Colloquium
Unless otherwise noted, the physics colloquia are held in Room 307 of the Science and Engineering Research Facility. Refreshments are served at 3:00 p.m. with the talk following at 3:30. Please note: Colloquia for Fall 2017 will not be webcast or recorded. The department is evaluating new cost models and options.
The Spring 2017 colloquia are available here, with the archives from previous semesters available Webcast archives.
Other Seminars for Fall 2017
August 28 |
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September 4: Holiday |
No Colloquium |
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September 11 |
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September 18 |
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September 25 |
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October 2 |
Nearly perfect fluidity: From cold atoms to hot quarks and gluons |
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October 9 |
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October 16 |
Joseph Carlson, Los Alamos National Laboratory |
Strongly Correlated Quantum Matter: Cold Atoms, Nuclei, and Neutron Stars |
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October 23 |
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October 30 |
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November 6 |
Relaxation of populations in nonequilibrium many-body physics: Breakdown of Mathiessen’s rule |
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November 13 |
Probing the helium dimer and trimer with fast, intense lasers |
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November 20 |
ν physics, new physics, and new ν physics with the Deep Underground Neutrino Experiment |
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November 27 |
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December 4 |
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Studies of exotic nuclear decays with digital photography
Photographic techniques played a key role in early triumphs of subatomic physics. The discovery of the positron or the first evidence for a hypernucleus are spectacular examples. Later, advantages of electronic recording and signal processing took over optical methods. Recently, however, a marriage of modern electronics with photography resulted in a bloom of digital imaging and boosted research in many branches of science. Taking advantage of this revolution, we introduced digital photography into nuclear physics research. We have developed a new type of ionization chamber with optical readout dedicated to studies of exotic and rare nuclear decays. It was successfully used to investigate new radioactive decays of nuclei very far form stability.
In my talk, I will briefly introduce our Optical Time Projection Chamber (OTPC) and illustrate its operation with selection of images obtained over the last years. These will include results of the two-proton radioactivity studies of 45Fe and 48Ni, beta-delayed multipaticle emission from 45Fe, 43Cr, and 31Ar, and very rare decay of 6He into an alpha particle and a deuteron. Finally, I will shortly discuss our plans and ideas for future experiments with the OTPC.
Exploration of spin-1/2 triangular lattice antiferromagnets
Quantum spin fluctuation plays a critical role to define exotic magnetic ground states in quantum magnets. From materials engineering view, geometrically frustrated lattice, low dimensionality, and low spin (spin-1/2) are three key ingredients to maximize the quantum spin fluctuations in a quantum magnet. In this talk, I will introduce our recent studies on two new layer compounds with spin-1/2 triangular lattice, which are designed by following this principle. I will demonstrate and discuss they are ideal materials to exhibit exotic properties, which are potential quantum spin liquid state for YbMgGaO4 and the approach of quantum melting point for Ba3CoSb2O9. I will also introduce the method to grow single crystals of both materials.
The Surprising Relevance of Exactly Solvable Models
Physics aims at an exact description of nature through precise laws. The methodology on the other hand, is dominated by approximations, since an exact solution of almost any generic problem is technically impossible. Despite the difficulties, our understanding of many complex phenomena owes much to simplified models- and in particular to a few exactly solvable models, which serve to benchmark approximations and provide deep insights. Celebrated exact results include the Heisenberg model of magnetism solved in 1-d by Hans Bethe, the 2-d Ising model solved by Lars Onsager, which were of crucial importance in the early years. In more recent years the number of solvable and physically relevant models has slowly increased. I will discuss in qualitative (non-technical) terms a particular exactly solvable model of quantum antiferromagnetism in 2-dimensions, the Shastry-Sutherland model, its exact solution, and two decades later, its re-emergence in the context of experimental magnetic systems such as SrCu(BO3)2 and TmB4.
Probing the quark gluon plasma
High energy collisions of heavy nuclei permit the study of nuclear matter at high temperatures and energy densities. At energy densities above about 1 GeV/fm^3 QCD predicts a phase transition in nuclear matter to a plasma of quarks and gluons. This matter, called a Quark Gluon Plasma (QGP), has different properties from normal nuclear matter due to its high temperature and density. Measurements at the Relativistic Heavy Ion Collider (RHIC) on Long Island and the Large Hadron Collider (LHC) in Geneva allow studies of nucleus-nucleus collisions over two orders of magnitude in center of mass energy. Hard parton scatterings lead to back-to-back jets, the collimated sprays of particles formed from a quark or gluon. These jets are ideal probes of the QGP and studies of their interactions with the medium can be used to constrain its properties.
Nearly perfect fluidity: From cold atoms to hot quarks and gluons
A dimensionless measure of fluidity is the ratio of shear viscosity to entropy density. In this talk I will argue that fluidity is a sensitive probe of the strength of correlations in a fluid. I will also discuss evidence that the two most perfect fluids ever observed are also the coldest and the hottest fluid ever created in the laboratory. The two fluids are cold atomic gases [~10^(-6) K] that can be probed in optical traps, and the quark gluon plasma [~10^(12)K] created in heavy ion collisions at RHIC and the LHC. Remarkably, both fluids come close to a bound on the shear viscosity that was first proposed based on calculations in string theory, involving the non-equilibrium evolution of black holes.
Strongly Correlated Quantum Matter: Cold Atoms, Nuclei, and Neutron Stars
Atomic nuclei offer unique opportunities for studying strongly correlated quantum matter. Analogies to cold atom experiments provide important incites intosuperfluid pairing, short-range correlations, and dynamic response.These nuclear properties can be very important in major experiments probing fundamental neutrino properties, and are also critical in determining the chemical evolution of the universe and the cold dense equation of state relevant to neutron stars.
The Dawn of Gravitational Wave Astronomy
A century after Einstein predicted the existence of gravitational waves and fifty years after the first detectors were built, we have finally entered the era of gravitational wave astronomy. Already there have been many surprises and lucky breaks, starting with the first detection of a binary black hole merger by the LIGO-Virgo collaboration days before the official start of the first observing run in September 2015, and capped-off by the discovery of a nearby neutron star collision just days before the end of the second observing run in August 2017. When the Virgo detector in Italy joined the LIGO detectors on August 1st 2017 it was thought it would be a good chance to test our combined analysis pipelines in preparation for the next observing campaign. Instead we soon had the first three detector observation of a black hole merger, allowing new tests of Einstein’s theory, followed a few days later by the first neutron star merger. The talk will summarize what we have learned and how the information is extracted, and provide a look-ahead to the next decade of gravitational wave astronomy.
Getting Antimatter out of Dark Matter
I discuss the possibility of dark matter conversion into our antimatter, if dark matter is represented by a matter of hypothetical parallel mirror world. Any neutral particle, elementary or composite, can have mixing with its mass degenerate mirror twin. In particular, mirror neutron can oscillate into our anti-neutron with experimentally accessible rates depending on environmental conditions as matter density and magnetic fields. Existing experimental and astrophysical bounds allow these oscillations to be rather fast, with oscillation time of the order of a second. Also mirror hydrogen atom could be converted into our anti-hydrogen, but with much longer oscillation time. These phenomena can have fascinating physical and astrophysical implications, and they can be tested in low cost experiments. If discovered, they can have far going consequences: matter exchange with parallel mirror world would provide an alternative source of energy.
Relaxation of populations in nonequilibrium many-body physics: Breakdown of Mathiessen’s rule
The lifetime of a quasiparticle of an equilibrium many-body system is determined by Mathiessen’s rule, where the total scattering rate is given by the sum of the scattering rates for all different scattering processes. The relaxation time is then represented by the inverse of the electronic self energy, which determines both the lifetime of the quasiparticle spectral function and the linear-response dc resistivity. In a pump/probe experiment, a high intensity pump excites electrons into a nonequilibrium distribution, and those excited populations decay and relax back towards a new equilibrium, with a characteristic relaxation time, that depends on the energy of the excitation above the Fermi energy. It turns out that this relaxation time is often significantly different from the equilibrium relaxation time. In this talk, I will describe what determines this nonequilibrium relaxation time. It does not satisfy Mathiessen’s rule, but instead depends in a complicated fashion on how energy is exchanged from electrons to phonons, as the populations relax. It also is often not given by the equilibrium relaxation time. One consequence of this analysis, is an explicit proof that a simple hot electron model is inconsistent with the exact equations of motion of a many-body system. We end with a discussion of some experiments that illustrate this behavior and some open challenges that remain in fully understanding nonequilibrium relaxation.
[1] Alexander Kemper and James Freericks, Relationship between Population Dynamics and the Self-Energy in Driven Non-Equilibrium Systems, Entropy 18, 180 (2016).
[2] A. F. Kemper, H. R. Krishnamurthy, and J. K. Freericks, The role of average time dependence on the relaxation of excited electron populations in nonequilibrium many-body physics, Fortschritte der Physik 65, 1600042 (2017).
Probing the Helium Dimer and Trimer with Fast, Intense Lasers
Helium is the only element that remains liquid under normal pressure down to zero temperature. Below 2.17K, the bosonic isotope helium-4 undergoes a phase transition to a superfluid. Motivated by this intriguing bulk behavior, the properties of finite-sized helium droplets have been studied extensively over the past 25 years or so. A number of properties of liquid helium-4 droplets are, just as those of nuclei, well described by the liquid drop model. The existence of the extremely fragile helium dimer was proven experimentally in 1994 in diffraction grating experiments. Since then, appreciable effort has gone into creating and characterizing trimers, tetramers and larger clusters. The ground state and excited state of the helium trimer are particularly interesting since these systems are candidates for Efimov states. The existence of Efimov states, which are unique due to scale invariance and an associated limit cycle, was predicted in 1971. However, till recently, Efimov states had -- although their existence had been confirmed experimentally -- not been imaged directly. Recently, ingenious experimental advances that utilize femtosecond lasers made it possible to directly image the static quantum mechanical density distribution of helium dimers and trimers. I will review some of these experiments and related theoretical calculations that led to the experimental detection of the excited helium trimer Efimov state. Extensions to the time domain will also be discussed. Intriguing laser-kick induced dynamics of the fragile helium dimer is observed experimentally and analyzed theoretically. These initial results open the door for future studies that probe scattering length dominated few-body systems using fast, intense lasers.
ν physics, new physics, and new ν physics with the Deep Underground Neutrino Experiment
The Deep Underground Neutrino Experiment (DUNE) is a mega-scale project that the international neutrino community is planning to construct, in the U.S., during the next decade. DUNE will use a multi-kiloton cryogenic liquid argon detector, placed deep underground in an abandoned gold mine in South Dakota, in order to image neutrino and other rare particle interactions with extremely high resolution. In doing so, it will study fundamental neutrino and antineutrino properties, and search for rare processes which may manifest as a consequence of new, yet-undiscovered physics. DUNE’s ultimate goal is to shed light on one of the biggest mysteries in physics today: why matter has prevailed over antimatter in our Universe. It will also serve as an observatory for atmospheric and extra-terrestrial neutrinos, such as those emitted during a supernova core-collapse. This talk will describe the DUNE experiment, highlighting some of the challenges associated with constructing and operating such a large-scale detector, and discuss its physics program.
Relationship Between Science and Public Policy
To most citizens science is an invisible hand that plays an important role in their lives. From the devices that make their lives easier to the drugs that help them recover from illness, and to the food that they consume science is there. In certain instances, from the magnificent discoveries made by astronomers to images of hurricanes, the science is brought to the public’s attention by the headlines in the various forms of media. However, science also plays a role in the policy decisions of our legislatures as scientists are called to advice on issues that face the nation and the public. These issues range from climate change to nano-technology, from human genomics to modified food crops, from fracking to immunization. How should scientists respond to requests for advice and/or solutions? Are there any problems or pitfalls in the process of advising and forming policy?
Simulating Disorder in Functional Materials
Inserting disordered impurity atoms is one of the most powerful ways to tune the functionality of advanced materials. In this talk I will demonstrate how disorder controls and reveals the underlying physics of heat conductance in thermo-electrics, electron pairing in superconductors and Anderson localization in intermediate band semiconductors. In particular I will illustrate how unbiased and materials-specific simulations shed light on complex experiments on disordered materials and allow for a fundamental understanding of their properties.