Motivated by the discovery of superconductivity in Sr-doped infinite-layer nickelate films on SrTiO_3(001), we explore the broader landscape of ABO_2 oxides through comprehensive high-throughput first-principles simulations. Specifically, delafossites and their ordered rock-salt (111) variants stand out as intriguing layered oxides that share the infinite-layer ABO_2 stoichiometry and simultaneously retain a perovskite-like octahedral motif. This positions them as a unique structural bridge between these two phases and as promising candidates for novel correlated electronic states. We compile a phase diagram that compares the relative stability of these four distinct oxides across the periodic table. Surprisingly, we find that the delafossite structure rivals the infinite-layer phase in thermodynamic stability for the nickelates, and even more for the recently suggested palladate and platinate analogs. Comparison of the respective electronic structures reveals that the delafossite compounds, which we find to be characterized by reversed cation order, exhibit a strongly d_z^2-dominated Fermi surface, in stark contrast to the d_x^2-y^2 character observed in the infinite-layer phases. Among all candidates, the La-Ni combination stands out as a thermodynamic optimum for stabilizing the infinite-layer motif. Furthermore, we show that hole doping via Ca, Sr, and Ba systematically enhances the stability of the infinite-layer phase in all three transition-metal families. These results reveal fundamental challenges in realizing bulk substrate-free infinite-layer oxides, and simultaneously offer guidance for future experimental synthesis efforts targeting novel superconducting compounds.

Atmospheric O 2 spontaneously reacts with MoS 2 , but little is known about the mechanism. Herein, in-situ Raman spectroscopy during temperature sweeps under controlled environments show that MoS 2 is stable in O 2 -free environments, but O-substitution and MoO 2 formation occur in the presence of parts-per-million levels of O 2 , and oxidation to MoO 3 occurs at higher O 2 -levels. MoO 2 is stable at room temperature, but time-resolved experiments and density functional theory (DFT)-generated phase diagrams reveal it to be a metastable kinetic product that sequentially oxidizes at elevated temperatures, forming Mo 4 O 11 then MoO 3 . DFT calculations reveal that O-substitution makes both S-vacancy formation and O-substitution more favorable, creating a positive feedback loop that destroys MoS 2 . Experimentally, this feedback loop is embodied as formation of MoO 2 at progressively lower temperatures when either O-substitution or O 2 partial pressure in the environment are increased. These findings reveal that MoS 2 is inherently unstable in O 2 -containing environments due to synergistic effects of two defect types.

Abstract The reduction of iron oxides using hydrogen as a reducing agent represents an alternative route toward the production of green steel. Based on density functional theory (DFT) calculations with a Hubbard U term, we explore the initial adsorption and incorporation of hydrogen in the near-surface region of hematite (0001). Starting with the Fe–O3–Fe termination, which is stable over a wide range of oxygen chemical potentials, we vary the H concentration and distribution in the surface layers. Adsorption of H on the surface or in the near-surface region is energetically favored, in contrast to incorporation in bulk hematite. H binds to an oxygen ion, while simultaneously a Fe3+ in the vicinity is reduced to Fe2+ with a distinct orbital polarization of the sixth electron at Fe2+ depending on the position and concentration of H: a1g in the surface vs a dxy orbital in the subsurface layer. Upon adsorption of 3H at the Fe–O3–Fe termination, a complete rearrangement of the top layers takes place with subsurface oxygen moving to the surface and forming hydroxyl groups, resulting in a 3OH/3Fe/stacking. A key finding is that H2 adsorption leading to H2O formation and desorption is an efficient way for removing lattice oxygen at a much lower energy cost than the direct formation of oxygen vacancies, thus highlighting a further mechanism toward reduction of the surface. Our results demonstrate migration of both anions and cations upon hydrogen adsorption and the emergence of lower coordinated Fe sites that can be regarded as a nucleus for the transformation from hematite to other reduced iron oxide phases such as magnetite.

Nanoscale miniaturization has revolutionized the field of spintronics by enabling exponential growth in areal bit density. A similar leap is also expected in device speeds through successfully harnessing femtosecond magnetization dynamics. However, combining this with the miniaturization of realistic devices is challenging. To address this, we studied the effect of dimensional confinement on the femtosecond demagnetization of Fe. By gradually increasing the level of confinement while keeping excitation conditions constant, we found that Fe layers thinner than 10 nm exhibit enlarged demagnetization amplitudes, reaching a ∼75% increase at 2 nm. By combining ultrafast experiments sensitive to the spins, the charge carriers, and the phonons, we establish that this finite-size effect is magnetic in origin and is not phonon-driven. With the support of ab-initio calculations and atomistic spin dynamics simulations, we identify the enhancement effect as due to local weakening of spin order at the Fe's interface, which becomes significant upon increased confinement.

The discovery of altermagnetism has initiated intensive research and opened new avenues for spin- tronic and transport applications. While current efforts are mostly focused on bulk materials which are typically insulating, here we propose design strategies to achieve a combination of altermag- netism and metallicity in oxide superlattices by exploiting symmetry breaking, electrostatic doping and confinement. While bulk SrCrO3 does not exhibit altermagnetism due to compensating effects between adjacent layers our density functional theory calculations with a Hubbard U parameter re- veal, that a single SrCrO3 layer confined in a (SrCrO3)1/(SrTiO3)1(001) superlattice (SL) exhibits a sizable non-relativistic spin splitting (NRSS) up to 350 meV with bulk d-wave nature due to the coexistence of orbital ordering and octahedral rotations (OORs). Since this system is insulating, we extend to SrCrO3/LaCrO3(001) SLs. In the (SrCrO3)4/(LaCrO3)4(001) SL the combination of a polar discontinuity at the interface and stronger OORs promotes metallic d-wave altermagnetism. The NRSS of up to 120 meV is contributed by the interfacial Cr d bands at the Fermi level with indications for a spin-selective Fermi surface nesting. These findings establish oxide superlattices as a promising platform to realize and explore altermagnetism for quantum transport and spintronic functionalities

Motivated by the technological relevance of S-doped few-layer graphene (FLG) in battery applications and in the oxygen reduction reaction, we systematically explore the effect of basal plane S-doping on the electronic properties of mono-, bi-, and four-layer graphene, using first-principles calculations with van der Waals corrections. In the monolayer we find a variety of effects ranging from a sustained Dirac cone with localized impurity bands away from the Fermi level for thiophenic doping (2V1S) to a band gap opening of 0.4 eV and impurity flat bands close to the Fermi-level for graphitic doping (1V1S) and an additional n-type doping together with spin-polarization for thiopyranic doping (4V3S). Incorporation in FLG leads to modification of the Dirac cone into a set of hyperbolic touching bands in 2V1S; reduction (bilayer) and closing of the band gap with additional hyperbolic touching bands in conjunction with an impurity flat band at the Fermi level in 1V1S and 4V3S and a reduction of spin polarization in the latter. Overall, S-doping enables design of the band structure and tuning the electronic behavior of FLG from metallic to insulating and from linear dispersion to impurity flat bands that makes S-doped FLG a promising material for versatile technological applications.

Light-matter interaction allows to achieve non-equilibrium states that are otherwise inaccessible. Motivated by recent experiments that report ferroelectricity – and even multiferroicity – in the prototypical diamagnetic band insulator SrTiO_3 induced by terahertz pulses, we investigate the carrier and magnetization dynamics of SrTiO_3 excited optically by linearly and circularly polarized light. Our real-time time-dependent density-functional theory (RT-TDDFT) results reveal a highly non-trivial, site- and orbital-dependent temporal evolution with charge transferred from O 2p to Ti 3d states. For linearly polarized light the orbitally polarized lobes of electron density at the oxygen and titanium sites fluctuate out-of-phase, resembling the soft transverse optical phonon mode, dynamically breaking inversion symmetry. In contrast, circularly polarized pulses induce a coherent rotation of the charge dipoles around O. This induces a helicity-dependent finite transient magnetization with opposite sign for oxygen and Ti even without ionic motion. Detailed analysis reveals that the dominant mechanism is the transfer of angular momentum of light to the electronic orbital angular momentum, while spin-orbit coupling plays a key role in the transfer from orbital to spin angular momentum, the former being an order of magnitude larger than the latter.

The inverse spinel is a promising anode material for electrochemical water splitting. Understanding how the surface orientation and composition affects the oxygen evolution reaction (OER) activity is of paramount importance for tailoring its functionality. To this end, we employ density functional theory (DFT) calculations with a Hubbard term to assess the OER performance at the iron‐ or cobalt‐rich (001) and the mixed (100) surfaces. While the overpotentials at Fe reaction sites are above 0.44 V and octahedral Co at the Co‐rich (001) surface exhibits an overpotential of 0.27 V, the lowest overpotential of 0.20 V is found for octahedral Co at the A‐layer of the mixed Co–Fe (100) surface. Taking into account implicit solvation shifts the most active site to octahedral iron at the B‐layer of the Fe‐rich (001) surface (=0.37 V). Our analysis reveals that a bulk‐like oxidation state during OER correlates with higher OER activity. Moreover, the reduced overpotential originates from the stabilization of the *OOH intermediate through an unusual bidentate bonding geometry to octahedral Co and neighboring Fe, highlighting the cooperative effect. This is in contrast to the monodentate binding typically assumed in the scaling relationship and demonstrates that the involvement of multiple surface sites can significantly improve the OER performance.

Controlling the direct growth of 2D materials onto dielectric substrates is considered as a key requirement for integrating these ultrathin functional materials into existing technology platforms. Here, a combined experimental and theoretical approach is presented to unravel the mechanism of low-temperature graphene growth on sapphire, a dielectric substrate widely used in the semiconductor industry. A clear dependence of the graphene growth rate on the crystal facet is found, with the highest growth rate for a-plane and ca-plane, and the lowest for r-plane sapphire. Density functional theory calculations reveal that the coordination environment of surface oxygen ions governs carbon adsorption energetics: lower coordinated oxygen sites on the a-plane markedly enhance carbon atom binding, driving nucleation and growth, while higher coordinated oxygen sites on the r-plane hinder adsorption and growth. Guided by these insights, it is demonstrated that tailoring substrate termination yields controllable graphene formation at temperatures as low as 670 °C and sheet resistances down to 1.65 kΩ □-1. This approach may establish a universal design principle to guide low-temperature growth of 2D materials on non-catalytic dielectrics.

Spin-polarized two-dimensional electron gases (2DEGs) are of particular interest for functional oxide electronics applications. The redox-created 2DEG residing on the strontium titanate, SrTiO3 (STO), side of a europium monoxide (EuO)/SrTiO3 (001) interface is expected to be significantly spin polarized due to the proximity to the strong (7 mu B/ f .u.) Heisenberg ferromagnet EuO. We apply magnetic circular dichroism in the angular distribution (MCDAD) of photoemitted electrons to investigate whether and how the induced spin polarization of the 2DEG depends on the dimensionality of the overlaying EuO layer. The experimental data are complemented by density functional theory calculations with a Hubbard U term (DFT + U). We show that the EuO/STO interfacial 2DEG is spin polarized even for ultrathin EuO overlayers, starting at an EuO threshold thickness of only two monolayers. Additional EuO monolayers even increase the induced magnetic Ti moment and thus the spin polarization of the 2DEG. Our results and the potential to enhance the magnetic order of EuO by other proximity effects indicate that the EuO/STO(001) interface is an ideal template for creating (multi-)functional spin-polarized 2DEGs for application in oxide electronics.

A central challenge in water electrolysis lies with the oxygen evolution reaction (OER) where the formation of molecular oxygen (O 2 ) is hindered by the constraint of angular momentum conservation. While the reactants OH − or H 2 O are diamagnetic (DM), the O 2 product has a paramagnetic (PM) triplet ground state, requiring a change in spin configuration when being formed. This constraint has prompted interest in spin‐selective catalysts as a means to facilitate OER. In this context, the roles of magnetism and chirality‐induced spin selectivity (CISS) in promoting the OER reaction have recently been investigated through both theoretical and experimental studies. However, pinpointing the key principles and their relative contribution in mediating spin‐enhancement remains a significant challenge. This roadmap offers a forward‐looking perspective on current experimental trends and theoretical developments in spin‐enhanced OER electrocatalysis and outlines strategic directions for integrating incisive experiments and operando approaches with computational modeling to disentangle key mechanisms. By providing a conceptual framework and identifying critical knowledge gaps, this perspective aims to guide researchers toward dedicated experimental and computational studies that will deepen the understanding of spin‐induced OER enhancement and accelerate the development of next‐generation catalysts.

Engineering efficient electron-transfer processes by tailoring the surface electronic state is a rational approach to developing active electrocatalysts. While advanced experimental techniques and theoretical calculations have progressively unveiled the relationship between electrocatalysis and electronic states, with a focus on perovskite-type metal oxides, many aspects of catalytic descriptors remain unclear. Transitioning from traditional perovskite frameworks, this study demonstrates infinite-layer nickelates, characterized by square-planar lattice and half-filled dx²-y² orbital configuration, as a novel model system for understanding the design of highly active electrocatalysts. Specifically, infinite-layer nickelates exhibit significantly higher activity for the oxygen evolution reaction compared to their perovskite counterparts, with an onset potential difference of 310 mV. This enhanced performance is evidenced by their large electric double-layer capacitance, which indicates an electrochemically active surface. Density functional theory calculations with a Hubbard U term (DFT+U) confirm the higher OER activity of the infinite-layer nickelates and relate it to the lower oxidation state of Ni at the infinite-layer surface compared to the perovskite. Thus, it is expected that our findings, highlighting high catalytic activity in square-planar NiO4 with a unique d-orbital configuration, will provide a new design strategy for exploring highly active electrocatalysts.

Cobalt spinel (Co3O4) is a dynamically restructuring catalyst under oxygen evolution reaction (OER) conditions. So far, little is known about the mechanistic complexity of the OER at different active sites of Co3O4 at the atomic level. Using the A- and B-terminations of a single-crystal Co3O4(001) model electrode, we apply a combination of density functional theory calculations and ab initio molecular dynamics simulations to identify three main types of active sites of Co3O4 under OER conditions. In addition to tetrahedral and octahedral surface sites, we report the formation of pseudo-octahedral sites due to a change in the local environment upon adsorption of intermediate species. For all these active sites, we analyze the elementary steps of the OER by descriptor-based analysis and the concept of degree of span control. While octahedral and pseudo-octahedral sites are catalytically more active than tetrahedral sites, we show structural sensitivity with respect to the key limiting reaction step, which ranges from O & horbar;O bond formation to O2 desorption and *OH oxidation. Our modeling strategy, which captures changes in the local environment, elementary steps of the OER, and the contribution of different reaction steps to the current density, provides an integrated and comprehensive framework for describing complex oxide materials under applied bias.

A two-dimensional electron gas (2DEG) forms at the interface of complex oxides like SrTiO3 (STO) and LaTiO3 (LTO), despite each material having a low native conductivity, as a band and a Mott insulator, respectively. The interface 2DEG hosts charge carriers with moderate charge carrier density and mobility that raised interest as a material system for applications like field-effect transistors or detectors. Of particular interest is the integration of these oxide systems in silicon technology. To this end we study the carrier dynamics in a STO/LTO/STO heterostructure epitaxially grown on Si(001) both experimentally and theoretically. Linear THz spectroscopy was performed to analyze the temperature dependent charge carrier density and mobility, which was found to be in the range of 1012 cm-2 and 1000 cm2 V-1 s-1, respectively. Pump-probe measurements revealed a very minor optical nonlinearity caused by hot carriers with a relaxation time of several 10 ps, even at low temperature. Density functional theory calculations with a Hubbard U term on ultrathin STO-capped LTO films on STO/Si(001) show an effective mass of 0.64-0.68 me.

A two-dimensional electron gas (2DEG) forms at the interface of complex oxides like SrTi O 3 (STO) and LaTi O 3 (LTO), despite each material having a low native conductivity, as a band and a Mott insulator, respectively. The interface 2DEG hosts charge carriers with moderate charge carrier density and mobility that raised interest as a material system for applications like field-effect transistors or detectors. Of particular interest is the integration of these oxide systems in silicon technology. To this end we study the carrier dynamics in a STO/LTO/STO heterostructure epitaxially grown on Si(001) both experimentally and theoretically. Linear THz spectroscopy was performed to analyze the temperature dependent charge carrier density and mobility, which was found to be in the range of 10 12 c m − 2 and 1000 c m 2 V − 1 s − 1 , respectively. Pump-probe measurements revealed a very minor optical nonlinearity caused by hot carriers with a relaxation time of several 10 ps, even at low temperature. Density functional theory calculations with a Hubbard U term on ultrathin STO-capped LTO films on STO/Si(001) show an effective mass of 0.64–0.68 m e .

A two-dimensional electron gas (2DEG) forms at the interface of complex oxides like SrTiO_3 (STO) and LaTiO_3 (LTO), despite each material having a low native conductivity, as a band and a Mott insulator, respectively. The interface 2DEG hosts charge carriers with moderate charge carrier density and mobility that raised interest as a material system for applications like field-effect transistors or detectors. Of particular interest is the integration of these oxide systems in silicon technology. To this end we study the carrier dynamics in a STO/LTO/STO heterostructure epitaxially grown on Si(001) both experimentally and theoretically. Linear THz spectroscopy was performed to analyze the temperature dependent charge carrier density and mobility, which was found to be in the range of 10^12 cm^2 and 1000 cm^2V^-1s^-1, respectively. Pump-probe measurements revealed a very minor optical nonlinearity caused by hot carriers with a relaxation time of several 10 ps, even at low temperature. Density functional theory calculations with a Hubbard U term on ultrathin STO-capped LTO films on STO(001) show an effective mass of 0.64-0.68 m_e.

By thermal decomposition of a crystalline hydroxycarbonate precursor with a Co:Fe ratio of 2:1, crystals with alternating ferrimagnetic and antiferromagnetic nano-domains were synthesized using a facile synthetic approach that combined bottom-up co-precipitation of the precursor with a self-assembled top-down nano-structuring during spinel formation. Due to the miscibility gap of the spinel phase diagram at this composition, a topotactic segregation into CoFe2O4-like and Co3O4-like domains takes place at 400 °C, giving rise to porous crystalline nano-flakes with spatial compositional fluctuations on a scale of approximately 5 nm. Experimental methods and density functional theory showed that the metastable nature of this interface-rich material is manifested in the unexpectedly low lattice parameter of the iron-rich domains, which can be explained by the compressive strain executed on this phase due to mosaicity. Investigations of the magnetic properties revealed an exchange bias effect, due to this unique microstructure, which is typically known for thin films or core/shell nanoparticles. Treatment at temperatures higher than 450 °C causes this microstructure to break down, the lattice strain to relax, and finally leads to properties expected for the thermodynamically stable phases according to the phase diagram.

Designing efficient electrocatalysts for clean energy conversion requires atomic-scale insight into the relationship between targeted surface modifications and catalytic performance. Using density functional calculations with an on-site Hubbard U term (DFT+U), we investigate the effect of Mn and V doping on the performance of Co3O4(001) as an anode material for the oxygen evolution reaction (OER). Our results show that Mn, which is preferentially incorporated at a surface octahedral site at both the B- and A-terminated Co3O4(001) surfaces, has a detrimental effect on the OER efficiency. V also prefers a surface octahedral site at the B-layer termination, acquiring a 5+ oxidation state and reducing the overpotential (η) of a Cooct reaction site from 0.48 V to 0.43 V. At the pristine A-termination, the tetrahedral surface Co reaction site has a lower OER activity (0.74 V) than the octahedral Co site (0.55 V). This trend is reversed with V-doping, where V favors incorporation at a subsurface octahedral site, adopting a 4+ oxidation state, resulting in the lowest overpotential of 0.18 V for a Cotet reaction site. Taking into account implicit solvation increases the overpotential for both undoped and Mn-doped surfaces, whereas it marginally decreases the overpotential for the V-doped A-layer termination. The deprotonation of *OH to *O constitutes the potential-determining step for all investigated surface reaction sites, thereby positioning them on the right leg of the volcano plot, while η of the Cotet reaction site at the V-doped A-layer lies beyond the top of the volcano. The enhancement in catalytic activity of the V-doped A-layer is largely attributed to the modification of the surface Cooct oxidation state from Co3+ (IS) to Co2+ due to the subsurface incorporation of V4+. This substantially reduces the binding energy of *OH and thereby the PDS by 0.56 eV. These insights demonstrate how selective doping can be used as a promising strategy to tailor catalyst’s performance.