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Due to coexisting electron-correlation and the spin-orbit interaction, 5d transition-metal oxides, e.g., complex iridates, have received a great deal of attention. However, our understanding of the system advances rather slowly owing to limited experimental approaches. In this colloquium, I will discuss our recent research on layered (2D) iridate thin-films of A₂IrO₄ (A = Ba, Sr, Ca). Note that the Ba₂IrO₄ and Ca₂IrO₄ phases do not exist in nature, yet their thermodynamically meta-stable phases can be synthesized using an epitaxy-stabilizing technique. Moreover, I will show artificial heterostructures of one-dimensional (1D) iridates consisting of linear Ir-O-Ir stripes. This approach of investigating artificial 1D systems opens a new avenue to explore low-dimensional quantum physics. Based on our recent experimental investigations of transport, optical spectroscopy, and resonant inelastic x-ray scattering, I will discuss our current understanding of the complex iridates and their topological properties due to coexisting strong electron-correlation and spin-orbit interaction.

Direct neutrino mass measurements probe Beyond Standard Model physics ex-

tensions which are employed to explain the finite neutrino masses, which are

in contradiction to the minimal Standard Model of Particle Physics but now

firmly established through the observation of neutrino flavor oscillations. While

being insensitive to the absolute neutrino mass scale oscillation experiments

provide lower limits, depending on the neutrino mass hierarchy. The upcom-

ing KATRIN experiment will improve the upper limits set by the Mainz and

Troitsk experiments (≤2 eV/c2 ) down to 200 meV/c2 and will probe the quasi

degenerate regime of neutrino mass hierarchy. I will discuss fundamentally new

laboratory approaches currently under development to either confirm a posi-

tive KATRIN result independently or to push the sensitivity limit towards the

40 meV/c2 range, the predicted lowest neutrino mass in case an inverted mass

hierarchy is realized in Nature. Major financial support by the U.S. Depart-

ment of Energy, Office of Science, Office of Nuclear Physics to the University of

Washington under Award Number DE-FG02-97ER41020 is acknowledged.

 

Conventional wisdom holds that quantum effects are washed out when the temperature becomes comparable to the quantized spacing between energy levels, and that the presence of a finite density of excitations above the ground state is antithetical to the stability of quantum coherent phenomena. However, the advent of modern techniques to cool and trap large collections of atoms, molecules and ions has stimulated a revival of interest in the behavior of isolated, interacting quantum systems. Recently it has been recognized that there exist "many-body localized" phases of matter that exhibit quantum dynamics even at infinite temperature, signalling a breakdown of conventional equilibrium statistical mechanics. I will discuss the intriguing and unusual properties of these inherently out-of-equilibrium systems, and outline their implications for our understanding of the phase structure of quantum many-particle systems.

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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