Intrinsic Heavy Wigner Crystal Forged by Transferred 4f Electrons

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 an 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 (2.02×10^{13}/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 electrons are accurately determined. Moreover, the collective tunneling is captured, underlining the quantum nature of a Wigner crystal. Our discovery presents the 4f system as a tunable platform with the charge-transfer degree of freedom to study the 2D many-body correlation of heavy electrons.