Relevant raw data for the main text Figures

As a quintessential quantum matter, realizing robust Wigner crystals and achieving their angstrom-scale imaging remain formidable challenges. Here, we devise an original charge-transfer-crystallization strategy to realize intrinsic electron solid made of 4f-electrons. The highly localized and correlated nature of 4f electrons gives rise to a heavy Wigner crystal. This unprecedented heaviness manifests as an electron solid with record-high density (2.02×10¹³/cm^-2) and melting temperature (>60 K). Q-plus atomic force microscopy and Kelvin probe force microscopy are employed to directly image the Wigner crystal, by which the sub-unit-cell localization and electrostatic influence of Wigner electron are accurately determined. Moreover, the collective tunneling is captured, underlining the quantum nature of Wigner crystal. Our discovery presents the 4f system as a tunable platform with the charge-transfer degree of freedom to study 2D many-body correlation of heavy electrons.