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This Week at Physics

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Events During the Week of February 24th through March 3rd, 2013

Monday, February 25th, 2013

Cosmology Journal Club
An Informal discussion about a broad variety of arXiv papers related to Cosmology
Time: 12:30 pm
Place: 5242 Chamberlin Hall
Abstract: Please visit the following link for more details:
Please feel free to bring your lunch!
If you have questions or comments about this journal club, would like to propose a topic or volunteer to introduce a paper, please email Le Zhang (lzhang263@wisc.edu)
Host: Peter Timbie
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R. G. Herb Condensed Matter Seminar
Orthogonal Metals - the simplest non-Fermi liquids
Time: 4:30 pm
Place: 5310 Chamberlin
Speaker: Rahul Nandkishore, Princeton University
Abstract: I present a fractionalized metallic phase which is indistinguishable from the Fermi liquid in conductivity and thermodynamics, but is sharply distinct in one electron properties, such as the electron spectral function. This phase is dubbed the `Orthogonal Metal.' The Orthogonal Metal and the transition to it from the Fermi liquid are naturally described using a slave particle representation wherein the electron is expressed as a product of a fermion and a slave Ising spin. I emphasize that when the slave spins are disordered the result is not a Mott insulator (as erroneously assumed in the prior literature) but rather the Orthogonal Metal. I present prototypical ground state wavefunctions for the Orthogonal Metal by modifying the Jastrow factor of Slater-Jastrow wave- functions that describe ordinary Fermi liquids. I further demonstrate that the transition from the Fermi liquid to the Orthogonal Metal can, in some circumstances, provide a simple example of a continuous destruction of a Fermi surface with a critical Fermi surface appearing right at the critical point. I present exactly soluble models that realize an Orthogonal Metal phase, and the phase transition to the Fermi liquid. These models thus provide valuable solvable examples for phase transitions associated with the death of a Fermi surface.

Reference: R. Nandkishore, M. Metlitski and T. Senthil, Phys. Rev. B 86, 045128 (2012)
Host: Chubukov
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Tuesday, February 26th, 2013

R. G. Herb Condensed Matter Seminar
Gate control of single electron spin in III-V semiconductor quantum dots: Anisotropy effects
Time: 10:00 am
Place: 5280 Chamberlin Hall
Speaker: Sanjay Prabhakar, Wilfrid Laurier University
Abstract: Among recent proposals for next-generation non-charge-based logic is the notion that a single electron can be trapped and its spin can be manipulated through the application of gate potentials. In the first part of my talk, I present numerical simulations of such spins in single-electron devices for realistic asymmetric confining potentials in two-dimensional electrostatically confined quantum dots. Using both analytical and numerical techniques, I show that breaking the in-plane rotational symmetry of the confining potential leads to a significant effect on the tunability of the g- factor and on the spin-flip rate mediated by phonon with applied gate potentials. In particular, anisotropy either extends the range of the tunability of the g-factor and spin-hot spot to larger quantum dots or viceversa. For example, anisotropy reduces the tunability of the g-factor and spin hot spot to smaller quantum dots radius as well as to smaller magnetic fields if we keep the area of the symmetric and asymmetric quantum dots same. It is well known that the cusp-like structure due to accidental degeneracy in the phonon mediated spin-flip rate can be seen only for the case of pure Rashba spin-orbit coupling in symmetric quantum dots. I present new analytical and numerical results which show that the cusp-like structure can be seen for pure Dresselhaus spin-orbit coupling case in asymmetric quantum dots.

In the second part of my talk, I investigate the geometric phase induced on the spin states during the adiabatic movement of the III-V semiconductor quantum dots in the plane of two-dimensional electron gas under the influence of applied gate potential along the lateral direction. Here, I present the spin-flip probabilities during the adiabatic evolution in the presence of the Rashba and the Dresselhaus linear spin-orbit interactions. I use the Feynman disentanglement technique to determine the non-Abelian Berry phase and find exact analytical expressions for three special cases: (a) the pure Rashba spin-orbit coupling, (b) the pure Dresselhause linear spin-orbit coupling, and (c) the mixture of the Rashba and Dresselhaus spin-orbit couplings with equal strength. For a mixture of the Rashba and the Dresselhaus spin-orbit couplings with unequal strengths, I obtain numerical results by solving the Riccati equation originating from the disentangling procedure. I find that the spin-flip probability in the presence of the mixed spin-orbit couplings is generally larger than those for the pure Rashba case and for the pure Dresselhaus case, and that the complete spin-flip takes place only when the Rashba and the Dresselhaus spin-orbit couplings are mixed symmetrically.


Gate control of a quantum dot single-electron spin in realistic confining potentials: Anisotropy effects; Sanjay Prabhakar and James Raynolds, phys. Rev. B 79, 195307 (2009).

Manipulation of single electron spin in a GaAs quantum dot through the application of geometric phases: The Feynman disentangling technique; Sanjay Prabhakar, James E Raynolds, Akira Inomata and Roderick Melnik, Phys. Rev. B 82, 195306 (2010).

Manipulation of the Lande g-factor in InAs quantum dots through the application of anisotropic gate potentials; Sanjay Prabhakar, James E Raynolds and Roderick Melnik, Phys. Rev. B 84, 155208 (2011).

The influence of anisotropic gate potentials on the phonon induced spin-flip rate in GaAs quantum dots; Sanjay Prabhakar, Roderick Melnik and Luis L Bonilla, Applied Physics Letters 100, 023108 (2012).
Host: Friesen
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Chaos & Complex Systems Seminar
The brave cyberworld of science communication
Time: 12:05 pm
Place: 4274 Chamberlin (Refreshments will be served)
Speaker: Dominique Brossard, UW Department of Life Sciences and Communication
Abstract: As more and more individuals turn to online environments to follow scientific issues and find information about science, recent research is science communication has focused on examining how these online environments may shape public attitudes toward science and public understanding of science. This introduction will discuss patterns of science communication online and present recent research findings in that area. Notably, I will introduce results of an experiment with a national sample of the American population testing the effects of comments on science blogs on readersaEuroTM attitudes toward the scientific issues covered in such blogs. I will also explain the effects of current practices related to online searches on public understanding of science. In light of these results, challenges and opportunities for science as an institution as well as for science and society will be discussed.
Host: Sprott
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Wednesday, February 27th, 2013

R. G. Herb Condensed Matter Seminar
Title to be announced
Time: 10:00 am
Place: 5310 Chamberlin Hall
Speaker: Smitha Vishveshwara from the University of Urbana-Champaign.
Host: Perkins
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Department Meeting
Time: 12:15 pm
Place: 5310 Chamberlin Hall
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Thursday, February 28th, 2013

R. G. Herb Condensed Matter Seminar
Engineering Synthetic Quantum Materials from Cold Atoms: Mott Insulators to Emergent Polariton Crystals
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Jonathan Simon, University of Chicago
Abstract: The tools of atomic physics provide a unique and powerful toolbox for studies of quantum many-body physics. Using such systems it has recently become possible to engineer strongly-correlated materials from the ground up and probe them with single-atom resolution. I will describe experiments in which we have synthesized the first magnetic material composed of ultracold atoms, and watched it undergo a quantum phase transition from a paramagnet to an antiferromagnet. I will then introduce a new algorithmic cooling scheme that we have demonstrated, pointing the way to yet more exotic quantum phases that exist at lower temperatures. Finally, I will describe ongoing efforts to develop materials composed of strongly correlated photons whose long-range anisotropic interactions will open new horizons, permitting studies of quantum soft-matter.
Host: Vavilov & Saffman
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Astronomy Colloquium
"Prospects for X-ray constraints on the local super-massive black hole occupation Fraction"
Time: 3:30 pm
Place: 4421 Sterling Hall
Speaker: Elena Gallo, Universtiy of Michigan
Abstract: An issue of crucial relevance in understanding the connection between super-massive black holes and their host galaxies is the "occupation fraction" of massive black holes in the present day universe. While the occupation fraction is expected to be close to 100% in high mass galaxies, predictions differ dramatically at the low mass end, with "light" seeds (i.e. remnants from the first generation of stars) producing a greater nuclear occupation fraction compared to direct collapse models below a few billion solar masses. For an unbiased sample, the local active fraction represents a strong lower limit to the occupation fraction, and X-ray observations of nearby, formally inactive galaxies over a wide range in stellar masses can provide observational constraints to the very mechanism by which the first black holes formed. Adopting a Monte Carlo approach, we make use of the Chandra AMUSE-surveys to characterize simultaneously the black hole occupation fraction and the scaling of nuclear activity with host mass. Further, we discuss future prospects for improving the precision of these parameters as a function of sample size, as well as desired sensitivity and spatial resolution of future missions.
Host: Prof Elena D'onghia
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Friday, March 1st, 2013

Theory Seminar (High Energy/Cosmology)
Indirect Probes of the MSSM after the Higgs Discovery
Time: 2:15 pm
Place: 5280 Chamberlin Hall
Speaker: Felix Yu, Fermilab
Abstract: I will present results from a study of the minimal supersymmetric standard model (MSSM) with minimal flavor violation (MFV), imposing constraints from flavor physics observables and MSSM Higgs searches, in light of the recent discovery of a 125 GeV Higgs boson by ATLAS and CMS. We analyzed the electroweak vacuum stability conditions to further restrict the MSSM parameter space. In addition, we showed a connection to ultraviolet physics via an implementation of renormalization group running, which determines the TeV-scale spectrum from a small set of minimal supergravity parameters. Finally, we investigated the impact from dark matter direct detection searches. Our work highlights the complementarity of collider, flavor and dark matter probes in exploring the MSSM, and shows that even in a MFV framework, flavor observables constrain the MSSM parameter space well beyond the current reach of direct SUSY particle searches.
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Physics Department Colloquium
Life after Lorentz: Quantum Mechanics, Gravity, and the Crisis of Falsifiability
Time: 3:30 pm
Place: 2241 Chamberlin Hall (coffee at 4:30 pm)
Speaker: Niayesh Afshordi, Perimeter Institute for Theoretical Physics & the University of Waterloo
Abstract: In the last decade of 19th century, Hendrik Lorentz discovered a group of transformations of space and "local time" that left Maxwell equations of electromagnetism unchanged. In the ensuing decades, this revelation led to the development of special and general theories of relativity by Einstein, and has been the cornerstone of much of theoretical physics and astrophysics ever since. In spite of its tremendous success over the past century, in this talk I entertain the possibility that Lorentz invariance might have been a "glorious historical accident", rather than a fundamental symmetry of nature! (My favorite) motivations for this line of argument come from a need for falsifiable theories of quantum gravity, early universe, dark energy, and black hole physics.
Host: Chung
Poster: http://www.physics.wisc.edu/twap/posters/2013/2917.pdf
Video: https://vod.physics.wisc.edu/media/2013_03_01.m4v
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"This Week at Physics" poster: http://www.physics.wisc.edu/twap/posters/2013/2013-02-25.pdf

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