Broken-symmetry states involving electronic degrees of freedom are extensively investigated in condensed-matter physics. In particular, long-range ordered phases involving spin or charge are routinely detected directly and can be visualized in real space. However, the orbital degree of freedom is more elusive and the experimental detection of orbital ordering generally relies on its indirect signatures.
Now, Kim et al. report on the direct visualization of Co orbitals in CeCoIn5 — a prototypal heavy fermion material — by means of scanning tunnelling microscopy. The researchers map the topography of cleaved CeIn- and Co-terminated (001) surfaces with atomic resolution, highlighting different properties of these two configurations. In particular, the shape of atoms is gradually modified from circular to dumbbell-like upon decreasing the tip–sample distance (or, equivalently, increasing the tunnelling current value) only for the Co-terminated surface.
Phenomenologically, the observed dumbbell orientation is only along  or  directions — a fact interpreted by the researchers as evidence for the electronic occupancy of Co dxz or dyz orbitals. Remarkably, occupied Co dxz and dyz orbitals alternate for nearest-neighbour lattice points, resulting in an antiferro-orbital ordered pattern along both the  and  directions. The researchers associate the origin of this state to surface-specific properties and, in particular, to the increased Coulomb repulsion energy due to the reduced amount of electrostatic screening.
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Prando, G. Orbital ordering mapped. Nature Nanotech (2017). https://doi.org/10.1038/nnano.2017.227