arXiv subjects

H. Yamauchi

Publications and source records attributed to H. Yamauchi.

At least 19 recordsLinked to original sources

arXiv2016

A new search for the $K_{L} \to π^0 ν\overlineν$ and $K_{L} \to π^{0} X^{0}$ decays

We searched for the $CP$-violating rare decay of neutral kaon, $K_{L} \to π^0 ν\overlineν$, in data from the first 100 hours of physics running in 2013 of the J-PARC KOTO experiment. One candidate event was observed while $0.34\pm0.16$ background events were expected. We set an upper limit of $5.1\times10^{-8}$ for the branching fraction at the 90\% confidence level (C.L.). An upper limit of $3.7\times10^{-8}$ at the 90\% C.L. for the $K_{L} \to π^{0} X^{0}$decay was also set for the first time, where $X^{0}$ is an invisible particle with a mass of 135 MeV/$c^{2}$.

hep-ex↗

arXiv2012

Overdoped Cuprates With High Temperature Superconducting Transitions

Evidence for High Tc cuprate superconductivity is found in a region of the phase diagram where non-superconducting Fermi liquid metals are expected. Cu valences estimated independently from both x-ray absorption near-edge structure (XANES) and bond valence sum (BVS) measurements are > 2.3 for structures in the homologous series (Cu0.75Mo0.25)Sr2(Y,Ce)sCu2O5+2s+δ with s = 1, 2, 3, and 4. The s = 1 member, (Cu0.75Mo0.25)Sr2YCu2O7+δ, 0 \leq δ \leq 0.5, is structurally related to YBa2Cu3O7 in which 25% of the basal Cu cations [i.e. those in the chain layer] are replaced by Mo, and the Ba cations are replaced by Sr. After oxidation under high pressure the s = 1 member becomes superconducting with Tc = 88K. The Cu valence is estimated to be ~2.5, well beyond the ~2.3 value for which other High-Tc cuprates are considered to be overdoped Fermi liquids. The increase in valence is attributed to the additional 0.5 oxygen ions added per chain upon oxidation. The record short apical oxygen distance, at odds with current theory, suggests the possibility of a new pairing mechanism but further experiments are urgently needed to obtain more direct evidence. From the structural point of view the members with s \geq 2 are considered to be equivalent to single-layer cuprates. All have Tc ~ 56 K which is significantly higher than expected because they also have higher than expected Cu valences. The XANES-determined valences normalized to give values in the CuO2 layers are 2.24, 2.25, and 2.26 for s = 2, 3, and 4, while the BVS values determined for the valence in the CuO2 layer alone are 2.31-2.34 for the s = 2 and 3 members. No evidence for periodic ordering has been detected by electron diffraction and high resolution imaging studies. The possibility that the charge reservoir layers are able to screen long range coulomb interactions and thus enhance Tc is discussed.

cond-mat.supr-con↗

arXiv2011

Impact of lithium composition on the thermoelectric properties of the layered cobalt oxide system LixCoO2

Thermoelectric properties of the layered cobalt oxide system LixCoO2 were investigated in a wide range of Li composition, 0.98 >= x >= 0.35. Single-phase bulk samples of LixCoO2 were successfully obtained through electrochemical deintercalation of Li from the pristine LiCoO2 phase. While LixCoO2 with x >= 0.94 is semiconductive, the highly Li-deficient phase (0.75 >= x >= 0.35) exhibits metallic conductivity. The magnitude of Seebeck coefficient at 293 K (S293K) significantly depends on the Li content (x). The S293K value is as large as +70 ~ +100 uV/K for x >= 0.94, and it rapidly decreases from +90 uV/K to +10 uV/K as x is lowered within a Li composition range of 0.75 >= x >= 0.50. This behavior is in sharp contrast to the results of x <= 0.40 for which the S293K value is small and independent of x (+10 uV/K), indicating that a discontinuous change in the thermoelectric characteristics takes place at x = 0.40 ~ 0.50. The unusually large Seebeck coefficient and metallic conductivity are found to coexist in a narrow range of Li composition at about x = 0.75. The coexistence, which leads to an enhanced thermoelectric power factor, may be attributed to unusual electronic structure of the two-dimensional CoO2 block.

cond-mat.str-el↗

arXiv2011

Magnetic and transport properties of the spin-state disordered oxide La0.8Sr0.2Co_{1-x}Rh_xO_{3-δ}

We report measurements and analysis of magnetization, resistivity and thermopower of polycrystalline samples of the perovskite-type Co/Rh oxide La$_{0.8}$Sr$_{0.2}$Co$_{1-x}$Rh$_x$O$_{3-δ}$. This system constitutes a solid solution for a full range of $x$,in which the crystal structure changes from rhombohedral to orthorhombic symmetry with increasing Rh content $x$. The magnetization data reveal that the magnetic ground state immediately changes upon Rh substitution from ferromagnetic to paramagnetic with increasing $x$ near 0.25, which is close to the structural phase boundary. We find that one substituted Rh ion diminishes the saturation moment by 9 $μ_B$, which implies that one Rh$^{3+}$ ion makes a few magnetic Co$^{3+}$ ions nonmagnetic (the low spin state), and causes disorder in the spin state and the highest occupied orbital. In this disordered composition ($0.05\le x \le 0.75$), we find that the thermopower is anomalously enhanced below 50 K. In particular, the thermopower of $x$=0.5 is larger by a factor of 10 than those of $x$=0 and 1, and the temperature coefficient reaches 4 $μ$V/K$^2$ which is as large as that of heavy-fermion materials such as CeRu$_2$Si$_2$.

cond-mat.str-el↗

arXiv2010

Observation of lattice softening at $T_{\rm c}$ in the FeSe$_{0.5}$Te$_{0.5}$ superconductor

Polycrystalline samples of FeSe$_{0.5}$Te$_{0.5}$ were synthesized using a conventional solid-state reaction method. The onset of bulk superconductivity transition was confirmed by SQUID magnetometry at 12.5~K. $^{57}$Fe Mössbauer spectra in transmission geometry were recorded at temperatures between 6.0 and 320 K. Both the isomer shift and the total absorption started to drop about $T_c$, indicating a softening of the lattice. The drop is estimated to correspond to at least 60~K from the original Debye temperature $θ_{\rm D}\approx 460$~K. Seebeck measurements indicate that the samples are $n$-type conductors at low temperatures with a cross-over to $p$-type conductivity around 135 K. The zero Seebeck coefficient is seen below $10.6$~K.

cond-mat.supr-con↗

arXiv2009

Electronic phase diagram of the layered cobalt oxide system, LixCoO2 (0.0 <= x <= 1.0)

Here we report the magnetic properties of the layered cobalt oxide system, LixCoO2, in the whole range of Li composition, 0 <= x <= 1. Based on dc-magnetic susceptibility data, combined with results of 59Co-NMR/NQR observations, the electronic phase diagram of LixCoO2 has been established. As in the related material NaxCoO2, a magnetic critical point is found to exist between x = 0.35 and 0.40, which separates a Pauli-paramagnetic and a Curie-Weiss metals. In the Pauli-paramagnetic regime (x <= 0.35), the antiferromagnetic spin correlations systematically increase with decreasing x. Nevertheless, CoO2, the x = 0 end member is a non-correlated metal in the whole temperature range studied. In the Curie-Weiss regime (x >= 0.40), on the other hand, various phase transitions are observed. For x = 0.40, a susceptibility hump is seen at 30 K, suggesting the onset of static AF order. A magnetic jump, which is likely to be triggered by charge ordering, is clearly observed at Tt = 175 K in samples with x = 0.50 (= 1/2) and 0.67 (= 2/3), while only a tiny kink appears at T = 210 K in the sample with an intermediate Li composition, x = 0.60. Thus, the phase diagram of the LixCoO2 system is complex, and the electronic properties are sensitively influenced by the Li content (x).

cond-mat.str-el↗

arXiv2009

Measurement of electron correlations in LixCoO2 (x=0.0 - 0.35) using 59Co nuclear magnetic resonance and nuclear quadrupole resonance techniques

CoO2 is the parent compound for the superconductor NaxCoO2\cdot1.3H2O and was widely believed to be a Mott insulator. We performed 59Co nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) studies on LixCoO2 (x = 0.35, 0.25, 0.12, and 0.0) to uncover the electronic state and spin correlations in this series of compounds which was recently obtained through electrochemical de-intercalation of Li from pristine LiCoO2. We find that although the antiferromagnetic spin correlations systematically increase with decreasing Li-content (x), the end member, CoO2 is a non-correlated metal that well satisfies the Korringa relation for a Fermi liquid. Thus, CoO2 is not simply located at the limit of x->0 for AxCoO2 (A = Li, Na) compounds. The disappearance of the electron correlations in CoO2 is due to the three dimensionality of the compound which is in contrast to the highly two dimensional structure of AxCoO2.

cond-mat.supr-con↗

arXiv2008

Electrochemical synthesis and properties of CoO2, the x = 0 phase of the AxCoO2 systems (A = Li, Na)

Single-phase bulk samples of the "exotic" CoO2, the x = 0 phase of the AxCoO2 systems (A = Li, Na), were successfully synthesized through electrochemical de-intercalation of Li from pristine LiCoO2 samples. The samples of pure CoO2 were found to be essentially oxygen stoichiometric and possess a hexagonal structure consisting of stacked triangular-lattice CoO2 layers only. The magnetism of CoO2 is featured with a temperature-independent susceptibility of the magnitude of 10-3 emu/mol Oe, being essentially identical to that of a Li-doped phase, Li0.12CoO2. It is most likely that the CoO2 phase is a Pauli-paramagnetic metal with itinerant electrons.

cond-mat.str-el↗

arXiv2007

Photoproduction of neutral kaons on the liquid deuterium target in the threshold region

The photoproduction process of neutral kaons on a liquid deuterium target is investigated near the threshold region, Egamma = 0.8-1.1 GeV. K0 events are reconstructed from positive and negative pions, and differential cross sections are derived. Experimental momentum spectra are compared with those calculated in the spectator model using a realistic deuteron wave function. Elementary amplitudes as given by recent isobar models and a simple phenomenological model are used to study the effect of the new data on the angular behavior of the elementary cross section. The data favor a backward-peaked angular distribution of the elementary n(gamma,K0)Lambda process, which provides additional constraints on current models of kaon photoproduction. The present study demonstrates that the n(gamma,K0)Lambda reaction can provide key information on the mechanism of the photoproduction of strangeness.

nucl-ex↗

arXiv2007

Synthesis and properties of CoO2, the x = 0 end member of the LixCoO2 and NaxCoO2 systems

We report here the synthesis of single-phase bulk samples of CoO2, the x = 0 end member of the AxCoO2 systems (A = Li, Na), from a pristine LiCoO2 sample using an electrochemical technique to completely de-intercalate lithium. Thus, synthesized CoO2 samples were found to be oxygen-stoichiometric and possess a crystal structure consisting of stacked triangular-lattice CoO2 layers only. The magnetic susceptibility of the CoO2 sample was revealed to be relatively large in its initial value and then level off as the temperature increases, suggesting that CoO2 is a Pauli-paramagnetic metal with itinerant electrons.

cond-mat.str-el↗

arXiv2006

Photo-production of neutral kaons on 12C in the threshold region

Kaon photo-production process on $^{12}$C has been studied by measuring neutral kaons in a photon energy range of 0.8$-$1.1 GeV. Neutral kaons were identified by the invariant mass constructed from two charged pions emitted in the $K^{0}_{S}\toπ^{+}π^{-}$ decay channel. The differential cross sections as well as the integrated ones in the threshold photon energy region were obtained. The obtained momentum spectra were compared with a Spectator model calculation using elementary amplitudes of kaon photo-production given by recent isobar models. Present result provides, for the first time, the information on $n(γ,K^{0})Λ$ reaction which is expected to play an important role to construct models for strangeness production by the electromagnetic interaction. Experimental results show that cross section of $^{12}{\rm C}(γ,K^0)$ is of the same order to that of $^{12}{\rm C}(γ,K^+)$ and suggest that slightly backward $K^0$ angular distribution is favored in the $γn\to K^0Λ$ process.

nucl-ex↗

arXiv2006

Room-temperature ferromagnetism in Sr_(1-x)Y_xCoO_(3-delta) (0.2 < x < 0.25)

We have measured magnetic susceptibility and resistivity of Sr$_{1-x}$Y$_x$CoO$_{3-δ}$ ($x=$ 0.1, 0.15, 0.2, 0.215, 0.225, 0.25, 0.3, and 0.4), and have found that Sr$_{1-x}$Y$_x$CoO$_{3-δ}$ is a room temperature ferromagnet with a Curie temperature of 335 K in a narrow compositional range of 0.2 $\leq x\leq$ 0.25. This is the highest transition temperature among perovskite Co oxides. The saturation magnetization for $x=$ 0.225 is 0.25 $μ_B$/Co at 10 K, which implies that the observed ferromagnetism is a bulk effect. We attribute this ferromagnetism to a peculiar Sr/Y ordering.

cond-mat.str-el↗

arXiv2005

The Parent of Misfit-Layered Cobalt Oxides: [Sr2O2]qCoO2

Misfit-layered (ML) cobalt oxides of the general formula of [MmA2Om+2]qCoO2 have been proven to be efficient thermoelectric materials as the structure is capable in accommodating the two seemingly contradictory characteristics of high electrical conductivity and large thermo-electric power. They are also potential hosts for other oxymoron-like functions. The known phases all contain one or two square-planar MO (M = Co, Bi, Pb, Tl, etc.) layers sandwiched together with AO (A = Ca, Sr, Ba, etc.) planes of square symmetry and CoO2 layers of hexagonal symmetry. Here we report realization of the simplest (m = 0) ML phase forming in the Sr-Co-O system with the cation ratio, Sr/Co = 1. Atomic-resolution TEM imaging confirms for the new phase the parent three-layer crystal structure, SrO-SrO-CoO2, which is compatible with the formula of [Sr2O2]qCoO2. Electron diffraction reveals that the phase is rather commensurate, i.e. the "misfit parameter" q is 0.5. Nevertheless, in terms of the transport-property characteristics the new ML parent is comparable to its earlier-established and more complex derivatives.

cond-mat.str-el↗

arXiv2004

Micro-structure and Magnetization of the 80-K Superconductor, TbSr2Cu2.7Mo0.3O7+d

The results of micro-structural and detailed magnetization studies are reported here for our recently published TbSr2Cu2.7Mo0.3O7+d superconductor with a transition temperature (Tc) at as high as 80 K [1]. From XRD and EDX the sample was confirmed to be of single-phase and possess the nominal cation stoichiometry. The SEM images showed well-developed grains for this ceramic high-Tc superconductor. Magnetization measurements at various temperatures and fields revealed bulk superconductivity below 80 K. Magnetization versus applied field (M vs. H) loops at various temperatures below the Tc exhibited clear full penetration field HP. In normal state (> 100 K) the susceptibility follows the Curie-Weiss behavior with an effective paramagnetic moment of 9.83 mB.

cond-mat.supr-con↗

arXiv2004

Effect of Sr-for-Ba isovalent substitution on the local structure, hole distribution and magnetic irreversibility of Cu(Ba,Sr)2YbCu2O6.95(2)

The effect of Sr(II)-for-Ba(II) isovalent substitution on the magnetic irreversibility field (Hirr) of Cu(Ba1-ySry)2YbCu2O6.95(2) (Cu-1212) sample series (y = 0 ~ 0.4) is studied to reveal guiding rules for tailoring the intrinsic Hirr characteristics of high-Tc superconductors. It has been assumed that substitution of the larger alkaline earth cation, Ba(II), by the smaller, Sr(II), might improve the Hirr characteristics as a consequence of the decrease in the thickness of nonsuperconductive blocking block (BB). However, results of the present work show that Sr substitution rather depresses the Hirr characteristics of the Cu-1212-phase superconductors even though the thickness of BB decreases. Both the amount of excess oxygen and the overall positive charge are confirmed to remain constant upon the Sr substitution by wet-chemical and XANES analyses, respectively. However, from neutron diffraction data analysis it is found that Sr substitution breaks the conductive CuO chains in BB by shifting part of the excess oxygen atoms from the characteristic b-axis lattice site to the a-axis site. This is believed to decrease the concentration of mobile holes in the BB, as supported by the results of TEP measurements. The lower Hirr(T) lines of the Sr-substituted samples may thus be attributed to the lower concentration of mobile holes in BB.

cond-mat.supr-con↗

arXiv2004

Oxygen and Cation Ordered Perovskite, Ba2Y2Mn4O11

A three-step route has been developed for the synthesis of a new oxygen-ordered double perovskite, BaYMn2O5.5 or Ba2Y2Mn4O11. (i) The A-site cation ordered perovskite, BaYMn2O5+d, is first synthesized at d ~ 0 by an oxygen-getter-controlled low-O2-pressure encapsulation technique utilizing FeO as the getter for excess oxygen. (ii) The as-synthesized, oxygen-deficient BaYMn2O5.0 phase is then readily oxygenated to the d ~ 1 level by means of 1-atm-O2 annealing at low temperatures. (iii) By annealing this fully-oxygenated BaYMn2O6.0 in flowing N2 gas at moderate temperatures the new intermediate-oxygen-content oxide, BaYMn2O5.5 or Ba2Y2Mn4O11, is finally obtained. From thermogravimetric observation it is seen that the final oxygen depletion from d ~ 1.0 to 0.5 occurs in a single sharp step about 600 C, implying that the oxygen stoichiometry of BaYMn2O5+d is not continuously tunable within 0.5 < d < 1.0. For BaYMn2O5.5 synchrotron x-ray diffraction analysis reveals an orthorhombic crystal lattice and a long-range ordering of the excess oxygen atoms in the YO0.5 layer. The magnetic behavior of BaYMn2O5.5 (with a ferromagnetic transition at ~ 133 K) is found different from those previously reported for the known phases, BaYMn2O5.0 and BaYMn2O6.0.

cond-mat.str-el↗

arXiv2003

Structural and magnetic properties of MSr2Y1.5Ce0.5Cu2Oz (M-1222) compounds with M = Fe and Co

MSr2Y1.5Ce0.5Cu2Oz (M-1222) compounds, with M = Fe and Co, have been synthesized through a solid-state reaction route. Both compounds crystallize in a tetragonal structure (space group I4/mmm). A Rietveld structural refinement of room-temperature neutron diffraction data for Fe-1222 reveals that nearly half the Fe remains at the M site, while the other half goes to the Cu site in the CuO2 planes. Existence of Fe at two different lattice sites, is also confirmed by 57Fe Mossbauer spectroscopy from which it is inferred that nearly 50% of the total Fe occupies the Cu site in the CuO2 planes as Fe3+, whereas the other 50 % is located at the M site with nearly 40 % as Fe4+ and around 10% as Fe3+. For the M = Co compound, nearly 84 % of Co remains at its designated M site, while the rest occupies the Cu site in the CuO2 planes. The oxygen content, z, based on oxygen occupancies refined from the neutron diffraction data, comes close to 9.0 for both the samples The ZFC and FC magnetization curves as a function of temperature show a complex behavior for both Fe-1222 and Co-1222 compounds.

cond-mat.str-el↗

arXiv2003

Kinetics and thermodynamics of the degree of order of the B cations in double-perovskite Sr2FeMoO6

Here we show that the B-site cation (Fe/Mo) ordering in the double-perovskite magnetoresistor, Sr2FeMoO6, is controlled by either kinetics or thermal equilibrium depending on the temperature range. In order to enhance the ordering, long synthesis periods at moderate temperatures are thus required. Employment of a special oxygen-getter-controlled low-O2-pressure encapsulation technique for the sample synthesis enabled us to use long heating periods without evident evaporation of Mo, and thereby reach the thermal equilibrium state and a high degree of B-site cation order. For samples fired at 1150C for long periods (> 36 hours), record-high Ms values of 3.7 ~ 3.9 muB were obtained.

cond-mat.str-el↗