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. 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.
Prof. Dipanjan Mazumdar, Southern Illinois University, Carbondale
Growth and physical properties of novel electronic and spintronic materials
We are interested in the growth of new materials and investigate physical properties affected by size, chemical doping, interface, heterostructure, and external tuning parameters (light, magnetic field, etc.). My talk consists of two separate topics that have involved several graduate and undergraduate students in the past three years. First, I will discuss our work on the influence finite-size in Topological insulator Bi2Se3 thin-films. Through a combination of quantum confinement and Burstein-Moss effect, we discovered a large optical blue-shift in Bi2Se3 as it approaches the 2D limit. We also correlate this increase in bulk band gap to enhanced resistivity and low Hall mobility in films below six quintuple layers. In the second part of my talk, I will present results on a theory-driven experimental search of new half-metallic materials that can be useful in emerging spintronics technologies such as spin-transfer torque random access memory. Several new and thermodynamically-stable half-metallic materials have been identified in the inverse-Heusler family (general formula X2YZ). Synthesis of such materials is underway both in bulk and thin-film form. I shall share some recent results on Manganese-based inverse-Heuslers.
Novel emergent inhomogeneous phases in electron-hole graphene bilayers
I propose two strongly coupled two-dimensional (2D) bilayers of graphene, one bilayer containing electrons and the other holes separated by hexagonal Boron Nitride dielectric, as an experimentally accessible system to observe novel emergent many-body phases. This system is currently attracting a lot of experimental interest. I will first demonstrate that coupled electron-hole bilayer graphene as well as coupled few-layer graphene sheets with carrier densities in a range accessible to experiments, can access the regime of strong pairing necessary for superfluidity. For the coupled bilayer graphene system, we find two new inhomogeneous ground states, a one-dimensional Charge Density Wave (1D-CDW) phase, i.e. density modulations in one planar direction, and a coupled electron-hole Wigner crystal (c-WC) in association with the superfluid phase. To account for the strong inter-layer correlation energy accurately, I introduce a new approach which is based on a random phase approximation at high densities and an interpolation between the weakly- and strongly-interacting regimes. The approach gives excellent agreement with available Quantum Monte Carlo calculations for single layer two-dimensional-electron-gas systems. In addition to a fundamental interest in observing these new exotic inhomogeneous phases, demonstration of the existence of inhomogeneous phases in conjunction with a superfluid phase within such a crystallographically simple system, should add to our insight into the role of striped phases in High-Temperature superconductors.
Quantum fluctuation due to artificial spin-1/2 in perovskite
Understanding the high performance thermoelectrics from electronic structures
Thermoelectric effect is an attracting technology which enables direct conversion between thermal and electrical energy, thus providing an alternative for power generation and refrigeration. However, thermoelectric performance is a contraindicated property of matter that requires combinations of transport properties that do not occur in ordinary materials. Finding new high performance thermoelectrics requires methods for efficiently screening compounds to detect the unusual electronic structures that can decouple transport quantities, especially for power factor, the Seebeck coefficient and the conductivity. Here we present and test a simple transport function that can be efficiently calculated using standard methods and which identifies materials that decouple these transport quantities. It is large for band structures that overcome the inverse relationship between σ and S regardless of the specific characteristic that does this, i.e. complex band shapes, multiple valleys, heavy-light band mixtures, band convergence, valley anisotropy, and other features. Thus this transport function provides a simple and easily used way to screen materials for potential TE performance. Several selected promising thermoelectric materials which are screened by the function will be discussed as well.
Dr. Ching-Kai Chiu, University of Maryland, College Park
Topological Quantum Matter With Symmetries
Topological insulators and superconductors are fermionic systems with bulk energy gaps separating the valence and conduction bands. They possess gapless boundary states that are topologically protected and are related to physical quantities, such as quantized Hall conductivity. The 2016 Nobel Prize in physics has made this fact rather universally celebrated. One type of the topologically protected bound states leading to many implementations, such as non-abelian braiding and quantum computing, is Majorana bound states. A Majorana bound states, which is its own anti-particle, has zero energy protected by particle-hole symmetry stemming from superconductivity. In this talk, I will first review the family of topological phases by studying protected bound states and then focus on the recent development of topological nodal-line semimetals.
Prof. Eun-Ju Moon, University of Missouri, Kansas City
Interfacial ferromagnetism engineering by octahedral modulations in complex oxide superlattices
The distortions and rotations of the corner-connected BO6 octahedra in ABO3 perovskite heterostructures play a crucial role to design and to control multifunctional properties. However, isolating the effect of the subtle octahedral interface coupling is challenging and it is difficult to identify the length scale. Here I will discuss how magnetic behavior can be tailored by structural interfacial coupling of the BO6 octahedra. In epitaxial ultrathin manganite films, different substrate-induced octahedral distortions are shown to affect the magnetic and electronic properties of the ultrathin films. Isovalent manganite superlattices will be presented in which magnetic behavior can be spatially confined by tuning the superlattice period relative to the length scale of interfacial octahedral coupling. Next, I will demonstrate a structural “delta doping” approach, non-charge-based, for controlling magnetism in ultrathin layers within superlattices. Polarized neutron reflectivity and temperature dependent magnetization measurements was used to correlate enhanced magnetization with local regions of suppressed octahedral rotations in the heterostructures. The atomic-scale modulations of the magnetism in oxide interfaces derived solely from structural effects highlights the design of local rotational gradients as routes to spatial control over novel electronic or ferroic states in oxide superlattices.