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Past Visitors

Josephson Detection of Multiband Effects in Superconductors

Talk time: 
Thursday, January 16, 2020 - 4:00pm
Talk location: 
321 Allen Hall
Abstract: 

The study of unconventional superconducting materials remains an active frontier of condensed matter physics. Exotic superconductivity, such as high TC, topological, and heavy-fermion superconductors, often rely on phase sensitive measurements to determine the underlying pairing and/or the nature of novel excitations. In this talk I will detail the use of Josephson effect to detect novel properties of two multiband superconducting systems: NbSe2 [1] and SnTe [2]. Focus will primarily be given modification of conventional Josephson effects due to the loss of time reversal symmetry found to exist in proximity-induced Josephson junctions of SnTe nanowires [2]. These works open new routes to exploration of multiband effects in superconductors and have important implications for topological states in superconducting materials. 

[1] S. Tran, J. Sell and J. R. Williams, “Dynamical Josephson Effects in NbSe2”, arXiv:1903.00453 (2019).

[2] C. J. Trimble et al., “Josephson Detection of Time Reversal Symmetry Broken Superconductivity in SnTe Nanowires”, arXiv:1907.04199 (2019). 

Chromophore Packing and Singlet Fission Rates

Talk time: 
Thursday, December 5, 2019 - 4:00pm
Talk location: 
321 Allen Hall
Abstract: 

Results of a theoretical examination of the effect of crystal packing on singlet fission (SF) rate are presented.  For a model system (pair of ethylenes) and several known or suspected SF materials: tetracene (1), cibalackrot (2), and 1,3-diphenylisobenzofuran (3), we predict molecular pair arrangements that are especially favorable for the rate of SF.  The predictions are obtained from an approximate evaluation of squares of SF eectronic matrix elements, based on a complete search of all possible pair packing geometries.  This is refined by an evaluation of intermolecular contributions to the energy balance of the SF process, followed by Marcus theory evaluation of SF rates, which are used to reoptimize the best geometries.  We also evaluate biexciton binding energies. Going beyond the prediction of optimal geometries, the model has been applied to a series of variously fluorinated derivatives of 3 and found to determine successfully which ones perform SF and which ones yield excimers instead.  Although a consideration of molecular pairs thus appears to be a useful first step and may be sufficient in some cases, experimental evidence  shows convincingly a need to consider at least three molecules at a time in the future.  Evaluation of triplet yields will require an additional evaluation of rates of competing processes, especially charge separation and excimer formation.

Quantum sensing and quantum nanophotonics at ORNL

Talk time: 
Thursday, November 21, 2019 - 4:00pm
Talk location: 
321 Allen Hall
Abstract: 

Two-mode squeezed light sources exhibiting continuous variable entanglement allow us to reduce the noise floor in optically transduced sensors below the standard quantum limit, enabling greater signal to noise ratios than are possible in the best possible classical sensors.  I will present some of our recent results demonstrating quantum enhanced sensitivity for applications ranging from magnetometry to plasmonic sensing to atomic force microscopy. I will also discuss some of our recent research efforts exploring quantum nanophotonics with plasmonic nanostructures and single photon emitters in low-dimensional materials.  I will close with a summary of our new research programs centering on nano- and mesoscale characterization of quasiparticle, vortex, and two-level-system interactions with light in superconducting quantum devices.

Quantum quench and nonequilibrium dynamics in lattice-confined spinor Bose-Einstein condensates

Talk time: 
Monday, October 21, 2019 - 4:00pm
Talk location: 
102 Thaw Hall
Abstract: 

Bose-Einstein condensates (BECs) are ultra-cold gases, in which all atoms have a single collective wavefunction for their spatial degrees of freedom. With an additional spin degree of freedom, spinor BECs constitute a collective quantum system offering an unprecedented degree of control over such parameters as spin, density, temperature, and the dimensionality of the system. Spinor BECs have thus been considered as good quantum simulators for verifying and optimizing condensed matter models. In this talk, I will discuss a novel quantum phase transition realized in our antiferromagnetic spinor BEC system. I will also present our experimental study on nonequilibrium dynamics of a spinor BEC after it is quenched across a superfluid to Mott insulator phase transition in cubic lattices. Intricate few-body dynamics consisting of spin-mixing oscillations at multiple frequencies are observed after distinct quantum quench sequences. We confirm these observed spin-mixing spectra can be utilized to reveal atom number distributions of an inhomogeneous system, to study transitions from two-body to many-body spin dynamics, and to precisely measure two key parameters determining the spinor physics.

Band Engineering for Quantum Simulation in Circuit QED

Talk time: 
Thursday, October 10, 2019 - 4:00pm
Talk location: 
104 Thaw Hall
Abstract: 

The field of circuit QED has emerged as a rich platform for both quantum computation and quantum simulation. Lattices of coplanar waveguide (CPW) resonators realize artificial photonic materials in the tight-binding limit. Combined with strong qubit-photon interactions, these systems can be used to study dynamical phase transitions, many-body phenomena, and spin models in driven-dissipative systems. I will show that these waveguide cavities are uniquely deformable and can produce lattices and networks which cannot readily be obtained in other systems, including periodic lattices in a hyperbolic space of constant negative curvature. Furthermore, I will show that the one-dimensional nature of CPW resonators leads to degenerate flat bands and that criteria for when they are gapped can be derived from graph-theoretic techniques. The resulting gapped flat-band lattices are difficult to realize in standard atomic crystallography, but readily realizable in superconducting circuits.

Artificial Atoms: Quantum Optics and Spin Physics of Quantum Dots

Talk time: 
Thursday, October 3, 2019 - 4:00pm
Talk location: 
321 Allen Hall
Abstract: 

Semiconductor quantum dots (QDs) are nanoscopic crystals that are often called artificial atoms. Charge carriers trapped within them have discrete energy levels in the fashion of single atoms, and they absorb and emit light at discrete wavelengths corresponding to those energy levels. Because of this, in many ways QDs behave like the canonical two-level quantum system, which makes them suitable for experiments involving the quantum nature of light, which is called quantum optics. For this reason and for their potential uses in quantum information applications, QDs attract great scientific interest.
 
I will describe how QDs demonstrate quantum optical behaviors exhibited by single atoms, showing that they really do act like artificial atoms. Then I'll discuss ways in which the solid-state environment of the QDs complicates the situation in ways that single atoms do not experience. Finally, I'll present some recent work in controlling the spin of an electron trapped in a QD with the aim of using it as a quantum bit.

SEISMIC: The Sloan Equity and Inclusion in STEM Introductory Courses Project

Talk time: 
Monday, September 23, 2019 - 4:00pm
Talk location: 
102 Thaw Hall
Abstract: 

Equity and inclusion are important goals for higher education. Data can play a central role in achieving these goals. First, data are essential for probing equity. To provide an example, I will describe the discovery of a pattern of gendered performance differences in large foundational courses, both at Michigan and at an array of other Universities. Data can also help create solutions, as when we test new course designs and develop tools that personalize education. 
Over the last year, a group of ten large public research universities have launched the Sloan Equity and Inclusion in STEM Introductory Courses, or SEISMIC project. Dozens of faculty, staff, and students from these institutions are working to connect STEM education research to practice in a national “learning laboratory.” Together, we hope to provide the evidence necessary to motivate change, and find practical ways to make our courses more equitable and inclusive. 

Phase-field Modeling of Polar States in Ferroelectric Heterostructures

Talk time: 
Thursday, March 28, 2019 - 4:00pm
Talk location: 
321 Allen Hall
Abstract: 

This presentation will discuss the applications of the phase-field method to understanding and discovering new mesoscale polar states that might emerge from nanoscale ferroelectric heterostructures subject to different mechanical and electric boundary conditions. As an example, the determination of thermodynamic conditions and geometric length scales leading to the formation of ordered polar vortex lattice as well as mixed states of regular domains and vortices in ferroelectric superlattices of PbTiO 3 /SrTiO 3 using phase-field simulations and analytical theory will be presented. Switching of these vortex lattice states might produce other transient polar states such as polar skyrmions. It is shown that the stability of these vortex lattices involves an intimate competition between long-range electrostatic, long-range elastic,
and short-range polarization gradient-related interactions leading to both an upper- and a lower- bound to the length scale at which these states can be observed. We further predicted the periodicity phase diagrams that show excellent agreements with experimental observations by collaborators.

Chen is the Donald W. Hamer Professor of Materials Science and Engineering, Professor of Engineering Science and Mechanics, and Professor of Mathematics at Penn State. He received his Ph.D. from MIT in Materials Science and Engineering in 1990 and joined the faculty at Penn State in 1992. He has published over 600 papers in the area of computational microstructure evolution and multiscale modeling of structural metallic alloys, functional oxides, and energy materials. For his research accomplishments, he has received numerous awards including the 2014 Materials Research Society (MRS) Materials Theory Award, a Guggenheim Fellowship, and a Humboldt Research Award. He is the Editor-in-Chief for npj Computational Materials published by Springer- Nature.

PRL at 60: You have your physics results, now what?

Talk time: 
Monday, March 25, 2019 - 4:00pm
Talk location: 
A301D Doherty Hall
Abstract: 

In a talk that I am really hoping will morph into a free-flowing Q and A session, I will discuss the role that PRL plays in disseminating your physics results. The process is a cascading sequence that entails interacting with journal editors, referees, conference chairs, journalists, department chairs, deans, funding agencies, and others. The tools however have changed in recent years; the arrival of social media, search engines, and electronic repositories has us in a state of flux. PRL published its first paper 60 years ago. Let's look back and forward.

My escape from the lab: scientific publishing

Talk time: 
Thursday, March 14, 2019 - 4:00pm
Talk location: 
321 Allen Hall
Abstract: 

Across the world and across disciplines, numbers reveal that the term “alt-ac” – referring to positions within higher education and research alternative to the professoriate – is a misnomer. Permanent academic jobs are, in fact, the “alt-ac”. In this talk, I’ll share my (happy) experience going from a computational chemistry lab to my current career on the “other side” of scientific publishing, and explores roles for STEM Ph.D.s in the publishing industry.

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