Externally Controlled Magnetism and Band Gap in Twisted Bilayer Graphene.

We theoretically study the effects of electron-electron interaction in twisted bilayer graphene in a transverse dc electric field. When the twist angle is not very small, the electronic spectrum of the bilayer consists of four Dirac cones inherited from each graphene layer. An applied bias voltage leads to the appearance of two holelike and two electronlike Fermi surface sheets with perfect nesting among electron and hole components. Such a band structure is unstable with respect to the exciton band-gap opening due to the screened Coulomb interaction. The exciton order parameter is accompanied by spin-density-wave order. The gap depends on the twist angle and can be varied by a bias voltage. This result correlates well with recent transport measurements [J.-B. Liu et al., Sci. Rep. 5, 15285 (2015)SRCEC32045-232210.1038/srep15285]. Our proposal allows the coexistence of (i) an externally controlled semiconducting gap and (ii) a nontrivial multicomponent magnetic order. This is interesting for both fundamental research and applications.

Spin-Valley Half-Metal as a Prospective Material for Spin Valleytronics.

Half-metallicity (full spin polarization of the Fermi surface) usually occurs in strongly correlated electron systems. We demonstrate that doping a spin-density wave insulator in the weak-coupling regime may also stabilize half-metallic states. In the absence of doping, the spin-density wave is formed by four nested bands [i.e., each band is characterized by charge (electron or hole) and spin (up or down) labels]. Of these four bands, only two accumulate the charge carriers introduced by doping, forming a half-metallic two-valley Fermi surface. Depending on the parameters, the spin polarizations of the electronlike and holelike valleys may be either (i) parallel or (ii) antiparallel. The Fermi surface of (i) is fully spin polarized (similar to usual half-metals). Case (ii), referred to as "a spin-valley half-metal," corresponds to complete polarization with respect to the spin-valley operator. The properties of these states are discussed.

Instabilities of the AA-stacked graphene bilayer.

Tight-binding calculations predict that the AA-stacked bilayer graphene has one electron and one hole conducting band, and that the Fermi surfaces of these bands coincide. We demonstrate that as a result of this degeneracy, the bilayer becomes unstable with respect to a set of spontaneous symmetry violations. Which of the symmetries is broken depends on the microscopic details of the system. For strong on-site Coulomb interaction we find that antiferromagnetism is the most stable order parameter. For an on-site repulsion energy typical for graphene systems, the antiferromagnetic gap can exist up to room temperature.

Asymmetric long josephson junction acting as a ratchet for a quantum field.

We study the escape rate of flux quanta in a long Josephson junction having an asymmetric spatial inhomogeneous critical current density. Such a junction can behave as a ratchet when driven by an ac current in the presence of a magnetic field. This rectification gives rise to a dc voltage V(dc) across the junction. The usual approach of particlelike tunneling cannot describe this rectification, and a quantum field theory description is required. We also show that, under specific conditions, the rectification direction, and consequently V(dc), can change its sign when varying the temperature T near the crossover temperature T* between the quantum and classical regimes.

Effect of electron-lattice interaction on the phase separation in strongly correlated electron systems with two types of charge carriers.

The effect of electron-lattice interaction is studied for a strongly correlated electron system described by the two-band Hubbard model. A two-fold effect of electron-lattice interaction is taken into account: in non-diagonal terms, it changes the effective bandwidth, whereas in diagonal terms, it shifts the positions of the bands and the chemical potential. It is shown that this interaction significantly affects the doping range corresponding to the electronic phase separation and can even lead to a jump-like transition between states with different values of strains.

Phase separation in Jahn-Teller systems with localized and itinerant electrons.

The phase diagram for doped manganites and related compounds is analyzed in terms of the Kondo-lattice model taking into account an interplay between electrons localized due to lattice distortions and those in the band states. It is shown that the number of itinerant charge carriers can be significantly lower than that implied by the doping level. The competition between the homogeneous (ferromagnetic or antiferromagnetic) and phase-separated states is discussed and a strong tendency to the phase separation was revealed for a wide doping range.