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This Week's Colloquium: Andrey Rogachev, Sept 6, 2012

Andrey Rogachev
University of Utah

Thursday, Sept 6, 2012
102 JFB

Refreshments: 3:30 pm in 219 JFB
Lecture 4:00pm (102 JFB)

Title: Superconductor – Insulator Transition in One-dimensional Nanowires


The properties of one-dimensional superconducting wires depend on physical processes with different characteristic lengths. To identify the process dominant in the critical regime we have studied the transport properties of very narrow (9–20 nm) MoGe wires fabricated by advanced electron-beam lithography in a wide range of length. We observed that the wires undergo a superconductor-insulator transition (SIT) that is controlled local processes. A qualitatively similar superconductor-insulator transition can be induced by an external magnetic field. Our results are not consistent with any currently known theory of the SIT. In the talk I will also briefly describe several collaborative projects that utilize the group expertise in nanofabrication and dynamical transport measurements.


This Week's Colloquium: Christoph Boehme, Aug 30, 2012

Christoph Boehme
University of Utah

Thursday, Aug 30, 2012
102 JFB

Refreshments: 3:30 pm in 219 JFB
Lecture 4:00pm (102 JFB)

Title: Spintronics of weakly spin-orbit coupled semiconductors


While the term "Spintronics" was originally introduced as label for technologies that represent information through spin states rather than charge states, it is nowadays oftentimes used solely in the context of spin-polarization, spin-injection, spin-transport and spin-orbit effects. Silicon and carbon based semiconductors display only weak spin-orbit coupling and - in the case of organic semiconductors - charge transport via hopping through strongly localized states. These materials appear at first glance therefore to be entirely unsuitable for spintronics. However, they also exhibit spin related effects not seen in materials with strong spin-orbit coupling which can be used for an alternative, radically different approach to spintronics which is based on spin-permutation symmetry states of charge carrier pairs rather than spin-polarization states. Reading spin-permutation symmetry is straight forward when pronounced spin-selection rules exist. In contrast to spin-polarization, permutation symmetry does not depend directly on temperature and magnetic field strength. Furthermore, the absence of spin-orbit coupling can also allow for long spin-coherence times and thus, the possibility to connect spintronics to an all spin based memory which could be important for spin-based quantum information concepts. While spin-orbit coupling is needed in traditional spintronics for electric field controlled spin-manipulation, low-spin-orbit coupled devices may achieve the same via electric field controlled spin-exchange interaction. In this talk, our work on the development of this alternative organic spintronics concept will be presented and the state of its experimental implementation will be discussed


This Week's Colloquium: Dave Kieda, Aug. 23, 2012

Dave Kieda
University of Utah

Thursday, Aug 23, 2012
102 JFB

Refreshments: 3:30 pm in 219 JFB
Lecture 4:00pm (102 JFB)

Title: State of the Department


Gary Finnegan Thesis Defense 08/22/12

Thesis Defense

Gary Finnegan

Monday, August 22, 2012
2:00pm (110 INSCC)

Title: High Energy Gamma-Ray Astronomy Observations Of Geminga With The VERITAS Array


The closest known super-nova remnant and pulsar is Geminga. The Geminga pulsar is the first pulsar to have ever been detected initially by gamma rays and the first pulsar in a class of radio-quiet pulsars. In 2007 a detection of very high energy gamma rays (∼ 20 TeV), that are positionally coincident with Geminga, was reported by the Milagro collaboration, with a large angularly extended emission (∼ 2.6° ). The Very Energetic Radiation Imaging Telescope Array System (VERITAS) is a ground- based observatory with four imaging Cherenkov telescopes with an energy range between 100 GeV to more than 30 TeV. The imaging Cherenkov telescopes detect the Cherenkov light from charged particles in an electromagnetic air shower initiated by high energy particles such as gamma rays and cosmic rays. The field of view (FOV) of the VERITAS telescopes is approximately 3.5°. Most gamma-ray sources detected by VERITAS are point like sources, which have an angular extension smaller than the resolution of the telescopes (∼ 0.1°). For an angularly extended object, such as Geminga, an external FOV from the source must be used to estimate the background noise to avoid contamination from the source itself. In this dissertation, I will describe a new analysis procedure that is designed to increase the sensitivity of angularly extended objects like Geminga. I will present the results of my analysis, which conclude with the detection of very high energy emission from the Geminga region at the level of a few percent of the Crab nebula and a possible extension less than one degree wide. This detection however awaits a confirmation by the VERITAS collaboration. To conclude, I will present the implications of the detection of Geminga.


[1] G. Finnegan for the VERITAS Collaboration, ”Orbit Mode Observation Technique Developed for VERITAS”, Proceedings of the 2011 Fermi Symposium, arXiv:1111.0121v1 (Nov 2011).

[2] G. Finnegan for the VERITAS Collaboration, “Search for TeV Emission from Geminga by VERITAS”, Proceeding of the 31st ICRC, arXiv:0907.5237v3 (July 2009).

[3] V.A. Acciari, et al, “Observation of Extended Very High Energy Emission from the Supernova Remnant 1C443 with VERITAS”, Astrophysical Journal Letters 698, L133 (2009).


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