Hole Transport in p-Type ZnO

A two-band model involving the A- and B-valence bands was adopted to analyze the temperature dependent Hall effect measured on N-doped \textit{p}-type ZnO. The hole transport characteristics (mobilities, and effective Hall factor) are calculated using the ``relaxation time approximation'' as a function of temperature. It is shown that the lattice scattering by the acoustic deformation potential is dominant. In the calculation of the scattering rate for ionized impurity mechanism, the activation energy of 100 or 170 meV is used at different compensation ratios between donor and acceptor concentrations. The theoretical Hall mobility at acceptor concentration of $7 \times 10^{18}$ cm$^3$ is about 70 cm$^2$V$^{-1}$s$^{-1}$ with the activation energy of 100 meV and the compensation ratio of 0.8 at 300 K. We also found that the compensation ratios conspicuously affected the Hall mobilities.Comment: 5page, 5 figures, accepted for publication in Jpn. J. Appl. Phy

In-situ photoemission study of Pr_{1-x}Ca_xMnO_3 epitaxial thin films with suppressed charge fluctuations

We have performed an {\it in-situ} photoemission study of Pr_{1-x}Ca_xMnO_3 (PCMO) thin films grown on LaAlO_3 (001) substrates and observed the effect of epitaxial strain on the electronic structure. We found that the chemical potential shifted monotonically with doping, unlike bulk PCMO, implying the disappearance of incommensurate charge fluctuations of bulk PCMO. In the valence-band spectra, we found a doping-induced energy shift toward the Fermi level (E_F) but there was no spectral weight transfer, which was observed in bulk PCMO. The gap at E_F was clearly seen in the experimental band dispersions determined by angle-resolved photoemission spectroscopy and could not be explained by the metallic band structure of the C-type antiferromagnetic state, probably due to localization of electrons along the ferromagnetic chain direction or due to another type of spin-orbital ordering.Comment: 5 pages, 4 figure

Chaotic hybrid new inflation in supergravity with a running spectral index

We propose an inflation model in supergravity, in which chaotic and hybrid inflation occurs successively, followed by new inflation. During hybrid inflation, adiabatic fluctuations with a running spectral index with \ns >1 on a large scale and \ns <1 on a smaller scale are generated, as favored by recent results of the first year Wilkinson Microwave Anisotropy Probe. The initial condition of new inflation is also set dynamically during hybrid inflation, and its duration and the amplitude of density fluctuations take appropriate values to help early star formation to realize early reionization.Comment: 15 pages, to appear in Phys. Rev.

Critical behavior of self-assembled rigid rods on triangular and honeycomb lattices

Using Monte Carlo simulations and finite-size scaling analysis, the critical behavior of self-assembled rigid rods on triangular and honeycomb lattices at intermediate density has been studied. The system is composed of monomers with two attractive (sticky) poles that, by decreasing temperature or increasing density, polymerize reversibly into chains with three allowed directions and, at the same time, undergo a continuous isotropic-nematic (IN) transition. The determination of the critical exponents, along with the behavior of Binder cumulants, indicate that the IN transition belongs to the q=1 Potts universality class.Comment: 6 pages, 5 figure

Cosmological Constraints on Late-time Entropy Production

We investigate cosmological effects concerning the late-time entropy production due to the decay of non-relativistic massive particles. The thermalization process of neutrinos after the entropy production is properly solved by using the Boltzmann equation. If a large entropy production takes place at late time t$\simeq$ 1 sec, it is found that a large fraction of neutrinos cannot be thermalized. This fact loosens the tight constraint on the reheating temperature T_R from the big bang nucleosynthesis and T_R could be as low as 0.5 MeV. The influence on the large scale structure formation and cosmic microwave background anisotropies is also discussed.Comment: 4 pages, using RevTeX and five postscript figures, comments added, to appear in Phys. Rev. Let

Gradual Disappearance of the Fermi Surface near the Metal-Insulator Transition in La1−x_{1-x}Srx_{x}MnO3_{3}

We report the first observation of changes in the electronic structure of La$_{1-x}$Sr$_{x}$MnO$_{3}$ (LSMO) across the filling-control metal-insulator (MI) transition by means of in situ angle-resolved photoemission spectroscopy (ARPES) of epitaxial thin films. The Fermi surface gradually disappears near the MI transition by transferring the spectral weight from the coherent band near the Fermi level ($E_{F}$) to the lower Hubbard band, whereas a pseudogap behavior also exists in the ARPES spectra in the close vicinity of $E_{F}$ for the metallic LSMO. These results indicate that the spectral weight transfer derived from strong electron-electron interaction dominates the gap formation in LSMO associated with the filling-control MI transition.Comment: 11 pages, 4 figure

Spectrum of Background X-rays from Moduli Dark Matter

We examine the $X$-ray spectrum from the decay of the dark-matter moduli with mass $\sim {\cal O}(100)$keV, in particular, paying attention to the line spectrum from the moduli trapped in the halo of our galaxy. It is found that with the energy resolution of the current experiments ($\sim 10$%) the line intensity is about twice stronger than that of the continuum spectrum from the moduli that spread in the whole universe. Therefore, in the future experiments with higher energy resolutions it may be possible to detect such line photons. We also investigate the $\gamma$-ray spectrum emitted from the decay of the multi-GeV moduli. It is shown that the emitted photons may form MeV-bump in the $\gamma$-ray spectrum. We also find that if the modulus mass is of the order of 10 GeV, the emitted photons at the peak of the continuum spectrum loses their energy by the scattering and the shape of the spectrum is significantly changed, which makes the constraint weaker than that obtained in the previous works.Comment: 14 pages (RevTeX file) including four postscript figures, reviced version to be published in Physical Review