Proceedings of nanoGe September Meeting 2017 (NFM17)
Publication date: 20th June 2016
Geometry, whether on the atomic or nanoscale, is a key factor for the electronic band structure of materials. Some specific geometries give rise to novel and potentially useful electronic bands. For example, a honeycomb lattice leads to Dirac-type bands where the charge carriers behave as massless particles. Theoretical predictions are triggering the exploration of novel two-dimensional (2D) geometries, such as graphynes and the kagomé and Lieb lattices.
The Lieb lattice is the 2D analogue of the 3D lattice exhibited by perovskites: it is a square-depleted lattice, which is characterized by a band structure featuring Dirac cones intersected by a flat band. Whereas photonic and cold-atom Lieb lattices have been demonstrated1, an electronic equivalent in 2D is difficult to realize in an existing material. In principle, lithography could be used to impose a Lieb pattern on a 2D electron gas. Alternatively, a strategy similar to the one employed for generating artificial graphene could be used2.
Here, we present an electronic Lieb lattice formed by the surface state electrons of Cu(111) confined by an array of carbon monoxide molecules positioned with a scanning tunnelling microscope3. Using scanning tunnelling microscopy, spectroscopy and wavefunction mapping, we confirm the predicted characteristic electronic structure of the Lieb lattice. The experimental findings are corroborated by muffin-tin and tight-binding calculations. At higher energies, second-order electronic patterns are observed, which are equivalent to a super-Lieb lattice.
The Cu(111)/CO system is an ideal model system, as it allows one to tune parameters that cannot be easily varied in a real solid-state material. The inherent versatility and the direct access to structural and electronic characterization allow a reality check for advanced theory and a first step in the design of truly novel electronic materials.
[1] D. Guzmán-Silva et al., New J. Phys. 16, 063061 (2014); S. Taie et al., Science Adv. 1, 1500854 (2015)
[2] K.K. Gomes et al., Nature 483, 306–310 (2012)
[3] M.R. Slot et al., Nat. Phys. (2017), doi:10.1038/nphys4105
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