Charge-density-wave states in double-layer graphene structures in a high magnetic field

We study the phases of correlated charge-density waves that form at a high magnetic field in two parallel graphene flakes separated by a thin insulator. The predicted phases include the square and hexagonal charge-density-wave bubbles, and a quasi-one-dimensional stripe phase. We find that the transition temperature for such phases is within the experimentally accessible range and that formation of interlayer-correlated states produces a negative compressibility contribution to the differential capacitance of this system.Comment: 6+3 pages, 7 figure

Signature of electronic excitations in the Raman spectrum of graphene

Inelastic light scattering from Dirac-type electrons in graphene is shown to be dominated by the generation of the inter-band electronic modes which are odd in terms of time-inversion symmetry and belong to the irreducible representation A$_2$ of the point group C$_{6v}$ of the honeycomb crystal. At high magnetic fields, these electron-hole excitations appear as peculiar $n^- \to n^+$ inter-Landau-level modes with energies $\omega_n=2\sqrt{2n} \hbar v/\lambda_B$ and characteristically crossed polarisation of in/out photons.Comment: 4 pages, 2 figures, revised and improve

Intra-Landau level magnetoexcitons and the transition between quantum Hall states in undoped bilayer graphene

We study the collective modes of the quantum Hall states in undoped bilayer graphene in a strong perpendicular magnetic and electric field. Both for the well-known ferromagnetic state that is relevant for small electric field $E_\perp$ and the analogous valley/layer polarized one suitable for large $E_\perp$, the low energy physics is dominated by magnetoexcitons with zero angular momentum that are even combinations of excitons that conserve Landau orbitals. We identify a long wave length instability in both states, and argue that there is an intermediate range of the electric field $E^{(1)}_\text{c} < E_\perp < E^{(2)}_\text{c}$ where a gapless phase interpolates between the incompressible quantum Hall states. The experimental relevance of this crossover via a gapless state is discussed.Comment: 7 pages, 5 figure

Magnetothermopower and magnon-assisted transport in ferromagnetic tunnel junctions

We present a model of the thermopower in a mesoscopic tunnel junction between two ferromagnetic metals based upon magnon-assisted tunneling processes. In our model, the thermopower is generated in the course of thermal equilibration between two baths of magnons, mediated by electrons. We predict a particularly large thermopower effect in the case of a junction between two half-metallic ferromagnets with antiparallel polarizations, $S_{AP} \sim - (k_B/e)$, in contrast to $S_{P} \approx 0$ for a parallel configuration.Comment: 3 pages, 1 eps figur

Selection rules for Raman-active electronic excitations in carbon nanotubes

Raman measurements in carbon allotropes are generally associated with the exploration of the vibrational modes. Here, we present a theory of the non-resonant inelastic light scattering accompanied by the excitations of intersubband electron-hole pairs in carbon nanotubes and predict the selection rules and polarization properties of the dominant intersubband Raman active modes.Comment: 4 pages, 3 figure

Hierarchy of gaps and magnetic minibands in graphene in the presence of the Abrikosov vortex lattice

We determine the structure of band and gaps in graphene encapsulated in hexagonal boron nitride and subjected to magnetic field of Abrikosov lattice of vortices in the underlying superconducting film. The spectrum features one non-dispersive magnetic miniband at zero energy, separated by the largest gaps in the miniband spectrum from a pair of minibands resembling slightly broadened first Landau levels in graphene, suggesting the persistence of $\nu = \pm 2$ quantum Hall effect states. Also, we identify occasional merging point of magnetic minibands which feature Dirac-type dispersion at the consecutive miniband edges.Comment: 5 pages, 3 figure

Cooling of chiral heat transport in the quantum Hall effect graphene

In the quantum Hall effect (QHE) regime, heat is carried by electrons in the edge states of Landau levels. Here, we study cooling of hot electrons propagating along the edge of graphene at the filling factor $\nu=\pm2$, mediated by acoustic phonons. We determine the temperature profile extended from a hot spot, where the Hall current is injected into graphene from a metallic contact, taking into account specifics of boundary conditions for lattice displacements in graphene in a van der Waals heterostructure with an insulating substrate. Our calculations, performed using generic boundary conditions for Dirac electrons, show that emission of phonons can explain a short cooling length observed in graphene-based QHE devices by Nahm, Hwang and Lee [PRL 110, 226801 (2013)].Comment: 4+2 pages, accepted to Phys.Rev.

Quantum conductance fluctuations in 3D ballistic adiabatic wires.

Quantum conductance of 3D ballistic wires with idealy flat boundaries obeys fluctuations with the properties quite distinguishable from those of universal conductance fluctuations: Both their amplitude and the sensitivity to the magnetic field flux $\Phi =HS$ penetrated into the sample cross-sectional area $S$ are different and depend on details of the cross-sectioanl shape of the wire. When the latter is integrable, conductance fluctuations have the enlarged amplitude $\delta G\sim\left[(e^2/h)^3G\right]^{1/4}$. When the cross-sectional shape of a wire is non-integrable, the irregular part of a conductance has the $e^ 2/h$ scale, whereas the correlation field is reduced to the value of $H_S\sim (\lambda_F/\sqrt S)^{1/2}(\Phi_0/S)$ and the correlation voltage of the nonlinear conductance fluctuations has the scale of $eV_c\sim\hbar^2/mS\sim E_F/(S/\lambda_F)$, where $\lambda_F=1/p_F$ is the Fermi wavelength.Comment: 5 pages, no pictures, to be published in "Coulomb and Interference Effects in Small Electronic Structures", ed. by D.Glattli, M.Sanquer and J.T.T.Van