Yasuyuki Hikita

Electrodes with a maximal active site density are critical for high‐performance high‐temperature electrochemical cells (HTECs). One widely employed approach involves the use of porous nanostructures with a high surface‐to‐volume ratio. However, their active site densities inevitably decrease owing to particle aggregation induced at high temperatures, necessitating further development of electrode processing techniques. Taking Pt/yttria‐stabilized zirconia (YSZ) interface as a model system, a Pt nanostructured textile akin to the cantaloupe‐rind pattern with high mechanical integrity is fabricated. Application of an AC voltage to this textile electrode at an elevated temperature reduces the Pt particle size from submicron to 10–80 nm forming a nanocomposite with YSZ, accompanied by a 40‐fold increase in current density under high‐temperature water electrolysis conditions. Furthermore, the AC voltage application to a partially aggregated electrode restores its nano‐blended structure associated with the recovery of its activity. This technique is effective in counteracting particle aggregation on demand, providing an alternative approach to achieve high performance and extended lifetimes in HTECs.

A single crystalline thin film of Ba₂Ti₆O₁₃ was grown on the (210) surface of a SrTiO₃ single-crystal substrate by pulsed laser deposition. Scanning transmission electron microscopy shows that the Ba₂Ti₆O₁₃ thin film grew epitaxially with the a-plane, and the b-/c-axes of Ba₂Ti₆O₁₃ parallel to the (210) plane and the [00 1 ¯ ]/[ 1 ¯ 20] directions of SrTiO₃, respectively. The electrical conductance of the Ba₂Ti₆O₁₃ film was approximately one order larger than that of the reduced SrTiO_3−δ single crystal with oxygen deficiency of roughly δ = 0.05. Electronic structure calculations predict that Ba₂Ti₆O₁₃ is an n-type degenerate semiconductor with spin-split states primarily of Ti 3d orbital.

Designing acid‐stable oxygen evolution reaction electrocatalysts is key to developing sustainable energy technologies such as polymer electrolyte membrane electrolyzers but has proven challenging due to the high applied anodic potentials and corrosive electrolyte. This work showcases advanced nanoscale microscopy techniques supported by complementary structural and chemical characterization to develop a fundamental understanding of stability in promising SrIrO₃ thin film electrocatalyst materials. Cross‐sectional high‐resolution transmission electron microscopy illustrates atomic‐scale bulk and surface structure, while secondary ion mass spectrometry imaging using a helium ion microscope provides the nanoscale lateral elemental distribution at the surface. After accelerated degradation tests under anodic potential, the SrIrO₃ film thins and roughens, but the lateral distribution of Sr and Ir remains homogeneous. A layer‐wise dissolution mechanism is hypothesized, wherein anodic potential causes the IrO_x‐rich surface to dissolve and be regenerated by Sr leaching. The characterization approaches utilized herein and mechanistic insights into SrIrO₃ are translatable to a wide range of catalyst systems.

Straining an oxide membrane: Perovskite manganites, such as La _0.7 Ca _0.3 MnO ₃ , have complex phase diagrams with many competing states. Among the knobs that can be used to control their properties are magnetic field and strain. Hong et al. placed membranes of La _0.7 Ca _0.3 MnO ₃ on a flexible polymer layer (see the Perspective by Beekman). Stretching the flexible layer resulted in large strains of up to 8% on the membrane. By varying the magnitude and direction of the strain, the researchers were able to explore the phase diagram of the system and influence its magnetic and transport properties. Science , this issue p. 71 ; see also p. 32

Oxide interfaces, including the LaAlO₃/SrTiO₃ interface, have been a subject of intense interest for over a decade due to their rich physics and potential as low dimensional nanoelectronic systems. The field has reached the stage where efforts are invested in developing devices. It is critical now to understand the functionalities and limitations of such devices. Recent scanning probe measurements of the LaAlO₃/SrTiO₃ interface have revealed locally enhanced current flow and accumulation of charge along channels related to SrTiO₃ structural domains. These observations raised a key question regarding the role these modulations play in the macroscopic properties of devices. Here we show that the microscopic picture, mapped by scanning superconducting quantum interference device, accounts for a substantial part of the macroscopically measured transport anisotropy. We compared local flux data with transport values, measured simultaneously, over various SrTiO₃ domain configurations. We show a clear relation between maps of local current density over specific domain configurations and the measured anisotropy for the same device. The domains divert the direction of current flow, resulting in a direction dependent resistance. We also show that the modulation can be significant and that in some cases up to 95% of the current is modulated over the channels. The orientation and distribution of the SrTiO₃ structural domains change between different cooldowns of the same device or when electric fields are applied, affecting the device behavior. Our results, highlight the importance of substrate physics, and in particular, the role of structural domains, in controlling electronic properties of LaAlO₃/SrTiO₃ devices. Further, these results point to new research directions, exploiting the STO domains ability to divert or even carry current.

We present depth resolved X-ray micro-Laue diffraction experiments on the low temperature domain structure of SrTiO₃. At 80 K, monochromatic X-ray diffraction shows an elongated out-of-plane unit cell axis within a matrix of in-plane oriented tetragonal unit cells. Full deviatoric strain mappings from white beam diffraction show a dominance of two tetragonal domain orientations (x- and z-axes) over a large area of sample surface. This information sets an upper bound on domain wall widths and offers a method for studying 3D domain structure at low temperatures.

We demonstrate the selective fabrication of Ruddlesden-Popper (RP) type SrIrO₃, Sr₃Ir₂O₇, and Sr₂IrO₄ epitaxial thin films from a single SrIrO₃ target using pulsed laser deposition (PLD). We identified that the growth conditions stabilizing each phase directly map onto the phase diagram expected from thermodynamic equilibria. This approach allows precise cation stoichiometry control as evidenced by the stabilization of single phase Sr₃Ir₂O₇ for the first time, overcoming the close thermodynamic stability between neighboring RP phases. Despite the non-equilibrium nature of PLD, these results highlight the importance of thermodynamic guiding principles to strategically synthesize the targeted phase in complex oxide thin films.

One of the crucial parameters dictating the efficiency of photoelectrochemical water‐splitting is the semiconductor band edge alignment with respect to hydrogen and oxygen redox potentials. Despite the importance of metal oxides in their use as photoelectrodes, studies to control the band edge alignment in aqueous solution have been limited predominantly to compound semiconductors with modulation ranges limited to a few hundred mV. The ability to modulate the flat band potential of oxide photoanodes by as much as 1.3 V, using the insertion of subsurface electrostatic dipoles near a Nb‐doped SrTiO₃/aqueous electrolyte interface is reported. The tunable range achieved far exceeds previous reports in any semiconductor/aqueous electrolyte system and suggests a general design strategy for highly efficient oxide photoelectrodes.

The observed magnetoresistance (MR) in three-terminal (3T) ferromagnet-nonmagnet (FM-NM) tunnel junctions has historically been assigned to ensemble dephasing (Hanle effect) of a spin accumulation, thus offering a powerful approach for characterizing the spin lifetime of candidate materials for spintronics applications. However, due to crucial discrepancies of the extracted spin parameters with known materials properties, this interpretation has come under intense scrutiny. By employing epitaxial artificial dipoles as the tunnel barrier in oxide heterostructures, the band alignments between the FM and NM channels can be controllably engineered, providing an experimental platform for testing the predictions of the various spin accumulation models. Using this approach, we have been able to definitively rule out spin accumulation as the origin of the 3T MR. Instead, we assign the origin of the magnetoresistance to spin-dependent hopping through defect states in the barrier, a fundamental phenomenon seen across diverse systems. A theoretical framework is developed that can account for the signal amplitude, linewidth, and anisotropy.