Thermoelectric power ͑TEP͒ is measured in bilayer graphene for various temperatures and charge-carrier densities. At low temperatures, measured TEP well follows the semiclassical Mott formula with a hyperbolic dispersion relation. TEP for a high carrier density shows a linear temperature dependence, which demonstrates a weak electron-phonon interaction in the bilayer graphene. For a low carrier density, a deviation from the Mott relation is observed at high temperatures and is attributed to the low Fermi temperature in the bilayer graphene. Oscillating TEP and the Nernst effect for varying carrier density, observed in a high magnetic field, are qualitatively explained by the two dimensionality of the system.

Fano resonances and their strong doping dependence are observed in Raman scattering of single-layer graphene (SLG). As the Fermi level is varied by a back-gate bias, the Raman G band of SLG exhibits an asymmetric line shape near the charge neutrality point as a manifestation of a Fano resonance, whereas the line shape is symmetric when the graphene sample is electron or hole doped. However, the G band of bilayer graphene (BLG) does not exhibit any Fano resonance regardless of doping. The observed Fano resonance can be interpreted as interferences between the phonon and excitonic many-body spectra in SLG.The absence of a Fano resonance in the Raman G band of BLG can be explained in the same framework since excitonic interactions are not expected in BLG.

Four-terminal resistances, both longitudinal and diagonal, of a locally gated graphene device are measured in the quantum-Hall (QH) regime. In sharp distinction from previous two-terminal studies [J. R. Williams et al., Science 317, 638 (2007); B.Özyilmaz et al., Phys. Rev. Lett. 99, 166804 (2007)], asymmetric QH resistances are observed, which provide information on reflection as well as transmission of the QH edge states. Most quantized values of resistances are well analyzed by the assumption that all edge states are equally populated. Contrary to the expectation, however, a 5/2 transmission of the edge states is also found, which may be caused by incomplete mode mixing and/or by the presence of counter-propagating edge states. This four-terminal scheme can be conveniently used to study the edge-state equilibration in locally gated graphene devices as well as mono-and multi-layer graphene hybrid structures.

A spin injection is achieved in a direct-contact cobalt−single-layer graphene nonlocal spin-valve system, overlaid with a top gate. The spin signal is retained even in bipolar configurations of graphene. Hanle spin-precession analysis demonstrates that proportionality between spin and momentum relaxation times, which supports the Elliot-Yafet-type spin relaxation, holds consistently only when the carrier-density dependence of the density of states is taken into account. The corresponding strong spin-orbit coupling (∼10 meV) suggests that covalently bonded adsorbates, rather than charged impurities, govern the spin relaxation in diffusive graphene.

|Ate present the results of measurements on proximitywN'ect arrays with debberate positional disorder, characterized by a parameter h, . This system provides an experimental realization of an XF magnet with random Dzyaloshinskii-Moriya interactions. In measurements of resistance versus perpendicular magnetic fieid, R(fo), we Snd strong evidence for s critical field f,~l /5 beyond which phase coherence is destroyed. Our Monte Carlo simulations show evidence for reentrant behavior in the heHcity modulus F, ' but with F always 6nite at the lowest temperatures.Such a critical Seld and reentrance vrere predicted by Granato and Kosterlitz.A single Josephson junction is isomorphic to a pair of classical XYspins, and an array of junctions is therefore a realization of an XY spin system. This equivalence has prompted the use of two-dimensional junction arrays, in zero magnetic field, as model statistical mechanical systems for studying roblems such as the Kosterlitz-Thouless transition. ' A uniform array in a perpendicular magnetic field provides a realization of the uniformly frustrated XYmagnet, with tunable frustration parametrized by f, the magnetic Sux per plaquette in units of the Sux quantum. The particular case of full frustration, where f n+ T (n an integer), has received a great deal of attention as a realization of Villain's "odd model, "3 although theory and experiment' are still far from complete in this area.