We present a complete photonic logic gate architecture in a compact solid-state system, making use of the nonlinear and spintronic properties of exciton polaritons in semiconductor microcavities. The dynamics of the system is modeled using the spinor Gross-Pitaevskii equations, and it is shown that the proposal fulfills all the necessary criteria for fully functioning information processing devices without the use of any external electric fields.

Using the tight-binding approximation we calculated the magnetic susceptibility of graphene quantum dots (GQDs) of different geometrical shapes and characteristic sizes of 2-10 nm, when the magnetic properties are governed by the electron edge states. Two types of edge states can be discerned: the zero-energy states (ZES) located exactly at the zero-energy Dirac point, and the dispersed edge states (DES) with the energy close, but not exactly equal to zero. DES are responsible for the temperature independent diamagnetic response, while ZES provide the temperature dependent spin Curie paramagnetism. The hexagonal, circular and randomly shaped GQD contain mainly DES and, as a result, they are diamagnetic. The edge states of the triangular GQD are of ZES type. These dots reveal the crossover between spin paramagnetism, dominating for small dots and at low temperatures, and orbital diamagnetism, dominating for large dots and at high temperatures.

We study theoretically the effect of the fermion and boson densities on the superconductivity transition critical temperature (Tc) of a two dimensional electron gas (2DEG), where superconductivity is mediated by a Bose-Einstein condensate of exciton-polaritons. The critical temperature is found to increase with the boson density, but surprisingly it decreases with the 2DEG density increase. This makes doped semiconductor structures with shallow Fermi energies better adapted for observation of the exciton-induced superconductivity than metallic layers. For the realistic GaAsbased microcavities containing-doped and neutral quantum wells we estimate Tc as close to 50K. Superconductivity is suppressed by magnetic fields of the order of 4T due to the Fermi surface renormalisation.PACS numbers: 71.36.+c, 74.78.Fk, 71.35.Gg High temperature superconductivity (HTSC) has been desperately searched for during decades since the appearance of the seminal work of Bardeen-Cooper-Schrieffer (BCS) [1] in the early 50s. Among many different paths physicist have tried to achieve it, the excitonic mechanism of superconductivity (SC) deserves a particular attention [2][3][4]. According to Ginsburg [5,6], excitons are expected to be suitable for realization of HTSC because the characteristic energy above which the electron attraction mediated by excitons vanishes is several orders of magnitude larger than the Debye energy limiting the attraction mediated by phonons.Despite optimistic expectations, to the best of our knowledge, the exciton mechanism of SC has never worked until now, most likely due the reduced retardation effect [4,7]. Phonons in the BCS model are very slow compared to electrons on the Fermi surface. Hence there is a strong retardation effect in phonon-mediated electronelectron attraction, so that the size of a Cooper pair is very large (of the order of 100nm), and the Coulomb repulsion can be neglected at such distances. In contrast, an exciton is a very fast quasi-particle once it is accelerated to the wave-vectors comparable with the Fermi wave-vector in a metal. Therefore the replacement of phonons by excitons leads to the loss of retardation and the smaller sizes of Cooper pairs, that is why the Coulomb repulsion starts playing an important role. In realistic multilayer structures the Coulomb repulsion appears to be stronger than the exciton-mediated attraction so that Cooper pairs cannot be formed. In literature [8, 9] one find reports on layered metal-insulator structures where SC at 50K in layered metal-insulator structures, nevertheless there is still no evidence that the excitonic mechanism is responsible for this effect.Recently, the novel mechanism to achieve superconductivity mediated by exciton-polaritons has been proposed in references [10,11]. Exciton-polaritons are quasiparticles that arise due to the strong coupling of excitons with light. Particularly interesting exciton-polariton related phenomena have been observed in semiconductor quantum wells (QW) embedded in microcavity [12,13]. Bose-Einste...

We study the electronic and magnetic properties of multilayer quantum dots (MQDs) of graphite in the nearest-neighbor approximation of tight-binding model. We calculate the electronic density of states and orbital susceptibility of the system as function of the Fermi level location. We demonstrate that properties of MQD depend strongly on the shape of the system, on the parity of the layer number and on the form of the cluster edge. The special emphasis is given to reveal the new properties with respect to the monolayer graphene quantum dots (GQD). The most interesting results are obtained for the triangular MQD with zig-zag edge at near-zero energies. The asymmetrically smeared multi-peak feature is observed at Dirac point within the size-quantized energy gap region, where monolayer graphene flakes demonstrate the highly-degenerate zero-energy state. This feature, provided by the edge-localized electronic states results in the splash-wavelet behavior in diamagnetic orbital susceptibility as function of energy.