Observation of Dirac electrons in germanene on diboride thin films

Germanene is a single atom thick honeycomb lattice of Ge atoms. Just like silicene, free-standing germanene is predicted to feature $\pi $ bands forming graphene-like Dirac cones [1]. Epitaxial germanene was already claimed to form on number of substrates [2-4], but no evidence for the existence of a $\pi $ electronic system has been reported yet. In the present work, we demonstrate experimentally that Ge atoms segregated on the (0001) surface of zirconium diboride (ZrB$_{\mathrm{2}})$ thin films grown on Ge(111) form a germanene layer. ZrB$_{\mathrm{2}}$(0001) with germanene is (3$\surd $3X3$\surd $3)-reconstructed at low-temperature and ($\surd $3X$\surd $3)-reconstructed at room temperature. The (3$\surd $3X3$\surd $3) reconstruction originates from the matching of this unit cell with the (4X4) unit cell of a Ge honeycomb lattice. Evidence for its germanene nature stems from the observation of the Dirac cone-like dispersion at the K point of its Brillouin zone. [1] S. Cahangirov et al., Phys Rev. Lett. 102, 236804 (2009). [2] L. Li et al., Adv. Mater. 26, 4820 (2014). [3] M. E. D\'{a}vila et al., New J. Phys. 16, 095002 (2014). \newline [4] M. Derivaz et al., Nano Lett. 15, 2510 (2015).