Magnetic Moments in Superconductors: From Assassin to Facilitator

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Date/Time:Monday, 24 Sep 2012 from 4:10 pm to 5:00 pm
Location:Physics 0005
Phone:515-294-5441
Channel:Colloquium
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Jeffrey Lynn (NIST Center for Neutron Research, National Institute of Standards and Technology)

Abstract: The magnetic properties of superconductors have a rich and interesting history. Early work showed that even tiny concentrations of magnetic impurities destroyed the superconducting pairing through the exchange-driven spin depairing mechanism, prohibiting any possibility of magnetic order coexisting with superconductivity. The first exceptions to this rule were provided by the cubic rare-eath substituted CeRu2 alloys, followed by the ternary Chevrel-phase superconductors (e.g. HoMo6S8) and related compounds, which exhibit long range magnetic order coexisting with superconductivity. The very low magnetic ordering temperatures (~1 K) suggested that dipolar rather than exchange interactions dominate, thus (it was thought) allowing the coexistence. These materials also provided the first examples of the competition between ferromagnetism and superconductivity. In the newer borocarbide class of magnetic superconductors (e.g. ErNi2B2C), however, it became clear that the magnetic order is in fact exchange driven. The borocarbides also provided the first example of the spontaneous formation of flux quanta (vortices). For the high-TC cuprate and iron-based superconductors we now have come full circle, as the spins are not only tolerated but are intimately tied to the superconductivity. The "parent" cuprate systems are Mott-Hubbard antiferromagnetic insulators with very strong magnetic interactions that are two-dimensional in nature. These strong exchange interactions survive into the superconducting state, yielding highly correlated electrons that participate directly in the superconducting pairing. The "parent" materials of the new iron-based high TC superconductors are also antiferromagnets with very energetic spin excitations, and in the superconducting regime they form a "magnetic resonance" that is directly tied to the superconducting order parameter, just like the cuprates. Our understanding of this new family of systems is now arguably better than for the cuprates, and perhaps the iron-based superconductors will provide the key to a full understanding of both classes of materials.

Further information and references can be found at http://www.ncnr.nist.gov/staff/jeff. For a recent review of the Iron-based Family of High TC Magnetic Superconductors, see Jeffrey W. Lynn and Pengcheng Dai, Physica C 469. 469 (2009).