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

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.

💡 Research Summary

CeSiI is a newly discovered exfoliable van‑der‑Waals (vdW) heavy‑fermion metal that exhibits a Kondo coherence temperature T*≈50 K and a long‑range incommensurate antiferromagnetic (AF) order at TN≈7.5 K under ambient conditions. To elucidate the interplay between Kondo hybridization, magnetism, and possible unconventional superconductivity, the authors performed systematic high‑pressure (HP) electrical‑transport measurements on high‑quality single crystals (RRR≈8) up to 14 GPa and down to 50 mK.

In the low‑pressure regime (2–6 GPa) both T* and TN shift to lower temperatures: T* drops from ~36 K at 2 GPa to ~21 K at 6 GPa, while TN is reduced from ~5 K to ~3 K. This simultaneous suppression cannot be explained by a simple enhancement of Kondo screening, which normally pushes T* upward as pressure increases; instead the authors suggest that crystal‑field effects or spin‑scattering processes may be involved.

Above ~6 GPa the trend reverses: T* rises sharply, reaching ~110 K at 11 GPa, whereas the AF anomaly disappears completely near 6 GPa, indicating a pressure‑driven AF quantum critical point (QCP). Concomitantly, a superconducting (SC) dome emerges in a narrow pressure window of 6–9 GPa. At 7 GPa the onset of SC appears at Tc,onset≈0.24 K with zero resistance at Tc,zero≈0.19 K; the dome peaks at Tc≈0.24 K around 7 GPa and vanishes above 11 GPa.

Magnetic‑field studies reveal an upper critical field Bc2(0) of about 3 T at 7 GPa, which exceeds the Pauli paramagnetic limit Bp=1.84 Tc by a factor of 4–7, strongly indicating an unconventional pairing state (e.g., spin‑triplet or strong spin‑orbit coupling). The temperature dependence of Bc2 follows the Werthamer‑Helfand‑Hohenberg (WHH) model, supporting the robustness of the extracted Bc2 values.

The normal‑state resistivity was analyzed using ρ(T)=ρ0+ATⁿ. The exponent n evolves from the Fermi‑liquid value n≈2 at low pressure, to a non‑Fermi‑liquid linear‑in‑T behavior (n≈1) at the QCP, and back to n≈2 at higher pressures. The coefficient A, proportional to the square of the quasiparticle effective mass, diverges near Pc≈7 GPa: A increases by more than an order of magnitude when approaching Pc from below, then drops sharply for P>9 GPa. Power‑law fits A∝|P−Pc|⁻ᵝ give asymmetric exponents (β≈0.5 below Pc, β≈1.25 above Pc), suggesting different critical regimes on either side of the QCP, possibly reflecting a change in Fermi‑surface topology or a transition from a localized‑f to itinerant‑f regime.

From the slope of Bc2(T) near Tc, the effective mass enhancement m*/m0 is estimated to be ≈8.5 at 7 GPa, consistent with the A‑coefficient divergence. The close correlation between the peak of the SC dome and the maximum of A indicates that critical spin fluctuations associated with the AF QCP likely mediate the pairing.

The authors discuss two possible QCP scenarios: (i) a spin‑density‑wave (SDW) type where the AF order stems from a Fermi‑surface instability of a heavy‑fermion band (as in CeCoIn5), and (ii) a Kondo‑breakdown type where the f‑electrons remain localized up to the QCP and the Fermi surface reconstructs abruptly (as proposed for CeRhIn5 and YbRh2Si2). The similarity of the T‑P phase diagram of CeSiI to that of CeRhIn5 favors the latter, but magnetic frustration inherent to the triangular lattice of CeSiI could also broaden the critical region into a spin‑liquid‑like state.

Beyond the fundamental physics, CeSiI represents the first vdW heavy‑fermion superconductor, opening a pathway to study heavy‑fermion phenomena in truly two‑dimensional systems. Its exfoliable nature enables mechanical thinning, strain engineering, surface doping, and heterostructure fabrication, offering unprecedented control over dimensionality and quantum criticality. The work thus establishes CeSiI as a versatile platform for exploring the interplay of strong correlations, Kondo hybridization, magnetism, and unconventional superconductivity in low‑dimensional quantum materials.