Discovery of parity-violating chiral polar-nematic charge density wave and superconductivity in kagome metals

TL;DR

This study uncovers a spontaneously broken inversion symmetry and chiral multipolar order in kagome metals, revealing new electronic phases and controllable chiral states linked to unconventional superconductivity.

Contribution

It provides direct microscopic evidence of parity-violating chiral polar-nematic charge density waves and their coupling to superconductivity in kagome metals.

Findings

Inversion symmetry is spontaneously broken in the CDW state.

Chiral polar-nematic order exhibits ferroelectric and nematic multipolar moments.

Parity-violating pair density modulations are observed below the superconducting transition.

Abstract

Nonmagnetic kagome metals and superconductors AV3Sb5 (A = K, Rb, Cs) host unconventional charge density wave (CDW) and superconducting (SC) phases accompanied by multiple electronic symmetry breaking. Due to the centrosymmetric crystal structure, inversion symmetry has generally been assumed to hold. Here, using scanning tunneling microscopy complemented by atomic force microscopy and optical second-harmonic generation, we directly reveal that inversion symmetry in the kagome plane is spontaneously broken in the CDW state. The mixed-parity CDW state exhibits ferroelectric dipolar and nematic quadrupolar ordered moments. The coexistence and coupling between the dipole and quadrupole favor noncollinear ferro-polar and nematic alignment that breaks all mirror symmetries and gives rise to robust electronic chirality in the 3Q CDW. The multipolar coupling to in-plane electric field enables electric field control and manipulation of the chiral polar-nematic CDW state, including its chirality. Below the SC transition, we observe parity-violating pair density modulations at both the original and the CDW lattice wavevectors. Our findings of parity-violating electronic chiral multipolar order provide microscopic insights into the magnetoelectric and nonreciprocal transport, loop current order, pairing density waves, and unconventional superconductivity in kagome metals and related quantum materials.