Pairing Mechanism in Hund's Metal Superconductors and the Universality of the Superconducting Gap to Critical Temperature Ratio

Tsung Han Lee, Andrey Chubukov, Hu Miao, Gabriel Kotliar

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Abstract

We analyze a simple model containing the physical ingredients of a Hund's metal, the local spin fluctuations with power-law correlators, (Ω0/|Ω|)γ, with γ greater than one, interacting with electronic quasiparticles. While the critical temperature and the gap change significantly with varying parameters, the 2Δmax/kBTc remains close to twice the BCS value in agreement with experimental observations in the iron-based superconductors (FeSC).

Original languageEnglish (US)
Article number187003
JournalPhysical review letters
Volume121
Issue number18
DOIs
StatePublished - Nov 1 2018

Bibliographical note

Funding Information:
T.-H.L. and G.K. were supported by the NSF Grant No. DMR-1733071. A.C. was supported by the NSF Grant No. DMR-1523036. H.M. is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Early Career Award Program under Award No. 1047478.

Funding Information:
In this work, we build on the recent understanding of the physics of the Hund’s metal and studied a phenomenological γ -model describing the superconductivity mediated by a bosonic propagator with a power-law frequency dependence, λ ( Ω ) ∝ 1 / | Ω | γ . This model captures the essence of the transition from a Hund’s metal to a superconductor at a temperature comparable to or higher than a crossover temperature between non-Fermi-liquid and Fermi-liquid behavior [9,11] . We use the model to explore the main characteristics of the pairing gap and T c , ignoring the complications such as the multiorbital or multiband structure of FeSCs. We find 2 Δ max / T c to be independent of the interaction strength and equal to 7.2–7.3 if we use γ = 1.2 obtained from the three-band Hubbard model. These results are in surprisingly good agreement with recent experiments which argued that 2 Δ max / T c ≈ 7.2 is the same in at least two FeSCs: LiFeAs and FeTe 0.55 Se 0.45 [14] . It would be interesting to extend these observations to a more realistic description of the materials, taking into account the multiorbital nature of the problem, and the fact that, in Hund’s metals, the power-law behavior of local spin susceptibility holds in an intermediate temperature range between a Fermi-liquid regime at low temperatures and a high temperature regime where the orbitals and the spins are both quasi-atomic-like. We would like to thank Ar. Abanov, K. Haule, K. Stadler, J. VonDelft, and Y. Wu for numerous discussions on the subject of Hund’s metals and superconductivity in the γ -model. T.-H. L. and G. K. were supported by the NSF Grant No. DMR-1733071. A. C. was supported by the NSF Grant No. DMR-1523036. H. M. is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Early Career Award Program under Award No. 1047478. [1] 1 Y. Kamihara , T. Watanabe , M. Hirano , and H. Hosono , J. Am. Chem. Soc. 130 , 3296 ( 2008 ). 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