Quasi-Phase Transition and Many-Spin Kondo Effects in Graphene Nanodisk

TL;DR

This paper studies the thermodynamic behavior of trigonal zigzag graphene nanodisks, revealing a quasi-phase transition and many-spin Kondo effects influenced by different types of leads, with implications for spintronics.

Contribution

It introduces the concept of a quasi-phase transition in nanodisks and demonstrates many-spin Kondo effects with different lead materials.

Findings

Identification of a quasi-phase transition via specific heat and susceptibility peaks.

Observation of a new specific heat peak indicating many-spin Kondo effects.

Evidence of spin-singlet formation between lead electrons and nanodisk spins.

Abstract

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 quasi-phase transition between the quasi-ferromagnet and quasi-paramagnet states, as signaled by a sharp peak in the specific heat and in the susceptability. Lead effects are described by the many-spin Kondo Hamiltonian. A new peak emerges in the specific heat. Furthermore, the band width 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.