Eric Sembrat's Test Bonanza

In frustrated magnetic materials, geometry and magnetic interactions combine to suppress conventional magnetic order. Instead, disordered "spin liquid" states can host exotic magnetic phenomena which persist to the lowest measurable temperatures. Neutron scattering is an ideal experimental technique to understand spin-liquid states, but the absence of conventional magnetic order means that standard data-analysis methods cannot be used. Two limitations have traditionally restricted our understanding spin liquids at the atomic scale: (i) the magnetic interactions must be anticipated, and (ii) single-crystal samples must be available.

In my presentation, I will show how neutron-scattering data can be converted robustly into a three-dimensional model of the spin-liquid state. Using an atomistic refinement approach, I show that it is possible to recover accurate three-dimensional information from powder-averaged data, without making any assumptions about the underlying magnetic interactions. I will present experimental results for two materials. First, I present evidence for a "hidden order" state in the canonical frustrated magnet Gd3Ga5O12. Second, I use single-crystal neutron-scattering data to understand the origin of magnetic disorder in beta-Mn and its alloys. Finally, I will discuss the implications of these results for understanding disorder in other materials.

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I will try to explain, in elementary terms, the deep connection between space-time geometry and quantum entropy, uncovered in the work of Bekenstein, Hawking, 't Hooft, Gibbons Jacobson, Fischler, Susskind and Bousso. This leads to the conclusion that many of the fundamental degrees of freedom, which describe our world, are inaccessible to direct local measurement. Indeed, local excitations are constrained low entropy states of the fundamental degrees of freedom. These insights give us clues to the nature of a fundamental theory of quantum gravity, and have implications for early universe cosmology, inflation, and the particle physics at the TeV scale (I won't have time to discuss the last of these claims, which is highly speculative).

The dawn of gravitational wave astronomy is upon us as Advanced LIGO and Advanced Virgo begin to come on line later this year. With the first detection of gravitational waves, the cosmic cacophony of the gravitational universe will be open to us, allowing us to probe some of the densest regions in the universe as well as some of the most energetic astronomical phenomena (eg. gamma-ray bursts). In order to perform gravitational wave astronomy, one must decipher the astrophysical information encoded in the detected gravitational wave signals. This seminar will give a brief overview of the methods for performing gravitational wave astronomy, based on Bayesian inference, and highlight some examples of gravitational wave astronomy for "unmodelled" burst gravitational wave signals and other transients signals.

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The emergence of attosecond spectroscopy has offered researchers a new experimental tool, which provides unprecedented temporal resolution for the direct measurement and control of electronic processes. Pioneering studies have obtained unique physical insights by taking snapshots of atomic and molecular phenomena on the natural timescale of electrons. Ongoing efforts aim to expand the scope of attosecond XUV research for exploration of complex dynamics in molecules and materials. My lab has conducted frontline investigations to probe the electronic complexity stemming from coupled nuclear motion, electronic correlations, external fields, or a combination thereof. I will present our latest results on the coherent evolution of electron hole near a conical intersection of a polyatomic molecule. Such conical intersections induce nuclear-motion-mediated electron dynamics and serve as nature’s energy funnels in many biochemical processes (e.g. DNA repair, light harvesting etc.). In a similar context, we have investigated electron-phonon coupling and electronic correlations in carbon nanomaterials. I will show that apart from the sensitive measurements of electronic couplings, we can also monitor the evolution of quantum coherence in charge transfer processes. Finally, I will discuss our ongoing work on the generation of ‘isolated’ attosecond pulses and their application in study of correlation-driven charge migration in molecules and magnetic materials. Such interdisciplinary efforts will open the attosecond field for a broad set of applications in physics, chemistry and material sciences.  

The integer quantum Hall effect (IQHE) is often described in terms of skipping orbits: 2-D electrons begin nascent cyclotron orbits, only to be interrupted by the material’s boundary, and instead reflect from the edge, beginning a partial orbit anew.  These classical skipping trajectories follow the systems boundary, and in the quantum limit connect to the quantum Hall system’s conducting edge channels.  Even though decades of measurements have confirmed this overall picture, the intrinsic difficulty of imaging electrons has precluded all attempts to image these orbits.

We have directly realized a cold-atom lattice in the extreme quantum limit, where each lattice plaquette contains about 4/3 of the quantum of flux: unthinkable in crystalline materials (where it would take 10_­⁴ T fields), but achievable using engineered tunneling phases in an optical lattice.  To achieve such fields, we use a hybrid lattice geometry, where one dimension of the lattice is a normal optical lattice and the other dimension is the internal hyperfine states of the atoms; this gives us perfect single-site imaging resolution along one dimension. We then dynamically prepared our ultracold bosons in several different initial states, which allowed us to observe edge and bulk properties in two ways: (1) we directly imaged the edge states (associated with quantized conductance in IQHE systems) and the localized bulk states (associated with the insulating bulk of IQHE systems); (2) we then created excitations, those on the edge were analogues to edge magnetoplasmons in quantum Hall systems and directly follow their chirally oriented skipping orbits.

Superfluidity, or flow without resistance, is a macroscopic quantum effect that is present in a multitude of systems, including liquid helium, superconductors, and ultra-cold atomic gases.  Here, I will present our work studying superfluid flow in a Bose-Einstein condensate (BEC) of sodium atoms.  By manipulating optical potentials, we are able to form BECs into any shape, including rings and targets.  Ring condensates are unique in that they can support quantized, persistent currents.  We drive transitions between persistent current states using a rotating perturbation, or weak link.  This ring and rotating potential form a circuit, which is analogous to an rf superconducting quantum interference device (SQIUD).  Our circuit shows the essential features of an rf-SQUID, including tunable transitions between quantized persistent current states and hysteresis.  Such features make an rf-SQUID a sensitive magnetometer; by analogy, our device could act as a rotation sensor.  In addition to these experiments, we have also realized other geometries such as a dumbbell and a dc-SQUID, that allow us to study critical velocities and resistive flow in superfluids.  These, and similar experiments with tunable geometries, shed new light onto the details of quantum transport and superfluidity, and may pave the way for new ‘atomtronic’ devices.

Cold atoms and ions provide an interesting playground for a variety of measurements of fundamental physics.  Using RF traps, experiments become possible with both large ensembles of ions, e.g. in cold chemistry, and few/single ions, such as in quantum computations/simulations or optical clocks, where ultimate quantum control is required.  In the first part of the talk, recent results from our work on cold chemistry and cold molecular ions using a hybrid atom--ion experiment will be presented.  We have developed an integrated time-of-flight mass spectrometer, which allows for the analysis of the complete ion ensemble with isotopic resolution.  Using this new setup, we have significantly enhanced previous studies of cold reactions in our system.  Potential routes towards ultra-cold reactions at the quantum level will be presented.  Current work aims at demonstrating rotational cooling of molecular ions and photo-associating molecular ions.

The second part of the talk reports on our results of the search for the low-energy isomeric transition in thorium-229.  This transition in the vacuum-ultraviolet regime (around 7.8 eV) has a lifetime of tens of minutes to several hours and is better isolated from the environment than electronic transitions.  This makes it a very promising candidate for future precision experiments, such as a nuclear clock or tests of variation of fundamental constants, which could outperform implementations based on electronic transitions.  Our approach of a direct search for the nuclear transition uses thorium-doped crystals and, in a first experiment, synchrotron radiation (ALS, LBNL) to drive this transition.  We were able to exclude a large region of possible transition frequencies and lifetimes. Currently, we continue our efforts with enhanced sensitivity using a pulsed VUV laser system.