arXiv subjects

M. Enoki

Publications and source records attributed to M. Enoki.

12 recordsLinked to original sources

arXiv2019

Thermodynamic origin of solute-enriched stacking-fault in dilute Mg-Zn-Y alloys

We investigate thermodynamic behaviors of dilute Mg-Zn-Y ternary alloys to form a unique solute-enriched stacking-fault (SESF), which is an intrinsic-II type stacking-fault (I2-SF) enriched by the Zn and Y atoms and represents the structural-unit of the long-period stacking/order (LPSO) phase. SESF in the hexagonal-close-packed (hcp) Mg matrix forms a local face-centered-cubic (fcc) environment, and hence our thermodynamic analysis is based on the Gibbs energy comparison between hcp and fcc phases over the Mg-Zn-Y ternary composition ranges, using the calculation of phase diagrams (CALPHAD) method aided by the first principles calculations. Segregation behaviors of solute Zn/Y atoms into the SESF are firstly estimated according to the Hillert's parallel tangent law, followed by the possible disorder-order phase transformation within the SESF using the multiple-sublattice model. We find that the Zn/Y co-segregations at the SESF provide a remarkable condition that the fcc layers become more stable than the hcp-Mg matrix. Besides, within the SESF, the following spinodal-like decomposition into the Mg-rich solid-solution and the Zn/Y-rich L12-type order phase causes a significant reduction of the total Gibbs energy of the system. These thermodynamic behaviors explain fairly well a phenomenological origin of the Zn-Y clustering with the L12-type short-range order, which is known to occur for the LPSO phases and also confirmed for the present SESF by electron microscopy experiments. Therefore, strong Zn-Y interactions even in dilute conditions play a key role to stabilize firmly the SESF in the Mg-Zn-Y alloys.

cond-mat.mtrl-sci↗

arXiv2017

The Hyper Suprime-Cam SSP Survey: Overview and Survey Design

Hyper Suprime-Cam (HSC) is a wide-field imaging camera on the prime focus of the 8.2m Subaru telescope on the summit of Maunakea in Hawaii. A team of scientists from Japan, Taiwan and Princeton University is using HSC to carry out a 300-night multi-band imaging survey of the high-latitude sky. The survey includes three layers: the Wide layer will cover 1400 deg$^2$ in five broad bands ($grizy$), with a $5\,\sigma$ point-source depth of $r \approx 26$. The Deep layer covers a total of 26~deg$^2$ in four fields, going roughly a magnitude fainter, while the UltraDeep layer goes almost a magnitude fainter still in two pointings of HSC (a total of 3.5 deg$^2$). Here we describe the instrument, the science goals of the survey, and the survey strategy and data processing. This paper serves as an introduction to a special issue of the Publications of the Astronomical Society of Japan, which includes a large number of technical and scientific papers describing results from the early phases of this survey.

astro-ph.IM↗

arXiv2017

Unravelling local spin polarization of Zhang-Rice singlet in lightly hole-doped cuprates using high-energy optical conductivity

Unrevealing local magnetic and electronic correlations in the vicinity of charge carriers is crucial in order to understand rich physical properties in correlated electron systems. Here, using high-energy optical conductivity (up to 35 eV) as a function of temperature and polarization, we observe a surprisingly strong spin polarization of the local spin singlet with enhanced ferromagnetic correlations between Cu spins near the doped holes in lightly hole-doped La$_{1.95}$Sr$_{0.05}$Cu$_{0.95}$Zn$_{0.05}$O$_{4}$. The changes of the local spin polarization manifest strongly in the temperature-dependent optical conductivity at ~7.2 eV, with an anomaly at the magnetic stripe phase (~25 K), accompanied by anomalous spectral-weight transfer in a broad energy range. Supported by theoretical calculations, we also assign high-energy optical transitions and their corresponding temperature dependence, particularly at ~2.5 ~8.7, ~9.7, ~11.3 and ~21.8 eV. Our result shows the importance of a strong mixture of spin singlet and triplet states in hole-doped cuprates and demonstrates a new strategy to probe local magnetic correlations using high- energy optical conductivity in correlated electron systems.

cond-mat.str-el↗

arXiv2015

The Quasar-LBG Two-point Angular Cross-correlation Function at z ~ 4 in the COSMOS Field

In order to investigate the origin of quasars, we estimate the bias factor for low-luminosity quasars at high redshift for the first time. In this study, we use the two-point angular cross-correlation function (CCF) for both low-luminosity quasars at $-24<M_{\rm 1450}<-22$ and Lyman-break galaxies (LBGs). Our sample consists of both 25 low-luminosity quasars (16 objects are spectroscopically confirmed low-luminosity quasars) in the redshift range $3.1<z<4.5$ and 835 color-selected LBGs with $z^{\prime}_{\rm LBG}<25.0$ at $z\sim4$ in the COSMOS field. We have made our analysis for the following two quasar samples; (1) the spectroscopic sample (the 16 quasars confirmed by spectroscopy), and (2) the total sample (the 25 quasars including 9 quasars with photometric redshifts). The bias factor for low-luminosity quasars at $z\sim4$ is derived by utilizing the quasar-LBG CCF and the LBG auto-correlation function. We then obtain the $86\%$ upper limits of the bias factors for low-luminosity quasars, that are 5.63 and 10.50 for the total and the spectroscopic samples, respectively. These bias factors correspond to the typical dark matter halo masses, log $(M_{\rm DM}/(h^{-1}M_{\odot}))=$$12.7$ and $13.5$, respectively. This result is not inconsistent with the predicted bias for quasars which is estimated by the major merger models.

astro-ph.GA↗

arXiv2012

Constraints on the faint end of the quasar luminosity function at z~5 in the COSMOS field

We present the result of our low-luminosity quasar survey in the redshift range of 4.5 < z < 5.5 in the COSMOS field. Using the COSMOS photometric catalog, we selected 15 quasar candidates with 22 < i' < 24 at z~5, that are ~ 3 mag fainter than the SDSS quasars in the same redshift range. We obtained optical spectra for 14 of the 15 candidates using FOCAS on the Subaru Telescope and did not identify any low-luminosity type-1 quasars at z~5 while a low-luminosity type-2 quasar at z~5.07 was discovered. In order to constrain the faint end of the quasar luminosity function at z~5, we calculated the 1sigma confidence upper limits of the space density of type-1 quasars. As a result, the 1sigma confidence upper limits on the quasar space density are Phi< 1.33*10^{-7} Mpc^{-3} mag^{-1} for -24.52 < M_{1450} < -23.52 and Phi< 2.88*10^{-7} Mpc^{-3} mag^{-1} for -23.52 < M_{1450} < -22.52. The inferred 1sigma confidence upper limits of the space density are then used to provide constrains on the faint-end slope and the break absolute magnitude of the quasar luminosity function at z~5. We find that the quasar space density decreases gradually as a function of redshift at low luminosity (M_{1450} ~ -23), being similar to the trend found for quasars with high luminosity (M_{1450}<-26). This result is consistent with the so-called downsizing evolution of quasars seen at lower redshifts.

astro-ph.CO↗

arXiv2012

Spin-stripe density varies linearly with hole content in single-layer Bi2201 cuprate

We have performed inelastic neutron scattering measurements on the single-layer cuprate Bi2+xSr2-xCuO6+y (Bi2201) with x=0.2, 0.3, 0.4 and 0.5, a doping range that spans the spin-glass (SG) to superconducting (SC) phase boundary. The doping evolution of low energy spin fluctuations was found to be characterized by a change of incommensurate modulation wave vector from the tetragonal [110] to [100]/[010] directions, while maintaining a linear relation between the incommensurability and the hole concentration, delta p. In the SC regime, the spectral weight is strongly suppressed below 4 meV. Similarities and differences in the spin correlations between Bi2201 and the prototypical single-layer system La2-xSrxCuO4 are discussed.

cond-mat.str-el↗

arXiv2010

Probing the faint end of the quasar luminosity function at z ~ 4 in the COSMOS field

We searched for quasars that are ~ 3 mag fainter than the SDSS quasars in the redshift range 3.7 < z < 4.7 in the COSMOS field to constrain the faint end of the quasar luminosity function. Using optical photometric data, we selected 31 quasar candidates with 22 < i' < 24 at z ~ 4. We obtained optical spectra for most of these candidates using FOCAS on the Subaru telescope, and identified 8 low-luminosity quasars at z ~ 4. In order to derive the quasar luminosity function (QLF) based on our spectroscopic follow-up campaign, we estimated the photometric completeness of our quasar survey through detailed Monte Carlo simulations. Our QLF at z ~ 4 has a much shallower faint-end slope beta = -1.67^{+0.11}_{-0.17} than that obtained by other recent surveys in the same redshift. Our result is consistent with the scenario of downsizing evolution of active galactic nuclei inferred by recent optical and X-ray quasar surveys at lower redshifts.

astro-ph.CO↗

arXiv2010

Hidden itinerant-spin phase in heavily-overdoped La2-xSrxCuO4 revealed by dilute Fe doping: A combined neutron scattering and angle-resolved photoemission study

We demonstrated experimentally a direct way to probe a hidden propensity to the formation of spin density wave (SDW) in a non-magnetic metal with strong Fermi surface nesting. Substituting Fe for a tiny amount of Cu (1%) induced an incommensurate magnetic order below 20 K in heavily-overdoped La2-xSrxCuO4 (LSCO). Elastic neutron scattering suggested that this order cannot be ascribed to the localized spins on Cu or doped Fe. Angle-resolved photoemission spectroscopy (ARPES), combined with numerical calculations, revealed a strong Fermi surface nesting inherent in the pristine LSCO that likely drives this order. The heavily-overdoped Fe-doped LSCO thus represents the first plausible example of the long-sought "itinerant-spin extreme" of cuprates, where the spins of itinerant doped holes define the magnetic ordering ground state. This finding complements the current picture of cuprate spin physics that highlights the predominant role of localized spins at lower dopings. The demonstrated set of methods could potentially apply to studying hidden density-wave instabilities of other "nested" materials on the verge of density wave ordering.

cond-mat.supr-con↗

arXiv2010

Investigation of the spin-glass regime between the antiferromagnetic and superconducting phases in Fe$_{1+y}$Se$_x$Te$_{1-x}$

Using bulk magnetization along with elastic and inelastic neutron scattering techniques, we have investigated the phase diagram of Fe$_{1+y}$Se$_{x}$Te$_{1-x}$ and the nature of magnetic correlations in three nonsuperconducting samples of Fe$_{1.01}$Se$_{0.1}$Te$_{0.9}$, Fe$_{1.01}$Se$_{0.15}$Te$_{0.85}$ and Fe$_{1.02}$Se$_{0.3}$Te$_{0.7}$. A cusp and hysteresis in the temperature dependence of the magnetization for the $x=0.15$ and 0.3 samples indicates spin-glass (SG) ordering below $T_{\rm sg} = 23$K. Neutron scattering measurements indicate that the spin-glass behavior is associated with short-range spin density wave (SDW) ordering characterized by a static component and a low-energy dynamic component with a characteristic incommensurate wave vector of ${\bf Q}_m = (0.46, 0, 0.50)$ and an anisotropy gap of $\sim$ 2.5 meV. Our high ${\bf Q}$-resolution data also show that the systems undergo a glassy structural distortion that coincides with the short-range SDW order.

cond-mat.supr-con↗

arXiv2009

Gigantic Impact of Magnetic Impurity on Stripe Order in La$_{2-x}$Sr$_{x}$CuO$_4$ ($x$$\sim$1/8)

Precise neutron-scattering experiments on La$_{1.88-y}$Sr$_{0.12+y}$Cu$_{0.99}$R$_{0.01}$O$_4$ ($y$=0 for R=Cu, Zn and $y$=0.01 for R=Ga, Fe) were performed to examine the stability of stripe order in cuprate oxides through impurity substitution effects. For all the impurity-substituted samples a static charge-density-wave order similar to that reported in the low-temperature-tetragonal phase (LTT) was discovered in the low-temperature-orthorhombic (LTO) phase for the first time. Fe substitution significantly enhanced both the charge- and spin-density-wave orders. These results indicate that dynamical charge stripes can be stabilized into static ones around the Cu-site defects regardless of the type of crystal structure. Additionally, large magnetic moments at the Cu-site may induce a decrease in the Cu spin fluctuations around the defects, which in turn, enhances the static charge-density-wave order.

cond-mat.str-el↗

arXiv2008

Neutron-scattering study of spin correlations in La1.94-xSrxCe0.06CuO4

We performed a neutron-scattering experiment to investigate the effect of distortion of CuO2 planes on the low-energy spin correlation of La1.94-xSrxCe0.06CuO4 (LSCCO). Due to the carrier-compensation effect by co-doping of Sr and Ce, LSCCO has a smaller orthorhombic lattice distortion compared to La2-xSrxCuO4 (LSCO) with comparable hole concentration p. A clear gap with the edge-energy of 6~7 meV was observed in the energy spectrum of local dynamical susceptibility c"(w) for both x=0.18 (p~0.14) and x=0.24 (p~0.20) samples as observed for optimally-doped LSCO (x=0.15~0.18). For the x=0.14 (p~0.10) sample, in addition to the gap-like structure in c"(w) we observed a low-energy component within the gap which develops below 2~3meV with decreasing the energy. The low-energy component possibly coincides with the static magnetic correlation observed in this sample. These results are discussed from a view point of relationship between the stability of low-energy spin fluctuations and the distortion of CuO2 planes.

cond-mat.supr-con↗

arXiv2008

Star formation history of galaxies from z=0 to z=0.7 A backward approach to the evolution of star-forming galaxies

We investigate whether the mean star formation activity of star-forming galaxies from z=0 to z=0.7 in the GOODS-S field can be reproduced by simple evolution models of these systems. In this case, such models might be used as first order references for studies at higher z to decipher when and to what extent a secular evolution is sufficient to explain the star formation history in galaxies. We selected star-forming galaxies at z=0 and at z=0.7 in IR and in UV to have access to all the recent star formation. We focused on galaxies with a stellar mass ranging between 10^{10} and 10^{11} M_sun for which the results are not biased by the selections. We compared the data to chemical evolution models developed for spiral galaxies and originally built to reproduce the main characteristics of the Milky Way and nearby spirals without fine-tuning them for the present analysis. We find a shallow decrease in the specific star formation rate (SSFR) when the stellar mass increases. The evolution of the SSFR characterizing both UV and IR selected galaxies from z=0 to z=0.7 is consistent with the models built to reproduce the present spiral galaxies. There is no need to strongly modify of the physical conditions in galaxies to explain the average evolution of their star formation from z=0 to z=0.7. We use the models to predict the evolution of the star formation rate and the metallicity on a wider range of redshift and we compare these predictions with the results of semi-analytical models.

astro-ph↗