LaCNS Seminars Spring 2018
1) Monday, January 22, 3:00 pm, 1008B Digital Media Center
Dr. Dustin Gilbert (Research Physicist, NIST Center for Neutron Research), host John DiTusa
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. 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. 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. 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.
 S. Mühlbauer, B. Binz, F. Jonietz et al., Skyrmion Lattice in a Chiral Magnet, Science 323, 915 (2009).
 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).
 D. A. Gilbert, A. J. Grutter, P. Neves et al., Precipitating Ordered Skyrmion Lattices from Helical Spaghetti. Under Review (2018).
2) 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
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.
3) Monday, March 19, 3:00 pm, 1008B Digital Media Center
Dr. Tom Berlijn (Research Staff, Oak Ridge National Laboratory), host Rongying Jin
Abstract: 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.
4) 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
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.
5) Monday, April 23, 3:00 pm, 1008B Digital Media Center
Dr. Marcel Baer (Chemical Physics & Analysis Scientist, Pacific Northwest National Laboratory), host Revati Kumar
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.
6) Thursday, May 3, 3:00 pm, 1008B Digital Media Center
Dr. Thomas Weiss (Senior Research Engineer, Stanford Synchrotron Radiation, Stanford University), host Gerald Schneider