Charge-order-assisted topological surface states and flat bands in the kagome superconductor CsV$_3$Sb$_5$

1Hefei National Laboratory for Physical Sciences at the Microscale, Department of Physics and CAS Key Laboratory of Strongly-coupled Quantum Matter Physics, University of Science and Technology of China, Hefei, Anhui 230026, China 2Swiss Light Source, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland 3Materials Department and California Nanosystems Institute, University of California Santa Barbara, Santa Barbara, California 93106, USA 4Department of Physics, City University of Hong Kong, Kowloon, Hong Kong