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.)
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.
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.
Prof. Jian “Javen” Lin, Department of Mechanical & Aerospace Engineering, MU
Artificial Intelligence Powered Autonomous Materials Design and Development
As the emerging of deep learning algorithms and the resounding successes of data-driven informatics, artificial intelligence (AI) enabled approaches are beginning to take shape within materials science. These approaches enable to help correlate the synthesis/processing, structures, and property relationship, which is central research goal for material research. Especially, they are very powerful in predicting properties of materials that are hard to measure or compute using traditional approaches—due to the cost, time or effort involved. It is anticipated that AI would push the material revolution to a paradigm of full autonomy in the next five to ten years. This talk will first introduce AI and its application in materials science. Then, some scientific and technological challenges will be discussed followed by our recent attempt in developing AI algorithms for predicting electronic properties of topologically doped 2D materials. Finally, future outlook in this emerging field will be provided.
Prof. Zheng Yan, Department of Biomedical, Biological & Chemical Engineering and Department of Mechanical & Aerospace Engineering, University of Missouri
Materials and Manufacturing Innovations for Next-Generation Soft Electronics
Innovations that eliminate the mismatch between soft biological tissues and rigid conventional devices will create soft electronic systems, which can find wide applications in clinical human healthcare, fundamental biomedical studies, human-machine interfaces, robotics, athletic training, and many others. Currently, soft electronics can be achieved by employing intrinsically-soft organic electronic materials, flexible and stretchable forms of inorganic electronic materials, or emerging nanomaterials. The further development of soft electronics needs the introduction of some new attributes. For example, they need to be gas-permeable to facilitate perspiration evaporation and biofluids transport, thereby minimizing immune responses and inflammation risks. They should have programmed three-dimensional (3D) structures to interact with biological tissues through the volume. In this context, I will first introduce our recent research results of developing gas-permeable, multifunctional on-skin bioelectronic sensing platforms using porous graphene as device components and using PDMS sponges as substrates. The device examples include electrophysiological sensors, temperature sensors, hydration sensors and joule-heating elements, which demonstrate comparable performances to conventional, rigid, gas-impermeable devices. Secondly, I will introduce our recently-developed mechanically-guided assembly approach of building 3D structures and devices with programmed geometries and unprecedented flexibility and stretchability. The 3D assembly process is naturally compatible with existing planar micro/nanosystems technologies. It provides a fast, powerful means for building complex, programmed 3D structures and devices of advanced materials in a parallel fashion spanning length scales from sub-micrometer to meter dimensions. The application demonstrations include reconfigurable inductors, 3D photodetectors, electronic cellular scaffolds, implantable biomechanical energy harvesters, 3D supercapacitors, and several others.
Dr. Churna Bhandari, University of Missouri, Columbia
Spin-orbital entangled 2DEG at the polar LaAlO3 and non-polar iridates interface
The combination of a large spin-orbit coupling (SOC) and a reduced Coulomb interaction (U) in the 4d and 5d transition metal oxides has made these compounds hosts for a number of exotic quantum states, such as the spin-orbit driven Mott insulators, Weyl semimetals, axion insulators, and Kitaev spin liquids. In this talk, I would like to introduce 5d Sr2IrO4 /SrIrO3 (SIO 214/113) iridates, an example of large SOC quasi-two dimensional oxide material. The octahedral crystal field splits Ir (5d5) states into lower t2g and upper eg states, where the six-fold degeneracy of the t2g states is further lifted by strong SOC effect resulting in the formation of spin-orbital entangled quartet Jeff= 3/2 (occupied) and doublet Jeff= 1/2 (half-filled). Because of the Coulomb interaction, the half-filled Jeff= 1/2 band splits into lower and upper Hubbard band forming a Mott-insulating (MI) state, which, if doped can lead interesting physics. Then, I will discuss an interface between the polar ordinary band insulator LaAlO3 (LAO) and non-polar SIO (214) or (113), which provides a way for electron doping in the SIO. Using density functional studies, we predict the formation of a novel spin-orbital entangled two-dimensional electron gas (2DEG) at the (001) interface between LAO and SIO (214) or (113), by the combined effect of the SOC, U, and polar catastrophe. Quite remarkably the 2DEG is found to be localized on a single IrO2 monolayer, unlike other polar interfaces such as LaAlO3/SrTiO3, where the 2DEG is several monolayers thick. The electron gas occupies the upper Jeff = 1/2 Hubbard band in the interface layer, which becomes half-filled with a simple square-like Fermi surface. If successfully grown, this would be the first candidate material to host the spin-orbital entangled 2DEG.
Prof. Yew-San Hor, Missouri S&T, Rolla
Doped Bi2Se3 Topological Superconductors
Topological superconductors are predicted to have a full superconducting pairing gap in the bulk and gapless surface states consisting Majorana fermions which are spinless quasiparticles with no charge. This Majorana fermionic surface state, if detectable, could be useful for quantum computer. However, topological superconductors and the associated Majorana quasiparticles have not been conclusively established in real materials so far. This presentation will show by chemical doping, Bi2Se3 topological insulator can be tuned into a bulk superconductor that could be a candidate for topological superconductor. The first example i.e. CuxBi2Se3 was discovered few years ago to be a promising one. Recently, NbxBi2Se3 is found to be another promising system for the topological superconductivity studies. Physical properties of the superconductors will be shown.
Dr. John Nichols, Department of Physics, University of Arkansas at Little Rock
Exotic magnetism at 5d-3d interfaces
Transition metal oxides (TMOs) involving 3d metals have proven to give rise to numerous exotic properties such as high temperature superconductivity, colossal magnetoresistance, and multiferroics all of which are known to result from strong on-site Coulomb interactions (U). Conversely in 5d TMOs, such as the iridates, U is significantly weaker and has a comparable energy scale with relativistic spin-orbit interactions (SOI) where these fundamental interactions collectively govern the ground states of these materials. Even though nature essentially limits us to these two extremes of strong U with negligible SOI and weak U with strong SOI, artificial nanostructures such iridate-manganite heterostructures provide an ideal platform to investigate interfaces where both U and SOI are significantly stronger than what is typically found in bulk crystals. Thus, here I will present the structural, electronic, and magnetic properties of a series of (AMnO3)m/(SrIrO3)n (A = Sr or La) superlattices and will discuss the implications of these results.
Prof. Risheng Wang, MS&T
Shiyuan Gao, Washington University
Understanding excitons in 2D materials and van der Waals heterostructures from first-principles
Two-dimensional materials and their heterostructures have attracted much interests recently as promising candidates for photonics, optoelectronics, and valleytronics devices, where excitons dominate the optical properties. In this talk, I will first briefly review the theoretical efforts that went into understanding excitons in two-dimensional materials, focusing on the first-principles GW+BSE method. Then I will discuss a particular system of great experimental interest, the bilayer transition metal dichalcogenide heterostructure. Due to the interlayer coupling, the dipole oscillator strength and radiative lifetime of the low energy excitons in such bilayer heterostructure can be tuned by over an order of magnitude with an external gate field. I’ll show a simple model that captures the essential physics behind this tunability and allows the extension of the first-principles results to arbitrary external gate fields.
Dr. Guang Bian, University of Missouri
Recent Progress in Novel Topological and Functional Materials
Ever since the experimental discovery of graphene and topological insulators, there have been intense research activities in searching and identifying new topological phases of condensed matter. Low energy properties of these crystalline solids have deep connections to fundamental topics in high-energy physics such as Weyl fermions, Majorana fermions, chiral anomaly and supersymmetry. Thanks to the on-going progress in synthesizing high-quality bulk crystals and improving the capabilities of photoemission spectroscopy, vast opportunities have been created for exploring new fundamental physics in solid materials. In this talk, we will discuss recent theoretical and experimental advances in Weyl semimetals, nodal-line semimetals, and topological superconductors. The observed Fermi arcs, drumhead surface states, and transport chiral anomaly suggest potential applications of these materials in electronic and spintronic devices.
Shulei Zhang, Argonne National Laboratory
Formation of Skyrmion Lattice Without Inversion Symmetry Breaking
Two-dimensional magnetic skyrmions are nanoscale spin textures that are topologically protected: the spin structure of an individual skyrmion is associated with an integer winding number which cannot be continuously changed into another integer number without overcoming a finite energy barrier. The creation, annihilation and transport of magnetic skyrmions strongly rely on their topological properties, which make them promising candidates as spin information carriers in future spintronic devices. Formation of skyrmions and skyrmion lattices (SkXs) in chiral magnets with inversion symmetry breaking relies on the Dzyaloshinskii-Moriya interaction (DMI) which favors perpendicular alignment of neighboring spins. However, most chiral magnets are known to host SkX only at low temperature due to their low Curie temperatures, which hinders the application of skyrmions at room temperature. In this talk, I will demonstrate theoretically that, in ferromagnetic thin films with inversion symmetry (and hence no DMI), SkX can be stabilized by a spatially varying uniaxial magnetic anisotropy with easy axis periodically rotating from in-plane to out-of-plane. The phase diagrams calculated by a Ginzburg-Landau approach indeed show that SkX state is energetically favorable at room temperature with a very small magnetic field of the order of 10 Oe. Remarkably, the size of the skyrmions is determined by the ratio of the exchange length and the period of the spatial modulation of the anisotropy, at variance with conventional skyrmions stabilized by dipolar interaction and DMI. If time permits, I will also discuss, from a theoretical point of view, the current issues in electric detection of skyrmions by using topological Hall effect – a kind of Hall effect that depends on the topology of the spin texture.
Udo Schwingenschloegl, KAUST
Elemental 2D materials beyond graphene: Insights from computational theory
The presentation will address recent developments related to elemental 2D materials beyond graphene, with a focus on silicene, germanene, and arsenene. Several examples will be discussed in order to illustrate how computational theory based on first-principles calculations can contribute to understanding basic physical and chemical phenomena in 2D condensed matter. Silicene is of particular interest due to its compatibility with established Si technology. Regrettably, strong interaction with common substrates eliminates the Dirac states. Alternative substrates will be analyzed and the effects on silicene evaluated with respect to technological requirements. Germanene attracts more and more attention, because effects of spin-orbit coupling are accessible in contrast to lighter 2D materials. While the same is true for arsenene, the material's strongly buckled structure is not compatible with Dirac physics. Recovering the sp2 bonding, on the other hand, makes it possible to realize unusual properties.
Arnab Banerjee, Oak Ridge National Laboratory
Topological Excitations in a Honeycomb Kitaev Spin-Liquid Candidate
The Kitaev model on a honeycomb lattice predicts a special quantum spin liquid (QSL) ground state with excitations resembling Majorana Fermions and gauge flux excitations. These emergent features are exciting prospects to both basic physics and applications towards a lossless technology for quantum qubits. In this talk, I will describe our recent range of experiments on the magnetic Mott insulator alpha-RuCl3 which has honeycomb layers held together with weak van-der-Waals interactions. A strong spin-orbit coupling and an octahedral crystal field makes the Kitaev interactions the leading order term in the Hamiltonian. Prominently, despite a long-range ordered ground state, our inelastic neutron scattering measurements reveal a continuum of fractionalized excitations resembling predictions from Majorana Fermions, confirming that the material is proximate to a QSL. In a 8T magnetic field the long-range order vanishes revealing a true QSL with gapped excitations raising hopes of a state where non-Abelian excitations can reside. This opens prospects for future experiments which can directly probe the signatures of quantum coherence in this material.