Abstract: Brown dwarfs are a recent addition to the list of objects studied in Astronomy. With masses between 13 and 75 times the mass of Jupiter, they lack sustained stable Hydrogen burning so they never join the stellar main sequence. They have properties similar to both planets and low-mass stars so studies of their population inform on both. The distances and kinematics of brown dwarfs provide key constraints on their ages, membership in local Galactic associations, intrinsic brightness, and multiplicity. Coupling our knowledge of their distance with other observables (such as photometry and spectroscopy) yields detailed information about a brown dwarfs atmosphere - a place rife with thick exotic clouds and chaotic storms. In this talk I will describe an important ongoing effort to measure brown dwarf distances for the purpose of understanding Jupiter-like atmospheres and analogs to the giant exoplanets we are just starting to directly image.

Abstract: Transition metal oxides are an important class of materials as they exhibit a wide variety of both interesting and useful properties (e.g. magnetism, superconductivity, ferroelectricity) that can be controlled by external parameters. One important control parameter is strain, or how much a material is compressed or pulled. At first glance, SrCoO₃ may seem relatively ordinary: in bulk it is a ferromagnetic metal, a behavior that has also been observed in moderately strained thin films. Recent theoretical work, however, has predicted a transition to an antiferromagnetic state under large compressive (“pushing”) or tensile (“pulling”) strains. This would also be accompanied by a metal-insulator transition as well as the emergence of a ferroelectric state. If this behaviour can be experimentally realized, then “strain engineering” SrCoO₃ will be an avenue to developing new types of multiferroic materials, where robust magnetism and ferroelectricity not only coexist, but are strongly coupled. In this talk I will discuss recent experimental investigations searching for the existence of these novel magnetic phases in thin films of SrCoO₃ grown under varying strain conditions.

Abstract: The solar wind has often been called the “Best Turbulence Laboratory“ known to science. However, study of the solar wind using remote observation and direct measurements from satellites can be difficult and expensive. In this talk, I will introduce a different approach to examining astrophysical or magnetohydrodynamic (MHD) turbulence using a laboratory-based plasma experiment. Laboratory techniques offer many advantages to studying plasma turbulence including the ability to control parameters, make multiple point spatial measurements, and employ a wider variety of diagnostic tools. I will discuss how an astrophysically-relevant turbulent plasma can be produced and what analysis methods are used to understand the characteristics of this turbulence. By combining discoveries made in the lab with observations made in space, we can more effectively tackle the myriad of questions that persist about the nature of the solar wind.

Abstract: When cooled to temperatures below approximately 2 K (-273.15 C) liquid helium starts behaving in very peculiar ways. This behavior has come to be known as superfluidity and is a manifestation of the quantum nature of the system (known as a quantum liquid). Superfluids have been discovered in several other systems as well, some surprising, and have become an important link between many branches of physics. In this talk I will give a broad overview of superfluids, mainly in the context of liquid helium. I will show how superfluids can be used to study a range of topics in widely varying fields, and finally discuss the possibility of the existence of superfluidity in a solid system

Abstract: Assembly of metal nanoparticles (NPs) into ordered structures is one of the main pursuits of nanoscience, especially for applications in nanophotonics. Assembly of metal nanoparticles into ordered structures can give rise to many interesting optical properties, such as negative refraction and Fano resonance. Control over the chemical interactions between colloidal nanoparticles has recently led to the formation of new heterostructures and even quasi-crystals. In this work, we developed a new concept to create assembly of colloidal metal nanoparticles by using only light field (i.e. no chemical interactions)!! This is performed by exploiting light-matter interactions amongst the constituent nanoparticles on meso-scale, known as “optical-binding” interactions. ‘Optical binding’ occurs on intermediate scales $(d \sim \lambda)$ between near-field $(d \ll \lambda)$ and far-field $(d \gg \lambda)$ limits due to inter-particle forces produced by light scattering between multiple particles. In this talk, I will show that optical binding interactions can be used to enable controlled formation of 1D and 2D clusters and arrays of metal nanoparticles as well as hybrid assemblies of metal and semiconductor nanocrystals. I will also show that that the intermediate-scale coupling can be exploited to tune the collective plasmon resonance as well as introduce optical anisotropy in these structures.

Abstract: Optically pumped atomic vapors are some of the most sensitive magnetometers created to date. The ultimate limit of the magnetometer sensitivity is the quantum spin projection noise limit — a limit posed by the Uncertainty Principle. I will describe our efforts in developing an array of optically pumped Rb-87 magnetometers for use in fetal magnetocardiography. This is a measurement that involves detecting the magnetic fields generated by the heart of a developing fetus and serves as a useful diagnostic for congenital heart diseases. I will also discuss progress we have made in understanding and controlling different sources of noise and interference as we push the sensitivity towards the quantum noise limit in samples with approximately 10¹⁴ atoms.

Abstract: Granular materials exist all around us, from avalanches in nature to the mixing of pharmaceuticals, yet their behavior remains poorly understood. They resist shear like solids, flow like liquids, and exist in energetic gas-like states, but additionally exhibit properties that are uniquely granular. As a complex system, relatively simple interactions between grains produce surprising emergent, collective behavior, including catastrophic failures, shear banding, and de-mixing. One framework to understand these systems is through a jamming/unjamming transition and the continuous forming and breaking of a strong force network based on geometrical constraints resisting flow. This generic transition is common in soft condensed matter systems, from foams to traffic flow. Jamming depends on factors including packing, applied shear, and fluctuations, but additional complications such as cohesion or the presence of an interstitial fluid provide additional impediments to developing a general predictive equation of state. I'll present experiments on quasi-static shear and free-surface granular flows aimed at understanding the striking behavior of these systems. Specific experiments focus on flow under the influence of external vibrations, statistics of laboratory scale avalanches, and granular-fluid mixtures in which surface chemistry effects serve a primary role. By using photoelastic grains in a subset, we are able to measure both particle trajectories and the local force network. These experiments provide insight into the nonlinear statistical properties of granular systems, the importance of vibration in destabilizing force networks, and the role of collective behavior due to geometry and cohesion.

Abstract: Medical physics involves the application of physics in modern medicine. There are a variety of specialties including therapeutic, diagnostic, nuclear and health physics. The focus of this talk will be the educational opportunities, the requirements for certification, and the role of the physicist in the workflow of a therapeutic radiation oncology department.

Abstract: We live in an ecological system where multiple species coexist as a consequence of various interactions. Unlike predator-prey, the latter is not killed by the former in a parasite-host system. In this talk, I will first give a brief introduction to population dynamics. I will then present a study on simple parasite-host interaction. Both Monte Carlo simulation and theoretical analyses are utilized to understand the non-trivial stationary state of the parasite spatial distribution with a stationary host. When the host moves with uniform velocity, solving the problem becomes much more challenging. Instead, we switch a frame of reference and consider a stationary host with parasites performing biased diffusion, for which our theoretical predictions (with no fitting parameters) also agree with simulation results. In the appropriate continuum limit, the two processes are identical but interesting differences emerge in our lattice model. The most notable phenomenon is that the stationary parasite population generally increases with the bias, reaching a maximum before vanishing at some critical value. This model also lends itself to investigating a class of “contact-dependent” interactions.

Abstract: Our notions of thermal equilibrium are reinforced by intuition gained from the world around us. A hot cup of tea cools by radiation, conduction, and convection, and reaches equilibrium with the air around it. But what happens if we disturb an isolated system? The energy we put into it has nowhere to go. Does this system ever relax? What kind of equilibrium does it relax to - can it be described simply by a temperature? What happens when the components of the system interact with each other? Over the last couple of decades, we have started understanding these questions due to several breakthroughs in theory, computation, and experiment. In this talk, I will motivate this question, discuss some basic concepts surrounding it, and pose a more specific question. I will then talk about an experimental system that is particularly suited to studying these problems, and describe some aspects of my research and where it fits into the bigger picture.

Abstract: Swimming bacteria play integral roles in processes ranging from biogeochemical cycling in the oceans to the spread of infections in the human body to engineered systems (bioremediation, bioreactors). The goal of our research is to understand the biophysical mechanisms governing the interactions between swimming bacteria and their fluid environment. To accomplish this, we take a multi-physics approach, which often includes fluid, solid, and statistical mechanics, as well as high-speed imaging and microfluidics. In this seminar, I will describe how some bacteria have evolved to take advantage of a mechanical buckling instability in their 20 nanometer thick flagellum to steer their swimming trajectories. In the second part of the talk, I will show how fluid flow (shear) can trap swimming bacteria, producing striking spatial heterogeneity for suspensions of otherwise randomly-swimming cells. The results of these studies have potential implications for broad-ranging applications including micro-robotics, biomedical devices, and ocean ecology.

Abstract: The body maintains two extensive vascular networks: the blood vessels that deliver nutrients and remove waste products, and the lymphatic vessels that maintain fluid balance and support the immune system. Traditionally, these systems have be viewed as relatively static ‘plumbing’, but recently the endothelial and smooth muscle cells that make up the walls of these vessels have been shown to respond dynamically to gradients in diffusible growth factors and to the shear and stretch forces arising from the fluid motion. As a result, individual vessels can vary their diameters to induce and regulate fluid pumping, and networks of vessels can adapt their flow patterns and connectivity to more efficiently carry out their transport functions. In this talk, I will present several new results developed by applying network theory and nonlinear dynamics to problems of vessel behavior. In particular, we have discovered a new mechanobiological oscillator in the lymphatic system and some new features of flow in optimal flow networks. These results shed light on medical issues such as tissue swelling from lymphatic malfunction and blood flow in tumors. In short, I will present some really cool time-lapse images obtained by optical frequency domain methods, some colorful computational fluid dynamics and some delightful stability analyses of nonlinear differential equations.

Abstract: Inherently out of equilibrium soft materials such as the cytoskeleton of a cell or a bacterial colony have come to be referred to as active materials. These systems are driven at the microscale by energy consuming entities (such as molecular motors) that do work or produce autonomous motion. In this talk I will consider a simple model of self propelled particles and describe lessons learnt both in terms of a theoretical framework to understand active materials and as a tool to discover design principles to extract macroscopic work from such systems.

Abstract: Recently, a number of strict equalities have been developed for far from equilibrium statistical mechanical systems that relate work and free energy. We develop a field- theoretic description of non-equilibrium work relations using Doi-Peliti field theory. Specifically, we create the Doi-Peliti field theory for thermal systems and use it to derive the well-known Jarzynski equality. Our resulting framework can be extended to other non-equilibrium relations. We consider classical particles on a lattice that experience pair-wise interactions and a local potential. These particles hop with rates determined by coupling to a thermal bath. Work protocols are imposed by varying the local potential, which drives the system out of equilibrium. In this framework, work relations appear simply as the result of a gauge-like transformation combined with a time-reversal. We present the derivation with a one-dimensional system on a lattice and conclude with the generalization to multiple dimensions and the continuum limit.

Abstract: Thermoelectricity can be used to generate electricity from waste heat, or for cooling. Crystalline ZnO is a versatile semiconductor with moderate thermoelectric (TE) properties. The TE behavior of ZnO could possibly be improved if the crystalline order is lost. We use computational tools to understand and predict the TE behavior of amorphous ZnO.

Computation is a powerful tool to explain materials properties. Computation also provides a promising route to discover and invent exotic materials. We use density functional theory to understand how electrons interact with each other, and with ions. Molecular dynamics-based methods are used to model the ionic behavior. Results from these methods are combined to estimate heat conduction, resistivity, and thermopower of ZnO. Based on our calculations, we predict that ZnO would have better TE properties in amorphous state, rather than in the crystalline form. Moreover, we may be able to apply this understanding to other compounds to improve their TE behavior.