University of Minnesota
School of Physics & Astronomy

Physics and Astronomy Calendar

Thursday, February 26th 2015
1:25 pm:
Condensed Matter Seminar in 210 Physics
Speaker: Brian Andersen, University of Copenhagen
Subject: Recent developments of cuprates and pnictides; pairing symmetry, competing order, and nematicity

In this talk I will present some of our recent theoretical efforts to understand the fascinating interplay between magnetic, charge, and superconducting order in cuprates and Fe-based superconductors. The talk will include a general discussion of spin fluctuations in iron pnictides, including the evidence and modelling of nematic (anisotropic) spin fluctuations. These can have profound influence on the transport properties of these systems both through inelastic scattering and emergent new static defect states significantly contributing to anisotropies in the measured quantities. This is true even above the magnetic transition where the anisotropic spin fluctuations can be frozen by disorder, to create elongated magnetic droplets whose anisotropy grows as the magnetic transition is approached. Such states act as strong anisotropic defect potentials that scatter with much higher probability perpendicular to their length than parallel, although the actual crystal symmetry breaking is tiny. From the calculated scattering potentials, relaxation rates, and conductivity in this region we conclude that such emergent defect states are essential for the transport anisotropy observed in experiments. I will end this part of the talk by presenting a general scenario for the transport anisotropy throughout the whole phase diagram.

Next, I will turn to a discussion of competing magnetic phases in the pnictides relevant to recent experiments finding magnetic order in a tetragonal crystal lattice. This points to the existence of other so-called C4 symmetric magnetic phases with, for example, non-collinear moments. I will present a theoretical microscopic study of these phases and their general electronic properties. A discussion will be included on the role of superconductivity and disorder in destroying and stabilizing these novel magnetic C4 states, respectively. Finally, if time allows, I will also discuss some recent studies of the doping dependence of the pairing symmetry of the cuprates in the presence of spin-density wave order.

Faculty Host: Rafael Fernandes

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