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.)
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
To be announced
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
To be announced
Prof. Fahad Mahmood
Prof. Aihua Xie, Oklahoma State University
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.
Dr. Christopher Moore, Clemson University
Quasi-Majorana bound states in semiconductor-superconductor heterostructures
Originally proposed in high energy physics as particles that are their own anti-particles, Majorana fermions have yet to be observed experimentally. However, possible signatures of their condensed matter analog, zero energy, charge neutral, quasiparticle excitations, known as Majorana zero modes (MZMs), are beginning to emerge in experimental data. Recent tunneling transport experiments involving quantum dot-semiconductor nanowire systems with proximity-induced superconductivity, strong Rashba spin-orbit coupling, and an applied magnetic field, were capable for the first time of measuring quantized zero bias conductance peaks (ZBCPs), which remain quantized over a range of experimental control parameters. These robust quantized conductance plateaus have long been believed to be the smoking gun signature of the existence of MZMs. In this work, through numerical calculations on a tight-binding model, quantized conductance plateaus of height 2e2/ h are shown to identically arise in these systems as a result of low-energy bound states, generically induced by the quantum dot, in which the component Majorana bound states are partially separated in space without being topological MZMs. These results establish that the observation of quantized conductance plateaus of height 2e2/ h in local charge tunneling experiments does not represent sufficient evidence for the existence of topologically protected MZMs localized at the opposite ends of a wire. Finally, I will outline a two-terminal experiment involving charge tunneling at both ends of the wire capable of distinguishing between the generic quasi-Majorana bound states and the non-Abelian MZMs. So while claims of an experimental breakthrough may be extremely encouraging, they are somewhat premature.
Johnson Lu, University of Missouri, Columbia
Photoemission Studies of Au/Ag Hybrid Quantum Well States
The quantum-well states (QWSs) associated with electron confinement on the nanometer scale have attracted considerable interest due to their importance in low-dimensional physics and in magnetic/electronic device applications. To study QWSs in noble metal systmes, we grew atomically uniform Au films on Ag(111) surface. The Au QWSs are observed by using Angle-resolved photoemission spectroscopy (ARPES). The Au QWSs hybridize with the Ag states and thus, show an energy dependence on the thickness of the supporting Ag film and the thickness of the Au overlayer. Furthermore, a sandwich structure of Ag/Au/Ag is fabricated with different thicknesses of constituent layers. The Au d-states, though buried under the Ag overlayers, are still observable through hybridization with the Ag states. In this heterostructure, Au and Ag orbitals hybridize and form hybrid QWSs of which the charge density spread across the whole film. Interestingly, the hybrid quantum well states with Au d-orbitals possess a large effective mass as a consequence of partially localized nature of d-states. Detailed first-principles simulations are performed to illustrate the orbital components of hybrid QWSs and provide an insight into the band dispersion of hybrid states.
Prof. Sashi Satpathy, University of Missouri, Columbia
Anomalous and Spin Hall Effects: Introduction and Current Research
This talk will start with the basics and will cover some of the state-of-the-art research topics in this area. The well-known textbook Hall effect  in solids is due to the Lorentz force on the electron placed in a magnetic field. A much stronger effect, the anomalous Hall effect (AHE), which is not driven by the Lorentz force, occurs in ferromagnets and other systems with broken time-reversal symmetry and spin-orbit coupling. It was as long as seventy years after its discovery that AHE was explained by Karplus and Luttinger  to be due to the spin-orbit interaction in solids. More recently, it has been formulated in terms of the Berry curvature in momentum space . A second type, the topological Hall effect (THE), occurs due to the conduction electrons following the local spin configurations of atoms in real space, e. g., in skyrmions and magnetic domain walls. Finally, the spin Hall effect (SHE) can occur in materials due to spin-asymmetric scattering in non-spin-polarized solids, where a spin imbalance develops rather than a charge imbalance. We will illustrate these effects using simple models as well as density-functional calculations . We will also show that the AHE can be tuned in the oxide heterostructures by an applied electric field. Apart from new fundamental physics, these effects may have potential applications in spintronic devices. References E. H. Hall, Am. J. Math. 2, 287 (1879; E. H. Hall, Philos. Mag. 10, 301 (1880) R. Karplus and J.M. Luttinger, Phys. Rev. 95, 1154 (1954) T. Jungwirth, Q. Niu, and A. H. MacDonald, Phys. Rev. Lett. 88, 207208 (2002) S. Bhowal and S. Satpathy, Electric Field Tuning of the Anomalous Hall Effect: SrIrO3/SrMnO3, Nature Comp. Mater. (2019); Dirac Nodal Lines and Density-Functional Prediction of a Large Spin Hall Effect in the Perovskite Iridate Ba3TiIr2O9 (in preparation)
Prof. Paweł Kowalczyk, University of Lodz, Poland
Antimonene and Bismuthene: growth and electronic properties
Recently, growing interest in 2D materials is observed initiated by exfoliation of graphene followed by investigations of silicane, germanene and stanene all located in 14th group of periodic table. Elements located in 15th group are also capable to crystalize in 2D form in layered A17 structure (black phosphorus structure, α-form). The widely investigated α-phosphorene is best known example, however, α-arsenen, α-antimonen, α-bismuthene and α-bismuth antimoniden can be synthesized. Interestingly, Sb and Bi can also form stable hexagonal form i.e. β phase based on A7 structure (blue phosphorus). Our recent LEEM/PEEM experiments allowed us for the first time to observe growth of bismuthene. Surprisingly, we discovered that growth of Bi islands is characterized by their anomalous diffusion with islands jump length distribution described by truncated Levy statistics. For thicker depositions we observed transformation of α-Bi to β-Bi. Moreover, for the first time we were able to investigate electronic structure of single 2 µm wide bismuthene island using µARPES. These experiments further supported by extensive STM/STS measurements and DFT calculations allowed us to understand both crystallographic and electronic structure of bismuthene. We decided to use bismuthene islands as a support for antimony films in order to engineer antimonene on bismuthene heterostructure. Surprisingly we fabricated two Van der Waals heterostructure systems: α- and β-antimonene grown on top of bismuthene. It is worth pointing out here that our α-antimonene was first experimental realization of this allotrope. Crystallographic structure of both Van der Waals heterostructres is investigated using STM and is further confirmed by moiré pattern simulations. Their electronic structure is probed experimentally using STS and theoretically using DFT. In particular DFT results suggest that α-Sb is 2D topological insulator while β-Sb is topologically trivial semiconductor. We note that the α-phases of Bi and Sb have a black phosphorus-like structure which has not previously been employed in van der Waals heterostructures. *This work is supported by the Polish National Science Centre under project DEC-2015/17/B/ST3/02362.
High performance computing in materials science - nonlocal correlation and electrostatics
Prof. Tay-Rong Chang, National Cheng-Kung University
Topological crystalline insulator: from symmetry indicators to material discovery
Topological crystalline insulators (TCI) are insulating electronic phases of matter with nontrivial topology originating from crystalline symmetries. In the past decade,many materials have been demonstrated to be the topological insulators or the topological semimetals, however the exotic TCI states have remained elusive. Building upon recent theoretical works, we develop a feasible method to uniquely determine the novel rotational symmetry topological invariants based on first-principles calculations. In this talk, I will show how we predict new TCI materials and display their unusual electronic structures that entirely distinct from traditional topological materials .  Xiaoting Zhou et al., Topological crystalline insulator states in the Ca2As family, arXiv:1805.05215 (2018).
Dr. Wolfgang Kreuzpaintner, TU Munich
Development of In situ Thin Film Growth for Polarized Neutron Reflectometry
Thin magnetic films and heterostructures thereof are the basic building blocks of a large number of electronic devices whose fabrication is based on physical vapor deposition techniques. The sample structure, stoichiometry, and defect population are defined by and evolve with the deposition process and their precise control and optimization are of great importance. It is, hence, highly desirable to directly analyze the development of the physical properties of magnetic heterostructures, e.g. the magnetization, during the growth process and to correlate them with the structural parameters of the sample. While, the in situcharacterization of thin films by electron- and photon-based probes as well as by scanning probe techniques is common practice, the in situmeasurement of the magnetic properties of thin films using (polarized) neutron reflectometry ((P)NR) is an extremely challenging task. The collaborative effort between TUM, University Augsburg and MPI Stuttgart in constructing a mobile sputtering facility for the growth and in situmonitoring of magnetic multilayers, which can be installed at suitable neutron beamlines, will be presented: In particular, the current state in development will be shown, ranging from unpolarized and polarized proof of principle neutron reflectivity measurements on Ni/Cr and Fe thin films carried out at the ToF reflectometer REFSANS at the FRM II neutron source to the latest fast in situPNR measurements at the AMOR beamline at PSI. For the latter, the “Selene” neutron optical concept, based on elliptic neutron mirrors is essential. An overview over the latest developments and future modifications as well as the completion work carried out to allow the setup to be applied for even broader scientific research will conclude the talk.