Winter 2016 Colloquium Series
January 7th, 2016
Speaker:Jayson Paulose
Leiden University, Netherlands
Title: Topological Design of Mechanical Metamaterials
Abstract:
Topological phenomena lie at the forefront of condensed matter physics. When a physical observable is linked to a topological index characterizing a system, it is unaffected by local changes and thus robust against a range of perturbations. This concept of topological protection, originally developed in electronic systems, has recently been applied to mechanical systems as well. In this talk, I'll show how a recent mapping between spring lattices and electronic topological insulators can be exploited to design topologically protected mechanical response in artificial repetitive structures, or metamaterials. First, I'll demonstrate localized flexible regions in otherwise rigid lattices that arise due to an interplay between crystal defects and a bulk topological index characterizing lattice vibrations. I'll then show how topological stress states single out regions for buckling in the interior of a structure. Finally, I'll extend the mapping from central-force spring networks to a broader class of structures with more complex couplings among elements, encompassing gear networks and frictional disc packings. The results will be demonstrated in real-world prototypes of topological mechanical metamaterials, and point towards new ways of designing robust mechanical phenomena across different scales.
January 14th, 2016
Speaker: Yi Li
Princeton Center for Theoretical Science, Princeton University
Title:Non-perturbative Results for Itinerant Ferromagnetism in Multi-orbital Systems
Abstract:
Itinerant ferromagnetism (FM) is intrinsically a strongly correlated phenomenon, which remains a major challenge of condensed matter physics. Most FM materials are orbital-active with prominent Hund’s coupling. However, the local physics of Hund’s rule usually does not lead to the FM long-range order. Furthermore, the magnetic phase transitions of itinerant electrons are also long-standing problems difficult to handle by using perturbative methods. In this talk, I will present non-perturbative studies on itinerant FM. Exact theorems are established for a stable itinerant FM phase in a large region of electron densities in multi-orbital systems, which provide sufficient conditions for Hund’s rule to build up global FM coherence. In addition, thermodynamic properties and magnetic phase transitions of itinerant electrons are studied via sign-problem-free quantum Monte Carlo simulations at generic fillings. Without introducing local moments as a priori, the Curie-Weiss metal behavior is identified in a wide range of temperatures. These results will provide important guidance to the current experimental search for novel itinerant FM states in a large class of systems ranging from the transition-metal-oxide heterostructures (e.g. LaAlO3/SrTiO3) to the p-orbital bands in optical lattices filled with ultra-cold fermions.
January 21, 2016
Speaker:Thomas Vojta
Missouri University of Science and Technology
Title:Emerging phases and phase transitions in quantum matter
Abstract:
Condensed matter physics deals with the complex behavior of
many-particle systems. Novel phases of matter can emerge as a
result of strong interactions between the constituent
particles. A natural place to look for these phenomena are
quantum phase transitions, the boundaries between different
quantum ground states of matter.
This talk first gives an introduction into quantum phase
transitions and then discusses several novel phases of matter
that have been discovered in their vicinity in solids and in
ultracold atomic gases. These include exotic superconductors
and magnets as well as Griffiths phases that are dominated
by strong disorder.
January 28, 2016
Speaker:ŽeljkoIvezić
University of Washington
Title: LSST: a Color Movie of the Universe Coming Near You!
Abstract:
The Large Synoptic Survey Telescope (LSST) will carry out an imaging survey covering
the sky that is visible from Cerro Pachon in Northern Chile, with first light in 2019.
With close to 1000 observations in ugrizy bands over a 10-year period, these data will enable deep coadded maps across half the sky reaching hundred times fainter flux level than the Sloan Digital Sky Survey (SDSS). About 20 billion galaxies and a similar number of stars will be detected in these maps -- for the first time in history, the number of cataloged celestial objects will exceed the number of living people. The time-resolved observations will open a movie-like window on objects that change brightness, or move, on timescales ranging from 10 seconds to 10 years. With a raw data rate of about 15 TB per night (about the same as one SDSS per night), LSST will collect over 100 PB of data over its lifetime, resulting in an incredibly rich and extensive public archive that will be a treasure trove for breakthroughs in many areas of astronomy and physics, ranging from the properties of near-Earth asteroids to characterizations of dark energy and dark matter. I will provide an overview of the main science drivers and a status report for the federally-funded construction project that started in 2014.
February 4th, 2016
Speaker:Michael Mulligan
Stanford
Title:Do Two-Dimensional Metals Exist?
Abstract:
Conventional wisdom teaches us that electrons confined to a two-dimensional quantum well will do one of three things as the temperature is lowered to zero: superconduct, insulate, or exhibit the so-called quantum Hall effect. (Here, I am concentrating on the types of order as revealed in electrical charge transport; finer distinctions can be made, e.g., magnetic ordering.) Nature, however, is stubborn and doesn't always listen. In this talk, I will describe two experimental systems that surprisingly appear to violate the conventional wisdom and instead exhibit a metallic phase at zero temperature. I will argue that there is a deep analogy between the two systems that relates their behaviors and discuss how such novel metallic phases can explain other unconventional low-temperature quantum orders.
February 11th, 2016
Speaker: Yuhai Tu
IBM T. J. Watson Research Center
Title:Physics of information processing in living systems
Abstract:
Living organisms need to obtain and process information that are crucial for their survival. These information processes, ranging from signal transduction in a single cell to image processing in the human brain, are performed by biological circuits (networks). However, these biochemical or neural circuits are inherently noisy. Yet, certain accuracy is required to carry out proper biological functions. How do biological networks process information accurately and efficiently? What is the energy cost of biological computing? Is there a fundamental limit for its performance? In this talk, we will describe our recent work in trying to address these questions in the context of two basic cellular computing tasks: sensory adaptation for memory encoding [1,2]; biochemical oscillation for accurate timekeeping [3].
[1] “The energy-speed-accuracy trade-off in sensory adaptation”, G. Lan, P. Sartori, S. Neumann, V. Sourjik, and Yuhai Tu, Nature Physics 8, 422-428, 2012.
[2] “Free energy cost of reducing noise while maintaining a high sensitivity”, Pablo Sartori and Yuhai Tu, Phys. Rev. Lett. 2015. 115: 118102.
[3] “The free-energy cost of accurate biochemical oscillations”, Y. Cao, H. Wang, Q. Ouyang, and Yuhai Tu, Nature Physics 11, 772, 2015.
February 18th, 2016
Speaker:Anushya Chandran, Perimeter Institute
Title: Shaking up Statistical Physics in Interacting Quantum Systems
Abstract:
Statistical mechanics is a central pillar of modern science with applications ranging from sociology to economics. At its core is the idea of thermal equilibrium, which allows for a simple description of an interacting quantum system in terms of a few properties like temperature, without keeping track of the entire wavefunction. But what if a quantum system fails to equilibrate?
In this talk, I will discuss how we are discovering the answer to this question theoretically and experimentally. I’ll focus on two settings: disordered systems and periodically driven systems. In the former, many-body localization can prevent thermalization even at very high energy densities. The transition between the localized and the thermal phase is a fascinating dynamical quantum transition about which little is known. I will derive a rigorous constraint on this transition and apply it to current numerical studies and cold atomic experiments. Clean periodically driven systems, on the other hand, generically heat indefinitely. I will present one physical setting of interacting bosons in which this expectation fails.
February 25th, 2016
Hubert de Guise, Lakehead University
Raymer: Host
March 3rd, 2016
Speaker:Chiu-Fan Lee,
Imperial College, London
Title: Universalities in Biology – “Cutting through the chaos”
Abstract:
The interior of a cell can seem like a bag of unstructured multi-component fluid, yet the cell as a whole can perform well-coordinated and precise tasks such as coordinated motility. How does sharply defined system-level behaviour arise from a seemingly chaotic microenvironment? Such emergence may in fact be familiar to physicists from the studies of critical phenomena, in which case short wavelength fluctuations of the system get averaged out in such a way that a precise law dictating the long wavelength (system-level) behaviour arises. In addition, diverse physical systems can exhibit identical behaviour if they belong to the same universality class. I will argue in this talk that the concept of universality is equally fitting for biology. I will specifically discuss two non-equilibrium systems of biological relevance: motility-induced phase separating systems and incompressible active fluids.
March 10th, 2016
TBA