Quasiphase transition and many-spin Kondo effects in a graphene nanodisk

The trigonal zigzag nanodisk with size N has N localized spins. We investigate its thermodynamical properties with and without external leads. Leads are made of zigzag graphene nanoribbons or ordinary metallic wires. There exists a quasiphase transition between the quasiferromagnet and quasiparamagnet states, as signaled by a sharp peak in the specific heat and in the susceptibility. Lead effects are described by the many-spin Kondo Hamiltonian. An additional peak emerges in the specific heat. Furthermore, the bandwidth of free electrons in metallic leads becomes narrower. By investigating the spin-spin correlation it is argued that free electrons in the lead form spin singlets with electrons in the nanodisk. They are indications of many-spin Kondo effects.