Ultrafast dephasing of coherent optical phonons in atomically controlled GeTe/Sb2_{2}Te3_{3} superlattices

Femtosecond dynamics of coherent optical phonons in GeTe/Sb$_{2}$Te$_{3}$ superlattices (SLs), a new class of semiconductor SLs with three different states, have been investigated by using a reflection-type pump-probe technique at various lattice temperatures. The time-resolved transient reflectivity (TR) obtained in as-grown SLs exhibits the coherent A$_{1}$ optical modes at 5.10 THz and 3.78 THz, while only the single A$_{1}$ mode at 3.68 THz is observed in annealed SLs. The decay rate of the A$_{1}$ mode in annealed SLs is strongly temperature dependent, while that in as-grown SLs is not temperature dependent. This result indicates that the damping of the coherent A$_{1}$ phonons in amorphous SLs is governed by the phonon-defect (vacancy) scattering rather than the anharmonic phonon-phonon coupling.Comment: 5 pages, 5 figure

Nucleosynthesis in Core-Collapse Supernovae and GRB--Metal-Poor Star Connection

We review the nucleosynthesis yields of core-collapse supernovae (SNe) for various stellar masses, explosion energies, and metallicities. Comparison with the abundance patterns of metal-poor stars provides excellent opportunities to test the explosion models and their nucleosynthesis. We show that the abundance patterns of extremely metal-poor (EMP) stars, e.g., the excess of C, Co, Zn relative to Fe, are in better agreement with the yields of hyper-energetic explosions (Hypernovae, HNe) rather than normal supernovae. We note that the variation of the abundance patterns of EMP stars are related to the diversity of the Supernova-GRB connection. We summarize the diverse properties of (1) GRB-SNe, (2) Non-GRB HNe/SNe, (3) XRF-SN, and (4) Non-SN GRB. In particular, the Non-SN GRBs (dark hypernovae) have been predicted in order to explain the origin of C-rich EMP stars. We show that these variations and the connection can be modeled in a unified manner with the explosions induced by relativistic jets. Finally, we examine whether the most luminous supernova 2006gy can be consistently explained with the pair-instability supernova model.Comment: 15 pages, 9 figures. To appear in "Supernova 1987A: 20 Years After: Supernovae and Gamma-Ray Bursters", eds. S. Immler, K. Weiler, & R. McCray (American Institute of Physics) (2007

The Connection between Gamma-Ray Bursts and Extremely Metal-Poor Stars as Nucleosynthetic Probes of the Early Universe

The connection between the long GRBs and Type Ic Supernovae (SNe) has revealed the interesting diversity: (i) GRB-SNe, (ii) Non-GRB Hypernovae (HNe), (iii) X-Ray Flash (XRF)-SNe, and (iv) Non-SN GRBs (or dark HNe). We show that nucleosynthetic properties found in the above diversity are connected to the variation of the abundance patterns of extremely-metal-poor (EMP) stars, such as the excess of C, Co, Zn relative to Fe. We explain such a connection in a unified manner as nucleosynthesis of hyper-aspherical (jet-induced) explosions Pop III core-collapse SNe. We show that (1) the explosions with large energy deposition rate, $\dot{E}_{\rm dep}$, are observed as GRB-HNe and their yields can explain the abundances of normal EMP stars, and (2) the explosions with small $\dot{E}_{\rm dep}$ are observed as GRBs without bright SNe and can be responsible for the formation of the C-rich EMP (CEMP) and the hyper metal-poor (HMP) stars. We thus propose that GRB-HNe and the Non-SN GRBs (dark HNe) belong to a continuous series of BH-forming stellar deaths with the relativistic jets of different $\dot{E}_{\rm dep}$.Comment: 8 pages, 6 figures. To appear in "Massive Stars as Cosmic Engines", Proceedings of IAU Symposium 250 (December 2007, Kauai), eds. F. Bresolin, P.A. Crowther, & J. Puls (Cambridge Univ. Press

The utilization of YAP Scintillation Detector for Soft Gamma Radiation Measurement in Backscatter Thickness Gauge

ABSTRACT Interesting properties of a YAP(Ce) scintillator make it an alternative solution for low energy gamma measurement in high counting rates that previously employed GM counter tubes and/or Nal(Tl) scintillators. Some characteristics of the YAP(Ce) crystal combined with a photomultiplier tube have been successfully demonstrated in a backscatter mode of thickness gauging with 241Am gamma-source.

Presupernova evolution and explosive nucleosynthesis of zero metal massive stars

We present a new set of zero metallicity models in the range 13-80 $\rm M_\odot$ together to the associated explosive nucleosynthesis. These models are fully homogeneous with the solar metallicity set we published in Limongi & Chieffi (2006) and will be freely available at the web site http://www.iasf-roma.inaf.it./orfeo/public{\_}html. A comparison between these yields and an average star that represents the average behavior of most of the very metal poor stars in the range $\rm -5.0<[Fe/H]<-2.5$ confirms previous findings that only a fraction of the elemental [X/Fe] may be fitted by the ejecta of $\it standard$ core collapse supernovae.Comment: 39 pages, 8 figures, 2 tables, accepted for publication in ApJ

Dynamical Susceptibility in KDP-type Crysals above and below Tc II

The path probability method (PPM) in the tetrahedron-cactus approximation is applied to the Slater-Takagi model with dipole-dipole interaction for KH2PO4-type hydrogen-bonded ferroelectric crystals in order to derive a small dip structure in the real part of dynamical susceptibility observed at the transition temperature Tc. The dip structure can be ascribed to finite relaxation times of electric dipole moments responsible for the first order transition with contrast to the critical slowing down in the second order transition. The light scattering intensity which is related to the imaginary part of dynamical susceptibility is also calculated above and below the transition temperature and the obtained central peak structure is consistent with polarization fluctuation modes in Raman scattering experiments.Comment: 8 pages, 11 figure

SN 2006aj Associated with XRF 060218 At Late Phases: Nucleosynthesis-Signature of A Neutron Star-Driven Explosion

Optical spectroscopy and photometry of SN 2006aj have been performed with the Subaru telescope at t > 200 days after GRB060218, the X-ray Flash with which it was associated. Strong nebular emission-lines with an expansion velocity of v ~ 7,300 km/s were detected. The peaked but relatively broad [OI]6300,6363 suggests the existence of ~ 2 Msun of materials in which ~1.3 Msun is oxygen. The core might be produced by a mildly asymmetric explosion. The spectra are unique among SNe Ic in (1) the absence of [CaII]7291,7324 emission, and (2) a strong emission feature at ~ 7400A, which requires ~ 0.05 Msun of newly-synthesized 58Ni. Such a large amount of stable neutron-rich Ni strongly indicates the formation of a neutron star. The progenitor and the explosion energy are constrained to 18 Msun < Mms < 22 Msun and E ~ (1 - 3) 10^{51} erg, respectively.Comment: Accepted for Publication in the Astrophysical Journal Letters (2007, ApJ, 658, L5). 8 pages, including 1 table and 3 figures. Typos correcte