arXiv subjects

Suguru Iwasaki

Publications and source records attributed to Suguru Iwasaki.

9 recordsLinked to original sources

arXiv2025

Diffusion-Controlled Anion Conversion into Dense Polycrystalline and Single-Crystalline Oxyhydrides

Oxyhydrides represent a new class of functional materials, yet the synthesis of dense polycrystals or single-crystals suitable for transport studies remains a significant challenge due to hydrogen desorption at elevated temperatures. The co-diffusion of oxygen and hydrogen in densely sintered BaTiO3 enables the topochemical formation of millimeter-scale bulk BaTiO3-xHx via high-pressure diffusion control (HPDC). Hydride ions selectively occupy oxygen-deficient sites, as confirmed by neutron diffraction, TPD, TG, and NMR. Systematic tuning of the hydrogen content and precise control of the electronic conductivity were achieved via HPDC. Hydrogen desorption analysis reveals distinct bonding states between near-surface and interior-bulk regions, which significantly affect the oxynitride conversion under N2 flow. Importantly, the diffusion-based nature of HPDC allows direct anion conversion even in single-crystalline oxides, as demonstrated by the synthesis of SrTiO3-xHx single crystals. These results establish HPDC as a general platform for accessing dense, metastable oxyhydrides with tunable anionic composition and transport properties.

cond-mat.mtrl-sci↗

arXiv2025

Compositional Tuning in NaxAlB14 via Diffusion Control

A uniform Na distribution in NaxAlB14 was achieved using high-pressure diffusion control (HPDC), which promotes Na deintercalation through enhanced diffusion under high pressure, combined with post-annealing. NaxAlB14 with a non-stoichiometric Na composition is thermodynamically metastable, and conventional solid-state reactions with adjusted starting compositions typically result in the formation of stoichiometric NaAlB14 and side products. While HPDC alone typically leads to concentration gradients, intentionally halting the Na removal process before complete extraction, followed by annealing, enabled a uniform composition across the bulk. This allowed structural and electronic properties to be examined over a wide range of Na concentrations. As Na content decreased, electrical conductivity increased, and the optical band gap narrowed. NMR measurements showed an increase in the density of states at the Fermi level, consistent with DFT calculations predicting boron-related in-gap states. Boron vacancies at specific sites were found to generate deep levels near the band gap center, which can explain experimentally observed optical gap reduction. These results demonstrate that diffusion-controlling methods can be effectively applied to synthesize metastable compounds with tunable compositions in covalent frameworks. Furthermore, they provide a foundation for designing functional boride-based materials with adjustable electronic properties by controlling Na extraction and inducing defect formation.

cond-mat.mtrl-sci↗

arXiv2022

Adiabatic temperature change in ErAl2/metal PIT wires: A practical method for estimating the magnetocaloric response of magnetocaloric composites

We report the adiabatic temperature change in ErAl2 magnetocaloric wires fabricated by a powder-in-tube (PIT) process. The adiabatic temperature change of the PIT wires is found to be determined by not only the volume fraction of ErAl2 core but also the magnitude relationship between the specific heat of the ErAl2 core and the metal sheath. We propose a quantitative analysis method for calculating the temperature and core volume fraction dependence of adiabatic temperature change in the PIT wire, whose formula is applicable to also various magnetocaloric composites, useful to estimate the magnetocaloric response prior to fabrication.

cond-mat.mtrl-sci↗

arXiv2021

SuperMat: Construction of a linked annotated dataset from superconductors-related publications

A growing number of papers are published in the area of superconducting materials science. However, novel text and data mining (TDM) processes are still needed to efficiently access and exploit this accumulated knowledge, paving the way towards data-driven materials design. Herein, we present SuperMat (Superconductor Materials), an annotated corpus of linked data derived from scientific publications on superconductors, which comprises 142 articles, 16052 entities, and 1398 links that are characterised into six categories: the names, classes, and properties of materials; links to their respective superconducting critical temperature (Tc); and parametric conditions such as applied pressure or measurement methods. The construction of SuperMat resulted from a fruitful collaboration between computer scientists and material scientists, and its high quality is ensured through validation by domain experts. The quality of the annotation guidelines was ensured by satisfactory Inter Annotator Agreement (IAA) between the annotators and the domain experts. SuperMat includes the dataset, annotation guidelines, and annotation support tools that use automatic suggestions to help minimise human errors.

cond-mat.supr-con↗

arXiv2020

Enhancement of giant refrigerant capacity in Ho$_{1-x}$Gd$_{x}$B$_{2}$ alloys (0.1 $\leq$ x $\leq$ 0.4)

Intending to optimize the giant magnetocaloric properties of HoB$_{2}$, we synthesized and magnetocalorically characterized Ho$_{1-x}$Gd$_{x}$B$_{2}$(0.1 $\leq$ x $\leq$ 0.4) alloys. We found out that Gd enters stoichiometrically and randomly into the Ho site, leading to a Vegard-type structural change. The addition of spherical S$^{7/2}$ Gd$^{3+}$ moments prompts an enhancement in Curie temperature, a reduction in peak value of the magnetic entropy change while still being relatively high, and a broadening of the magnetic entropy change curves. The overall influence is a relatively high refrigerant capacity and relative cooling power, and an extension of the thermal working range to higher temperatures; thus, electing Ho$_{1-x}$Gd$_{x}$B$_{2}$(as potential candidates for cryogenic refrigeration applications.

cond-mat.mtrl-sci↗

arXiv2020

Effect of Dy substitution in the giant magnetocaloric properties of HoB$_{2}$

Recently, a massive magnetocaloric effect near the liquefaction temperature of hydrogen has been reported in the ferromagnetic material HoB$_{2}$. Here we investigate the effects of Dy substitution in the magnetocaloric properties of Ho$_{1-x}$Dy$_{x}$B$_{2}$ alloys ($\textit{x}$ = 0, 0.3, 0.5, 0.7, 1.0). We find that the Curie temperature ($\textit{T}$$_{C}$) gradually increases upon Dy substitution, while the magnitude of the magnetic entropy change |$Δ\textit{S}_{M}$| at $\textit{T}$ = $\textit{T}_{C}$ decreases from 0.35 to 0.15 J cm$^{-3}$ K$^{-1}$ for a field change of 5 T. Due to the presence of two magnetic transitions in these alloys, despite the change in the peak magnitude of |$Δ\textit{S}_{M}$|, the refrigerant capacity ($\textit{RC}$) and refrigerant cooling power ($\textit{RCP}$) remains almost constant in all doping range, which as large as 5.5 J cm$^{-3}$ and 7.0 J cm$^{-3}$ for a field change of 5 T. These results imply that this series of alloys could be an exciting candidate for magnetic refrigeration in the temperature range between 10-50 K.

cond-mat.mtrl-sci↗

arXiv2020

Demonstration of Electric Double Layer Gating under High Pressure by the Development of Field-Effect Diamond Anvil Cell

We have developed an approach to control the carrier density in various material under high pressure by the combination of an electric double layer transistor (EDLT) with a diamond anvil cell (DAC). In this study, this EDLT-DAC was applied to a Bi thin film, and here we report the field-effect under high pressure in the material. Our EDLT-DAC is a promising device for exploring unknown physical phenomena such as high transition-temperature superconductivity (HTS).

cond-mat.mtrl-sci↗

arXiv2020

Machine Learning Guided Discovery of Gigantic Magnetocaloric Effect in HoB$_{2}$ Near Hydrogen Liquefaction Temperature

Magnetic refrigeration exploits the magnetocaloric effect which is the entropy change upon application and removal of magnetic fields in materials, providing an alternate path for refrigeration other than the conventional gas cycles. While intensive research has uncovered a vast number of magnetic materials which exhibits large magnetocaloric effect, these properties for a large number of compounds still remain unknown. To explore new functional materials in this unknown space, machine learning is used as a guide for selecting materials which could exhibit large magnetocaloric effect. By this approach, HoB$_{2}$ is singled out, synthesized and its magnetocaloric properties are evaluated, leading to the experimental discovery of gigantic magnetic entropy change 40.1 J kg$^{-1}$ K$^{-1}$ (0.35 J cm$^{-3}$ K$^{-1}$) for a field change of 5 T in the vicinity of a ferromagnetic second-order phase transition with a Curie temperature of 15 K. This is the highest value reported so far, to our knowledge, near the hydrogen liquefaction temperature thus it is a highly suitable material for hydrogen liquefaction and low temperature magnetic cooling applications.

cond-mat.mtrl-sci↗

arXiv2020

Magnetic entropy change of ErAl2 magnetocaloric wires fabricated by a powder-in-tube method

We report the fabrication of ErAl2 magnetocaloric wires by a powder-in-tube method (PIT) and the evaluation of magnetic entropy change through magnetization measurements. The magnetic entropy change of ErAl2 PIT wires exhibits similar behavior to the bulk counterpart, while its magnitude is reduced by the decrease in the volume fraction of ErAl2 due to the surrounding non-magnetic sheaths. We find that another effect reduces the magnetic entropy change of the ErAl2 PIT wires around the Curie temperature, and discuss its possible origin in terms of a correlation between magnetic properties of ErAl2 and mechanical properties of sheath material.

cond-mat.mtrl-sci↗