LaCNS Seminars Spring 2018

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Dustin Gilbert photo1) Monday, January 22, 3:00 pm, 1008B Digital Media Center

Dr. Dustin Gilbert (Research Physicist, NIST Center for Neutron Research), host John DiTusa

"Unique Uses of Neutrons in the Search for Magnetic Skyrmions"

Abstract: Magnetic skyrmions exhibit topologically protected quantum states which offer exciting new mechanisms for ultrahigh density and low dissipation information storage and also provide an ideal platform for explorations of unique topological phenomena and magnetic quasiparticles. Neutron scattering has played a crucial role in the scientific investigation of skyrmions, including providing the first evidence of their discovery.[1] Here I will discuss two projects in which neutron scattering have been quintessential in advancing our understanding of skyrmion spin textures. In the first of these works we demonstrate the realization of artificial Bloch skyrmion lattices over extended areas in their ground state at room temperature and zero magnetic field.[2] These artificial skyrmion structures are generated by patterning vortex-state magnetic nanodots with controlled circularity on an underlayer film with perpendicular magnetic anisotropy (PMA). Key to this work was demonstrating the imprinting of the chiral skyrmion structure from the vortex into the underlayer film. The imprinted feature, buried underneath the nanodots, was directly probed by specular and off-specular polarized neutron reflectometry measurements. The neutron measurements proved to be intriguing in their own right as these structures are comparable to the neutron coherence length, and the off-specular reflectometry is a relatively rarely used technique.

In the second work we prepare a chiral jammed state in chemically disordered, B20 structured (Fe, Co)Si consisting of skyrmion lattices, multi-q helices and labyrinth domains. Using small angle neutron scattering (SANS) we demonstrate a symmetry-breaking magnetic field sequence which disentangles the jammed state, resulting in an ordered, oriented skyrmion lattice.[3] This sequence is independent of the initial orientation of the crystal, suggesting it could be applied to realize ordered lattices even in systems with overwhelming structural disorder such as powders. Indeed, ordered oriented skyrmion arrays are realized in powdered Cu2OSeO3 using the same sequence. Disentangling the jammed state changes the topological charge of the system and is accompanied by the nucleation of charged and un-charged magnetic monopoles. Micromagnetic simulations confirm the experimental results and suggest skyrmion-skyrmion interactions may be responsible for the observed ordering. Beyond the important physics of these results this approach makes the rapid screening of candidate skyrmion materials possible by allowing the measurement of powder samples.

[1]  S. Mühlbauer, B. Binz, F. Jonietz et al., Skyrmion Lattice in a Chiral Magnet, Science 323, 915 (2009).

[2]  D. A. Gilbert, B. B. Maranville, A. L. Balk et al., Realization of ground-state artificial skyrmion lattices at room temperature, Nature Commun. 6, 8462 (2015).

[3]  D. A. Gilbert, A. J. Grutter, P. Neves et al., Precipitating Ordered Skyrmion Lattices from Helical Spaghetti. Under Review  (2018).

Email: dustin.gilbert@nist.gov

 

Meunier photo2) Monday, March 12, 3:00 pm, 1008B Digital Media Center

Prof. Vincent Meunier (Gail and Jeffrey L. Kodosky ’70 Chair, Department of Physics, Applied Physics and Astronomy, Rensselaer Polytechnic Inst.), host Bill Shelton

“Breathe, shear, repeat…lattice vibration signatures of 2D materials"

 Abstract:  2D materials (2DMs) such as graphene, transition metal dichalcogenides (TMDs) and black phosphorus have attracted significant attention as emerging low-dimensional materials. These materials feature an array of properties that offer many promises in terms of potential electronic and optoelectronic applications. Many characterization techniques have been employed to improve the understanding of these materials, to establish their crystal structure, purity, number of layers, and internal arrangements. In particular, Raman spectroscopy, has demonstrated that the vibrations can be used as solid indicators of the structural properties of 2DMs. However, due to the emergence of new properties, the interpretation of experimental features requires a dedicated modeling effort based on quantum-mechanics. In this talk, I will overview how quantum mechanical properties and non-resonant Raman scattering are combined to determine the fundamental structural properties in a broad array of 2D materials. I will discuss the importance of low-frequency modes in the study of layer-layer interactions in 2DMs, and how relative twisting angles between layers can be determined by monitoring relative shifts in Raman active mode. I will also show how vibrational signatures can be exploited to understand in-plane anisotropy in phosphorene.

E-mail: meuniv@rpi.edu


Tom Berlijn photo3) Monday, March 19, 3:00 pm, 1008B Digital Media Center

Dr. Tom Berlijn (Research Staff, Oak Ridge National Laboratory), host Rongying Jin

"Understanding Disordered Materials via Unbiased Simulations"

Berlijn image for talkAbstract: Inserting disordered impurity atoms is one of the most powerful ways to tune the functionality of advanced materials. In this talk I will demonstrate how disorder controls and reveals the underlying physics of heat conductance in thermo-electrics, electron pairing in superconductors and Anderson localization in intermediate band semiconductors. In particular I will illustrate how unbiased and materials-specific simulations shed light on complex experiments on disordered materials and allow for a fundamental understanding of their properties.

E-mail: berlijnt@ornl.gov


Raghavan photo4) Monday, April 2, 3:00 pm, 1008B Digital Media Center

Prof. Srinivasa Raghavan (Professor and Patrick and Marguerite Sung Chair, Department of Chemical & Biomolecular Engineering, Univ. of Maryland), host Bhuvnesh Bharti

“Nature-Inspired 'Smart' Materials: Ability to Move, Morph, Destroy and Heal”

Abstract:  Our laboratory seeks to engineer the assembly of polymers, surfactants, and nanoparticles into micro- or nanostructured materials. We seek to create “smart” or responsive materials whose properties can be transformed by an external stimulus. The inspiration for our work frequently comes from nature, and extends across the range of length scales.

At the nanoscale, we study molecular self-assembly into structures such as vesicles and micelles. In addition, we have created self-assembling biopolymers that are able to convert liquid blood into a gel; thereby, the materials stop bleeding from serious injuries. A startup company is attempting to commercialize these “hemostatic” materials.

At the microscale, we create polymeric capsules inspired by the architecture and properties of biological cells. Examples include: capsules with many inner compartments; capsules that can “swim” in water in the presence of a chemical fuel; and capsules that can destroy other microscale structures.

At the macroscale, we are developing polymer hydrogels inspired by the responsive properties of plant leaves and aquatic creatures. For example, we have designed hydrogels that transform from a flat sheet to a folded tube in response to a specific cue. We have also designed hydrogel-membranes with the ability to regulate water flow based on temperature, pH, and light. 

E-mail: sraghava@umd.edu

 

 Siebenburger photo5) Monday, April 16, 3:00 pm, 1008B Digital Media Center

Dr. Miriam Siebenbürger (Department of Polymer Science and Engineering, University of Massachusetts Amherst), host Phillip Sprunger

"Mechanics of Colloidal Suspensions driven either by dynamics or by structure"

Abstract: The colloidal scale, the size range between nanometer and micrometer, is a fascinating world of foams, emulsions and particles of any shape and of any type of materials. The applications reach from cosmetics and wall paint, to membranes and concrete. At the colloidal length scale, experimental methods of the millisecond to minutes range can be applied due to the slowdown of a factor of 109 of dynamics compared to molecular systems. Colloids are driven by Brownian motion and can be prepared in various shapes and surface properties. Colloidal suspensions can be used as a model system of utmost simplicity in order to compare it to theory: the hard-sphere suspension. The special focus of this talk is located on concentrated colloidal suspensions, their phase transitions from the fluid to the glassy or the crystalline state and structural changes resulting in enhanced mechanical properties. These fluid-to-solid transitions are characterized by the mechanics and flow properties with rheological approaches, the structure by scattering methods, and the combination of these techniques (here: rheology and small angle neutron scattering (rheo-SANS)).

The first part will focus on the dynamical arrest of a concentrated suspension resulting in the glassy state. Resulting rheological properties will be compared to the Mode Coupling Theory. This theory and its underlying mechanism will be confirmed by diverse rheological experiments. In combination with the theory, predictions of complex history dependent glassy mechanics are possible and can be transferred to other types of glasses. 

The second part will focus on enhancing the mechanical properties by structural changes: the crystallization of colloidal suspensions and shear-aligning of crystals are followed by means of rheo-SANS and compared with simulations.  Finally, neutron small angle scattering combined with cryo-TEM revealed the mechanism of the build-up of a very strong self-healing gel.

E-mail: m.siebenbuerger@umass.edu

 

Marcel Baer photo6) Monday, April 23, 3:00 pm, 1008B Digital Media Center

Dr. Marcel Baer (Chemical Physics & Analysis Scientist, Pacific Northwest National Laboratory), host Revati Kumar

DFT and Force Field Study on the Effect of Ions on Structure and Side-Chain Interactions in Peptoids

Abstract: The description of peptides and the use of molecular dynamics simulations to refine structures and investigate the dynamics on an atomistic scale are well developed. A consensus in this community over multiple decades has resulted in the availability of parameterized force fields that only require the sequence of amino-acids and an initial guess for the three-dimensional structure. The recent discovery of peptoids, that are designed with functionality attached to the nitrogen instead of the Ca is a significant departure from the standard force fields for peptides and will require a retooling of the currently available interaction potentials in order to have the same level of confidence in the predicted structures and pathways as there is presently in the peptide counterparts. Here we present modeling of peptoids using a combination of ab initio molecular dynamics (AIMD), atomistic resolution classical FF and coarse-grained models (CG) to span the relevant time and length scales. To make contact with experiments and identify features of the peptoid monomers that promote formation of stable/ordered nanostructures, both nucleation and aggregation will be explored using CG simulations. To properly account for the dominant forces that stabilize ordered structures of peptoids, namely steric-, electrostatic, and hydrophobic interactions mediated through sidechain-sidechain interactions in the CG model those have to be first mapped out using high fidelity atomistic representations. A key feature here is not only to use gas phase quantum chemistry tools, but also account for solvation effects in the condensed phase through ab initio molecular dynamics simulation. One major challenge is to elucidate ion binding to charged or polar regions of the peptoid and its concomitant role in the creation of local order. Here, similar to proteins, a specific ion effect is observed suggesting that both the net charge and the precise chemical nature of the ion will need to be described.

E-mail: marcel.baer@pnnl.gov

 

Weiss photo7) Thursday, May 3, 3:00 pm, 1008B Digital Media Center

Dr. Thomas Weiss (Senior Research Engineer, Stanford Synchrotron Radiation Light Source, SLAC National Accelerator Laboratory), host Gerald Schneider

"Small-Angle X-ray Scattering and Its Application to Structural Molecular Biology"

Abstract: Small angle X-ray scattering (SAXS) is a versatile and powerful tool to investigate the structure of matter on the nanometer scale. Its application especially in the field of structural biology has seen a tremendous growth over the last two decades and the technique has been well established as one of the main experimental tools for the structural biologist. While conceptually simple, the experimental realization is not trivial and typically requires a dedicated setup offering a highly collimated beam and extremely low residual background scattering. In addition, sophisticated sample handling and preparation is often necessary to perform these experiments error-free and efficiently in terms of sample consumption and beam time and obtain high quality data. The small-angle x-ray scattering station BL4-2 at the Stanford Synchrotron Radiation Light Source (SSRL) provides such state-of-the-art experimental facilities for biological SAXS studies. Over the past few years we have concentrated on developing and optimizing our instrument and sample handling equipment to save time and sample amount without compromising data quality. Examples are our fully automated high-throughput solution sample delivery robot, a highly automated size-exclusion chromatography coupled solution scattering setup (SEC-SAXS), as well as our customized stopped-flow device for reduced sample consumption to perform millisecond range time-resolved mixing experiments. In this presentation I will introduce the basics of the SAXS technique as they relate to structural studies of biological and soft matter systems, discuss several of the technical developments to facilitate the experiments at the synchrotron beam line and showcase some of the scientific work they enabled.

Email: weiss@slac.stanford.edu

 

Jianwei Sun photo8) Monday, May 14, 3:00 pm, 1008B Digital Media Center

Dr. Jianwei Sun (Assistant Professor, Department of Physics and Engineering Physics, Tulane University), host Rongying Jin

"The SCAN density functional and its surprising performance in cuprates"

Abstract: The accuracy and computational efficiency of the widely used Kohn-Sham density functional theory (DFT) is limited by the approximation to its exchange-correlation energy Exc. The earliest local density approximation (LDA) overestimates the strengths of all bonds near equilibrium (even the vdW bonds). By adding the electron density gradient to model Exc, generalized gradient approximations (GGAs) generally soften the bonds to give robust and overall more accurate descriptions, except for the vdW interaction which is largely lost. Further improvement for covalent, ionic, and hydrogen bonds can be obtained by the computationally more expensive hybrid GGAs, which mix GGAs with the nonlocal exact exchange. Meta-GGAs are still semilocal in computation and thus efficient. Compared to GGAs, they add the kinetic energy density that enables them to recognize and accordingly treat different bonds, which no LDA or GGA can [2]. In this talk, I will present an advance in DFT, the recently developed non-empirical strongly constrained and appropriately normed (SCAN) meta-generalized gradient approximation (meta-GGA) [1]. SCAN predicts accurate geometries and energies of diversely-bonded molecules and materials (including covalent, metallic, ionic, hydrogen, and van der Waals bonds), significantly improving over its predecessors, the GGAs that dominate materials computation, at comparable efficiency [2]. SCAN’s excellent performance on cuprates, traditionally regarded as strongly-correlated systems out of reach of DFT, will be highlighted, exemplified by its accurate prediction of the metal insulator transition of La2CuO4 under doping [3]. I will further explain how SCAN was constructed [1], why it can improve over GGAs [2], and where it should fail [4]. At the end, efforts to improve SCAN via nonlocal corrections will be discussed.

[1] J. Sun, A. Ruzsinszky, and J.P. Perdew, Strongly constrained and appropriately normed semilocal density functional, PRL 115, 036402 (2015).
[2] J. Sun, R.C. Remsing, Y. Zhang, Z. Sun, A. Ruzsinszky, H. Peng, Z. Yang, A. Paul, U. Waghmare, X. Wu, M.L. Klein, and J.P. Perdew, Accurate First-principles structures and energies of diversely-bonded systems from an efficient density functional, Nat. Chem. 8, 831 (2016).
[3] J.W. Furness, Y. Zhang, C. Lane, I.G. Buda, B. Barbiellini, R.S. Markiewicz, A. Bansil, and J. Sun, An accurate first-principles treatment of doping-dependent electronic structure of high-temperature cuprate superconductors, Nature Communication Physics, 1, 11 (2018).
[4] H. Peng, Z. Yang, J.P. Perdew, and J. Sun, Versatile van der Waals density functional based on a meta-generalized gradient approximation, PRX 6, 041005 (2016). 

Email: jsun@tulane.edu

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