Heavy electrons, Charge Density Fluctuations, and Superconductivity in KNi2X2 (X = S, Se)

Tyrel McQueen (Johns Hopkins University)

Materials with the ThCr2Si2 structure type host myriad examples of many-body physics, including high-temperature superconductivity and heavy-fermion behavior. In these compounds, the emergence of the collective states frequently occurs near magnetic instabilities, suggesting that magnetic fluctuations underlie the electronic phenomena. In this talk, I will provide evidence for similar many-body physics in the structurally related, but non-magnetic, compounds KNi2S2 and KNi2Se2. From the analysis of high-resolution synchrotron X-ray diffraction and neutron total scattering data, we observe spatially incoherent charge density wave fluctuations that disappear on cooling. Along with the implied and unusual increase in local symmetry, we find that there is negative thermal expansion and enhancement of electronic band mass below T ~ 15 K, with superconductivity emerging below 1 K. These findings demonstrate that collective electronic phenomena occur in ThCr2Si2-type materials without direct proximity to magnetism, and highlight the importance in understanding charge in driving the emergence of coherent or many-body electronic states. If time permits, I will also present some of our recent results regarding superconductivity in a new class of materials based on BiS2 layers.

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