Geometrically frustrated lattices can display a range of correlated phenomena, ranging from spin frustration and charge order to dispersionless flat bands due to quantum interference. One particularly compelling family of such materials is the half-valence spinel LiB_2O_4 materials. On the B-site frustrated pyrochlore sublattice, the interplay of correlated metallic behavior and charge frustration leads to a superconducting state in LiTi_2O_4 and heavy fermion behavior in LiV_2O_4. To date, however, LiTi_2O_4 has primarily been understood as a conventional BCS superconductor despite a lattice structure that could host more exotic groundstates. Here, we present a multimodal investigation of LiTi_2O_4, combining ARPES, RIXS, proximate magnetic probes, and ab-initio many-body theoretical calculations. Our data reveals a novel mobile polaronic ground state with spectroscopic signatures that underlie co-dominant electron-phonon coupling and electron-electron correlations also found in the lightly doped cuprates. The cooperation between the two interaction scales distinguishes LiTi_2O_4 from other superconducting titanates, suggesting an unconventional origin to superconductivity in LiTi_2O_4. Our work deepens our understanding of the rare interplay of electron-electron correlations and electron-phonon coupling in unconventional superconducting systems. In particular, our work identifies the geometrically frustrated, mixed-valence spinel family as an under-explored platform for discovering unconventional, correlated ground states.

Electrical control of magnetic order remains one of the fundamental pursuits in condensed matter physics and spintronics, offering transformative potential for energy-efficient, high-density information technologies. While current-induced switching via spin-transfer and spin-orbit torques is well established in ferromagnets, electrically driving a transition between distinct magnetic phases, specifically from a ferromagnetic to an antiferromagnetic state, remains largely unexplored experimentally. Here, we demonstrate a reversible, electrically-driven inverse metamagnetic transition in an epitaxial thin film of the correlated manganite Sm1-xSrxMnO₃. Above a critical current threshold, the system abruptly switches from a low-resistance ferromagnetic state to a high-resistance antiferromagnetic-like phase. We exploit this phenomenon in nanoscale (250 × 250 nm²) spin-filter tunnel junctions based on LaNiO₃/Sm₀.₇₅Sr₀.₂₅MnO₃/ SrTiO₃/La₀.₇Sr₀.₃MnO₃ heterostructures, realizing robust, bistable resistance switching with unconventional magnetoresistance exceeding 200 %, tunable by current, temperature, and magnetic field. These findings open a phase-transition-based route for electrically driven spintronic devices beyond conventional torque-based strategies.

Abstract Electrical control of magnetic order remains one of the fundamental pursuits in condensed matter physics and spintronics, offering transformative potential for energy-efficient, high-density information technologies. While current-induced switching via spin-transfer and spin-orbit torques is well established in ferromagnets, electrically driving a transition between distinct magnetic phases, specifically from a ferromagnetic to an antiferromagnetic state, remains largely unexplored experimentally. Here, we demonstrate a reversible, electrically-driven inverse metamagnetic transition in an epitaxial thin film of the correlated manganite Sm 1-x Sr x MnO₃. Above a critical current threshold, the system abruptly switches from a low-resistance ferromagnetic state to a high-resistance antiferromagnetic-like phase. We exploit this phenomenon in nanoscale (250 × 250 nm²) spin-filter tunnel junctions based on LaNiO₃/Sm₀.₇₅Sr₀.₂₅MnO₃/ SrTiO₃/La₀.₇Sr₀.₃MnO₃ heterostructures, realizing robust, bistable resistance switching with unconventional magnetoresistance exceeding 200 %, tunable by current, temperature, and magnetic field. These findings open a phase-transition-based route for electrically driven spintronic devices beyond conventional torque-based strategies.

Layered nickelates have been studied extensively over the last three decades due to their structural similarities to the high-${T}_{c}$ superconducting cuprates. Using reactive oxide molecular beam epitaxy (MBE), we synthesize ${\mathrm{Nd}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{NiO}}_{4}$ thin films for $x=0\ensuremath{-}1.4$ to probe the properties and electronic structure as a function of hole doping. The samples with lower doping show semiconducting behavior across the temperatures probed with an onset of metallic conductivity at $x=1.4$. We also present polarization-dependent O $K$ and Ni ${L}_{2,3}$ x-ray absorption spectra to track the evolution of the oxygen-nickel hybridization, distribution of holes between O $2p$ and Ni $3d$ states and the nickel oxidation state across the series. Angle-resolved photoemission spectroscopy (ARPES) measurements reveal a Fermi surface that comprises a cupratelike hole pocket of ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ character with an additional electron pocket of ${d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ character at $\mathrm{\ensuremath{\Gamma}}$. The emergence of a quasiparticle peak at the Fermi vector for $x=1.4$ corroborates the insulator-to-metal transition at $x\ensuremath{\sim}1$. Finally, observe a fully two-dimensional Fermi surface with no momentum-dependent pseudogap, in contrast to measurements of the related bulk compound, ${\mathrm{Eu}}_{0.9}{\mathrm{Sr}}_{1.1}{\mathrm{NiO}}_{4}$.

Magnetic interactions are thought to play a key role in the properties of many unconventional superconductors, including cuprates, iron pnictides, and square-planar nickelates. Superconductivity was also recently observed in the bilayer and trilayer Ruddlesden-Popper nickelates, the electronic structure of which is expected to differ from that of cuprates and square-planar nickelates. Here we study how electronic structure and magnetic interactions evolve with the number of layers, n, in thin film Ruddlesden-Popper nickelates Ndn+1NinO3n+1 with n = 1, 3, and 5 using resonant inelastic x-ray scattering (RIXS). The RIXS spectra are consistent with a high-spin |3d8L) electronic configuration, resembling that of La2-xSrxNiO4 and the parent perovskite, NdNiO3. The magnetic excitations soften to lower energy in the structurally self-doped, higher-n films. Our observations confirm that structural tuning is an effective route for altering electronic properties, such as magnetic superexchange, in this prominent family of materials.

Conventional racetrack memories move information by pushing magnetic domain walls or other spin textures with spin-polarized currents, but the accompanying Joule heating inflates their energy budget and can hamper scaling. Here we present a voltage-controlled, magnetoelectric racetrack in which transverse electric fields translate coupled ferroelectric-antiferromagnetic walls along BiFeO3 nanostrips at room temperature. Because no charge traverses the track, the switching dissipates orders of magnitude less energy than the most efficient spin-torque devices with more favourable scaling, making the scheme significantly more attractive at the nanoscale. We further uncover noncollinear topological magnetoelectric textures that emerge at domain walls in BiFeO3, where the nature of these topologies influences their stability upon translation. Among these are polar bi-merons and polar vertices magnetoelectrically coupled with magnetic cycloid disclinations and previously unobserved, topological magnetic cycloid twist topologies. We observe domain wall velocities of at least kilometres per second - matching or surpassing the fastest ferrimagnetic and antiferromagnetic racetracks and approaching the acoustic-phonon limit of BiFeO3 - while preserving these topologies over tens of micrometres. The resulting high velocity, low-energy racetrack delivers nanosecond access times without the thermal overhead of current-driven schemes, charting a path toward dense, ultralow-power racetrack devices which rely on spin texture translation.

The discovery of high-temperature superconductivity in bulk La_3Ni_2O_7 under high hydrostatic pressure and, more recently, biaxial compression in epitaxial thin films has ignited significant interest in understanding the interplay between atomic and electronic structure in these compounds. Subtle changes in the nickel-oxygen bonding environment are thought to be key drivers for stabilizing superconductivity, but specific details of which bonds and which modifications are most relevant remains so far unresolved. While direct, atomic-scale structural characterization under hydrostatic pressure is beyond current experimental capabilities, static stabilization of strained La_3Ni_2O_7 films provides a platform well-suited to investigation with new picometer-resolution electron microscopy methods. Here, we use multislice electron ptychography to directly measure the atomic-scale structural evolution of La_3Ni_2O_7 thin films across a wide range of biaxial strains tuned via substrate. By resolving both the cation and oxygen sublattices, we study strain-dependent evolution of atomic bonds, providing the opportunity to isolate and disentangle the effects of specific structural motifs for stabilizing superconductivity. We identify the lifting of crystalline symmetry through modification of the nickel-oxygen octahedral distortions under compressive strain as a key structural ingredient for superconductivity. Rather than previously supposed c-axis compression, our results highlight the importance of in-plane biaxial compression in superconducting thin films, which suggests an alternative – possibly cuprate-like – understanding of the electronic structure. Identifying local regions of inhomogeneous oxygen stoichiometry and high internal strain near crystalline defects, we suggest potential pathways for improving the sharpness and temperature of the superconducting transition.

We examine the bulk electronic structure of Nd3Ni2O7 using Ni 2p core-level hard x-ray photoemission spectroscopy combined with density functional theory + dynamical mean-field theory. Our results reveal a large deviation of the Ni 3d occupation from the formal Ni2.5+ valency, highlighting the importance of the charge-transfer from oxygen ligands. We find that the dominant d8 configuration is accompanied by nearly equal contributions from d7 and d9 states, exhibiting an unusual valence state among Ni-based oxides. Finally, we discuss the Ni dx2-y2 and dz2 orbital-dependent hybridization, correlation and local spin dynamics.

Layered perovskites such as the Dion-Jacobson, Ruddlesden-Popper, and Aurivillius families host a wide range of correlated electron phenomena, from high-temperature superconductivity to multiferroicity. Here we report a new family of layered perovskites, realized through topotactic oxygen intercalation of La_n+1Ni_nO_3n+1 (n=1-4) Ruddlesden-Popper nickelate thin films grown by ozone-assisted molecular-beam epitaxy. Post-growth ozone annealing induces a large c-axis expansion - 17.8 decreases with increasing n. Surface X-ray diffraction coupled with Coherent Bragg Rod Analysis reveals that this structural expansion arises from the intercalation of approximately 0.7 oxygen atoms per formula unit into interstitial sites within the rock salt spacer layers. The resulting structures exhibit a spacer layer composition intermediate between that of the Ruddlesden-Popper and Aurivillius phases, defining a new class of layered perovskites. Oxygen-intercalated nickelates exhibit metallicity and significantly enhanced nickel-oxygen hybridization, a feature linked to high-temperature superconductivity. Our work establishes topotactic oxidation as a powerful synthetic approach to accessing highly oxidized, metastable phases across a broad range of layered oxide systems, offering new platforms to tune properties via spacer-layer chemistry.

Indium tin oxide (Sn-doped In2O3; ITO) is a well-studied transparent conductor where doping can be used to stabilize a superconducting state. In this work, we use a combination of thin film deposition of ITO and solution-phase chemistry using n-BuLi (C4H9Li) to realize superconductivity in Li-doped ITO. Solution-phase intercalation is commonly employed for bulk, polycrystalline materials but has rarely been applied to thin films. Using x-ray diffraction, atomic force microscopy, electronic transport, and optical transmission measurements, we characterize the optical transparency and superconductivity of lithium intercalated ITO thin films. After 72 hours of lithium intercalation, we find a critical temperature, T c , of 0.49 K and an optical transparency of at least 73% in the visible optical range-all while maintaining crystallinity and nanometer surface roughness.

Among the pyrochlore oxides, the frustrated magnet Tb2Ti2O7 has drawn intense interest as a spin liquid candidate. Its unusual magnetic properties rely on a careful balance of interactions on the frustrated pyrochlore lattice and are exquisitely sensitive to composition. Leveraging the precise stoichiometry control of reactive-oxide molecular beam epitaxy, we manipulate the defect chemistry of Tb2Ti2O7 thin films on (111)-oriented YSZ substrates and probe their structural and magnetic properties. Single-phase pyrochlore thin films are realized within a large growth window at up to 25% off-composition. Using both scanning transmission electron microscopy and electron energy loss spectroscopy, we demonstrate the preferential formation of distinct defects with varying film stoichiometry. Titanium-rich films incorporate titanium excess via antistuffing of both Ti3+ and Ti4+ locally along slanted < 112 > antiphase boundaries. In contrast, excess terbium is directly incorporated into the film through homogeneous stuffing and terbium-rich films exhibit mostly < 111 > antiphase boundaries. DC magnetic susceptibility measurements suggest that defects reduce the frustration index of Tb2Ti2O7, with titanium and terbium excess both leading to an enhanced saturated magnetic moment and less negative Curie-Weiss temperature compared to the stoichiometric film, while all films remain paramagnetic down to 1.8 K.

The layered square-planar nickelates, Nd_n+1Ni_nO_2n+2, are an appealing system to tune the electronic properties of square-planar nickelates via dimensionality; indeed, superconductivity was recently observed in Nd_6Ni_5O_12 thin films. Here, we investigate the role of epitaxial strain in the competing requirements for the synthesis of the n=3 Ruddlesden-Popper compound, Nd_4Ni_3O_10, and subsequent reduction to the square-planar phase, Nd_4Ni_3O_8. We synthesize our highest quality Nd_4Ni_3O_10 films under compressive strain on LaAlO_3 (001), while Nd_4Ni_3O_10 on NdGaO_3 (110) exhibits tensile strain-induced rock salt faults but retains bulk-like transport properties. A high density of extended defects forms in Nd_4Ni_3O_10 on SrTiO_3 (001). Films reduced on LaAlO_3 become insulating and form compressive strain-induced c-axis canting defects, while Nd_4Ni_3O_8 films on NdGaO_3 are metallic. This work provides a pathway to the synthesis of Nd_n+1Ni_nO_2n+2 thin films and sets limits on the ability to strain engineer these compounds via epitaxy.

Fluoride-ion batteries have several potential advantages over lithium-ion batteries. Materials development is still needed, however, to realize electrolytes with sufficiently high anion conductivity and compatibility with anode and cathode layers. Fluoride compounds are difficult to synthesize directly as single crystals but can be realized from oxide film precursors via topotactic chemistry techniques. Here, we create crystalline alkaline earth bismuth fluoride films BaBiF5 and SrBiF5 through oxide molecular beam epitaxy and topotactic fluorination. We characterize their ionic conductivities and demonstrate their potential as electrolytes. Finally, we realize epitaxial synthesis of BaBiF5 on BaF2 substrates, providing a route to thin film fluoride-ion battery devices.

Superconducting nickelates are a new family of strongly correlated electron materials with a phase diagram closely resembling that of superconducting cuprates. While analogy with the cuprates is natural, very little is known about the metallic state of the nickelates, making these comparisons difficult. We probe the electronic dispersion of thin-film superconducting five-layer (n = 5) and metallic three-layer (n = 3) nickelates by measuring the Seebeck coefficient S. We find a temperature-independent and negative S/T for both n = 5 and n = 3 nickelates. These results are in stark contrast to the strongly temperaturedependent S/T measured at similar electron filling in the cuprate La1.36Nd0.4Sr0.24CuO4. The electronic structure calculated from density-functional theory can reproduce the temperature dependence, sign, and amplitude of S/T in the nickelates using Boltzmann transport theory. This demonstrates that the electronic structure obtained from first-principles calculations provides a reliable description of the fermiology of superconducting nickelates and suggests that, despite indications of strong electronic correlations, there are well-defined quasiparticles in the metallic state. Finally, we explain the differences in the Seebeck coefficient between nickelates and cuprates as originating in strong dissimilarities in impurity concentrations. Our study demonstrates that the high elastic scattering limit of the Seebeck coefficient reflects only the underlying band structure of a metal, analogous to the high magnetic field limit of the Hall coefficient. This opens a new avenue for Seebeck measurements to probe the electronic band structures of relatively disordered quantum materials.

Here, we report the development of a hydrogen-free, topotactic oxygen deintercalation technique using alkali metal aluminum gallium amalgams (A(x)AlGa, where A = Li, Na, K). These amalgams provide a uniquely tunable system where the choice of alkali metal, its concentration, and the Al:Ga composition alter its reductive properties. We demonstrate the utility of this method in topotactically removing oxygen from bulk and thin film specimens of LnNiO(3) (Ln = La, Nd) to form the infinite layer of nickelate LnNiO(2) (Ln = La, Nd). For example, Na0.25AlGa affords bulk LaNiO2 from LaNiO3 at 300 degrees C for 120 h, while the same amalgam at 265 degrees C for 48 h affords the intermediate La2Ni2O5 (LaNiO2.5). Other alterations in time and temperature as well as the choice of alkali metal (A) and its concentration (x) in A(x)AlGa allow further exploration of the topotactic reduction. Compared to standard techniques based on hydrogen gas or hydrides such as LiH, NaH, and CaH2, these amalgams offer an elegant tunability of the reduction potential, enabling control over the rate and degree of oxygen removal without the risk of hydrogen intercalation.