Search

Common belief is that the large band shifts observed in incommensurate misfit compounds, e.g. (LaSe)1.14(NbSe2)2, are due to interlayer charge transfer. In contrast, our analysis, based on both ARPES measurements and a specialized ab initio framework employing only quantities well defined in incommensurate materials, demonstrates that the large band shifts instead reflect changes in valence band hybridization and interlayer bonding. The strong alignment of our ab initio predictions and ARPES measurements confirms our understanding of the incommensurate electronic structure and charge transfer., Comment: 6 pages, 4 figures, Supplemental Materials included as pdf in LaTeX package

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 hydrogen incorporation is not generally required to achieve a superconducting state in layered square-planar nickelates., Comment: 23 pages, 21 figures (including supplemental information)

Infinite-layer nickelates $R_{1-x}$Sr$_{x}$NiO$_{2}$ ($R$ = La, Pr, Nd) are a class of superconductors with structural similarities to cuprates. Although long-range antiferromagnetic order has not been observed for these materials, magnetic effects such as antiferromagnetic spin fluctuations and spin-glass behavior have been reported. Different experiments have drawn different conclusions about whether the pairing symmetry is $s$- or $d$ wave. In this paper, we applied a scanning superconducting quantum interference device (SQUID) to probe the magnetic behavior of film samples of three infinite-layer nickelates (La$_{0.85}$Sr$_{0.15}$NiO$_2$, Pr$_{0.8}$Sr$_{0.2}$NiO$_2$, and Nd$_{0.775}$Sr$_{0.225}$NiO$_2$) grown on SrTiO$_3$ (STO), each with a nominal thickness of 20 unit cells. In all three films, we observed a ferromagnetic background. We also measured the magnetic susceptibility above the superconducting critical temperature in Pr$_{0.8}$Sr$_{0.2}$NiO$_2$ and La$_{0.85}$Sr$_{0.15}$NiO$_2$ and identified a non-Curie-Weiss dynamic susceptibility. Both magnetic features are likely due to NiO$_x$ nanoparticles. Additionally, we investigated superconductivity in Pr$_{0.8}$Sr$_{0.2}$NiO$_2$ and Nd$_{0.775}$Sr$_{0.225}$NiO$_2$, which exhibited inhomogeneous diamagnetic screening. The superfluid density inferred from the diamagnetic susceptibility in relatively homogeneous regions shows $T$-linear behavior in both samples. Finally, we observed superconducting vortices in Nd$_{0.775}$Sr$_{0.225}$NiO$_2$. We determined a Pearl length of 330 $\upmu$m for Nd$_{0.775}$Sr$_{0.225}$NiO$_2$ at 300 mK both from the strength of the diamagnetism and from the size and shape of the vortices. These results highlight the importance of considering NiO$_x$ particles when interpreting experimental results for these films.

Charge order pervades the phase diagrams of many quantum materials where it competes with superconducting and magnetic phases, hosts electronic phase transitions and topological defects, and couples to the lattice generating intricate structural distortions. Incommensurate charge order is readily stabilized in manganese oxides where it is associated with anomalous electronic and magnetic properties, but its nanoscale structural inhomogeneity complicates precise characterization and understanding of these phases. Leveraging atomic-resolution variable temperature cryogenic scanning transmission electron microscopy (cryo-STEM), we characterize the thermal evolution of charge order in a model manganite system, finding that mobile networks of topological defects generate phase inhomogeneity and induce incommensuration in an otherwise lattice-locked modulation. The melting of charge order at high temperatures is accompanied by generation of additional defects which decouple large areas of the order from the lattice., Comment: 14 pages, 6 figures

Progress in the study of infinite-layer nickelates has always been highly linked to materials advances. In particular, the recent development of superconductivity via hole-doping was predicated on the controlled synthesis of Ni in a very high oxidation state, and subsequent topotactic reduction to a very low oxidation state, currently limited to epitaxial thin films. Here we demonstrate a process to combine these steps with a heterostructure which includes an epitaxial soluble buffer layer, enabling the release of freestanding membranes of (Nd,Sr)NiO2 encapsulated in SrTiO3, which serves as a protective layer. The membranes have comparable structural and electronic properties to that of optimized thin films, and range in lateral dimensions from millimeters to ~100 micron fragments, depending on the degree of strain released with respect to the initial substrate. The changes in the superconducting transition temperature associated with membrane release are quite similar to those reported for substrate and pressure variations, suggestive of a common underlying mechanism. These membranes structures should provide a versatile platform for a range of experimental studies and devices free from substrate constraints.

We present an integrated procedure for the synthesis of infinite-layer nickelates using molecular-beam epitaxy with gas-phase reduction by atomic hydrogen. We first discuss challenges in the growth and characterization of perovskite NdNiO$_3$/SrTiO$_3$, arising from post growth crack formation in stoichiometric films. We then detail a procedure for fully reducing NdNiO$_3$ films to the infinite-layer phase, NdNiO$_2$, using atomic hydrogen; the resulting films display excellent structural quality, smooth surfaces, and lower residual resistivities than films reduced by other methods. We utilize the in situ nature of this technique to investigate of the role that SrTiO$_3$ capping layers play in the reduction process, illustrating their importance in preventing the formation of secondary phases at the exposed nickelate surface. A comparative bulk- and surface-sensitive study indicates formation of a polycrystalline crust on the film surface serves to limit the reduction process., Comment: Main text: 12 pages, 7 figures. Supplemental Materials: 11 pages, 11 figures

The charge density wave (CDW) material 1T-TaS$_2$ exhibits a pulse-induced insulator-to-metal transition, which shows promise for next-generation electronics such as memristive memory and neuromorphic hardware. However, the rational design of TaS$_2$ devices is hindered by a poor understanding of the switching mechanism, the pulse-induced phase, and the influence of material defects. Here, we operate a 2-terminal TaS$_2$ device within a scanning transmission electron microscope (STEM) at cryogenic temperature, and directly visualize the changing CDW structure with nanoscale spatial resolution and down to 300 {\mu}s temporal resolution. We show that the pulse-induced transition is driven by Joule heating, and that the pulse-induced state corresponds to nearly commensurate and incommensurate CDW phases, depending on the applied voltage amplitude. With our in operando cryo-STEM experiments, we directly correlate the CDW structure with the device resistance, and show that dislocations significantly impact device performance. This work resolves fundamental questions of resistive switching in TaS$_2$ devices critical for engineering reliable and scalable TaS$_2$ electronics.

Charge density waves are emergent quantum states that spontaneously reduce crystal symmetry, drive metal-insulator transitions, and precede superconductivity. In low-dimensions, distinct quantum states arise, however, thermal fluctuations and external disorder destroy long-range order. Here we stabilize ordered two-dimensional (2D) charge density waves through endotaxial synthesis of confined monolayers of 1T-TaS$_2$. Specifically, an ordered incommensurate charge density wave (oIC-CDW) is realized in 2D with dramatically enhanced amplitude and resistivity. By enhancing CDW order, the hexatic nature of charge density waves becomes observable. Upon heating via in-situ TEM, the CDW continuously melts in a reversible hexatic process wherein topological defects form in the charge density wave. From these results, new regimes of the CDW phase diagram for 1T-TaS$_2$ are derived and consistent with the predicted emergence of vestigial quantum order.

The layered square-planar nickelates, Nd$_{n+1}$Ni$_{n}$O$_{2n+2}$, are an appealing system to tune the electronic properties of square-planar nickelates via dimensionality; indeed, superconductivity was recently observed in Nd$_{6}$Ni$_{5}$O$_{12}$ 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$_{4}$Ni$_{3}$O$_{10}$, and subsequent reduction to the square-planar phase, Nd$_{4}$Ni$_{3}$O$_{8}$. We synthesize our highest quality Nd$_{4}$Ni$_{3}$O$_{10}$ films under compressive strain on LaAlO$_{3}$ (001), while Nd$_{4}$Ni$_{3}$O$_{10}$ 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$_{4}$Ni$_{3}$O$_{10}$ on SrTiO$_{3}$ (001). Films reduced on LaAlO$_{3}$ become insulating and form compressive strain-induced $c$-axis canting defects, while Nd$_{4}$Ni$_{3}$O$_{8}$ films on NdGaO$_{3}$ are metallic. This work provides a pathway to the synthesis of Nd$_{n+1}$Ni$_{n}$O$_{2n+2}$ thin films and sets limits on the ability to strain engineer these compounds via epitaxy.

Exploiting the emerging nanoscale periodicities in epitaxial, single-crystal thin films is an exciting direction in quantum materials science: confinement and periodic distortions induce novel properties. The structural motifs of interest are ferroelastic, ferroelectric, multiferroic, and, more recently, topologically protected magnetization and polarization textures. A critical step towards heterostructure engineering is understanding their nanoscale structure, best achieved through real-space imaging. X-ray Bragg coherent diffractive imaging visualizes sub-picometer crystalline displacements with tens of nanometers spatial resolution. Yet, it is limited to objects spatially confined in all three dimensions and requires highly coherent, laser-like x-rays. Here we lift the confinement restriction by developing real-space imaging of periodic lattice distortions: we combine an iterative phase retrieval algorithm with unsupervised machine learning to invert the diffuse scattering in conventional x-ray reciprocal-space mapping into real-space images of polar and elastic textures in thin epitaxial films. We first demonstrate our imaging in PbTiO3/SrTiO3 superlattices to be consistent with published phase-field model calculations. We then visualize strain-induced ferroelastic domains emerging during the metal-insulator transition in Ca2RuO4 thin films. Instead of homogeneously transforming into a low-temperature structure (like in bulk), the strained Mott insulator splits into nanodomains with alternating lattice constants, as confirmed by cryogenic scanning transmission electron microscopy. Our study reveals the type, size, orientation, and crystal displacement field of the nano-textures. The non-destructive imaging of textures promises to improve models for their dynamics and enable advances in quantum materials and microelectronics.

The layer stacking order in 2D materials strongly affects functional properties and holds promise for next generation electronic devices. In bulk, octahedral MoTe$_2$ possesses two stacking arrangements, the Weyl semimetal T$_d$ phase, and the higher-order topological insulator 1T' phase; however, it remains unclear if thin exfoliated flakes of MoTe$_2$ follow the T$_d$, 1T', or an alternative stacking sequence. Here, we resolve this debate using atomic-resolution imaging within the transmission electron microscope. We find that the layer stacking in thin flakes of MoTe$_2$ is highly disordered and pseudo-random, which we attribute to intrinsic confinement effects. Conversely, WTe$_2$, which is isostructural and isoelectronic to MoTe$_2$, displays ordered stacking even for thin exfoliated flakes. Our results are important for understanding the quantum properties of MoTe$_2$ devices, and suggest that thickness may be used to alter the layer stacking in other 2D materials., Comment: 6 figures, 2 tables

The discrete Fourier transform is among the most routine tools used in high-resolution scanning / transmission electron microscopy (S/TEM). However, when calculating a Fourier transform, periodic boundary conditions are imposed and sharp discontinuities between the edges of an image cause a cross patterned artifact along the reciprocal space axes. This artifact can interfere with the analysis of reciprocal lattice peaks of an atomic resolution image. Here we demonstrate that the recently developed Periodic Plus Smooth Decomposition technique provides a simple, efficient method for reliable removal of artifacts caused by edge discontinuities. In this method, edge artifacts are reduced by subtracting a smooth background that solves Poisson's equation with boundary conditions set by the image's edges. Unlike the traditional windowed Fourier transforms, Periodic Plus Smooth Decomposition maintains sharp reciprocal lattice peaks from the image's entire field of view.

This article introduces a training simulator for electron beam alignment using Ronchigrams. The interactive web application, www.ronchigram.com, is an advanced educational tool aimed at making scanning transmission electron microscopy (STEM) more accessible and open. For experienced microscopists, the tool offers on-hand quantification of simulated Ronchigrams and their resolution limits., Comment: 6 pages, 3 figures

The Ruddlesden-Popper ($A_{n+1}B_{n}\text{O}_{3n+1}$) compounds are a highly tunable class of materials whose functional properties can be dramatically impacted by their structural phase $n$. The negligible energetic differences associated with forming a sample with a single value of $n$ versus a mixture of $n$ makes the growth of these materials difficult to control and can lead to local atomic-scale structural variation arising from small stoichiometric deviations. In this work, we present a Python analysis platform to detect, measure, and quantify the presence of different $n$-phases based on atomic-resolution scanning transmission electron microscopy (STEM) images in a statistically rigorous manner. We employ phase analysis on the 002 Bragg peak to identify horizontal Ruddlesden-Popper faults which appear as regions of high positive compressive strain within the lattice image, allowing us to quantify the local structure. Our semi-automated technique offers statistical advantages by considering effects of finite projection thickness, limited fields of view, and precise sampling rates. This method retains the real-space distribution of layer variations allowing for a spatial mapping of local $n$-phases, enabling both quantification of intergrowth occurrence as well as qualitative description of their distribution, opening the door to new insights and levels of control over a range of layered materials., Comment: 8 pages, 5 figures

The rare-earth nickelates possess a diverse set of collective phenomena including metal-to-insulator transitions, magnetic phase transitions, and, upon chemical reduction, superconductivity. Here, we demonstrate epitaxial stabilization of layered nickelates in the Ruddlesden-Popper form, Nd$_{n+1}$Ni$_n$O$_{3n+1}$, using molecular beam epitaxy. By optimizing the stoichiometry of the parent perovskite NdNiO$_3$, we can reproducibly synthesize the $n = 1 - 5$ member compounds. X-ray absorption spectroscopy at the O $K$ and Ni $L$ edges indicate systematic changes in both the nickel-oxygen hybridization level and nominal nickel filling from 3$d^8$ to 3$d^7$ as we move across the series from $n = 1$ to $n = \infty$. The $n = 3 - 5$ compounds exhibit weakly hysteretic metal-to-insulator transitions with transition temperatures that depress with increasing order toward NdNiO$_3$ ($n = \infty)$., Comment: 11 pages, 4 figures with Supplemental Information

The occurrence of superconductivity in proximity to various strongly correlated phases of matter has drawn extensive focus on their normal state properties, to develop an understanding of the state from which superconductivity emerges. The recent finding of superconductivity in layered nickelates raises similar interests. However, transport measurements of doped infinite-layer nickelate thin films have been hampered by materials limitations of these metastable compounds - in particular, a relatively high density of extended defects. Here, by moving to a substrate (LaAlO$_{3}$)$_{0.3}$(Sr$_{2}$TaAlO$_{6}$)$_{0.7}$ which better stabilizes the growth and reduction conditions, we can synthesize the doping series of Nd$_{1-x}$Sr$_{x}$NiO$_{2}$ essentially free from extended defects. This enables the first examination of the 'intrinsic' temperature and doping dependent evolution of the transport properties. The normal state resistivity exhibits a low-temperature upturn in the underdoped regime, linear behavior near optimal doping, and quadratic temperature dependence for overdoping. This is strikingly similar to the copper oxides, despite key distinctions - namely the absence of an insulating parent compound, multiband electronic structure, and a Mott-Hubbard orbital alignment rather than the charge-transfer insulator of the copper oxides. These results suggest an underlying universality in the emergent electronic properties of both superconducting families.

Nickel-based superconductors provide a long-awaited experimental platform to explore possible cuprate-like superconductivity. Despite similar crystal structure and $d$ electron filling, these systems exhibit several differences. Nickelates are the most polar layered oxide superconductor, raising questions about the interface between substrate and thin film -- thus far the only sample geometry to successfully stabilize superconductivity. We conduct a detailed experimental and theoretical study of the prototypical interface between Nd$_{1-x}$Sr$_x$NiO$_2$ and SrTiO$_3$. Atomic-resolution electron energy loss spectroscopy in the scanning transmission electron microscope reveals the formation of a single intermediate Nd(Ti,Ni)O$_3$ layer. Density functional theory calculations with a Hubbard $U$ term show how the observed structure alleviates the strong polar discontinuity. We explore effects of oxygen occupancy, hole doping, and cation structure to disentangle the contributions of each for reducing interface charge density. Resolving the nontrivial interface structure will be instructive for future synthesis of nickelate films on other substrates and in vertical heterostructures.

Since the discovery of high-temperature superconductivity in the copper oxide materials, there have been sustained efforts to both understand the origins of this phase and discover new cuprate-like superconducting materials. One prime materials platform has been the rare-earth nickelates and indeed superconductivity was recently discovered in the doped compound Nd$_{0.8}$Sr$_{0.2}$NiO$_2$. Undoped NdNiO$_2$ belongs to a series of layered square-planar nickelates with chemical formula Nd$_{n+1}$Ni$_n$O$_{2n+2}$ and is known as the 'infinite-layer' ($n = \infty$) nickelate. Here, we report the synthesis of the quintuple-layer ($n = 5$) member of this series, Nd$_6$Ni$_5$O$_{12}$, in which optimal cuprate-like electron filling ($d^{8.8}$) is achieved without chemical doping. We observe a superconducting transition beginning at $\sim$13 K. Electronic structure calculations, in tandem with magnetoresistive and spectroscopic measurements, suggest that Nd$_6$Ni$_5$O$_{12}$ interpolates between cuprate-like and infinite-layer nickelate-like behavior. In engineering a distinct superconducting nickelate, we identify the square-planar nickelates as a new family of superconductors which can be tuned via both doping and dimensionality., Comment: 21 pages, 4 figures