The anomalous stress dependence of multiband superconductors: Sr2RuO4 compared with AFe2As2 (A = K, Rb, Cs)

Sr2RuO4 and AFe2As2 are strongly correlated Hund’s metals that are close to a strain-driven electronic instability, mainly manifested in the dxy-orbital character of their band structure.1,2 Both systems exhibit the typical features of multiband superconductors. The superconducting order causes a distortion of the crystal lattice, which determines its stress dependence and is related to the superconducting gap structure.3 We have studied the lattice distortion of Sr2RuO4 and AFe2As2 single crystals by measuring their thermal expansion as a function of temperature between 20 mK and 4 K. Unlike measurements under finite uniaxial pressure, the thermal expansion does not break the symmetry, but allows to track the stress dependence below the superconducting transition temperature Tc. The AFe2As2 crystals show below Tc the usually observed behavior, where the anisotropy of the lattice distortion remains approximately constant with temperature. In Sr2RuO4, however, there is a clear change from an almost isotropic to a highly anisotropic stress dependence as temperature decreases. This suggests that the superconductivity in Sr2RuO4, in contrast to AFe2As2, is dominated by bands with significantly different gap symmetry.
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[1] A. Chronister et al., npj Quantum Mater. 7, 113 (2022)
[2] F. Eilers et al., Phys. Rev. Lett. 116, 237003 (2016)
[3] R. Joynt et al., Phys. Rev. B 32, 6074 (1985)