Superconductivity in FeSe enhanced and modulated by quantum geometetry*

While the dispersion relation in the band structure is a fundamental property of electron systems in solids, quantum geometry in the band structure is essential for various many-body states and unconventional responses in quantum materials. In particular, the real part of the quantum geometric tensor, namely the Fubini-Study quantum metric, is recently attracting attention, although the imaginary part is well known as the Berry curvature.

While research of quantum geometry in superconductors was triggered by the studies of flat band systems [1], quantum geometry is essential in a broader class of superconductors. We show that Fe-based superconductors are playgrounds of quantum geometry in superconductors.

We formulate the superfluid weight in unconventional superconductors based on the geometric properties of Bloch electrons. We apply the formula to a model of monolayer FeSe obtained by first-principles calculation. Our numerical calculations point to a significant enhancement of the Berezinskii-Kosterlitz-Thouless transition temperature due to the geometric contribution to the superfluid weight [2], which is not included in the Fermi liquid theory. This means that superconductivity in monolayer FeSe is significantly enhanced by quantum geometry. The quantum geometric effect on the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state acquiring a finite center of mass momentum in Cooper pairs is also studied [3]. The quantum metric gives a negative contribution to the superfluid weight in some unconventional superconducting states and stabilizes the FFLO state.

These results reveal that the geometric properties of Bloch electrons play an essential role in superconductors and pave the way for clarifying hidden aspects of superconductivity from the viewpoint of quantum geometry.

 

[1] S. Peotta and P. Törmä, Nat. Commun. 6, 8944 (2015).

[2] T. Kitamura, T. Yamashita, J. Ishizuka, A. Daido, Y. Yanase, Phys. Rev. Research 4, 023232 (2022).

[3] T. Kitamura, A. Daido, Y. Yanase, Phys. Rev. B 106, 184507 (2022).