Cornell University

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Tuesday, December 9, 2014 at 4:00pm to 5:00pm

Clark Hall, 700
Central Campus

Rafael Fernandes, Department of Physics and Astronomy, University of Minnesota. Professor Eun-Ah Kim, host.

Seminar Title: Impact of electronic nematicity on the unconventional superconductivity of the iron pnictides

Abstract: Five years after their discovery, much of the interest in the iron pnictides remains in understanding not only their high-temperature superconducting phase, but also the nature of their normal state. In this context, recent experiments have provided strong evidence for the existence of an unusual correlated state in the phase diagram of these materials, dubbed electronic nematic. Below the nematic transition temperature, the tetragonal symmetry of the system is broken down to orthorhombic not by lattice vibrations, but by electronic degrees of freedom. However, two questions remain open: What is the origin of this nematic state? What is its relationship to the superconducting state? In this talk we will explore these two issues via a microscopic electronic model in which the nematic instability is caused by magnetic fluctuations arising from a degenerate ground state. A key consequence of this model is that lattice fluctuations and magnetic fluctuations are not independent. Instead, they follow a simple scaling relation, which we will show to be satisfied by elastic modulus and NMR experimental data. We will also demonstrate that, in general, nematic order competes with the unconventional sign-changing s+- superconducting state, although they may coexist under certain conditions. When the s+- instability is in close competition with a d-wave instability – as it has been suggested in several iron pnictides – we will show that nematic and superconducting degrees of freedom are strongly coupled. As a result, not only Tc can be significantly enhanced by nematic order, but also nematicity itself can be used as a diagnostic tool to search for more exotic superconducting states – such as states that spontaneously break time-reversal or tetragonal symmetries.

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Mr. Douglas E. Milton, Sr.

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Rafael Fernances

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U. Minnesota

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