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
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