arXiv subjects
Publications and source records attributed to N. Watanabe.
In quantum information theory, it is important to find modulations with low information loss for noisy channels. In this paper, using the quantum dynamical entropy and the quantum dynamical mutual entropy, we investigate the transmission efficiency of two quantum modulators through attenuation channels.
In molecular clouds at temperatures as low as 10 K, all species except hydrogen and helium should be locked in the heterogeneous ice on dust grain surfaces. Nevertheless, astronomical observations have detected over 150 different species in the gas phase in these clouds. The mechanism by which molecules are released from the dust surface below thermal desorption temperatures to be detectable in the gas phase is crucial for understanding the chemical evolution in such cold clouds. Chemical desorption, caused by the excess energy of an exothermic reaction, was first proposed as a key molecular release mechanism almost 50 years ago. Chemical desorption can, in principle, take place at any temperature, even below the thermal desorption temperature. Therefore, astrochemical net- work models commonly include this process. Although there have been a few previous experimental efforts, no infrared measurement of the surface (which has a strong advantage to quantify chemical desorption) has been performed. Here, we report the first infrared in situ measurement of chemical desorption during the reactions H + H2S -> HS + H2 (reaction 1) and HS + H -> H2S (reaction 2), which are key to interstellar sulphur chemistry. The present study clearly demonstrates that chemical desorption is a more efficient process for releasing H2S into the gas phase than was previously believed. The obtained effective cross-section for chemical desorption indicates that the chemical desorption rate exceeds the photodesorption rate in typical interstellar environments.
We present the discovery and confirmation of two new transiting giant planets from the Kepler extended mission K2. K2-260 b is a hot Jupiter transiting a $V=12.7$ F6V star in K2 Field 13, with a mass and radius of $M_{\star}=1.39_{-0.06}^{+0.05} M_{\odot}$ and $R_{\star}=1.69 \pm 0.03 R_{\odot}$. The planet has an orbital period of $P=2.627$ days, and a mass and radius of $M_P=1.42^{+0.31}_{-0.32} M_J$ and $R_P=1.552^{+0.048}_{-0.057} R_J$. This is the first K2 hot Jupiter with a detected secondary eclipse in the Kepler bandpass, with a depth of $71 \pm 15$ ppm, which we use to estimate a geometric albedo of $A_g\sim0.2$. We also detected a candidate stellar companion at 0.6" from K2-260; we find that it is very likely physically associated with the system, in which case it would be an M5-6V star at a projected separation of $\sim400$ AU. K2-261 b is a warm Saturn transiting a bright ($V=10.5$) G7IV/V star in K2 Field 14. The host star is a metal-rich ([Fe/H]$=0.36 \pm 0.06$), mildly evolved $1.10_{-0.02}^{+0.01} M_{\odot}$ star with $R_{\star}=1.65 \pm 0.04 R_{\odot}$. Thanks to its location near the main sequence turn-off, we can measure a relatively precise age of $8.8_{-0.3}^{+0.4}$ Gyr. The planet has $P=11.633$ days, $M_P=0.223 \pm 0.031 M_J$, and $R_P=0.850^{+0.026}_{-0.022} R_J$, and its orbit is eccentric ($e=0.39 \pm 0.15$). Its brightness and relatively large transit depth make this one of the best known warm Saturns for follow-up observations to further characterize the planetary system.
We propose a dynamical (quintessence) model of dark energy in the current universe with a renormalizable (Higgs-like) scalar potential. We prove the viability of our model (after fine tuning) for the certain range of the average scalar curvature values, and study the cosmological signatures distinguishing our model from the standard description of dark energy in terms of a cosmological constant.
The present study experimentally demonstrated that solid H2O is formed through the surface reaction OH + H2 at 10 K. This is the first experimental evidence of solid H2O formation using hydrogen in its molecular form at temperatures as low as 10 K. We further found that H2O formation through the reaction OH + H2 is about one order of magnitude more effective than HDO formation through the reaction OH + D2. This significant isotope effect results from differences in the effective mass of each reaction, indicating that the reactions proceed through quantum tunneling.
We review our recently developed electronic structure calculation methods used for the dynamics of large-scale solids or liquids with an efficient algorithm for large scale simultaneous linear equations. The electronic structure calculation method is the `atomic superposition and electron delocalization molecular orbitals theory' (ASED), using the Mulliken charge density. Very crucial algorithm is the generalized shifted COCG (conjugate orthogonal conjugate gradient) method based on the Krylov subspace extended to non-orthogonal basis set. The most important techniques for applications are the shifted equation and the seed switching method, which make the computational cost be reduced much. We, then, present some applications to electronic structure calculations with MD simulation. The applications are given to the fracture propagation in nano-scale Si crystals and the proton transfer in water.
We revisit the old (fourth-order or quadratically generated) gravity model of Starobinsky in four space-time dimensions, and derive the (inflaton) scalar potential in the equivalent scalar-tensor gravity model via a Legendre-Weyl transform. The inflaton scalar potential is used to compute the (CMB) observables of inflation associated with curvature perturbations (namely, the scalar and tensor spectral indices, and the tensor-to-scalar ratio), including the new next-to-leading-order terms with respect to the inverse number of e-foldings. The results are compared to the recent (WMAP5) experimental bounds. We confirm both mathematical and physical equivalence between f(R) gravity theories and the corresponding scalar-tensor gravity theories.
We reconsider the toy-model of topological inflation, based on the R*4-modified gravity. By using its equivalence to the certain scalar-tensor gravity model in four space-time dimensions, we compute the inflaton scalar potential and investigate a possibility of inflation. We confirm the existence of the slow-roll inflation with an exit. However, the model suffers from the eta-problem that gives rise to the unacceptable value of the spectral index n_s of scalar perturbations.
This paper reports the result of our evaluation of the tenth-order QED correction to the lepton g-2 from Feynman diagrams which have sixth-order light-by-light-scattering subdiagrams, none of whose vertices couple to the external magnetic field. The gauge-invariant set of these diagrams, called Set II(e), consists of 180 vertex diagrams. In the case of the electron g-2 (a_e), where the light-by-light subdiagram consists of the electron loop, the contribution to a_e is found to be - 1.344 9 (10) (α/π)^5. The contribution of the muon loop to a_e is - 0.000 465 (4) (α/π)^5. The contribution of the tau-lepton loop is about two orders of magnitudes smaller than that of the muon loop and hence negligible. The sum of all of these contributions to a_e is - 1.345 (1) (α/π)^5. We have also evaluated the contribution of Set II(e) to the muon g-2 (a_μ). The contribution to a_μfrom the electron loop is 3.265 (12) (α/π)^5, while the contribution of the tau-lepton loop is -0.038 06 (13) (α/π)^5. The total contribution to a_μ, which is the sum of these two contributions and the mass-independent part of a_e, is 1.882 (13) (α/π)^5.
We have evaluated the contribution to the anomalous magnetic moment of the electron from six tenth-order Feynman diagrams which contain eighth-order vacuum-polarization function formed by two light-by-light scattering diagrams connected by three photons. The integrals are constructed by two different methods. In the first method the subtractive counter terms are used to deal with ultraviolet (UV) singularities together with the requirement of gauge-invariance. In the second method, the Ward-Takahashi identity is applied to the light-by-light scattering amplitudes to eliminate UV singularities. Numerical evaluation confirms that the two methods are consistent with each other within their numerical uncertainties. Combining the two results statistically and adding small contribution from the muons and/or tau leptons, we obtain $ 0.000 399 9 (18) (α/π)^5$. We also evaluated the contribution to the muon $g-2$ from the same set of diagrams and found $ -1.263 44 (14) (α/π)^5$.
Experiments on the hydrogenation of CO on crystalline and amorphous ice at 15 K were carried out to investigate the structural effects of the ice surface. The effective rate of H atom addition to CO on the amorphous ice was found to be larger than that on the crystalline ice, while CO depletion on crystalline ice became larger after long exposure.We demonstrated that the CO-coverage on the ice surfaces dominates the effective reaction rate rather than the surface structure. The larger depletion of CO on crystalline ice, as compared to amorphous ice, suggests easier desorption of CO and/or products by the heat of the reaction.
The reactions of cold H atoms with solid O2 molecules were investigated at 10 K. The formation of H2O2 and H2O has been confirmed by in-situ infrared spectroscopy. We found that the reaction proceeds very efficiently and obtained the effective reaction rates. This is the first clear experimental evidence of the formation of water molecules under conditions mimicking those found in cold interstellar molecular clouds. Based on the experimental results, we discuss the reaction mechanism and astrophysical implications.
The effects of nonmagnetic Zn and magnetic Ni substitution for Cu site on magnetism are studied by measurements of uniform magnetic susceptibility for lightly doped La_{2-x}Sr_xCu_{1-z}M_zO_4 (M=Zn or Ni) polycrystalline samples. For the parent x=0, Zn doping suppresses the Néel temperature T_N whereas Ni doping hardly changes T_N up to z=0.3. For the lightly doped samples with T_N~0, the Ni doping recovers T_N. For the superconducting samples, the Ni doping induces the superconductivity-to-antiferromagnetic transition (or crossover). All the heavily Ni doped samples indicate a spin glass behavior at \~15 K.
We studied local electronic states near Zn in optimally doped YBa$_2$(Cu$_{1-x}$Zn_x)$_3$O$_{7-δ}$ and underdoped YBa$_2$(Cu$_{1-x}$Zn_x)$_4$O$_8$ via satellite signals of plane-site Cu(2) nuclear quadrupole resonance (NQR) spectra. From the relative intensity of Cu NQR spectra, the satellite signals are assigned to Zn-neighbor Cu NQR lines. The Cu nuclear spin-lattice relaxation time of the satellite signal is shorter than that of the main signal, which indicates that the magnetic correlation is locally enhanced near Zn both for the underdoped and the optimally doped systems. The pure YBa$_2$Cu$_4$O$_8$ is a stoichiometric, homogenous, underdoped electronic system; nevertheless, the Zn-induced inhomogeneous magnetic response in the CuO$_2$ plane is more marked than that of the optimally doped YBa$_2$Cu$_3$O$_{7-δ}$.
We estimated the spread of the magnetically enhanced regions (wipeout regions) around Zn ions in high-Tc superconductors YBa2(Cu1-xZnx)4O8 (x=0, 0.005, 0.01, and 0.022) at T=4.2 K, via the planar 63Cu nuclear spin-lattice relaxation study with Cu nuclear quadrupole resonance (NQR) spin-echo technique. From the analysis of nonexponential planar Cu nuclear spin-lattice relaxation curves, we found that the wipeout region per a Zn ion shrinks with Zn doping in the superconducting state. The shrinkage is associated with suppression of the host antiferromagnetic spin correlation, as a result from a dilution effect on the magnetic CuO2 network.
We present the planar 63Cu nuclear spin-lattice relaxation study of impurity-doped YBa2(Cu1-xMx)4O8 (M=Ni, Zn) at T=4.2-300 K with Cu nuclear quadrupole resonance (NQR) spin-echo technique. In the light of an impurity-induced NQR relaxation theory, we estimated the host Cu nuclear spin-lattice relaxation time (T1)HOST, the impurity-induced Cu nuclear spin-lattice relaxation time (t1), and the wipeout region around the impurity in the superconducting state. The pseudo spin-gap behavior of the host Cu antiferromagnetic spin fluctuation above Tc is not readily removed by a small amount of the impurity. The Zn-induced non-Korringa behavior of (T1)HOST below Tc is in sharp contrast to the doping effect of the magnetic impurity Ni.
We studied low-frequency spin dynamics near the impurity-induced superconductor-to-insulator transition for underdoped high-Tc superconductor YBa2(Cu1-xMx)4O8 (M=Ni, Zn) using the Cu nuclear quadrupole resonance (NQR) spin-echo technique. We observed remarkable suppression of the normal-state pseudo spin-gap and a loss of Cu NQR spectrum intensity at low temperatures around the critical impurity concentration.