Layered nickelates host a variety of correlated electronic phenomena that can be tuned through doping, strain, and dimensionality. Here, we explore anion engineering as an alternative tuning knob to modify the properties of layered nickelate thin films. First, we synthesize epitaxial thin films of the n = 1 Ruddlesden-Popper nickelate, La2NiO4. We then achieve transformation to crystalline La2NiO3F2 thin films through redox-neutral, topochemical fluorination. X-ray diffraction and electron microscopy confirm the atomic structure and crystallinity of La2NiO4 and La2NiO3F2. X-ray absorption spectroscopy further confirms a NiO4F2 coordination environment and Ni2+ oxidation state following fluorination, while electronic transport measurements reveal semiconducting behavior across a range of compressive strain states (ϵ = -1.9% to -5.8%). High dynamic range reciprocal space mapping reveals nanoscale periodicity that emerges upon fluorination, and computational analysis indicates that La2NiO3F2 is susceptible to transverse structural distortions. Overall, we illustrate topochemical fluorination and anion engineering as a tuning knob to modify the chemical and electronic properties of complex oxide thin films.

We report the crystal structure and electric transport and/or dielectric properties of the Ba(PO4)2M2O3 compounds, for M = Ta, Nb as well as the mixtures Ta/Nb, W/ Nb and W/Ta. While the existence of these phases has already been reported in parts, their accurate crystal structures remained unclear but suggest the R-3m space group for M = Ta and Nb. Here the crystal symmetry was revised to the chiral R32 space group, as confirmed by SHG. Furthermore, looking at them from the new viewpoint of the only recently discovered tungsten counterpart (certified R-3m symmetry) opens wide perspectives on materials properties of this family of compounds. For the d0 Nb/Ta5+ phases, we confirm that they are large-gap semiconductors (between 3.7 and 4.8 eV) with weak frequency dependence of the permittivity and low dielectric loss. For mixed Nb/Ta semiconductors the compositional dependence of the bandgap (Eg) and the dielectric constant (E'r) have been analyzed using a bowing parameter and a Clausius-Mosotti law, respectively. For M = Ta, the inversely related Eg and E'r values stand out of the global behavior of the solid solution. The analogy with the M = W case is all the more interesting, since Ba(PO4)2W2O3 is a rare example of a W compound in a single 5+ valence. It constitutes the m = 2 member of the recently-discovered [Ba(PO4)2][WmO3-m] layered-monophosphate bronze series (L-MPTB's). In this subclass, the occurrence of genuine 2D robust metallicity confined in thin areas of their crystal structures validate exotic electronic features hold by the (d0/d1) mixed W5/6+ valence imposed by the stoichiometry. Our work suggests the miscibility of electronically inactive d0 Ta/Nb5+ and d1 W5+ species in an extended compositional phase diagram. The possibility to tune and further confine the electron density of chemically modified L-MPTB is discussed towards intriguing opportunities.

Altermagnets are collinear antiferromagnets with non-relativistic spin splitting of the electronic states. We identify the Ruddlesden-Popper chromates Sr_{n+1}Cr_{n}O_{3n+1}, including the perovskite SrCrO_{3}, as candidate materials in which altermagnetism can emerge from spontaneous orbital ordering rather than crystal symmetry. First-principles calculations reveal a layer-dependent spin splitting whose net effect depends on the relative alignment of spin and orbital order: when spin and orbital orders align in adjacent layers, the system exhibits overall spin splitting and thus altermagnetism. When either the spin or orbital order is reversed between adjacent layers, the sign of the spin splitting reverses as well. As a result, the splitting remains finite within each layer but cancels overall. We refer to this compensated state, consisting of two altermagnetic sublattices with opposite spin splitting, as antialtermagnetism. In the Ruddlesden-Popper chromates, odd-n members can host both altermagnetic and antialtermagnetic states, whereas even-n compounds and the perovskite limit are strictly antialtermagnetic.

The observation of superconductivity in undoped infinite-layer nickelates RNiO_2 (R = rare earth) challenges our current understanding and calls for a re-examination of the underlying electronic structure of this family of materials. In this context, it is particularly important to extend the investigation of RNiO_2 compounds from the intensively studied hole-doped regime to the almost unexplored electron-doped one. Here, we use a combination of density-functional theory and dynamical mean-field theory to study the evolution of the electronic structure of infinite-layer nickelates in these two doping regimes. We find a striking asymmetry in the self-doping of the Ni-d_x^2-y^2 band due to the R(5d) states: while this effect is strongly suppressed upon hole doping, electron doping instead leads to an increase in the size of the R(5d) electron pockets, but without effectively hole-doping the Ni-d_x^2-y^2 band. This asymmetry has an important impact on the magnetic response as antiferromagnetism is rapidly suppressed upon hole doping, whereas it remains the ground state upon electron doping. Despite these differences, electronic correlations on both sides of the phase diagram are dominated by the Ni d_x^2-y^2 orbital, suggesting that a single-band description may be appropriate for infinite-layer nickelates in both the electron- and hole-doped regimes.

The subclass collinear antiferromagnets that break spin Kramers degeneracy—thereby exhibiting ferromagnet-like properties—offer exciting opportunities in magnetism, which motivates the expansion of the material base for these so-called altermagnets. Here, we demonstrate that Ruddlesden–Popper and perovskite phases offer a rich material platform for altermagnetic behavior. Using first-principles calculations, we demonstrate altermagnetism in prototypical nickel-based compounds such as La2NiO4 and identify additional candidates, including the superconducting La3Ni3O7 and the multiferroic BiFeO3. These materials span insulating, semiconducting, and metallic conduction types, with computed nonrelativistic spin splittings reaching up to 250 meV. Our analysis further reveals the presence of accidental nodes and distinct spin-momentum texture topologies at the Brillouin-zone boundary, suggesting a refined classification beyond the initial d-wave and higher even-parity wave classes. Additionally, we address formal inconsistencies in the traditional classification of magnetically ordered systems, proposing resolutions within the altermagnetic framework. Finally, we highlight the potential for altermagnetic behavior of ferrimagnets and weak ferromagnets, broadening the scope for future exploration.

Altermagnets are collinear antiferromagnets with spin-split electronic states. We introduce Ruddlesden-Popper chromates Sr_n+1Cr_nO_3n+1 (including SrCrO_3) as candidate materials in which altermagnetism can emerge from spontaneous orbital ordering rather than crystal symmetry. First-principles calculations reveal a layer-dependent spin splitting: if the spin and orbital orders align in adjacent layers, the system exhibits non-relativistic spin splitting, and thus altermagnetism. In contrast, if either the spin or the orbital order is reversed in adjacent layers, we observe a layerwise uncompensated spin splitting, that is compensated in the adjacent layer, giving rise to the concept of anti-altermagnetism. In the RP series, odd n members support coexistence of altermagnetism and anti-altermagnetism, whereas even n and the perovskite limit are strictly anti-altermagnetic. In both cases, larger n favors metallicity, and in odd n compounds strain can further stabilize altermagnetism.

The newly discovered series of layered monophosphate tungsten bronzes (L-MPTB) [Ba(PO4)2]WmO3m-3 consist of m-layer-thick slabs of WO6 octahedra separated by barium-phosphate spacers. They display a 2D metallic behavior confined in the central part of the perovskite slabs. Here, we report the missing m = 2 member of this series, containing the rather uncommon W5+ oxidation state. We have analyzed its structure-property relationships in relation to the other members of the L-MPTB family. In particular, we have determined its crystal structure by means of single-crystal X-ray and electron diffraction and investigated its physical properties from resistivity, Seebeck-coefficient and heat-capacity measurements combined with first-principles calculations. All the L-MPTB compounds show metallic behavior down to 1.8 K without any clear charge-density-wave (CDW) order. The m = 2 member, however, displays an increased influence of the spacer that translates into anisotropic negative thermal expansion, reversed thermopower and reversed crystal-field splitting of the tungsten t2g orbitals. Our analysis of the full [Ba(PO4)2]WmO3m-3 series reveals a systematic and significant W off-centering in their octahedral coordination. We identify the resulting anti-polar character of these W displacements as the crucial aspect behind the 2D metallicity of these systems: It leads to the presence of bound charges whose screening determines the distribution of mobile charges, tending to accumulate at the center of the [WmO3-m] block. We argue that this mechanism is analogous to enhanced conductivity observed for charged domain walls in ferroelectrics, thus providing a general design rule to promote 2D metallicity in layered systems.

The lattice dynamics of the superconducting materials LaFeSiH and LaFeSiO 1 - delta as well as their intermetallic precursor LaFeSi are investigated by polarized Raman spectroscopy and first-principles calculations, together with X-ray and advanced electron diffraction techniques for their structural analysis. We find that the Fe-dominated Raman-active modes reflect the chemical peculiarities of these silicides compared to their pnictide counterparts, with enhanced structural couplings between the FeSi layer and the spacer that can be related to the ionic vs . covalent character of the latter. In addition, we find signatures of enhanced electron-phonon coupling for some of the Raman-active modes. Beyond that, our study reveals intriguing Fe-based Raman features as well as structural subtleties in LaFeSiH suggesting that this superconductor may formally be non-centrosymmetric.

Transition-metal oxides host a wide variety of electronic phenomena that can be significantly influenced by the effective dimensionality of the system under consideration. These include charge, spin, and orbital orderings, as well as unconventional superconductivity. In this context, the Ruddlesden-Popper chromates Srn+1CrnO3n+1 emerge as a particularly intriguing series of materials. Formally, the chromium atom displays a rather special 4+ oxidation state throughout the entire series. However, the effective dimensionality changes from quasi-2D to 3D as n increases from 1 to infinity. As a result, the insulating antiferromagnetic behavior observed for n = 1, 2, 3 transforms into itinerant antiferromagnetism with reduced transition temperature for the n = infinity end member of the series, i.e., the perovskite SrCrO3. Further, distinct orbital orderings with exotic singlet states have been predicted for these systems. However, the lack of single-crystal bulk or thin-film samples has made experimental progress difficult. Here we demonstrate the synthesis of thin films of the perovskite SrCrO3 and the associated layered chromates via oxide molecular beam epitaxy for n = 1 to n = 5. Our electrical transport measurements reveal a gradual evolution from a strongly insulating state in Sr2CrO4 to a metallic state in the end member SrCrO3. X-ray absorption spectroscopy measurements demonstrate a varying hybridization strength of the Cr4+ valence electrons across the series, helping to explain the trend in conduction. Density functional theory calculations further confirm the observed transport trend and identify additional distortions present in the system.

Nickelate superconductors are outstanding materials with intriguing analogies with the cuprates. These analogies suggest that their superconducting mechanism may be unconventional, although this fundamental question is currently under debate. Here, we scrutinize the role played by electronic correlations in enhancing the electron-phonon coupling in the infinite-layer nickelates and the extent to which this may promote superconductivity. Specifically, we use $ab$ $initio$ many-body perturbation theory to perform state-of-the-art $GW$ and Eliashberg-theory calculations. We find that the electron-phonon coupling is effectively enhanced compared to density-functional-theory calculations. This enhancement may lead to low-$T_c$ superconductivity in the parent compounds already. However, it remains marginal in the sense that it cannot explain the record $T_c$s obtained with doping. This circumstance implies that conventional superconductivity is preempted by another pairing mechanism in the infinite-layer nickelates.

The magnetic and structural properties of the recently discovered pnictogen/chalcogen-free superconductor LaFeSiH ($T_c\simeq10$~K) have been investigated by $^{57}$Fe synchrotron M{\"o}ssbauer source (SMS) spectroscopy, x-ray and neutron powder diffraction and $^{29}$Si nuclear magnetic resonance spectroscopy (NMR). No sign of long range magnetic order or local moments has been detected in any of the measurements and LaFeSiH remains tetragonal down to 2 K. The activated temperature dependence of both the NMR Knight shift and the relaxation rate $1/T_1$ is analogous to that observed in strongly overdoped Fe-based superconductors. These results, together with the temperature-independent NMR linewidth, show that LaFeSiH is an homogeneous metal, far from any magnetic or nematic instability, and with similar Fermi surface properties as strongly overdoped iron pnictides. This raises the prospect of enhancing the $T_c$ of LaFeSiH by reducing its carrier concentration through appropriate chemical substitutions. Additional SMS spectroscopy measurements under hydrostatic pressure up to 18.8~GPa found no measurable hyperfine field.

Superconductivity in infinite layer nickelates Nd_{1-x}Sr_{x}NiO_{2} has so far been achieved only in thin films, raising questions on the role of substrates and interfaces. Given the challenges associated with their synthesis it is imperative to identify their intrinsic properties. We use resonant inelastic x-ray scattering to investigate the influence of the SrTiO_{3} capping layer on the excitations of Nd_{1-x}Sr_{x}NiO_{2} (x=0 and 0.2). Spin excitations are observed in parent and 20% doped Nd_{1-x}Sr_{x}NiO_{2} regardless of capping, proving that magnetism is intrinsic to infinite-layer nickelates and appears in a significant fraction of their phase diagram. In parent and superconducting Nd_{1-x}Sr_{x}NiO_{2}, the spin excitations are slightly hardened in capped samples compared to the noncapped ones. Additionally, a weaker Ni-Nd charge transfer peak at ∼0.6  eV suggests that the hybridization between Ni 3d and Nd 5d orbitals is reduced in capped samples. From our data, capping induces only minimal differences in Nd_{1-x}Sr_{x}NiO_{2} and we phenomenologically discuss these differences based on the reconstruction of the SrTiO_{3}-NdNiO_{2} interface and other mechanisms such as crystalline disorder.

Superconductivity in infinite layer nickelates Nd1-xSrxNiO2 has so far been achieved only in thin films, raising questions on the role of substrates and interfaces. Given the challenges associated with their synthesis it is imperative to identify their intrinsic properties. We use resonant inelastic x-ray scattering to investigate the influence of the SrTiO3 capping layer on the excitations of Nd1-xSrxNiO2 (x = 0 and 0.2). Spin excitations are observed in parent and 20% doped Nd1-xSrxNiO2 regardless of capping, proving that magnetism is intrinsic to infinite-layer nickelates and appears in a significant fraction of their phase diagram. In parent and superconducting Nd1-xSrxNiO2, the spin excitations are slightly hardened in capped samples compared to the noncapped ones. Additionally, a weaker Ni-Nd charge transfer peak at similar to 0.6 eV suggests that the hybridization between Ni 3d and Nd 5d orbitals is reduced in capped samples. From our data, capping induces only minimal differences in Nd1-xSrxNiO2 and we phenomenologically discuss these differences based on the reconstruction of the SrTiO3-NdNiO2 interface and other mechanisms such as crystalline disorder.

When a system is driven across a second-order phase transition, defects can form because it cannot respond quickly enough to the new conditions. The Kibble–Zurek mechanism explains this physics, and has now been invoked for Ising-type domains.

Phosphate tungsten and molybenum bronzes represent an outstanding class of materials displaying textbook examples of charge-density-wave (CDW) physics among other fundamental properties. Here we report on the existence of a novel structural branch with the general formula [Ba(PO4)(2)][WmO3m-3] (m=3, 4 and 5) denominated ' layered monophosphate tungsten bronzes ' (L-MPTB). It results from thick [Ba(PO4)(2)](4-) spacer layers disrupting the cationic metal-oxide 2D units and enforcing an overall trigonal structure. Their symmetries are preserved down to 1.8 K and the compounds show metallic behaviour with no clear anomaly as a function of temperature. However, their electronic structure displays the characteristic Fermi surface of previous bronzes derived from 5d W states with hidden nesting properties. By analogy with previous bronzes, such a Fermi surface should result into CDW order. Evidence of CDW order was only indirectly observed in the low-temperature specific heat, giving an exotic context at the crossover between stable 2D metals and CDW order.

Ferroelectric domain walls are quasi-2D systems that show great promise for the development of nonvolatile memory, memristor technology, and electronic components with ultrasmall feature size. Electric fields, for example, can change the domain wall orientation relative to the spontaneous polarization and switch between resistive and conductive states, controlling the electrical current. Being embedded in a 3D material, however, the domain walls are not perfectly flat and can form networks, which leads to complex physical structures. In this work, the importance of the nanoscale structure for the emergent transport properties is demonstrated, studying electronic conduction in the 3D network of neutral and charged domain walls in ErMnO3 . By combining tomographic microscopy techniques and finite element modeling, the contribution of domain walls within the bulk is clarified and the significance of curvature effects for the local conduction is shown down to the nanoscale. The findings provide insights into the propagation of electrical currents in domain wall networks, reveal additional degrees of freedom for their control, and provide quantitative guidelines for the design of domain-wall-based technology.

Intermetallics represent an important family of compounds, in which insertion of light elements (H, B, C, N) has been widely explored for decades to synthesize novel phases and promote functional materials such as permanent magnets or magnetocalorics. Fluorine insertion, however, has remained elusive so far since the strong reactivity of this atypical element, the most electronegative one, tends to produce the chemical decomposition of these systems. Here, we introduce a topochemical method to intercalate fluorine atoms into intermetallics, using perfluorocarbon reactant with covalent C-F bonds. We demonstrate the potential of this approach with the synthesis of non-stoichiometric mixed anion (Si-F) LaFeSiFx single-crystals, which are further shown to host FeSi-based superconductivity. Fluorine topochemistry on intermetallics is thus proven to be an effective route to provide functional materials where the coexistence of ionic and metallo-covalent blocks, and their interactions through inductive effects, is at the root of their functional properties.

Pnictogens and chalcogens are both viable anions for promoting Fe-based superconductivity, and intense research activity in the related families has established a systematic correlation between the Fe-anion height and the superconducting critical temperature T c , with an optimum Fe-anion height of ~1.38 Å. Here, we report the discovery of superconductivity in the compound LaFeSiO 1− δ that incorporates a crystallogen element, Si, and challenges the above picture: considering the strongly squeezed Fe–Si height of 0.94 Å, the superconducting transition at T c = 10 K is unusually high. In the normal state, the resistivity displays non-Fermi-liquid behavior while NMR experiments evidence weak antiferromagnetic fluctuations. According to first-principles calculations, the Fermi surface of this material is dominated by hole pockets without nesting properties, which explains the strongly suppressed tendency toward magnetic order and suggests that the emergence of superconductivity materializes in a distinct set-up, as compared to the standard s ± - and d -wave electron-pocket-based situations. These properties and its simple-to-implement synthesis make LaFeSiO 1− δ a particularly promising platform to study the interplay between structure, electron correlations, and superconductivity.