Odd-parity quadrupole order and induced nonreciprocal transport in the kagome metal CsTi${}_3$Bi${}_5$ driven by quantum interference

Kagome metals present a fascinating platform of quantum phases thanks to the interplay between the geometric frustration and strong electron correlation. Here, we propose the emergence of the electric odd-parity bond order (BO) that originates from the intra-unit-cell odd-parity configuration in recently discovered kagome metal CsTi${}_3$Bi${}_5$. The predicted E1u BO is induced by the beyond-mean-field mechanism, that is, the quantum interference among different sublattice spin fluctuations. Importantly, the accompanied nematic deformation of the Fermi surface is just ~1% while the intensity of the quasiparticle interference signal exhibits drastic nematic anisotropy, consistent with the scanning tunneling microscope measurements in CsTi${}_3$Bi${}_5$. The present odd-parity BO triggers interesting phenomena, such as the non-linear Hall effect and emergent electromagnetism.