Unidirectional coherent quasiparticles in the high-temperature rotational symmetry broken phase of AV₃Sb₅ kagome superconductors

Kagome metals AV₃Sb₅¹ (where the A can stand for K, Cs or Rb) display a rich phase diagram of correlated electron states, including superconductivity^2–4 and density waves^5–7. Within this landscape, recent experiments have revealed signs of a transition below approximately 35 K attributed to an electronic nematic phase that spontaneously breaks the rotational symmetry of the lattice⁸. Here we show that the rotational symmetry breaking initiates universally at a high temperature in these materials, towards the 2 × 2 charge density wave transition temperature. We do this via spectroscopic-imaging scanning tunnelling microscopy and study the atomic-scale signatures of the electronic symmetry breaking across several materials in the AV₃Sb₅ family: CsV₃Sb₅, KV₃Sb₅ and Sn-doped CsV₃Sb₅. Below a substantially lower temperature of about 30 K, we measure the quantum interference of quasiparticles, a key signature for the formation of a coherent electronic state. These quasiparticles display a pronounced unidirectional feature in reciprocal space that strengthens as the superconducting state is approached. Our experiments reveal that high-temperature rotation symmetry breaking and the charge ordering states are separated from the superconducting ground state by an intermediate-temperature regime with coherent unidirectional quasiparticles. This picture is phenomenologically different compared to that in high-temperature superconductors, shedding light on the complex nature of rotation symmetry breaking in AV₃Sb₅ kagome superconductors.