Kagome superconductors AV3βSb5β (A = K, Rb, Cs) provide a fertile
playground for studying various intriguing phenomena such as non-trivial band
topology, superconductivity, giant anomalous Hall effect, and charge density
wave (CDW). Remarkably, the recent discovery of C2β symmetric nematic phase
prior to the superconducting state in AV3βSb5β has drawn enormous
attention, as the unusual superconductivity might inherit the symmetry of the
nematic phase. Although many efforts have been devoted to resolve the charge
orders using real-space microscopy and transport measurements, the direct
evidence on the rotation symmetry breaking of the electronic structure in the
CDW state from the reciprocal space is still rare. The underlying mechanism is
still ambiguous. Here, utilizing the micron-scale spatially resolved
angle-resolved photoemission spectroscopy, we observed the fingerprint of band
folding in the CDW phase of KV3βSb5β, which yet demonstrates the
unconventional unidirectionality, and is indicative of the rotation symmetry
breaking from C6β to C2β. We then pinpointed that the interlayer coupling
between adjacent planes with Ο-phase offset in the 2Γ2Γ2 CDW
phase would lead to the preferred twofold symmetric electronic structure.
Time-reversal symmetry is further broken at temperatures below βΌ 40 K as
characterized by giant anomalous Hall effect triggered by weak magnetic fields.
These rarely observed unidirectional back-folded bands with time-reversal
symmetry breaking in KV3βSb5β may provide important insights into its
peculiar charge order and superconductivity