Abstract

Alkali-doped fullerides are multiorbital electron systems with molecular orbital degrees of
freedom, and exhibit s-wave superconductivity with transition temperatures as high as 38 K
[1]. In these systems, the coupling between electrons in the threefold-degenerate t1u orbitals
and intramolecular Jahn–Teller phonons plays an essential role. Jahn–Teller phonons generate
an effective antiferromagnetic Hund’s coupling, thereby stabilizing low-spin states and spinsinglet superconductivity. Since the corresponding phonon-mediated interactions are
intrinsically frequency dependent, retardation effects can strongly affect response functions in
the superconducting state. In this talk, I discuss electronic and phononic angular-momentum
responses in fulleride superconductors based on the Jahn–Teller–Hubbard model and
Eliashberg theory.
In the first part, I focus on the electronic spin and orbital magnetic susceptibilities in the
superconducting state. By analyzing their temperature dependence, we find that the spin
susceptibility vanishes at low temperatures, whereas the orbital magnetic susceptibility remains
finite [2]. This contrasting behavior originates from the combination of multiorbital physics and
phonon retardation effects. We further show that field-induced odd-frequency pairing plays an
important role in the complete suppression of the spin magnetization.
In the second part, I focus on phonon angular momentum (PAM) carried by Jahn–Teller
phonons[3]. While the generation and control of PAM by heat currents, phonon Zeeman effects,
and related mechanisms have been actively discussed [4], the impact of superconductivity on
PAM remains unclear. We analyze the temperature dependence of PAM induced by an orbital
Zeeman field applied to the electronic system, and find that PAM changes sign across the
superconducting transition and is strongly enhanced in the superconducting phase. A lowenergy effective theory reveals that the sign reversal originates from the Fermi-surface
contribution in the normal state, while the enhancement is characterized by the ratio −D/w1
between the electronic bandwidth and the Jahn–Teller phonon energy.
These results indicate that Jahn–Teller phonons are not merely mediators of
superconductivity, but also key degrees of freedom governing electronic and phononic angularmomentum responses in the superconducting state.
[1] Y. Nomura et al., J. Phys. : Condens. Matter 28, 153001 (2016).
[2] N. Okada et. al., PRB 113, 024517 (2026).
[3] L. Zhang et al., Phys. Rev. Lett. 112, 085503 (2014).
[4] D. M. Juraschek et al., Nat. Phys. 21, 1532 (2025).