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This Week at Physics

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Events on Thursday, October 2nd, 2014

R. G. Herb Condensed Matter Seminar
The Zeeman Effect in Confined Superconductors: A new probe of absolute spin polarization on the atomic scale
Time: 10:00 am
Place: 5310 Chamberlin Hall
Speaker: Christian Ast, Max Planck Institute for Solid State Research
Abstract: When a dimensionally confined superconductor is subjected to a magnetic field, the compensating currents are suppressed such that superconductivity persists to much higher magnetic fields than in the bulk. This effect has been observed first in planar tunnel junctions by Meservey, Tedrow and Fulde [1]. The resulting Zeeman splitting of the quasiparticle density of states can be exploited as a probe for the absolute spin polarization of a spin polarized tunnelling current [2]. We have transferred this concept to the scanning tunnelling microscope (STM) to probe the spin polarization of the tunnelling current on the atomic scale by using a superconducting tip. As the superconducting tip carries no magnetic moment itself, the information in the spin polarized tunnelling current can be attributed to the magnetic properties of the sample. This is a decisive advantage over other spin-polarized STM techniques. First measurements on magnetic cobalt islands on a Cu(111) substrate are in excellent agreement with literature. Measuring the spin polarization of the tunnelling current as a function of tip-island distance, we find that the spin polarization increases by 65% when the distance is increased by only 2.3Å. This can be attributed to the different exponential decay of majority and minority states into the vacuum. This means that we can exploit the vacuum barrier as a tunable spin filter.

References
[1] R. Meservey et al., Phys. Rev. Lett. 25, 1270 (1970).
[2] P. M. Tedrow and R. Meservey, Phys. Rev. Lett. 26, 192 (1971).
Host: Franz Himpsel
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NPAC (Nuclear/Particle/Astro/Cosmo) Forum
Astrophysical neutrinos, PeV events at IceCube, and the Direct Detection of Dark Matter
Time: 2:30 pm
Place: 5280 Chamberlin
Speaker: Raj Gandhi, HRI, India and Fermi National Accelerator Laboratory
Abstract: Given present day constraints, dark matter (DM) may not comprise of thermal weakly interacting massive particles (WIMPS). Drawing closely upon the fact that neutral current neutrino nucleon interactions are indistinguishable from DM-nucleon interactions at low energies, we study the consequences of extending this feature to high energies for a small, non-thermal but highly energetic population of DM particle χ, created via the decay of a significantly more massive and long-lived non-thermal relic φ, which forms the bulk of DM. If χ interacts with nucleons, it is possible that its cross-section, like the neutrino-nucleus coherent cross-section, may rise sharply with energy leading to deep inelastic scattering, similar to neutral current neutrino-nucleon interactions at high energies. Thus, its direct detection may be possible via cascades in very large neutrino detectors. IceCube (IC) has recently reported three ultra-high energy PeV cascade events clustered around 1-2 PeV. We apply this notion to these events and discuss the features which may help discriminate this scenario from one in which only astrophysical neutrinos constitute the IC event sample.
Host: Barger
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Astronomy Colloquium
Filaments in simulations of Molecular Cloud FOrmation
Time: 3:30 pm
Place: 4421 Sterling Hall
Speaker: Gilberto Gomez, UNAM Mexico
Abstract: In this talk, we report on the characterization of filaments that develop self-consistently in a numerical simulation of cloud formation by colliding flows. The filaments are not in equilibrium at any time, but instead are long-lived flow features. The filaments are long-lived because they accrete from their environment while simultaneously accreting onto the clumps within them; they are essentially the locus where the flow changes from accreting in two dimensions to accreting in one dimension.

In the simulation, two prominent filaments are formed, with lengths ∼15 pc and masses ∼600Msun above density n ∼ 1e3 cm−3. The density profile exhibits a central flattened core of size ∼0.3 pc and an envelope that decays as r^−2.5 in reasonable agreement with observations. Accretion onto the filament reaches a maximum linear density rate of ∼30Msun Myr^−1 pc^−1.
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