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February 15, 2016

Shaking Up Statistical Physics in Interacting Quantum Systems

Physics Colloquium, Thursday, February 18th, 2016

Speaker: Anushya Chandran, Perimeter Institute


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 absorb heat indefinitely. I will present one physical setting of interacting bosons in which this expectation fails.

Time: 4:00-5:00pm
Location: 100 Willamette Hall

Host: Dietrich Belitz

Colloquium Archive

February 9, 2016

Physics of Information Processing in Living Systems

Physics Colloquium, Thursday, February 11th, 2016

Speaker: Yuhai Tu, IBM T.J. Watson Research Center


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.

Time: 4:00-5:00pm
Location: 100 Willamette Hall

Host: Dietrich Belitz

Colloquium Archive

February 1, 2016

Do two-dimensional metals exist?

Physics Colloquium, Thursday, February 4th, 2016

Speaker: Michael Mulligan, Stanford University

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.

Host: Dietrich Belitz

January 22, 2016

LSST: a color movie of the Universe coming near you!

Physics Colloquium, Thursday, January 28th, 2016

Speaker: Željko Ivezić,, University of Washington


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, this 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.

Host: Spencer Chang

Colloquium Archive


January 15, 2016

Emerging Phases and Phase Transitions in Quantum Matter

Physics Colloquium, Thursday, January 21st, 2016

Speaker: Thomas Vojta
Missouri University of Science and Technology


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.

Host: Dietrich Belitz



December 29, 2015

Non-perturbative Results for Itinerant Ferromagnetism in Multi-orbital Systems

January 14th, 2016

Speaker: Yi Li
Princeton Center for Theoretical Science, Princeton University

Title: Non-perturbative Results for Itinerant Ferromagnetism in Multi-orbital Systems


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.


November 20, 2015

Is there a Dark Sector or was Einstein Wrong?

Physics Colloquium, Thursday, December 3rd, 2015

Speaker:  Scott Dodelson, Fermilab


Observations over the past decade suggest that most of the energy in the Universe is in the form of Dark Energy and Dark Matter. The hunt is on for the identity of these new substances as we strive to understand how they fit in to the rest of physics. Recently, attention has turned to another possibility: there is no dark sector but Einstein’s theory of gravity needs to be modified. We are now faced (again!) with a contest between two competing ideas: Change the fundamental laws of Nature OR Introduce new substances. Which will win and how will we find out?

Host: Graham Kribs

Click here to view Fall 2015 schedule:


November 16, 2015

Title: Advanced Particle Accelerator R&D

Physics Colloquium, Thursday, November 19th, 2015

Speaker: Mark Hogan, SLAC National Accelerator Laboratory


Particle accelerators are the ultimate microscopes. They produce high-energy beams of particles – or, in some cases, generate X-ray laser pulses – to probe the fundamental particles and forces that make up the universe and explore the building blocks of life. But it takes huge accelerators, like the 2-mile-long SLAC linac, to generate beams with enough energy and resolving power. If we could achieve the same thing with accelerators just a few feet long, accelerators and particle colliders could be much smaller and cheaper. Eighteen years ago, SLAC began an exciting series of experiments aimed at accelerating electrons and positrons to high energies in a much shorter distance by having them “surf” on waves of hot, ionized gas like that found in fluorescent light tubes. These “plasma wakefield acceleration” experiments have been mounted by a diverse group of accelerator, laser and plasma researchers from national laboratories and universities at FACET, the Facility for Advanced Accelerator Experimental Tests, which uses the first two kilometers of the SLAC linac. Mark Hogan reviews the history of these experiments, the current program at FACET and plans for the future at FACET-II.

Host: Jim Brau

Click here to view Fall 2015 schedule:




November 9, 2015

Title: The Black Hole Information Paradox, Alive and Kicking

Physics Colloquium, Thursday, November 12th, 2015

Speaker:   Joe Polchinski, UC Santa Barbara


Thought experiments have played an important role in figuring out the laws of physics. For the unification of quantum mechanics and gravity, where the phenomena take place in extreme regimes, they are even more crucial.

Hawking’s 1976 paper “Breakdown of Predictability in Gravitational Collapse” presented one of the great thought experiments in the history of physics, arguing that black holes destroy information in a way that requires a modification of the laws of quantum mechanics. Skeptics for years failed to poke holes in Hawking’s argument, but concluded that if quantum mechanics is to be saved then our understanding of spacetime must break down in a radical way. For a time it seemed that Maldacena’s discovery of gauge/gravity duality had resolved the issue, but the recent firewall argument has opened many new questions.

Host: G. Kribs

Click here to view Fall 2015 schedule:


November 2, 2015

Birds, magnets, soap, and sandblasting: surprising connections in the theory of incompressible flocks

Speaker: John Toner, University of Oregon


In this talk I’ll describe the hydrodynamic theory of the motion of incompressible flocks: that is, collections of self-propelled entities (“birds”) that are packed so tightly together that their density cannot change as they move. In two dimensions, this problem can be mapped onto an equilibrium magnet with a peculiar constraint. This problem, in turn, can be shown to be equivalent to a 2d smectic (“soap”), with the flow lines of the flock playing the role of the smectic layers. Finally, this smectic problem can be mapped onto the 1+1 dimensional KPZ equation, which describes the growth or corrosion (“sandblasting”) of a one dimensional interface. The scaling properties of this last system, which have been known exactly for a long time, can thereby be used to determine those of incompressible 2d flocks. One important implication of the resulting scaling laws is that such flocks can exhibit long-ranged order in two dimensions, unlike their equilibrium counterparts.

Click here to view Fall 2015 schedule:


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