Eric Sembrat's Test Bonanza

In February 2021, NASA's Perseverance rover landed successfully at the Octavia E. Butler Landing Site in the Jezero Crater, the site of an ancient open-system lake on Mars. Perseverance is seeking signs of ancient life, and is collecting Martian rock and soil samples for possible return to Earth by a future mission. Upon completing a 90-day commissioning phase, Perseverance embarked on an exploration and sampling campaign of the rocks and outcrops comprising the present-day floor of Jezero Crater. This abstract summarizes the mission results from Perseverance's first eight months on Mars, including the lead-up to the collection of the mission's first sample. 

Virtual talk: 11:25 a.m.-12:15 p.m. via Blue Jeans:  http://primetime.bluejeans.com/a2m/live-event/udqsqwfd

Social Event: 4-6 p.m., Molecular Science and Engineering Building, ground floor outdoor patio

Optically-trapped, ultra-cold gases of spin ½-up and spin ½-down ⁶Li atoms enable “designer” interactions, offering a versatile environment for simulating exotic quantum systems that span a vast range of energy. A strongly interacting gas is a scale-invariant, nearly perfect hydrodynamic system with universal transport coefficients, enabling parameter-free comparison with predictions, where there is currently some tension.  I will discuss our latest measurements in a “box” potential, where these coefficients are directly extracted from the time-dependent response of a cloud to small perturbations. Then I will discuss measurements of information scrambling in the very weakly interacting regime, where the cloud behaves as a large spin lattice in energy space, with effective long-range interactions.

Bio:

John E. Thomas received his B. S. degree in Physics at MIT in 1973 and his Ph. D. in Physics at MIT in 1979. John joined the Physics Department at Duke University in 1986 and was named the Fritz London Distinguished Professor in 2004. In 2011, John received the Jessie Beams Award for Research from SESAPS and moved his research group (JETlab) to North Carolina State University, where he is currently the John S. Risley Distinguished Professor of Physics. John is a Fellow of the American Physical Society, a member of the Optical Society of America and a Fellow of the American Association for the Advancement of Science. In 2018, he received the APS Davisson-Germer Prize for his research on unitary Fermi gases.

Abstract

Quantum Hall states are a prominent example of exotic topological states of matter. The signature property of QH states, namely, the quantization of Hall conductance, is well-appreciated, and independent of sample-specific details, to the extent that it is used for precise measurements of fundamental constants. Less well understood, and at the frontier of current research, is how the geometry of these states responds to gravitational perturbations, i.e., deformations to the real space manifold they are embedded in, and what if any universal signatures characterize this response. In this talk I will discuss how remarkable new universal behaviors emerge when probing the gravitational response of quantum Hall states. By exploiting novel aspects of the quantum geometry of charged particles in a magnetic field, I will show that these responses can be characterized not only by considering QH states in curved spaces, but equivalently, by placing them in non-uniform electric fields, thus facilitating experimental tests of these results. I will conclude by noting how the quantum geometry can be combined with the theory of coherent states to provide an analytical route, hitherto elusive, for deriving the properties of fractional quantum Hall phases from experimentally relevant microscopic Hamiltonians.

Abstract

The field of nanophotonics is based on the ability to confine light to nanoscale dimensions. Within the mid- to far-infrared, such confinement inherently implies overcoming the diffraction limit due to the long free-space wavelengths. Through the implementation of polaritons one can overcome the diffraction limit through the formation of quasiparticles formed by coupling of coherent oscillating charges with photons. A whole suite of potential polaritons can be realized through careful choice of the charge, with two predominant types being the surface plasmon (free carriers) and phonon (bound charge on ionic lattice) polaritons in the infrared. While each form offers significant advantages, significant restrictions also remain. As such, identifying novel materials with unique optical functionalities and the creation of hybrid materials where the properties and function can be designed are imperative for advanced IR devices to be realized. This talk will discuss recent advancements from our group including low-loss plasmonic conducting oxides for novel infrared sources through hybridization of optical modes, the observation and exploitation of the natural hyperbolic response of hexagonal boron nitride and molybdenum trioxide for on-chip photonics, as well as the implementation of hybridization of polaritonic modes and manipulation of the phonon dispersion and density of states as a means to design infrared nanophotonic materials. Beyond this, methods to improve material lifetime, realize active modulation, control polariton propagation with nanoscale precision and to provide additional functionality will be discussed.

Bio

Professor Joshua Caldwell is the Flowers Family Chancellor Faculty Fellow and Associate Professor of Mechanical Engineering at Vanderbilt University. He was awarded his Bachelor of Chemistry from Virginia Tech in 2000 before heading to the University of Florida where he received his PhD in Physical Chemistry in 2004. There he used magnetic resonance methods to investigate electron-nuclear spin coupling within low-dimensional quantum wells and heterostructures. He accepted a postdoctoral fellowship at the Naval Research Laboratory in 2005, using optical spectroscopy as a means of understanding defects within wide-band gap semiconductors. He was transitioned to permanent staff in 2007, where he began work in the field of nanophotonics, investigating coupling phenomena within plasmonic materials. Prof. Caldwell merged his prior work in wide band gap semiconductor materials with his efforts in nanophotonics, leading to his work exploiting undoped, polar dielectric crystals for low-loss, sub-diffractional infrared optics. He is a three-time recipient of the highly competitive NRL Nanoscience Institute grants and was promoted to senior (supervisory) staff at NRL in 2012. He was awarded a sabbatical at the University of Manchester with Prof. Kostya Novoselov in 2013-2014, investigating the use of van der Waals crystals such as hexagonal boron nitride for mid-IR to THz nanophotonics, where he demonstrated the natural hyperbolic response of this material. During his time at NRL he was a 4-time recipient of the Alan Berman Best Pure Science Paper Award and received the Thomas Edison Best Patent Award for his dry transfer technique for 2D materials. In 2017 he accepted a tenured Associate Professorship at Vanderbilt University within the Mechanical Engineering Department. He was elected as a Fellow of the Materials Research Society in 2020 and has published over 150 papers, >7100 citations and 11 patents, with two more pending.

Abstract

In recent years, the thermalization of quantum systems has been the subject of intense study. Particularly interesting are systems which exhibit slow or completely arrested thermalization, with many-body localized systems being a prime example of the latter. Recent theoretical work has identified Hilbert space fragmentation in clean, kinetically-constrained systems as another mechanism for the breakdown of ergodicity in many-body quantum systems. Motivated by engineering such systems with ultracold atoms, I will discuss two recent experiments we have performed. In the first, we studied tilted Fermi-Hubbard systems and discovered a slow thermalization mechanism due to an interplay of charge and heat transport. Modified versions of this system may be used to explore prethermal Hamiltonians with a fragmented Hilbert space. In the second experiment, we studied the short-time quench dynamics of charge-density wave states in a spinless fermionic lattice gas with off-site interactions realized with Rydberg-dressing. We again observed a slowdown of the dynamics for strong, off-site interactions. We discuss connections of this experiment to theoretical work on fragmentation in t-V models.

How does learning occur? In the context of neural networks, learning occurs via optimization, where a loss function is minimized to achieve the desired result. But physical networks such as mechanical spring networks or flow networks cannot minimize such a loss function by themselves—they need the help of a computer.

An alternative is to encode local rules into those networks so that they can evolve under external driving to develop function. For example, if the springs in a mechanical network have equilibrium lengths that grow if the springs are stretched, and shrink when the springs are compressed, the network will naturally evolve under applied stresses.

I will describe how both of these strategies—global minimization of a loss function as well as training by local rules--can be used to teach materials how to perform functions inspired by biology, such as the ability of proteins (e.g. hemoglobin) to change their conformations upon binding of an atom (oxygen) or molecule, or the ability of the brain’s vascular network to send enhanced blood flow and oxygen to specific areas of the brain associated with a given task.

Abstract

The quantum theory of magnetism has provided many durable paradigms for quantum phases of matter, including intrinsically quantum disordered states, symmetry-protected topological phases, and quantum spin liquids.  It also served as a birthing ground for many important developments in the theory of quantum phase transitions.  In this lecture, I will review some of the history and highlights of this very rich field.

Abstract

Graphene, an atom thick layer of carbon atoms, was first isolated in 2004. By now its electronic properties are very well known, in particular it is well known that graphene does not have any tendency to be a superconductor. However, in the last two years experimentalists have shown that a system formed by two layers of graphene, when stacked with a specific --"magic"-- relative twist angle, can become superconducting. One of the key features of twisted bilayer graphene is the fact that its electronic bands are extremely flat. The flat nature of the bands favors the formation of collective ground states. However, standard results suggest that for a system with flat bands the superconducting current should be very small and therefore that superconductivity could be observed only at vanishingly small temperatures. This is not the case in magic-angle twisted bilayer graphene. In this talk I will first introduce the key properties of "magic-angle twisted bilayer graphene". I will then discuss how it is possible for such a system to show all the hallmarks of superconductivity despite the fact that, due to the extreme flatness of the bands, results valid for standard superconductors would lead to conclude that the macroscopic signatures of superconductivity should be absent.

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References:

"Geometric and conventional contribution to superfluid weight in twisted bilayer graphene"

Xiang Hu, Timo Hyart, Dmitry I. Pikulin, Enrico Rossi

Phys. Rev. Lett, 123, 237002 (2019)

Abstract

In this talk, I will present results from a high-precision DMRG study of universal finite-size corrections to the central charge at Berezenskii-Kosterlitz-Thouless transitions. The central charge is an important property of conformal field theories in two spatial dimensions that corresponds roughly to the number of bosonic degrees of freedom in the theory. I provide a quick review of John Cardy’s derivation of the logarithmic corrections of interest and how a DMRG study of the 1D Heisenberg model is the best way to verify them numerically. Then I will review my carefully performed analysis and numerical results. I will end my talk by proposing to test the performance limitations of a MERA network on this problem.

Bio

Dr. Spalding graduated with a physics PhD from University of California Riverside in 2020 with a thesis entitled "Extended hubbard model: Studies on quantum information and disorder".

Absract 

Typical quantum systems prepared in an out-of-equilibrium state will quickly approach a state of thermal equilibrium, even when there is no coupling to an external thermal bath. The approach to equilibrium is governed by a classical hydrodynamics of diffusing charges without significant quantum effects. A growing number of new scenarios that escape this classical description and remain out of equilibrium forever (or for parametrically long times) have been discovered, and are newly accessible to coherent quantum experiments. In certain 1D systems, closeness to an "integrable point" with an extensive number of extra symmetries delays equilibrium, and hydrodynamics is substituted by a strange hydrodynamic-like theory with many anomalous effects. In systems with strong disorder, localization completely kills transport altogether. I will describe our efforts to understand the dynamics in these new phases.