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.)
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.
Kanokporn Chattrakun, Department of Physics MU
Bridging biochemical activities with conformational dynamics observed in atomic force microscopy
Single molecule atomic force microscopy (AFM) provides a means to probe molecular dynamics such as conformational dynamics, oligomeric state changes of proteins, intermolecular interactions, etc. Some advantages of this technique include its stability, precision and the ability to probe biological systems in near-native environments. Despite these advantages, there is one unavoidable restriction: A typical AFM measurement requires a supporting surface which can raise question on the viability of the proteins under study. We have adapted traditional biochemical assays to quantify the activity of surface adsorbed translocases, the machinery E. coli uses to selectively translocate certain proteins across the membrane, as prepared for an AFM measurement. Two assays were used to verify such activities: ATP hydrolysis and translocation. ATP hydrolysis activities revealed that translocases on the surface were biochemically active with three distinguishable levels of activity. Each of these levels was comparable to those achieved in solution. Translocation activities demonstrated that translocases on the surface translocated precursor proteins at a lower kinetic rate, likely due to frictional coupling between the surface and the precursor. Two surfaces were incorporated for comparison purposes. Glass provides an environment that promotes more translocation activity owing to its inherent surface roughness. Neutron reflectometry (NR) provided additional insights to the submembrane space, which is inaccessible to AFM measurements. NR results suggested that the submembrane space is sufficient for precursor proteins to occupy after being translocated, corroborating the measured translocation activity. AFM investigations unveiled nucleotide dependent conformational dynamics of translocase complexes on glass surface. The dynamics were observed at high temporal resolution via kymographs. Collectively, this data suggests that one can associate the observed conformational dynamics via AFM to the proteins’ biochemical activities.
Dr. Raina Olsen, Aurora Quantum Technologies
Problems in Quantum Computing: Inspiration from Bell Labs
Bell Labs is arguably the most famous industrial research organization in the world. In its heyday – from about the 1940s to 1980s – it produced 9 Nobel prizes, 33,000 patents, and laid much of the foundation for the information revolution. In Jon Gertner’s book about Bell Labs, “The Idea Factory”, he argues that Bell Labs management understood early on that the most important challenge wasn’t to find good ideas. Any explosively emerging technology makes it too easy to invent new ideas. Instead, the challenge is first to find the good problems – the most important, fundamental problems holding the technology back – and only then to come up with inventive ideas to solve these problems. Quantum technologies currently stand poised between academia and industry, with the world watching to see if quantum can and will bring about the next technological revolution. We discuss the biggest challenges to developing quantum into a truly revolutionary technology, including the major technical problems, as well as the structural challenges in today’s academic, industrial, and governmental institutions.
Sayantika Bhowal, Department of Physics MU
Orbital texture and orbital Hall effect in solids with broken inversion symmetry: 2D transition metal dichalcogenides
Ever since its discovery the notion of Berry phase has permeated through all branches of physics1. Over the past few decades, the Berry phase of the electronic wave function is realized to be indispensable in describing a broad number of phenomena, which includes polarization, orbital magnetism, various type of Hall effects [charge (anomalous), spin, and orbital Hall effects]2,3,4 in solid state physics. A relatively new type of Hall effect is the orbital Hall effect (OHE), where an imbalance of orbital moment develops rather than a charge imbalance (as in the standard Hall effect), when an electric field is applied. While the OHE is proposed to be the origin of anomalous and spin Hall effects in several materials, this effect is yet to be observed experimentally. The mechanism of this effect has been discussed only recently for centro-symmetric systems5, where an orbital texture generated by the longitudinal applied electric field plays a crucial role. In my talk, we venture the OHE in broken inversion symmetric 2D materials, the quintessential candidate of which are the monolayer transition metal diachalcogenides (TMDC), MoX2; X= S, Se, Te. We show that in contrast to the centro-symmetric systems6, an intrinsic momentum-space orbital texture is present in these systems even before the electric field is applied. We explore the crucial roles of the intrinsic orbital texture and the Berry curvature (not present in the centrosymmetric systems) in driving the large OHE in broken inversion symmetric systems. We show that the orbital texture and the OHE appear even in absence of spin-orbit coupling (SOC) and give rise to the well known “valley dependent spin splitting" and the weak spin Hall effect in presence of SOC. The large magnitude of the orbital Hall conductivity in TMDC, opens up the possibility of application of these materials in “orbitronics" devices, beyond spintronics, to carry information. We further propose the plausible ways to tune the observed OHE and suitable experiments to probe the computed orbital texture as well as the OHE. Our work emphasizes the intrinsic orbital transport properties (OHE) of TMDC based on the more fundamental principle of orbital-valley coupling, in contrast to the well studied spin, valley and exciton Hall effects in these materials, which can be achieved only by extrinsic means (doping, light illuminating etc.) and rely on the coupled spin-valley physics. References: 1. D. Xiao, M.-C. Chang and Q. Niu, RMP 82, 1959 (2010). 2. S. Bhowal and S. Satpathy, npj Computational Materials 5 (1), 61 (2019). 3. S. Bhowal and S. Satpathy, Phys. Rev. B 99, 245145 (2019). 4. S. Bhowal and S. Satpathy, Phys. Rev. B 100, 115101 (2019). 5. D. Go, D. Jo, C. Kim, and H.-W. Lee, Phys. Rev. Lett. 121, 086602 (2018). 6. S. Bhowal and S. Satpathy (Manuscript in preparation)
Prof. Fahad Mahmood
Measuring and manipulating quantum materials with THz light
Uncovering and controlling emergent phenomena in quantum materials through external stimuli is a central goal of modern condensed matter physics. In this talk, I will demonstrate how ultrafast THz techniques can alter collective behavior and selectively decipher interactions in topological insulators and a frustrated magnet quantum spin liquid (QSL) candidate. For topological insulators, time-and-angle resolved photoemission spectroscopy (Tr-ARPES) based on THz excitation is used to dynamically engineer light-induced ‘Floquet-Bloch’ electronic states. For the QSL candidate YbMgGaO4 time-domain THz spectroscopy is used to probe spin-wave excitations at the Brillouin zone center, ideally complementing neutron scattering to determine a hierarchy of exchange interactions. I will also give a short perspective on how an extension of THz spectroscopy might be able detect fractionalized excitations in such frustrated magnetic systems. These results lay the foundation for utilizing coherent light-matter interaction to steer materials towards a desired quantum phase.
Prof. Aihua Xie, Oklahoma State University, Stillwater, OK
Infrared Structural Biology: Exploring Protein Function in Space and Time
The biological function of a protein is often carried out through a succession of structural transformations. Thus, technologies that capture 3D-structures of proteins, mainly X-ray crystallography, nuclear magnetic resonance (NMR), and cryogenic electron microscopy (cryo-EM), have made immeasurable contributions to our understanding of protein functions. Do we have enough structural information for deep understanding on protein functions? In this talk, I will discuss what desirable structural information of proteins remain missing, and show how infrared structural biology, an emerging technology, offers distinctive and complementary structural information that enhances our understanding of protein functions.
Nicolae Atodiresei, Peter Grünberg Institut and Institute for Advanced Simulation Forschungszentrum Jülich and JARA
Ab initio Simulations of Organic Molecules and 2D Systems Interacting with Magnetic Surfaces
The density functional theory provides a framework with predictive power that can be used to describe hybrid systems in a realistic manner. In this respect, ab initio studies elucidate how the subtle interplay between the electrostatic, the weak van der Waals and the strong chemical interactions determine the geometric, electronic and magnetic structure of hybrid interfaces formed between organic molecules and 2D materials with metallic surfaces. More precisely, the interaction between the π-like electronic cloud of organic materials or the lone electron pairs of the 2D systems with the magnetic states of a metal influences the (i) spin-polarization, (ii) magnetic exchange coupling, (iii) magnetic moments and (iv) their orientation at the hybrid interfaces. In this talk I will briefly summarize how first-principles calculations (i) provide the basic insights needed to interpret surface-science experiments and (ii) are a key tool to design novel materials with tailored properties that can be integrated in spintronic devices. References:  N. Atodiresei, J. Brede, P. Lazić, V. Caciuc, G. Hoffmann, R. Wiesendanger, S. Blügel, Phys. Rev. Lett. 105, 066601 (2010).  M. Callsen, V. Caciuc, N. Kiselev, N. Atodiresei, S. Blügel, Phys. Rev. Lett. 111, 106805 (2013).  K. V. Raman, A. M. Kamerbeek, A. Mukherjee, N. Atodiresei, T. Sen, P. Lazic, V. Caciuc, R. Michel, D. Stalke, S. K. Mandal, S. Blügel, M. Münzenberg, J. S. Moodera, Nature 493, 509 (2013).  J. Brede, N. Atodiresei, V. Caciuc, M. Bazarnik, A. Al-Zubi, S. Blügel, R. Wiesendanger, Nature Nanotechnology 9, 1018 (2014).  R. Friedrich, V. Caciuc, N. Atodiresei, S. Blügel, Phys. Rev. B 92, 195407 (2015).  F. Huttmann, A. J. Martínez-Galera, V. Caciuc, N. Atodiresei, S. Schumacher, S. Standop, I. Hamada, T. O. Wehling, S, Blügel, and T. Michely, Phys. Rev. Lett. 115, 236101 (2015).  B. Warner, F. El Hallak, N. Atodiresei, Ph. Seibt, H. Prüser, V. Caciuc, M. Waters, A. J. Fisher, S. Blügel, J. van Slageren, C. F. Hirjibehedin, Nature Communications 7, 12785 (2016).  F. Huttmann, N. Schleheck, N. Atodiresei, T. Michely, J. Am. Chem. Soc. 139, 9895 (2017).  M. Paβens, V. Caciuc, N. Atodiresei, M. Feuerbacher, M. Moors, R. E. Dunin-Borkowski, S. Blügel, R. Waser and S. Karthäuser, Nature Communications 8, 15367 (2017).  V. Caciuc, N. Atodiresei, S. Blügel, Phys. Rev. Mat. 3, 094002 (2019).
Ping Yu, MU Department of Physics
Photothermal Effect of Semiconductor Nanoparticles
Photothermal therapy is a clinical method to treat tumors in neurosurgery by using needle like fiber optic applicator, as evidenced by the US Food and Drug Administration approved instruments and several clinical trials currently in the US. However, this method is limited by the ability to deliver the applicator to the tumor sites, thus only applicable to localized and near surface disease or lesions. In addition, the thermal damage of healthy tissue is serious that additional methods are needed to cool down the applicator and define the thermal boundary during the therapy. Nanoparticles targeting cancer cells provide a new potential for photothermal therapy. In this talk, I will present our recent work on the development of bio-compatible copper sulfide (CuS) nanoparticles (NPs) that have a strong photothermal effect at 980 nm based on localized surface plasmon resonances. Due to a unique binding affinity to the cancer cells, CuS NPs coated with the functional layers have a high selectivity to the cancer cells and maintain an efficient photothermal effect from the CuS cores. In this talk I will also discuss different factors that influence the photothermal conversion efficiency.
Dr. Predrag Lazic, Rudjer Boskovic Institute
High performance computing in materials science - nonlocal correlation and electrostatics
In this talk we will present two projects from materials science that are closely related to HPC. The first project which resulted in the code called JuNoLo was one of the first implementations of the vdW-DF density functional which by inclusion of nonlocal correlation covered for the van der Waals interaction in ab initio calculations. The code was implemented for massively parallel runs which were done on a BlueGene/P machines. The second project will present the Robin Hood metod which has origins in materials science problem - but ended up as a numerical method for solving electrostatic problems via Boundary Elements Methods. The RH method proved to be so superior to the state of the art methods that we raised some 650k USD and founded a company (Artes Calculi) based on the idea. Short overview of company will be presented including software development and collaboration with companies such as Nokia.
Christopher J. Arendse, University of the Western Cape
Hybrid perovskite thin films by sequential low-pressure vapour deposition in a single reactor
We demonstrate a two-step low-pressure vapour deposition method for the deposition of methyl ammonium lead iodide perovskite thin films directly on Corning glass substrates in a single reactor. Continuous, polycrystalline lead iodide (PbI2) thin films were deposited in the first step by evaporation of a PbI2 powder in a chemical vapour deposition reactor (CVD) at low-pressure, which produced high quality perovskite during the subsequent CVD conversion step in the same reactor. Perovskite conversion is complete after 90 minutes of exposure to methyl ammonium iodide (MAI) vapours at 130 °C. The perovskite conversion starts with the PbI2 framework at the sample surface and progresses towards the substrate-interface with prolonged conversion time. X-ray photoelectron spectroscopy depth profiling confirms the perovskite compositional homogeneity along the growth direction. The high absorption coefficient and refractive index confirms the superior density of the perovskite thin films. The optical properties of the fully converted perovskite remain stable after 16 days of exposure to harsh humidity conditions, ascribed to its superior morphology and grain size. Finally, we will report on the incorporation of the perovskite thin film into a photovoltaic cell.
William G. Fahrenholtz, Missouri University of Science and Technology
Materials Research Center and Ultra-High Temperature Ceramics Research at Missouri S&T
This presentation will provide an overview of materials research at Missouri S&T and then highlight some recent accomplishments in research related to ultra-high temperature ceramics. The Materials Research Center at Missouri S&T has two missions: 1) serve as a campus resource for major research instrumentation; and 2) promote interdisciplinary materials research. A short overview will describe capabilities within the center and highlight some on-going research. The second part of the presentation will focus on research on ultra-high temperature ceramics (UHTC). Professors Hilmas and Fahrenholtz in the Materials Science and Engineering Department at Missouri S&T have collaborated for more than 15 years on UHTC research. The presentation will review some past accomplishments, describe some of the unique research infrastructure, and highlight some on-going projects related to UHTCs.
Suraj Hegde, University of Illinois, Urbana
Majorana wavefunction physics in Kitaev chains: Manifestations in disorder and non-Equilibrium dynamics
In this talk, I will focus on the wavefunction features of the Majorana zero modes in topological superconductors, that have received immense attention in current day research. Recent works with my collaborators show how the oscillations appearing in the Majorana wavefunctions have drastic effects in non-equilibrium dynamics and disordered systems. Some of these results reveal novel features in the phase diagram of the Kitaev chain unexplored before and they also relate to few exact results in the spin chain physics. In the end, I hope to talk about some of our recent work on novel time-decay and relativistic effects in quantum Hall systems in a saddle potential, that relate to features in black hole physics. I will also indicate some future directions that could be pursued in these topics.