Scaling behavior of spin transport in hydrogenated graphene

We calculate the spin transport of hydrogenated graphene using the Landauer-B\"uttiker formalism with a spin-dependent tight-binding Hamiltonian. The advantages of using this method is that it simultaneously gives information on sheet resistance and localization length as well as spin relaxation length. Furthermore, the Landauer-B\"uttiker formula can be computed very efficiently using the recursive Green's function technique. Previous theoretical results on spin relaxation time in hydrogenated graphene have not been in agreement with experiments. Here, we study magnetic defects in graphene with randomly aligned magnetic moments, where interference between spin-channels is explicitly included. We show that the spin relaxation length and sheet resistance scale nearly linearly with the impurity concentration. Moreover, the spin relaxation mechanism in hydrogenated graphene is Markovian only near the charge neutrality point or in the highly dilute impurity limit

Electronic and phononic Raman scattering in detwinned YBa2_2Cu3_3O6.95_{6.95} and Y0.85_{0.85}Ca0.15_{0.15}Ba2_2Cu3_3O6.95_{6.95}: s-wave admixture to the dx2−y2d_{x^2-y^2}-wave order parameter

Inelastic light (Raman) scattering has been used to study electronic excitations and phonon anomalies in detwinned, slightly overdoped YBa$_2$Cu$_3$O$_{6.95}$ and moderately overdoped Y$_{0.85}$Ca$_{0.15}$Ba$_2$Cu$_3$O$_{6.95}$ single crystals. In both samples modifications of the electronic pair-breaking peaks when interchanging the a- and b-axis were observed. The lineshapes of several phonon modes involving plane and apical oxygen vibrations exhibit pronounced anisotropies with respect to the incident and scattered light field configurations. Based on a theoretical model that takes both electronic and phononic contributions to the Raman spectra into account, we attribute the anisotropy of the superconductivity-induced changes in the phonon lineshapes to a small s-wave admixture to the $d_{x^2-y^2}$ pair wave-function. Our theory allows us to disentangle the electronic Raman signal from the phononic part and to identify corresponding interference terms. We argue that the Raman spectra are consistent with an s-wave admixture with an upper limit of 20 percent.Comment: accepted in Phys. Rev. B, 11 page

The phonon dispersion of graphite by inelastic x-ray scattering

We present the full in-plane phonon dispersion of graphite obtained from inelastic x-ray scattering, including the optical and acoustic branches, as well as the mid-frequency range between the $K$ and $M$ points in the Brillouin zone, where experimental data have been unavailable so far. The existence of a Kohn anomaly at the $K$ point is further supported. We fit a fifth-nearest neighbour force-constants model to the experimental data, making improved force-constants calculations of the phonon dispersion in both graphite and carbon nanotubes available.Comment: 7 pages; submitted to Phys. Rev.

Ion observations from geosynchronous orbit as a proxy for ion cyclotron wave growth during storm times

[1] There is still much to be understood about the processes contributing to relativistic electron enhancements and losses in the radiation belts. Wave particle interactions with both whistler and electromagnetic ion cyclotron (EMIC) waves may precipitate or accelerate these electrons. This study examines the relation between EMIC waves and resulting relativistic electron flux levels after geomagnetic storms. A proxy for enhanced EMIC waves is developed using Los Alamos National Laboratory Magnetospheric Plasma Analyzer plasma data from geosynchronous orbit in conjunction with linear theory. In a statistical study using superposed epoch analysis, it is found that for storms resulting in net relativistic electron losses, there is a greater occurrence of enhanced EMIC waves. This is consistent with the hypothesis that EMIC waves are a primary mechanism for the scattering of relativistic electrons and thus cause losses of such particles from the magnetosphere

Efficient qubit detection using alkali earth metal ions and a double STIRAP process

We present a scheme for robust and efficient projection measurement of a qubit consisting of the two magnetic sublevels in the electronic ground state of alkali earth metal ions. The scheme is based on two stimulated Raman adiabatic passages (STIRAP) involving four partially coherent laser fields. We show how the efficiency depends on experimentally relevant parameters: Rabi frequencies, pulse widths, laser linewidths, one- and two-photon detunings, residual laser power, laser polarization and ion motion.Comment: 14 pages, 15 figure

The Distance to the Coma Cluster from Surface Brightness Fluctuations

We report on the first determination of the distance to the Coma Cluster based on surface brightness fluctuation (SBF) measurements obtained from Hubble Space Telescope WFPC2 observations of the bright E0 galaxy NGC 4881 in the Coma Cluster and ground-based observations of the standard E1 galaxy NGC 3379 in the Leo-I group. Relative distances based on the I-band fluctuation magnitude, I(SBF), are strongly dependent on metallicity and age of the stellar population. However, the radial changes in the stellar populations of the two giant ellipticals, NGC 3379 and NGC 4881, are well described by published Mg_2 gradients, and the ground-based measurements of I(SBF) at several radial points in NGC 3379 are used to calibrate I(SBF) in terms of the Mg_2 index. The distance to NGC 3379, assumed to be identical to the average SBF distance of the Leo-I group, is combined with the new SBF measurements of NGC 4881 to obtain a Coma Cluster distance of 102+-14 Mpc. Combining this distance with the cosmic recession velocity of Coma (7186+-428 km/s), we find the Hubble constant to be H_0 = 71+-11 km/s/Mpc.Comment: 12 pages, LaTex, includes aaspp4.sty and 3 eps figures. To appear in ApJ Letter

States for phase estimation in quantum interferometry

Ramsey interferometry allows the estimation of the phase $\phi$ of rotation of the pseudospin vector of an ensemble of two-state quantum systems. For $\phi$ small, the noise-to-signal ratio scales as the spin-squeezing parameter $\xi$, with $\xi<1$ possible for an entangled ensemble. However states with minimum $\xi$ are not optimal for single-shot measurements of an arbitrary phase. We define a phase-squeezing parameter, $\zeta$, which is an appropriate figure-of-merit for this case. We show that (unlike the states that minimize $\xi$), the states that minimize $\zeta$ can be created by evolving an unentangled state (coherent spin state) by the well-known 2-axis counter-twisting Hamiltonian. We analyse these and other states (for example the maximally entangled state, analogous to the optical "NOON" state $|\psi> = (|N,0>+|0,N>)/\sqrt{2}$) using several different properties, including $\xi$, $\zeta$, the coefficients in the pseudo angular momentum basis (in the three primary directions) and the angular Wigner function $W(\theta,\phi)$. Finally we discuss the experimental options for creating phase squeezed states and doing single-shot phase estimation.Comment: 8 pages and 5 figure

The Determination of Temperature in Methane-Air, Propane-Air and Butane-Air Flames Using the Raman Q-Branch Spectra of Nitrogen

The unresolved Q-branch profiles in the Raman spectra of\u27 nitrogen in premixed, laminar, methane-air, propane-air and butane- air flames with lean to stoichiometric fuel-air equivalence ratios have been recorded experimentally and used to determine flame temperature s with a precision of about 1% using an iterative computer program which fitted calculated nitrogen spectra to the experimentally recorded spectra. Axial and radial temperature profiles in these flames are presented and the results compared with calculated adiabatic flame temperatures

Boundary effects in finite size plasmonic crystals: Focusing and routing of plasmonic beams for optical communications

Plasmonic crystals, which consist of periodic arrangements of surface features at a metal-dielectric interface, allow the manipulation of optical information in the form of surface plasmon polaritons. Here we investigate the excitation and propagation of plasmonic beams in and around finite size plasmonic crystals at telecom wavelengths, highlighting the effects of the crystal boundary shape and illumination conditions. Significant differences in broad plasmonic beam generation by crystals of different shapes are demonstrated, while for narrow beams, the propagation onto the smooth metal film is less sensitive to the crystal boundary shape. We show that by controlling the boundary shape, the size and the excitation beam parameters, directional control of propagating plasmonic modes and associated beam parameters such as angular beam splitting, focusing power and beam width can be efficiently achieved. This provides a promising route for robust and alignment-independent integration of plasmonic crystals with optical communication components