Artificial spin ice is a model nanomagnetic system, where nanoscale bar magnets are fabricated on a periodic lattice with dimensions and geometry intended to model magnetic materials. On some lattices, the bar magnets can assume a regular ordered ground state, but on other lattices, the configuration does not permit such orderings and the material is said to be frustrated. It has been show through thermalization that square lattice artificial spin ice structures composed of various specialized materials can obtain the antiferromagnetic ground state of the lattice, sometimes with the occurrence of domain walls separating domains of order.  Using this as a starting structure, we seek to understand the effect of the introduction of lattice defects, namely edge dislocations.  In two dimensions, an edge dislocation is a point defect, but we show that it can create an extended region of frustration, in the form of an antiphase domain wall terminating at the defect. I will discuss computational modelling of the behaviour of the ice structure and the behaviours observed when the lattice contains two defects as a function of their relative distance and orientation.  I will also discuss the reversal behaviour of frozen artificial spin ice structures on the kagome lattice.  In these specimens, we observe avalanches during the reversal process that take on various stochastic sizes, which fall along a power law distribution.  This behaviour suggests the occurrence of criticality in these systems, and I will present a statistical analysis within that context. Numerical simulation results and the possibility of universal behaviour will be presented and discussed.

The laws of nature that emerge from string theory are in essence discrete: they contain no continuous parameters.  The possible laws correspond to the possible topologies of spacetime.  I will first explain what quantum gravity is, and why it is essential for interpreting measurements of the polarization of the cosmic microwave background (CMB). I will then develop the connection between discrete laws and topology, and show how we have exploited this connection to systematically enumerate inflationary cosmologies in string theory.  The brightness of the primordial gravitational wave signal in the CMB is controlled by integers that count the number of holes in the spacetime.

Understanding the fundamental relationship between atomic structure and material properties is the holy grail of the science of materials. Towards this goal we are working to develop a real-time and atomistic understanding of the mechanistic steps taken during the growth and transformation of crystalline materials. To do this we employ a combination of complementary synthetic and characterization approaches, in particular using in situ ultra-high resolution transmission electron microscopy (TEM) to observe key structural transformations in real-time. Our in situ experiments include directly performing nanomaterial synthesis in the TEM as well as determining the kinetics of structural phase transformations of as-synthesized inorganic nanocrystals. Further, based on an unexpected observation made during one of these in situ measurements, we have developed a new approach to directly synthesize arrays of crystallographically well-defined nanoscale interfaces. Several examples will be presented to illustrate our approach, including: the real-time observation of the solid-state reaction of an individual nanowire; a post-synthetic structural phase transformation within an individual nanorod, and finally, the creation of new nanostructured architectures using liquid metal nanodroplets.

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For most wireless communication systems, the signal environment is dominated by ambient noise and interference, which means that improving the efficiency of the antenna does not increase performance much. When the signal comes from the sky (think radio astronomy and satellite communications), the situation is very different. High aperture efficiency, radiation efficiency, spillover efficiency, and low noise electronics are everything in terms of the performance of a receiver. Bent metal antennas (horns and parabolic dishes) are very efficient and for the last century have been working just fine. The catch is that these kinds of receivers are “dumb” and offer only a fixed beam pattern. We would like to use smart antennas, phased arrays, and adaptive antennas for astronomy and satellite applications to have more control over the beam and more flexibility in selectively receiving signals of interest, but existing phased array technologies are too expensive, lossy, noisy, and most of all, too inefficient. Over more than a decade, my group has used numerical modeling, antenna design optimization, network theory, microwave noise analysis, and array signal processing theory to produce some of the most sensitive phased arrays ever built. This presentation will tell the story of this research field and show how the results have enabled new sensors, satellite receivers, scientific instruments, and influenced the IEEE’s latest version of the governing standard for definitions of antenna terms.

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Understanding the fundamental relationship between atomic structure and material properties is the holy grail of the science of materials. Towards this goal we are working to develop a real-time and atomistic understanding of the mechanistic steps taken during the growth and transformation of crystalline materials. To do this we employ a combination of complementary synthetic and characterization approaches, in particular using in situ ultra-high resolution transmission electron microscopy (TEM) to observe key structural transformations in real-time. Our in situ experiments include directly performing nanomaterial synthesis in the TEM, as well as determining the kinetics of structural phase transformations of as-synthesized inorganic nanocrystals. Further, based on an unexpected observation made during one of these in situ measurements, we have developed a new approach to directly synthesize arrays of crystallographically well-defined nanoscale interfaces. Several examples will be presented to illustrate our approach, including: the real-time observation of the solid-state reaction of an individual nanowire; a post-synthetic structural phase transformation within an individual nanorod; and finally, the creation of new nanostructured architectures using liquid metal nanodroplets.

Progress over the past fifteen years in the digitization and analysis of text found in cultural objects (inscriptions, manuscripts, scrolls) has led this past year to a new and astonishing discovery. This talk will tell the story of emerging methods for imaging and analysis culminating in a personal account of the discovery, the people involved, and the technical approaches used.  The digitization of damaged objects supports a new era of collaboration and exploration that has enabled compelling new discoveries and solutions to long-standing problems.

Rydberg atoms were the focus of much of Keith MacAdam's scientific life. Why he found them so fascinating becomes obvious with a brief summary of their properties. Their properties are exaggerated compared to normal atoms, allowing the introduction of novel detection techniques and clear manifestiations of unexpected physical phenomena. Keith was one of the pioneers in the study of Rydberg atoms. His background in radio frequency resonance spectroscopy was the perfect preparation for his initial Rydberg atom work, high resolution microwave spectroscopy, and his measurements are still the best. He is best known, though, for his beautiful work on  collisions of charged particles with Rydberg atoms, particularly charge exchange, the process in which the Rydberg electron hops from its initial ion core to the incoming ion. In spite of the fact that the "modern" Rydberg atom experiments began forty years ago, Rydberg atoms, today at microKelvin temperatures, remain a subject of intense interest.