arXiv subjects

N. Ikeda

Publications and source records attributed to N. Ikeda.

11 recordsLinked to original sources

arXiv2024

Ultrafast Anisotropic Polarization Dynamics of Electronic Ferroelectrics LuFe2O4

Femtosecond time-resolved second harmonic generation (SHG) measurements were performed at room temperature on a single crystal of LuFe2O4, which has recently attracted attention as an electronic ferroelectric material in which polarization occurs due to the real-space arrangement of Fe2+ and Fe3+. We irradiated the crystal with 800 nm laser pulses, traced the subsequent SHG changes, and observed anisotropic polarization dynamics: immediately after photoexcitation, the in-plane (ab-plane) component of the SHG greatly reduced, while the inter-plane (c-axis) SHG increased. This means that a hidden photoexcited state, in which the in-plane charge ordering is broken but the polarization along c-axis direction enhanced, has been created by photoexcitation, indicating that the electronic ferroelectricity can be controlled on a femtosecond scale.

cond-mat.mtrl-sci↗

arXiv2011

AKARI-CAS --- Online Service for AKARI All-Sky Catalogues

The AKARI All-Sky Catalogues are an important infrared astronomical database for next-generation astronomy that take over the IRAS catalog. We have developed an online service, AKARI Catalogue Archive Server (AKARI-CAS), for astronomers. The service includes useful and attractive search tools and visual tools. One of the new features of AKARI-CAS is cached SIMBAD/NED entries, which can match AKARI catalogs with other catalogs stored in SIMBAD or NED. To allow advanced queries to the databases, direct input of SQL is also supported. In those queries, fast dynamic cross-identification between registered catalogs is a remarkable feature. In addition, multiwavelength quick-look images are displayed in the visualization tools, which will increase the value of the service. In the construction of our service, we considered a wide variety of astronomers' requirements. As a result of our discussion, we concluded that supporting users' SQL submissions is the best solution for the requirements. Therefore, we implemented an RDBMS layer so that it covered important facilities including the whole processing of tables. We found that PostgreSQL is the best open-source RDBMS products for such purpose, and we wrote codes for both simple and advanced searches into the SQL stored functions. To implement such stored functions for fast radial search and cross-identification with minimum cost, we applied a simple technique that is not based on dividing celestial sphere such as HTM or HEALPix. In contrast, the Web application layer became compact, and was written in simple procedural PHP codes. In total, our system realizes cost-effective maintenance and enhancements.

astro-ph.IM↗

arXiv2011

Fabrication of InP nano pillars by ECR Ar ion irradiation

Regular arrays of InP nano pillars have been fabricated by low energy Electron Cyclotron Resonance (ECR) Ar+ ion irradiation on InP(111) surface. Several scanning electron microscopy (SEM) images have been utilized to invetsigate the width, height, and orientation of these nano pillars on InP(111) surfaces. The average width and length of these nano-pillars are about 50 nm and 500 nm, respectively. The standing angle with respect to the surface of the nano-pillars depend on the incidence angle of the Ar ion irradiation during the fabrication process. Interestingly, the growth direction of the nano pillars are along the reflection direction of the ion beam and the standing angles are nearly same as the ion incidence angle with the surface normal. This nano-pillas are easily transferred from the InP surface to double sided carbon tape without any damage. High Resolution Transmission Electron Microscopy (HRTEM) study of single nano-pillar reveals that this nano-pillar are almost crystalline in nature except 2-4 nm amorphous layer on the outer surface. The transmission electron microscopy combined with energy-dispersive x-ray spectroscopy (TEM-EDS) analysis of these nano pillars exhibit that the ratio of In and P is little higher compared to the bulk InP.

cond-mat.mtrl-sci↗

arXiv2011

Planck Early Results: The submillimetre properties of a sample of Galactic cold clumps

(abridged) We perform a detailed investigation of sources from the Cold Cores Catalogue of Planck Objects (C3PO). Our goal is to probe the reliability of the detections, validate the separation between warm and cold dust emission components, provide the first glimpse at the nature, internal morphology and physical characterictics of the Planck-detected sources. We focus on a sub-sample of ten sources from the C3PO list, selected to sample different environments, from high latitude cirrus to nearby (150pc) and remote (2kpc) molecular complexes. We present Planck surface brightness maps and derive the dust temperature, emissivity spectral index, and column densities of the fields. With the help of higher resolution Herschel and AKARI continuum observations and molecular line data, we investigate the morphology of the sources and the properties of the substructures at scales below the Planck beam size.

astro-ph.GA↗

arXiv2010

Large-scale distributions of mid- and far-infrared emission from the center to the halo of M82 revealed with AKARI

The edge-on starburst galaxy M82 exhibits complicated distributions of gaseous materials in its halo, which include ionized superwinds driven by nuclear starbursts, neutral materials entrained by the superwinds, and large-scale neutral streamers probably caused by a past tidal interaction with M81. We investigate detailed distributions of dust grains and polycyclic aromatic hydrocarbons (PAHs) around M82 to understand their interplay with the gaseous components. We performed mid- (MIR) and far-infrared (FIR) observations of M82 with the Infrared Camera and Far-Infrared Surveyor on board AKARI. We obtain new MIR and FIR images of M82, which reveal both faint extended emission in the halo and very bright emission in the center with signal dynamic ranges as large as five and three orders of magnitude for the MIR and FIR, respectively. We detect MIR and FIR emission in the regions far away from the disk of the galaxy, reflecting the presence of dust and PAHs in the halo of M82. We find that the dust and PAHs are contained in both ionized and neutral gas components, implying that they have been expelled into the halo of M82 by both starbursts and galaxy interaction. In particular, we obtain a tight correlation between the PAH and H$α$ emission, which provides evidence that the PAHs are well mixed in the ionized superwind gas and outflowing from the disk.

astro-ph.GA↗

arXiv2009

AKARI Far-Infrared All Sky Survey

We demonstrate the capability of AKARI for mapping diffuse far-infrared emission and achieved reliability of all-sky diffuse map. We have conducted an all-sky survey for more than 94 % of the whole sky during cold phase of AKARI observation in 2006 Feb. -- 2007 Aug. The survey in far-infrared waveband covers 50 um -- 180 um with four bands centered at 65 um, 90 um, 140 um, and 160 um and spatial resolution of 3000 -- 4000 (FWHM).This survey has allowed us to make a revolutionary improvement compared to the IRAS survey that was conducted in 1983 in both spatial resolution and sensitivity after more than a quarter of a century. Additionally, it will provide us the first all-sky survey data with high-spatial resolution beyond 100 um. Considering its extreme importance of the AKARI far-infrared diffuse emission map, we are now investigating carefully the quality of the data for possible release of the archival data. Critical subjects in making image of diffuse emission from detected signal are the transient response and long-term stability of the far-infrared detectors. Quantitative evaluation of these characteristics is the key to achieve sensitivity comparable to or better than that for point sources (< 20 -- 95 [MJy/sr]). We describe current activities and progress that are focused on making high quality all-sky survey images of the diffuse far-infrared emission.

astro-ph.GA↗

arXiv2008

Bunching visibility for correlated photons from single GaAs quantum dots

We study photon bunching phenomena associated with biexciton-exciton cascade in single GaAs self-assembled quantum dots. Experiments carried out with a pulsed excitation source show that significant bunching is only detectable at very low excitation, where the typical intensity of photon streams is less than the half of their saturation value. Our findings are qualitatively understood with a model which accounts for Poissonian statistics in the number of excitons, predicting the height of a bunching peak being determined by the inverse of probability of finding more than one exciton.

cond-mat.mtrl-sci↗

arXiv2004

Relationship between charge stripe order and structural phase transitions in La$_{1.875}$Ba$_{0.125-x}$Sr$_{x}$CuO$_{4}$

The nature of charge stripe order and its relationship with structural phase transitions were studied using synchrotron X-ray diffraction in La$_{1.875}$Ba$_{0.125-x}$Sr$_{x}$CuO$_{4}$ ($0.05\leq x\leq 0.10$). For $x=0.05$, as temperature increased, incommensurate superlattice peaks associated with the charge order disappeared just at the structural phase transition temperature, $T_{\rm d2}$. However, for $x=0.075$ and 0.09, the superlattice peaks still existed as a short range correlation even above $T_{\rm d2}$, indicating a precursor of charge ordering. Furthermore, temperature dependences of the superlattice peak intensity, correlation length, and incommensurability for $x=0.05$ are different from those for $x=0.075$ and 0.09. These results suggest that the transition process into the charge stripe order strongly correlates with the order of the structural phase transitions. A quantitative comparison of the structure factor associated with the charge order have been also made for all the samples.

cond-mat.supr-con↗

arXiv2003

Measurements of Proton, Helium and Muon Spectra at Small Atmospheric Depths with the BESS Spectrometer

The cosmic-ray proton, helium, and muon spectra at small atmospheric depths of 4.5 -- 28 g/cm^2 were precisely measured during the slow descending period of the BESS-2001 balloon flight. The variation of atmospheric secondary particle fluxes as a function of atmospheric depth provides fundamental information to study hadronic interactions of the primary cosmic rays with the atmosphere.

astro-ph↗

arXiv2003

Synchrotron X-ray diffraction study of a charge stripe order in 1/8-doped La$_{1.875}$Ba$_{0.125-x}$Sr$_{x}$CuO$_{4}$

Lattice distortions associated with charge stripe order in 1/8 hole-doped La$_{1.875}$Ba$_{0.125-x}$Sr$_{x}$CuO$_{4}$ are studied using synchrotron X-ray diffraction for $x=0.05$ and $x=0.075$. The propagation wave vector and charge order correlation lengths are determined with a high accuracy, revealing that the oblique charge stripes in orthorhombic $x=0.075$ crystal are more disordered than the aligned stripes in tetragonal $x=0.05$ crystal. The twofold periodicity of lattice modulations along the c-axis is explained by long-range Coulomb interactions between holes on neighboring CuO$_{2}$ planes.

cond-mat.supr-con↗

arXiv2001

The "devil's staircase" type phase transition in NaV2O5 under high pressure

The "devil's staircase" type phase transition in the quarter-filled spin-ladder compound NaV2O5 has been discovered at low temperature and high pressure by synchrotron radiation x-ray diffraction. A large number of transitions are found to successively take place among higher-order commensurate phases with 2a * 2b * zc type superstructures. The observed temperature and pressure dependence of modulation wave number qc, defined by 1/z, is well reproduced by the Axial Next Nearest Neighbor Ising (ANNNI) model. The qc is suggested to reflect atomic displacements coupled with charge ordering in this system. The experimental fact implies that two competitive inter-layer interactions between the Ising spins, i.e., the nearest neighbor J1>0 (ferro) and the next nearest neighbor J2<0 (antiferro) along the c-axis, are intrinsic in this compound. A microscopic origin of the inter-layer interaction is not yet known; however, the nearest neighbor interaction J1>0 between the V2O5 layers is especially interesting. It is very surprising that the phase transition of such a complicated charge-lattice-spin coupled system NaV2O5 can be described by the simple ANNNI model.

cond-mat.str-el↗