Magnetism and Superconductivity in the iron-arsenides: The peculiar case of CaFe2As2

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Date/Time:Thursday, 29 Oct 2009 from 4:10 pm to 5:00 pm
Location:Physics 5
Channel:Condensed Matter Physics
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Alan I. Goldman (Iowa State University and Ames Laboratory)

The discovery of pressure-induced superconductivity in the AFe2As2 (A=Ba, Sr, Ca) family has opened an exciting new avenue for investigations of the relationship between magnetism, superconductivity, and lattice instabilities in the iron arsenide family of superconductors. Superconductivity in the parent AFe2As2 compounds under applied pressure has been reported by several groups using liquid-media pressure cells. CaFe2As2 is unusual since, under modest hydrostatic pressure, a new non-magnetic "collapsed tetragonal" (cT) phase emerges from the antiferromagnetically ordered low-temperature orthorhombic phase. Recent measurements under hydrostatic pressure conditions have revealed that superconductivity, if present at all, occurs in a very narrow range of pressure close to the reported orthorhombic-to-collapsed tetragonal transition. Furthermore, both the antiferromagnetic order in the low-temperature orthorhombic phase and the correlated spin-fluctuations in the higher temperature tetragonal phase are quenched upon the transformation to the cT phase. I will discuss recent elastic and inelastic neutron scattering measurements that have elucidated the nature of the magnetic order at ambient pressure, its relationship to the tetragonal-orthorhombic structural transition at ambient pressure, the quenching of magnetism in the cT phase and the identification of the superconducting phase in CaFe2As2 under applied uniaxial pressure.