Graphene-based quantum heterospin graphs

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 also identify a double degeneracy in the total spin quantum number $S$ in the first excited state for three-leg spin graphs (3-LSGs). This degeneracy depends on both the number of sites and the spin species that compose the 3-LSG. We identify the double degeneracy of the first excited state as a consequence of swapping transformation symmetry of the Hamiltonian.