Mixed Valence Driven Heavy-Fermion Behavior and Superconductivity in KNi2Se2
and
Charge density wave fluctuations, heavy electrons, and superconductivity in KNi2S2
Phys. Rev. B (2012) [Editor's Suggestion], doi:10.1103/PhysRevB.86.054512
Phys. Rev. B (2013), doi:10.1103/PhysRevB.87.045124


Executive Summary

The emergence of complex behavior from simple interactions in many-body systems results in the most fascinating phenomena of the world around us. What distinguishes emergent behavior is that the collective response of the many-body system appears to be more than the sum of the individual parts. Examples range from the flocking of birds to life itself.

Emergence also occurs due to simple interactions between electrons in solids, and is responsible for many useful materials properties, such as superconductivity, which is the ability to carry an electrical current with zero resistance and is used in devices from MRIs to cell towers. However, our understanding of the fundamental origins of these behaviors, and thus our ability to create and utilize such states in other applications (such as in energy capture or storage), is currently limited.

Relevant to these manuscripts is a class of materials called heavy-fermions, which are metals in which the conduction electrons behave as though they have a mass many (10-1000) times that of a normal electron. This is known to happen in systems where individual magnetic electrons become indistinguishable as a result of strong interactions with a bath of non-magnetic conduction electrons; the product is an overall non-magnetic state that is coherent across the material, and gives rise to a number of spectacular deviations from traditional solid state theory and behavior. However, the intrinsically low energy scale of the magnetic interactions (usually < 10 K) that give rise to this state has precluded widespread applicability.

These papers presents the discovery of a new class of heavy-fermion materials in which the fundamental interaction giving rise to the emergent electronic state is electrostatically driven. This increases the energy scale, and hence temperature, at which the state forms. Immediately, this is important in the quest for a new exotic states of matter, but more generally our results demonstrate a new strategy for the design and manipulation of large-scale quantum states.

More recent ARPES experiments on single crystals (e.g. arXiv:1411.6455 and arXiv:1412.7016) have demonstrated that the above interpretation is too simplistic and that the heavy mass can be explained in the framework of traditional DFT-LDA with a small mass renormalization. However, since DFT-LDA is capable of capturing charge driven phenomena, it is not clear whether the ARPES observations are the proposed charge driven behavior or not.