  The recently discovered AV3Sb5 (A=K, Rb, Cs) family, possessing V kagome
nets, has received considerable attention due to the topological electronic
structure and intriguing correlated phenomena, including an exotic charge
density wave (CDW) and superconductivity. Detailed electronic structure studies
are essential to unravel the characteristics and origin of the CDW as well as
its interplay with superconductivity. Here, we present angle-resolved
photoemission spectroscopy (ARPES) measurements for CsV3Sb5 at multiple
temperatures and photon energies to reveal the nature of the CDW from an
electronic structure perspective. We present evidence for a three-dimensional
(3D) CDW order. In the process we also pinpoint a surface state attributed to a
Cs terminated surface. This state was previously attributed to band folding
band due to a CDW along the c direction or a quantum well state from quantum
confinement. The CDW expected 2-fold lattice reconstruction along c axis is
observed to be a quadrupling of the unit cell, thus for the first time directly
demonstrating the 3D nature of the CDW from the electronic structure
perspective. Moreover, this 3D CDW configuration originates from two distinct
types of distortions in adjacent kagome layers. These present results not only
provide key insights into the nature of the unconventional CDW in CsV3Sb5 but
also provides an important reference for further studies on the relationship
between the CDW and superconductivity.
