Relaxing Isocurvature Bounds on String Axion Dark Matter

If inflation scale is high, light scalars acquire large quantum fluctuations during inflation.If sufficiently long-lived, they will give rise to CDM isocurvature perturbations, which are highly constrained by the Planck data. Focusing on string axions as such light scalars, we show that thermal inflation can provide a sufficiently large entropy production to dilute the CDM isocurvature perturbations. Importantly, efficient dilution is possible for the string axions, because effectively no secondary coherent oscillations are induced at the end of thermal inflation, in contrast to the moduli fields. We also study the viability of the axion dark matter with mass of about 7 keV as the origin of the 3.5 keV X-ray line excess, in the presence of large entropy production.Comment: 18 pages, 2 figure

Forcibly driven coherent soft phonons in GeTe with intense THz-rate pump fields

We propose an experimental technique to generate large amplitude coherent phonons with irradiation of THz-rate pump pulses and to study the dynamics of phase transition in GeTe ferroelectrics. When a single pump pulse irradiates the sample at various pump power densities, the frequency of the soft phonon decreases sub-linearly and saturates at higher pump powers. By contrast, when THz-rate pump pulse sequence irradiates the sample at matched time intervals to forcibly drive the oscillation, a large red-shift of the phonon frequency is observed without saturation effects. After excitation with a four pump pulse sequence, the coherent soft phonon becomes strongly damped leading to a near critical damping condition. This condition indicates that the lattice is driven to a precursor state of the phase transition.Comment: 4 pages, 3 figure

Photoinduced Fano-resonance of coherent phonons in zinc

Utilizing femtosecond optical pump-probe technique, we have studied transient Fano-resonance in zinc. At high excitation levels the Fourier spectrum of the coherent E$_{2g}$ phonon exhibits strongly asymmetric line shape, which is well modeled by the Fano function. The Fano parameter (1/Q) was found to be strongly excitation fluence dependent while depending weakly on the initial lattice temperature. We attribute the origin of the Fano-resonance to the coupling of coherent phonon to the electronic continuum, with their transition probabilities strongly renormalized in the vicinity of the photoinduced structural transition.Comment: 5 pages, 3 figures, to be published in Physical Review

Inflation from a Supersymmetric Axion Model

We show that a supersymmetric axion model naturally induces a hybrid inflation with the waterfall field identified as a Peccei-Quinn scalar. The Peccei-Quinn scale is predicted to be around 10^{15}GeV for reproducing the large-scale density perturbation of the Universe. After the built-in late-time entropy-production process, the axion becomes a dark matter candidate. Several cosmological implications are discussed.Comment: 5 pages; to appear in PR

Fabrication of submicron alumina ceramics by pulse electric current sintering using M(2+) (M = Mg, Ca, Ni)-doped alumina nanopowders

Dense submicron-grained alumina ceramics were fabricated by pulse electric current sintering (PECS) using M(2+)(M: Mg, Ca, Ni)-doped alumina nanopowders at 1250 degrees C under a uniaxial pressure of 80 MPa. The M(2+)-doped alumina nanopowders (0-0.10 mass%) were prepared through a new sol-gel route using high-purity polyhydroxoaluminum (PHA) and MCl(2) solutions as starting materials. The composite gels obtained were calcined at 900 degrees C and ground by planetary ball milling. The powders were re-calcined at 900 degrees C to increase the content of a-alumina particles, which act as seeding for low-temperature densification. Densification and microstructural development depend on the M(2+) dopant species. Dense alumina ceramics (relative density >= 99.0%) thus obtained had a uniform microstructure composed of fine grains, where the average grain size developed for non-doped, Ni-doped, Mg-doped and Ca-doped samples was 0.67, 0.67, 0.47 and 0.30 mu m, respectively, showing that Ca-doping is the most promising method for tailoring of nanocrystalline alumina ceramics. (c) 2008 Elsevier Ltd and Techna Group S.r.l. All rights reserved.ArticleCERAMICS INTERNATIONAL. 35(5):1845-1850 (2009)journal articl