Unconventional superconductivity in ScIr 2 chiral crystal with a kagome lattice

The gist The study reports evidence of unconventional superconductivity in the ScIr2−xSix family by combining muon-spin spectroscopy measurements with band-structure calculations, revealing that its pairing is well described by a two-gap model where one gap has nodes, and it is classified as a topological chiral crystal due to the presence of an Ir chiral chain in the low-T phase.

Structural Phase Transition

The parent compound ScIr2 undergoes a structural phase transition from a high-T cubic phase to a low-T rhombohedral phase, which is suppressed by Si substitution in ScIr2−xSix (Page 1). This structural transition can also be viewed as a nematic transition that breaks the rotational symmetry (Page 3). The low-T rhombohedral phase not only hosts a kagome layer formed by Ir atoms but it also exhibits a chiral chain of Ir atoms (Page 3). The details of SXRD refinements suggest that the actual Si concentration is slightly lower than the nominal value, namely, ScIr1.82Si0.18 (Page 3).

Superconducting Pairing Symmetry

The superconducting pairing of ScIr2−xSix is well described by a two-gap model where one of the gaps has nodes, indicating an unconventional SC nature (Page 3). The temperature-dependent magnetic penetration depth measurements via transverse-field (TF-) µSR show that the fitting models rule out a d-wave model with line nodes and that the s-wave model without gap nodes shows better agreement with the experimental data (Page 3). The two-gap scenario, specifically the (s + d)-wave model, exhibits much better agreement with electronic specific heat data across the full temperature range, resulting in the smallest deviation (Page 4).

Topological and Chiral Properties

ScIr2 can be classified as a topological chiral crystal that admits a unique band topology, implying fermionic excitations with large Chern numbers (Page 1). In the low-T rhombohedral phase, the Ir atoms on 9d sites form a chiral chain along the [001]-direction (Page 3). This structure breaks inversion symmetry and belongs to the noncentrosymmetric superconductor (NCSC) family, where mixed-parity pairing may occur (Page 8). The presence of a large band splitting due to the antisymmetric SOC suggests that ScIr2 is a promising candidate for exploring mixed-parity pairing (Page 8). Furthermore, ScIr2 shows spin-polarized Fermi arc states spanning the entire surface Brillouin zone, which can be verified by photoemission techniques (Page 8).

Electronic Structure and Correlations

In both phases, the Ir-5d and Sc-4d orbitals dominate the DOS close to the Fermi level EF (Page 6). The dominance of the Ir 5d-orbitals suggests a large SOC effect, which plays an important role in determining the electronic properties of ScIr2−xSix (Page 6). Band splitting due to the SOC is significant along the Γ − L direction of ScIr2, where also the flat bands exist (Page 6). The rhombohedral phase lacks an inversion center and its antisymmetric SOC lifts the degeneracy of electronic bands, causing each flat band to split into two bands along the Γ − L direction (Page 6). The electrons on the flat bands exhibit strong correlations and large effective masses, which may explain the unconventional superconducting pairing revealed by both TF-µSR (Page 4) and specific-heat measurements (Page 4).

Conclusion

ScIr2−xSix represents a very rare case where Ir atoms host both a kagome lattice and structural chirality, providing an ideal platform for investigating the interplay between unconventional and topological SC, flat bands, and chirality (Page 9). The study concludes that the multigap characteristics are further supported by band-structure calculations (Page 4). The ScIr2−xSix family is a valuable platform for investigating both bulk- and surface SC (Page 9).

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arXiv:2607.16689v1 [cond-mat.supr-con] 18 Jul 2026

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1 Introduction

Kagome lattice, a geometrically frustrated network of corner-sharing triangles, has become one of the hottest topics in condensed matter physics (Page 2). Materials with a kagome lattice harbor nontrivial electronic structures generated by its unique geometry, including Dirac fermionic excitations, dispersionless flat bands, van Hove singularities (VHSs), etc. Flat bands arise from the destructive quantum interference of localized states which, together with the VHSs, imply strong electronic correlations (Page 2). In kagome materials, the interplay of band topology, spin-orbit coupling (SOC), electronic correlations, and magnetism leads to rich and exotic quantum phenomena [1–10], such as, superconductivity (SC), charge-density waves (CDW), spin-density waves (SDW), topological spin textures, quantum spin liquid (QSL), etc. Magnetic kagome materials that exhibit magnetic frustration, QSL states [11–14], or anomalous Hall effect [1, 3, 10, 15, 16] have been widely studied (Page 2). However, their superconducting counterparts, in particular unconventional, remain largely unexplored due to the limited availability of suitable kagome materials (Page 2). Although different families of superconducting kagome materials have been experimentally discovered or theoretically predicted, most of them either behave as conventional superconductors or have not yet been synthesized (Page 2). One of the exceptions is the AV3Sb5 (A = K, Rb, Cs) family [7, 17, 18], which becomes superconducting below Tc ≈ 0.9-2.5 K and exhibits a variety of exotic properties, including unconventional SC [19, 20], chiral CDW order [21], pair-density wave [22], nematic order [23], etc. While, in (K,Rb)V3Sb5, the evolution of nodal to nodeless SC can be tuned by external pressure [20], growing evidence reveals nodeless SC with multiple gaps in CsV3Sb5 [24–30]. The spin-lattice relaxation rate shows a Hebel-Slichter coherence peak just below Tc, indicating that CsV3Sb5 is a conventional superconductor [25], consistent with the absence of spontaneous magnetic fields in the superconducting state [28, 31]. Moreover, the SC order parameter of the AV3Sb5 family is most likely a spin singlet at ambient pressure [25], reflected in a distinct drop of the Knight shift below Tc (Page 2).

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2.1 Structural Phase Transition

Since the ScIr2−xSix family exhibits a two-dome-like superconducting phase diagram with maxima located at x = 0 and 0.25 (Figure 1a), we synthesized the corresponding ScIr2 and ScIr1.75Si0.25 compounds and systematically investigated their superconducting properties (Page 3). Room-temperature powder x-ray diffraction (XRD) refinements confirm their cubic crystal structure with a space group F d3¯m (No. 227) (Figure S1, Supporting Information) for ScIr1.82Si0.18, and each compound exhibits a distinct temperature-dependent electrical resistivity ρ(T) (Figure 1b) (Page 3). SXRD refinements of ScIr2 at T = 80 K show a significantly better agreement with the rhombohedral space group R32 (No. 155) than with the cubic group F d3¯m (No. 227) (Figure 1c) compared to ScIr1.82Si0.18 at both 80 K and 280 K are consistent with the F d3¯m space group (Figure 1e) (Page 3). The low-T rhombohedral phase is a distorted variant of the high-T cubic phase, reflecting the displacement of Ir atoms from the ideal planar kagome layer (Page 3). Interestingly, at 9d sites also form chiral chains in the low-T phase (Page 3). Therefore, ScIr2 can be classified as a topological chiral crystal that admits a unique band topology, implying fermionic excitations with large Chern numbers (Page 3).

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2.2 Characterization of Superconductivity

The SC of ScIr2−xSix was characterized via measurements of magnetic susceptibility and electrical resistivity (Page 4). The upper critical fields determined from the electrical resistivity measured under various magnetic fields are µ0Hc2(0) = 0.65(3) and 4.8(1) T for ScIr2 and ScIr1.82Si0.18, respectively (Page 4). For both compounds, the upper critical fields are below the Pauli-limiting field (i.e., µ0HP = 1.86Tc), meaning the orbital pair-breaking mechanism is dominant in the ScIr2−xSix family (Page 4). The calculated ξ(0) values are 22.5 and 8.3 nm for ScIr2 and ScIr1.82Si0.18, respectively (Page 4).

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2.3 TF-µSR and Superconducting Pairing

The symmetry of superconducting pairings is crucial to understand the mechanism of SC (Page 5).

1. textbf Intellectual Property Mapping for Novel Materials Development: Improved AI can perform Electronic Band Structure and Topology Aspects calculations using DFT to predict band splitting ESOC, which is about 275 meV in the cubic phase (Figure 6c) and 264 meV in the rhombohedral phase (Figure 6g) for ScIr2. This allows AI to guide material synthesis toward target electronic structures by predicting how Si substitution introduces a large amount of hole carriers into the ScIr2−xSix family.

2. textbf Superconducting Pairing Symmetry Classification: Improved AI can utilize the results from TF-µSR and Superconducting Pairing analysis, which shows that the (s + d)-wave model exhibits a much better agreement with the Ce/T data across the full temperature range, to automatically classify SC pairing symmetries for new kagome systems.

3. textbf Structural Phase Transition Prediction: Improved AI can correlate structural order with superconducting properties, noting that the low-T rhombohedral phase lacks an inversion center and its antisymmetric SOC lifts the degeneracy of electronic bands. This enables the AI to predict when a structural transition, like from a high-T cubic phase (centrosymmetric)- to a low-T rhombohedral phase (noncentrosymmetric), is likely to occur based on doping levels.

4. textbf Topological Crystal Identification: Improved AI can analyze band topology features, such as the observation that the rhombohedral phase of ScIr2 belongs to one of the chiral space groups, to identify materials that qualify as a topological chiral crystal exhibiting phenomena like giant helicoid Fermi arcs.

5. textbf Model Comparison and Validation: Improved AI can systematically compare fitting results across multiple theoretical models, noting that the (s+d)-wave model yields a zero-temperature effective magnetic penetration depth λ0 = 425 and 680 nm for ScIr2 and ScIr1.82Si0.18. This capability allows the AI to validate which pairing mechanism is most likely dominant by assessing the reduced least-squares deviations χ2r derived from both µSR and Ce/T data analysis.