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May 22, 2014

May 29 Colloquium

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Vera Luth, SLAC National Accelerator Laboratory

Observation of Time Reversal Violation in B Meson Decays

Host: Dietrich Belitz

Abstract: While discrete symmetries and conservation laws are basic concepts of physics, the search for broken symmetries has been a very interesting topic for both experimentalists and theorists. In particle physics, the violation of charge conjugation and parity in weak interactions was observed 60 years ago, and CP violation was found to be violated in neutral kaon decays and more recently in B mesons. It has been much more difficult to detect the violation of time reversal at the microscopic level. The BABAR Collaboration has recently found first and very convincing evidence for T Violation in neutral B mesons. The result is consistent with equal CP and T violation and with CPT invariance.

May 15, 2014

May 22 Colloquium

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Kelly Miller, Harvard

Physics Education Research and Practice; From Lecture Demonstrations to the Flipped Classroom

Host: Stan Micklavzina

Abstract:This talk will discuss both recent research on the efficacy of physics lecture demonstrations as well as the implementation of a “flipped class” in the context of introductory physics curricula.   Research has shown that roughly one out of every five observations of a demonstration is inconsistent with the actual outcome. Furthermore, correct observation of a lecture demonstration appears to be related to how well a student understands the underlying physics concept before being shown the demo. These findings, as well as the role of prediction-making in demonstration pedagogy will be discussed in more detail.

Recent trends towards flipped classrooms raise interesting questions regarding the teaching of physics to large introductory classes.  The implementation and logistics of Applied Physics 50, a new flipped, ‘studio style’ physics class at Harvard, will be discussed. This course combines best practices to deliver a learning experience that helps students develop team and problem solving skills as well as a solid conceptual understanding of physics.

May 8, 2014

May 15 Colloquium

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Alan Petersen, Spectra Physics

Review of Diode-Pumped Alkali Vapor Lasers

Host: Bryan Boggs

Since its first demonstration about 10 years ago, the diode-pumped alkali vapor laser (DPAL) has occupied a unique, sometimes controversial position within the laser field.  Originally conceived as large aperture, CW brightness converter for materials processing and directed energy applications these systems have also been pursued as potential short wavelength sources and have stimulated renewed interest in relevant gas phase atomic physics and solid state materials technology.  In this talk I will present the DPAL concept, summarize some early experiments including our own, and consider the challenges to high average power realization.

May 1, 2014

May 8 Colloquium

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Sarah Ballard, University of Washington

Directions to the Nearest Alien Earth-like Planet: Walk from Here to Knight Library

Host: Jim Brau

Abstract: The landscape of exoplanet science has been dramatically reshaped since the launch of NASA’s Kepler mission in 2009. While the mission’s primary science driver was to uncover the frequency of Earth-like planets orbiting Sun-like stars, in fact the vast majority of rocky planets in their stellar habitable zones reside in very different environments. M dwarf stars, half the mass of the Sun and smaller, host most of the galaxy’s terrestrial worlds. The small stature of these stars, the most prolific type in the universe, render exoplanet detection and characterization easier for upcoming missions. However, they furnish very different conditions for life than have nourished it on Earth. I’ll summarize the key findings of what Kepler has revealed about planet occurrence, and how it informs our search for signatures of life on other planets.

 

April 25, 2014

May 1 Colloquium

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Walt Ogburn, Kavli Institute for Particle Astrophysics and Cosmology, Stanford

BICEP2 and The Echoes of Cosmic Inflation

Host: Jim Brau

The BICEP2 team has announced the detection of degree-scale B-mode polarization of the CMB, consistent with the imprint of inflationary gravitational waves created an instant after the big bang.  The telescope is a compact refractor with a 26 cm aperture and 512 antenna-coupled TES bolometers observing at 150 GHz (2 mm).  BICEP2 observed from the South Pole for three seasons from 2010 to 2012. A low-foreground region of sky with an effective area of 380 square degrees was observed to a depth of 87 nK-degrees in Stokes Q and U.  We have just announced an excess of B-mode power over the base lensed-LCDM expectation in the range 30<l<150, inconsistent with the null hypothesis at a significance of >5σ. The observed B-mode power spectrum is well fit by a lensed-LCDM + tensor theoretical model with tensor/scalar ratio r=0.20+0.07-0.05, with r=0 disfavored at 7.0σ.

April 18, 2014

Arpil 24 Colloquium

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Jeff Lundeen, University of Ottawa

Seeing is Believing: Direct Observation of a General Quantum State

Host: Michael Raymer

Abstract: Central to quantum theory, the wavefunction is a complex distribution associated with a quantum system. Despite its fundamental role, it is typically introduced as an abstract element of the theory with no explicit definition. Rather, physicists come to a working understanding of it through its use to calculate measurement outcome probabilities through the Born Rule. Tomographic methods can reconstruct the wavefunction from measured probabilities. In contrast, I present a method to directly measure the wavefunction so that its real and imaginary components appear straight on our measurement apparatus. At the heart of the method is a joint measurement of position and momentum that is made possible by weak measurement (a concept that I will attempt to demystify). I will describe an experimental example of the method in which we directly measured the transverse spatial wavefunction of a single photon. New experimental work extending this to mixed states will be presented as well. Our direct measurement method gives the wavefunction a plain and general meaning in terms of a specific set of operations in the lab.

Related Papers:

Nature, 474, 188 (2011). http://arxiv.org/abs/1112.5471

Phys. Rev. Lett. 108, 070402 (2012). http://arxiv.org/abs/1112.3575

Physical Review Letters, 112, 070405 (2014). http://arxiv.org/abs/1309.1491

April 11, 2014

April 17 Colloquium

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Alex Small, Cal State Panoma

Host: Raghu Parthasarathy

Theoretical Limits to Superresolution Fluorescence Microscopy

Superresolution microscopy techniques enable fluorescence imaging of biological specimens with sub-wavelength resolution, down to as low as 10 nanometers in some cases.  Interestingly, in essentially all techniques considered thus far, the resolution scales as the wavelength of light divided by the square root of some measure of the number of photons used.  We have developed a proof that the relationship between image resolution and measures of photon count has a universal 1/sqrt(photon count) scaling for very wide classes of superresolution techniques.  Additionally, we have developed a theoretical framework for benchmarking the performance of the algorithms used to analyze the images, and have developed fast image analysis algorithms and mathematical approaches for optimizing image acquisition.

April 7, 2014

April 10 Colloquium

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Marc Janoscheck, Los Alamos National Laboratory

Investigating Strongly Correlated Electron Materials by Means of Neutron Scattering

Abstract: Materials governed by strong electronic correlations exhibit a broad set of interesting properties and ground states, such as heavy fermion behavior, complex magnetism, the Mott insulator, and unconventional superconductivity, and furthermore show potential for application spanning from thermoelectricity to spintroncis. Here it is generally believed that being able to manipulate and control these states that emerge through the cooperative behavior of in the order of 1023 electrons is critical to develop tomorrow’s devices and applications. Using two examples we will demonstrate that neutron scattering represents a powerful tool in shedding light on the properties of such complex materials.
First, we will report our work on the B20 helimagnet MnSi. The discovery of a new topological form of magnetic order in the family of cubic Dzyaloshinskii-Moriya helimagnets crystallizing in the B20 structure has recently attracted increasing scientific interest. This new magnetic state consists of a lattice of skyrmions, i.e., magnetic vortices characterized by a topological winding number [1], which is however only thermodynamically stable in a little pocket of the phase diagram at finite field close to the critical temperature Tc. In the skyrmion phase of MnSi spin transfer torques have been observed at record low current densities j = 106A m-2 making these materials promising for spintronic applications [2]. Here it was argued in Ref. [1] that the skyrmion lattice phase only exists due to strong renormalizations attributed to thermal fluctuations, which in turn motivated our detailed small angle neutron scattering (SANS) study of the magnetic fluctuations in MnSi. Our study demonstrates that the properties of MnSi are indeed governed by strongly-interacting critical fluctuations, resulting in a fluctuation-induced first-order transition at Tc. Notably, we show that within a self-consistent Hartree-Fock-Brazovskii theory the critical magnetic fluctuations determined by our SANS measurements quantitatively reproduce the observed specific heat and magnetic susceptibility data [3]. While our study contributes to our better understanding of the skyrmion-lattice compounds, we note that fluctuation-induced first-order transitions are of interest for a wide range of topics such liquid crystals, superconductors, cold atom systems or even phase transitions in the early universe (see references in Ref. [3]).

In the second part of the talk, we turn to our recent inelastic neutron scattering of δ-Pu. Pu is arguably the most complex elemental metal known because its 5f electrons are tenuously poised at the edge between localized and itinerant configurations [4]. Despite its 60-year old history, our collective understanding of this complicated element is still vague. However, it is clear that Pu’s complex structure leads to emergent behavior—all a direct consequence of its 5f electrons—including six allotropic phases, large volumetric changes associated with these transitions of up to 25%, and mechanical properties ranging from brittle α-Pu to ductile δ-Pu [5]. Pu also exhibits a Pauli-like magnetic susceptibility, electrical resistivity and a Sommerfeld coefficient of the specific heat that are an order of magnitude larger than in any other elemental metal [6]. Finally, while experiments find no sign for static magnetism in Pu, most theories that use the correct volume predict a magnetically ordered state [4]. This discrepancy might be reconciled by recent DMFT calculations that suggest that Pu fluctuates between localized and itinerant valence configurations [7]. Our measurements demonstrate the existence of magnetic fluctuations centered around 84 meV, in agreement with these calculations putting the understanding of the “missing” magnetism and electronic structure of Pu within reach for the first time.
[1] S. Mühlbauer et al., Science 323, 915 (2009) [2] F. Jonietz et al., Science 330, 1648, (2010); T. Schulz et al. Nature Physics 2231 (2012). [3] M. Janoschek et al., Phys. Rev B 87, 134407 (2013). [4] A. M Boring and J. L. Smith, in Challenges in Plutonium Science Vol. I, Los Alamos Science, No. 26, p. 91. (2000). [5] J. L. Smith, and E. A. Kmetko, J. Less-Common Met. 90, 83 (1983). [6] J. C. Lashley et al., Phys. Rev. B 72, 054416 (2005). [7] J. H. Shim , K. Haule, G. Kotliar Nature 446, 513–516 (2007).

Host: Dietrich Belitz

March 27, 2014

April 3 Colloquium

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John Toner, University of Oregon

Swarming in the dirt: Flocking in the presence of quenched disorder

Abstract: The effect of quenched (frozen) orientational disorder on the collective motion of active particles is analyzed. We find that, as with annealed disorder (Langevin noise), active polar systems are far more robust against quenched disorder than their equilibrium counterparts. In particular, long ranged order (i.e., the existence of a non-zero average velocity <\vec{v}>) persists in the presence of quenched disorder even in spatial dimension d=3, while it is destroyed even by arbitrarily weak disorder in d \le 4 in equilibrium systems. Furthermore, in d=2, quasi-long-ranged order (i.e., spatial velocity correlations that decay as a power law with distance) occur when quenched disorder is present, in contrast to the short-ranged order that is all that can survive in equilibrium.  These predictions are born out by simulations in both 2 and 3 dimensions.

 

March 7, 2014

March 13 Colloquium

Kwonmoo Lee, Harvard Medical School

Integrating Protein Dynamics and Cell Mechanics: Mechanosensitivity of Dynamic Actin Remodeling During Cell Migration

Migrating cells are mechanochemical machines where biochemistry and mechanics integrate to control cellular motion. The polymerization of the filamentous protein called actin provides force generation for membrane protrusion, an initial step of cell migration. In turn, mechanical force sensed by cells leads to dynamic responses of the intracellular biochemical pathways. The dissection of systems with such mechanochemical pathways has remained a fundamental challenge in biological investigation.

To unravel the complex nature of interplay between biochemistry and mechanics in space and time, we developed a novel statistical approach based on local image sampling and registration to directly visualize the dynamics of molecular/cellular events at the subcellular level. This method revealed distinct dynamics of molecular factors and traction force involved in cell protrusion, allowing us to establish in situ their differential functions. This quantitative framework also allowed us to exploit natural heterogeneity of cell protrusion to extract statistical relationships between different molecular/cellular events. Using this method we suggest that linear actin filaments along with adhesion formation initiate new protrusions. Then, the linear filaments undergo a structural transition to branched networks, mediated by exponential actin polymerization. Furthermore, we showed that increasing membrane tension during protrusion promotes this structural tension which leads to stronger force generation to support edge advancement against the membrane tension. This suggests that migrating cells can reorganize their biochemical machinery in response to mechanical cues in a highly dynamic manner at a time scale of 10 seconds. It also highlights that mechanical processes are tightly integrated with biochemical pathways and modulate protein dynamics, thereby playing critical roles in cell physiology.

REFRESHMENTS:  3:40 p.m. in the Willamette Atrium COLLOQUIUM:   4:00 p.m. in Willamette 100

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