Condensed Matter Physics Seminar
Magnetic Ordering in Superconductors
National Institute of Standards and Technology
The effects of magnetic impurities and the possibility of magnetic ordering in superconductors has had a rich and interesting history. Early work showed that magnetic impurities substituted into a superconductor quickly suppress superconductivity due to the strong spin scattering that disrupts the Cooper pairs. The first exceptions to this rule were provided by the ternary Chevrel-phase superconductors (RMo6S8) and related (RRh4B4) compounds, which exhibited long range magnetic order that coexisted with superconductivity The magnetic ordering temperatures were low, and the antiferromagnetism had only a weak influence on the superconducting state. Similar behavior is observed for the rare earth magnetic order in the cuprates (e.g. RBa2Cu3O7) and borocarbides (RNi2B2C), which are all antiferromagnets. One of the most interesting aspects of the cuprates concerns the magnetism associated with the Cu spins, which is believed to cause the d-wave superconducting pairing and can also simultaneously order magnetically. In the rare and more interesting situation where the magnetic interactions are ferromagnetic, there is strong coupling to the superconducting state that originates from the internally generated magnetic field. The competition with the superconducting order parameter gives rise to long wavelength oscillatory magnetic states and/or reentrant superconductivity in the ternary superconductors such as HoMo6S8. This behavior will be contrasted with results on the new "ferromagnetic superconductors"� such as RuSr2GdCu2O8, UGe2, and ErNi2B2C. We will then discuss recent measurements of the induced magnetic order observed in both hole-doped and electron-doped cuprate superconductors. If time permits, we will also discuss our very recent work on the NaxCoO2 system, where the Co ions carry S=1/2 as in the square-planar cuprates, but with the Co ions occupying a triangular lattice.
Thursday, March 3, 2005
Gant Science Complex
(The seminar is followed by coffee/tea, cookies, and an informal discussions.)