Observation of high-Tc superconductivity in rectangular FeSe/STO(110) monolayer

TL;DR

This study demonstrates high-temperature superconductivity in FeSe monolayers grown on STO(110) substrates, challenging the belief that C4 symmetry is necessary for high Tc in Fe-based superconductors.

Contribution

It shows that breaking C4 symmetry does not prevent high Tc superconductivity in FeSe/STO(110), expanding understanding of symmetry's role in superconductivity.

Findings

FeSe/STO(110) exhibits a 16 meV superconducting gap.

Superconductivity persists with nearly isotropic gap despite symmetry breaking.

Tc reaches around 60 K in the monolayer system.

Abstract

It is well known that superconductivity in Fe-based materials is favoured under tetragonal symmetry, whereas competing orders such as spin-density-wave (SDW) and nematic orders emerge or are reinforced upon breaking the fourfold (C4) symmetry. Accordingly, suppression of orthorhombicity below the superconducting transition temperature (Tc) is found in underdoped compounds. Epitaxial film growth on selected substrates allows the design of crystal specific lattice distortions. Here we show that despite the breakdown of the C4 symmetry induced by a 5% difference in the lattice parameters, monolayers of FeSe grown by molecular beam epitaxy (MBE) on the (110) surface of SrTiO3 (STO) substrates [FeSe/STO(110)] exhibit a large nearly isotropic superconducting (SC) gap of 16 meV closing around 60 K. Our results on this new interfacial material, similar to those obtained previously on FeSe/STO(001), contradict the common belief that the C4 symmetry is essential for reaching high Tc's in Fe-based superconductors.