped case, where the Wiedemann-Franz ratio is reduced, and in the undoped case, where it is greatly enhanced. In the latter case an interesting scenario emerges, in which a small amount of disorder helps to expose an underlying singularity of the transport coefficients: vanishing thermal resistivity, finite electric resistivity, and diverging Wiedemann-Franz ratio and Seebeck coefficient.
Guang Bian, Univ. of Missouri
Symmetry-Enforced Dirac Fermions in Nonsymmorphic α-BismutheneSymmetry-Enforced Dirac Fermions in Nonsymmorphic α-Bismuthene
The discovery of graphene and topological insulators has stimulated enormous interest in two-dimensional electron gases with linear band dispersion. The vanishing effective mass and non-zero Berry phase of Dirac fermion-like states give rise to many remarkable physical properties such as extremely high mobility and zero-energy Landau levels. According to recent theoretical works, nonsymmorphic crystal symmetries enforce the formation of Dirac cones, providing a new route to establishing Dirac states in 2D materials. Here we will discuss our recent work on the realization of the symmetry-enforced Dirac fermions in nonsymmorphic α-bismuthene (Bi monolayer). The bismuthene was synthesized by the method of molecular beam epitaxy (MBE). The Dirac band structure was observed by the micro-angle-resolved photoemission (μ-ARPES) experiment. The Dirac points are located at high-symmetry momentum points which are entirely determined by the lattice symmetry. This correspondence of Dirac states to the nonsymmorphic symmetry group can potentially lead to the discovery of a range wide of new 2D Dirac materials. In addition, the Dirac fermions in α-bismuthene is of spin-orbit type in contrast to the spinless Dirac states in graphene. The result will accelerate the search of 2D Dirac materials and extend “graphene” physics into new territory where strong spin-orbit coupling is present.
Ping Yu, MU Department of Physics
Airy Beam and its Application in Coherent Domain Imaging
Gaussian beam has been extensively used in optical imaging. A Gaussian beam is a beam of electromagnetic radiation with a transverse Gaussian field distribution. In most biomedical coherence domain imaging modalities, the light beam is focused into a small diameter so that high lateral resolution is possible. In addition, the beam should maintain a small size in axial direction for a distance called Rayleigh length or depth of field (DOF). Gaussian beam has a fundamental limitation in the DOF due to its diffraction, which gives a trade-off between the lateral resolution and the total image depth. Structured light beams such as Bessel beams and Airy beams have been developed in optical science. In this talk, I will give our recent results on the development of optical coherence tomography using a finite energy Airy beam to improve the DOF. I will discuss technical challenges and how to use modified Airy beam for imaging applications. I will also show some results of using the developed Airy beam technique to study biological tissue.
Ioan Kosztin, Department of Physics
Theoretical and Computational Modeling of the Rupture Force Distribution in Peptide-Lipid Interactions
Peptide-lipid interactions are essential for understanding various cellular processes and their mechanisms. High resolution AFM based dynamic force spectroscopy is most suitable to investigate peptide-lipid membrane interactions by measuring the detachment (last-rupture) force distribution, P(F), and the corresponding force dependent rupture rate, k(F). In general, the measured quantities, which differ considerably for different peptides, lipid-membranes, AFM tips (prepared under identical conditions), and retraction speeds of the AFM cantilever, cannot be described in terms of the standard theory, according to which peptide-lipid membrane detachment occurs along a single pathway, corresponding to a diffusive escape process across a free energy barrier. In particular, the prominent retraction speed dependence of k(F) is a clear indication that peptide-lipid membrane dissociation occurs stochastically along several detachment pathways. Thereby, we have formulated a general theoretical approach for describing P(F) and k(F), by assuming that peptide detachment from lipid membranes occurs, with certain probability, along a few dominant diffusive pathways. This new method was validated through a consistent interpretation of the experimental data. Furthermore, we have found that for moderate retraction speeds at intermediate force values, k(F) exhibits "catch-bond" behavior (i.e. decreasing detachment rate with increasing force). According to the proposed model this behavior is due to the stochastic mixing of individual detachment pathways which do not convert or cross during rupture. To our knowledge, such catch-bond mechanism has not been proposed and demonstrated before for a peptide-lipid interaction.