Emergence of unidirectional coherent quasiparticles from high-temperature rotational symmetry broken phase of AV3Sb5 kagome superconductors

Kagome metals AV3Sb5 display a rich phase diagram of correlated electron states, including superconductivity and novel density waves. Within this landscape, recent experiments reveal signs of a new transition below T ~ 35 K attributed to the highly sought-after electronic nematic phase that spontaneously breaks rotational symmetry of the lattice. We use spectroscopic-imaging scanning tunneling microscopy to study atomic-scale signatures of electronic symmetry breaking as a function of temperature across several materials in this family: CsV3Sb5, KV3Sb5 and Sn-doped CsV3Sb5. We find that rotational symmetry breaking onsets universally at a high temperature in these materials, toward the 2 x 2 charge density wave (CDW) transition temperature T*. At a significantly lower temperature of about 30 K, we discover a striking emergence of the quantum interference of coherent quasiparticles, a key signature for the formation of a coherent electronic state. These quasiparticles display a pronounced unidirectional reciprocal-space fingerprint, which strengthens on approaching the superconducting state. Our experiments reveal that the high-temperature charge ordering states are separated from the superconducting ground state by an intermediate-temperature regime with coherent unidirectional quasiparticles. Their emergence that occurs significantly below the onset of rotational symmetry breaking is phenomenologically different compared to high-temperature superconductors, shedding light on the complex nature of electronic nematicity in AV3Sb5 kagome superconductors.