Emergence of large quantum oscillation frequencies in thin flakes of the kagome superconductor CsV₃Sb₅

Kagome metals AV₃Sb₅ (A = K, Rb, and Cs) are recently discovered platforms featuring an unusual charge-density-wave (CDW) order and superconductivity. The electronic band structure of a kagome lattice can host both flat bands as well as Dirac-like bands, offering the possibility to stabilize various quantum states. Here, we probe the band structure of CsV₃Sb₅ via Shubnikov-de Haas quantum oscillations on both bulk single crystals and thin flakes. Although our frequency spectra are broadly consistent with the published data, we unambiguously reveal the existence of new frequencies with large frequencies ranging from ∼2085 to ∼2717 T in thin flakes when the magnetic field is along the c axis. These quasi-two-dimensional frequencies correspond to ∼52% to 67% of the CDW-distorted Brillouin-zone volume. The Lifshitz-Kosevich analysis further uncovers surprisingly small cyclotron effective masses, of the order of ∼0.1m_e for these frequencies. Consequently, a large number of high-velocity carriers exists in the thin flake of CsV₃Sb₅. Comparing with our band-structure calculations, we argue that an orbital-selective modification of the band structure is active. Our results provide indispensable information for understanding the fermiology of CsV₃Sb₅, paving a way for understanding how an orbital-selective mechanism can become an effective means to tune its electronic properties.