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Physics & Astronomy Department

Upcoming Colloquia

photo of Jason Gardner

Jason Gardner, Songshan Lake Materials Laboratory

Date: February 27, 2020. Time: 4:10-5:00 p.m. Location: Physical Sciences Rm. 234. Title: “Slow Spin Dynamics in Geometrically Frustrated Magnets.” Abstract: Frustration is a general concept in physics to describe the effects that occur when interactions of similar strength, compete and prevent a system from settling into a unique ground state. Geometrically frustrated magnets can exhibit unusual phenomena at low temperatures such as concentrated spin glass, partial order, spin ice and spin liquid states. Over the past 25 years a significant amount of work has been performed on these materials as the physics community grew interested. Today, physicists realize such magnets might be the core of many quantum technologies. Within this presentation, Jason will concentrate specifically on the slow spin dynamics found in these systems; the presence of such dynamics at the lowest temperatures is a signature of quantum spin liquids. Using neutron spin echo, backscattering spectrometers he has probed the electronic and nuclear spin systems of several geometrically frustrated magnets. He will focus on several quantum spin liquids, but will also discuss how the study the low energy properties can differentiate the properties of the spin ices, Ho2Ti2O7 and Dy2Ti2O7.

Photo of Paul Huang

Paul Huang, University of Oklahoma

Date: February 19, 2020. Time: 12:00-1:00 p.m. Location: Classroom Building Rm 103. Title: "Water at Solid Interfaces: Structure, Property and Manipulation." Abstract: A molecular-level understanding of near-surface water structure and a handy manipulation of its properties are relevant to a wide range of scientific and technological phenomena. In this presentation, we report the observation of a solid-like first adsorbed water layer (FAWL) and its tunable wetting transition at three metal surface models, namely, Au (100), Pd (100) and a Pd (100)/Au (100) bimetallic junction. Molecular dynamics simulation results reveal that: (a) there is a formation of the FAWL, resulting from competitive water-water hydrogen bonding and water-solid interactions, which in turn dictates the wettability at water/metal interfaces; (b) applying compressive lattice strain to metal substrates can induce interfacial wettability transition, which is mediated by subtle packing changes of the FAWL; (c) by adjusting the lattice strains, the bimetallic junction can host a switchable wettability transition. Besides this case, the behavior of water at two other interfaces (TiO2 and carbon nitride) will be also briefly discussed. We anticipate that those findings provide a rigorous fundamental understanding of how water wets a surface and how that wettability could be utilized purposely.

Picture of Xufeng Zhang

Xufeng Zhang, Argonne National Laboratory

Date: February 17, 2020. Time: 4:10-5:00 p.m. Location: Physical Sciences Building 234. Title: "Advancing Quantum Information Science with Hybrid Cavity Magnonics." Abstract: With recent demonstration of quantum computing and quantum communication, quantum information science has been changing our world in an unprecedented way. To fully explore the power of quantum information processing, it is important to further combine discrete quantum elements and build distributed quantum networks. However, this poses significant technical challenges because quantum coherence can be easily destroyed as the signal propagates through different systems. In this talk, I will show that magnons—the collective excitations of magnetization—provide a promising solution for efficiently converting quantum information among different forms while preserving the coherence. Specifically, cavity magnonics exhibits excellent compatibilities with other physical systems in the microwave, mechanical and optical domains. Thanks to the large spin density in our system, the interactions between magnons and other information carriers such as photons and phonons are drastically boosted which is critical for protecting the signal coherence. Most importantly, the tunability of magnons provides unparalleled controllability in the transduction process. Therefore, coherent magnon-based signal transduction can be achieved. I will finish the talk by describing opportunities toward on-chip integration of quantum magnonics.

Photo of Nicholas Borys

Nicholas Borys, Montana State University

Date: February 13, 2020. Time: 4:10-5:00 p.m. Location: Classroom Building Room 222. Title: “Excitons in 2D Atomically Thin Semiconductors." Abstract: Transition metal dichalcogenide semiconductors, such as monolayer MoS2, are an emergent class of ultrathin thin semiconductors that are only three atomic layers thick yet host a rich suite of photophysical phenomena that provides new opportunities ranging from fundamental investigations of many-body physics to the development of new optoelectronic and quantum devices. In these atomically thin semiconductors, the absorption of light creates an “exciton,” which is an excited electronic state composed of a negatively charged conduction band electron that is tightly bound to a positively charged valence band hole. Like molecules, excitons govern light-matter interactions such as absorption and emission in 2D semiconductors and are fundamental packets of energy that can be leveraged for next-generation technologies. Using time-resolved and nano-optical spectroscopy techniques to access excitonic physics at extreme length and time scales, a striking diversity of excitonic phenomena has been identified in these 2D materials. Building on previous results, our newest findings in these regimes highlight how exciton populations can be coerced into interacting to form bound states of multiple electrons and holes as well as how strain localizes excitons on length scales that are commensurate with their size. These new results demonstrate the exciting potential of monolayer semiconductors to be utilized for model optoelectronic and quantum devices with unique functionalities derived from 2D excitonic physics.

Photo of Xin Fan

Xin Fan, University of Denver

Date: January 30, 2020. Time: 4:10-5:00 p.m. Location: Physical Sciences Building 234. Title: “Transverse Spin-Orbit Effects in Magnetic Materials.” Abstract: The anomalous Hall effect, discovered by Edwin Hall in 1880, describes a phenomenon that an electric current perpendicular to magnetization of a magnetic material can produce a charge accumulation in the direction orthogonal to both electric current and magnetization. Through century-long theoretical and experimental efforts, it is now understood that the anomalous Hall effect arises from the spin-orbit coupling. The understanding of the anomalous Hall effect has also led to the discovery of new spin-orbit effects, such as the spin Hall effect, where an electric current generates spin accumulations in nonmagnetic materials. However, despite the comprehensive understanding, I will argue that the anomalous Hall effect is incomplete in describing the spin-orbit coupling-induced phenomenology in magnetic materials. There exist a group of spin-orbit effects associated with transverse spins – spins polarized perpendicular to the magnetization, which have been previously overlooked. We refer to this group of spin-orbit effects as transverse spin-orbit effects in magnetic materials. In my talk, I will discuss our experimental observations of two unique transverse spin-orbit effects: (1) spin-to-charge interconversion with unconventional spin rotation symmetry and (2) the generation of anomalous spin-orbit torque in a single layer magnetic film – a hidden counterpart to the anomalous Hall effect.

Photo of Neil Cornish

Neil Cornish, Montana State University

Date: November 22, 2019. Time: 3:10-4:00 p.m. Location: Physical Sciences Building 234. Title: “Gravitational Wave Astronomy: Now and When.” Abstract: It is now four years since the first detection of gravitational waves from a binary black hole merger, and two years since the first detection of a binary neutron star merger and subsequent kilonova explosion. The LIGO and Virgo detectors are now recording events on a weekly basis, and providing unique insights into the properties of some of the most extreme phenomena in the Universe. In the near future, a galactic-scale gravitational wave detector - built from pulsars - will detect the signals from supermassive black hole binaries, and a space based detector will open up the source-rich milli-Hertz frequency band. Please join me for a whirlwind tour of the past, present and future of gravitational wave astronomy.

Natalie Gosnell

Natalie Gosnell, Colorado College

Date: November 1, 2019. Time: 3:10-4:00 p.m. Location: Physical Sciences Building 234. Title: "Telling the Story of Blue Straggler Mass Transfer Through their White Dwarf Companions." Abstract: Our understanding of stellar evolution is built upon a synergy between observations and theory. Open star clusters (groups of stars in our galaxy that formed at the same time out of the same material) provide a laboratory for testing stellar evolution theories and illuminate where our current understanding fails. In depth studies of open clusters reveal that evolved stars frequently do not agree with simple models, and instead follow alternative pathways in stellar evolution. These stars are often excluded from studies for being rare or anomalous, but we now know that they make up a considerable fraction of evolved stars. The majority of these alternative pathway stellar products are blue straggler stars, whose dominant formation mechanism was an outstanding question for almost six decades, but we now know that most blue straggler stars form through mass transfer from a giant companion, resulting in a binary system with what is now a blue straggler and a white dwarf. I will discuss my work on constraining the histories of blue straggler stars using ultraviolet spectroscopy from the Hubble Space Telescope, illuminating areas where our current understanding of mass transfer is insufficient.

Nan Jiang

Nan Jiang, University of Illinois

Date: October 11, 2019. Time: 4:10-5:00 p.m. Location: Physical Sciences Building 234. Title: “Probing Intermolecular and Molecule-Substrate Interactions at Angstrom Scale via Scanning Tunneling Microscopy and Tip-Enhanced Raman Spectroscopy." Abstract: My research group is broadly interested in spectroscopically determining how local chemical environments affect single molecule behaviors. We focus on highly heterogeneous systems such as molecular self-assembly and bimetallic catalysis, developing and using new imaging and spectroscopic approaches to probe structure and function on nanometer length scales. This talk will focus on Tip-Enhanced Raman Spectroscopy (TERS), which affords the spatial resolution of traditional Scanning Tunneling Microscopy (STM) while collecting the chemical information provided by Raman spectroscopy. By using a plasmonically-active material for our scanning probe, the Raman signal at the tip-sample junction is incredibly enhanced, allowing for single-molecule probing. This method, further aided by the benefits of ultrahigh vacuum, is uniquely capable of obtaining (1) single molecules chemical identification; (2) the molecular mechanism of chemical bond formation under near-surface conditions using self-assembly concepts; (3) adsorbate-substrate interactions in the ordering of molecular building blocks in supramolecular nanostructures. By investigating substrate structures, superstructures, and the adsorption orientations obtained from vibrational modes, we extract novel surface-chemistry information at an unprecedented spatial (<1nm) and energy (<10 wavenumber) resolution. We are able to interrogate the impact of changes in the chemical environment on the properties of supramolecular nanostructures, and thereby lay the foundation for controlling their size, shape and composition.

Vivek Amin

Vivek Amin, University of Maryland

Date: October 10, 2019. Time: 3:10-4:00 p.m. Location: Physical Sciences Building 234. Title: "Anatomy of Spin-Orbit Torque." Abstract: Information and communications technology is predicted to account for 10% to 20% of the world’s power consumption within a decade. Alleviating this rise in power consumption requires rethinking the way we electronically process and store information. Spintronics, or spin electronics, offers a possible solution to this problem by using spin currents or spin waves rather than conventional charge currents to manipulate information. A key ingredient in spintronics is spin-orbit coupling: the relativistic coupling between a particle’s spin and orbital moments. Spin-orbit coupling permits conduction electrons to extract a virtually unlimited amount of angular momentum from the crystal lattice, potentially enabling energy efficient information processing. In this talk, I will discuss the electrical manipulation of a ferromagnet’s magnetization through spin-orbit coupling. This phenomenon, known as spin-orbit torque, could help harness all the advantages of different electronic memories (e.g. speed, nonvolatility, radiation hardness) into one device. The present understanding of spin-orbit torque is incomplete because there is no consensus among theory and experiment over the important mechanisms. We review the traditional spin-orbit torque mechanisms and then show that novel interfacial or bulk effects are needed to explain recent experiments. Shedding light on these mechanisms will help clarify the nature of spin-orbit torque, creating exciting new possibilities for current-controlled magnetization dynamics with attractive applications for information processing.

Upcoming Physics & Astronomy Colloquium Information

Physics & Astronomy Colloquia

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