arXiv subjects

Kazuhide Ichikawa

Publications and source records attributed to Kazuhide Ichikawa.

At least 37 records · Page 2Linked to original sources

arXiv2011

Electronic stress tensor analysis of hydrogenated palladium clusters

We study the chemical bonds of small palladium clusters Pd_n (n=2-9) saturated by hydrogen atoms using electronic stress tensor. Our calculation includes bond orders which are recently proposed based on the stress tensor. It is shown that our bond orders can classify the different types of chemical bonds in those clusters. In particular, we discuss Pd-H bonds associated with the H atoms with high coordination numbers and the difference of H-H bonds in the different Pd clusters from viewpoint of the electronic stress tensor. The notion of "pseudo-spindle structure" is proposed as the region between two atoms where the largest eigenvalue of the electronic stress tensor is negative and corresponding eigenvectors forming a pattern which connects them.

physics.chem-ph↗

arXiv2011

Inverted-sandwich-type and open-lantern-type dinuclear transition metal complexes: theoretical study of chemical bonds by electronic stress tensor

We study the electronic structure of two types of transition metal complexes, the inverted-sandwich-type and open-lantern-type, by the electronic stress tensor. In particular, the bond order b_e measured by the energy density which is defined from the electronic stress tensor is studied and compared with the conventional MO based bond order. We also examine the patterns found in the largest eigenvalue of the stress tensor and corresponding eigenvector field, the "spindle structure" and "pseudo-spindle structure". As for the inverted-sandwich-type complex, our bond order b_e calculation shows that relative strength of the metal-benzene bond among V, Cr and Mn complexes is V > Cr > Mn which is consistent with the MO based bond order. As for the open-lantern-type complex, we find that our energy density based bond order can properly describe the relative strength of Cr--Cr and Mo--Mo bonds by the surface integration of the energy density over the "Lagrange surface" which can take into account the spatial extent of the orbitals.

physics.chem-ph↗

arXiv2011

Theoretical Study of Hydrogenated Tetrahedral Aluminum Clusters

We report on the structures of aluminum hydrides derived from a tetrahedral aluminum Al4 cluster using ab initio quantum chemical calculation. Our calculation of binding energies of the aluminum hydrides reveals that stability of these hydrides increases as more hydrogen atoms are adsorbed, while stability of Al-H bonds decreases. We also analyze and discuss the chemical bonds of those clusters by using recently developed method based on the electronic stress tensor.

physics.atm-clus↗

arXiv2011

Microwave Emission from the Edgeworth-Kuiper Belt and the Asteroid Belt Constrained from WMAP

Objects in the Edgeworth-Kuiper belt and the main asteroid belt should emit microwaves that may give rise to extra anisotropy signals in the multipole of the cosmic microwave background (CMB) experiment. Constraints are derived from the absence of positive detection of such anisotropies for ell < 50, giving the total mass of Edgeworth-Kuiper belt objects to be smaller than 0.2 earth mass. This limit is consistent with the mass extrapolated from the observable population with the size of a > 15 km, assuming that the small-object population follows the power law in size dN/da ~ a^{-q} with the canonical index expected for collisional equilibrium, q ~ 3.5, with which 23% of the mass is ascribed to objects smaller than are observationally accessible down to grains. A similar argument applied to the main asteroid belt indicates that the grain population should not increase faster than q ~ 3.6 towards smaller radii, if it follows the power law continued to observed asteroids with larger radii. It is underlined that both cases are at or only slightly above the limit that can be physically significant, implying the importance of tightening further the CMB anisotropy limit, which may be attained with the observation at higher radio frequencies.

astro-ph.CO↗

arXiv2010

Electronic stress tensor of the hydrogen molecular ion: Comparison between the exact wave function and approximate wave functions using Gaussian basis sets

We investigate the electronic stress tensor of the hydrogen molecular ion for the ground state using the exact wave function and wave functions approximated by gaussian function basis set expansion. The spatial distribution of the largest eigenvalue, corresponding eigenvectors, tension and kinetic energy density are compared. We find that the cc-pV6Z basis set gives the spindle structure very close to the one calculated from the exact wave function. Similarly, energy density at the Lagrange point is very well approximated by the cc-pV5Z or cc-pV6Z basis sets.

physics.chem-ph↗

arXiv2009

Constraining the time variation of the coupling constants from cosmic microwave background: effect of Λ_{QCD}

We investigate constraints on the time variation of the fine structure constant between the recombination epoch and the present epoch, Δα/α\equiv (α_{rec} - α_{now})/α_{now}, from cosmic microwave background (CMB) taking into account simultaneous variation of other physical constants, namely the electron mass m_{e} and the proton mass m_{p}. In other words, we consider the variation of Yukawa coupling and the QCD scale Λ_{QCD} in addition to the electromagnetic coupling. We clarify which parameters can be determined from CMB temperature anisotropy in terms of singular value decomposition. Assuming a relation among variations of coupling constants governed by a single scalar field (the dilaton), the 95% confidence level (C.L.) constraint on Δα/αis found to be -8.28 \times 10^{-3} < Δα/α< 1.81 \times 10^{-3}, which is tighter than the one obtained by considering only the change of αand m_{e}. We also obtain the constraint on the time variation of the proton-to-electron mass ratio μ\equiv m_{p}/m_{e} to be -0.52 < Δμ/μ< 0.17 (95% C.L.) under the same assumption. Finally, we also implement a forecast for constraints from the PLANCK survey.

astro-ph.CO↗

arXiv2009

Stress Tensor of the Hydrogen Molecular Ion

The electronic stress tensor of the hydrogen molecule ion H_2^+ is investigated for the ground state (sigma_g 1s) and the first excited state (sigma_u^* 1s) using their exact wave functions. A map of its largest eigenvalue and corresponding eigenvector is shown to be closely related to the nature of chemical bonding. For the ground state, we also show the spatial distribution of interaction energy density to describe in which part of the molecule stabilization and destabilization take place.

physics.chem-ph↗

arXiv2009

Neutrino mass from cosmology: Impact of high-accuracy measurement of the Hubble constant

Non-zero neutrino mass would affect the evolution of the Universe in observable ways, and a strong constraint on the mass can be achieved using combinations of cosmological data sets. We focus on the power spectrum of cosmic microwave background (CMB) anisotropies, the Hubble constant H_0, and the length scale for baryon acoustic oscillations (BAO) to investigate the constraint on the neutrino mass, m_nu. We analyze data from multiple existing CMB studies (WMAP5, ACBAR, CBI, BOOMERANG, and QUAD), recent measurement of H_0 (SHOES), with about two times lower uncertainty (5%) than previous estimates, and recent treatments of BAO from the Sloan Digital Sky Survey (SDSS). We obtained an upper limit of m_nu < 0.2eV (95% C.L.), for a flat LambdaCDM model. This is a 40% reduction in the limit derived from previous H_0 estimates and one-third lower than can be achieved with extant CMB and BAO data. We also analyze the impact of smaller uncertainty on measurements of H_0 as may be anticipated in the near term, in combination with CMB data from the Planck mission, and BAO data from the SDSS/BOSS program. We demonstrate the possibility of a 5 sigma detection for a fiducial neutrino mass of 0.1eV or a 95% upper limit of 0.04eV for a fiducial of m_nu = 0eV. These constraints are about 50% better than those achieved without external constraint. We further investigate the impact on modeling where the dark-energy equation of state is constant but not necessarily -1, or where a non-flat universe is allowed. In these cases, the next-generation accuracies of Planck, BOSS, and 1% measurement of H_0 would all be required to obtain the limit m_nu < 0.05 - 0.06eV (95% C.L.) for the fiducial of m_nu = 0eV. The independence of systematics argues for pursuit of both BAO and H_0 measurements.

astro-ph.CO↗

arXiv2009

A Theoretical Study on a Reaction of Iron(III) Hydroxide with Boron Trichloride by Ab Initio Calculation

We investigate a reaction of boron trichloride (BCl3) with iron(III) hydroxide (Fe(OH)3) by ab initio quantum chemical calculation as a simple model for a reaction of iron impurities in BCl3 gas. We also examine a reaction with water. We find that compounds such as Fe(Cl)(OBCl2)2(OHBCl2) and Fe(Cl)2(OBCl2)(OHBCl2) are formed while producing HCl and reaction paths to them are revealed. We also analyze the stabilization mechanism of these paths using newly-developed interaction energy density derived from electronic stress tensor in the framework of the Regional DFT (Density Functional Theory) and Rigged QED (Quantum ElectroDynamics).

physics.chem-ph↗

arXiv2009

Constraining Light Gravitino Mass from Cosmic Microwave Background

We investigate the possibilities of constraining the light gravitino mass m_{3/2} from future cosmic microwave background (CMB) surveys. A model with light gravitino with the mass m_{3/2}<O(10) eV is of great interest since it is free from the cosmological gravitino problem and, in addition, can be compatible with many baryogenesis/leptogenesis scenarios such as the thermal leptogenesis. We show that the lensing of CMB anisotropies can be a good probe for m_{3/2} and obtain an expected constraint on m_{3/2} from precise measurements of lensing potential in the future CMB surveys, such as the PolarBeaR and CMBpol experiments. If the gravitino mass is m_{3/2}=1 eV, we will obtain the constraint for the gravitino mass as m_{3/2} < 3.2 eV (95% C.L.) for the case with Planck+PolarBeaR combined and m_{3/2}=1.04^{+0.22}_{-0.26} eV (68% C.L.) for CMBpol. The issue of Bayesian model selection is also discussed.

astro-ph.CO↗

arXiv2009

Measuring the History of Cosmic Reionization using the 21-cm PDF from Simulations

The 21-cm PDF (i.e., distribution of pixel brightness temperatures) is expected to be highly non-Gaussian during reionization and to provide important information on the distribution of density and ionization. We measure the 21-cm PDF as a function of redshift in a large simulation of cosmic reionization and propose a simple empirical fit. Guided by the simulated PDF, we then carry out a maximum likelihood analysis of the ability of upcoming experiments to measure the shape of the 21-cm PDF and derive from it the cosmic reionization history. Under the strongest assumptions, we find that upcoming experiments can measure the reionization history in the mid to late stages of reionization to 1-10% accuracy. Under a more flexible approach that allows for four free parameters at each redshift, a similar accuracy requires the lower noise levels of second-generation 21-cm experiments.

astro-ph.CO↗

arXiv2009

Constraints on neutrino masses from WMAP5 and BBN in the lepton asymmetric universe

In this paper, we put constraints on neutrino properties such as mass $m_ν$ and degeneracy parameters $ξ_i$ from WMAP5 data and light element abundances by using a Markov chain Monte Carlo (MCMC) approach. In order to take consistently into account the effects of the degeneracy parameters, we run the Big Bang Nucleosynthesis code for each value of $ξ_i$ and the other cosmological parameters to estimate the Helium abundance, which is then used to calculate CMB anisotropy spectra instead of treating it as a free parameter. We find that the constraint on $m_ν$ is fairly robust and does not vary very much even if the lepton asymmetry is allowed, and is given by $\sum m_ν< 1.3 \rm eV$ ($95 % \rm C.L.$).

astro-ph.CO↗

arXiv2008

Probing the Effective Number of Neutrino Species with Cosmic Microwave Background

We discuss how much we can probe the effective number of neutrino species N_nu with cosmic microwave background alone. Using the data of WMAP, ACBAR, CBI and BOOMERANG experiments, we obtain a constraint on the effective number of neutrino species as 0.96< N_nu <7.94 at 95% C.L. for a power-law LCDM flat universe model. The limit is improved to be 1.39 < N_nu < 6.38 at 95% C.L. if we assume that the baryon density, N_nu and the helium abundance are related by the big bang nucleosynthesis theory. We also provide a forecast for the PLANCK experiment using a Markov chain Monte Carlo approach. In addition to constraining N_nu, we investigate how the big bang nucleosynthesis relation affects the estimation for these parameters and other cosmological parameters.

astro-ph↗

arXiv2008

Primordial Helium Abundance from CMB: a constraint from recent observations and a forecast

We studied a constraint on the primordial helium abundance Y_p from current and future observations of CMB. Using the currently available data from WMAP, ACBAR, CBI and BOOMERANG, we obtained the constraint as Y_p = 0.25^{+0.10}_{-0.07} at 68% C.L. We also provide a forecast for the Planck experiment using the Markov chain Monte Carlo approach. In addition to forecasting the constraint on Y_p, we investigate how assumptions for Y_p affect constraints on the other cosmological parameters.

astro-ph↗

arXiv2008

Non-Gaussianity, Spectral Index and Tensor Modes in Mixed Inflaton and Curvaton Models

We study non-Gaussianity, the spectral index of primordial scalar fluctuations and tensor modes in models where fluctuations from the inflaton and the curvaton can both contribute to the present cosmic density fluctuations. Even though simple single-field inflation models generate only tiny non-Gaussianity, if we consider such a mixed scenario, large non-Gaussianity can be produced. Furthermore, we study the inflationary parameters such as the spectral index and the tensor-to-scalar ratio in this kind of models and discuss in what cases models predict the spectral index and tensor modes allowed by the current data while generating large non-Gaussianity, which may have many implications for model-buildings of the inflationary universe.

astro-ph↗

arXiv2008

Primordial Curvature Fluctuation and Its Non-Gaussianity in Models with Modulated Reheating

We investigate non-Gaussianity in the modulated reheating scenario where fluctuations of the decay rate of the inflaton generate adiabatic perturbations, paying particular attention to the non-linearity parameters $f_{\rm NL}, τ_{\rm NL}$ and $g_{\rm NL}$ as well as the scalar spectral index and tensor-to-scalar ratio which characterize the nature of the primordial power spectrum. We also take into account the pre-existing adiabatic perturbations produced from the inflaton fluctuations. It has been known that the non-linearity between the curvature perturbations and the fluctuations of the decay rate can yield non-Gaussianity at the level of $f_{\rm NL} \sim \mathcal{O}(1)$, but we find that the non-linearity between the decay rate and the modulus field which determines the decay rate can generate much greater non-Gaussianity. We also discuss a consistency relation among non-linearity parameters which holds in the scenario and find that the modulated reheating yields a different one from that of the curvaton model. In particular, they both can yield a large positive $f_{\rm NL}$ but with a different sign of $g_{\rm NL}$. This provides a possibility to discriminate these two competitive models by looking at the sign of $g_{\rm NL}$. Furthermore, we work on some concrete inflation models and investigate in what cases models predict the spectral index and the tensor-to-scalar ratio allowed by the current data while generating large non-Gaussianity, which may have many implications for model-buildings of the inflationary universe.

astro-ph↗

arXiv2008

The Hubble constant and dark energy from cosmological distance measures

We study how the determination of the Hubble constant from cosmological distance measures is affected by models of dark energy and vice versa. For this purpose, constraints on the Hubble constant and dark energy are investigated using the cosmological observations of cosmic microwave background, baryon acoustic oscillations and type Ia suprenovae. When one investigates dark energy, the Hubble constant is often a nuisance parameter, thus it is usually marginalized over. On the other hand, when one focuses on the Hubble constant, simple dark energy models such as a cosmological constant and a constant equation of state are usually assumed. Since we do not know the nature of dark energy yet, it is interesting to investigate the Hubble constant assuming some types of dark energy and see to what extent the constraint on the Hubble constant is affected by the assumption concerning dark energy. We show that the constraint on the Hubble constant is not affected much by the assumption for dark energy. We furthermore show that this holds true even if we remove the assumption that the universe is flat. We also discuss how the prior on the Hubble constant affects the constraints on dark energy and/or the curvature of the universe.

astro-ph↗

arXiv2008

On the Determination of Neutrino Masses and Dark Energy Evolution from the Cross Correlation of CMB and LSS

We discuss the possibilities of the simultaneous determination of the neutrino masses and the evolution of dark energy from future cosmological observations such as cosmic microwave background (CMB), large scale structure (LSS) and the cross correlation between them. Recently it has been discussed that there is a degeneracy between the neutrino masses and the equation of state for dark energy. It is also known that there are some degeneracies among the parameters describing the dark energy evolutions. We discuss the implications of these on the cross correlation of CMB with LSS in some details. Then we consider to what extent we can determine the neutrino masses and the dark energy evolution using the expected data from CMB, LSS and their cross correlation.

astro-ph↗