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This Week at Physics

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Events During the Week of January 30th through February 6th, 2011

Monday, January 31st, 2011

Plasma Physics (Physics/ECE/NE 922) Seminar
"Equilibrium Reconstruction and Measurement of Currents in the HSX Stellarator"
Time: 12:05 pm
Place: 2241 Chamberlin Hall
Speaker: John Schmitt, UW-Madison Dept. of Electrical Engineering
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Condensed Matter Theory Group Seminar
Realistic quantum critical points
Time: 4:00 pm
Place: 5280 Chamberlin
Speaker: Munehisa Matsumoto, University of California-Davis
Abstract: Quantum criticality has been discussed to play a key role in interesting phenomena in strongly-correlated systems, such as high-Tc superconductivity in cuprates (here Tc is the superconducting transition temperature), recently-discovered iron pnictides/chalcogenides, and heavy-fermion materials. In the main part of the talk I will show how the magnetic quantum critical point (QCP) in heavy-fermion materials can be quantitatively predicted by combining electronic-stricture calculations based on local-density approximation (LDA) and dynamical-mean field theory (DMFT) for the LDA-derived effective low-energy Hamiltonian. We utilize state-of-the-art continuous-time quantum Monte Carlo method to solve the impurity problem in DMFT formulated on the basis of localized f-electrons, which enables us to obtain numerically-exact solutions at low temperatures down to O(1) [K] within DMFT. Thus we reach at a good position to address the quantum critical point quantitatively and we find the followings: 1) striking multiple quantum critical points are found in a realistic phase diagram for Plutonium-based compounds, which is attributed to the strong-coupling nature of the effective Kondo-lattice model. PuCoGa5, with the highest Tc = 18.5 [K] among f-electron based materials, is found be located in the proximity to the third QCP [1]. 2) CeCoIn5, which has the highest Tc = 2.3 [K] among Cerium-based heavy-fermion compounds, its parent material CeIn3, and its new two-dimensional (2D) analogue CePt2In7 are concentrated around a QCP where CeCoIn5 is found to be right on top of QCP. The reason the most 2D one does not come closest to QCP is attributed to the subtlety in the competition between the dimensionality and hybridization effects along the c-axis [2]. In the final part of the talk I will discuss the possible subtle nature of what has been called QCP, which still challenges realistic numerics but careful numerical analyses of an effective field theory [3] tells us QCP might not truly be critical. Possible consequence for having the resonating valence bond state around what has been QCP [4] is revisited.

[1] MM, Q. Yin, J. Otsuki, S. Y. Savrasov, preprint [arXiv:1101.1582].
[2] MM, M. J. Han, J. Otsuki, S. Y. Savrasov, Phys. Rev. B 82, 180515(R) (2010) [arXiv:1004.5457].
[3] A. B. Kuklov, MM, N. V. Prokof'ev, B. V. Svistunov, M. Troyer, Phys. Rev. Lett. 101, 050405 (2008) [arXiv:0805.4334].
[4] P. Coleman and N. Andrei, J. Phys.: Condens. Matter 1, 4057 (1990).
Host: Robert Joynt
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Tuesday, February 1st, 2011

Chaos & Complex Systems Seminar
"Spatial and Temporal Variability in Groundwater Chemistry: Is There Any Such Thing as a "Representative" Sample?"
Time: 12:00 pm
Place: 4274 Chamberlin Hall
Speaker: Jean Bahr, UW-Madison Dept of Geoscience
Abstract: The dissolved constituents found in groundwater have been of interest for over two centuries. In fact many of the early developments in analytical chemistry were motivated by requests from physicians who were interested in the composition of springs and spas that had presumed therapeutic benefits. More recently, public attention to groundwater chemistry has focused on constituents that are associated with health hazards such as arsenic and hexavalent chromium. In addition to its relevance to human health, water chemistry data provide hydrogeologists with clues to the complex subsurface structures and processes that control groundwater flow and water-rock interactions. A major challenge to interpreting these data is posed by the spatial and temporal variability of measured concentrations. This talk will discuss several case studies in which high resolution sampling and tracer experiments have been used to document the effects of complex flow fields and subsurface reactions on the chemical signatures we observe in groundwater samples.
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"How do Galaxies get their Gas?"
Time: 3:30 pm
Place: 4421 Sterling Hall
Speaker: Dusan Keres, UC Berkeley
Abstract: Most galaxies are actively star forming at all epochs. However, observations of dense, galactic gas indicate that, at any epoch, there is not enough gas in galaxies to support evolution of star formation activity over time. This suggests that galactic gas is being replenished from the intergalactic medium.&lt;br&gt;<br>
I use fully cosmological hydrodynamic simulations to study the gas supply into galactic component from high redshift to present. At high redshift &amp;quot;smooth&amp;quot; infall of cold filamentary gas dominates the gas supply of all galaxies. This &amp;quot;cold mode accretion&amp;quot; is unlike the accretion in the standard model of galaxy formation in which cooling of the hot halo atmospheres is a source of gas supply to galaxies. Cold mode accretion is a major driver of very active star formation of high-z galaxies enabling such activity to proceed for a significant fraction of the Hubble time. Gas accretion rates at a given halo and galaxy mass decrease with time, causing the drop in star formation rates. At low redshift hot virialized gas can cool in some of the halos, but cold gaseous clouds that form from infalling filaments can dominate gas supply in galaxies such as Milky Way.&lt;br&gt;<br>
In this talk I will describe properties, physics and consequences of cold gas accretion as well as predictions for the observational probes of cold halo gas that can provide strong constraints on the models. I will also discuss remaining open questions and future directions in the studies of galactic gas accretion, including additional physical processes, new computational methods and observations with upcoming facilities.&amp;quot;&lt;br&gt;<br>
Host: Professor Heinz
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Wednesday, February 2nd, 2011

Department Meeting
Time: 12:15 pm
Place: 5310 Chamberlin Hall
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Thursday, February 3rd, 2011

R. G. Herb Condensed Matter Seminar
Two-dimensional electron gas with universal subbands at the surface of SrTiO3
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Andres Santander, CSNSM - Université Paris-Sud, France
Abstract: Similar to silicon that is the basis of conventional electronics, strontium titanate (SrTiO3) is the bedrock of the emerging field of oxide electronics. SrTiO3 is the preferred template to create exotic two-dimensional (2D) phases of electron matter at oxide interfaces, exhibiting metal-insulator transitions, superconductivity, or large negative magnetoresistance. However, the physical nature of the electronic structure underlying these 2D electron gases (2DEGs) remains elusive, although its determination is crucial to understand their remarkable properties. In this talk, we present our angle-resolved photoemission spectroscopy (ARPES) results showing that there is a highly metallic universal 2DEG at the vacuum-cleaved surface of SrTiO3, independent of bulk carrier densities over more than seven decades, including the undoped insulating material [1]. Our data unveil a remarkable electronic structure consisting on multiple subbands of heavy and light electrons. The analysis of the data shows that this 2DEG is confined within a region of ~5 unit cells with a sheet carrier density of ~0.33 electrons per a2 (a is the cubic lattice parameter). The similarity of this 2DEG with those reported in SrTiO3-based heterostructures and field-effect transistors suggests that different forms of electron confinement at the surface of SrTiO3 lead to essentially the same 2DEG. Our discovery provides a model system for the study of the electronic structure of 2DEGs in SrTiO3-based devices, and a novel route to generate 2DEGs at surfaces of functional oxides.

[1] A. F. Santander-Syro, O. Copie, T. Kondo, et al. "Two-dimensional electron gas with universal subbands at the surface of SrTiO3". Nature 469, 189 (2011).
Host: Natalia Perkins & Andrey Chubukov
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NPAC (Nuclear/Particle/Astro/Cosmo) Forum
Peculiar Velocity and its Effects on Cosmology with 21cm Tomography
Time: 4:00 pm
Place: 4274 Chamberlin Hall
Speaker: Yi Mao, UT Austin
Abstract: The peculiar velocity of the intergalactic gas responsible for the cosmic 21cm background from the epoch of reionization and beyond introduces an anisotropy in the three-dimensional power spectrum of brightness temperature fluctuations. We review how measurement of this anisotropy by future 21cm surveys is a promising tool for separating cosmology from 21cm astrophysics. A more careful treatment of the effects of peculiar velocity than previously attempted is necessary, however, to fulfill this promise. In this talk, we set out to account for peculiar velocity in every detail, and clarify the roles of thermal vs. velocity broadening and finite optical depth.
We will also find whether nonlinear effect of peculiar velocity may spoil future 21cm measurements. The discussion in this talk, although in the context of the Epoch
of Reionization, may affect the interpretation of the 21cm intensity mapping in the post-reionization epoch.
Host: Peter Timbie and Vernon Barger
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Friday, February 4th, 2011

Astronomy Colloquium
Special Astronomy Colloquium
Direct Imaging of Exoplanets: Prospects for comparative Exoplanetology
Time: 3:15 pm
Place: 4421 Sterling Hall
Speaker: Beth Biller, MPIA Heidelberg
Abstract: Direct detection, and direct spectroscopy in particular,
have great potential for advancing our understanding of extrasolar
planets. In combinations with other methods of planet detection,
direct imaging and spectroscopy will allow us to eventually: 1) fully map out the architecture of typical planetary systems and 2) study the physical properties of exoplanets (colors, temperatures, etc.) in depth. I will discuss initial results from the ongoing 500 hour NICI Planet-Finding Campaign using the novel Near-IR Coronagraphic Imager (NICI) at the 8-m Gemini South telescope. NICI combines a number of techniques to attenuate starlight a suppress superspeckles for direct detection of exoplanets: 1) Lyot coronagraphic imaging, 2) dual channel imaging for Spectral Differential Imaging (SDI) and 3)operation in a fixed Cassegrain rotator mode for Angular Differential Imaging (ADI). The combination of these techniques allows unprecedented contrasts of dmag > 14 (median value) at 1" in H band, sufficient to image giant planets (<5 MJup) around stars in nearby young moving groups and super jupiters (<10 MJup) around stars in the immediate solar neighborhood. I will also discuss the discovery of a tight substellar companion to the young solar analog PZ Tel(Biller et al. 2010), a member of the &#946; Pic moving group observed as part of the Gemini Near-Infrared Coronagraphic Imager Planet-Finding Campaign.

PZ Tel B is one of the few young substellar companions
directly imaged at orbital separations similar to those of giant
planets in our own solar system.

Planetary mass companions to brown dwarfs are an important counterpart to planets around stars and provide key benchmark objects for evolutionary models of substellar objects. Therefore, I will also discuss results from a systematic Keck Laser Guide Star (LGS) adaptive optics search to directly image planetary mass companions to young brown dwarfs in the Upper Sco embedded cluster (Biller et al. 2011).
Host: Prof Amy Barger
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Physics Department Colloquium
Multiferroic Vortices and Graph Theory
Time: 4:00 pm
Place: 2241 Chamberlin Hall (coffee at 3:30 pm)
Speaker: Sang-Wook Cheong, Rutgers Center for Emergent Materials
Abstract: The fascinating concept of topological defects permeates ubiquitously our understanding of the early-stage universe, hurricanes, quantum matters such as superfluids and superconductors, and also technological materials such as liquid crystals and magnets. Large-scale spatial configurations of these topological defects have been investigated only in a limited degree. Exceptions include the cases of supercurrent vortices or liquid crystals, but they tend to exhibit either trivial or rather-irregular configurations.Hexagonal REMnO3 (RE= rare earths) with RE=Ho-Lu, Y, and Sc, is an improper ferroelectric where the size mismatch between RE and Mn induces a trimerization-type structural phase transition, and this structural transition leads to three structural domains, each of which can support two directions of ferroelectric polarization. We reported that domains in h-REMnO3 meet in cloverleaf arrangements that cycle through all six domain configurations [1], Occurring in pairs, the cloverleafs can be viewed as vortices and antivortices, in which the cycle of domain configurations is reversed. Vortices and antivortices are topological defects: even in a strong electric field they won't annihilate. Recently we have found intriguing, but seemingly irregular configurations of a zoo of topological vortices and antivortices in h-REMnO3 [2]. These configurations can be neatly analyzed in terms of graph theory and this graph theoretical analysis reflects the nature of self-organized criticality in complexity phenomena as well as the condensation and eventual annihilation processes of topological vortex-antivortex pairs.

[1] Insulating Interlocked Ferroelectric and Structural Antiphase Domain Walls in Multiferroic YMnO3, T. Choi, Y. Horibe, H. T. Yi, Y. J. Choi, Weida. Wu, and S-W. Cheong, Nature Materials 9, 253-258 (2010).
[2] Self-Organization, Condensation and Annihilation of Topological Vortices and Antivortices in a Multiferroic, S. C. Chae, Y. Horibe, D. Y. Jeong, S. Rodan, N. Lee, and S.-W. Cheong, PNAS, in print.
Host: Perkins
Poster: http://www.physics.wisc.edu/twap/posters/2011/1888.pdf
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