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
3) 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
4) Monday, April 23, 3:00 pm, 1008B Digital Media Center
Dr. Marcel Baer (Chemical Physics & Analysis Scientist, Pacific Northwest National Laboratory), host Revati Kumar
1) Tuesday, September 12, 12:30 pm, 215 Williams Hall (joint LaCNS – Organic Chemistry seminar)
Prof. Jared Delcamp (Dept. of Chemistry & Biochemistry, University of Mississippi), host Donghui Zhang
Abstract: Recently, dye-sensitized solar cells (DSCs) were shown to be the highest power conversion efficiency technology of any solar cell technology when using photons from the beginning of the solar spectrum until 700 nm. Two key directions are apparent in further elevating this technology: (1) broadening the spectral window used, and (2) efficiently subdividing the spectrum further for multijunction devices which can be used in combination with many solar cell technologies. Progress toward designing optimal panchromatic organic sensitizers to use NIR photons based on physical organic concepts such as proaromaticity and cross conjugation will be discussed. Additionally, the design and realization of a series sequential multijunction dye-sensitized solar cell (SSM-DSC) system for effective photon management will be discussed. Ongoing research to optimize this system based on transition metal redox shuttle design and high voltage organic dye design will be analyzed. The SSM-DSC system coupled with electrocatalysts as solar-to-fuel systems has been shown to power water splitting and CO2 reduction coupled with water oxidation from a single illuminated area without external bias.
2) Monday, November 13, 3:00 pm, 1008B Digital Media Center
Prof. Kenneth Jordan (Richard King Mellon Professor and Distinguished Professor of Computational Chemistry, Department of Chemistry, Univ. of Pittsburgh), host Revati Kumar
Abstract: It is well known that certain metals and graphene support Rydberg-type series of excess electron states, where the binding of the electron is due to the interaction with its image potential. Sufficiently, polarizable molecules and clusters possess very-extended non-valence anion stats that can be viewed as finite system analogs to image potential states. In this talk, I discuss the development of one electron Hamiltonians for describing these excess electron species. These are generated by coupling the excess electron to a many-body polarizable force field.
3) Monday, November 20, 3:00 pm, 1008B Digital Media Center
Prof. Norman Wagner (Robert L. Pigford Chaired Professor of Chemical & Biomolecular Engineering, University of Delaware, Newark), host Bhuvnesh Bharti
Abstract: Shear thickening colloidal and/or nanoparticle suspensions are commonly encountered in chemical and materials processing, and are also the basis of a technology platform for advanced, field responsive nanocomposites. In this presentation, I will review some of the experimental methods and key results concerning the micromechanics of colloidal suspension rheology. Micromechanics is the ability to predict the properties of complex systems from a colloidal or microscopic level description of the structure and forces. A fundamental understanding of colloidal suspension rheology and in particular, shear thickening, has been achieved through a combination of model system synthesis, rheological, rheo-optical and rheo-small angle neutron scattering (SANS) measurements, as well as simulation and theory (Colloidal Suspension Rheology, Mewis and Wagner, Cambridge Univ. Press, 2012).
Shear thickening fluids (STFs) are novel field-responsive materials that can be engineered to be useful nanocomposites for enhanced ballistic and impact protection, puncture resistant medical gloves, energy absorbing materials for mitigating impacts and concussions, as well as in systems for mitigating micrometeoroid and orbital debris threats in space applications. The development of commercial applications of STFs will be discussed. The rheological investigations and micromechanical modeling serve as a framework for the rational design of STF-based materials to meet specific performance requirements not easily achieved with more conventional materials (Phys. Today, Oct. 2009, p. 27-32). I will illustrate some technological applications of STFs under commercial development, including use in astronaut protection and possible application in the manned mission to Mars.