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

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 the π-electron PariserParr-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.

Graphene quantum dots with Stone-Wales defect as a topologically tunable platform for visible-light harvesting

“…1, with the parameters U and V ij representing the on-site and long-range Coulomb interactions, respectively. The one-electron hopping matrix elements t ij are restricted between nearest-neighbor carbon atoms i and j, with the value t 0 = -2.4 eV corresponding to uniform carbon-carbon bond-length r 0 = 1.4 Å, in accordance with our earlier calculations on conjugated polymers, polyaromatic hydrocarbons, and graphene quantum dots [18][19][20][21][22][23][24] . For the non-uniform bond-lengths, the values of corresponding t ij are determined from the exponential formula t ij = t 0 e (r 0 −r ij )/δ , extensively used by us earlier 25 , in which r ij is the distance between ith and jth carbon atoms (in Å), t 0 = -2.4 eV r 0 = 1.4 Å, and δ = 0.73 Å is a parameter depicting electron-phonon coupling.…”

“…where κ i,j is the dielectric constant of the system representing the screening effects, and R i,j is the distance between ith and jth carbon atoms (in Å). In the present set of computations, we have adopted the "screened parameters" 27 with U = 8.0 eV, κ i,j = 2.0 (i = j), and κ i,i = 1.0, consistent with our earlier works on π-conjugated systems and graphene quantum dots [18][19][20][21][22][23][24] .…”

Excited States and Optical Properties of Hydrogen-Passivated Rectangular Graphenes: A Computational Study

“…(1) denotes the one-electron hopping processes connecting i -th and j -th atoms, quantified by matrix elements t ij . We assume that the hopping connects only the nearest-neighbor carbon atoms, with the value t 0 = 2.4 eV, consistent with our earlier calculations on conjugated polymers 14–20 , polyaromatic hydrocarbons 21,22 , and graphene quantum dots 24,25 . The next two terms in Eq.…”

“…In our theoretical approach we consider RGMs to be systems whose low-lying excited states are determined exclusively by their π electrons, with negligible influence of σ electrons. As a result we adopt a computational approach employing the Pariser-Parr-Pople (PPP) π –electron Hamiltonian 12,13 , and the configuration interaction (CI) method, used in several of our earlier works on conjugated polymers 14–20 , polycyclic aromatic hydrocarbons 21,22 , and graphene quantum dots 23–25 . We adopt this approach to study RGMs with the number of carbon atoms ranging from 28 to 56, corresponding to structures with increasing edge lengths in both armchair, and zigzag, directions.…”

Ferrimagnetic and antiferromagnetic phase in bilayer graphene nanoflake controlled with external electric fields

“…It might be mentioned that the antiferromagnetic orderings controlled with external field and other means in monolayer graphene nanostructures (nanoflakes, quantum dots, nanoribbons) were studied in Refs. [54,[56][57][58][59][60][61]. However, extending the system to bilayer one provides an additional possibility of applying independently in-plane and perpendicular electric field, enriching the phase diagram and enabling the extra control parameter.…”