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This Week at Physics

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Events During the Week of March 24th through March 31st, 2013

Sunday, March 24th, 2013

Distinguished Lecture Series
"Cosmology on a Moving Mesh"
Time: 4:00 pm
Place: 1310 Sterling Hall
Speaker: Lars Hernquist, Harvard University
Abstract:
Understanding the formation and evolution of galaxies in a cosmological context using numerical simulations remains an elusive goal. In this talk, I describe a new approach to modeling the hydrodynamics of galaxy formation in which the equations of motion are solved on a moving mesh. The use of a moving mesh makes the scheme fully Lagrangian, unlike popular particle-based codes which are quasi-Lagrangian in nature, and mitigates against advection errors when a spatially fixed grid is used. I present results from an initial study comparing results for a moving mesh with those obtained using a smoothed particle hydrodynamics solver. This preliminary work suggests that the new approach offers promise for resolving the long-standing problems which have plagued this field for nearly two decades.
Host: Prof Elena D'Onghia
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Monday, March 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:
    http://cmb.physics.wisc.edu/journal/index.html
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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Tuesday, March 26th, 2013

No events scheduled

Wednesday, March 27th, 2013

No events scheduled

Thursday, March 28th, 2013

R. G. Herb Condensed Matter Seminar
Control and possible applications of valley degree of freedom: Valleytronics?
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Belita Koiller, Federal University of Rio de Janeiro
Abstract: The conduction electrons in Si are not in a well-defined single Bloch state. Instead, the Si conduction band is six-fold degenerate, with minima (valleys) along the x, y and z crystallographic directions. This imposes limitations to the spin manipulation and coherence. It was recently proposed to encode quantum information directly into the valley degree of freedom, converting the spurious valley Hilbert subspace into a useful ingredient for a quantum computer. In this talk, valley degrees of freedom in Si are addressed in 3 different contexts.

1) Based on an atomistic pseudopotential theory, we demonstrate that ordered Ge-Si layered barriers confining a Si slab can be optimized to enhance the VS in the active Si region by up to one order of magnitude compared to the random alloy barriers adopted so far. We identify Ge/Si layer sequences leading to a VS as large as ~9 meV. The splitting is "protected" even if some mixing occurs at the interfaces.

2) Interface states form spontaneously at some semiconductor-barrier interfaces and they may improve or hinder the electron control and coherence for semiconductor-based qubits. From a simple 1D Tight-binding model, new insights emerge regarding the interface state's energy, as well as the exponential longer (shorter) localization lengths into the Si (barrier) material. The interface state may be probed experimentally by an external electric field, which modulates the capacitance of the system and the lowest level spacing (valley splitting).

3) We analyze the valley composition of one electron bound to a shallow donor close to a Si/barrier interface. A full six-valley effective mass model Hamiltonian is adopted. For low fields, the electron ground state is essentially confined at the donor. At high fields the ground state is such that the electron is drawn to the interface, leaving the donor practically ionized. Valley splitting at the interface occurs due to the valley-orbit coupling, taken here as a complex parameter. A sequence of two anti-crossings takes place and the complex phase affects the symmetries of the eigenstates and level anti-crossing gaps.

References:
1) L. Zhang, J-Wi Luo, A. L. Saraiva, B. Koiller, A. Zunger, arXiv:1303.4932.
2) A. L. Saraiva, B.Koiller, M. Friesen, Phys. Rev. B 82, 245314 (2010).
3) A. Baena, A. L. Saraiva, B.Koiller, M. J. Calderón Phys. Rev. B 86, 035317 (2012).
Host: Coppersmith
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Friday, March 29th, 2013

No events scheduled

"This Week at Physics" poster: http://www.physics.wisc.edu/twap/posters/2013/2013-03-25.pdf

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