Condensed Matter Seminars

Superconductors which conduct electric current without dissipation (zero resistance) and expel magnetic field (perfect diamagnetism) were discovered 100 years ago in metal at a few Kelvin above absolute zero. Stimulated by the discovery of cuprate superconductors in doped Mott antiferromagnetic insulator, over the last two decades research has been focused on discovery of unconventional superconductors of high transition temperature (Tc) in magnetic materials. Iron-based laminar materials in several related structure families with Tc as high as 56 K have generated much excitement in the last three years, and new discovery continues to appear. Using neutron scattering technique, we have determined crystal and magnetic structure of several families of the new superconductors and the sample composition [1-5], which provide solid foundation for further investigation on electronic structure and processes. Structural and magnetic transitions have been investigated to yield phase-diagrams which show a rich variety of relationship between superconducting and magnetic orders [6,7]. Such investigation also reveals the shortcoming of widely accepted spin-density-wave scenario and provides first experimental indication of important role of the orbital order [2]. The symmetry of superconducting order parameter has strong signature in magnetic excitation spectrum. We observed with inelastic neutron scattering method the telltale spin resonance mode of the unconventional s+/- symmetry in the superconducting state of the 11 superconductor [8]. The normal state was shown to exhibit the single-lobed incommensurate excitation continuum of a typical itinerant antiferromagnet, in contrast to spin-wave cone of a localized antiferromagnet [8,9], supporting a Fermi liquid description of the normal state. [1] Y. Qiu, W. Bao, Q. Huang et al., Phys. Rev. Lett. 101, 257002 (2008). [2] Q. Huang, Y. Qiu, W. Bao et al., Phys. Rev. Lett. 101, 257003 (2008); M. Kofu, Y. Qiu, W. Bao et al., New J. Phys. 11, 055001 (2009). [3] W. Bao, Y. Qiu, Q. Huang et al., Phys. Rev. Lett. 102, 247001 (2009). [4] W. Bao et al., arXiv:1102.0830 (2010). [5] F. Ye et al., arXiv:1102.2882 (2010). [6] H. Chen et al., Europhys. Lett. 85, 17006 (2009). [7] W. Bao et al., arXiv:1102.3674 (2010). [8] Y. Qiu, W. Bao, Y. Zhao et al., Phys. Rev. Lett. 103, 067008 (2009). [9] D.N. Argyriou et al., Phys. Rev. B 81, 220503 (R) (2010).