和達 大樹

<p>Background: Magneto-optical Kerr effect (MOKE) microscopes are powerful experimental tools to observe magnetic domains in magnetic materials. These devices are, however, typically large, unportable, and expensive (∼ several million yen), and therefore prevent many researchers in the field of materials science from easy access to study real-space images of magnetic domains.</p><p>Methods: To overcome these issues, we utilize data from ”The OpenFlexure Project” developed by the University of Bath and the University of Cambridge. The purpose of this project is to make high-precision mechanical positioning of the studied sample available to anyone with a 3D printer, especially for use in microscopes. We built a low-cost and portable MOKE microscope device by a 3D printer. We redesigned the 3D modeling data of an ordinary optical microscope provided by The OpenFlexure project and incorporated additional elements such as optical polarizers and an electro-magnetic coil into the primarily designed microscope that did not originally have these</p><p> elements.</p><p>Results: We successfully observed magnetic domains and their real-space motions induced by magnetic fields using the palm-sized low-cost MOKE microscope, which costs approximately 20,000 yen in raw materials to construct.</p><p>Conclusions: Our methodology to assemble a low-cost MOKE microscope will enable researchers working in the field of materials science to more easily observe magnetic domains without commercial equipment.</p>

<jats:p> We studied the relationship between the lattice constant and magnetism of [Formula: see text]-ordered FePt under high pressure by means of first-principles calculations and synchrotron x-ray measurements. Based on our calculations, we found that the c/ a ratio shows a local maximum at ∼20 GPa and that the Pt magnetic moment first remains almost unchanged and is sharply suppressed at ∼60 GPa. As for the c/ a, we experimentally verified the local maximum at ∼20 GPa by powder x-ray diffraction. We also measured the x-ray magnetic circular dichroism at the Pt L edge up to ∼20 GPa. Any significant change in the Pt magnetic moment was not observed in agreement with the calculations. These results, thus, indicate that magnetic states, where the magnetization of Fe decreased and that of Pt did not change, can be created in [Formula: see text]-ordered FePt by lattice deformation under high pressure. </jats:p>

Energy sustainability is critical for social activities in the human world. The quaternary compound Cu2ZnSnSe4 (CZTSe), as a promising candidate for thin-film solar cell absorption with medium-level thermoelectric performance, is of interest for the purpose of utilizing solar energy. The defect chemistry and atomic ordering in this particular compound also triggers interests in understanding its crystallographic structure as well as defects. Hereby, high energy resolution X-ray absorption spectroscopy is employed to investigate the electronic and geometric structural complexity in pristine and cobalt-doped Cu2ZnSnSe4. The occupational atomic sites of Cu are found to be mixed with the Zn atoms, forming CuZn anti-defects, which serve as a knob to tune local electronic structures. With proper doping, the band structure can be manipulated to improve the optical and thermoelectric properties of the CZTSe compounds.

<title>Abstract</title>Investigation of ultrafast dynamic behaviors can provide novel insights about the coupling mechanisms among multiple degrees of freedom in condensed matters, such as lattice, magnetism and electronic structure. Here we investigate both the ferromagnetic (FM) and antiferromagnetic (AFM) dynamics of a strongly correlated oxide system, GdBaCo₂O_5.5 thin film by time-resolved x-ray magnetic circular dichroism in reflectivity (XMCDR) and resonant magnetic x-ray diffraction (RMXD). A photo-induced AFM-FM transition characterized by an increase of the transient XMCDR (sensitive to FM order) beyond the unpumped value and a decay of RMXD (sensitive to AFM order) was observed. The photon-energy dependence of the transient XMCDR and reflectivity could be interpreted as a concomitant photo-induced spin-state transition (SST). The AFM-FM transition and SST couple with each other in the time domain, resulting in unusual dynamic behaviors of the magnetism.

The perovskite-type iron oxide Sr2/3La1/3FeO3 is known to show characteristic spin-charge ordering (SCO), where sixfold collinear spin ordering and threefold charge ordering are coupled with each other. Here, we report the discovery of a spin-charge decoupling and an antiferromagnetic (AFM) state competing with the SCO phase in perovskites (Sr1-xBax)2/3La1/3FeO3. By comprehensive measurements including neutron diffraction, Mössbauer spectroscopy, and x-ray absorption spectroscopy, we found that the isovalent Ba2+ substitution systematically reduces the critical temperature of the SCO phase and additionally yields the spin-charge decoupling in x>0.75. Whereas the ground state remains in the SCO phase in the whole x region, an unexpected G-type AFM phase with incoherent charge ordering or charge fluctuation appears as the high-temperature phase in the range of x>0.75. Reflecting the competing nature between them, the G-type AFM phase partially exists as a metastable state in the SCO phase at low temperatures. We discuss the origin of the spin-charge decoupling and the emergence of the G-type AFM phase with charge fluctuation in terms of the bandwidth reduction by the Ba substitution.

The interface properties of strongly correlated electron system/band semiconductor junctions were investigated in the La_1−xSr_xVO₃/p-Si(100) structure. Spectroscopic observations show that the electronic structure of the interface is typical of insulator/semiconductor junctions for x ≤ 0.2 and Schottky junctions for x ≥ 0.25. For the forward current density–bias voltage (J–V) characteristics, energy barriers that were otherwise unexplainable by spectroscopy were estimated from the thermionic emission model fitting of the J–V characteristics, indicating that the injected carriers from Si to La_1−xSr_xVO₃ can also feel the correlation force of Coulomb repulsion at the interface.

Soft X-ray absorption study for low-crystallinity metal organic frameworks to investigate the ligand field.

Ultrafast electron delocalization induced by a femtosecond laser pulse is a well-known process in which electrons are ejected from the ions within the laser pulse duration. However, very little is known about the speed of electron localization out of an electron gas in correlated metals, i.e., the capture of an electron by an ion. Here, we demonstrate by means of pump-probe x-ray techniques across the Eu L-3 absorption edge that an electron localization process in the EuNi2(Si0.27Ge0.79)(2) intermetallic material occurs within a few hundred femtoseconds after the optical excitation. Spectroscopy and diffraction data collected simultaneously at low temperature and for various laser fluences show that the localization dynamics process is much faster than the thermal expansion of the unit cell along the c direction which occurs within picoseconds. Nevertheless, this latter process is still much slower than pure electronic effects, such as screening, and the subpicosecond timescale indicates an optical phonon driven origin. In addition, comparing the laser fluence dependence of the electronic response with that found in other intermediate 4f valence materials, we suggest that the electron localization process observed in this Eu-based correlated metal is mainly related to changes in the 4f hybridization. The observed ultrafast electron localization process sparks fundamental questions for our understanding of electron correlations and their coupling to the lattice.

We demonstrate that total reflection hard x-ray photoelectron spectroscopy (TR-HAXPES) is a versatile method for elucidating a difference between surface and bulk electronic states of strongly correlated electron systems, complementing conventional bulk sensitive hard x-ray photoelectron spectroscopy (HAXPES). To demonstrate the experimental feasibility of the method, we investigated La0.6Sr0.4MnO3 and the electron-doped high-TC superconductor La1.9Ce0.1CuO4. From the incident angle dependence of the spectral line shapes, we found that the surface-sensitive TR-HAXPES measurement equivalent to soft x-ray photoelectron spectroscopy is possible in the total reflection condition. The results strongly suggest that this method allows us to measure both surface and bulk electronic states without making any changes to experimental setup such as the energy resolution, x-ray energy, and the beamline.

Copper tungstate (CuWO4) is an important semiconductor with a sophisticated and debatable electronic structure that has a direct impact on its chemistry. Using the PAL-XFEL source, we study the electronic dynamics of photoexcited CuWO4. The Cu L-3 X-ray absorption spectrum shifts to lower energy upon photoexcitation, which implies that the photoexcitation process from the oxygen valence band to the tungsten conduction band effectively increases the charge density on the Cu atoms. The decay time of this spectral change is 400 fs indicating that the increased charge density exists only for a very short time and relaxes electronically. The initial increased charge density gives rise to a structural change on a time scale longer than 200 ps.

Amorphous coordination polymers and metal-organic frameworks (MOFs) have attracted much attention owing to their various functionalities. Here, we demonstrate the tunable water adsorption behavior of a series of amorphous cyanide-bridged MOFs with different metals (M[Ni(CN)(4)]: MNi; M = Mn, Fe, and Co). All three compounds adsorb up to six water molecules at a certain vapor pressure (P-ads) and undergo conversion to crystalline Hofmann-type MOFs, M(H2O)(2)[Ni(CN)(4)]center dot 4H(2)O (MNi-H2O; M = Mn, Fe, and Co). The P-ads of MnNi, FeNi, and CoNi for water adsorption is P/P-0 = 0.4, 0.6, and 0.9, respectively. Although the amorphous nature of these materials prevented structural elucidation using X-ray crystallography techniques, the local-scale structure around the N-coordinated M(2+ )centers was analyzed using L-2,L-3-, K-edge X-ray absorption fine structure, and magnetic measurements. Upon hydration, the coordination geometry of these metal centers changed from tetrahedral to octahedral, resulting in significant reorganization of the MOF local structure. On the other hand, Ni[Ni(CN)(4)] (NiNi) containing square-planar Ni2+ centers did not undergo significant structural transformation and therefore abruptly adsorbed H2O in the low-pressure region. We could thus define how changes in the bond lengths and coordination geometry are related to the adsorption properties of amorphous MOF systems.

In this study, the electronic structures of A-site layer-ordered double perovskite YBaCo2O6 and YBaCo2O5.3 epitaxial thin films were investigated by x-ray photoemission spectroscopy (PES), x-ray absorption spectroscopy (XAS), and x-ray magnetic circular dichroism (XMCD) measurements. The valence band PES spectrum of the YBaCo2O6 thin film exhibited a finite density of states (DOS) at the Fermi energy (EF), while that of the YBaCo2O5.3 thin film exhibited no DOS at EF, corresponding well with the metallic and insulating transport properties, respectively. The Co L-edge XAS measurements revealed that the Co ions in YBaCo2O6 are in a mixed valence state of high-spin (HS) Co3+ (50%) and intermediate spin (IS) Co4+ (50%), while HS Co3+ (80%) and HS Co2+ (20%) coexist in YBaCo2O5.3. The XMCD measurements led to the conclusion that both HS Co3+ and IS Co4+ contribute to the ferromagnetism of YBaCo2O6.

Double-perovskite GdBaCo2O5.5 (GBCO) exhibits various fascinating features, including temperature- and magnetic-field-induced ferromagnetic to antiferromagnetic phase transition associated with the two ionic orders: an A-site Gd/Ba order along the c-axis, and an octahedral CoO6/pyramidal CoO5 order along the b-axis. However, the fine control of ionic orders remains a challenging issue. Herein, we demonstrate the engineering of ionic orders in GBCO films through appropriate selection of substrates. Specifically, we prepared four kinds of films comprising only the Gd/Ba order [on SrTiO3(001)], only the CoO6/CoO5 order [on LaSrGaO4(001)], and both the Gd/Ba and CoO6/CoO5 orders [on NdGaO3(110) and LaSrAlO4(001) (LSAO)]. In addition to the selective ionic orders through substrate selection, the GBCO film on LSAO also contained multiple domains with domain boundaries of [CoOx]-[CoOy] stacking. Magnetic, magnetoresistance, and resonant X-ray magnetic diffraction measurements revealed that the magnetic properties of the above GBCO films were widely modulated. With the ionic order control of the GBCO film, different magnetic phase transition behaviors and magnetic anisotropy switching from the in-plane to perpendicular direction were observed.

We have synthesized polycrystalline samples of Eu2Pt6(Al1-xGax)(15 )(x = 0, 0.05. 0.1, 0.2, 0.3, and 1) and examined electrical resistivity, magnetic susceptibility, high-field magnetization, and X-ray absorption spectra at the Eu L-3-edge. With increasing x up to 0.1, the first-order valence transition is shifted to low temperatures. For x >= 0.1. a nearly divalent state is stabilized down to the lowest temperature, and an antiferromagnetic order emerges below T-N = 13-16 K. For x = 0 and 0.05, magnetization curves in pulsed magnetic fields at T = 4.2 K clarify a field-induced valence transition at B-v= 29 and 19 T-v respectively. The ratio of B-v to the valence transition temperature is close to a universal value of other Eu-based systems with a different crystal structure (ThCr2Si2 type). The phase boundary curve in the T-B phase diagram is discussed thermodynamically.

Overexpression of human epidermal growth factor receptor 2 (HER2) is associated with more frequent cancer recurrence and metastasis. Sensitive sensing of HER2 in living breast cancer cells is crucial in the early stages of cancer and to further understand its role in cells. Biomedical imaging has become an indispensable tool in the fields of early cancer diagnosis and therapy. In this study, we designed and synthesized platinum (Pt) nanocluster bionanoprobes with red emission (Ex/Em = 535/630 nm) for fluorescence imaging of HER2. Our Pt nanoclusters, which were synthesized using polyamidoamine (PAMAM) dendrimer and preequilibration, exhibited approximately 1% quantum yield and possessed low cytotoxicity, ultrasmall size, and excellent photostability. Furthermore, combined with ProteinA as an adapter protein, we developed Pt bionanoprobes with minimal nonspecific binding and utilized them as fluorescent probes for highly sensitive optical imaging of HER2 at the cellular level. More importantly, molecular probes with long-wavelength emission have allowed visualization of deep anatomical features because of enhanced tissue penetration and a decrease in background noise from tissue scattering. Our Pt nanoclusters are promising fluorescent probes for biomedical applications.

Artificially fabricated 3$d$/5$d$ superlattices (SLs) involve both strong electron correlation and spin-orbit coupling in one material by means of interfacial 3$d$-5$d$ coupling, whose mechanism remains mostly unexplored. In this work we investigated the mechanism of interfacial coupling in LaMnO$_3$/SrIrO$_3$ SLs by several spectroscopic approaches. Hard x-ray absorption, magnetic circular dichroism and photoemission spectra evidence the systematic change of the Ir ferromagnetism and the electronic structure with the change of the SL repetition period. First-principles calculations further reveal the mechanism of the SL-period dependence of the interfacial electronic structure and the local properties of the Ir moments, confirming that the formation of Ir-Mn molecular orbital is responsible for the interfacial coupling effects. The SL-period dependence of the ratio between spin and orbital components of the Ir magnetic moments can be attributed to the realignment of electron spin during the formation of the interfacial molecular orbital. Our results clarify the nature of interfacial coupling in this prototypical 3$d$/5$d$ SL system and the conclusion will shed light on the study of other strongly correlated and spin-orbit coupled oxide hetero-interfaces.

SmO thin film is a new Kondo system showing a resistivity upturn around 10 K and was theoretically proposed to have a topologically nontrivial band structure. We have performed hard x-ray and soft x-ray photoemission spectroscopy to elucidate the electronic structure of SmO. From the Sm 3$d$ core-level spectra, we have estimated the valence of Sm to be $\sim$2.96, proving that the Sm has a mixed valence. The valence-band photoemission spectra exhibit a clear Fermi edge originating from the Sm 5$d$-derived band. The present finding is consistent with the theory suggesting a possible topological state in SmO and show that rare-earth monoxides or their heterostructures can be a new playground for the interplay of strong electron correlation and spin-orbit coupling.

<p>Ultrafast Fe L₃ XAS and 2p3d RIXS elucidate the photoexcitation process of hematite.</p>

The discoveries of intrinsic ferromagnetism in atomically-thin van der Waals crystals have opened up a new research field enabling fundamental studies on magnetism at two-dimensional (2D) limit as well as development of magnetic van der Waals heterostructures. To date, a variety of 2D ferromagnetism has been explored mainly by mechanically exfoliating 'originally ferromagnetic (FM)' van der Waals crystals, while bottom-up approach by thin film growth technique has demonstrated emergent 2D ferromagnetism in a variety of 'originally non-FM' van der Waals materials. Here we demonstrate that V5Se8 epitaxial thin films grown by molecular-beam epitaxy (MBE) exhibit emergent 2D ferromagnetism with intrinsic spin polarization of the V 3d electrons despite that the bulk counterpart is 'originally antiferromagnetic (AFM)'. Moreover, thickness-dependence measurements reveal that this newly-developed 2D ferromagnet could be classified as an itinerant 2D Heisenberg ferromagnet with weak magnetic anisotropy, broadening a lineup of 2D magnets to those potentially beneficial for future spintronics applications.

We examined the photo-induced dynamics of ferromagnetic Co/Pt thin films demonstrating perpendicular magnetic anisotropy with element specificity using resonant polar magneto-optical Kerr effect measurements at Pt~N${}_{6,7}$ and Co~M${}_{2,3}$ edges with an x-ray free electron laser. The obtained results showed a clear element dependence of photo-induced demagnetization time scales: $\tau_\textrm{demag.}^\textrm{Co}=80\pm60~\textrm{fs}$ and $\tau_\textrm{demag.}^\textrm{Pt}=640\pm140~\textrm{fs}$. This dependence is explained by the induced moment of the Pt atom by current flow from the Co layer through the interfaces. The observed magnetization dynamics of Co and Pt can be attributed to the characteristics of photo-induced Co/Pt thin film phenomena including all-optical switching.

We performed a resonant inelastic X-ray scattering (RIXS) study of La$_{2-x}$Sr$_{x}$NiO$_{4+{\delta } }$ (LSNO) at the oxygen $K$ edge to investigate the nature of the doped holes with regard to charge excitations. Charge excitations of the hole-doped nickelates are found to be almost independent of momentum transfer, indicating that the doped holes are strongly localized in character. Additionally, conspicuous changes in energy position are in temperature dependence. These characters are observed in stark contrast to those of the high-$T_{c}$ cuprate La$_{2-x}$Sr$_{x}$CuO$_{4}$ (LSCO), where delocalized doped holes form charge excitations with sizable momentum dependence in the CuO$_2$ plane. This distinct nature of charge excitations of doped holes is consistent with the metallicity of the materials and could be caused by strong electron-phonon coupling and weak quantum spin fluctuation in the nickelates.

Perovskite rhodates are characterized by intermediate strengths of both electronic correlation as well as spin-orbit coupling (SOC) and usually behave as moderately correlated metals. A recent publication (Phys. Rev. B 95, 245121(2017)) on epitaxial SrRhO$_3$ thin films unexpectedly reported a bad-metallic behavior and suggested the occurrence of antiferromagnetism below 100 K. We studied this SrRhO$_3$ thin film by hard x-ray photoemission spectroscopy and found a very small density of states (DOS) at Fermi level, which is consistent with the reported bad-metallic behavior. However, this negligible DOS persists up to room temperature, which contradicts with the explanation of antiferromagnetic transition at around 100 K. We also employed electronic structure calculations within the framework of density functional theory and dynamical mean-field theory. In contrast to the experimental results, our calculations indicate metallic behavior of both bulk SrRhO$_3$ and the SrRhO$_3$ thin film. The thin film exhibits stronger correlation effects than the bulk, but the correlation effects are not sufficient to drive a transition to an insulating state. The calculated uniform magnetic susceptibility is substantially larger in the thin film than that in the bulk. The role of SOC was also investigated and only a moderate modulation of the electronic structure was observed. Hence SOC is not expected to play an important role for electronic correlation in SrRhO$_3$.

Recently, hole-doped superconducting cuprates with the T'-structure La1.8-xEu0.2SrxCuO4 (LESCO) have attracted a lot of attention. We have performed x-ray photoemission and absorption spectroscopy measurements on as-grown and reduced T0-LESCO. Results show that electrons and holes were doped by reduction annealing and Sr substitution, respectively. However, it is shown that the system remains on the electron-doped side of the Mott insulator or that the charge-transfer gap is collapsed in the parent compound.

Recent advances in the molecular design of organic materials have uncovered various novel functional properties. One of them is the coupling of proton dynamics and electrical conductivity, which can only be achieved in 3D organic crystals. However, reduction of dimensionality to two dimensions is essential in organic electronics application. In this study, we prepared and characterized a 2D organic bilayer with "proton-electron" concerted functionality on a solid surface. It consisted of catechol-fused bis (methylthio) tetrathiafulvalene (H(2)Cat-BMT-TTF) deposited onto an imidazole-terminated alkanethiolate self-assembled monolayer (Im-SAM) on a Au surface. Direct evidence of interfacial hydrogen bonding(H-bonding) was obtained by scanning tunneling microscopy(STM), infrared reflection absorption spectroscopy (IRAS), and near edge X-ray absorption fine structure(NEXAFS) spectroscopy. STM images showed the deposited H(2)Cat-BMT-TTF molecules as grains with the thickness of a single molecular layer. The OH stretching vibrational modes of H(2)Cat-BMT-TTF in the IRAS spectra showed a large red shift and substantial broadening upon adsorption on Im-SAM, indicating that the OH groups of H(2)Cat-BMT-TTF act as the H+ donor sites. The counterpart H+ acceptor sites were pinpointed by N K-edge NEXAFS. The pi* peak of the imino N atoms of the imidazole rings in Im-SAM shifted to higher energy upon the adsorption of H(2)Cat-BMT-TTF. Therefore, H-bonds form between the imino N atoms (H+ acceptor sites) of Im-SAM and the OH groups (H+ donor sites) of H(2)Cat-BMT-TTF. The present work is a steady step toward the realization of 2D organic functional materials, and the experimental methods adopted herein will serve as powerful tools for the detection of their functions.