Graphene-based quantum heterospin graphs

TL;DR

This paper explores low-dimensional quantum spin systems based on magnetic nanographene structures, analyzing their energy spectra and proposing experimental realizations of antiferromagnetic spin chains with potential degeneracies.

Contribution

It introduces a first-principles study of graphene nanostructures as quantum spin systems and identifies a robust degeneracy in their excited states related to symmetry properties.

Findings

Identification of a double degeneracy in the first excited state of certain spin graphs.

Robustness of degeneracy under realistic experimental perturbations.

Proposal for experimental realization of antiferromagnetic spin chains using nanographene structures.

Abstract

We investigate from first principles a variety of low-dimensional open quantum spin systems based on magnetic nanographene structures that contain spin-1/2 and spin-1 triangulenes and/or olympicenes. These graphene nanostructures behave as localized spins and can be effectively described by a quantum bilinear-biquadratic Heisenberg Hamiltonian, for which we will compute the energy spectrum and the quantum numbers associated with the low-energy eigenstates. We propose the experimental realization of antiferromagnetic alternating spin chains using these graphene nanostructures, which result in ferrimagnetic systems whose ground state spin and degeneracy depend on the length of the chain. We identify a double degeneracy in the total spin quantum number $S$ of the first excited state in three-leg spin graphs (3-LSGs) and other heterospin nanostructures, which depends on both the number of sites and the spin species, and originates from the swapping transformation symmetry of the Hamiltonian. Numerical simulations indicate that this degeneracy remains largely robust for $N=7$ spin-1 3-LSGs under realistic perturbations present in experimental conditions.