Equations-of-motion method for triplet excitation operators in graphene

Abstract

The particle-hole continuum in the Dirac sea of graphene has a unique window underneath, which in principle leaves room for bound state formation in the triplet particle-hole channel (Baskaran and Jafari 2002 Phys. Rev. Lett. 89 016402). In this work, we construct appropriate triplet particle-hole operators and, using a repulsive Hubbard-type effective interaction, we employ equations of motion to derive approximate eigenvalue equations for such triplet operators. While the secular equation for the spin density fluctuations gives rise to an equation which is second order in the strength of the short range interaction, the explicit construction of the triplet operators obtained here shows that, in terms of these operators, the second-order equation can be factorized to two first-order equations, one of which gives rise to a solution below the particle-hole continuum of Dirac electrons in undoped graphene.

Observation of spin-polarized Anderson state around charge neutral point in graphene with Fe-clusters

“…For the disorder graphene showing insulating behavior, the temperature dependence of resistance can be described by either variable-range hopping (VRH) 13,17 or the universal function of the Kondo model 47 www.nature.com/scientificreports www.nature.com/scientificreports/ in general much higher than that of s-wave Kondo system because hybridization of conduction electrons with localized states in graphene lead to p-wave hybridization, which typically results in higher Kondo temperature as discussed in S. A. Jafari et al 42 . In addition, excitation of spin-1 boson due to inter-band particle-hole processes in graphene could further enhance the Kondo effect 48,49 . Figure 2(d) displays temperature dependent ρ xx at CNP in the presence of the high magnetic field 9 T. Results shows that the applied high magnetic field convert the system into conducting state at CNP.…”

Collective excitations and the nature of Mott transition in undoped gapped graphene

“…Then we extend the collective mode analysis of Ref. [11] to the case with ∆ = 0, whereby the singleparticle spectrum changes from ± v F |k| to the one given by Eq. ( 2).…”

Electromagnetic modes from Stoner enhancement: Graphene as a case study

“…For any model Hamiltonian, the properties of the system decide fluctuations in which channel are to be enhanced or suppressed. The nature of particle-hole continuum in graphene is such that provides a unique opportunity of which the spin-flip fluctuations can take advantage and develop a coherent pole which indicates that they bind into triplet bound states of particle-hole pairs below the continuum of free particle-hole excitations [29,30,31,34]. The very same ladder summation mechanism when accounted for in the EM response of the system, generates a singularity in the polarization function that arises from proliferation of Stoner PH excitations along the ladder rung.…”

“…This may happen in both undoped [29,30,31,32,33] and doped graphene [34]. In this work we would like to study the effect of such ladder diagrams in the electromagnetic response of graphene, and in particular to focus on the special role played by the spin-flip channel of particle-hole fluctuations.…”