Julia A. Mundy

The discovery of superconductivity in square-planar nickelates has offered a rich materials platform to explore the origins of high-temperature superconductivity. However, experimental investigations have largely been limited to the infinite-layer R NiO 2 ( R , rare earth) nickelates. We constructed a phase diagram of multilayer square-planar Nd n +1 Ni n O 2 n +2 compounds and found signatures of superconductivity for dimensionality n = 4 to 8. Upon decreasing n , the superconducting anisotropy evolves owing to 4 f electron effects, and electronic structure characteristics approach cuprate-like behavior. Magnetic fluctuations persist from within the superconducting regime and into the overdoped, nonsuperconducting regime. The superconducting regime overlaps with that of chemically doped infinite-layer nickelates, demonstrating underlying commonalities as well as differences across varying structural realizations of square-planar nickelates. Our work establishes this layered template for creating new nickel-based superconductors.

A new family of high-temperature superconductors was recently discovered in the $n=2,3$ Ruddlesden-Popper nickelates, where superconductivity emerges concomitant with suppression of parent density waves and structural octahedral rotations under hydrostatic pressure. Intriguingly, compressive strain mimics the structural effects of pressure in the $n=2$ phase, yielding ambient-pressure superconductivity. However, analogous strain-stabilized superconductivity has not been realized in the $n=3$. Here, we use atomically-precise synthesis, transport, picoscale electron microscopy, and synchrotron X-ray diffraction to probe $n=3$ La$_4$Ni$_3$O$_{10}$ thin films. Although compressive strain suppresses density wave order, we do not observe superconductivity even under the largest strain state. Importantly, we identify a structural distortion unique to strained $n=3$ thin films that may inhibit superconductivity: persistent, layer-inequivalent octahedral rotations around the $c$-axis. Our results highlight key differences between the $n=3$ and $n=2$ systems, suggesting that ambient-pressure superconductivity in the $n=3$ may require new methods beyond epitaxial strain engineering.

A new family of high-temperature superconductors was recently discovered in the $n=2,3$ Ruddlesden-Popper nickelates, where superconductivity emerges concomitant with suppression of parent density waves and structural octahedral rotations under hydrostatic pressure. Intriguingly, compressive strain mimics the structural effects of pressure in the $n=2$ phase, yielding ambient-pressure superconductivity. However, analogous strain-stabilized superconductivity has not been realized in the $n=3$. Here, we use atomically-precise synthesis, transport, picoscale electron microscopy, and synchrotron X-ray diffraction to probe $n=3$ La$_4$Ni$_3$O$_{10}$ thin films. Although compressive strain suppresses density wave order, we do not observe superconductivity even under the largest strain state. Importantly, we identify a structural distortion unique to strained $n=3$ thin films that may inhibit superconductivity: persistent, layer-inequivalent octahedral rotations around the $c$-axis. Our results highlight key differences between the $n=3$ and $n=2$ systems, suggesting that ambient-pressure superconductivity in the $n=3$ may require new methods beyond epitaxial strain engineering.

We present a comparative Ni core level photoemission study of La$_3$Ni$_2$O$_7$, Nd$_3$Ni$_2$O$_7$, and LaNiO$_3$ using both the Ni $2p$ and the Ni $1s$. We address the challenges in analyzing the widely investigated Ni $2p$ spectra arising from the substantial overlap in energy of the Ni $2p$ with the La $3d$. We show that on the other hand the deep Ni $1s$ core level does provide a clean view on the intrinsic electronic excitations and we highlight its potential to resolve detailed differences in the electronic structure within the strongly correlated Ruddlesden-Popper series La$_{n+1}$Ni$_n$O$_{3n+1}$.

We present a comparative Ni core level photoemission study of La$_3$Ni$_2$O$_7$, Nd$_3$Ni$_2$O$_7$, and LaNiO$_3$ using both the Ni $2p$ and the Ni $1s$. We address the challenges in analyzing the widely investigated Ni $2p$ spectra arising from the substantial overlap in energy of the Ni $2p$ with the La $3d$. We show that on the other hand the deep Ni $1s$ core level does provide a clean view on the intrinsic electronic excitations and we highlight its potential to resolve detailed differences in the electronic structure within the strongly correlated Ruddlesden-Popper series La$_{n+1}$Ni$_n$O$_{3n+1}$.

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, La 2 NiO 4 . We then achieve transformation to crystalline La 2 NiO 3 F 2 thin films through redox-neutral, topochemical fluorination. X-ray diffraction and electron microscopy confirm the atomic structure and crystallinity of La 2 NiO 4 and La 2 NiO 3 F 2 . X-ray absorption spectroscopy further confirms a NiO 4 F 2 coordination environment and Ni 2+ 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 La 2 NiO 3 F 2 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.

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 <i>n</i> = 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.

Abstract The discovery of high-temperature superconductivity in bulk La 3 Ni 2 O 7 under high hydrostatic pressure 1–4 and biaxial compression in epitaxial thin films 5–8 has generated substantial 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 remain unresolved so far. Although direct, atomic-scale structural characterization under hydrostatic pressure is beyond present experimental capabilities, static stabilization of strained La 3 Ni 2 O 7 films provides a platform well suited to investigation with new picometre-resolution electron microscopy methods. Here we use multislice electron ptychography (MEP) 9,10 to directly measure the atomic-scale structural evolution of La 3 Ni 2 O 7 thin films across a wide range of biaxial strains tuned by substrate choice. By resolving both the cation and oxygen sublattices, we study the 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 and identify in-plane lattice compression as a common attribute between bulk and thin-film superconductivity. Building on the detailed structures obtained by MEP, we introduce a theoretical framework to disentangle coupled structural distortions in corner-sharing octahedra 11 , which suggest that both known superconducting geometries of La 3 Ni 2 O 7 (hydrostatic pressure and compressive strain) suppress local t 2 g orbital mixing in the low-energy Ni bands by raising the octahedral symmetry.

Superconducting nickelate films are typically fabricated via post-processing of a parent perovskite or Ruddlesden-Popper film, most commonly a high-temperature anneal in the presence of a strong reducing agent such as CaH2, which removes oxygen from the apical sites and facilitates a topotactic transformation to the superconducting phase. Achieving uniform and highly crystalline reduced films has posed a longstanding fabrication challenge. Using neutron reflectometry and SIMS, the authors reveal the interplay between reduction conditions, vertical uniformity, defect distribution, and amorphization of the film. They find evidence for decreased amorphization near the film/substrate interface and competition between crystal quality and vertical uniformity.

for thermal control and memory applications.

Minimizing the energy of a many body system tends to favor order, but classical frustration and quantum fluctuations destabilize that order. The tension between these effects can produce exotic quantum states of matter. Quantum spin liquid (QSL) states emerge in models of localized magnetic moments where the crystal lattice connectivity frustrates ordering, and the exchange interaction of neighboring spins strengthens quantum fluctuations. Experimentally identifying a QSL in a real material is challenging from the lack of an order parameter. Piecing together evidence from varied techniques is necessary for diagnosing the nature of the ground state -- QSL or otherwise -- of a frustrated spin system. In this work, we use coplanar superconducting resonators to probe magnetic excitations in epitaxially grown thin films of a spin liquid candidate TbInO3. Adapting microwave techniques from the field of circuit quantum electrodynamics, we measure responses of these thin films whose volume is too low for applying conventional bulk techniques. In-plane susceptibility extracted from the spin resonance signal indicates extreme frustration of magnetic order down to 20 mK, over two orders of magnitude lower than the Curie-Weiss energy scale. Through a crystal field analysis, we identify the doublet eigenstates comprising the ground state. As a consequence of improper ferroelectricity, Tb moments split into two flavors with distinct g-factors reflecting the local crystal field environment of each site. Spin-orbit coupling, crystal fields, magnetic frustration and improper ferroelectricity distinctively combine to shape the magnetic ground state of TbInO3. This work establishes a measurement technique using superconducting resonators to probe thin films of frustrated magnets, and applies this technique towards building a coherent understanding of the magnetic properties of TbInO3.

Minimizing the energy of a many body system tends to favor order, but classical frustration and quantum fluctuations destabilize that order. The tension between these effects can produce exotic quantum states of matter. Quantum spin liquid (QSL) states emerge in models of localized magnetic moments where the crystal lattice connectivity frustrates ordering, and the exchange interaction of neighboring spins strengthens quantum fluctuations. Experimentally identifying a QSL in a real material is challenging from the lack of an order parameter. Piecing together evidence from varied techniques is necessary for diagnosing the nature of the ground state -- QSL or otherwise -- of a frustrated spin system. In this work, we use coplanar superconducting resonators to probe magnetic excitations in epitaxially grown thin films of a spin liquid candidate TbInO3. Adapting microwave techniques from the field of circuit quantum electrodynamics, we measure responses of these thin films whose volume is too low for applying conventional bulk techniques. In-plane susceptibility extracted from the spin resonance signal indicates extreme frustration of magnetic order down to 20 mK, over two orders of magnitude lower than the Curie-Weiss energy scale. Through a crystal field analysis, we identify the doublet eigenstates comprising the ground state. As a consequence of improper ferroelectricity, Tb moments split into two flavors with distinct g-factors reflecting the local crystal field environment of each site. Spin-orbit coupling, crystal fields, magnetic frustration and improper ferroelectricity distinctively combine to shape the magnetic ground state of TbInO3. This work establishes a measurement technique using superconducting resonators to probe thin films of frustrated magnets, and applies this technique towards building a coherent understanding of the magnetic properties of TbInO3.

The pyrochlore vanadates are compelling candidates for next-generation dissipationless devices. Lu2V2O7 and Y2V2O7 are ferromagnetic insulators (Tc ~ 70 K) that are believed to exhibit the magnon Hall effect and are expected to host topological magnons. Their completely dissipationless magnon edge states could be harnessed to realize low-power information transport in spintronic or magnonic devices. As a crucial step in the realization of devices, we synthesize the first thin films of pyrochlore Y2V2O7 on isostructural Y2Ti2O7 substrates and explore the evolution of their magnetic properties down to the ultrathin limit. All films are insulating ferromagnets with transition temperatures of up to the bulk value (Tc ~ 68 K) that decrease with thickness according to finite-size effects. Our films also exhibit a change in anisotropy from in-plane to out-of-plane easy axis coincident with the development of partial strain relaxation and nonzero magnetic hysteresis in an applied field. This evolution demonstrates the impact of strain on magnetic anisotropy and paves the way to tunable magnon topology.

The pyrochlore vanadates are compelling candidates for next-generation dissipationless devices. Lu2V2O7 and Y2V2O7 are ferromagnetic insulators (Tc ~ 70 K) that are believed to exhibit the magnon Hall effect and are expected to host topological magnons. Their completely dissipationless magnon edge states could be harnessed to realize low-power information transport in spintronic or magnonic devices. As a crucial step in the realization of devices, we synthesize the first thin films of pyrochlore Y2V2O7 on isostructural Y2Ti2O7 substrates and explore the evolution of their magnetic properties down to the ultrathin limit. All films are insulating ferromagnets with transition temperatures of up to the bulk value (Tc ~ 68 K) that decrease with thickness according to finite-size effects. Our films also exhibit a change in anisotropy from in-plane to out-of-plane easy axis coincident with the development of partial strain relaxation and nonzero magnetic hysteresis in an applied field. This evolution demonstrates the impact of strain on magnetic anisotropy and paves the way to tunable magnon topology.

The discovery of superconductivity in square-planar nickelates has offered a rich materials platform to explore the origins of cuprate-like superconductivity. Experimental investigations however have largely been limited to the infinite-layer $R$NiO$_2$ ($R$=rare-earth) nickelates. Here, we construct a phase diagram of multi-layer square-planar Nd$_{n+1}$Ni$_n$O$_{2n+2}$ compounds and discover signatures of superconductivity for $n$ = 4 - 8. Upon decreasing the dimensionality $n$, the superconducting anisotropy evolves due to 4$f$ electron effects, and electronic structure characteristics approach cuprate-like behavior. Magnetic fluctuations persist from within the superconducting regime and into the over-doped, non-superconducting regime. Remarkably, the superconducting regime overlaps with that of chemically-doped infinite-layer nickelates, demonstrating underlying commonalities and distinct differences across varying structural realizations of square-planar nickelates. Our work establishes this layered template for creating new nickel-based superconductors.

Layered perovskites─including the Dion–Jacobson, Ruddlesden–Popper, and Aurivillius families─exhibit a wide range of correlated electron phenomena, from high-temperature superconductivity to multiferroicity. Here, we report a new family of layered perovskites realized through topochemical oxidation of La n +1 Ni n O 3 n +1+δ ( n = 1–4) Ruddlesden–Popper nickelate thin films. Postgrowth ozone annealing induces a substantial c -axis expansion─17.8% for La 2 NiO 4+δ ( n = 1)─that monotonically decreases with increasing n . Surface synchrotron X-ray diffraction and coherent Bragg rod analysis (COBRA) reveal that this structural expansion arises from the intercalation of approximately δ ≈ 0.7–1.0 oxygen atoms into interstitial sites within the rock salt spacer layers, far exceeding the previous record of δ ≈ 0.3 for any Ruddlesden–Popper oxide. These oxygen-intercalated phases form a new class of layered perovskites with a spacer layer composition intermediate between the Ruddlesden–Popper and Aurivillius phases. Furthermore, oxygen intercalation induces metallicity, enhances nickel–oxygen hybridization, and suppresses oxygen octahedral rotations, a feature associated with high-temperature superconductivity in Ruddlesden–Popper nickelates. Our work establishes topochemical oxidation as a powerful approach to accessing highly oxidized, metastable phases across a broad range of layered oxide systems, offering new platforms to engineer electronic properties via intercalation chemistry.

Thin film BaTiO$_3$ has one of the highest known Pockels coefficients (&gt;1200 pm/V), making it an attractive material for use in electro-optic devices. It is advantageous to integrate BaTiO$_3$ on silicon to enable complementary metal-oxide-semiconductor (CMOS) compatible processing. However, synthesis of high-quality BaTiO$_3$ directly on silicon remains a challenge. Here, we synthesize BaTiO$_3$ using hybrid metal-organic molecular beam epitaxy (hMBE) and demonstrate its transfer onto silicon using thermocompression bonding and chemical lift-off. Hybrid metal-organic MBE enables self-regulated synthesis of highly stoichiometric thin films at high growth rates (&gt;100nm/hr). Our transfer method results in millimeter-scale areas of atomically flat, crack-free BaTiO$_3$ making it a potentially scalable method. Finally, we demonstrate the applicability of our process to device fabrication through characterization of lithographically-patterned and etch-transferred sub-micron features.

Thin film BaTiO$_3$ has one of the highest known Pockels coefficients (>1200 pm/V), making it an attractive material for use in electro-optic devices. It is advantageous to integrate BaTiO$_3$ on silicon to enable complementary metal-oxide-semiconductor (CMOS) compatible processing. However, synthesis of high-quality BaTiO$_3$ directly on silicon remains a challenge. Here, we synthesize BaTiO$_3$ using hybrid metal-organic molecular beam epitaxy (hMBE) and demonstrate its transfer onto silicon using thermocompression bonding and chemical lift-off. Hybrid metal-organic MBE enables self-regulated synthesis of highly stoichiometric thin films at high growth rates (>100nm/hr). Our transfer method results in millimeter-scale areas of atomically flat, crack-free BaTiO$_3$ making it a potentially scalable method. Finally, we demonstrate the applicability of our process to device fabrication through characterization of lithographically-patterned and etch-transferred sub-micron features.

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 LiB2O4 materials. On the B-site frustrated pyrochlore sublattice, the interplay of correlated metallic behavior and charge frustration leads to a superconducting state in LiTi2O4 and heavy fermion behavior in LiV2O4. To date, however, LiTi2O4 has primarily been understood as a conventional BCS superconductor despite a lattice structure that could host more exotic ground states. Here, we present a multimodal investigation of LiTi2O4, 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 LiTi2O4 from other superconducting titanates, suggesting an unconventional origin to superconductivity in LiTi2O4. 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. The authors study epitaxial thin films of the pyrochlore-sublattice compound LiTi2O4 by RIXS and ARPES. They observe cooperation between strong electron correlations and strong electron-phonon coupling, giving rise to a mobile polaronic ground state in which charge motion and lattice distortions are coupled.

The discovery of superconductivity in square-planar nickelates has offered a rich materials platform to explore the origins of cuprate-like superconductivity. Experimental investigations however have largely been limited to the infinite-layer $R$NiO$_2$ ($R$=rare-earth) nickelates. Here, we construct a phase diagram of multi-layer square-planar Nd$_{n+1}$Ni$_n$O$_{2n+2}$ compounds and discover signatures of superconductivity for $n$ = 4 - 8. Upon decreasing the dimensionality $n$, the superconducting anisotropy evolves due to 4$f$ electron effects, and electronic structure characteristics approach cuprate-like behavior. Magnetic fluctuations persist from within the superconducting regime and into the over-doped, non-superconducting regime. Remarkably, the superconducting regime overlaps with that of chemically-doped infinite-layer nickelates, demonstrating underlying commonalities and distinct differences across varying structural realizations of square-planar nickelates. Our work establishes this layered template for creating new nickel-based superconductors.

Hexagonal ${\mathrm{TbInO}}_{3}$ exhibits spin liquid behavior at low temperatures and improper ferroelectricity at room temperature, and as such may host novel electronic and magnetic states upon carrier doping. This paper presents density functional theory (DFT) calculations of the electronic, dielectric, and defect properties of ${\mathrm{TbInO}}_{3}$. We study ${\mathrm{Ti}}^{4+}$ and ${\mathrm{Ca}}^{2+}$ as substitutional dopants replacing ${\mathrm{In}}^{3+}$ and ${\mathrm{Tb}}^{3+}$ to introduce electron- and hole-doping, respectively. Point defect calculations reveal that ${\mathrm{Ti}}^{4+}$ dopants introduce shallow defect states near the conduction band minimum, suggesting the possibility of $n$-type conductivity, whereas ${\mathrm{Ca}}^{2+}$ dopants result in deep in-gap states. We also analyze changes to structural properties with relatively large (8%--16%) Ca and Ti concentrations. We utilize molecular beam epitaxy (MBE) to synthesize epitaxial thin films of ${\mathrm{TbInO}}_{3}$ doped with ${\mathrm{Ti}}^{4+}$ and ${\mathrm{Ca}}^{2+}$, and use scanning transmission electron microscopy (STEM) imaging to show that the experimental films corroborate the doping-induced structural changes found with DFT. However, all samples remain electrically insulating, which we attribute to the localization of added carriers on the Tb cation. Finally, we propose charge-transfer doping as an alternative strategy to induce conductivity in ${\mathrm{TbInO}}_{3}$, and identify several possible substrates to achieve this.

Abstract Quantum spin liquids, where the frustrated magnetic ground state hosts highly entangled spins resisting long-range order to 0 K, are exotic quantum magnets proximate to unconventional superconductivity and candidate platforms for topological quantum computing. Although several quantum spin liquid material candidates have been identified, thin films crucial for device fabrication and further tuning of properties remain elusive. Recently, hexagonal TbInO 3 has emerged as a quantum spin liquid candidate which also hosts improper ferroelectricity and exotic high-temperature carrier transport. Here, we synthesize thin films of TbInO 3 and characterize their magnetic and electronic properties. Our films present a highly frustrated magnetic ground state without long-range order to 0.4 K, consistent with bulk crystals. We further reveal a rich ferroelectric domain structure and unconventional non-local transport near room temperature, suggesting hexagonal TbInO 3 as a promising candidate for realizing exotic magnetic and transport phenomena in epitaxial heterostructures.

Quantum spin liquids, where the frustrated magnetic ground state hosts highly entangled spins resisting long-range order to 0 K, are exotic quantum magnets proximate to unconventional superconductivity and candidate platforms for topological quantum computing. Although several quantum spin liquid material candidates have been identified, thin films crucial for device fabrication and further tuning of properties remain elusive. Recently, hexagonal TbInO $_{3}$ has emerged as a quantum spin liquid candidate which also hosts improper ferroelectricity and exotic high-temperature carrier transport. Here, we synthesize thin films of TbInO $_{3}$ and characterize their magnetic and electronic properties. Our films present a highly frustrated magnetic ground state without long-range order to 0.4 K, consistent with bulk crystals. We further reveal a rich ferroelectric domain structure and unconventional non-local transport near room temperature, suggesting hexagonal TbInO $_{3}$ as a promising candidate for realizing exotic magnetic and transport phenomena in epitaxial heterostructures.

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.

Abstract Barium titanate (BaTiO 3 ) is a material of interest for photonic device applications due to its strong optical non‐linearity. However, BaTiO 3 ‐based devices have not found widespread adoption, in part due to the challenges associated with synthesizing high quality thin‐films. Here, high‐resolution scanning transmission electron microscope (STEM) imaging is used to investigate the atomic structure of both on‐ and off‐stoichiometric BaTiO 3 synthesized by molecular beam epitaxy (MBE). This investigation reveals an asymmetry in the way the BaTiO 3 atomic lattice accommodates off‐stoichiometry growth and unveils features beyond what is expected from diffraction or surface characterization techniques. Excess titanium incorporates into the BaTiO 3 lattice to form pervasive defects despite titanium‐rich films having a low surface roughness and high‐quality appearance in diffraction. Excess barium forms a rough, water‐soluble surface layer but does not significantly impact the quality of the BaTiO 3 lattice. STEM is used to map titanium atom displacement in real‐space. The average displacement distance is 30–60 pm in the strained thin‐films, higher than the &lt;20 pm displacement in bulk BaTiO 3 . Additionally, the titanium atom displacement direction deviates from the c ‐axis of the unit cell, which may have implications for the material's electro‐optic tensor and thus for electro‐optic device design.

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+1}Ni_{n}O_{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% for La_{2}NiO_{4} (n=1) - that monotonically 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.

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 Li$B_2$O$_4$ materials. On the $B$-site frustrated pyrochlore sublattice, the interplay of correlated metallic behavior and charge frustration leads to a superconducting state in LiTi$_2$O$_4$ and heavy fermion behavior in LiV$_2$O$_4$. To date, however, LiTi$_2$O$_4$ 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$_2$O$_4$, 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$_2$O$_4$ from other superconducting titanates, suggesting an unconventional origin to superconductivity in LiTi$_2$O$_4$. 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.

The authors present here a resonant inelastic x-ray scattering study of magnetic and electronic excitations in the layered Ruddlesden-Popper nickelates Nd${}_{n+1}$Ni${}_{n}$O${}_{3n+1}$. As the number of layers increases, holes are doped into ligand sites and magnetic excitations soften modestly--an effect attributed to competing influences of hole doping and enhanced interlayer exchange. These findings highlight structural tuning as an effective route to manipulate electronic and magnetic properties, establishing a promising platform for unconventional superconductivity.

We examine the bulk electronic structure of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:msub> <a:mi>Nd</a:mi> <a:mn>3</a:mn> </a:msub> <a:msub> <a:mi>Ni</a:mi> <a:mn>2</a:mn> </a:msub> <a:msub> <a:mi mathvariant="normal">O</a:mi> <a:mn>7</a:mn> </a:msub> </a:mrow> </a:math> using Ni <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:mrow> <c:mn>2</c:mn> <c:mi>p</c:mi> </c:mrow> </c:math> core-level hard x-ray photoemission spectroscopy combined with density functional theory <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"> <d:mo>+</d:mo> </d:math> dynamical mean-field theory. Our results reveal a large deviation of the Ni <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"> <e:mrow> <e:mn>3</e:mn> <e:mi>d</e:mi> </e:mrow> </e:math> occupation from the formal <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"> <f:msup> <f:mrow> <f:mi>Ni</f:mi> </f:mrow> <f:mrow> <f:mn>2.5</f:mn> <f:mo>+</f:mo> </f:mrow> </f:msup> </f:math> valency, highlighting the importance of the charge transfer from oxygen ligands. We find that the dominant <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"> <g:msup> <g:mi>d</g:mi> <g:mn>8</g:mn> </g:msup> </g:math> configuration is accompanied by nearly equal contributions from <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"> <h:msup> <h:mi>d</h:mi> <h:mn>7</h:mn> </h:msup> </h:math> and <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"> <i:msup> <i:mi>d</i:mi> <i:mn>9</i:mn> </i:msup> </i:math> states, exhibiting an unusual valence state among Ni-based oxides. Finally, we discuss the Ni <j:math xmlns:j="http://www.w3.org/1998/Math/MathML"> <j:msub> <j:mi>d</j:mi> <j:mrow> <j:msup> <j:mi>x</j:mi> <j:mn>2</j:mn> </j:msup> <j:mo>−</j:mo> <j:msup> <j:mi>y</j:mi> <j:mn>2</j:mn> </j:msup> </j:mrow> </j:msub> </j:math> and <k:math xmlns:k="http://www.w3.org/1998/Math/MathML"> <k:msub> <k:mi>d</k:mi> <k:msup> <k:mi>z</k:mi> <k:mn>2</k:mn> </k:msup> </k:msub> </k:math> orbital-dependent hybridization, correlation and local spin dynamics.

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, ${\mathrm{Eu}}_{0.9}{\mathrm{Sr}}_{1.1}{\mathrm{NiO}}_{4}$.

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.

The discovery of high-temperature superconductivity in bulk La$_3$Ni$_2$O$_7$ 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$_3$Ni$_2$O$_7$ 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$_3$Ni$_2$O$_7$ 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.

Indium tin oxide (Sn-doped ${\mathrm{In}}_{2}{\mathrm{O}}_{3}$; 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 $n\text{-BuLi}$ (${\mathrm{C}}_{4}{\mathrm{H}}_{9}\mathrm{Li}$) 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, ${T}_{\text{c}}$, 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.

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 ( <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mi>n</a:mi> <a:mo>=</a:mo> <a:mn>5</a:mn> </a:math> ) and metallic three-layer ( <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mi>n</c:mi> <c:mo>=</c:mo> <c:mn>3</c:mn> </c:math> ) nickelates by measuring the Seebeck coefficient <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:mi>S</e:mi> </e:math> . We find a temperature-independent and negative <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"> <g:mi>S</g:mi> <g:mo>/</g:mo> <g:mi>T</g:mi> </g:math> for both <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"> <i:mi>n</i:mi> <i:mo>=</i:mo> <i:mn>5</i:mn> </i:math> and <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"> <k:mi>n</k:mi> <k:mo>=</k:mo> <k:mn>3</k:mn> </k:math> nickelates. These results are in stark contrast to the strongly temperature-dependent <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"> <m:mi>S</m:mi> <m:mo>/</m:mo> <m:mi>T</m:mi> </m:math> measured at similar electron filling in the cuprate <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"> <o:mrow> <o:msub> <o:mrow> <o:mi>La</o:mi> </o:mrow> <o:mrow> <o:mn>1.36</o:mn> </o:mrow> </o:msub> </o:mrow> <o:mrow> <o:msub> <o:mrow> <o:mi>Nd</o:mi> </o:mrow> <o:mrow> <o:mn>0.4</o:mn> </o:mrow> </o:msub> </o:mrow> <o:mrow> <o:msub> <o:mrow> <o:mi>Sr</o:mi> </o:mrow> <o:mrow> <o:mn>0.24</o:mn> </o:mrow> </o:msub> </o:mrow> <o:mrow> <o:msub> <o:mrow> <o:mi>CuO</o:mi> </o:mrow> <o:mrow> <o:mn>4</o:mn> </o:mrow> </o:msub> </o:mrow> </o:math> . The electronic structure calculated from density-functional theory can reproduce the temperature dependence, sign, and amplitude of <q:math xmlns:q="http://www.w3.org/1998/Math/MathML" display="inline"> <q:mi>S</q:mi> <q:mo>/</q:mo> <q:mi>T</q:mi> </q:math> 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. Published by the American Physical Society 2024

High Resolution Image Download MS PowerPoint Slide 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 BaBiF 5 and SrBiF 5 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 BaBiF 5 on BaF 2 substrates, providing a route to thin film fluoride-ion battery devices.

Abstract A key open question in the study of layered superconducting nickelate films is the role that hydrogen incorporation into the lattice plays in the appearance of the superconducting state. Due to the challenges of stabilizing highly crystalline square planar nickelate films, films are prepared by the deposition of a more stable parent compound which is then transformed into the target phase via a topotactic reaction with a strongly reducing agent such as CaH 2 . Recent studies, both experimental and theoretical, have introduced the possibility that the incorporation of hydrogen from the reducing agent into the nickelate lattice may be critical for the superconductivity. In this work, we use secondary ion mass spectrometry to examine superconducting La 1− x X x NiO 2 / SrTiO 3 ( X = Ca and Sr) and Nd 6 Ni 5 O 12 / NdGaO 3 films, along with non-superconducting NdNiO 2 / SrTiO 3 and (Nd,Sr)NiO 2 / SrTiO 3 . We find no evidence for extensive hydrogen incorporation across a broad range of samples, including both superconducting and non-superconducting films. Theoretical calculations indicate that hydrogen incorporation is broadly energetically unfavorable in these systems, supporting our conclusion that extensive hydrogen incorporation is not generally required to achieve a superconducting state in layered square-planar nickelates.

Oxide molecular beam epitaxy is a powerful synthesis technique capable of creating complex layered structures with elements in high oxidation states. The authros start with the Sr${}_{n+1}$Cr${}_{n}$O${}_{3n+1}$ Ruddlesden-Popper series. This system contains the magnetic Cr${}^{4+}$ cation which gives rise to electronic correlations that vary as a function of structural dimensionality: Sr${}_{2}$CrO${}_{4}$ and Sr${}_{3}$Cr${}_{2}$O${}_{7}$ possess enhanced spin and orbital ordering temperatures compared to the SrCrO${}_{3}$ end member. In this work, they synthesize films for $n=1$ to $n=5$, uncovering a metal to insulator transition. They seek the physical origins of the concomitant spin and orbital orderings -- both experimentally with x-ray absorption spectroscopy measurements, and theoretically with density functional theory calculations. Their results unveil the basis of these exotic ground states, including additional structural distortions that play a key role in the system and enable the metal-insulator transitions.

In the Acknowledgements section of this article the grant number relating to NSF was incorrectly given as DMR 2045826 and should have been DMR-2045826. The original article has been corrected.