, Renmin University of China
[Host: Seunghun Lee]
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 .
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 . 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.
 Y. Qiu, W. Bao, Q. Huang et al., Phys. Rev. Lett. 101, 257002 (2008).
 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).
 W. Bao, Y. Qiu, Q. Huang et al., Phys. Rev. Lett. 102, 247001 (2009).
 W. Bao et al., arXiv:1102.0830 (2010).
 F. Ye et al., arXiv:1102.2882 (2010).
 H. Chen et al., Europhys. Lett. 85, 17006 (2009).
 W. Bao et al., arXiv:1102.3674 (2010).
 Y. Qiu, W. Bao, Y. Zhao et al., Phys. Rev. Lett. 103, 067008 (2009).
 D.N. Argyriou et al., Phys. Rev. B 81, 220503 (R) (2010).
Condensed Matter Seminar
Tuesday, March 29, 2011
Physics Building, Room 313
Note special date.
Note special room.