Real-space imaging of an orbital Kondo resonance on the Cr(001) surface

The Kondo effect1 is usually connected with the interaction between a localized spin moment and itinerant electrons. This interaction leads to the formation of a narrow resonance at the Fermi level, which is called the Abrikosov–Suhl or Kondo resonance2. Scanning tunnelling microscopy is an ideal technique for real-space investigations of complicated electronic structures3,4 and many-body phenomena, such as the formation of the Kondo resonance5,6,7,8 or d-wave pairing in high-Tc superconductors9. Theory has predicted that similar, Kondo-like many-electron resonances are possible for scattering centres with orbital instead of spin degrees of freedom—the quadruple momenta in uranium-based compounds or two-level systems in metallic glasses are examples of such ‘pseudo-Kondo’ scattering centres10. Here we present evidence for the orbital Kondo resonance on a transition-metal surface. Investigations of an atomically clean Cr(001) surface at low temperature using scanning tunnelling microscopy reveal a very narrow resonance at 26 meV above the Fermi level, and enable us to visualize the orbital character of the corresponding state. The experimental data, together with many-body calculations, demonstrate that the observed resonance is an orbital Kondo resonance formed by two degenerate dxz, dyz surface states.