Physics & Astronomy Colloquium

Controlled Magnetic Reversal and Emergent Metamagnetism in Permalloy Films Patterned into Artificial Quasicrystals

Dr. Lance Delong University of Kentucky

Ferromagnetic (FM) thin films patterned into periodic lattices of nanoscale holes or dots are candidated for UHD data storage media, an drelated wire network patterns are of fundamental interest as examples of controlled phase transitions in "artificial spin ice". Our recent Physical Review Letter reported an experimental study of the static and dynamic magnetic properties of FM permalloy thin films patterned as Penrose P2 (quasicrystal) tilings that exhibit long-range order, but aperiodic translational symmetry. Our DC magnetization and ferromagnetic resonance data constitute, we believe, the first experimental study of th espin wave dynamics of an artificial FM quasicrystalline thin film. Ground-breaking efforts were required to both pattern and deposit the sample film materials, and to execute large-scale numerical simulations of their static and dynamic behavior. This work demonstrates a new method for controlling the evolution of FM domain walls and spin wave spectra in magnetic media, in spite of a lack of periodic symmetry in an artificial quasicrystalline pattern. Simulations reveal a remarkably controlled sequence of reversals of individual film segments located on sublattices of the quasicrystal pattern, which may signal the occurence of true metamagnetic phase transitions in larger-area samples. These results directly imply FM films patterned as Penrose P2 tilings constitute a novel class of magnonic crystals whose magnon frequency dispersion and physical properties were heretofore unknown.

 

 

Quantum Tapestries

Dr. Matthew Fisher University of California, Santa Barbara

Quantum Tapestries Within each of Nature's crystals is an exotic quantum world of electrons weaving to and fro. Each crystal has its own unique tapestry, as varied as the crystals themselves. In some crystals the electrons weave an orderly quilt. Within others the electrons are seemingly entwined in an entangled web of quantum motion. In thi stalk I will describe the ongoing efforts to disentangle even Nature's most intricate quantum embroidery. Cutting-edge quantum many-body simulations together with recent ideas from quantum information theory, such as entangelment entropy, are enabling a coherent picture to emerge.

Quantum Gravity with Anisotropic Scaling and the Multicritical Universe

 

 

Dr. Petr Horava University of California, Berkeley

The problem of understanding how gravity fits together with other fundamental interactions of matter has been at the forefront of theoretical research for many decades, leading to the rich framework of string theory and M-theory. In this framework, many fundamental questions are being resolved, but many remain quite mysterious, suggesting that some novel concepts may be needed. I review the recent concept of multicritical gravity with Lifshitz-type anisotropic scaling, and its applications in areas ranging from particle phenomenology beyond the standard model to non-relativistic versions of the AdS/CFT correspondence.

 

 

The Co-Evolution of Galaxies and Black Holes: A Local Perspective

 

 

I will summarize our current understanding of the formation and evolution of galaxies and supermassive black holes, and emphasize the underlying relationship between these two populations. I will pose several of the most fundamental shortcomings of our current models and then examine how they may be addressed by what we have learned from observations in the local universe of the effects that massive stars and supermassive black holes have on their surroundings. I will do my best to give a "physicist-friendly" talk that minimizes jargon and stresses the basic underlying physical processes.

 

 

Colloquium: Charge Transport in Organic Materials and Devices

Dr. Oana Jurchescu 
Wake Forest University


Charge Transport in Organic Materials and Devices 

Organic semiconductors are becoming increasingly attractive given their solution processability, which allows for low-cost production on flexible media like paper, plastic, or textiles. But in spite of these advantages, the complexity of film formation resulting from solution growth processes makes it challenging to control the device performance in a reliable way. In this talk I will discuss the growth, structure, and electronic properties of functionalized pentacene and anthradithiophene organic thin-film transistors deposited by scalable solution deposition methods, such as spray deposition or vibration-assisted crystrallization. The results will be compared with those obtained in single crystal devices and several approaches to improve film quality and device performance will be presented. The effect of processing parameters on charge carrier mobilities, on/off ratios and interfacial trap densities will be detailed. Transitioning from mono-mulecular crystals to multi-component materials, such as the organic charge transfer complexes, which are combinations of charge donating (D) and charge accepting (A), I will show examples on how novel functionalities can emerge from D/A intermolecular interactions. 

 

 

 

 

Colloquium: Cosmological Implications of Recent Low-noise, High-resolution Measurements of the Cosmic Microwave Background

Dr. Lloyd Knox University of California, Davis

Observing the sky in the microwave region of the spectrum allows us to directly image the universe when it was just a few hundred thousand years old. The universe was much simpler then, simple enough that its expected statistical properties, given a model, can be calculated with high accuracy. Recent improvements in measurement resolution and sensitivity, most notably from the Planck satellite, but also from the South Pole Telescope, have provided precision tests of the standard cosmological model. In this colloquium I will introduce the cosmic microwave background (CMB) and the standard cosmological model. I will explain the nature of these precision tests and what we are learning about the origin of all structure in the universe, and about the background of neutrinos thermally produced in the big bang. I will also cover how the improvements in resolution and sensitivity are opening up a new window on the dark universe, via gravitational lensing of the CMB.

Colloquium: Viscosity, Quark Gluon Plasma, and String Theory

Viscosity, quark gluon plasma, and string theory

Viscosity is a very old concept which was introduced to physics by Navier in the 19th century. However, in strongly coupled systems, viscosity is difficult to compute from first principle. In this talk I will describe some recent surprising developments in string theory which allow one to compute the viscosity for a class of strongly interacting quantum fluids not too dissimilar to the quark gluon plasma. I will describe efforts to measure the viscosity and other physical properties of the quark gluon plasma created in relativistic heavy ion collisions.

Cosmic Linear Accelerators: Extreme Reconnection and other Surprises from the Crab Nebula

The unexpected discovery of gamma-ray flares from the Crab Nebula may have surprising implications for plasma astrophysics. Standard particle acceleration mechanisms cannot account for the energies of the flaring photons. Instead, these observations point toward an acceleration process involving rapid destruction of magnetic field through reconnection. I will discuss the extreme particle acceleration process that may lead to the flares, and the likely role of current-driven instabilities in triggering reconnection in the Crab and elsewhere.

Colloquium: Glimpsing Color in the World of Black and White

Glimpsing color in a world of black and white

Protons, neutrons and all the many other strongly interacting subnuclear particles, known as hadrons, are made of quarks and gluons. These fundamental constituents are held together by a color force described by quantum chromodynamics (QCD). A detailed understanding of how the strong coupling regime of QCD, which is responsible for confinement and dynamical chiral symmetry breaking, determines the spectrum and structure of hadrons will be outlined. Such studies, both experimental and theoretical, color in the picture of strong dynamics. What we know now and the glimpses to come from accelerator facilities like that at Jefferson Lab will be described.

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