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Current Seminars

Nanoscale Phase Separation in an Iron Based Superconductor
Despina Louca, University of Virginia
  • February 15, 2018
  • 4pm in Dow 107
  • Reception at 3:30pm in Dow 208
  • Abstract: The AxFe2-ySe2 (A = K, Rb, Cs) iron selenide superconductors have been intensely studied since their discovery in part due to the interplay between magnetism and superconductivity, the possible role of Fe-vacancy order in the superconducting mechanism, and whether or not phase separation occurs between superconducting (SC) and non-superconducting (NSC) regions. In the high temperature phase, the vacancies are randomly distributed. Upon cooling below the structural transition, superlattice structures appear due to Fe vacancy and alkali metal order. Several scenarios have been proposed regarding the nature of the crystal structure below the transition, includig one that describes a lattice that is phase separated into a minority I4/mmm phase which is compressed in-plane and extended out-of-plane in comparison to the high temperature centrosymmetric phase and has no Fe vacancies, and an I4/m phase with the Fe vacancies ordered in different superlattice patterns. Moreover, magnetic ordering is characteristic of the vacancy-ordered phase. The antiferromagnetic state, commonly reported in the literature, is robust unlike what has been observed in all other Fe-based superconductors, and its coexistence with the superconducting state has raised concerns about the validity of the s+/- coupling mechanism coupled with the absence of hole pockets at the Fermi surface and the lack of nesting in this system. The I4/mmm phase with no Fe vacancy has largely been attributed to be the host of superconductivity in part because of the absence of magnetism and vacancies at least at the Fe site. It is understood at present that by post-annealing and quenching, superconductivity can be enhanced in this system even though the actual mechanism remains unknown. Magnetic refinement from neutron powder diffraction measurements revealed that magnetic order does not exclude the presence of a SC phase. To investigate this issue further, high energy X-ray scattering measurements were performed on two kinds of KxFe2-ySe2 single crystals, one annealed and SC, and the other as-grown and NSC. In combination with Monte Carlo simulation, it is shown that superconductivity in quenched crystal is most likely present in regions between the I4/m domain boundaries, bordering the I4/mmm domains with no Fe vacancies. Thus superconductivity in this system appears at the crossover of the vacancy order-disorder transition. Quenching increases the boundary walls around the I4/m domains, leading to an increase of the percolation paths and an enhancement of the SC volume fraction.
Making Science More LGBTQ+ Inclusive
MacKenzie Warren, Department of Physics & Astronomy, Michigan State University
  • February 26, 2018
  • 5pm in the Library Auditorium
  • Reception at 3:30pm in Dow 208
  • Abstract:  There are currently no data on the number of LGBTQ+ people in fields like physics and other hard science but research  shows that LGBTQ+ people face significant barriers and harassment in STEM fields.  There are significant steps that departments can take to counter this and build more inclusive communities.  I will present here the experiences of LGBTQ+ people in astronomy and physics and provide recommendations for how individuals and departments can best support LGBTQ+ students and scientists.
Advancing our knowledge of stellar structure and stellar systems with optical interferometry
Henrique Schmitt, Naval Research Laboratory
  • March 1, 2018
  • 4pm in Dow 107
  • Reception at 3:30pm in Dow 208
  • Abstract: With a few notable exceptions, resolving stars and circumstellar disks, or detecting nearby companions, either stars or planets, cannot be done with single telescopes. The solution to this problem requires the use of optical interferometry. This technique combines the light from multiple telescopes in order to synthesize a larger aperture and circumvent the resolution limitation imposed by single telescopes. In this talk I will present details about the optical interferometry technique, with particular emphasis on the Navy Precision Optical Interferometry and techniques currently being developed at this facility. I will discuss recent results, going from the structure of stars and circumstellar disks, to the observations of the nuclei of active galaxies, as well as the use of optical interferometry to image geostationary satellites.
String Theory
Speaker TBA
  • March 29, 2018
  • 4pm in Dow 107
  • Reception at 3:30pm in Dow 208
The Beautiful Invisible, Sigma Pi Sigma Event
Giovanni Vignale, University of Missouri
  • April 19, 2018
  • 4pm in the Park Library Auditorium
  • Sigma Pi Sigma Award Ceremony and Reception at 5pm in the Baber Room
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