Heavy Fermion Superconductors Violate Pauli Limit with Enhanced Critical Fields and Ratios Near Half Filling

Heavy-fermion superconductors present a fascinating puzzle, often exhibiting superconductivity at remarkably high magnetic fields that defy conventional limits, and a new investigation explores the underlying mechanisms driving this behaviour. Yan-Xiao Wang and Yin Zhong, both from Lanzhou University, lead a study demonstrating how the unique electronic structure of these materials enhances superconductivity’s resilience against magnetic fields. The team’s analysis reveals that approaching a ‘near-flat-band’ configuration, a region where electrons have very low energy and move slowly, significantly boosts the critical magnetic field at which superconductivity is destroyed. This enhancement arises because the reduced electron velocity weakens the disruptive effects of the magnetic field, effectively stabilising the superconducting state and offering a new understanding of the factors governing high-field superconductivity in these complex materials.

Flat Bands and Enhanced Superconducting Limits

Heavy-fermion superconductors often exhibit upper critical fields exceeding the conventional Pauli limit, a long-standing challenge to established theories of superconductivity. Recent investigations suggest that specific band structures, particularly those featuring near-flat bands, can significantly enhance the critical field. This research focuses on understanding how the topological properties of the electronic band structure influence superconductivity in heavy fermion systems, potentially paving the way for higher-temperature and higher-field superconducting materials. The study employs a theoretical framework combining first-principles calculations and a mean-field approach to investigate this interplay.

Researchers analysed the electronic structure of representative heavy fermion materials, identifying near-flat bands near the Fermi level. These bands, characterised by a small density of states, exhibit unique properties that suppress the effects of magnetic fields, enhancing the critical field. The calculations demonstrate that these near-flat bands effectively screen the magnetic field, allowing Cooper pairs to remain stable at field strengths exceeding the conventional Pauli limit, and provide a novel mechanism for understanding and potentially engineering high-field superconductivity. The research investigates the paramagnetic limit, revealing the essential role of strong correlations and hybridized quasiparticle bands in the process of pair-breaking. A self-consistent mean-field analysis within the two-dimensional Kondo-Heisenberg model examined spin-singlet s-, extended-s-, and d-wave pairing under applied Zeeman fields, computing the critical field, transition temperature, and the Clogston-Chandrasekhar ratio. The results demonstrate that the Clogston-Chandrasekhar ratio increases sharply as the conduction filling approaches half filling, attributed to a weakly dispersive region of the lower hybridized band where a strongly reduced Fermi velocity diminishes the normal-state paramagnetic energy.

Pauli Limit Broken in Superconductors

This research investigates the possibility of exceeding the Pauli-Clogston limit in superconductivity, a theoretical upper bound on the critical magnetic field a superconductor can withstand. Recent experimental observations in materials like heavy fermion superconductors and flat-band materials suggest it can be surpassed. The paper explores the theoretical mechanisms enabling this behaviour, focusing on strong correlations, flat bands, heavy fermion systems, and non-Fermi liquid behaviour. The authors propose that a combination of strong correlations, flat bands, heavy fermion behaviour, and non-Fermi liquid characteristics can suppress the Pauli-Clogston limit and enable superconductivity to persist at higher magnetic fields, with the interplay between itinerant and localized electrons playing a crucial role.

This is significant because it challenges a long-held belief in condensed matter physics, opens up new avenues for designing novel superconductors with higher critical fields crucial for applications like MRI machines and fusion reactors, and provides insights into unconventional superconductivity. Key takeaways include that the Pauli-Clogston limit is not absolute, strong correlations are key to exceeding it, materials with flat bands and heavy fermion characteristics are promising candidates, and new physics beyond the standard Fermi liquid theory is needed to fully understand these systems. This research suggests that the landscape of superconductivity is more complex and fascinating than previously thought.

Flat Bands Stabilize Superconducting Critical Fields

This research investigates the mechanisms behind enhanced superconductivity in heavy-fermion materials, specifically focusing on how strong electron correlations influence the upper critical field. By employing the Kondo-Heisenberg model and performing detailed calculations, scientists demonstrate that proximity to a nearly flat band in the material’s electronic structure plays a crucial role in stabilizing superconductivity against magnetic fields. The team explored different types of pairing and found that the sensitivity to the Pauli limiting depends on both the curvature of the electronic bands and the specific structure of the superconducting order parameter. These results provide microscopic evidence supporting the idea that materials with nearly flat bands offer a robust pathway to achieving higher upper critical fields, and therefore more resilient superconductivity.