First-principles investigation of magnetism and electronic structures of substitutional 3d3d transition-metal impurities in bcc Fe

The magnetic and electronic structures of $3d$ impurity atoms from Sc to Zn in ferromagnetic body-centered cubic iron are investigated using the all-electron full-potential linearized augmented plane-wave method based on the generalized gradient approximation (GGA). We found that in general, the GGA results are closer to the experimental values than those of the local spin density approximation. The calculated formation enthalpy data indicate the importance of a systematic study on the ternary Fe-C-$X$ systems rather than the binary Fe-$X$ systems, in steel design. The lattice parameters are optimized and the conditions for spin polarization at the impurity sites are discussed in terms of the local Stoner model. Our calculations, which are consistent with previous work, imply that the local spin-polarizations at Sc, Ti, V, Cu, and Zn are induced by the host Fe atoms. The early transition-metal atoms couple antiferromagnetically, while the late transition-metal atoms couple ferromagnetically, to the host Fe atoms. The calculated total magnetization ($M$) of bcc Fe is reduced by impurity elements from Sc to Cr as a result of the antiferromagnetic interaction, with the opposite effect for solutes which couple ferromagnetically. The changes in $M$ are attributed to nearest neighbor interactions, mostly between the impurity and host atoms. The atom averaged magnetic moment is shown to follow generally the well-known Slater-Pauling curve, but our results do not follow the linearity of the Slater-Pauling curve. We attribute this discrepancy to the weak ferromagnetic nature of bcc Fe. The calculated Fermi contact hyperfine fields follow the trend of the local magnetic moments. The effect of spin-orbit coupling is found not to be significant although it comes into prominence at locations far from the impurity sites.Comment: 26 pages, 11 figure

Non-monotonic temperature dependent transport in graphene grown by Chemical Vapor Deposition

Temperature-dependent resistivity of graphene grown by chemical vapor deposition (CVD) is investigated. We observe in low mobility CVD graphene device a strong insulating behavior at low temperatures and a metallic behavior at high temperatures manifesting a non-monotonic in the temperature dependent resistivity.This feature is strongly affected by carrier density modulation. To understand this anomalous temperature dependence, we introduce thermal activation of charge carriers in electron-hole puddles induced by randomly distributed charged impurities. Observed temperature evolution of resistivity is then understood from the competition among thermal activation of charge carriers, temperature-dependent screening and phonon scattering effects. Our results imply that the transport property of transferred CVD-grown graphene is strongly influenced by the details of the environmentComment: 7 pages, 3 figure

Discovery of a Second Millisecond Accreting Pulsar: XTE J1751-305

We report the discovery by the RXTE PCA of a second transient accreting millisecond pulsar, XTE J1751-305, during regular monitoring observations of the galactic bulge region. The pulsar has a spin frequency of 435 Hz, making it one of the fastest pulsars. The pulsations contain the signature of orbital Doppler modulation, which implies an orbital period of 42 minutes, the shortest orbital period of any known radio or X-ray millisecond pulsar. The mass function, f_x = (1.278 +/- 0.003) x 10^{-6} M_sun, yields a minimum mass for the companion of between 0.013 and 0.017 M_sun, depending on the mass of the neutron star. No eclipses were detected. A previous X-ray outburst in June, 1998, was discovered in archival All-Sky Monitor data. Assuming mass transfer in this binary system is driven by gravitational radiation, we constrain the orbital inclination to be in the range 30-85 deg, and the companion mass to be 0.013-0.035 M_sun. The companion is most likely a heated helium dwarf. We also present results from the Chandra HRC-S observations which provide the best known position of XTE J1751-305.Comment: Astrophysical Journal Letters, Accepted, (AASTeX

Comment on "Accelerated Detectors and Temperature in (Anti) de Sitter Spaces"

It is shown how the results of Deser and Levin on the response of accelerated detectors in anti-de Sitter space can be understood from the same general perspective as other thermality results in spacetimes with bifurcate Killing horizons.Comment: 5 pages, LaTe

Binary spreading process with parity conservation

Recently there has been a debate concerning the universal properties of the phase transition in the pair contact process with diffusion (PCPD) $2A\to 3A, 2A\to \emptyset$. Although some of the critical exponents seem to coincide with those of the so-called parity-conserving universality class, it was suggested that the PCPD might represent an independent class of phase transitions. This point of view is motivated by the argument that the PCPD does not conserve parity of the particle number. In the present work we pose the question what happens if the parity conservation law is restored. To this end we consider the the reaction-diffusion process $2A\to 4A, 2A\to \emptyset$. Surprisingly this process displays the same type of critical behavior, leading to the conclusion that the most important characteristics of the PCPD is the use of binary reactions for spreading, regardless of whether parity is conserved or not.Comment: RevTex, 4pages, 4 eps figure