Physics and Astronomy Calendar

Week of Monday, December 11th 2017

Monday, December 11th 2017
12:15 pm:
Speaker: Patrick Kelly, UMN
Subject:  Using A Highly Magnified Star and the Multiply Imaged Supernova Refsdal to Constrain the Outcomes of Star Formation, Stellar Evolution, and the Abundance of Primordial Black Holes

We recently detected an individual blue supergiant star in a multiply imaged spiral galaxy at redshift z=1.49 behind the MACS J1149 galaxy cluster (z=0.54). In the spring of 2016, the star (dubbed LS1) appeared to brighten by a factor of three in Hubble Space Telescope imaging due to microlensing by a star in the intracluster medium of the foreground cluster. Lens models of the cluster show that the star, which is adjacent to the galaxy cluster’s critical curve, likely became magnified briefly by more than a factor of 2000. Additional monitoring subsequently revealed a second transient object which may be LS1’s counterimage in October 2016. I will describe ongoing work to model LS1’s light curve and to draw conclusions about the stellar population in the intracluster medium, the outcomes of massive stellar evolution, and the abundance of primordial black holes. The multiply imaged supernova, SN Refsdal, has appeared as five separate images, and a new measurement of its time delay provides useful and complementary constraints on the cluster lens model.

1:25 pm:
Nuclear Physics Seminar in Tate 201-20
Speaker: Huaiyu Duan
Subject: Fast neutrino oscillations in supernovae
(Note Special Time)

Neutrinos dominate the energetics of core-collapse supernovae and are crucial to the evolution of the nascent neutron star. An important question of supernova physics is whether neutrino oscillations can occur deep enough in the supernova envelope to have a significant impact on the nucleosynthesis and even the shock revival of the supernova. It was pointed out recently that neutrino oscillations can occur on very short distance/time scales and even in the limit of (almost) zero neutrino masses. I will discuss the mechanism of this fast neutrino oscillation phenomenon and how one may understand it with a few simple but insightful toy models.

Faculty Host: Yong-Zhong Qian

Tuesday, December 12th 2017
11:15 am:
Speaker: Tomoya Nakatani
Subject: Recent advances in CPP-GMR materials: Heusler alloys and new non-magnetic spacers
Faculty Host: Paul Crowell
12:20 pm:
Space Physics Seminar in Tate 301-20
There will be no seminar this week.
2:00 pm:
Thesis Defense in PAN 110
Speaker: Ziran Wang, University of Minnesota
Subject: Magnetism for Data Storage: Magnetoresistance Enhancement by Wave Vector Filtering and Transition Shifts in HAMR
This is the public portion of Mr. Wang's thesis defense. His advisor is Randall Victora.

Magnetoresistive heterostructures have important applications in magnetic storage technology and spintronics. This thesis uses Green’s function techniques to calculate the transport properties of a novel structure Fe/Ag/Fe/InAs/Ag. We show that the magnetoresistance can be enhanced to above 1000% due to the wave-vector filtering effect imposed by the InAs layer; meanwhile, the resistance-area product is as low as ~10Ωµm^2. The magnetoresistance shows oscillations with the InAs thickness when the Fermi level is in the conduction band, and the oscillations are quantitatively explained by theory.
Heat-assisted magnetic recording (HAMR) is a leading technology for the next-generation magnetic data storage. This thesis uses micromagnetic simulations to study magnetization transition shifts induced by nonequilibrium spin dynamics in HAMR. We examine the effects of thermal profile, head velocity, damping, and head field rise-time. We also propose methods to determine spin temperature and its lag relative to lattice temperature. By quantifying switching time, spin temperature lag, and superparamagnetic writing, we show that superparamagnetic effects cause largest transition shifts and dominate in typical HAMR processes.

Wednesday, December 13th 2017
09:05 am:

Aaron Durgin & Chase McCabe, "Moessbauer Spectroscopy of Fe57 Compounds"

Jarod White, "NMR Spectroscopy"

Pedro Angulo-Umana & Kai Wang, "Optical Whispering Gallery Mode Resonances"

Jacob Christy & Mitchell Frand, "Single Photon Quantum Interference"

1:25 pm:
Speaker: Wolfram Brenig, Technical University Braunschweig
Subject: Thermal transport in Kitaev-Heisenberg spin systems

Mott insulators with strong spin orbit coupling have become a testbed for exotic quantum phases, spin liquids and emergent Majorana matter. In this context we present results for the thermal conductivity of the Kitaev-Heisenberg model on ladders and the Kitaev model on honeycomb lattices. In the pure Kitaev limit, and in contrast to other integrable spin systems, the ladder represents a perfect heat insulator. This is shown to be a direct fingerprint of fractionalization into mobile Majorana matter and a static Z2 gauge field. We find a full suppression of the Drude weight and a pseudogap in the conductivity. With Heisenberg exchange, we find a crossover from a heat insulator to conductor, due to recombination of fractionalized spins into triplons. Increasing the dimension, and for the 2D honeycomb lattice, we show that very similar behavior occurs with however dissipative heat transport resulting in the thermodynamic limit. Our findings rest on several approaches comprising a mean-field theory, complete summation over all gauge sectors, exact diagonalization, and quantum typicality calculations.

Faculty Host: Natalia Perkins
There is no seminar this week.

Thursday, December 14th 2017
10:10 am:
Biophysics Seminar in 120 PAN
There will be no seminar this week.
11:30 am:
Thesis Defense in 301-20 Tate
Speaker: Ryan Arneson
This is the public portion of Mr. Arneson's thesis defense. His advisor is Robert Gehrz.

This is the public portion of Mr. Arneson's thesis defense. His advisor is Robert Gehrz.

3:35 pm:
There will be no colloquium this week.

Friday, December 15th 2017
10:00 am:
Thesis Defense in 301-20 Tate
Speaker: Karlen Shahinyan
Subject: Much Ado About Blazars: Investigations of Extreme Variability Patterns in the Very High Energy Gamma-ray Blazar Emission
This is the public portion of Mr. Shahinyan's thesis defense. His advisor is Lucy Fortson.
Speaker: No colloquium this week.
There will be no Colloquiium this week.
4:00 pm:
Thesis Defense in 110 PAN
Speaker: Sean Kalafut, University of Minnesota
Subject: Search for a WR boson and heavy neutrinos using the LHC and the CMS experiment
This is the public portion of Mr. Kalafut's Thesis Defense. His advisor is Roger Rusack.
4:40 pm:
To be announced.

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