The antiferromagnetic $S=1/2$ Heisenberg model on the ${\text{C}}_{60}$ fullerene geometry

TL;DR

This study uses DMRG to analyze the magnetic properties of the antiferromagnetic $S=1/2$ Heisenberg model on the C$_{60}$ fullerene, revealing insights into frustration, correlations, and thermodynamics.

Contribution

It provides the first detailed DMRG analysis of the Heisenberg model on the C$_{60}$ geometry, highlighting the nature of excitations and correlations in this fullerene.

Findings

Lowest excited state is a triplet, not a singlet.

Spin correlations are stronger along hexagon bonds and decay exponentially.

Thermodynamic properties show features similar to kagome lattice behavior.

Abstract

We solve the quantum-mechanical antiferromagnetic Heisenberg model with spins positioned on vertices of the truncated icosahedron using the density-matrix renormalization group (DMRG). This describes magnetic properties of the undoped C$_{60}$ fullerene at half filling in the limit of strong on-site interaction $U$. We calculate the ground state and correlation functions for all possible distances, the lowest singlet and triplet excited states, as well as thermodynamic properties, namely the specific heat and spin susceptibility. We find that unlike smaller C$_{20}$ or C$_{32}$ that are solvable by exact diagonalization, the lowest excited state is a triplet rather than a singlet, indicating a reduced frustration due to the presence of many hexagon faces and the separation of the pentagonal faces, similar to what is found for the truncated tetrahedron. This implies that frustration may be tuneable within the fullerenes by changing their size. The spin-spin correlations are much stronger along the hexagon bonds and exponentially decrease with distance, so that the molecule is large enough not to be correlated across its whole extent. The specific heat shows a high-temperature peak and a low-temperature shoulder reminiscent of the kagome lattice, while the spin susceptibility shows a single broad peak and is very close to the one of C$_{20}$.