Carrier transport in 2D graphene layers

Carrier transport in gated 2D graphene monolayers is theoretically considered in the presence of scattering by random charged impurity centers with density $n_i$. Excellent quantitative agreement is obtained (for carrier density $n > 10^{12} \rm{cm}^{-2}$) with existing experimental data (Ref. \onlinecite{kn:novoselov2004, kn:novoselov2005, kn:zhang2005, kn:kim2006, kn:fuhrer2006}). The conductivity scales linearly with $n/n_i$ in the theory, and shows extremely weak temperature dependence. The experimentally observed asymmetry between electron and hole conductivities is explained by the asymmetry in the charged impurity configuration in the presence of the gate voltage, while the high-density saturation of conductivity for the highest mobility samples is explained as a crossover between the long-range and the point scattering dominated regimes. We argue that the experimentally observed saturation of conductivity at low density arises from the charged impurity induced inhomogeneity in the graphene carrier density which becomes severe for $n \lesssim n_i \sim 10^{12} \rm{cm}^{-2}$.Comment: 5 pages, 4 figures, published in Phys. Rev. Let

Cosmological Vorticity in a Gravity with Quadratic Order Curvature Couplings

We analyse the evolution of the rotational type cosmological perturbation in a gravity with general quadratic order gravitational coupling terms. The result is expressed independently of the generalized nature of the gravity theory, and is simply interpreted as a conservation of the angular momentum.Comment: 5 pages, revtex, no figure

Second-order Perturbations of the Friedmann World Model

We consider instability of the Friedmann world model to the second-order in perturbations. We present the perturbed set of equations up to the second-order in the Friedmann background world model with general spatial curvature and the cosmological constant. We consider systems with the completely general imperfect fluids, the minimally coupled scalar fields, the electro-magnetic field, and the generalized gravity theories. We also present the case of null geodesic equations, and the one based on the relativistic Boltzmann equation. In due stage a decomposition is made for the scalar-, vector- and tensor-type perturbations which couple each other to the second-order. Gauge issue is resolved to each order. The basic equations are presented without imposing any gauge condition, thus in a gauge-ready form so that we can use the full advantage of having the gauge freedom in analysing the problems. As an application we show that to the second-order in perturbation the relativistic pressureless ideal fluid of the scalar-type reproduces exactly the known Newtonian result. As another application we rederive the large-scale conserved quantities (of the pure scalar- and tensor-perturbations) to the second order, first shown by Salopek and Bond, now from the exact equations. Several other applications are made as well.Comment: 61 pages; published version in Phys. Rev.

Cosmological perturbations in a generalized gravity including tachyonic condensation

We present unified ways of handling the cosmological perturbations in a class of gravity theory covered by a general action in eq. (1). This gravity includes our previous generalized $f(\phi,R)$ gravity and the gravity theory motivated by the tachyonic condensation. We present general prescription to derive the power spectra generated from vacuum quantum fluctuations in the slow-roll inflation era. An application is made to a slow-roll inflation based on the tachyonic condensation with an exponential potential.Comment: 5 page

A conserved variable in the perturbed hydrodynamic world model

We introduce a scalar-type perturbation variable $\Phi$ which is conserved in the large-scale limit considering general sign of three-space curvature ($K$), the cosmological constant ($\Lambda$), and time varying equation of state. In a pressureless medium $\Phi$ is {\it exactly conserved} in all scales.Comment: 4 pages, no figure, To appear in Phys. Rev.

Conserved cosmological structures in the one-loop superstring effective action

A generic form of low-energy effective action of superstring theories with one-loop quantum correction is well known. Based on this action we derive the complete perturbation equations and general analytic solutions in the cosmological spacetime. Using the solutions we identify conserved quantities characterizing the perturbations: the amplitude of gravitational wave and the perturbed three-space curvature in the uniform-field gauge both in the large-scale limit, and the angular-momentum of rotational perturbation are conserved independently of changing gravity sector. Implications for calculating perturbation spectra generated in the inflation era based on the string action are presented.Comment: 5 pages, no figure, To appear in Phys. Rev.