Quantum phase transitions and superconductivity in the pressurized heavy-fermion compound CeCuP2

TL;DR

CeCuP2 exhibits unconventional pressure-induced phase transitions, including two superconducting phases with a record transition temperature, challenging traditional models of heavy fermion behavior.

Contribution

This study reveals a novel pressure-temperature phase diagram in CeCuP2, showing superconductivity beyond the Doniach paradigm with two distinct superconducting phases.

Findings

Kondo coherence temperature decreases with pressure, contrary to typical behavior.

Two superconducting phases observed at high pressure, with the highest T_c among Ce-based heavy fermions.

Unusual phase diagram suggests new mechanisms involving valence fluctuations and crystal structure effects.

Abstract

The tilted balance among competing interactions can yield a rich variety of ground states of quantum matter. In most Ce-based heavy fermion systems, this can often be qualitatively described by the famous Doniach phase diagram, owing to the competition between the Kondo screening and the Ruderman-Kittel-Kasuya-Yoshida exchange interaction. Here, we report an unusual pressure-temperature phase diagram beyond the Doniach one in CeCuP2. At ambient pressure, CeCuP2 displays typical heavy-fermion behavior, albeit with a very low carrier density. With lowering temperature, it shows a crossover from a non Fermi liquid to a Fermi liquid at around 2.4 K. But surprisingly, the Kondo coherence temperature decreases with increasing pressure, opposite to that in most Ce-based heavy fermion compounds. Upon further compression, two superconducting phases are revealed. At 48.0 GPa, the transition temperature reaches 6.1 K, the highest among all Ce-based heavy fermion superconductors. We argue for possible roles of valence tuning and fluctuations associated with its special crystal structure in addition to the hybridization effect. These unusual phase diagrams suggest that CeCuP2 is a novel platform for studying the rich heavy fermions physics beyond the conventional Doniach paradigm.