Optical signatures of electric field driven magnetic phase transitions in graphene quantum dots

TL;DR

This study demonstrates that external electric fields can induce magnetic phase transitions in graphene quantum dots and that their magnetic states can be optically identified through electro-absorption spectra, advancing spintronic device design.

Contribution

The paper introduces a method to detect magnetic states in graphene quantum dots via optical spectra and shows electric field-driven magnetic phase transitions using the PPP model.

Findings

Magnetic states in QDs undergo phase transitions under electric fields.

Electro-absorption spectra reveal signatures of magnetic ordering.

Electric fields cause spin-dependent gap splitting in QDs.

Abstract

Experimental challenges in identifying various types of magnetic ordering in graphene quantum dots (QDs) pose a major hurdle in the application of these nanostructures for spintronic devices. }Based upon phase diagrams obtained by employing {\normalsize{}the $\pi$-electron }Pariser-Parr-Pople (PPP) model Hamiltonian, we demonstrate that the magnetic states undergo phase transition under the influence of an external electric field. Our calculations of the electro-absorption spectra of these QDs indicate that the spectrum in question carries strong signatures of their magnetic state (FM vs AFM), thus suggesting the possibility of an all-optical characterization of their magnetic nature. Further, the gaps for the up and the down spins are the same in the absence of an external electric field, both for the antiferromagnetic (AFM), and the ferromagnetic (FM) states of QDs. But, once the QDs are exposed to a suitably directed external electric field, gaps for different spins split, and, exhibit distinct variations with respect to the strength of the field. The nature of variation exhibited by the energy gaps corresponding to the up and down spins is different for the AFM and FM configurations of QDs. This selective manipulation of the spin-polarized gap splitting by an electric field in finite graphene nanostructures can open up new frontiers in the design of graphene-based spintronic devices.