Cosmic ray spectral hardening due to dispersion in the source injection spectra

Recent cosmic ray (CR) experiments discovered that the CR spectra experience a remarkable hardening for rigidity above several hundred GV. We propose that this is caused by the superposition of the CR energy spectra of many sources that have a dispersion in the injection spectral indices. Adopting similar parameters as those of supernova remnants derived from the Fermi $\gamma$-ray observations, we can reproduce the observational CR spectra of different species well. This may be interpreted as evidence to support the supernova remnant origin of CRs below the knee. We further propose that the same mechanism may explain the "ankle" of the ultra high energy CR spectrum.Comment: 5 pages, 3 figures and 1 table. Updated with the diffusion propagation model, accepted by Phys. Rev.

Heavy-tailed statistics in short-message communication

Short-message (SM) is one of the most frequently used communication channels in the modern society. In this Brief Report, based on the SM communication records provided by some volunteers, we investigate the statistics of SM communication pattern, including the interevent time distributions between two consecutive short messages and two conversations, and the distribution of message number contained by a complete conversation. In the individual level, the current empirical data raises a strong evidence that the human activity pattern, exhibiting a heavy-tailed interevent time distribution, is driven by a non-Poisson nature.Comment: 4 pages, 4 figures and 1 tabl

Dielectric properties of hydrogen-incorporated chemical vapor deposited diamond thin films

Diamond thin films with a broad range of microstructures from a ultrananocrystalline diamond (UNCD) form developed at Argonne National Laboratory to a microcrystalline diamond (MCD) form have been grown with different hydrogen percentages in the Ar/CH4 gas mixture used in the microwave plasma enhanced chemical vapor deposition (CVD) process. The dielectric properties of the CVD diamond thin films have been studied using impedance and dc measurements on metal-diamond-metal test structures. Close correlations have been observed between the hydrogen content in the bulk of the diamond films, measured by elastic recoil detection (ERD), and their electrical conductivity and capacitance-frequency (C-f) behaviors. Addition of hydrogen gas in the Ar/CH4 gas mixture used to grow the diamond films appears to have two main effects depending on the film microstructure, namely, (a) in the UNCD films, hydrogen incorporates into the atomically abrupt grain boundaries satisfying sp2 carbon dangling bonds, resulting in increased resistivity, and (b) in MCD, atomic hydrogen produced in the plasma etches preferentially the graphitic phase codepositing with the diamond phase, resulting in the statistical survival and growth of large diamond grains and dominance of the diamond phase, and thus having significant impact on the dielectric properties of these films

Conductance fluctuations in a quantum dot under almost periodic ac pumping

It is shown that the variance of the linear dc conductance fluctuations in an open quantum dot under a high-frequency ac pumping depends significantly on the spectral content of the ac field. For a sufficiently strong ac field $\gamma\tau_{\phi}<< 1$, where $1/\tau_{\phi}$ is the dephasing rate induced by ac noise and $\gamma$ is the electron escape rate, the dc conductance fluctuations are much stronger for the harmonic pumping than in the case of the noise ac field of the same intensity. The reduction factor $r$ in a static magnetic field takes the universal value of 2 only for the white--noise pumping. For the strictly harmonic pumping $A(t)=A_{0}\cos\omega t$ of sufficiently large intensity the variance is almost insensitive to the static magnetic field $r-1= 2\sqrt{\tau_{\phi}\gamma} << 1$. For the quasi-periodic ac field of the form $A(t)=A_{0} [\cos(\omega_{1} t)+\cos(\omega_{2} t)]$ with $\omega_{1,2} >> \gamma$ and $\gamma\tau_{\phi} << 1$ we predict the novel effect of enchancement of conductance fluctuations at commensurate frequencies $\omega_{2}/\omega_{1}=P/Q$.Comment: 4 pages RevTex, 4 eps figures; the final version to appear in Phys.Rev.

Asymmetry of Strange Sea in Nucleons

Based on the finite-temperature field theory, we evaluate the medium effects in nucleon which can induce an asymmetry between quarks and antiquarks of the strange sea. The short-distance effects determined by the weak interaction can give rise to $\delta m\equiv \Delta m_s-\Delta m_{\bar s}$ where $\Delta m_{s(\bar s)}$ is the medium-induced mass of strange quark by a few KeV at most, but the long-distance effects by strong interaction are sizable. Our numerical results show that there exists an obvious mass difference between strange and anti-strange quarks, as large as 10-100 MeV.Comment: 15 latex pages, 3 figures, to appear in PR

Optical study of Dirac fermions and related phonon anomalies in the antiferromagnetic compound CaFeAsF

We performed optical studies on CaFeAsF single crystals, a parent compound of the 1111-type iron-based superconductors that undergoes a structural phase transition from tetragonal to orthorhombic at Ts=121 K and a magnetic one to a spin density wave (SDW) state at TN=110 K. In the low-temperature optical conductivity spectrum, after the subtraction of a narrow Drude peak, we observe a pronounced singularity around 300cm−1 that separates two regions of quasilinear conductivity. We outline that these characteristic absorption features are signatures of Dirac fermions, similar to what was previously reported for the BaFe2As2 system [Z.-G. Chen et al., Phys. Rev. Lett. 119, 096401 (2017)]. In support of this interpretation, we show that for the latter system this singular feature disappears rapidly upon electron and hole doping, as expected if it arises from a van Hove singularity in between two Dirac cones. Finally, we show that one of the infrared-active phonon modes (the Fe-As mode at 250cm−1) develops a strongly asymmetric line shape in the SDW state and note that this behavior can be explained in terms of a strong coupling with the Dirac fermions