National Synchrotron Light Source Seminar

The discovery of pressure-induced superconductivity in the AFe2As2 (A=Ba, Sr, Ca) family opened an exciting new avenue for investigations of the relationship between magnetism, superconductivity, and lattice instabilities in the iron arsenide family of superconductors. At ambient pressure, the undoped parent phases undergo a tetragonal-to-orthorhombic distortion accompanied by antiferromagnetic order at lower temperatures. CaFe2As2 is unusual since, under modest pressures (< 1GPa), a new non-magnetic “collapsed tetragonal” (cT) phase emerges from the antiferromagnetically ordered low-temperature orthorhombic phase. Superconductivity in the parent AFe2As2 compounds, under applied pressure, has been reported by several groups using liquid-media pressure cells. Measurements under hydrostatic pressure conditions, however, found that superconductivity, if present at all, occurs over a very narrow range close to the reported orthorhombic-to-cT transition. I will discuss neutron and x-ray diffraction measurements under uniaxial and hydrostatic pressure conditions that have elucidated the nature of the phase responsible for superconductivity in this system.