Role of Spin Vortex Crystal Fluctuations in the Pressure-induced Half-collapse Transition in CaKFe₄As₄ Superconductor

Structural transitions under pressure are studied for the recently discovered Fe-based superconductor CaKFe₄As₄ using density functional theory. Under a hydrostatic pressure of 4 GPa, this material undergoes a transition to a half-collapsed phase, which is the first known example of such transition [PRB 96, 140501(R) (2017)]. A half-collapsed phase is marked by an enhancement of only half of the As-As bonds, which is sufficient to fully suppress bulk superconductivity. Good agreement between the simulated structures and the measured ones is obtained when a spin vortex configuration is imposed on the Fe sublattice. Interestingly, spin fluctuations of similar nature are confirmed by the NMR analysis [PRB 96, 104512 (2017)]. Furthermore, electron doping leads to condensation of these fluctuations and the formation of a spin vortex crystal [arXiv:1706.01067], which is unique among iron pnictides.

This work was done in collaboration with Paul Canfield, Anna Böhmer, Sergey L. Bud'ko, Yuji Furukawa, Udhara Kaluarachchi, William Meier, Peter Orth (Ames Lab), Rafael Fernandes (Univ Minnesota) and Christian Batista (Univ Tennessee).