Quantum heavy-fermion metal superconducts at 240 milliKelvin

Researchers led by Tong Shi demonstrate that CeSiI becomes superconducting at 240 milliKelvin under pressure, revealing unusual behavior in this van der Waals heavy-fermion metal. The study details a V-shaped, non-monotonic dependence of the Kondo coherent state on applied pressure, a response differing from most materials. This proximity of superconductivity to antiferromagnetic instability, combined with a large upper critical field, suggests an unconventional pairing mechanism is at work within CeSiI, offering a new platform for exploring strong electron correlations.

CeSiI Exhibits Superconductivity at 240 milliKelvin under Pressure

CeSiI exhibits superconductivity at a remarkably low 240 milliKelvin when subjected to pressure, positioning the material as a new platform for exploring unconventional superconductivity. The study focused on understanding strong electron interactions within heavy-fermion systems, and the observed pressure-induced superconductivity provides crucial insight into these complex phenomena. The emergence of a superconducting state within CeSiI coincides with the suppression of its long-range antiferromagnetic order, suggesting a novel pairing mechanism is at play.

A large upper critical field accompanies this proximity to antiferromagnetic instability, further supporting the idea that superconductivity in CeSiI is unconventional, distinguishing it from more traditional superconducting materials. Normal-state transport measurements revealed non-Fermi-liquid behavior and a divergence of the effective electron mass, providing additional evidence for quantum criticality within the material’s structure.

This work, published on July 28, 2026, builds on previous investigations into two-dimensional heavy-fermion systems and offers a new avenue for understanding the interplay between strong electron correlations, Kondo hybridization, magnetism, and unconventional superconductivity. The data supporting these findings are openly available via the MatELab, allowing other researchers to verify and expand upon the results. The researchers state that their findings support CeSiI as a heavy-fermion superconductor and reveal an unconventional nature for its Kondo coherence at ambient pressure, highlighting the potential of CeSiI for future exploration in condensed matter physics.

V-Shaped Pressure Dependence of Kondo Coherence in CeSiI

CeSiI exhibits a peculiar response to applied pressure, diverging from the behavior expected in most materials. Investigations reveal the Kondo coherent state within this van der Waals heavy-fermion metal does not simply increase or decrease with pressure; instead, it follows a V-shaped, non-monotonic dependence, a finding detailed in work published on July 28, 2026. This unusual characteristic suggests a complex interplay of electron interactions within the material, prompting further examination of its quantum properties.

The team’s phase diagram of CeSiI demonstrates that as pressure increases, the Kondo coherence initially strengthens, reaches a minimum, and then surprisingly begins to recover. This observed quantum criticality suggests CeSiI is poised at a point where small changes in pressure can dramatically alter its electronic properties. The investigation builds on previous studies of two-dimensional heavy-fermion systems, but the V-shaped pressure dependence of CeSiI distinguishes it as a unique case worthy of continued study, potentially unlocking new insights into the fundamental physics of these exotic materials.

The proximity of superconductivity to antiferromagnetic instability is particularly noteworthy, as it implies these two normally competing states are closely linked in CeSiI. The researchers utilized a palm cubic anvil apparatus, coupled with a specialized integrated-fin gasket, to achieve the necessary high-pressure conditions for their experiments.

Non-Fermi-Liquid Behavior Signals Quantum Criticality in CeSiI

The heavy-fermion metal CeSiI exhibits behavior characteristic of quantum criticality, evidenced by the breakdown of conventional physics at low temperatures and under applied pressure. Researchers detailed a phase diagram revealing how the material transitions between different states, finding that the temperature defining the Kondo coherent state changes in an unusual, V-shaped manner as pressure increases. This non-monotonic response, strengthening initially and then weakening, suggests CeSiI is operating near a quantum critical point, where even minor adjustments can dramatically alter its properties.

This divergence implies that electrons within CeSiI are behaving collectively, rather than as independent particles, a hallmark of strongly correlated electron systems. The emergence of superconductivity at approximately 240 milliKelvin, coinciding with the suppression of long-range antiferromagnetic order, further underscores the material’s complex interplay of quantum phenomena; magnetic fluctuations may be playing a crucial role. This accessibility is intended to foster collaboration and accelerate progress in understanding these complex materials.

Large Upper Critical Field Suggests Unconventional Pairing in CeSiI

CeSiI challenges conventional understandings of superconductivity; most materials require significantly higher temperatures to achieve this state, while this compound exhibits the phenomenon at approximately 240 milliKelvin under applied pressure. This remarkably low transition temperature immediately signals the presence of exotic physics governing electron behavior within the material’s structure. This proximity, coupled with the observation of a particularly large upper critical field, strongly suggests that electron pairing within the material is occurring through a mechanism distinct from those observed in conventional superconductors.

The upper critical field represents the magnetic field strength above which superconductivity is destroyed; a large value indicates robust pairing, but in this case, it points towards an unconventional origin. Normal-state transport measurements further support the idea of quantum criticality within CeSiI.