# Unconventional orbital ordering and emergent dimensional reduction in fulleride superconductors

## TL;DR

This paper investigates the unconventional orbital order in fulleride superconductors, revealing a nonlocal order parameter, orbital-selective Mott state, and emergent two-dimensional behavior at low energies, explaining high critical fields.

## Contribution

It introduces a systematic study of nonlocal order parameters in fullerides, characterizing the Jahn-Teller metal and its orbital-selective Mott state with no local order parameter.

## Key findings

- Orbital-selective Mott state with localized and metallic orbitals.
- Effective dimensional reduction from three to two dimensions at low energies.
- High upper critical field explained by emergent two-dimensionality.

## Abstract

Non-local 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.