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This Week at Physics

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Events During the Week of February 23rd through March 2nd, 2014

Monday, February 24th, 2014

Plasma Physics (Physics/ECE/NE 922) Seminar
Transport Properties of Strongly Coupled Plasmas
Time: 12:00 pm
Place: 2241 Chamberlin
Speaker: Scott Baalrud, University of Iowa
Host: Cary Forest
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Condensed Matter Theory Group Seminar
Valleytronics in Silicon: the Principles and the Practice
Time: 4:30 pm
Place: Chamberlin 5310
Speaker: Andre Saraiva, University of Wisconsin
Abstract: Alternative schemes for information processing are being pursued by the industry. Spintronics - the most prominent candidate for alternative electronics - explores an internal degree of freedom of electrons, namely the spin, in order to encode information. Recent advances in the manipulation of the valley degree of freedom[1] of certain semiconductors are opening the possibility for a third option: Valleytronics. But in order to achieve useful control of valley, large enough valley-orbit induced valley splitting (VS) must be achieved.

We will present three ways to control valleys on a scale higher than 1 meV:

a) in a quantum well, the adoption of a barrier constituted of a layered heterostructure might lead to constructive reflection if the layer thicknesses match the electron wavelength, in analogy with a Bragg mirror [2];

b) the disparity between the high valley splitting in a impurity donor potential and the low splitting in a Si/Insulator interface may be harnessed controlling the tunneling between these two states, so that the valley splitting may be controlled digitally [3];

c) intrinsic Tamm/Shockley interface states might strongly hybridize with conduction states, leading to a much enhanced valley splitting[4,5], and its contribution to the 2DEG ground state may be experimentally identified [6].

1. D. Culcer, A. L. Saraiva, Belita Koiller, Xuedong Hu, and S. Das Sarma, PRL *108*, 126804 (2012).
2. L. Zhang, J.-W. Luo, A Saraiva, Belita Koiller, Alex Zunger, Nature Comm. *4*, 2396 (2013)
3. A. Baena, A. L. Saraiva, Belita Koiller, and M. J. CalderA3n, PRB 86, 035317 (2012).
4. K. Takashina, Y. Ono, A. Fujiwara, Y. Takahashi and Y. Hirayama, *PRL* *96, *236801 (2006).
5. A. Saraiva, Belita Koiller and M. Friesen, Phys. Rev. B *82*, 245314 (2010).
6. A. Dusko, A. Saraiva and Belita Koiller, arXiv:1310.6878 (2013).
Host: Perkins
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Tuesday, February 25th, 2014

Chaos & Complex Systems Seminar
Roving the Red Planet: A field geologist explores gale crater on Mars
Time: 12:05 pm
Place: 4274 Chamberlin (refreshments will be served)
Speaker: Rebecca Williams, Planetary Science Institute
Abstract: On August 5, 2012, NASA's Mars Science Laboratory rover Curiosity landed in northwest Gale Crater. With the most sophisticated suite of scientific instruments ever employed to investigate the Martian surface housed within a mini-Cooper-sized rover, Curiosity has been assessing the character of the ancient environments based on examination of clues contained within rocks. In this presentation, Williams will discuss why Gale Crater was selected as the landing site, the ingenious 'sky-crane' landing system developed for this mission, how the science team participates in daily operations from their home institutions, and science results after one year of surface activities.
Host: Clint Sprott
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"Physics Today" Undergrad Colloquium (Physics 301)
Dark Matter Searches
Time: 1:20 pm
Place: 2223 Chamberlin
Speaker: Yang Bai, UW Madison Department of Physics
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Wednesday, February 26th, 2014

Department Meeting
Time: 12:15 pm
Place: 5310 Chamberlin Hall
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Thursday, February 27th, 2014

R. G. Herb Condensed Matter Seminar
Topological phases, Majorana fermions and disorder in superconductors
Time: 10:00 am
Place: 5310 Chamberlin Hall
Speaker: Smitha Vishveshwara , University of Illinois at Urbana-Champaign
Abstract: In the hunt for Majorana particles, originally proposed in the context of particle physics, recent explorations have led to exciting prospects in superconducting wires, including potential experimental detection. Here, an introduction to topological properties of superconductors that result in Majorana modes will first be presented. It will then be argued that superconducting wires subject to various potential landscapes can exhibit rich topological behavior. As an example, it will be shown that the presence of quasiperiodic potentials can give rise to beautiful phase diagrams that mimic complex Hofstadter butterfly patterns. Finally, connections to localization physics will be made in the context of disordered topological superconductors.
Host: Perkins
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Atomic Physics Seminar
Coherent control of strongly interacting Rydberg gases in thermal vapor cells
Time: 2:00 pm
Place: 5310 Chamberlin Hall
Speaker: Prof. Tilman Pfau, University of Stuttgart
Abstract: Rydberg atoms are of great interest due to their prospects in quantum information processing. Coherent control of the strong Rydberg-Rydberg interaction allows for the realization of quantum operations and devices such as quantum gates and single-photon sources. To date, impressive experimental progress has been limited to the ultracold domain [1]. Being able to exploit this interaction in a coherent manner in thermal vapor would eliminate the need for cooling and trapping of atoms and thus offer new prospects for applications in terms of integration and scalability.&lt;br&gt;<br>
We present our progress on the coherent control and investigation of Rydberg atoms in small vapor cells. We show that we are able to drive Rabi oscillations on the nanosecond timescale to a Rydberg state by using a pulsed laser excitation and are therefore faster than the coherence time limitation given by the Doppler width [2].&lt;br&gt;<br>
A systematic investigation of the dephasing of these oscillations for different atomic densities and Rydberg S-states (n = 22-40) reveals a clear signature for Rydberg-Rydberg interaction which is the basis for quantum devices based on the Rydberg blockade. Due to the high excitation bandwidth we are probing interaction level shifts up to a few GHz which correspond to very small interatomic distances (&amp;lt; 1μm). Despite the complicated level structure for Rydberg molecular states at these distances we find that the scaling with principle quantum number is still consistent with van der Waals type interaction. The strength of the interaction exceeds the energy scale of thermal motion (i.e. the Doppler broadening) and therefore enables strong quantum correlations above room temperature [3].&lt;br&gt;<br>
Furthermore we present our latest results on the combination of the pulsed Rydberg excitation with a four-wave-mixing scheme [4] and our progress towards the creation of non-classical light. &lt;br&gt;<br>
[1] M. Saffman et al., RMP 82, 2313 (2010) and references therein &lt;br&gt;<br>
[2] B. Huber et al., PRL 107, 243001 (2011) &lt;br&gt;<br>
[3] T. Baluktsian et al., PRL 110, 123001 (2013)&lt;br&gt;<br>
[4] M. Saffman and T. G. Walker, Phys. Rev. A 66, 065403 (2002), M. M. Müller, et al. Phys. Rev. A 87, 053412 (2013)&lt;br&gt;<br>
Host: Saffman
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Astronomy Colloquium
"Scaling Computational Astrophysics"
Time: 3:30 pm
Place: 4421 Sterling Hall
Speaker: Matthew Turk, Columbia University
Abstract: The term "Big Data" means different things to different people; often it's used to describe unstructured or semi-structured records, or fast-moving data that has to be processed quickly to be of any use, or just a huge volume of data that stretches the limits of many computing systems. In this talk, I will present how simulation and analysis have attempted to respond to the challenges of "big data" not as a goal in and of itself, but as a by-product of trying to use increasingly rich simulation data to study complex physical processes.
I will describe new avenues in understanding how the first stars in the universe formed, the simulation platform Enzo (enzo-project.org)that enables us to study these objects, and where furthering our understanding requires advancing the state of the art in hydrodynamic studies. I will present the analysis and visualization platform yt(yt-project.org), and its aim to provide a lingua franca for astrophysical phenomena, empowering individuals to ask complex and detailed questions of data. Finally, I will discuss the communities that have grown around these platforms, how retaining a focus on self-directed scientific inquiry has allowed collaboration to flourish between researchers, and why collaboration and community is the next great scaling challenge for computational astrophysics.
Host: Prof Richard Townsend
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Friday, February 28th, 2014

Theory Seminar (High Energy/Cosmology)
Pure Gravity Mediation
Time: 2:00 pm
Place: 5280 Chamberlin Hall
Speaker: Jason Evans, University of Minnesota
Abstract: If low energy supersymmetry is realized in nature, the apparent discovery of a Higgs boson with mass around 125 GeV suggests a supersymmetric mass spectrum in the TeV or multi-TeV range. Multi-TeV scalar masses are a necessary component of supersymmetric models with pure gravity mediation or in any model with strong moduli stabilization. The simplest model of pure gravity mediation contains only two free parameters: the gravitino mass and $tan beta$. Scalar masses are universal at some high energy renormalization scale and gaugino masses are determined through anomalies and depend on the gravitino mass and the gauge couplings. This theory requires a relatively large graviton mass (m_{3/2} gtrsim 300 TeV) and a limited range in tan beta simeq 1.7--2.5. By allowing for non-unversalities in the Higgs soft masses, the allowed range in tan beta is greatly increased which then permits smallers values of m_{3/2} and makes detection of the gluino at the LHC possible. Furthermore, if one adopts a no-scale or partial no-scale structure for the K"ahler manifold, sfermion masses may vanish at the tree level. It is usually assumed that the leading order anomaly mediated contribution to scalar masses appears at 2-loops. However, there are at least two possible sources for 1-loop scalar masses. These may arise if Pauli-Villars fields are introduced as messengers of supersymmetry breaking. We consider the consequences of a spectrum in which the scalar masses associated with the third generation are heavy (order $m_{3/2}$) with 1-loop scalar masses for the first two generations. A similar spectrum is expected to arise in GUT models based on $E_7/SO(10)$ where the first two generations of scalars act as pseudo-Nambu-Goldstone bosons. Explicit breaking of this symmetry by the gauge couplings then generates one-loop masses for the first two generations. In particular, we show that it may be possible to reconcile the $g_mu - 2$ discrepancy with potentially observable scalars and gauginos at the LHC.
Host: Jordi Salvadó Serra
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Physics Department Colloquium
A single Rydberg electron in a Bose-Einstein Condensate
Time: 3:30 pm
Place: 2241 Chamberlin Hall (coffee at 4:30 pm)
Speaker: Prof. Tilman Pfau, University of Stuttgart
Abstract: Electrons attract polarizable atoms via a 1/r^4 potential. For slow electrons the scattering from that potential is purely s-wave and can be described by a Fermi pseudopotential. To study this interaction Rydberg electrons are well suited as they are slow and trapped by the charged nucleus. In the environment of a high pressure discharge Amaldi and Segre, already in 1934 observed a lineshift proportional to the scattering length [1], which was first introduced to explain their findings.
At ultracold temperatures and Rydberg states with medium size principle quantum numbers n, one or two ground state atoms can be trapped in the meanfield potential created by the Rydberg electron, leading to so called ultra-long range Rydberg molecules [2]. These molecules can show a linear Stark effect corresponding to a permanent dipole moment [3], which if seen from a standpoint of traditional molecular physics is surprising.
At higher Rydberg states the spatial extent of the Rydberg electron orbit is increasing. For principal quantum numbers n in the range of 100-200 and typical BEC densities, up to several ten thousand ground state atoms are located inside one Rydberg atom, leading again to a density dependent energy shift of the Rydberg state. This allows, together with the strong van-der-Waals blockade, to excite only one single Rydberg atom in a condensate. We excite a Rydberg electron with n upto 202 in the BEC, the size of which becomes comparable to the size of the BEC. We study their life time in the BEC and the coupling between the electron and phonons in the BEC [3]. So the single electron that we prepare in a quantum gas allows nicely to study the transition from two- to few- to many-body interaction.
As an outlook, the trapping of a full condensate inside a Rydberg atom of high principal quantum number and the imaging of the Rydberg electron's wavefunction by its impact onto the surrounding ultracold cloud seem to be within reach.

[1] E. Amaldi and E. Segre, Nature 133, 141 (1934)

[2] C. H. Greene, et al. PRL. 85, 2458 (2000); V. Bendkowsky et al., Nature 458, 1005 (2009)
[3] W. Li, et al., Science 334, 1110 (2011)
[4] J . B. Balewski, A. T. Krupp, A. Gaj, D. Peter, H. P. Büchler, R. Löw, S. Hofferberth, T. Pfau, Nature 502, 664 (2013)
Host: Saffman
Video: http://www.physics.wisc.edu/vod/2014/02/28.html
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"This Week at Physics" poster: http://www.physics.wisc.edu/twap/posters/2014/2014-02-24.pdf

©2013 Board of Regents of the University of Wisconsin System