Anisotropic Behavior of Knight Shift in Superconducting State of Na_xCoO_2yH_2O

The Co Knight shift was measured in an aligned powder sample of Na_xCoO_2yH_2O, which shows superconductivity at T_c \sim 4.6 K. The Knight-shift components parallel (K_c) and perpendicular to the c-axis (along the ab plane K_{ab}) were measured in both the normal and superconducting (SC) states. The temperature dependences of K_{ab} and K_c are scaled with the bulk susceptibility, which shows that the microscopic susceptibility deduced from the Knight shift is related to Co-3d spins. In the SC state, the Knight shift shows an anisotropic temperature dependence: K_{ab} decreases below 5 K, whereas K_c does not decrease within experimental accuracy. This result raises the possibility that spin-triplet superconductivity with the spin component of the pairs directed along the c-axis is realized in Na_xCoO_2yH_2O.Comment: 5 pages, 5 figures, to be published in Journal of Physical Society of Japan vol. 75, No.

Incidence of the Tomonaga-Luttinger liquid state on the NMR spin lattice relaxation in Carbon Nanotubes

We report 13C nuclear magnetic resonance measurements on single wall carbon nanotube (SWCNT) bundles. The temperature dependence of the nuclear spin-lattice relaxation rate, 1/T1, exhibits a power-law variation, as expected for a Tomonage-Luttinger liquid (TLL). The observed exponent is smaller than that expected for the two band TLL model. A departure from the power law is observed only at low T, where thermal and electronic Zeeman energy merge. Extrapolation to zero magnetic field indicates gapless spin excitations. The wide T range on which power-law behavior is observed suggests that SWCNT is so far the best realization of a one-dimensional quantum metal.Comment: 5 pages, 4 figure

Electron-Phonon mechanism for Superconductivity in Na0.35_{0.35}CoO2_2: Valence-Band Suhl-Kondo effect Driven by Shear Phonons

To study the possible mechanism of superconductivity in Na$_{0.35}$CoO$_2$, we examine the interaction between all the relevant optical phonons (breathing and shear phonons) and $t_{2g}(a_{1g}+e_g')$-electrons of Co-ions, and study the transition temperature for a s-wave superconductivity. The obtained $T_{\rm c}$ is very low when the $e_g'$-valence-bands are far below the Fermi level. However, $T_{\rm c}$ is strongly enhanced when the top of the $e_g'$-valence-bands is close to the Fermi level (say -50meV), thanks to interband hopping of Cooper pairs caused by shear phonons. This ``valence-band Suhl-Kondo mechanism'' due to shear phonons is significant to understand the superconductivity in Na$_{0.35}$CoO$_2$. By the same mechanism, the kink structure of the band-dispersion observed by ARPES, which indicates the strong mass-enhancement ($m^\ast/m\sim3$) due to optical phonons, is also explained.Comment: 5 pages, 4 figures; v2:Added references, published in J. Phys. Soc. Jp

CoO2-Layer-Thickness Dependence of Magnetic Properties and Possible Two Different Superconducting States in NaxCoO2.yH2O

In order to understand the experimentally proposed phase diagrams of NaxCoO2.yH2O, we theoretically study the CoO2-layer-thickness dependence of magnetic and superconducting (SC) properties by analyzing a multiorbital Hubbard model using the random phase approximation. When the Co valence (s) is +3.4, we show that the magnetic fluctuation exhibits strong layer-thickness dependence where it is enhanced at finite (zero) momentum in the thicker (thinner) layer system. A magnetic order phase appears sandwiched by two SC phases, consistent with the experiments. These two SC phases have different pairing states where one is the singlet extended s-wave state and the other is the triplet p-wave state. On the other hand, only a triplet p-wave SC phase with dome-shaped behavior of Tc is predicted when s=+3.5, which is also consistent with the experiments. Controversial experimental results on the magnetic properties are also discussed.Comment: 5 pages, 4 figures. Submitted to Journal of the Physical Society of Japa

17^{17}O NMR Measurements on Superconducting Na0.35_{0.35}CoO2⋅y_{2} \cdot yH2_{2}O

An $^{17}$O NMR measurement was performed on nonoriented polycrystalline Na$_{0.35}$CoO$_2 \cdot y$H$_2$O with superconducting (SC) transition temperature $T_\text{c} = 4.6$ K. A weak temperature dependence was observed in the Knight shift at the O site ($^{17}K$). The spin part of $^{17}K$ ($^{17}K_{\rm spin}$) is estimated from the plot of $^{17}K$ against bulk susceptibility $\chi$. The $^{17}K_{\rm spin}$ decreases in the SC state, indicative of the decrease in the in-plane component of the spin susceptibility. The nuclear spin-lattice relaxation rate $1/T_1$ at the O site $^{17}(1/T_1)$ shows a good scaling with $1/T_1$ at the Co site $^{59}(1/T_1)$. This indicates that the spin fluctuations at the O site originate from the Co spin dynamics. The relationships between $^{17}(1/T_1T)$ and $^{17}K_{\rm spin}$ and between $^{17}(1/T_1T)$ and $^{59}(1/T_1T)$ show the development of incommensurate fluctuations at {\bf q $\sim$ 0} other than {\bf q $=$ 0} below 30 K. A clear indication of ferromagnetic correlations at {\bf q $=$ 0} was not observed from the present $^{17}$O-NMR studies.Comment: 5 pages, 5 figures, to be published in Journal of physical Society of Japan vol.74, No.

On the fraction of intermediate-mass close binaries that explode as type-Ia supernovae

Type-Ia supernovae (SNe-Ia) are thought to result from a thermonuclear runaway in white dwarfs (WDs) that approach the Chandrasekhar limit, either through accretion from a companion or a merger with another WD. I compile observational estimates of the fraction eta of intermediate-mass stars that eventually explode as SNe-Ia, supplement them with several new estimates, and compare them self-consistently. The estimates are based on five different methods, each utilising some observable related to the SN-Ia rate, combined with assumptions regarding the IMF: the ratio of SN-Ia to core-collapse rates in star-forming galaxies; the SN-Ia rate per unit star-formation rate; the SN-Ia rate per unit stellar mass; the iron to stellar mass ratio in galaxy clusters; and the abundance ratios in galaxy clusters. The five methods indicate that a fraction in the range eta~2-40% of all stars with initial masses of 3-8 M_sun (the generally assumed SN-Ia progenitors) explode as SNe-Ia. A fraction of eta~15% is consistent with all five methods for a range of plausible IMFs. Considering also the binarity fraction among such stars, the mass ratio distribution, the separation distribution, and duplicity (every binary can produce only one SN-Ia explosion), this implies that nearly every intermediate mass close binary ends up as a SN-Ia, or possibly more SNe-Ia than progenitor systems. Theoretically expected fractions are generally one to two orders of magnitude lower. The problem could be solved: if all the observational estimates are in error; or with a ``middle-heavy'' IMF; or by some mechanism that strongly enhances the efficiency of binary evolution toward SN-Ia explosion; or by a non-binary origin for SNe-Ia.Comment: MNRAS, accepted versio

Deformation of Electronic Structures Due to CoO6 Distortion and Phase Diagrams of NaxCoO2.yH2O

Motivated by recently reported experimental phase diagrams, we study the effects of CoO6 distortion on the electronic structure in NaxCoO2.yH2O. We construct the multiband tight-binding model by employing the LDA result. Analyzing this model, we show the deformation of band dispersions and Fermi-surface topology as functions of CoO2-layer thickness. Considering these results together with previous theoretical ones, we propose a possible schematic phase diagram with three successive phases: the extended s-wave superconductivity (SC), the magnetic order, and the spin-triplet SC phases when the Co valence number s is +3.4. A phase diagram with only one phase of spin-triplet SC is also proposed for the s=+3.5 case.Comment: 4 pages, 5 figure

Habitual exercise program protects murine intestinal, skeletal, and cardiac muscles against aging

Aging and aerobic exercise are two conditions known to interfere with health and quality of life, most likely by inducing oxidative stress to the organism. We studied the effects of aging on the morphological and functional properties of skeletal, cardiac, and intestinal muscles and their corresponding oxidative status in C57BL/6 mice and investigated whether a lifelong moderate exercise program would exert a protective effect against some deleterious effects of aging. As expected, aged animals presented a significant reduction of physical performance, accompanied by a decrease of gastrocnemius cross-sectional area and cardiac hypertrophy. However, most interesting was that aging dramatically interfered with the intestinal structure, causing a significant thickening of the ileum muscular layer. Senescent intestinal myocytes displayed many mitochondria with disorganized cristae and the presence of cytosolic lamellar corpuscles. Lipid peroxidation of ileum and gastrocnemius muscle, but not of the heart, increased in aged mice, thus suggesting enhanced oxidative stress. With exception of the intestinal muscle responsiveness, animals submitted to a daily session of 60 min, 5 days/wk, at 13 up to 21 m/min of moderate running in treadmill during animal life span exhibited a reversion of all the observed aging effects on intestinal, skeletal, and heart muscles. the introduction of this lifelong exercise protocol prevented the enhancement of lipid peroxidation and sarcopenia and also preserved cellular and ultracellular structures of the ileum This is the first time that the protective effect of a lifelong regular aerobic physical activity against the deleterious effects of aging on intestinal muscle was demonstrated.Universidade Federal de São Paulo, Escola Paulista Med, Dept Biophys, BR-04023062 São Paulo, BrazilUniversidade Federal de São Paulo, Escola Paulista Med, Dept Physiol, BR-04023062 São Paulo, BrazilUniversidade Federal de São Paulo, Escola Paulista Med, Dept Pathol, BR-04023062 São Paulo, BrazilUniversidade Federal de São Paulo, Escola Paulista Med, Dept Morphol, BR-04023062 São Paulo, BrazilUniversidade Federal de São Paulo, Escola Paulista Med, Dept Biophys, BR-04023062 São Paulo, BrazilUniversidade Federal de São Paulo, Escola Paulista Med, Dept Physiol, BR-04023062 São Paulo, BrazilUniversidade Federal de São Paulo, Escola Paulista Med, Dept Pathol, BR-04023062 São Paulo, BrazilUniversidade Federal de São Paulo, Escola Paulista Med, Dept Morphol, BR-04023062 São Paulo, BrazilWeb of Scienc