December 7, 2016
11:00 am
Bldg. 440, A105-106

"Pushing the Frontiers of Atomistic Modeling Towards Predictive Design", Subramanian Sankaranarayanan, Nanoscience & Technology Division, Argonne National Laboratory. Host: Pierre Darancet

The ability to perform accurate calculations efficiently is crucial for computational materials design. Molecular dynamics (MD) is one such popular technique for materials design and provides information on structure and dynamical evolution of materials. The accuracy and efficiency of MD, however, hinges strongly on the quality of the force fields that describe the various inter/intramolecular interactions. Historically, force field (FF) development for MD can take months to several years while being limited to specific material systems. In this talk, we will discuss how our streamlined approach to FF development using first principles training data and machine learning algorithms is dramatically bringing down this timeframe to weeks to days. The procedure involves several steps including (a) generation and manipulation of extensive fitting data sets through electronic structure calculations and/or experiments, (b) defining functional forms, (c) formulating novel highly optimized training procedures, (d) coarse-graining to improve efficiency while retaining accuracy, and (e) subsequently coding and implementing these algorithms on high performance computers (HPCs). We will also discuss the validation of this approach on several diverse material systems ranging from precious metal/oxide nanocatalysts to newly discovered two dimensional materials such as stanene to more common yet complex system such as water.

November 9, 2016
11:00 am
Bldg. 440, A105-106

"Memcomputing: a Brain-inspired Topological Computing Paradigm", Massimiliano (Max) Di Ventra, University of California, San Diego. Host: Pierre Darancet

Which features make the brain such a powerful and energy-efficient computing machine? Can we reproduce them in the solid state, and if so, what type of computing paradigm would we obtain? I will show that a machine that uses memory to both process and store information, like our brain, and is endowed with intrinsic parallelism and information overhead - namely takes advantage, via its collective state, of the network topology related to the problem - has a computational power far beyond our standard digital computers [1]. We have named this novel computing paradigm “memcomputing” [2]. As an example, I will show the polynomial-time solution of prime factorization and the NP-hard version of the subset-sum problem using polynomial resources and self-organizing logic gates, namely gates that self-organize to satisfy their logical proposition [3]. I will also show that these machines are described by a Witten-type topological field theory and they compute via an instantonic phase where a transient long-range order develops due to the effective breakdown of topological supersymmetry [4]. The digital memcomputing machines that we propose are scalable and can be easily realized with available nanotechnology components, and may help reveal aspects of computation of the brain.

[1] F. L. Traversa and M. Di Ventra, Universal Memcomputing Machines, IEEE Transactions on Neural Networks and Learning Systems, 26, 2702 (2015).

[2] M. Di Ventra and Y.V. Pershin, Computing: the Parallel Approach, Nature Physics, 9, 200 (2013).

[3] F. L. Traversa and M. Di Ventra, Polynomial-time solution of prime factorization and NP-hard problems with digital memcomputing machines, arXiv:1512.05064.

[4] M. Di Ventra, F. L. Traversa and I.V. Ovchinnikov, Topological field theory and computing with instantons arXiv:1609.03230.

October 26, 2016

October 12, 2016
11:00 am
Bldg. 440, A105-106

"Imaging Irreversible Transformation Using Movie-Mode Dynamic Transmission Electron Microscopy", Tian Lu, Lawrence Livermore National Laboratory. Host: Jianguo J.G. Wen

In situ transmission electron microscopy (TEM) has been utilized for decades to image materials processes at high spatial resolution. Yet the relevant dynamics of many of these processes often remain elusive, as they unfold too rapidly to discern at small spatial scales using conventional TEM imaging conditions. Given the rapid microstructural evolution of many types of irreversible transformation fronts—on the order of mm/s to m/s—nanosecond temporal resolutions are required to capture these processes. The dynamic transmission electron microscope (DTEM) at LLNL was developed to enable imaging of transient states during irreversible transformations with nanometer spatial and nanosecond temporal resolutions using a single-shot acquisition mode. Recently, movie-mode DTEM was designed and implemented to provide multiple (up to 9) single-shot acquisitions in under ~1 μs, yielding frame rates that are on the order of 106 times higher than conventional in situ TEM frame rates. Movie-mode DTEM employs an arbitrary waveform generator to deliver a user-defined (pulse duration and spacing) series of electron pulses, allowing complex irreversible transformation events to be tracked across microsecond timescales. Movie-mode DTEM acquisitions result in higher data throughput with reduced uncertainty. Here, we will provide an overview of movie-mode DTEM instrumentation and operation, with examples of its application to various materials science problems, including amorphous-crystalline phase transformations of semiconductor , where amorphous-crystalline interface evolution is monitored, rapid alloy solidification, and observation of defect motion during high strain rate deformations.

September 28, 2016
11:00 am
Bldg. 440, A105-106

“Elucidating the Nanomaterial Genome with Scanning Probe Block Co-Polymer Lithography ”, Chad Mirkin, Northwestern University. Host: Xiao-Min Lin

The development of cantilever-free scanning probe techniques has allowed inexpensive, reproducible, and high-throughput patterning of both hard and soft nanomaterials over large areas. Specifically, scanning probe block copolymer lithography (SPBCL) allows one to generate nanoreactors consisting of polymers loaded with metal precursors, which upon thermal treatment can be converted into well-defined nanoparticles. Since one can finely tune both the size and composition of the polymer nanoreactors, nanoparticles of complex compositions can be synthesized with control over size from the 1 to 50 nm length scale. Furthermore, by using 1 million pyramidal pens in a single array, one can probe a wide variety of nanoparticles that systematically vary in size and composition. Nanoparticle libraries made in this manner can be metals, metal oxides, multimetallic alloys, and janus structures. This novel approach lays the foundation for creating new libraries of materials, where scale, in addition to composition becomes an important library parameter.

"Magnetic Skyrmions in Motions", Suzanne G.E. te Velthuis, Argonne National Laboratory, Materials Science Division. Host: Elena Rozhkova

Magnetic skyrmions are topologically stable spin textures that have been discovered in materials with Dzyaloshinskii-Moriya interactions. On one hand, the controlled manipulation of magnetic skyrmions in thin films at room temperature is envisioned to enable skyrmion-based spintronics, leading to energy-efficient device applications. On the other hand, there is the fundamental question as to whether for quasi-particles with a topological charge, like skyrmions, there is a Hall effect, analogous to the ordinary Hall effect of electrically charged particles. Experimental evidence for this skyrmion Hall effect has thus far remained elusive. In this presentation I will discus our work demonstrating how in heterostructures, inhomogeneous electric charge currents combined with the spin Hall effect in a heavy metal layer can be used to generate and manipulate magnetic skyrmions in an adjacent ferromagnetic layer [1]. Skyrmions, visualized using magneto-optical Kerr effect microscopy, are generated via diverging electric charge currents from stripe domains, with chiral domain walls, in a process that appears similar to droplet formation in surface-tension driven fluid flow. In the same materials system, we find experimentally that under application of sufficiently large homogeneous currents, the motion of magnetic skyrmions contains both longitudinal and transverse components, evidencing the skyrmion Hall effect [2]. Interestingly, the derived skyrmion Hall angle is shown to first increase with increasing current density, after which it saturates. The behavior indicates pinning due to defects plays a role in determining the dynamics. A maximum skyrmion Hall of 32 deg is obtained, close to the expected value for the relatively large skyrmions (1 micron diameter) in this system. The dependence of the sign of the skyrmion Hall angle on the topological charge (+/-1) is demonstrated, illustrating the potential for topological sorting.

"Dopants and Charge Carriers in Colloidal Quantum Dots", Daniel Gamelin, University of Washington, hosted by Elena Shevchenko

The capacity to control the physical properties of semiconductors by deliberate introduction of impurities or extra charge carriers has been exploited extensively in information processing devices, solar cells, and other semiconductor technologies. This talk will describe some of our group's recent research activities aimed at the development of fundamentally new materials via nanocrystal doping, and the discovery and description of new physical properties that emerge upon nanocrystal doping, with either electronic dopants, luminescence activators, or spin-bearing impurities. These new materials and properties have broad fundamental and practical implications that may impact future QD-based solar energy conversion, light-emission, or bio-imaging technologies.

April 6, 2016
4:00 pm
Bldg. 440, A105-106

"Structural and Chemical Manipulation of 2D Nanomaterials: Graphene, MoS2, Boron Nitride", Vikas Berry, University of Illinois at Chicago, hosted by Gary Wiederrecht

The presentation with have three parts: (a) Wrinkling of 2D nanomaterials, (b) oxide assisted growth of BN, and (c) MoS2 functionalization for electronic applications. (A) A graphene flap-valve will be demonstrated that allows only one-way expulsion of interfaced biological cell’s aqueous content to ‘indefinitely desiccate’ and shrink cells even under high humidity. We apply this process to form axially-aligned graphene-wrinkles to develop a device with anisotropic electrical properties. (B) Ultra-smooth hexagonal boron nitride (h-BN) can dramatically enhance the carrier/phonon transport of interfaced 2D nanomaterials. Boron-oxygen and Boron-nitrogen chemistries for oxide- and nitride- assisted nucleation and growth of large-area, uniform and ultrathin h-BN directly on oxidized substrates will be outlined. (C) Ultrathin two dimensional metal dichalogenide (MoS2, WS2, so forth) exhibits confinement of carriers, evolution of band structure, high on/off rectification, and high thermal absorption. However their incorporation into other systems requires controlled functionalization and/or interaction with other nanoscale entities. Stable sulfur/nobel metal functionalization via both diffusion limited aggregation and instantaneous reaction arresting (using microwaves) on MoS2 crystallographic edges (with 60o displacement) will be presented.

April 5, 2016
4:00 pm
Bldg. 440, A105-106

"XMCD Studies of Applied Magnetic Materials and the Related Instrumentation at SPring-8", Tetsuya Nakamura, Japan Synchrotron Radiation Research Institute

Magnetic materials have wide variety of applications in computing devices and energy saving products. X-ray magnetic circular dichroism (XMCD) has become a useful method for investigating these materials because of the advantageous element and shell specificities. The technique is capable of application in microscopy, time resolved measurements, and extreme sample environments such as high magnetic fields and low temperatures. Although the XMCD experiment itself has become very familiar and is no longer new, the importance of the method and the requirement of the instrumentation are still in high demand for studying various kinds of magnetic materials.

In this seminar, I will present XMCD studies which were conducted at the soft X-ray beamline, BL25SU, of SPring-8. These studies cover the topics of high magnetic field XMCD experiments with a 40 T pulsed magnet, and also of scanning XMCD microscopy using an 8 T superconducting magnet. In the pulsed magnet apparatus, the magnetic-field-induced valence transition in EuNi2(Si0.18Ge0.82)2 [1] and the valence-specific magnetization of LuFe2O4 and associated multiferroicity [2] have demonstrated the capabilities of the high magnetic field XMCD technique [3]. On the other hand, the scanning XMCD apparatus was developed mainly for the purpose of investigating magnetic domains in a Nd-Fe-B sintered magnet, which is the best permanent magnet with high coercivity. Since the magnetic anisotropy field of Nd2Fe14B is about 7.3 T, the 8 T superconducting magnet was introduced and combined with the scanning XMCD apparatus [3]. One of the main features of this observation technique is not only in high magnetic fields, but in observing irregular surfaces because other conventional magnetic domain observation methods under high magnetic fields need flat surfaces or transmittable thin films. This advantage of scanning XMCD is very crucial for our purpose of observing the fractured surface of Nd-Fe-B permanent magnets, in which the fractured surface keeps its original coercivity well, in contrast to polished surfaces which lose their high coercivity. The spatial resolution of about 100 nm in the present instruments is moderate in terms of modern scanning XMCD experiments in other facilities. However, the longer focusing depth coming from the longer focusing distance is very useful for observing the fractured surfaces of Nd-Fe-B sintered magnets without refocusing during the scanning. A low temperature option and a smaller beam are under consideration to further the application of scanning XMCD microscopy under high magnetic fields.

"Memoir of the Manhattan Project and a Few of its People", Murray Peshkin, Argonne National Laboratory, Physics Division

I was a young undergraduate student who was drafted into the U.S. army and assigned to Los Alamos during World War II. I was lucky to become Richard Feynman's personal calculator. That position enabled me to observe from below a remarkable project and a few of the great men who carried it out. I will give a very personal impression of what I observed and also of how this country looked to a then-young man during and shortly after the war. I will also briefly present reflections of a now-old man about the consequences and the wisdom of what we did.

A true inspiring story from a generation who have accomplished an extraordinary task in the most difficult time in history!

"Photonic Crystal Acoustic Sensors for Functional Characterization of Stem Cells", Olav Solgaard, Stanford University, hosted by Il Woong Jung

Photonic Crystal mirrors have unique properties, including monolithic crystalline structure, flexible phase response, mechanical strength and chemical robustness, that make them ideal for sensor applications where sensitivity, ability to operate in challenging environments, miniaturization, and long term stability are important. In this talk, we outline fundamental capabilities, scaling properties, design, and fabrication of Photonic Crystal sensors, and we report on their application to characterization and sorting of cardiomyocytes derived from pluripotent stem cells.

"Single-cell Transcriptomics and Biology using Droplet Microfluidics", Anindita Basu, Broad Institute of MIT and Harvard, hosted by Tijana Rajh/Supratick Guha

We developed ‘Drop-Seq’, a high-throughput technique to profile the transcriptomes of single mammalian cells using emulsion microfluidics and DNA barcodes. This is accomplished by (a) encapsulating and lysing one cell per emulsion droplet, and (b) barcoding RNA contents from each cell using unique DNA-barcoded micro-beads. This enables us to study the transcriptional behavior of a large number of cells at single-cell resolution. We then couple and/or extend this technique to study transcriptional responses of isolated mammalian cells to different bio-chemical and pathological stimuli provided in the microfluidics devices; some examples will be provided to illustrate applications.

"Multiple Phonon Scattering at Proximal Interfaces", Pawel Keblinksi, Rensselaer Polytechnic Institute, hosted by Pierre Darancet

An interface scatters phonons and thus poses resistance to the heat flow, in addition to the bulk resistance of the material. The associated interfacial thermal resistance can dominate the overall heat flow when the density of the interfaces is high, such as in nanoscale and interfacial materials. When two (or more) interfaces or junctions are at a distance smaller than the phonon mean path, the interfacial resistances of each interface are not independent. Using molecular dynamics simulations and phonon scattering based analysis we will study heat flow mechanisms across proximal interfaces in various systems including self-assembled organic monolayers between two solids, nanoscopic solid adlayer on a substrate and molecular junctions. We will demonstrate the presence and role of multiple phonon scattering and interference effects on individual phonons and overall interfacial thermal transport.

January 6, 2016