Targeted material design is at the center of research efforts on epitaxial, complex oxide heterostructures. Due to the strong electron-lattice coupling, small structural modifications are often decisive for the resulting macroscopic, physical properties of artificially layered materials. Here we report on a detailed structural analysis of a set of differently stacked LaNiO3−LaGaO3 superlattices by transmission electron microscopy. We find that the relative thickness ratio of the two superlattice sublayers affects the structural network, resulting in different Ni–O bond lengths and Ni–O–Ni angles under the same epitaxial strain conditions. Whereas the bond length values depend mainly on the LaNiO3 layer thickness, bond angles are mainly influenced by the LaGaO3 layer thickness. The orbital polarization determined from x-ray absorption spectroscopy measurements shows that the superlattice with the smallest deviation of both values compared to bulk shows the highest orbital polarization. Therefore, the thickness ratio of the two superlattice components can be regarded as an additional effective tool to tune the functional properties of nickelates. Published by the American Physical Society 2024

The essential building blocks of cuprate superconductors are two-dimensional CuO$_2$ sheets interspersed with charge reservoir layers. In bilayer cuprates, two closely spaced CuO$_2$ sheets are separated by a larger distance from the subsequent pair in the next unit cell. In contrast to single-layer cuprates, prior theoretical work on bilayer systems has predicted two distinct acoustic plasmon bands for a given out-of-plane momentum transfer. Here we report random phase approximation (RPA) calculations for bilayer systems which corroborate the existence of two distinct plasmons bands. We find that the intensity of the lower-energy band is negligibly small, whereas the higher-energy band carries significant spectral weight. We also present resonant inelastic x-ray scattering (RIXS) experiments at the O $K$-edge on the bilayer cuprate Y$_{0.85}$Ca$_{0.15}$Ba$_2$Cu$_3$O$_7$ (Ca-YBCO), which show only one dispersive plasmon branch, in agreement with the RPA calculations. In addition, the RPA results indicate that the dispersion of the higher-energy plasmon band in Ca-YBCO is not strictly acoustic, but exhibits a substantial energy gap of approximately 250 meV at the two-dimensional Brillouin zone center.

We report on the epitaxial growth of pyrochlore NRO thin films utilizing a reactive off-axis sputtering technique. The growth process employed Ru and Nd metal targets with a repetitive substrate temperature sequence. X-ray diffraction, magnetic susceptibility, and spectroscopic ellipsometry measurements confirm that the structural, magnetic, and electronic properties of near-stoichiometric NRO films match those of the bulk material. Our spin-polarized density functional calculations based on Nd2Ru2O7 structural parameters accurately describe the optical spectra and assign Hubbard bands to the interband transitions observed above the optical band gap of 0.2 eV. By utilizing the technique's capability to adjust the degree of ruthenium deficiency, we investigated Nd2Ru2O7 films across a wide range of stoichiometric variations. Decreasing the Ru/Nd ratio results in lattice expansion, an increase in the optical band gap, and the suppression of Ru 4d intersite optical transitions. Additionally, this adjustment facilitates the elimination of minor inclusions of a ferromagnetic NdOx impurity phase, influencing the magnetic properties of stoichiometric films at low temperatures. The successful growth of Nd2Ru2O7 films opens up promising opportunities for designing and exploring strain- and light-induced states in pyrochlore ruthenates.

The Ferrell-Glover-Tinkham (FGT) sum rule in superconductors defines the superfluid density, rho s, as the optical conductivity spectral weight (SW) that transfers into a delta function at omega = 0 due to the opening of the energy gap below Tc. In high-Tc superconductors, strong electron-boson coupling, self-energy effects, and intertwining of energy scales can link rho s to various high-energy processes, making the question of whether or not the FGT sum rule is valid in cuprates, and at what energy scale, central to a full understanding of the pairing mechanism. Here, we report high-precision measurements of the FGT sum rule in near-optimally doped DyBa2Cu3O7-delta thin films. We resolve the low-energy balance of SW by combining submillimeter-microwave interferometry, terahertz time-domain spectroscopy, and infrared ellipsometry to independently obtain the real and imaginary parts of the complex dielectric function between 0.8 meV and 1.1 eV (6-9000 cm-1). By applying a Kramers-Kronig consistency analysis to the measured spectra we find that the FGT sum rule is obeyed, and the total intraband SW is conserved to within +/- 0.2% below an energy scale similar to 0.6 eV. We attribute specific anomalies observed in the conductivity spectra below similar to 0.6 eV to coupling of charge carriers to the spectrum of collective antiferromagnetic spin fluctuations. The procedure presented here, applied to near-optimally doped DyBa2Cu3O7-delta, lays out a protocol for how the FGT sum rule should be studied in other doping levels and compounds.

We use resonant inelastic x-ray scattering (RIXS) at the O K- and Cu K-edges to investigate the doping dependence and temperature dependence of low-energy plasmon excitations in La2-xSrxCuO4. We observe a monotonic increase of the energy scale of the plasmons with increasing doping x in the underdoped regime, whereas a saturation occurs above optimal doping x greater than or similar to 0.16 and persists at least up to x = 0.4. Furthermore, we find that the plasmon excitations show only a marginal temperature dependence, and possible effects due to the superconducting transition and the onset of strange metal behavior are either absent or below the detection limit of our experiment. Taking into account the strongly correlated character of the cuprates, we show that layered t -JV model calculations accurately capture the increase of the plasmon energy in the underdoped regime. However, the computed plasmon energy continues to increase even for doping levels above x greater than or similar to 0.16, which is distinct from the experimentally observed saturation and reaches a broad maximum around x = 0.55. We discuss whether possible lattice disorder in overdoped samples, a renormalization of the electronic correlation strength at high dopings, or an increasing relevance of nonplanar Cu and O orbitals could be responsible for the discrepancy between experiment and theory for doping levels above x = 0.16.

Using x-ray absorption spectroscopy with lateral resolution from the submillimeter to submicrometer range, we investigate the homogeneity, the chemical composition, and the nickel 3d- oxygen 2p charge transfer in topotactically reduced epitaxial PrNiO2+δ thin films. To this end, we use x-ray absorption spectroscopy in a standard experimental setup and in a soft x-ray microscope to probe the element and spatially resolved electronic structure modifications through changes of the nickel-2p and oxygen-1s absorption spectrum upon soft-chemistry reduction. We find that the reduction process is laterally homogeneous across a partially reduced PrNiO2+δ thin film sample for length scales down to 50 nm.

Microscopic studies on thin film superconductors play an important role for probing non-equilibrium phase transitions and revealing dynamics at the nanoscale. However, magnetic sensors with nanometer scale spatial and picosecond temporal resolution are essential for exploring these. Here, we present an all-optical, microwave-free method, that utilizes the negatively charged nitrogen-vacancy (NV) center in diamond as a non-invasive quantum sensor and enables the spatial detection of the Meissner state in a superconducting thin film. We place an NV implanted diamond membrane on a superconducting LSCO thin film. The strong B-field dependence of the NV photoluminescence (PL) allows us to investigate the Meissner screening in LSCO under an externally applied magnetic field in a non-resonant manner.

Using ion-implanted $^8$Li $\beta$-detected NMR, we study the evolution of the correlated metallic state of LaNiO$_3$ in a series of LaNiO$_3$/LaAlO$_3$ superlattices as a function of bilayer thickness. Spin-lattice relaxation measurements in an applied field of 6.55 T reveal two equal amplitude components: one with metallic ($T$-linear) $1/T_{1}$, and a second with a more complex $T$-dependence. The metallic character of the slow relaxing component is only weakly affected by the LaNiO$_3$ thickness, while the fast component is much more sensitive, exhibiting the opposite temperature dependence (increasing towards low $T$) in the thinnest, most magnetic samples. The origin of this bipartite relaxation is discussed.

We report the fabrication of an epitaxial grown thin-film heterostructure YBa2Cu3O7–x/ PrNiO3/YBa2Cu3O7–x with a 6 nm thick praseodymium nickelate (PrNiO3) interlayer. X-ray diffraction studies demonstrate that obtained patterns of both the upper and bottom YBa2Cu3O7–x films, as well as PrNiO3, are in good agreement with the lattice parameters of materials. Measurements of the resistive characteristics show that, with decreasing temperature, the voltage dependence of the conductivity G(V) deviates from the symmetric, becoming noticeable at T = 77 K, a temperature lower than the critical temperature of the cuprate superconductor YBa2Cu3O7–x.

The recent observation of superconductivity in infinite-layer Nd_1-xSr_xNiO_2 thin films has attracted a lot of attention, since this compound is electronically and structurally analogous to the superconducting cuprates. Due to the challenges in the phase stabilization upon chemical doping with Sr, we synthesized artificial superlattices of LaNiO_3 embedded in insulating LaGaO_3, and used layer-selective topotactic reactions to reduce the nickelate layers to LaNiO_2. Hole doping is achieved via interfacial oxygen atoms and tuned via the layer thickness. We used electrical transport measurements, transmission electron microscopy, and x-ray spectroscopy together with ab initio calculations to track changes in the local nickel electronic configuration upon reduction and found that these changes are reversible. Our experimental and theoretical data indicate that the doped holes are trapped at the interfacial quadratic pyramidal Ni sites. Calculations for electron-doped cases predict a different behavior, with evenly distributed electrons among the layers, thus opening up interesting perspectives for interfacial doping of transition metal oxides.

Heteroepitaxy offers a new type of control mechanism for the crystal structure, the electronic correlations, and thus the functional properties of transition-metal oxides. Here, we combine electrical transport measurements, high-resolution scanning transmission electron microscopy (STEM), and density functional theory (DFT) to investigate the evolution of the metal-to-insulator transition (MIT) in NdNiO_3 films as a function of film thickness and NdGaO_3 substrate crystallographic orientation. We find that for two different substrate facets, orthorhombic (101) and (011), modifications of the NiO_6 octahedral network are key for tuning the transition temperature T_MIT over a wide temperature range. A comparison of films of identical thickness reveals that growth on [101]-oriented substrates generally results in a higher T_MIT, which can be attributed to an enhanced bond-disproportionation as revealed by the DFT+U calculations, and a tendency of [011]-oriented films to formation of structural defects and stabilization of non-equilibrium phases. Our results provide insights into the structure-property relationship of a correlated electron system and its evolution at microscopic length scales and give new perspectives for the epitaxial control of macroscopic phases in metal-oxide heterostructures.

Oxide heterostructures provide unique opportunities to modify the properties of quantum materials through a targeted manipulation of spin, charge, and orbital states. Here, we use resonant x-ray reflectometry to probe the electronic structure of thin slabs of YVO3 embedded in a superlattice with LaAlO3. We extend the previously established methods of reflectometry analysis to a general form applicable to t(2g) electron systems and extract quantitative depth-dependent x-ray linear dichroism profiles. Our data reveal an artificial, layered orbital polarization, where the average occupation of xz and yz orbitals in the interface planes next to LaAlO3 is inverted compared to the central part of the YVO3 slab. This phase is stable down to 30 K and the bulklike orbital ordering transitions are absent. We identify the key mechanism for the electronic reconstruction to be a combination of epitaxial strain and spatial confinement by the LaAlO3 layers, in good agreement with predictions from ab initio theory.

Using ion-implanted Li-8 beta-detected NMR, we study the evolution of the correlated metallic state of LaNiO3 in a series of LaNiO3/LaAlO3 superlattices as a function of bilayer thickness. Spin-lattice relaxation measurements in an applied field of 6.55 T reveal two equal amplitude components: one with metallic (T linear) 1/T-1 and a second with a more complex T dependence. The metallic character of the slow relaxing component is only weakly affected by the LaNiO3 thickness, while the fast component is much more sensitive, exhibiting the opposite temperature dependence (increasing toward low T ) in the thinnest, most magnetic samples. The origin of this bipartite relaxation is discussed in terms of electronic phase separation.

We present the electronic and structural properties of monolayer ${\mathrm{WSe}}_{2}$ grown by pulsed-laser deposition on monolayer graphene (MLG) on SiC. The spin splitting in the ${\mathrm{WSe}}_{2}$ valence band at $\overline{\mathrm{K}}$ was ${\mathrm{\ensuremath{\Delta}}}_{\mathrm{SO}}=0.469\ifmmode\pm\else\textpm\fi{}0.008$ eV, as determined by angle-resolved photoemission spectroscopy. Synchrotron-based grazing-incidence in-plane x-ray diffraction (XRD) revealed the in-plane lattice constant of monolayer ${\mathrm{WSe}}_{2}$ to be ${a}_{{\mathrm{WSe}}_{2}}=3.2757\ifmmode\pm\else\textpm\fi{}0.0008$ \AA{}. This indicates a lattice compression of $\ensuremath{-}0.19$% relative to bulk ${\mathrm{WSe}}_{2}$. By using the experimentally determined graphene lattice constant (${a}_{\mathrm{MLG}}=2.4575\ifmmode\pm\else\textpm\fi{}0.0007$ \AA{}), we found that a $3\ifmmode\times\else\texttimes\fi{}3$ unit cell of the slightly compressed ${\mathrm{WSe}}_{2}$ is perfectly commensurate with a $4\ifmmode\times\else\texttimes\fi{}4$ graphene lattice with a mismatch below 0.03%, which could explain why the monolayer ${\mathrm{WSe}}_{2}$ is compressed on MLG. From XRD and first-principles calculations, we conclude that the observed size of strain will affect ${\mathrm{\ensuremath{\Delta}}}_{\mathrm{SO}}$ only on the order of a few meV. In addition, angle-resolved, ultraviolet, and x-ray photoelectron spectroscopies shed light on the band alignment between ${\mathrm{WSe}}_{2}$ and MLG/SiC and indicate electron transfer from graphene to the ${\mathrm{WSe}}_{2}$ monolayer. As further revealed by atomic force microscopy, the ${\mathrm{WSe}}_{2}$ island size depends on the number of carbon layers on top of the SiC substrate. This suggests that the epitaxy of ${\mathrm{WSe}}_{2}$ favors the weak van der Waals interactions with graphene, while it is perturbed by the influence of the SiC substrate and its carbon buffer layer.

The microwave and magnetic parameters of Josephson Nb/Au/Sr2IrO4/YBa2Cu3Ox mesastructures with a layer of Sr2IrO4, a material representing a Mott antiferromagnetic insulator with a high spin–orbit interaction energy ESO ~ 0.4 eV, were studied. Shapiro steps, oscillating with radiation power, appeared under monochromatic electromagnetic radiation confirm the Josephson properties of these structures. In the presence of a weak magnetic field H < 15 Oe, the voltage–current characteristics (VCCs) had resonant current steps at voltages Vn, which were inversely proportional to the size L of structures in plane. Polarity reversal in the electrical current I led to asymmetry in the arrangement of resonant current steps. At a specified magnetic field H, the voltage Vn remained constant, and the amplitudes of resonant current steps nonmonotonically changed.

We have used atomic layer-by-layer molecular beam epitaxy to synthesize coherently lattice-matched thin films of the high-temperature superconductor DyBa2Cu3O7−x with minimal defect density. A systematic set of x-ray reciprocal-space maps reveals tetragonal and orthorhombic structures with different twinning patterns and elucidates their evolution with the thickness, the oxygenation state, and the epitaxial relationship with the substrate. We also show that films with more pronounced orthorhombicity exhibit lower normal-state resistivities and higher superconducting transition temperatures. These findings provide guidance for the synthesis of optimized superconducting heterostructures and devices.

The temperature dependence of the superfluid density ρ_{s}(T) has been measured for a series of ultrathin MBE-grown DyBa_{2}Cu_{3}O_{7-δ} superconducting (SC) films by submillimeter wave interferometry combined with time-domain terahertz spectroscopy and IR ellipsometry. We find that all films 10 u.c. and thicker show the same universal temperature dependence of ρ_{s}(T), which follows the critical behavior characteristic of single crystal YBa_{2}Cu_{3}O_{7-δ} as T approaches T_{c}. In 7 u.c. thick films, ρ_{s}(T) declines steeply upon approaching T_{c}, as expected for the Berezinskii-Kosterlitz-Thouless vortex unbinding transition. Our analysis provides evidence for a sharply defined 4 u.c. non-SC interfacial layer, leaving a quasi-2D SC layer on top. We propose that the SC state in this interfacial layer is suppressed by competing (possibly charge) order.