Superconductivity under pressure in the two-dimensional van der Waals heavy-fermion metal CeSiI

TL;DR

This study reveals pressure-induced superconductivity in the two-dimensional van der Waals heavy-fermion metal CeSiI, demonstrating unconventional pairing, quantum criticality, and establishing it as the first vdW heavy-fermion superconductor.

Contribution

It constructs the first detailed phase diagram of CeSiI under pressure, uncovering a superconducting dome and evidence of quantum criticality in a vdW heavy-fermion system.

Findings

Superconductivity emerges at 6 GPa with Tcmax = 240 mK.

Unconventional pairing suggested by large upper critical field.

Quantum critical behavior observed near the critical pressure.

Abstract

CeSiI is a newly discovered exfoliable van der Waals (vdW) heavy-fermion metal featured by a long-range antiferromagnetic (AF) order (TN =7.5 K) inside the Kondo coherent state below T* = 50 K. To gain a more profound understanding of the intriguing physics of this material and to uncover novel phenomena driven by quantum criticality, it is imperative to construct the phase diagram of CeSiI detailing the evolutions of T* and TN as a function of external tuning parameters such as pressure (P).In this study, we employ high pressure as an effective tuning knob to investigate this system, thereby generating a comprehensive T-P phase diagram of CeSiI. This diagram is characterized by an unusual V-shaped nonmonotonic evolution of T*(P) and the emergence of a superconducting dome with Tcmax = 240 mK upon suppression of AF order at Pc = 6 GPa, coinciding with the minimum of T*(P).The close proximity of the superconductivity (SC) to the AF instability and an unusually large upper critical field Bc2(0) exceeding 4-7 times the Pauli paramagnetic limit, suggests an unconventional pairing mechanism in CeSiI. Further analyses of normal-state transport properties provide evidence of quantum criticality, i.e., non-Fermi-liquid behavior and divergence of quasiparticle effective mass near Pc = 7 GPa. Our findings not only establish CeSiI as the first vdW heavy-fermion superconductor but also highlight an unconventional nature for the Kondo coherent state at T* at ambient pressure, hence opening a new avenue to study the interplay of strong electron correlation, Kondo hybridization, magnetism, and unconventional SC in the vdW heavy-fermion systems.