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

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.

Pseudo-magnetic field distribution and pseudo-Landau levels in suspended graphene flakes

Combining the tight-binding approximation and linear elasticity theory for a planar membrane, we investigate stretching of a graphene flake assuming that two opposite edges of the sample are clamped by the contacts. We show that, depending on the aspect ratio of the flake and its orientation, gapped states may form in the membrane in the vicinity of the contacts. This gap in the pre-contact region should be biggest for the armchair orientation of the flake and width to length ratio of around 1.Comment: 7 pages + 3 figure

Multifractality: generic property of eigenstates of 2D disordered metals.

The distribution function of local amplitudes of eigenstates of a two-dimensional disordered metal is calculated. Although the distribution of comparatively small amplitudes is governed by laws similar to those known from the random matrix theory, its decay at larger amplitudes is non-universal and much slower. This leads to the multifractal behavior of inverse participation numbers at any disorder. From the formal point of view, the multifractality originates from non-trivial saddle-point solutions of supersymmetric $\sigma$-model used in calculations.Comment: 4 two-column pages, no figures, submitted to PRL

Thermally excited spin-current in metals with embedded ferromagnetic nanoclusters

We show that a thermally excited spin-current naturally appears in metals with embedded ferromagnetic nanoclusters. When such materials are subjected to a magnetic field, a spin current can be generated by a temperature gradient across the sample as a signature of electron-hole symmetry breaking in a metal due to the electron spin-flip scattering from polarised magnetic moments. Such a spin current can be observed via a giant magneto-thermopower which tracks the polarisation state of the magnetic subsystem and is proportional to the magnetoresistance. Our theory explains the recent experiment on Co clusters in copper by S. Serrano-Guisan \textit{et al} [Nature Materials AOP, doi:10.1038/nmat1713 (2006)

Optical manifestations of symmetry breaking in bilayer graphene

We propose a spectroscopic method of identifying broken symmetry states of bilayer graphene. We demonstrate theoretically that, in contrast to gapped states, a strained bilayer crystal or nematic phase of the electronic liquid are distinguishable by the dependence of the lineshape of absorption on the polarization of the light. This property is characteristic for both the infrared and far-infrared spectral ranges, which correspond to the absorption by transitions between low-energy bands and split bands, and transitions between the low-energy valence and conduction bands, respectively.Comment: 4 pages, 2 figures. Updated with proof corrections and journal referenc

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