  Recently, competing electronic instabilities, including superconductivity and
density-wave-like order, have been discovered in vanadium-based kagome metals
AV3Sb5 (A = K, Rb, Cs) with a nontrivial band topology. This finding stimulates
wide interests to study the interplay of these competing electronic orders and
possible exotic excitations in the superconducting state. Here, in order to
further clarify the nature of density-wave-like transition in these kagome
superconductors, we performed 51V and 133Cs nuclear magnetic resonance (NMR)
measurements on the CsV3Sb5 single crystal. A first-order phase transition
associated with orbital ordering is revealed by observing a sudden splitting of
orbital shift in 51V NMR spectrum at the structural transition temperature Ts ~
94 K. In contrast, the quadrupole splitting from a charge-density-wave (CDW)
order on 51V NMR spectrum only appears gradually below Ts with a typical
second-order transition behavior, suggesting that the CDW order is a secondary
electronic order. Moreover, combined with 133Cs NMR spectrum, the present
result also confirms a three-dimensional structural modulation with a 2ax2ax2c
period. Above Ts, the temperature-dependent Knight shift and nuclear
spin-lattice relaxation rate (1/T1) further indicate the existence of
remarkable magnetic fluctuations from vanadium 3d orbitals, which are
suppressed due to orbital ordering below Ts. The present results strongly
support that, besides CDW order, the previously claimed density-wave-like
transition also involves a dominant orbital order, suggesting a rich orbital
physics in these kagome superconductors.
