Sommerfeld Lecture Series (ASC)

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ASC Theory Colloquium: Physics and Geometry of Morphogenesis Oct 15, 2015

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One hundred years ago, D’Arcy Thompson – a nineteenth century polymath, working at the turn of the twentieth century – wrote a beautiful monograph, “On Growth and Form”, in which he pondered the geometry of living forms and how it emerges in the process of Morphogenesis. Thompson was ahead of his time. Genetics and Developmental Biology have since come a long way in elucidating the general and particular aspects of Morphogenesis, uncovering the key genes and molecules that underlie the process in different animals and plants. Yet, Thompson’s agenda of understanding how developmental processes actually specify the geometry of tissues, limbs and organs is far from complete. A particular challenge is to bridge the gap between microscopic scales, where molecular mechanisms operate, and the macroscopic scales of animal “shape and form”. This challenge offers much for a Theoretical Physicist to think about. This talk will provide some examples, relating the study of order in the arrangement of fly wing hairs to ferromagnetism and uncovering an unexpected wealth of mechanical phenomena in the study of cellular flows in a fly embryo.

Public Lecture: On the Possibility of Evolutionary Forecasting Oct 14, 2015

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When we think about evolution, it is typically in the context of natural history, seeking an explanation for the amazing diversity of life. Yet evolution is not only the matter of the past, but an ongoing dynamical process linking the past with the future. Evolutionary dynamics is particularly apparent in rapidly mutating microbes and viruses. For example, the virus causing seasonal flu continuously evolves to escape human immunity generated by previous infections: because of this process, we get the flu again and again. Can we understand evolutionary dynamics well enough to predict the future, at least far enough to help with the flu vaccine updates? This talk will review basic mechanisms of evolutionary dynamics and discuss some of the old and new approaches to evolutionary forecasting and the challenges that they face. Surprisingly, ideas from Theoretical Physics can be helpful in understanding evolutionary dynamics.

Theory Colloquium: Towards Material Design Using Strongly Correlated Electron Materials Jan 23, 2015

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Our understanding of simple solids, is firmly grounded on the Fermi liquid concept and powerful computational techniques built around the density functional theory. These ideas form the basis of our “standard model” of solid state physics and have provided us with an accurate description of many materials of great technological significance. Correlated electron systems are materials for which the the standard model of solid state physics fails dramatically. The best known example being the copper oxide high temperature superconductors. Correlated electron materials continue to be discovered accidentally and surprise us with their exceptional physical properties and their potential for new applications. The most recent example is provided by the iron arsenide based high temperature superconductors. From a theoretical perspective describing strongly correlated electron systems pose one of the most difficult non-perturbative challenges in physics. In this colloquium I will give an elementary introduction to the field of strongly correlated electron materials and Dynamical Mean Field Theory (DMFT) a non perturbative method which provides a zeroth order picture of the strong correlation phenomena in close analogy with the Weiss mean field theory in statistical mechanics. Applications materials containing f and d electrons will be presented to show how the anomalous properties of correlated materials emerge from their atomic constituents. I will conclude with an outlook of the challenges ahead and the perspectives for a rational material design.

Condensed Matter Theory Seminar: Shining Light on Transition Metal Oxides: Resilient Quasiparticles and the Unveiling of the Hidden Fermi Liquid Jan 23, 2015

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Strongly correlated metals exhibit anomalous transport properties which have puzzled condensed matter physicists for many years. They are characterized by large resistivities which exceed the Mott Ioffe Reggel limit and large thermoelectric responses, which cannot be explained in terms of standard Fermi liquid quasiparticles. Dynamical Mean Field Theory (DMFT) calculations [1,2] carried out on a doped one band Hubbard model suggest that this behavior originate in the strong temperature dependence of thee parameters of the underlying resilient (non-Landau) quasiparticles. We will test these ideas by analyzing low energy optical spectroscopy measurements in several prototypical compounds starting with the archetypal correlated material Sesquioxide V2O3. We will also show first principles, material specific, LDA+DMFT calculations which are in very good agreement with the experiments [3].

Public Lecture: The Quest for High Temperature Superconductivity Jan 21, 2015

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Superconductivity is a state of matter where electrons can flow without resistance and where magnetic fields are expelled. It was discovered serendipitously more than a hundred years ago. Today, superconductors are essential components of medical imaging devices as well as high energy particles accelerators. Understanding this phenomena was one of the greatest intellectual challenges of the twentieth century. A dramatic advance was provided by the BCS (Bardeen Cooper Schrieffer) theory 45 years after. It posits that superconductivity is the result of macroscopic condensation of electron pairs, which are held together by the vibrations of the lattice. The condensate is a macroscopic quantum objects and its rigidity accounts for its striking macroscopic properties. The BCS theory was so successful that by the early 70’s superconductivity was considered a completely understood subject with the maximum achievable critical temperature having been reached experimentally around 30K. In the late 80’s this field of research took a dramatically turn with the discovery of new ceramic compounds which superconduct at temperatures as high as 160 K. These materials, cannot be described by straightforward extensions of the BCS theory. Scientists are still working on finding new explanations for these materials and we will describe the challenge they pose. The quest for room temperature superconductivity thus continues. A breakthrough in this field would have unimaginable consequences, changing the way we transmit electricity from its generation to its consumption to the way we design computers.

Public Lecture: The Principle of Least Action, from the “Vis-viva” to Quantum Black Holes Jul 08, 2014

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The Principle of Least Action is both profound and practical. Since its first formulation by Maupertuis and Euler nearly three centuries ago, the Principle has been, and continues to be, a formidable battlehorse for penetrating unchartered territory in theoretical physics. The Principle, its connection with, and implications for, our ideas of symmetry, space, time, quantum mechanics, thermodynamics and gravitation, are glanced at.

Sommerfeld Theory Colloquium: Sloppy Models and How Science Works Jan 20, 2014

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“With four parameters I can fit an elephant; with five I can make it wag its tail.” Systems biology models of the cell have an enormous number of reactions between proteins, RNA, and DNA whose rates (parameters) are hard to measure. Models of climate change, ecosystems, and macroeconomics also have parameters that are hard or impossible to measure directly. If we fit these unknown parameters, fiddling with them until they agree with past experiments, how much can we trust their predictions? Multiparameter fits are sloppy; the parameters can vary over enormous ranges and still agree with past experiments. Nonetheless, they can often make useful predictions about future experiments, even allowing for these huge parameter uncertainties: a few stiff combinations of parameters govern the behavior. Third, these sloppy models all appear strikingly similar to one another – for example, the stiffnesses in every case we’ve studied are spread roughly uniformly over a range of over a million. We will use ideas and methods from differential geometry to explain what sloppiness is and why it happens so often. Finally, we shall show that models in physics are also sloppy – that sloppiness makes science possible.

Biophysics Seminar: Statistical Physics in Biology: Twisting transitions for DNA, and Ising Models for cell Membranes Jan 15, 2014

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The theory of phase transitions splits between abrupt transitions (nucleation and growth, critical droplets) and continuous transitions (scaling and universality). I’ll discuss wonderful biophysics examples for each: Michelle Wang’s twisting single molecules of DNA, and with Sarah Veatch’s discovery of universal Ising critical fluctuations in living cell membranes. (1) Plectonemes are the helically wound loops formed in garden hoses and electrical cords when they are overtwisted. Wang's group studies their equilibrium formation in overtwisted DNA, where they observe reversible transitions over the free energy barrier. This system, with well-known continuum elasticity and a controlled disorder, forms an unusual opportunity to test our ideas about the nucleation of phase transitions, and to generalize them to include randomness. (2) Sarah Veatch in Baird's group has recently made an amazing discovery – cell membranes, when stripped from the cytoskeleton, sit just above an Ising critical point. Cooled by 5%, they phase separate into two components: differing mixtures of lipids and proteins. We've tried to answer three questions: Why don't intact cells undergo this phase separation? Why would a cell want to sit near a critical point? What does statistical mechanics tell us about lipid rafts and the formation of protein aggregates?

Public Lecture: Crackling Noise Jan 15, 2014

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A piece of paper or candy wrapper crackles when it is crumpled. A magnet crackles when you change its magnetization slowly. The earth crackles as the continents slowly drift apart, forming earthquakes. Crackling noise happens when a material, when put under a slowly increasing strain, slips through a series of short, sharp events with an enormous range of sizes. There are many thousands of tiny earthquakes each year, but only a few huge ones. The sizes and shapes of earthquakes show regular patterns that they share with magnets and many other systems. This suggests that there must be a shared scientific explanation. We shall hear about crackling noise and that it is a symptom of a surprising truth: the system behaves the same on small, medium, and large scales.

Public Lecture: Quantum Beauty May 29, 2013

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Does the world embody beautiful ideas? Pythagoras and Plato intuited that it should, Newton and Maxwell showed, in impressive examples, how it could. Modern physics demonstrates, in depth and detail, that it does. I will narrate, through notable examples, how the concept of beauty in physical law has evolved – and how it continues to guide our quest for ultimate understanding.

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