Hund?s rule magnetism in C $\mathsf{_{60}}$ ions?

Abstract

We investigate the occurrence of Hund's rule magnetism in C n± 60 molecular ions, by computing the ground-state spin for all charge states n from −3 to +5. The two competing interactions, electronvibration (e-v, including Jahn Teller, favoring low spin) and electron-electron (e-e, including Hund-rule exchange, favoring high spin), are accounted for based on previously computed ab-initio coupling parameters. Treating the ion coordinates as classical, we first calculate and classify the static Jahn-Teller distorted states for all n, inclusive of both e-v and e-e effects. We then correct the adiabatic result by including the zero-point energy lowering associated with softening of vibrations at the adiabatic Jahn-Teller minima. Our overall result is that while, like in previous investigations, low-spin states prevail in negative ions, Hund's rule high spin dominates all positive C n+ 60 ions. This suggests also that Hund-rule magnetism could arise in fullerene cation-based solid state compounds, particularly those involving C 2+ 60 .PACS. 36.40.Cg Electronic and magnetic properties of clusters -61.48.+c Fullerenes and fullerene-related materials (structure) -71.20.Tx Fullerenes and related materials; intercalation compounds (electronic structure) -75.75.+a Magnetic properties of nanostructures

Spin-triplets-wave local pairing induced by Hund’s rule coupling

“…Fullerene systems are known to have sizable strengths for both interactions, although the JT interaction is more dominant for alkali-doped region. A recent density functional calculation study 26 has suggested that the fullerene in the hole-doped region may have stronger Hund's rule interaction, with possible applications to cation-doped fullerenes 27 . Finally, we make an observation of an interesting property of the local interaction Eq.…”

Low-energy excitations of a linearly Jahn-Teller coupled orbital quintet

“…Also, the spectrum (Fig. 9b) is less dense: the 10 tunneling states (A + H + G) converge to zero energy at strong coupling; the 20 1-phonon tangential states converging to ω (1) τ are fairly well visible; the tangential ω (2) τ , radial ω, and overtone 2 ω A fairly general feature of the vibronic spectrum, especially visible in Figs. 2b, 9b, and 10b, is the rapid drop of several vibronic energies, then followed by a successive climb back at strong coupling where tunneling is suppressed and the motion gradually collapses to oscillations around the adiabatic wells.…”

“…The HamiltonianĤ depends now on two parameters. Each vibronic eigenvalue traces a 2-dimensional surface as g and α (or equivalently g (1) H and g fixed α, plus circular cuts at fixed g. Like for the G mode, the strong-coupling asymptotic vibronic energies, are determined by the normal frequencies of oscillation around the adiabatic JT minima, and these frequencies now depend on α. The five normal modes separate into a radial vibration of unchanged frequency ω and symmetry A 1 , plus two softer doublydegenerate (of E 1 and E 2 symmetry for D 5 wells and both of E symmetry for D 3 wells) tangential modes, represented in Fig.…”

“…A notable example is that of positive C 60 ions, with one (or several) holes in a h u molecular orbital [1,2]. At the adiabatic level, partly occupied quintet states are unstable against molecular distortions involving the vibrational modes of the molecule.…”

Surface Hubbard U of alkali fullerides

“…The JT interaction is stronger than the on-ball exchange coupling [9,17], leading to low-spin ground states and to the stabilization of even-charge anionic states [18,19]. In compounds of even stoichiometry (x = 2, 4) charge fluctuations are hindered both by correlation and JT effects [20][21][22], resulting in insulating and diamagnetic ground states [23][24][25], while in odd stoichiometries of cubic symmetry metallic behaviour and even phonon-mediated superconductivity (for x = 3) are observed [26][27][28].…”