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The electronic states of materials have an intrinsic angular momentum known as “spin”. The coupling between diffusive currents of spin and heat in materials is the basis of the emerging field of “spin caloritronics”. Analogous to a thermocouple where a temperature difference produces a voltage that can be used to measure temperature, heat currents in magnetic materials produce currents of spin that can be used to manipulate magnetization. Our work in this field takes advantage of recent advances in the measurement and understanding of heat transport at the nanoscale using ultrafast lasers. We use picosecond duration laser pulses as a source of heat (the pump) and detect changes in temperature and magnetization using a combination of thermoreflectance and magneto-optic Kerr effect (the probe). Our pump-probe optical methods enable us to generate enormous heat fluxes on the order of 100 GW m^-2 that persist for ~30 ps.

Spin caloritronics effects can be divided into two broad categories: effects arising from thermal excitations of independent electrons (spin-dependent Seebeck effect) and effects arising from collective excitations of spin waves (spin Seebeck effect). The spin-dependent Seebeck effect of a perpendicular ferromagnetic layer converts a heat current into a spin current, which in turn can be used to exert a thermal spin transfer torque on a second ferromagnetic layer with in-plane magnetization. Using a [Co,Ni] multilayer as the source of spin, an energy fluence of ≈4 J m^-2 creates thermal STT sufficient to induce ≈1% tilting of the magnetization of a 2 nm-thick CoFeB layer. We study the spin Seebeck effect driven by an interfacial temperature difference between electrons in a normal metal (Au or Cu) and spin-waves in a ferromagnetic insulator (Y₃Fe₅O₁₂). The spin Seebeck coefficient provides new insights on the coupling of excitations across material interfaces.

The need to store energy at high concentration in a way that is safe, efficient, and economical appears in several technologies: load balancing of the grid,  alternative energy schemes, and transportation beyond the standard model.   I will survey the proposed schemes, noting where the laws of physics put boundaries on what can be achieved.

Refreshments will be served in CP 179 at 3:15 PM

NASA's SOFIA (Stratospheric Observatory for Infrared Astronomy), an airborne observatory optimized for conducting astrophysical investigations across the infrared-to-sub-millimeter spectral range, is an international partnership between the U.S. and German space agencies. As an airborne telescope optimized for infrared data collection SOFIA offers the only regular access to the wide swath of infrared wavelengths obscured by Earth's lower atmosphere and unavailable to ground-based observatories.

The presentation will focus on scientific results, some surprise discoveries, and unique analysis techniques utilizing SOFIA data. One dominant theme is how stars are able to form in extreme environments such as in the Galactic Center, where energetic radiation fields and a hot, turbulent medium in the vicinity of a supermassive black hole would seemingly be unconducive to the observed prolific star production. SOFIA offers unique tools for such studies, such as the ability to reveal kinematic signatures showing the details of how a star forming cloud collapsed to its current state, as well as providing clocks capable of directly measuring the collapse timescales for comparison to theoretical predictions.

Refreshments will be served in CP 179 at 3:15 PM

In a quantum quench, system is prepared in some initial state (usually the ground state of some hamiltonian) and then allowed evolve in isolation with a different hamiltonian, for example, by rapidly quenching a parameter. This is most interesting for many-body systems where one can ask questions such as whether subsystems reach a stationary state, whether this state appears thermal, and how quickly does it reach this state. Although an obvious set of problems, they have only recently come to the fore with possibility of performing such experiments in ultra-cold atoms and other systems. In this talk I will try to address these questions in the context of some simple, and not-so-simple exactly solvable models.

 

Refreshments will be served in CP 179 at 3:15 PM

When photons scatter, their angular distribution and energy shift reveal information about the structure of the scattering target. As a result, photon scattering has long been used to study materials at the atomic and molecular level. By substantially increasing the photon energy, experiments can also be used to measure electromagnetic properties of the proton and neutron -- properties which are sensitively related to the interactions among the constituent quarks and gluons. We will discuss experiments which measure the electric and magnetic polarizabilities of the nucleon, and present new results for the neutron.

 

Refreshments will be served in CP 179 at 3:15 PM

Details of the chemical composition of stars provide information about the formation and evolution of the galaxies in which they form. I will outline some of the connections and provide some results for our Milky Way Galaxy from the Sloan Digital Sky Survey (SDSS) Apache Point Observatory Galactic Evolution Experiment (APOGEE). APOGEE is providing maps of the abundances of stars across the Milky Way. These suggest different timescales for star formation in different locations, both radially and vertically within the disk of the Galaxy, and also suggest that movement of stars within the Galaxy, through a process knows as radial migration, is important.

Refreshments will be served in CP 179 at 3:15 PM

Twenty years ago, the Material Chip's invention allowed 10^3-10^4 materials with different compositions to be synthesized under the same conditions and characterized for physical properties in one experiment. Ten years ago, the development of the Materials Phase Diagram Chip allowed ternary phase diagrams at a constant synthesis temperature to be mapped in one experiment. Today, a complete phase diagram (a 3D contour with a 2D composition map and a temperature axis) can be mapped in one experiment with a single Materials Phase Diagram Chip, increasing the efficiency of phase diagram mapping by million-fold. The physics and implications of these chips will be discussed.

Refreshments will be served in CP 179 at 3:15 PM