Phase-field Modeling of Polar States in Ferroelectric Heterostructures
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?
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
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.
Quantum Nanophotonics: Engineering Atom-Photon Interactions on a Chip
The ability to engineer controllable atom-photon interactions is at the heart of quantum optics and quantum information processing. In this talk, I will introduce a nanophotonic platform for engineering strong atom-photon interactions on a semiconductor chip. I will first discuss an experimental demonstration of a spin-photon quantum transistor , a fundamental building block for quantum repeaters and quantum networks. The device allows a single spin trapped inside a semiconductor quantum dot to switch a single photon, and vice versa, a single photon to flip the spin. I will discuss how the spin-photon quantum transistor realizes optical nonlinearity at the fundamental single quantum level, where a single photon could switch the transmission of multiple subsequent photons . I will next discuss the promise of realizing photon-mediated many-body interactions in an alternative solid-state platform based on a more homogeneous quantum emitter, silicon-vacancy (SiV) color centers in diamond. I will introduce our efforts in creating strong light-matter interactions through photonic crystal cavities fabricated in diamond , and the use of cavity-stimulated Raman emission to overcome the remaining frequency inhomogeneity of the emitters . Finally, I will outline the exciting prospects of applying inverse designed nanophotonic structures into quantum optics, and their potential applications in engineering photon-mediated atom-atom interactions.
 S. Sun et al., Nature Nanotech. 11, 539–544 (2016).
 S. Sun et al., Science 361, 57-60 (2018).
 J. L. Zhang* and S. Sun* et al., Nano Lett. 18, 1360–1365 (2018).
 S. Sun et al., Phys. Rev. Lett. 121, 083601 (2018)
Chemical and Physical Considerations in the Production of a Cup of Coffee
Despite coffee’s ubiquity and tremendous economic value (~1.5% of the USA GDP), there remains very little research in the field. Yet, numerous physical and chemical processes play a determining role in cup quality, ranging from agricultural practices, to roasting and brewing. This talk canvases the landscape of coffee research to date, detailing areas that require further study, as well as discussing our early efforts to better understand the key factors that determine cup quality and reproducibility.
Interfacial Coupling and Magnetic Competition in Magnetic and Magnetoelectric Systems
In the American economic system, competition is a critical driver of performance and innovation. The same can be said for materials physics. My group focuses on studying a variety of strongly correlated quantum systems, where the competition between charge, spin and orbital degrees of freedom can lead to novel or enhanced properties. It is this sensitivity that makes these materials useful for devices. A good device has a measured property (such as resistance or magnetization) that changes dramatically with an external stimulus (such as current, temperature or magnetic field). Competition is a valuable strategy for creating this interplay of parameters. Magnetic competition in magnetic systems, on the other hand, has often been seen as a hindrance. While it typically decreases the overall net magnetization, I will show that it can be utilized to generate novel phenomena useful for devices, such as giant negative magnetization and enhanced magnetization at small applied fields. While much research on magnetism utilizes large fields to strengthen the net magnetization, most devices will need to utilize small fields. While my group also collaborates on a wide range of other systems (such as topological insulators, delafossites and transition edge sensors), much of our focus has been to grow high-quality films and understand the interfacial interactions in magnetic and magnetoelectric layers. I will discuss our first observation of a magnetoelectric dead layer, which motivated our recent interest and successes in magnetic phase competition and then some of the interesting features we have discovered in complex oxide thin films.
mK to km: How Millikelvin Physics is Reused to Explore the Earth Kilometers Below the Surface
It is a common, but still surprising observation that many physics students have never met a physicist outside of an academic setting. Thus many undergraduate and graduate students know of few sources of information to help them understand what opportunities may exist beyond university environments. The purpose of the APS Distinguished Lecturer program is to show how some physicists have navigated the transition to the “real world”.
Investigations of the superfluid phases of liquid helium-3 would seem to have little application to the study of rock formations thousands of meters below the surface of the earth. However, the physicist’s tool box is versatile, and techniques used in one field of study can be reused, with appropriate adaptation, in very different circumstances.
The temperature of liquid helium-3 in the millikelvin range can be measured using an unbalanced-secondary mutual inductance coil set designed to monitor the magnetic susceptibility of a paramagnetic salt. The loss signal is discarded by phase sensitive detection. Now consider the task of measuring the electrical conductivity, at centimeter scale, of the earth surrounding a borehole. Turn the mutual inductance coil set inside out, with secondary coils arranged to be unbalanced with respect to the rock wall. Instead of discarding the loss signal, use it to measure conductivity. A sensor based on this principle has been implemented in a widely deployed borehole geophysical instrument, used to estimate the prevailing direction of the wind millions of years ago, or to decide where to drill the next well in an oilfield.
Nuclear magnetic resonance may seem a very improbable measurement of the rock surrounding a borehole. Conventionally, we place the sample (which might be a human being) inside the NMR apparatus. In borehole deployment, the instrument is placed inside the sample, the temperature is as high as 175°C, pressure ranges to 140 MPa, and measurements must be made while moving at 10 cm/s. Apparatus with these specifications have been deployed worldwide, and are used to measure a number of rock properties, including the distribution of the sizes of pores in sedimentary rock, and the viscosity of oil found therein. They have also been used for geological and oceanographic studies in northern Alaska, and at the seafloor offshore Monterey, California.
Synthesizing quantum matter with electrons and microwave photons
Experimental research at the nanoscale continues to challenge our ability to predict the behavior of quantum systems. Advances with lithographically patterned solid-state electronic devices have enabled multiple platforms for the simulation of quantum matter. In particular, semiconductor quantum dots and superconducting qubits provide tools for studying the wealth of physics induced by nonlinearities at the single electron and single microwave-photon level, respectively, and have been separately pursued as enabling technologies for qubits. In recent years, hybrid devices that combine such historically distinct lines of research have received greater attention, whether to enable novel sensing or measurement applications, or to couple small systems of qubits together at long range (e.g. quantum transduction). I will showcase the rich behaviors and phases of quantum matter that coupled quantum dots can exhibit, including a surprising transport mechanism called cotunneling drag [PRL 117, 066602 (2016)], signatures of Kondo physics with emergent symmetry [Nature Physics 10, 145 (2013)], and non-Fermi liquid states [Nature 536, 237–240 (2015)]. I will also discuss my work towards fabricating superconducting qubits on silicon-on-insulator substrates for hybrid device applications [Appl. Phys. Lett. 111, 042603 (2017)]. The integration of quantum dots and superconducting resonators promises to yield new probes for studying quantum matter, and superconducting qubits are coming of age in their own right for the implementation of many-body spin models.
Understanding Molecular and Hybrid Crystals from First Principles
Molecular crystals are crystalline solids composed of molecules bound together by relatively weak intermolecular interactions, typically consisting of van der Waals interactions and/or hydrogen bonds. Hybrid crystals combine molecular units and covalent/ionic networks.
Both classes of crystals play an important role in many areas of science and engineering, ranging from biology and medicine to mechanics and electronics. Therefore, much effort has been dedicated to understanding their structure and properties.
Predicting the behavior of such materials from first principles is highly desired for understanding their unique properties and for allowing rational design of novel materials and structures. Preferably, we would like to obtain such understanding from density functional theory (DFT), because the relative computational simplicity afforded by DFT allows us to attack realistic, experimentally accessible problems. Unfortunately, despite many other successes, DFT has traditionally struggled with useful prediction of properties of crystals that contain weakly-bound units.
Here, I will show how state-of-the-art DFT approaches allow us to overcome these limitations, quantitatively. I will focus on our recent progress in explaining and even predicting important classes of collective effects, i.e., phenomena that the individual units comprising the crystal do not exhibit, but arise through their interaction. Specifically, I will address unique structural, mechanical, electrical, and optical properties of both biogenic and synthetic crystals, with an emphasis on constructive interaction between theory and experiment.