Charge density wave fluctuations, heavy electrons, and superconductivity in KNi$_2$S$_2$

TL;DR

This study reveals that KNi$_2$S$_2$ exhibits unusual structural transitions, charge density wave fluctuations, heavy-fermion behavior, and superconductivity without magnetic order, challenging conventional understanding of correlated electron systems.

Contribution

It demonstrates the coexistence of charge density wave fluctuations and heavy-fermion behavior in a non-magnetic, mixed valence compound, expanding knowledge of correlated electron phenomena.

Findings

Charge density wave fluctuations disappear on cooling.

Heavy electron state with effective mass 24 times electron mass.

Superconductivity observed below 0.46 K.

Abstract

Understanding the complexities of electronic and magnetic ground states in solids is one of the main goals of solid-state physics. Materials with the canonical ThCr$_2$Si$_2$-type structure have proved particularly fruitful in this regards, as they exhibit a wide range of technologically advantageous physical properties described by "many-body physics," including high-temperature superconductivity and heavy fermion behavior. Here, using high-resolution synchrotron X-ray diffraction and time-of-flight neutron scattering, we show that the isostructural mixed valence compound, KNi$_2$S$_2$, displays a number of highly unusual structural transitions, most notably the presence of charge density wave fluctuations that disappear on cooling. This behavior occurs without magnetic or charge order, in contrast to expectations based on all other known materials. Furthermore, the low-temperature electronic state of KNi$_2$S$_2$ is found to exhibit many characteristics of heavy-fermion behavior, including a heavy electron state ($m^*/m_e \sim$ 24), with a negative coefficient of thermal expansion, and superconductivity below $T_c$ = 0.46(2) K. In the potassium nickel sulfide, these behaviors arise in the absence of localized magnetism, and instead appear to originate in proximity to charge order.