Unconventional orbital ordering and emergent dimensional reduction in fulleride superconductors

Abstract

Nonlocal order parameters in space-time are proposed to characterize the unconventional orbital-selective conducting state in fulleride superconductors, called the Jahn-Teller metal. In previous works, it has been argued that this state can be interpreted as a spontaneous orbital-selective Mott state, in which the electrons in two of the three ${t}_{1u}$ molecular orbitals are localized, while those in the third one are metallic. Here, based on the realistic band structure for fullerides, we provide a systematic study of nonlocal order parameters and characterize the Jahn-Teller metal, for which there exists no one-body local order parameter in contrast to conventional orderings. It is shown that the Mottness, or integer filling nature for each orbital due to strong correlation effects, is a relevant feature of the present orbital order. The local orbital moment thus vanishes and the static distortion associated with a conventional orbital moment is absent. Transport characteristics are also investigated, and it is found that the dimensionality is effectively reduced from three to two at low energies, while the cubic nature is recovered at high energies. This accounts for the high upper critical field observed in the superconducting state of the fcc fullerides inside the Jahn-Teller metal regime.