Jian Wang group and collaborators report orbital-selective high-temperature Cooper pairing developed in the two-dimensional limit
For superconductors with orbital multiplicity, considering Hund’s rule coupling, the interorbital charge fluctuations can be suppressed and orbital differentiation may emerge. Such orbital-decoupling effect dictates that the physics strongly depends on each orbital separately. Naturally, Cooper pairing can also be orbital-selective and accordingly anisotropic, where electrons of specific orbital(s) primarily bind to form Cooper pairs. The orbital-selective pairing is unique to spin fluctuations, and has thus become increasingly pivotal in clarifying the pairing ‘enigma’ particularly for multiband high-temperature superconductors.
To date, the orbitally selective pairing was discovered only in bulk FeSe (one kind of Fe-based superconductors), while the 2D-limit counterpart, one-unit-cell (1-UC) FeSe grown on SrTiO3(001) substrate, is scarcely explored. For 1-UC FeSe/SrTiO3(001), although the enhanced high Tc of 55–65 K comparing to Tc of ~9 K in bulk FeSe has encouraged extensive investigations, the central pairing issues remain debated partially due to the absence of Γ-hole pockets. One of the experimental challenges is how to effectively distinguish the different bosonic modes (e.g., spin fluctuations vs. phonon) for mediating the coherent Cooper pairs. If there exists orbital-selective pairing is crucial to the widely debated pairing in 1-UC FeSe.
Previously, in 1-UC FeSe, Jian Wang group observed the magnetic-excitation-like bosonic mode [Nano Letters 19, 3464 (2019)] and the sign-changing gap [PRL 123, 036801 (2019)], paving the way to further understanding the interfacial high-Tc superconductivity. Recently, Prof. Jian Wang, in collaboration with Prof. Peter J. Hirschfeld from University of Florida and Prof. Brian M. Andersen from University of Copenhagen, reports orbital-selective Cooper pairing developed in 1-UC FeSe/SrTiO3 using in situ STM. Based on high-resolution quasiparticle-interference imaging, E(q) dispersions are extracted along different directions. Further analysis reveals anisotropic self-energy, a ‘fingerprint’ of anti-ferromagnetic spin-fluctuations. Meanwhile, systematic Dynes-formula fittings to several sets of temperature-dependent spectra yield anisotropic gaps. Theoretically, to reproduce such gap anisotropy, it is necessary to incorporate orbitally selective effects of electronic correlations within a spin-fluctuation pairing calculation, where the dxy orbital becomes coherence-suppressed. These results support 1-UC FeSe as a high-Tc superconductor with orbital-selectivity in 2D limit despite quantum fluctuations. The self-energy data based on measured quasiparticle-interference patterns also independently confirm the orbitally selective paring scenario. Since FeSe is the building block of iron chalocogenides, our findings imply the general existence of orbital selectivity in iron-based superconductors and the universal significance of electron correlations in high-Tc superconductors.
The paper was online published in Nano Letters on Apr. 13, 2022. Chaofei Liu is the first author, and Prof. Jian Wang is the corresponding author. This work was financially supported by the National Natural Science Foundation of China, the National Key Research and Development Program of China, Beijing Natural Science Foundation, and Strategic Priority Research Program of Chinese Academy of Sciences.
Paper link: https://pubs.acs.org/doi/10.1021/acs.nanolett.1c04863
Figure: (a-b) Bogoliubov quasiparticle-interference patterns. (c) E(q) dispersions at selected directions, showing bosonic-coupling-induced kinks, which coincide in energy with the hump representing the bosonic mode in dI/dV spectrum. (d) Angle-dependent gap Δ(θ). (e) Schematic of gap structure.