arXiv subjects

Hidenori Hiramatsu

Publications and source records attributed to Hidenori Hiramatsu.

At least 19 recordsLinked to original sources

arXiv2026

Tellurium trioxide as an n-type wide-gap semiconductor with ultrahigh work functions from first principles

The exploration of next-generation wide-gap semiconductors with extreme work functions is highly desirable for the continued advancement of electronic and optoelectronic devices. By using first principles calculations, we show that \b{eta}-TeO3, with a wide band gap exceeding 3 eV, emerges as a promising n-type semiconductor exhibiting ultrahigh work functions of up to ~8 eV. Intentionally or unintentionally incorporated interstitial H, Li, Na, Cu, Mg, and Zn impurities and substitutional F impurities are identified as shallow donors, whereas O vacancies act as minor carrier-compensating defects that locally trap electrons at neighbouring Te sites. The work function is evaluated across a series of low-index nonpolar surfaces. The presence of ordered O vacancies is energetically favoured on these surfaces and tends to reduce the corresponding work functions. These findings underscore the significance of precise control over the O chemical potential to suppress surface O-vacancy formation, thereby preserving the exceptional electronic performance of \b{eta}-TeO3 associated with ultrahigh work functions. Collectively, the results highlight the potential of \b{eta}-TeO3 for hole injection/extraction/transport layers as well as transparent electrodes for applications such as light-emitting devices and photovoltaic cells, while providing useful guidance on carrier doping and surface tuning for these practical applications through elucidating the effects of the point-defect and surface configurations on the electronic properties.

cond-mat.mtrl-sci↗

arXiv2025

Nonreciprocal charge transport in an iron-based superconductor with broken inversion symmetry engineered by a hydrogen-concentration gradient

The breaking of spatial inversion symmetry in condensed matter gives rise to intriguing physical properties, such as ferroelectricity, piezoelectricity, spin-momentum locking, and nonreciprocal responses. Here we propose that a concentration gradient, which often persists as a quasi-stable nonequilibrium state with long relaxation times in solids, can serve as a general platform for inversion symmetry breaking. We demonstrate this concept in an epitaxial thin film of the hydrogen-doped SmFeAsO (Sm1111:H) superconductor with a depthwise hydrogen-concentration gradient introduced via an optimized topotactic reaction. This film exhibits nonreciprocal charge transport, meaning that the electrical resistance depends on the direction of the applied current, which serves as a key signature of broken inversion symmetry. A pronounced nonreciprocal signal emerges in the vicinity of the superconducting transition, which we attribute to vortex-motion nonreciprocity arising from an asymmetric pinning landscape created by the hydrogen-concentration gradient. Owing to the high critical temperature of Sm1111:H, vortex-origin nonreciprocity is observed above 40 K, representing the highest temperature reported to date among single bulk materials without an artificially hetero-layered structure. Our findings establish concentration-gradient engineering as a versatile and broadly applicable route for realizing inversion-broken states in otherwise centrosymmetric hosts, opening pathways toward a broader landscape of odd-parity-driven functionalities.

cond-mat.mtrl-sci↗

arXiv2022

High upper critical field (120 T) with small anisotropy of highly hydrogen-substituted SmFeAsO epitaxial film

The electronic transport properties of a highly hydrogen-substituted 1111-type SmFeAsO epitaxial film with high critical-temperature (Tc = 45 K) were investigated under high magnetic fields. By using a single-turn magnet generating up to 130 T, we clarified that the upper critical field (μ0Hc2) of SmFeAsO0.65H0.35 is 120 T at the nearly low-temperature limit of 2.2 K for μ0H || ab. The angular dependence of μ0Hc2 revealed that the anisotropic parameter (γ) around Tc is ~2, which is comparable with that of a practical candidate 122-type BaFe2As2 with lower Tc and much smaller than that of F-substituted SmFeAsO. The small γ mainly originates from the high hydrogen incorporation. The extremely high μ0Hc2 and small γ, together with the high Tc and high critical current density, suggest that SmFeAsO1-xHx has high potential for the superconducting electromagnets and cables.

cond-mat.supr-con↗

arXiv2022

Fully automated spectroscopic ellipsometry analyses of crystalline-phase semiconductors based on a new algorithm

One significant drawback of a spectroscopic ellipsometry (SE) technique is its time-consuming and often complicated analysis procedure necessary to assess the optical functions of thin-film and bulk samples. Here, to solve this inherent problem of a traditional SE method, we present a new general way that allows full automation of SE analyses for crystalline-phase semiconductors exhibiting complex absorption features. In particular, we have modified a scheme established in our previous study, which performs a non-linear SE fitting analysis only in a low energy region at the beginning, while the analyzed energy region is gradually expanded toward higher energy by incorporating addition optical transition peaks. In this study, we have further developed a unique analyzing-energy search algorithm, in which a proper analyzing-energy region is determined to incorporate the feature of a new transition peak. In the developed method, a drastic improvement over the previous simple approach has been confirmed for expressing complex dielectric functions consisting of sharp and broad absorption peaks. The proposed method (Delta M method) has been applied successfully to analyze perovskite-based crystalline samples, including hybrid perovskite (CH3NH3PbI3) and chalcogenide perovskites (SrHfS3 and BaZrS3). In the automated analyses of these semiconductors, 7-8 transition peaks are introduced automatically to describe sample dielectric functions, while structural parameters, such as thin-film and roughness thicknesses, are also determined simultaneously. The established method can drastically reduce an analysis time to a level that allows the automatic inspection of daily varying material optical properties and expands the application area of spectroscopic ellipsometry considerably.

cond-mat.mtrl-sci↗

arXiv2019

Superconductivity at 48 K of heavily hydrogen-doped SmFeAsO epitaxial films grown by topotactic chemical reaction using CaH2

High-critical-temperature (Tc) superconductivity at 48 K is reported for hydrogen-doped SmFeAsO epitaxial films on MgO single-crystal substrates. The key processes are pulsed laser deposition to grow undoped SmFeAsO epitaxial films and subsequent topotactic chemical reaction using CaH2 powders under evacuated silica-glass ampule atmosphere. Based on this post-deposition thermal annealing treatment that we have developed, a maximum hydrogen concentration x = ~0.35 was realized in SmFeAs(O1-xHx). Disordered hydrogen-substitution at O sites is experimentally confirmed directly by atomic-scale microstructural observations. Magnetization measurement validates the bulk nature of the high-Tc superconductivity in the films. This method will become an effective and general method to fabricate various high-quality oxyhydride epitaxial films.

cond-mat.supr-con↗

arXiv2019

Heteroepitaxial Thin-Film Growth of a Ternary Nitride Semiconductor CaZn2N2

Zinc-based nitride CaZn2N2 films grown by molecular beam epitaxy (MBE) with a plasma-assisted active nitrogen-radical source are promising candidates of next-generation semiconductors for light-emitting diodes and solar cells. This nitride compound has previously only been synthesized in a bulk form by ultrahigh-pressure synthesis at 5 GPa. Three key factors have been found to enable heteroepitaxial film growth: (i) precise tuning of the individual flux rates of Ca and Zn, (ii) the use of GaN template layers on sapphire c-plane as substrates, and (iii) the application of MBE with an active N-radical source. Because other attempts at physical vapor deposition and thermal annealing processes have not produced CaZn2N2 films of any phase, this rf-plasma-assisted MBE technique represents a promising way to stabilize CaZn2N2 epitaxial films. The estimated optical band gap is ~1.9 eV, which is consistent with the value obtained from bulk samples. By unintentional carrier doping, n- and p-type electronic conductions are attained with low carrier densities of the order of 1013 /cm3. These features represent clear advantages when compared with Zn-based oxide semiconductors, which usually have much higher carrier densities irrespective of their intentionally undoped state. The carrier mobilities at room temperature are 4.3 cm2/(Vs) for electrons and 0.3 cm2/(Vs) for hole carriers, which indicates that transport properties are limited by grain boundary scattering, mainly because of the low-temperature growth at 250 °C, which realizes a high nitrogen chemical potential.

cond-mat.mtrl-sci↗

arXiv2019

Stabilization and heteroepitaxial growth of metastable tetragonal FeS thin films by pulsed laser deposition

Pulsed laser deposition, a non-equilibrium thin-film growth technique, was used to stabilize metastable tetragonal iron sulfide (FeS), the bulk state of which is known as a superconductor with a critical temperature of 4 K. Comprehensive experiments revealed four important factors to stabilize tetragonal FeS epitaxial thin films: (i) an optimum growth temperature of 300 °C followed by thermal quenching, (ii) an optimum growth rate of ~7 nm/min, (iii) use of a high-purity bulk target, and (iv) use of a single-crystal substrate with small in-plane lattice mismatch (CaF2). Electrical resistivity measurements indicated that none of all the films exhibited superconductivity. Although an electric double-layer transistor structure was fabricated using the tetragonal FeS epitaxial film as a channel layer to achieve high-density carrier doping, no phase transition was observed. Possible reasons for the lack of superconductivity include lattice strain, off-stoichiometry of the film, electrochemical etching by the ionic liquid under gate bias, and surface degradation during device fabrication.

cond-mat.mtrl-sci↗

arXiv2019

Tunable light-emission through the range 1.8-3.2 eV and p-type conductivity at room temperature for nitride semiconductors, Ca(Mg1-xZnx)2N2 (x=0-1)

The ternary nitride CaZn2N2, composed only of earth-abundant elements, is a novel semiconductor with a band gap of 1.8 eV. First-principles calculations predict that continuous Mg substitution at the Zn site will change the optical band gap in a wide range from ~3.3 eV to ~1.9 eV for Ca(Mg1-xZnx)2N2 (x = 0-1). In this study, we demonstrate that a solid-state reaction at ambient pressure and a high-pressure synthesis at 5 GPa produce x = 0 and 0.12, and x = 0.12-1 polycrystalline samples, respectively. It is experimentally confirmed that the optical band gap can be continuously tuned from ~3.2 eV to ~1.8 eV, a range very close to that predicted by theory. Band-to-band photoluminescence is observed at room temperature in the ultravioletfired region depending on x. A 2% Na doping at the Ca site of CaZn2N2 converts its highly resistive state to a p-type conducting state. Particularly, the x = 0.50 sample exhibits intense green emission with a peak at 2.45 eV (506 nm) without any other emission from deep-level defects. These features meet the demands of the III-V group nitride and arsenide/phosphide light-emitting semiconductors.

cond-mat.mtrl-sci↗

arXiv2019

Particulate Generation on Surface of Iron Selenide Films by Air Exposure

Nanometer-sized particular structures are generated on the surfaces of FeSe epitaxial films directly after exposure to air; this phenomenon was studied in the current work because these structures are an obstacle to field-induced superconductivity in electric double-layer transistors using FeSe channel layers. Chemical analyses using field-effect scanning Auger electron spectroscopy revealed no clear difference in the chemical composition between the particular structures and the other flat surface region. This observation limits the possible origins of the particulate formation to light elements in air such as O, C, H, and N.

cond-mat.supr-con↗

arXiv2019

Insulator-like behavior coexisting with metallic electronic structure in strained FeSe thin films grown by molecular beam epitaxy

This paper reports that ~10-nm-thick FeSe thin films exhibit insulator-like behavior in terms of the temperature dependence of their electrical resistivity even though bulk FeSe has a metallic electronic structure that has been confirmed by photoemission spectroscopy and first-principles calculations. This apparent contradiction is explained by potential barriers formed in the conduction band. Very thin FeSe epitaxial films with various [Fe]/[Se] were fabricated by molecular beam epitaxy and classified into two groups with respect to lattice strain and electrical properties. Lattice parameter a increased and lattice parameter c decreased with increasing [Fe]/[Se] up to 1.1 and then a levelled off and c began to decrease at higher [Fe]/[Se]. Consequently, the FeSe films had the most strained lattice when [Fe]/[Se] was 1.1, but these films had the best quality with respect to crystallinity and surface flatness. All the FeSe films with [Fe]/[Se] of 0.8-1.9 exhibited insulator-like behavior, but the temperature dependences of their electrical resistivities exhibited different activation energies Ea between the Se-rich and Fe-rich regions; i.e., Ea were small (a few meV) up to [Fe]/[Se]=1.1 but jumped up to ~25 meV at higher [Fe]/[Se]. The film with [Fe]/[Se]=1.1 had the smallest Ea of 1.1 meV and exhibited an insulator-superconducting transition at 35 K with zero resistance under gate bias. The large Ea of the Fe-rich films was attributed to the unusual lattice strain with tensile in-plane and relaxed out-of-plane strains. The large Ea of films with [Fe]/[Se]>1.1 resulted in low mobility with a high potential barrier of ~50 meV in the conduction band for percolation carrier conduction compared with that of the [Fe]/[Se]=1.1 film (~17 meV). Therefore, the Fe-rich films exhibited remarkable insulator-like behavior similar to a semiconductor despite their metallic electronic structure.

cond-mat.str-el↗

arXiv2018

Superconducting transition temperatures in the electronic and magnetic phase diagrams of Sr2VFeAsO3-delta, a superconductor

We elucidate the magnetic phases and superconducting transition temperatures (Tc) in Sr2VFeAsO3-delta (21113V), an iron-based superconductor with a thick-blocking layer fabricated from a perovskite-related transition metal oxide. At low temperatures (T < 37.1 K), 21113V exhibited a superconducting phase in the range 0.031 =< delta =< 0.145 and an antiferromagnetic (AFM) iron sublattice in the range 0.267 =< delta =< 0.664. Mixed-valent vanadium exhibited a dominant AFM phase in 0.031 =< delta =< 0.088, and a partial ferrimagnetic (Ferri.) phase in the range 0.124 =< delta =< 0.664. The Ferri. phase was the most dominant at a delta value of 0.267, showing an AFM phase of Fe at T < 20 K. Increasing the spontaneous magnetic moments reduced the magnetic shielding volume fraction due to the superconducting phase. This result was attributed to the magnetic phase of vanadium, which dominates the superconductivity of Fe in 21113V. The Tc-delta curve showed two maxima. The smaller and larger of Tc maxima occurred at delta = 0.073 and delta = 0.145, respectively; the latter resides on the phase boundary between AFM and the partial Ferri. phases of vanadium. 21113V is a useful platform for verifing new mechanisms of Tc enhancement in iron-based superconductors.

cond-mat.supr-con↗

arXiv2018

Efficient Construction Method for Phase Diagrams Using Uncertainty Sampling

We develop a method to efficiently construct phase diagrams using machine learning. Uncertainty sampling (US) in active learning is utilized to intensively sample around phase boundaries. Here, we demonstrate constructions of three known experimental phase diagrams by the US approach. Compared with random sampling, the US approach decreases the number of sampling points to about 20%. In particular, the reduction rate is pronounced in more complicated phase diagrams. Furthermore, we show that using the US approach, undetected new phase can be rapidly found, and smaller number of initial sampling points are sufficient. Thus, we conclude that the US approach is useful to construct complicated phase diagrams from scratch and will be an essential tool in materials science.

cond-mat.mtrl-sci↗

arXiv2018

Phase transition in CaFeAsH: bridging 1111 and 122 iron-based superconductors

Iron-based superconductors can be categorized as two types of parent compounds by considering the nature of their temperature-induced phase transitions; namely, first order transitions for 122- and 11-type compounds and second-order transitions for 1111-type compounds. This work examines the structural and magnetic transitions (ST and MT) of CaFeAsH by specific heat, X-ray diffraction, neutron diffraction, and electrical resistivity measurements. Heat capacity measurements revealed a second-order phase transition accompanies an apparent single peak at 96 K. However, a clear ST from the tetragonal to orthorhombic phase and a MT from the paramagnetic to antiferromagnetic phase were detected. The structural (Ts) and Néel temperatures (TN) were respectively determined to be 95(2) and 96 K by X-ray and neutron diffraction and resistivity measurements. This small temperature difference, Ts - TN, was attributed to strong magnetic coupling in the inter-layer direction owing to CaFeAsH having the shortest lattice constant c among parent 1111-type iron arsenides. Considering that a first-order transition takes place in 11- and 122-type compounds with a short inter-layer distance, we conclude that the nature of the ST and MT in CaFeAsH is intermediate in character, between the second-order transition for 1111-type compounds and the first-order transition for other 11- and 122-type compounds.

cond-mat.supr-con↗

arXiv2017

Recent advances in iron-based superconductors toward applications

Iron with a large magnetic moment was widely believed to be harmful to the emergence of superconductivity because of the competition between the static ordering of electron spins and the dynamic formation of electron pairs (Cooper pairs). Thus, the discovery of a high critical temperature (Tc) iron-based superconductor (IBSC) in 2008 was accepted with surprise in the condensed matter community and rekindled extensive study globally. IBSCs have since grown to become a new class of high-Tc superconductors next to the high-Tc cuprates discovered in 1986. The rapid research progress in the science and technology of IBSCs over the past decade has resulted in the accumulation of a vast amount of knowledge on IBSC materials, mechanisms, properties, and applications with the publication of more than several tens of thousands of papers. This article reviews recent progress in the technical applications (bulk magnets, thin films, and wires) of IBSCs in addition to their fundamental material characteristics. Highlights of their applications include high-field bulk magnets workable at 15-25 K, thin films with high critical current density (Jc) > 1 MA/cm2 at ~10 T and 4 K, and an average Jc of 1.3*104 A/cm2 at 10 T and 4 K achieved for a 100-m-class-length wire. These achievements are based on the intrinsically advantageous properties of IBSCs such as the higher crystallographic symmetry of the superconducting phase, higher critical magnetic field, and larger critical grain boundary angle to maintain high Jc. These properties also make IBSCs promising for applications using high magnetic fields.

cond-mat.supr-con↗

arXiv2017

Highly hydrogen-sensitive thermal desorption spectroscopy system for quantitative analysis of low hydrogen concentration (~1 x 10^16 atoms/cm3) in thin-film samples

We developed a highly hydrogen-sensitive thermal desorption spectroscopy (HHS-TDS) system to detect and quantitatively analyze low hydrogen concentrations in thin films. The system was connected to an in situ sample-transfer chamber system, manipulators, and an rf magnetron sputtering thin-film deposition chamber under an ultra-high-vacuum (UHV) atmosphere of ~10^-8 Pa. The following key requirements were proposed in developing the HHS-TDS: (i) a low hydrogen residual partial pressure, (ii) a low hydrogen exhaust velocity, and (iii) minimization of hydrogen thermal desorption except from the bulk region of the thin films. To satisfy these requirements, appropriate materials and components were selected, and the system was constructed to extract the maximum performance from each component. Consequently, ~2000 times higher sensitivity to hydrogen than that of a commercially available UHV-TDS system was achieved using H+-implanted Si samples. Quantitative analysis of an amorphous oxide semiconductor InGaZnO4 thin film (1 cm x 1 cm x 1um thickness, hydrogen concentration of 4.5 x 10^17 atoms/cm3) was demonstrated using the HHS-TDS system. This concentration level cannot be detected using UHV-TDS or secondary ion mass spectroscopy (SIMS) systems. The hydrogen detection limit of the HHS-TDS system was estimated to be ~1 x 10^16 atoms/cm3, which implies ~2 orders of magnitude higher sensitivity than that of SIMS and resonance nuclear reaction systems (~10^18 atoms/cm3).

cond-mat.mtrl-sci↗

arXiv2017

BaFe2(As1-xPx)2 (x = 0.22-0.42) thin films grown on practical metal-tape substrates and their critical current densities

We optimized the substrate temperature (Ts) and phosphorus concentration (x) of BaFe2(As1-xPx)2 films on practical metal-tape substrates for pulsed laser deposition from the viewpoints of crystallinity, superconductor critical temperature (Tc), and critical current density (Jc). It was found that the optimum Ts and x values are 1050 degree C and x = 0.28, respectively. The optimized film exhibits Tc_onset = 26.6 and Tc_zero = 22.4 K along with a high self-field Jc at 4 K (~1 MA/cm2) and relatively isotropic Jc under magnetic fields up to 9 T. Unexpectedly, we found that lower crystallinity samples, which were grown at a higher Ts of 1250 degree C than the optimized Ts = 1050 degree C, exhibit higher Jc along the ab plane under high magnetic fields than the optimized samples. The presence of horizontal defects that act as strong vortex pinning centers, such as stacking faults, are a possible origin of the high Jc values in the poor crystallinity samples.

cond-mat.supr-con↗

arXiv2017

High-field transport properties of a P-doped BaFe2As2 film on technical substrate

High temperature (high-Tc) superconductors like cuprates have superior critical current properties in magnetic fields over other superconductors. However, superconducting wires for high-field-magnet applications are still dominated by low-Tc Nb3Sn due probably to cost and processing issues. The recent discovery of a second class of high-Tc materials, Fe-based superconductors, may provide another option for high-field-magnet wires. In particular, AEFe2As2 (AE: Alkali earth elements, AE-122) is one of the best candidates for high-field-magnet applications because of its high upper critical field, Hc2, moderate Hc2 anisotropy, and intermediate Tc. Here we report on in-field transport properties of P-doped BaFe2As2 (Ba-122) thin films grown on technical substrates (i.e., biaxially textured oxides templates on metal tapes) by pulsed laser deposition. The P-doped Ba-122 coated conductor sample exceeds a transport Jc of 10^5 A/cm^2 at 15 T for both major crystallographic directions of the applied magnetic field, which is favourable for practical applications. Our P-doped Ba-122 coated conductors show a superior in-field Jc over MgB2 and NbTi, and a comparable level to Nb3Sn above 20 T. By analysing the E-J curves for determining Jc, a non-Ohmic linear differential signature is observed at low field due to flux flow along the grain boundaries. However, grain boundaries work as flux pinning centres as demonstrated by the pinning force analysis.

cond-mat.supr-con↗

arXiv2016

Electron effective mass and mobility limits in degenerate perovskite stannate BaSnO$_3$

The high room temperature mobility and the electron effective mass in BaSnO$_3$ are investigated in depth by evaluation of the free carrier absorption observed in infrared spectra for epitaxial films with free electron concentrations from $8.3 \times 10^{18}$ to $7.3 \times 10^{20}$~cm$^{-3}$. Both the optical band gap widening by conduction band filling and the carrier scattering mechanisms in the low and high doping regimes are consistently described employing parameters solely based on the intrinsic physical properties of BaSnO$_3$. The results explain the current mobility limits in epitaxial films and demonstrate the potential of BaSnO$_3$ to outperform established wide band gap semiconductors also in the moderate doping regime.

cond-mat.mtrl-sci↗