Physics and Astronomy Colloquium

semester, 2015


Wednesday, January 21st 2015
3:35 pm:
There will be no Colloquium this week.

Wednesday, January 28th 2015
3:35 pm:
Speaker: Deborah Jin (JILA, University of Colorado)
Subject: Ultracold Polar Molecules
Refreshments served in Room 216 Physics after colloquium

Gases of atoms can be cooled to temperatures close to absolute zero, where intriguing quantum behaviors such as Bose-Einstein condensation and superfluidity emerge. A new direction in experiments is to try to produce an ultracold gas of molecules, rather than atoms. In particular, polar molecules, which have strong dipole-dipole interactions, are interesting for applications ranging from quantum information to modeling condensed matter physics. I will describe experiments that produce and explore an ultracold gas of polar molecules.


Wednesday, February 4th 2015
3:35 pm:
Speaker: Hugh Hudson, UC Berkeley
Subject: Solar Flares, X-ray Astronomy, and Black Swans
Refreshments served in Room 216 Physics after colloquium

The atmosphere of the Sun (and that of many another star) hosts catastrophic disruptions - flares - that involve a very wide range of phenomena. Much of the development of our basic understanding began with John Winckler (Minnesota) and his students, and their students. The physics of space plasmas plays a major role in modern analysis. I will discuss new wrinkles in the physics of flares, emphasizing the X-ray observations, and assess some recent new information regarding extreme events - the "black swans" - detected now in tree-ring fossil records and Kepler astronomical photometry.

Faculty Host: John Wygant

Wednesday, February 11th 2015
3:35 pm:
Speaker: Jennifer Thomas, University College London
Subject: Neutrino Oscillations at Work
Refreshments served in Room 216 Physics after colloquium

The observation that the three types of neutrino flavor oscillate among themselves led to the realisation that neutrinos have a very small but non-zero mass. This is extremely important because the supremely successful Standard Model of particle physics had expected, and indeed needed, the neutrinos to have exactly zero mass. Since the discovery of neutrino oscillations over the last 15 years, the parameters of the oscillations have been sufficiently well measured to turn neutrino oscillations into a tool for learning more about the elusive neutrino. I will explain the concept of neutrino oscillations, and report on the recent results from around the world in context with the new challenges now facing researchers of inferring the remaining unknown neutrino properties. I will talk briefly about an exciting new project on the horizon for the very near future.

Faculty Host: Marvin Marshak

Wednesday, February 18th 2015
3:35 pm:
Speaker: Michel Janssen, University of Minnesota
Subject: The centenary of general relativity: How did Einstein find his gravitational field equations?
Refreshments served in Room 216 Physics after colloquium

In his search for gravitational field equations from late 1912 to late 1915, Einstein vacillated between two different strategies. Following a "mathematical strategy," he extracted candidate field equations from the Riemann curvature tensor and checked whether these equations were compatible with energy-momentum conservation and reproduced Newton’s theory of gravity in the appropriate limit. Following a "physical strategy," he constructed field equations for the gravitational field in close analogy with those for the electromagnetic field. In his later years, Einstein routinely claimed that he brought his search for gravitational field equations to a successful conclusion in November 1915 by switching to the mathematical strategy at the eleventh hour. Most commentators have accepted this later assessment but we have argued that Einstein achieved his breakthrough of November 1915 by doggedly pursuing the physical strategy. In a lecture in Vienna in September 1913, Einstein clearly laid out this physical strategy. As long as one took the older Einstein’s word for it that the mathematical strategy was responsible for the success of November 1915, one could quickly pass over the Vienna lecture. But if it was really the physical strategy that was responsible for this success, as we believe, the Vienna lecture deserves a much more prominent place in the account of the genesis of general relativity than it has been given so far.


Wednesday, February 25th 2015
3:35 pm:
Speaker: Adam Cohen, Harvard University
Subject:  Bringing Bioelectricity to Light
Refreshments served in Room 216 Physics after colloquium

In the wild, microbial rhodopsin proteins convert sunlight into biochemical signals in their host organisms. Some microbial rhodopsins convert sunlight into changes in membrane voltage. We engineered a microbial rhodopsin to run in reverse: to convert changes in membrane voltage into fluorescence signals that are readily detected in a microscope. Archaerhodopsin-derived voltage-indicating proteins enable optical mapping of bioelectric phenomena with unprecedented speed and sensitivity. We are applying these tools to study the role of voltage across biology: in bacteria, plant roots, fish hearts, mouse brains, and human induced pluripotent stem cell (hiPSC)-derived neurons and cardiomyocytes. We are engineering new functionality into microbial rhodopsins by taking advantage of their strong optical nonlinearities.

Faculty Host: Jochen Mueller

Wednesday, March 4th 2015
3:35 pm:
There will be no Colloquium this week.

Wednesday, March 11th 2015
3:35 pm:
Speaker: Philip Schuster, Perimeter Institute for Theoretical Physics
Subject: Accelerating our Understanding of Dark Matter
Refreshments served in Room 216 Physics after colloquium

Most of the matter in the Universe is dark; determining the composition and interactions of this dark matter are among the defining challenges in particle physics today. I will summarize the present status of dark matter searches and the case for exploration beyond the WIMP paradigm, particularly the motivations for “light dark matter” close to but beneath the weak scale. I will also describe sharp milestones in sensitivity needed to decisively explore the best-motivated light dark matter scenarios, and comment on experimental techniques to reach these milestones.

Faculty Host: Priscilla Cushman

Wednesday, March 18th 2015
3:35 pm:
Subject: Spring Break - No colloquium
Refreshments served in Room 216 Physics after colloquium

Wednesday, March 25th 2015
3:35 pm:
Speaker: Allan MacDonald, University of Texas, Austin
Subject: Exciton Condensates are Super!
Refreshments served in Room 216 Physics after colloquium

Electronic systems can have a type of order in which coherence is spontaneously established between two distinct groups of electrons. So far this (particle-hole or exciton condensate) type of order has been found only in double-layer two-dimensional electron gas systems, and only in certain strong magnetic field limits. I will review some of the surprising superfluid transport effects that have been observed in double-layer exciton condensates, and speculate on the possibility of realizing similar effects at room temperature either by enhancing the stability of bilayer exciton condensate states or by designing ferromagnetic materials with appropriate properties.

Faculty Host: Alex Kamenev

Wednesday, April 1st 2015
3:35 pm:
Speaker: Laura Greene, University of Illinois at Urbana-Champaign
Subject: Deciphering Electron Matter in Unconventional Superconductors
Refreshments served in Room 216 Physics after colloquium

Superconductors may be grouped into two major classes. The first is conventional metallic, whose pairing mechanism is explained by the BCS theory and electron-phonon coupling. The second we call unconventional, and the precise pairing mechanism has still to be worked out. All of the unconventional superconductors have electronic properties that are highly tunable, either by doping or pressure, from a non-superconducting parent compound, to a superconductor, to a non-superconducting Fermi liquid, thus defining a superconducting ‘dome’ in the phase diagram. More than 40 families of such materials, including the high-temperature superconductors, exhibit this ubiquitous phase diagram. In the underdoped phases, all of these materials show intriguing correlated electron states above the dome, and researches agree that the understanding of this “electron matter” holds the key to the pairing mechanism. Finally, I will show how we have found that point contact spectroscopy is exquisitely sensitive detecting electron matter.

Faculty Host: Rafael Fernandes

Wednesday, April 8th 2015
3:35 pm:
Speaker: Steven Kivelson, Stanford University
Subject: Quenched disorder and vestigial nematicity in correlated electronic systems
Refreshments served in Room 216 Physics after colloquium

Intermediate phases with “vestigial order” occur when the spontaneously broken symmetries of a “fully ordered” groundstate are restored sequentially as a function of increasingly strong thermal or quantum fluctuations, or of increasing magnitude of quenched randomness. From this perspective, a large number of developments in the field of highly correlated systems – in particular the remarkable proliferation of cases in which electron nematic phases and their relatives appear as significant players in the physics of “interesting” materials - can be treated as variations on the same underlying theme. Recent experiments probing charge order in the pseudo-gap regime of the hole-doped cuprate high-temperature superconductors and nematic order in the Fe based superconductors are interpreted in light of these results.

Faculty Host: Rafael Fernandes

Wednesday, April 15th 2015
3:35 pm:
Speaker:  Reinhardt Schuhmann, Physical Review Letters
Subject: Physical Review Letters - managing a peer review journal
Refreshments served in Room 216 Physics after colloquium

Dr. Schuhmann is the Managing Editor of Physics Review Letters. Physical Review Letters receives ~11000 submissions per year, and publishes about 1/4 of them. Editors decide what to publish with extensive input from peer review, with roughly 70% of manuscripts reviewed. My talk will outline of how PRL manages peer review for such volume, with examples from correspondence, while it remains the premier physics journal. It is the most cited physics journal, with a Letter cited roughly every 90 seconds. PRL faces many challenges, however, as the publishing trends in some areas of physics shift, for example to smaller, less comprehensive, or more interdisciplinary venues. I will discuss some of these challenges, and what PRL is doing, and plans to do, to maintain a competitive journal that covers the full arc of physics. I would greatly appreciate your feedback and questions during and after the talk.


Wednesday, April 22nd 2015
3:35 pm:
Speaker: Margaret J. Geller, Harvard-Smithsonian Center for Astrophysics
Subject: The Ultimate Mass of Galaxy Clusters
Refreshments served in Tate Foyer after colloquium. This is the more technical portion of the Van Vleck Lecture series. This lecture is free and open to the public.

Clusters of galaxies are still forming at the current epoch. At the turnaround radius, the infall velocity induced by the mass concentration in the cluster just balances the Hubble flow. In the standard cosmology, galaxies within the turnaround radius will remain bound to the cluster and the mass within this radius is a good estimator of the ultimate cluster mass. Our redshift survey of nearly 100 massive clusters (HeCS: Hectospec Cluster Survey) in the redshift range 0.1 to 0.3 enables a number of important cosmological probes. These include a test of the cluster mass proxy derived from observations of the Sunyaev-Zeldovich effect. These data also enable measurement of mass profiles to large radius using the caustic technique. These profiles enable direct measurement of the accretion rate of galaxy clusters and of their ultimate mass. These measurements are new and direct tests of our understanding
of the growth of structure in the universe.


Wednesday, April 29th 2015
3:35 pm:
Speaker: Leslie Rosenberg, University of Washington
Subject: Searching for Dark-Matter Axions
Refreshments served in Room 216 Physics after colloquium

The axion is a hypothetical elementary particle whose existence would explain the baffling absence of CP violation in the strong interactions. Axions also happen to be a compelling dark-matter candidate. Even if dark-matter axions were to comprise the overwhelming majority of mass in the universe, they would be extraordinarily difficult to detect. However, several experiments, either under construction or taking data, would be sensitive to even the more pessimistically coupled axions. This talk describes the current state of these searches.


Wednesday, May 6th 2015
Speaker: Frank Pollmann, Max Planck Institute for the Physics of Complex Systems
Subject: Efficient numerical simulations of quantum many-body systems
Refreshments served in Room 216 Physics after colloquium. 3:35 pm Award Ceremony. 3:50 - 4:50 pm Colloquium

The simulation of quantum many-body systems on classical computers is notoriously difficult because of the exponential growth of the Hilbert space with the size of the system. This makes the study of some of the most fascinating problems in condensed matter physics extremely challenging. These include for example the understanding of high TC superconductors, as well as frustrated quantum magnets which might realize exotic phases of matter. In my talk, I will discuss how numerical approaches based on quantum information concepts allow for an efficient simulation of quantum many-body systems. In particular, I will introduce tensor-product state based methods that provide an optimized representation of the relevant corner of the Hilbert space. I will then demonstrate applications of matrix-product state based methods to obtain the ground state as well as the dynamics of strongly correlated quantum systems.

Faculty Host: Fiona Burnell

Thursday, September 10th 2015
3:35 pm:
Physics and Astronomy Colloquium in 230 STSS/Bruininks Hall
There will be no colloquium this week.

Thursday, September 17th 2015
3:35 pm:
Physics and Astronomy Colloquium in 230 STSS/Bruininks Hall
Speaker: Chandralekha Singh, University of Pittsburgh
Subject: Improving student understanding of quantum mechanics
Refreshments to be served outside 230 SSTS after the colloquium.

Learning quantum mechanics can be challenging, in part due to the non-intuitive nature of the subject matter. I will describe investigations of the difficulties that students have in learning quantum mechanics. We find that the patterns of reasoning difficulties in learning quantum mechanics are often universal, similar to the universal nature of reasoning difficulties found in introductory physics. Moreover, students often fail to monitor their learning while learning quantum mechanics. To help improve student understanding of quantum concepts, we are developing quantum interactive learning tutorials (QuILTs) as well as tools for peer-instruction. The goal of QuILTs and peer-instruction tools is to actively engage students in the learning process and to help them build links between the formalism and the conceptual aspects of quantum physics without compromising the technical content.

Faculty Host: Kenneth Heller

Thursday, September 24th 2015
3:35 pm:
Physics and Astronomy Colloquium in 230 STSS/Bruininks Hall
Speaker: Mikhail Voloshin, University of Minnesota
Subject: Exotic Mesons and Baryons
Refreshments to be served outside 230 SSTS after the colloquium.

According to the original quark model template there are mesons consisting of a quark and an antiquark, and baryons made from three quarks. Strongly interacting particles that do not fit this template are called exotic. Experimental findings of recent years have uncovered existence of exotic mesons: tetraquarks, and baryons: pentaquarks, containing an additional heavy quark-antiquark pair. I discuss properties of such particles and the current theoretical approaches to understanding their internal dynamics.


Thursday, October 1st 2015
3:35 pm:
Physics and Astronomy Colloquium in 230 STSS/Bruininks Hall
Speaker: Lance Dixon, SLAC/Stanford
Subject: New Approaches to Quantum Scattering and the Payoff for the LHC
Refreshments to be served outside 230 SSTS after the colloquium.

The Large Hadron Collider is renewing its exploration of the energy frontier of particle physics, searching for new particles and interactions beyond the Higgs boson. For the LHC to uncover many types of new physics, the "old physics" produced by the Standard Model must be understood very well. For decades the central theoretical tool for this job was the Feynman diagram. However, Feynman diagrams are just too slow, even on fast computers, to allow adequate precision for complicated events with many jets of hadrons in the final state. Such events constitute formidable backgrounds at the LHC to many searches for new physics. Over the past few years, alternative methods to
Feynman diagrams have come to fruition. The new "on-shell" methods are based on the old principle of unitarity. They can be much more efficient because they exploit the underlying simplicity of scattering amplitudes, and recycle lower-loop information. Farther afield, the
new methods have led to intriguing new results in quantum gravity and in supersymmetric analogs of the Standard Model. I'll explain how and why these methods work, and present recent state-of-the-art results obtained with them.

Faculty Host: Tony Gherghetta

Thursday, October 8th 2015
3:35 pm:
Physics and Astronomy Colloquium in 230 STSS/Bruininks Hall
Speaker: Giovanni Zocchi, UCLA
Subject: Nature’s molecular machines: a materials science approach
Refreshments to be served outside 230 SSTS after the colloquium.

Living matter, at the molecular scale, is different from usual matter. Biological molecules, specifically enzymes, deform without breaking, couple chemical reactions to molecular tasks. Nature’s molecular machines are not scaled down versions of macroscopic machines: molecular motors have nothing to do with Carnot cycles, and molecular pumps have nothing to do with hydrodynamics. So how do these molecules work. I will describe our advances in extracting universal mechanical properties of enzymes, and come to the surprising conclusion that the molecules of life are visco-elastic ! Enzymes “flow” from one solid – like conformation to another. As it turns out, the molecules we are made of behave dynamically like “silly putty”. Another universal conclusion is that any enzyme can be controlled mechanically, opening mechanical control for thousands of chemical reactions.

Faculty Host: Jochen Mueller

Thursday, October 15th 2015
3:35 pm:
Physics and Astronomy Colloquium in 230 STSS/Bruininks Hall
Speaker: Oleg Tchernyshyov, Johns Hopkins University
Subject: Mechanics of magnetic solitons
Refreshments to be served outside 230 SSTS after the colloquium.

Magnets host a variety of solitons that are stable for topological reasons: domain walls, vortices, and skyrmions, to name a few. Because of their stability, topological solitons can potentially be used for storing and processing information. This motivates us to build economic, yet realistic models of soliton dynamics in magnets. E.g., a domain wall in a cylindrical ferromagnetic wire can be pictured as a bead on a string, which can move along the string and rotate about its axis. Its mechanics is counterintuitive: it rotates if pushed and moves if twisted. I will review basic models of ferro- and antiferromagnetic domain walls in one dimension and discuss examples from higher dimensions, e.g., vortices and skyrmions.

Faculty Host: Natalia Perkins

Thursday, October 22nd 2015
3:35 pm:
Physics and Astronomy Colloquium in 230 STSS/Bruininks Hall
Speaker: James Bullock, University of California, Irvine
Subject: Dwarf Galaxies, the Local Group, and Cosmology
Refreshments to be served outside 230 SSTS after the colloquium.

The Local Group and the tiny galaxies that surround the Milky Way provide unique and detailed data sets for testing ideas in cosmology and galaxy formation. In this talk I will discuss how numerical simulations coupled with local "near-field" observations are informing our understanding of dark matter, the formation of the first galaxies, and the physical processes that act at the threshold of galaxy formation.

Faculty Host: Evan Skillman

Thursday, October 29th 2015
3:35 pm:
Physics and Astronomy Colloquium in 230 STSS/Bruininks Hall
Speaker: Claudia Scarlata, University of Minnesota
Subject: Galaxy formation from a different perspective
Refreshments to be served outside 230 SSTS after the colloquium.

Our understanding of the formation and evolution of galaxies has been revolutionized in the past decade. Galaxies' growth is now thought to be regulated by the physics of baryons, through a self-regulating process wherein the star-formation rate, the gas accretion rate, and the gas outow rate all satisfy a slowly evolving equilibrium condition. However, there are still a number of problems/open questions with these baryon-dominated models, particularly when low-mass galaxies are looked at in detail. I will present recent results from the WFC3 Infrared Spectroscopic Parallel Survey (WISP) that we are conducting on the Hubble Space Telescope. This large program is identifying thousands of galaxies across a wide range of redshifts, spanning more that two thirds of the age of the universe. Our survey provides a selection function that is independent of galaxy stellar mass, and thus allows the study of those low mass objects that are mostly affected by energy feedback. These are the galaxies that provide the strongest constraints on galaxy formation models. I will also discuss our results in the context of future space based surveys such as Euclid and WFIRST-AFTA.


Thursday, November 5th 2015
3:35 pm:
Physics and Astronomy Colloquium in 230 STSS/Bruininks Hall
Speaker: Uwe Kortshagen, University of Minnesota
Subject: Luminescence, Doping, and Transport Properties of Silicon Nanocrystals produced via Nonthermal Plasma Synthesis

Nonthermal plasma synthesis of nanocrystals is particularly suited for covalently bonded materials that require high temperatures to be produced with good crystallinity. Several years ago, we showed that plasma produced silicon nanocrystals are capable of high-efficiency photoluminescence, different from bulk silicon material. More recently, the capability of nonthermal plasmas to produce substitutionally doped nanocrystal materials has attracted attention, as substitutional doping had presented a significant challenge both for liquid and gas phase synthesis due to effects such as self-purification.

This presentation discusses the physics of plasma synthesis process. High photoluminescense quantum yields are achieved by careful surface functionalization through grafting alkene ligands to the nanocrystal surfaces. We also discuss the substitutional doping of silicon nanocrystals with boron and phosphorous using a nonthermal plasma technique. While the synthesis approach is identical in both cases, the activation behavior of these two dopants is found to be dramatically different. Finally, we present some experimental work on transport in films of highly phosphorous-doped nanocrystals, which indicates the approach to the metal-to-insulator transition.

This work was supported in part by the NSF Materials Research Science and Engineering Center under grant DMR-1420013, the DOE Energy Frontier Research Center for Advanced Solar Photophysics, and the Army Office of Research under MURI grant W911NF-12-1-0407.

Faculty Host: Boris Shklovskii

Thursday, November 12th 2015
3:35 pm:
Physics and Astronomy Colloquium in 230 STSS/Bruininks Hall
Speaker: James Wells, University of Michigan
Subject: Higgs bosons and superconductors in the lab and throughout the universe
Refreshments to be served outside 230 SSTS after the colloquium.

I take an historical look at the developments of elementary
particle physics and condensed matter physics over the last century,
with focus on the interaction of these two grand areas of physics in
the context of spontaneous symmetry breaking. In particular, I compare
the discoveries of mutual interest in superconductivity and the
electroweak theory of particle physics and how the fields have
inspired each other. I describe how important the discovery of the
Higgs boson has been to particle physics and what it means for the
future. In the process I give an in-depth response to Anderson’s
recent statement in Nature: “Maybe the Higgs boson [of particle
physics] is fictitious!”

Faculty Host: Tony Gherghetta

Thursday, November 19th 2015
3:35 pm:
Physics and Astronomy Colloquium in 230 STSS/Bruininks Hall
Speaker: Joe Lykken, Fermilab
Subject: Neutrinos are Everywhere
Refreshments to be served outside 230 SSTS after the colloquium.

The story of Fermi's "little neutral ones" has already many surprises and inspiring examples of daring experimental initiative. Today a host of new experiments are trying to unlock the secrets of these elusive particles.

Faculty Host: Tony Gherghetta

Thursday, November 26th 2015
Speaker: No Colloquium today due to Thanksgiving Holiday

Thursday, December 3rd 2015
3:35 pm:
Physics and Astronomy Colloquium in 230 STSS/Bruininks Hall
Speaker: Heather Wilkens, Manager of Business Development, ATA Engineering
Subject: Examining an Industrial Career Path in Physics
Refreshments to be served outside 230 SSTS after the colloquium.

Heather Wilson is Manager of Business Development at ATA Engineering

Researchers estimate that more PhD physicists in the US work in the private sector than in academia, so thinking through an industrial career path is a very useful exercise for current physics graduate students. In this talk, Dr. Wilkens will present recent career statistics of physicists in the private sector, along with the story of her industrial career after receiving her PhD from the U of MN Physics Department in 2003. She will also share thoughts on the industrial career paths chosen by her colleagues, and delve into the new and exciting careers she sees opening up for physicists in less traditional sectors.

Faculty Host: E. Dan Dahlberg

Thursday, December 10th 2015
3:35 pm:
Physics and Astronomy Colloquium in 230 STSS/Bruininks Hall
Speaker: Cancelled--No Colloquium today

Thursday, December 17th 2015
3:35 pm:
Physics and Astronomy Colloquium in 230 STSS/Bruininks Hall
There will be no colloquium this week.

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