Large Jahn-Teller shifts and splittings observed in halide perovskite CsPbBr3

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Despite the fundamental role of Jahn-Teller effects of first and second order in shaping structural and electronic properties, there is hardly any observation in angle-resolved photoemission of solids. In oxide and halide perovskites, band structure replicas have occasionally been reported as fingerprints of Jahn-Teller effects, but no accompanying energy shifts or splittings that would allow a conclusion about their origin. In CsPbBr3, we uncover both key signatures: Upon cooling, orthorhombic replica bands emerge which had eluded earlier studies. This includes an extra valence band maximum at Γ which can even be distinguished at room temperature. Most importantly, band narrowing along Γ-M, a splitting at Γ, and the lifting of degeneracy between nonequivalent M points, all of several 100 meV, become apparent. Temperature-dependent x-ray diffraction, used as input for band structure calculations, links these effects directly to tilts and rotations of the PbBr6 octahedra. Our results uncover a strong electron-lattice interaction which is at the heart of so-far unresolved questions concerning polaronic transport, dynamic disorder, and exciton trapping in halide perovskites.