  Eugene Wigner predicted long ago that when the Coulomb interactions between
electrons become much stronger than their kinetic energy, electrons crystallize
into a closely packed lattice. A variety of two-dimensional systems have shown
evidence for Wigner crystals; however, a spontaneously formed classical or
quantum Wigner crystal (WC) has never been directly visualized. Neither the
identification of the WC symmetry nor direct investigation of its melting has
been accomplished. Here we use high-resolution scanning tunneling microscopy
(STM) measurements to directly image a magnetic field-induced electron WC in
Bernal-stacked bilayer graphene (BLG), and examine its structural properties as
a function of electron density, magnetic field, and temperature. At high fields
and the lowest temperature, we observe a triangular lattice electron WC in the
lowest Landau Level (LLL) of BLG. The WC possesses the expected lattice
constant and is robust in a range of filling factors between $\nu\sim$ 0.13 and
$\nu\sim$ 0.38 except near fillings where it competes with fractional quantum
Hall (FQH) states. Increasing the density or temperature results in the melting
of the WC into a liquid phase that is isotropic but has a modulated structure
characterized by the WC's Bragg wavevector. At low magnetic fields, the WC
unexpectedly transitions into an anisotropic stripe phase, which has been
commonly anticipated to form in higher LLs. Analysis of individual lattice
sites reveals signatures that may be related to the quantum zero-point motion
of electrons in the WC lattice.
