New superconductors: iron chalcogenides
Postdoctoral fellow Jamie Nelson (McQueen group) has synthesized two superconductors from a new family of iron chalcogenides, KNi2Se2 and KNi2S2 with the onset of superconductivity at 0.8 K and 0.5 K. Although the critical temperatures are too low for the materials to be technologically useful, they present two puzzling features: heavy-fermion behavior and a symmetry increase on cooling. The former is surprising because heavy fermions are usually found in lanthanide and actinide compounds possessing both localized magnetic f-electrons and a metallic bath of itinerant electrons. KNi2Se2 not only lacks occupied 4f or 5f states, but also does not exhibit any signatures of localized magnetic moments.
Density-functional theory (DFT) calculations of the phonon spectrum at room temperature were confirmed by neutron scattering experiments conducted by graduate student Jiajia Wen (Broholm group). However, the DFT calculations predict a state with lower symmetry than was actually observed. Generally, higher symmetry upon cooling is unusual, as it naively suggests an increase in configurational entropy, when considering only the degeneracy of lattice modes. A higher symmetry of the lattice at low temperatures was confirmed by research associate Natalia Drichko, who conducted Raman scattering experiments on KNi2Se2.
Graduate student James Murray (theory group) hypothesized that the heavy-fermion behavior is linked to the unusual increase in local symmetry on cooling. His model may provide a state in which the total entropy is decreased by such a raising of lattice symmetry and explain why our DFT calculations fail to correctly predict the ground state of this material.