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Sphaerotilus natans is a filamentous sheath-forming bacterium commonly found in activated sludge. Its sheath is assembled from a thiolic glycoconjugate called thiopeptidoglycan. S. montanus ATCC-BAA-2725 is a sheath-forming member of stream biofilms, and its sheath is morphologically similar to that of S. natans. However, it exhibits heat susceptibility, which distinguishes it from the S. natans sheath. In this study, chemical composition and solid-state NMR analyses suggest that the S. montanus sheath is free of cysteine, indicating that disulfide linkage is not mandatory for sheath formation. The S. montanus sheath was successfully solubilized by N-acetylation, allowing solution-state NMR analysis to determine the sugar sequence. The sheath was susceptible to thiopeptidoglycan lyase prepared from the thiopeptidoglycan-assimilating bacterium, Paenibacillus koleovorans. The reducing ends of the enzymatic digests were labeled with 4-aminobenzoic acid ethyl ester, followed by HPLC. Two derivatives were detected, and their structures were determined. We found that the sheath has no peptides and is assembled as follows: [→4)-β- d -GlcA-(1→4)-β- d -Glc-(1→3)-β- d -GalNAc-(1→4)-α- d -GalNAc-(1→4)-α- d -GalN-(1→]n (β- d -Glc and α- d -GalNAc are stoichiometrically and substoichiometrically 3-O-acetylated, respectively). Thiopeptidoglycan lyase was thus confirmed to cleave the 1,4 linkage between α- d -GalN and β- d -GlcA, regardless of the peptide moiety. Furthermore, vital fluorescent staining of the sheath demonstrated that elongation takes place at the tips, as with the S. natans sheath.

The diode effect is fundamental to electronic devices and is widely used in rectifiers and AC-DC converters. At low temperatures, however, conventional semiconductor diodes possess a high resistivity, which yields energy loss and heating during operation. The superconducting diode effect (SDE), which relies on broken inversion symmetry in a superconductor may mitigate this obstacle: in one direction a zero-resistance supercurrent can flow through the diode, but for the opposite direction of current flow, the device enters the normal state with ohmic resistance. The application of a magnetic field can induce SDE in Nb/V/Ta superlattices with a polar structure, in superconducting devices with asymmetric patterning of pinning centres, or in superconductor/ferromagnet hybrid devices with induced vortices. The need for an external magnetic field limits their practical application. Recently, a field-free SDE was observed in a NbSe2/Nb3Br8/NbSe2 junction, and it originates from asymmetric Josephson tunneling that is induced by the Nb3Br8 barrier and the associated NbSe2/Nb3Br8 interfaces. Here, we present another implementation of zero-field SDE using noncentrosymmetric [Nb/V/Co/V/Ta]20 multilayers. The magnetic layers provide the necessary symmetry breaking and we can tune the SDE by adjusting the structural parameters, such as the constituent elements, film thickness, stacking order, and number of repetitions. We control the polarity of the SDE through the magnetization direction of the ferromagnetic layers. Artificially stacked structures, as the one used in this work, are of particular interest as they are compatible with microfabrication techniques and can be integrated with devices such as Josephson junctions. Energy-loss-free SDEs as presented in this work may therefore enable novel non-volatile memories and logic circuits with ultralow power consumption., 31 pages, 11 figures

To electrochemically control structural and transport properties of oxygen-deficient perovskite SrFeOy (2.5 ≦ y ≦ 3) (SFO) epitaxial films, we employed electric-field-effect transistor structures in which the proton-conducting solid electrolyte Nafion is used as a gate insulator. When a positive gate voltage (VGS) is applied and protons are injected toward the film channel layer, the SFO films are electrochemically reduced, leading to increases in the channel resistance. On the other hand, when a negative VGS is applied and protons are removed, the SFO films are oxidized, and as a result, the channel resistances decrease. In addition, we found that the electrochemically reduced SFO films accommodate protons, forming the proton-containing oxide HxSrFeO₂.₅ whose proton concentration is determined by elastic recoil detection analysis to be x ∼ 0.11. Our results indicate the usefulness of the proton-conducting solid electrolyte for electrochemically controlling transition metal oxides and for exploring proton-containing oxides., Archived file replaced from Accepted Manuscript to Version of Record. (2023-03-02)

Complex transition-metal oxides offer an interesting class of materials for use as oxygen reduction reaction (ORR) catalysts. A recent study proposed a volcano-shape relationship between ORR activi...

Lithium-oxide-halide and lithium-hydroxide-halide antiperovskites were explored for potential electrolytes in all-solid Li-ion batteries. A single-phase sample of the Ruddlesden–Popper (RP) series of compounds, LiBr(Li2OHBr)2 with double antiperovskite Li2OHBr layers and rigid rock-salt type LiBr layers, was obtained. Li+-ion vacancies are introduced in the double antiperovskite Li2OHBr layers but not in the LiBr layers and induce two-dimensional Li-ion conduction with low activation energy by mediating Li-ion hopping. In contrast to the Br-containing RP phase, Cl-containing Li-oxide-halide and Li-hydroxide-halide RP phases cannot be crystallized due to the structural mismatch between the antiperovskite layers and rigid LiCl layers.

Recently, all-optical magnetization control has been garnering considerable attention in realizing next-generation ultrafast magnetic information devices. Here, employing a magneto-optical Kerr effect (MOKE) microscope, we observed the laser-induced magnetization switching of ferrimagnetic oxide NiCo2O4 (NCO) epitaxial thin films with perpendicular magnetic anisotropy, where the sample was pumped at 1030-nm laser pulses, and magnetic domain images were acquired via the MOKE microscope with a white light emitting diode. Laser pulses irradiated an NCO thin film at various temperatures from 300 K to 400 K while altering the parameters of pulse interval, fluence, and the number of pulses with the absence of the external magnetic field. We observed accumulative all-optical switching at 380 K and above. Our observation of oxide NCO thin films facilitates the realization of chemically stable magnetization switching using ultrafast lasers, and without applying a magnetic field., Comment: 6 pages, 6 figures

Electrochemical reactions with insertions of ions in solids depend on crystallographic orientations. We investigated electrochemical responses of (100), (110) and (111)-oriented oxygen-deficient perovskite SrFeO2.5+y epitaxial films in electric-field-effect transistor structures with the proton-conducting electrolyte Nafion as a gate insulator. We found that only (100)-oriented SrFeO2.5+y films exhibit changes associated with gate-voltage-induced electrochemical reductions. Furthermore, elastic recoil detection analysis shows that electrochemically reduced (100) films can accommodate protons, forming the proton-containing oxide H0.11SrFeO2.5+y . Our results show that oxygen vacancies form preferentially along the {100} axes and ion diffusion in electrochemical reactions occurs dominantly along the {100} directions in SrFeO2.5+y .

We investigate the anomalous Hall effect in perpendicularly magnetized ferrimagnet $\mathrm{Ni}{\mathrm{Co}}_{2}{\mathrm{O}}_{4}$ epitaxial films 3 to 30 nm thick. We find that the anomalous Hall conductivity ${\ensuremath{\sigma}}_{\mathrm{AHE}}$ has complex relations with the longitudinal electrical conductivity ${\ensuremath{\sigma}}_{\mathrm{xx}}$. When the ${\ensuremath{\sigma}}_{\mathrm{xx}}$ is relatively large (larger than ${10}^{3}\phantom{\rule{0.16em}{0ex}}{\mathrm{\ensuremath{\Omega}}}^{--1}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{--1}$), the ${\ensuremath{\sigma}}_{\mathrm{AHE}}$ is almost independent of the ${\ensuremath{\sigma}}_{\mathrm{xx}}$, but scales with the magnetization. This unique scaling behavior of ${\ensuremath{\sigma}}_{\mathrm{AHE}}$ indicates that the anomalous Hall effect is of intrinsic origin dominated by the Berry curvature of occupied electronic states. On the other hand, when the ${\ensuremath{\sigma}}_{\mathrm{xx}}$ is lowered, the scaling relation is changed to ${\ensuremath{\sigma}}_{\mathrm{AHE}}\ensuremath{\sim}{\ensuremath{\sigma}}_{\mathrm{xx}}^{1.6}$, indicating a crossover of the dominant contribution to ${\ensuremath{\sigma}}_{\mathrm{AHE}}$, from the intrinsic Berry phase to the impurity scatterings. Our results show the intrinsic contribution from Berry curvature plays a crucial role in the anomalous Hall effect in $\mathrm{Ni}{\mathrm{Co}}_{2}{\mathrm{O}}_{4}$ films.

We experimentally investigated the magnetic properties of ${\mathrm{NiCo}}_{2}{\mathrm{O}}_{4}$ epitaxial films known to be conductive oxides with perpendicular magnetic anisotropy (PMA) at room temperature. Both magnetotorque and magnetization measurements at various temperatures provide clear experimental evidence of the spin reorientation transition at which the MA changes from PMA to easy-cone magnetic anisotropy (ECMA) at a certain temperature (${T}_{\mathrm{SR}}$). ECMA was commonly observed in films grown by pulsed laser deposition and reactive radio frequency magnetron sputtering, although ${T}_{\mathrm{SR}}$ is dependent on the growth method as well as the conditions. The cone angles measured from the $c$ axis increased successively at ${T}_{\mathrm{SR}}$ and approached a maximum of 45--50 degrees at the lowest measurement temperature of 5 K. Calculation with the cluster model suggests that the ${\mathrm{Ni}}^{3+}$ ions occupying the ${T}_{d}$ site could be the origin of the ECMA. Both the magnetic properties and the results of the calculation based on the cluster model indicate that the ECMA is attributable to the cation antisite distribution of ${\mathrm{Ni}}^{3+}$, which is possibly formed during the growth process of the thin films.

Using a time-resolved magneto-optical Kerr effect (TR-MOKE) microscope, we observed ultrafast demagnetization of inverse-spinel-type NiCo2O4 (NCO) epitaxial thin films of the inverse spinel type ferrimagnet NCO with perpendicular magnetic anisotropy. This microscope uses a pump-probe method, where the sample is pumped at 1030 nm, and magnetic domain images are acquired via MOKE microscopy at 515 nm (the second harmonic). We successfully observed the dynamics of the magnetic domain of the NCO thin film via laser irradiation, and obtained a demagnetization time constant of approximately 0.4 ps. This time constant was significantly smaller than the large time constants reported for other half-metallic oxides. These results, combined with the results of our x-ray photoemission spectroscopy study, indicate that this NCO thin film is a ferrimagnetic metal whose electronic structure deviates from the theoretically predicted half-metallic one., 5 pages, 5 figures

Recently, Ag–Bi–I system has attracted significant attention as an emergent material for absorbers in solar cells. We have investigated the single-phase compositions, analyzed the detailed crystal structures, measured optical and electronic properties, and determined the structure–property relations of Ag2−3xBixI2. Single-phase samples with CdCl2-type rhombohedral structures were obtained with Ag-rich compositions (x = 0.45–0.48), while defect-spinel-type cubic structures were stabilized with Bi-rich compositions (x = 0.52–0.57). Both the rhombohedral and cubic structures consisted of the cubic close-packed I-ion sublattices, and the small difference in the Ag/Bi composition resulted in distinct crystal structures. Both the CdCl2-type rhombohedral and defect-spinel-type cubic compounds exhibited semiconducting behavior with proper band gap energies, which indicates that both materials are potentially useful as absorbers in solar cells. The rhombohedral structure compounds have shallower valence band energies, larger indirect band gap energies, and higher electrical conductivity with lower activation energy than the cubic structure compounds. The distinct properties result from the difference in the defect structures, additional Ag occupation in the rhombohedral phase and deficiencies of the octahedral site in the cubic phase.

Chiral antiferromagnets are regarded as a new class of antiferromagnets owing to its intriguing spin transport properties such as a sizable anomalous Hall effect (AHE). In this work, we fabricated chiral antiferromagnetic L12-Mn3Ir (111)-oriented films and investigated the anomalous Hall conductivity with respect to the chemical order parameter S . It is found that the anomalous Hall conductivity in (111)-oriented films is much more pronounced compared to (001)-oriented films, which is consistent with the theoretical prediction. We also found that the anomalous Hall conductivity strongly depends on the longitudinal conductivity, which could suggest an extrinsic contribution to the AHE in L12-Mn3Ir.

Crystal growth techniques using mist precursors, such as mist chemical vapor deposition (mist CVD), are useful for growing epitaxial thin films of various functional oxides under non-vacuum conditions and at relatively low temperatures. Their growth mechanism remains elusive, however, so we developed a home-made reaction chamber for mist CVD and investigated the epitaxial growth mechanism of magnetite Fe3O4 using alcoholic mists of iron acetylacetonate precursors. We show here that grown epitaxial films’ structural and physical properties depend on mist flow rates (or N2 carrier gas flow rates). Increasing the mist flow and promoting mists’ reactions on substrate surfaces produce Fe3O4 epitaxial films having larger magnetizations and lower electrical resistivities and exhibiting the Verwey transition. We also show that films’ properties are modified by changing either water additive concentrations in alcoholic mist solutions or the distance (the joint pipe length) between the mist bottle and the reaction chamber. Our results highlight the significance of the mist-flow-induced reactions for mist CVD epitaxial growth, providing new insight into the mechanism of epitaxial growth by mist CVD.

Caloric effects of solids can provide us with innovative refrigeration systems more efficient and environment-friendly than the widely-used conventional vapor-compression cooling systems. Exploring novel caloric materials is challenging but critically important in developing future technologies. Here we discovered that the quadruple perovskite structure ferrimagnet BiCu₃Cr₄O₁₂ shows large multiple caloric effects at the first-order charge transition occurring around 190 K. Large latent heat and the corresponding isothermal entropy change, 28.2 J K⁻¹ kg⁻¹, can be utilized by applying both magnetic fields (a magnetocaloric effect) and pressure (a barocaloric effect). Adiabatic temperature changes reach 3.9 K for the 50 kOe magnetic field and 4.8 K for the 4.9 kbar pressure, and thus highly efficient thermal controls are achieved in multiple ways., 磁場と圧力でマルチに冷却可能な酸化物新材料 --フェリ磁性電荷転移酸化物におけるマルチ熱量効果の実証--. 京都大学プレスリリース. 2021-06-22.

We grew epitaxial thin films of hematite (α-Fe2O3) on (0 0 0 1) Al2O3 substrates by pulsed laser deposition and investigated their magnetic properties. α-Fe2O3 films grown at lower temperatures are found to undergo the Morin transition at higher temperatures, implying that lowering the growth temperature and managing lattice defects associated with strain relaxation are key to realizing Morin transition. We also characterized films' magnetic properties by spin Hall magnetoresistance (SMR). We show that tri-axial magnetic anisotropy can be detected as SMR features having a 60° period with respect to in-plane magnetic field rotations. Furthermore, a large change in SMR ratio associated with Néel vector re-orientation due to Morin transition is seen. Details of SMR properties for α-Fe2O3 films and their analysis results, including the influence of the fabrication process on SMR behaviors, are discussed.

We carried out x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) spectroscopy and investigated cation valence states and spin and orbital magnetic moments in inverse-spinel ferrimagnet $\mathrm{N}{\mathrm{i}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{o}}_{2+y}{\mathrm{O}}_{4\ensuremath{-}z}$ (NCO) epitaxial films with perpendicular magnetic anisotropy. We show that the oxygen pressure ${P}_{\mathrm{O}2}$ during film growth by pulsed laser deposition influences not only the cation stoichiometry (site occupation) but also the cation valence state. Our XAS results show that the Ni in the ${O}_{h}$-site is in the intermediate valence state between +2 and +3, $\mathrm{N}{\mathrm{i}}^{(2+\ensuremath{\delta})+}$ ($0l\ensuremath{\delta}l1$), whose nominal valence state (the \ensuremath{\delta} value) varies depending on ${P}_{\mathrm{O}2}$. On the other hand, the Co in the octahedral (${O}_{h}$) and tetrahedral (${T}_{d}$) sites, respectively, have a valence state close to +3 and +2. We also find that the XMCD signals originate mainly from the ${T}_{d}$-site $\mathrm{C}{\mathrm{o}}^{2+}$ ($\mathrm{C}{\mathrm{o}}_{\mathrm{Td}}$) and ${O}_{h}$-site $\mathrm{N}{\mathrm{i}}^{(2+\ensuremath{\delta})+}$ ($\mathrm{N}{\mathrm{i}}_{\mathrm{Oh}}$), indicating that these cation valence states are the key to determining the magnetic and transport properties of NCO films. Interestingly, the valence state of $\mathrm{N}{\mathrm{i}}^{(2+\ensuremath{\delta})+}$ that gives rise to the XMCD signal remains unchanged independent of ${P}_{\mathrm{O}2}$. The electronic structure of $\mathrm{N}{\mathrm{i}}^{(2+\ensuremath{\delta})+}$ that is responsible for the magnetic moment and electrical conduction differs from those of $\mathrm{N}{\mathrm{i}}^{2+}$ and $\mathrm{N}{\mathrm{i}}^{3+}$. In addition, the orbital magnetic moment originating from $\mathrm{C}{\mathrm{o}}_{\mathrm{Td}}$ is as large as $0.14\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{C}{\mathrm{o}}_{\mathrm{Td}}$ and parallel to the magnetization, while the $\mathrm{N}{\mathrm{i}}_{\mathrm{Oh}}$ orbital moment is as small as $0.07\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{N}{\mathrm{i}}_{\mathrm{Oh}}$ and is rather isotropic. $\mathrm{C}{\mathrm{o}}_{\mathrm{Td}}$, therefore, plays the key role in the perpendicular magnetic anisotropy of the films. Our results demonstrate the significance of the site-dependent cation valence states for the magnetic and transport properties of $\mathrm{NiC}{\mathrm{o}}_{2}{\mathrm{O}}_{4}$ films.

Optically generated excitonic states (excitons and trions) in transition metal dichalcogenides are highly sensitive to the electronic and magnetic properties of the materials underneath. Modulation and control of the excitonic states in a novel van der Waals (vdW) heterostructure of monolayer MoSe2 on double-layered perovskite Mn oxide ((La0.8 Nd0.2 )1.2 Sr1.8 Mn2 O7 ) is demonstrated, wherein the Mn oxide transforms from a paramagnetic insulator to a ferromagnetic metal. A discontinuous change in the exciton photoluminescence intensity via dielectric screening is observed. Further, a relatively high trion intensity is discovered due to the charge transfer from metallic Mn oxide under the Curie temperature. Moreover, the vdW heterostructures with an ultrathin h-BN spacer layer demonstrate enhanced valley splitting and polarization of excitonic states due to the proximity effect of the ferromagnetic spins of Mn oxide. The controllable h-BN thickness in vdW heterostructures reveals a several-nanometer-long scale of charge transfer as well as a magnetic proximity effect. The vdW heterostructure allows modulation and control of the excitonic states via dielectric screening, charge carriers, and magnetic spins.

Cation distribution in transition-metal oxides is closely tied to their physical properties. Here we employed resonant x-ray-diffraction technique and quantitatively evaluated the cation distribution in inverse spinel $\mathrm{NiC}{\mathrm{o}}_{2}{\mathrm{O}}_{4}$ films that were epitaxially grown by pulsed laser deposition. Our results reveal that oxygen partial pressure during growth ${P}_{\mathrm{O}2}$ is a key parameter determining cation distribution in the films, with a larger ${P}_{\mathrm{O}2}$ leading to an increase in the Ni concentration occupying the octahedral $({O}_{h})$ sites. We further show that the ${P}_{\mathrm{O}2}$-dependent ${O}_{h}$-site Ni concentration impacts the magnetic properties of the films: Films having the cation distribution close to the stoichiometric exhibit ferrimagnetism with a transition temperature higher than 400 K and enhanced perpendicular magnetic anisotropy. On the other hand, the reductions in the ${O}_{h}$-site Ni concentration leads to the deterioration of the ferrimagnetic properties such as magnetization, transition temperature, and anisotropy energy. Our study demonstrates that the ferrimagnetic properties and the anisotropy can be tuned through the cation distribution, which would provide an additional degree of freedom in designing spintronic devices based on spinel oxides including $\mathrm{NiC}{\mathrm{o}}_{2}{\mathrm{O}}_{4}$.

We investigated the field-sweep-rate and time dependence of the transverse resistivity anomalies in ultrathin $\mathrm{SrRu}{\mathrm{O}}_{3}$ epitaxial thin films the behaviors of which resemble what has been claimed to be the topological Hall effect. We find that the magnitudes of the anomalies are independent of the field sweep rate, while the magnetic field at which the anomalies appear $({H}_{\mathrm{hump}})$ and the coercive field $({H}_{c})$ exhibit identical sweep-rate dependence. We also show that the transverse resistivity under a fixed external magnetic field exhibits transient behavior, eventually (\ensuremath{\approx}5000 s) approximating the value of the anomalous Hall resistivity that is obtained after the magnetization reversal processes are completed. These results indicate that ${H}_{c}$ is inhomogeneous within the films and activation energies of the magnetization reversal at ${H}_{\mathrm{hump}}$ and ${H}_{c}$ are identical. The observed field-sweep-rate and time dependence of the transverse resistivity anomalies can be understood in the framework of conventional magnetic switching dynamics, and topological interpretation would not be necessarily taken into account. Our results are in line with a model in which inhomogeneity of ${H}_{c}$ and anomalous Hall resistivity are responsible for the transverse resistivity anomalies in $\mathrm{SrRu}{\mathrm{O}}_{3}$.

Artificially engineered superlattice offers highly flexible control of the quantized electronic state by manipulating its lattice structure. We fabricated artificially engineered superlattices [Nb/V/Ta] n as a new class of superconducting superlattices and investigated the upper critical field H c2 in terms of the structural difference. It is found that the out-of-plane upper critical field H c 2 ⊥ significantly increases compared with that of a Nb single layer film as the thickness of the constituent Nb, V, and Ta layers becomes thin. In the case of the superlattice [Nb/V/Ta] n , the H c 2 ⊥ is basically governed by the orbital pair-breaking effect rather than Pauli pair-breaking effect and increases only when the crystalline size of the superlattice decreases by the insertion of V layers. Thus, we conclude that the shortening of the coherence length of Cooper pairs due to the crystalline disorder predominantly contributes to the increase of the H c 2 ⊥ in the superlattice [Nb/V/Ta] n .

Compositional tuning of correlations between electrons, spins, and lattices in solids is a key strategy for exploring quantum phenomena. In contrast to uniform chemical substitutions that are widely used for the compositional tunings, continuous variations in compositions along a given direction of materials (composition gradients) are expected to modulate translational crystal symmetries and electronic band structures, leading to unique properties not seen in homogeneously substituted materials. Preparing compositionally graded materials, however, is still challenging as it requires elaborate fabrication processes, and how composition gradients affect materials’ properties remains elusive. Here, we show that composition gradients can be made by simply depositing films under continuously varying oxygen pressures by pulsed laser deposition. Applying this technique to the inverse-spinel-structured ferrimagnet NiCo2O4, we found that composition gradients in NiCo2O4 epitaxial films tune the magnetic anisotropy and stabilize the magnetizations modulated along the gradient, which are not seen in compositionally uniform films. The spatially modulated magnetization can also be reversed in controlled manners through distinct antiferromagnetically coupled magnetization configurations in the single material. Our results demonstrate the potential of composition gradients to tune materials’ properties and explore a new class of materials with added functionalities.

We quantitatively evaluated oxygen octahedral distortions in epitaxial thin films of the itinerant ferromagnet, ${\mathrm{La}}_{0.5}{\mathrm{Sr}}_{0.5}\mathrm{Co}{\mathrm{O}}_{3}$ (LSCO), and investigated their impact on strucure-property relationships. The compressively strained film on a $\mathrm{LaAl}{\mathrm{O}}_{3}$ substrate has a lower electrical resistivity and higher ferromagnetic transition temperature than the tensilely strained film on a ${\mathrm{La}}_{0.30}{\mathrm{Sr}}_{0.70}{\mathrm{Al}}_{0.65}{\mathrm{Ta}}_{0.35}{\mathrm{O}}_{3}$ substrate. The magnetic anisotropy is also found to depend on the type of strain, with perpendicular magnetic anisotropy induced in the compressively strained film and in-plane magnetization seen in the tensilely strained film. Our synchrotron x-ray-diffraction measurements and quantitative analysis reveal distinct oxygen octahedral distortions accommodated in these films and show that the out-of-plane and in-plane Co--O bond lengths of the compressively strained film are comparable to the in-plane and out-of-plane bond lengths of the tensilely strained film. These results indicate that the bond-length changes in LSCO modify hybridization between Co $3d$ and O2p orbitals, leading to the strain-dependent properties. These results highlight the significant role of octahedral distortions for structure-property relationships in the strained LSCO films.

We fabricated electric double-layer transistors with perovskite nickelate LaNiO3 epitaxial films as a channel and an ionic liquid as a gate insulator and investigated the channel's transport behavior at negative gate bias voltages. We found that when the negative VG is low enough, the channel resistance undergoes reversible changes due to electrostatic carrier accumulation. On the other hand, with increasing VG, electrochemical decomposition of the channels due to the VG-induced instability of the unusually high Ni valence state [Ni(3+δ)+] occurs and the channels are etched, which is seen as an irreversible increase in the channel resistance. We also show that channel layers electrochemically etched to a few nanometers thick exhibit metallic transport behavior and the electrostatic carrier accumulation in the etched metallic channels results in 30% reductions in the channel resistance at 220 K, which are about twice as large as those seen for the ultra-thin insulating film channels. These observations indicate that surface defects and dangling bonds in the electrochemically etched channel layers differ from those in ultrathin films. Our results highlight that the electrochemical decomposition and etching in electric double-layer transistors is a unique approach for fabricating ultrathin oxide films and exploring electric field-induced effects on their functional properties.