Discovery of d-orbital order in Tb2CoAl4Ge2

Orbital order describes a quantum state where occupied orbitals line up in a periodic pattern. While orbital physics play a fundamental and universal role in strongly correlated electron systems, the existence and particularly the band structure fingerprint of orbital order remain a long-standing mystery . Here, we report the discovery of rare earth 5d-orbital order developed by the surface states of intermetallic alumogermanide Tb2CoAl4Ge2. Angle resolved photoemission spectroscopy (ARPES) reveals characteristic nematic features like Fermi surface deformation and band split, which are decently reproduced by a ferro-orbital order term in mean-field Hamiltonian. The structural and magnetic origin of such order is excluded by systematic high-resolution neutron powder diffraction (NPD) and scanning tunnelling microscopy (STM) measurements. Uniform orbital polarization from linear dichroism ARPES and spontaneous symmetry breaking domains observed by spatial resolved ARPES and STM conclusively validate the orbital order scenario. Our results not only highlight a methodology that distinguishes orbital physics, but also benchmark a pure d-orbital order avoiding complications from structural distortion as in colossal magnetoresistance manganites, magnetic order as in iron-based superconductors, and charge transfer p-orbital order in cuprates.