Isil Ozfidan

Biexciton Binding of Dirac fermions Confined in Colloidal Graphene Quantum Dots

We present transient absorption measurements and microscopic theory of biexciton binding in triangular colloidal graphene quantum dots consisting of 168 sp(2)-hybridized C atoms. We observe optical transitions from the lowest orbitally dark singlet exciton states to states below the energy of an unbound dark+bright singlet-exciton pair. Through microscopic calculations of the low-energy exciton and biexciton states via tight-binding, Hartree-Fock, and configuration interaction methods, the spectra reveal a biexciton consisting primarily of a dark-bright singlet-pair bound by ∼0.14 eV.

Electron-Electron interactions, topological phase and optical properties of a charged artificial benzene ring

We present a theory of the electronic and optical properties of a charged artificial benzene ring (ABR). The ABR is described by the extended Hubbard model solved using exact diagonalization methods in both real and Fourier space as a function of tunneling matrix element t, Hubbard on-site repulsion U and inter-dot interaction V. In the strongly interacting case we present exact analytical results for the spectrum of the hole in a half-filled ABR dressed by spin excitations of remaining electrons. The spectrum is interpreted in terms of the appearance of a topological phase associated with an effective gauge field piercing through the ring. We show that the maximally spin polarized (S=5/2) and maximally spin depolarised (S=1/2) states are the lowest energy, orbitally non degenerate, states. We discuss the evolution of the phase diagram and level crossings as interactions are switched off and the ground state becomes spin non-degenerate but orbitally degenerate S=1/2. We present a theory of optical absorption spectra and show that the evolution of the ground and excited states, level crossings and presence of artificial gauge can be detected optically.

Graphene-based integrated electronic, photonic and spintronic circuit

To create carbon-based nanoscale integrated electronic, photonic, and spintronic circuit one must demonstrate the three functionalities in a single material, graphene quantum dots (GQDs), by engineering lateral size, shape, edges, number of layers and carrier density. We show theoretically that spatial confinement in GQDs opens an energy gap tunable from UV to THz, making GQDs equivalent to semiconductor nanoparticles. When connected to leads, GQDs act as single-electron transistors. The energy gap and absorption spectrum can be tuned from UV to THz by size and edge engineering and by external electric and magnetic fields. The sublattice engineering in, e.g., triangular graphene quantum dots (TGQDs) with zigzag edges generates a finite magnetic moment. The magnetic moment can be controlled by charging, electrical field, and photons. Addition of a single electron to the charge-neutral system destroys the ferromagnetic order, which can be restored by absorption of a photon. This allows for an efficient spin-photon conversion. These results show that graphene quantum dots have potential to fulfill the three functionalities: electronic, photonic, and spintronic, realized with different materials in current integrated circuits, as well as offer new functionalities unique to graphene.