Houston, TX 77005
4:00 p.m. Wednesday, Jan. 30, 2013
On Campus | Alumni
In conventional Bardeen-Cooper-Schrieffer (BCS) superconductors, superconductivity occurs when electrons form coherent Cooper pairs below the superconducting transition temperature Tc. Although the kinetic energy of paired electrons increases in the superconducting state relative to the normal state, the reduction in the ion lattice energy is sufficient to give the superconducting condensation energy (Ec = ?N(0)2/2 and 2¯h!De?1/N(0)V0 , where N(0) is the electron density of states at zero temperature, ¯h!D is the Debye energy, and V0 is the strength electron-lattice coupling). For iron pnictide superconductors derived from electron or hole doping of their antiferromagnetic (AF) parent compounds, the microscopic origin for supercnductivity is unclear. Here we use neutron scattering to show that high-Tc superconductivity only occurs for iron pnictides with low-energy ( 25 meV or 6.5kBTc) itinerant electron-spin excitation coupling and high energy (> 100 meV) spin excitations. Since our absolute spin susceptibility measurements for optimally hole-doped iron pnictide reveal that the change in magnetic exchange energy below and above Tc can account for the superconducting condensation energy, we conclude that the presence of both high-energy spin excitations giving rise to a large magnetic exchange coupling J and low-energy spin excitations coupled to the itinerant electrons is essential for high-Tc superconductivity in iron pnictides.