Mixed-Valence-Driven Heavy-Fermion Behavior and Superconductivity in KNi$_2$Se$_2$

TL;DR

This study uncovers a heavy-fermion-like electronic state and superconductivity in KNi$_2$Se$_2$, driven by mixed valence and charge fluctuations, without magnetic order, highlighting a novel mechanism for heavy fermion behavior.

Contribution

It demonstrates that KNi$_2$Se$_2$ exhibits heavy-fermion behavior and superconductivity driven by charge fluctuations rather than magnetic interactions.

Findings

Heavy electronic state below 20 K with increased effective mass

Superconductivity at 0.80 K linked to heavy fermion behavior

Presence of charge-density-wave distortions that diminish at low temperature

Abstract

Based on specific heat and magnetoresistance measurements, we report that a "heavy" electronic state exists below $T \approx$ 20 K in KNi$_2$Se$_2$, with an increased carrier mobility and enhanced effective electronic band mass, $m$* = 6$m_b$ to 18$m_b$. This "heavy" state evolves into superconductivity at $T_c$ = 0.80(1) K. These properties resemble that of a many-body heavy-fermion state, which derives from the hybridization between localized magnetic states and conduction electrons. Yet, no evidence for localized magnetism or magnetic order is found in KNi$_2$Se$_2$ from magnetization measurements or neutron diffraction. Instead, neutron pair-distribution-function analysis reveals the presence of local charge-density-wave distortions that disappear on cooling, an effect opposite to what is typically observed, suggesting that the low-temperature electronic state of KNi$_2$Se$_2$ arises from cooperative Coulomb interactions and proximity to, but avoidance of, charge order.