arXiv subjects

Berit H. Goodge

Publications and source records attributed to Berit H. Goodge.

49 records · Page 3Linked to original sources

arXiv2022

Reconstructing the polar interface of infinite-layer nickelate thin films

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.

cond-mat.supr-con↗

arXiv2021

Superconductivity in a quintuple-layer square-planar nickelate

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.

cond-mat.supr-con↗

arXiv2021

Nickelate superconductivity without rare-earth magnetism: (La,Sr)NiO$_{2}$

The observation of superconductivity in infinite layer nickelate (Nd,Sr)NiO$_{2}$ thin films has led to rapid theoretical and experimental investigations of these copper-oxide-analogue systems [1-15]. Superconductivity has also been found in (Pr,Sr)NiO$_{2}$ [16,17], but not previously in (La,Sr)NiO$_{2}$ [2], raising a fundamental question whether superconductivity is associated with the presence of rare-earth moments [18,19]. Here we show that with significant materials optimization, substantial portions of the La$_{1-x}$Sr$_{x}$NiO$_{2}$ phase diagram can enter the regime of coherent low-temperature transport ($x$ = 0.14 - 0.20), with subsequent superconducting transitions and a maximum onset of ~ 9 K at $x$ = 0.20. Additionally, we observe the unexpected indication of a superconducting ground state in undoped LaNiO$_{2}$, which likely reflects the self-doped nature of the electronic structure. Combining the results of (La/Pr/Nd)$_{1-x}$Sr$_{x}$NiO$_{2}$ reveals a generalized superconducting dome, characterized by systematic shifts in the unit cell volume and in the relative electron-hole populations across the lanthanides.

cond-mat.supr-con↗

arXiv2020

Isotropic Pauli-Limited Superconductivity in the Infinite Layer Nickelate Nd$_{0.775}$Sr$_{0.225}$NiO$_{2}$

The recent observation of superconductivity in thin film infinite-layer nickelates$^{1-3}$ offers a different angle to investigate superconductivity in layered oxides$^{4}$. A wide range of candidate models have been proposed$^{5-10}$, emphasizing single- or multi-orbital electronic structure, Kondo or Hund's coupling, and analogies to cuprates. Clearly, further experimental characterization of the superconducting state is needed to develop a full understanding of the nickelates. Here we use magnetotransport measurements to probe the superconducting anisotropy in Nd$_{0.775}$Sr$_{0.225}$NiO$_{2}$. We find that the upper critical field is surprisingly isotropic at low temperatures despite the layered crystal structure. In a magnetic field the superconductivity is strongly Pauli-limited, such that the paramagnetic effect dominates over orbital de-pairing. Underlying this isotropic response is a substantial anisotropy in the superconducting coherence length, which is at least four times longer in-plane than out-of-plane. A prominent low-temperature upturn in the upper critical field indicates the presence of an unconventional ground state.

cond-mat.supr-con↗

arXiv2020

a-axis YBa2Cu3O7-x/PrBa2Cu3O7-x/YBa2Cu3O7-x trilayers with subnanometer rms roughness

We demonstrate a-axis YBa2Cu3O7-x/PrBa2Cu3O7-x/YBa2Cu3O7-x trilayers grown on (100) LaAlO3 substrates with improved interface smoothness. The trilayers are synthesized by ozone-assisted molecular-beam epitaxy. The thickness of the PrBa2Cu3O7-x layer is held constant at 8 nm and the thickness of the YBa2Cu3O7-x layers is varied from 24 nm to 100 nm. X-ray diffraction measurements show all trilayers to have >95% a-axis content. The rms roughness of the thinnest trilayer is < 0.7 nm and this roughness increases with the thickness of the YBa2Cu3O7-x layers. The thickness of the YBa2Cu3O7-x layers also affects the transport properties: while all samples exhibit an onset of the superconducting transition at and above 85 K, the thinner samples show wider transition widths, ΔTc. High-resolution scanning transmission electron microscopy reveals coherent and chemically sharp interfaces, and that growth begins with a cubic (Y,Ba)CuO3-x perovskite phase that transforms into a-axis oriented YBa2Cu3O7-x as the substrate temperature is ramped up.

cond-mat.supr-con↗

arXiv2020

Atomic-resolution elemental mapping at cryogenic temperatures enabled by direct electron detection

Spectroscopic mapping by scanning transmission electron microscopy coupled with electron energy loss spectroscopy (STEM-EELS) is a powerful technique for determining the structure and chemistry of a wide range of materials and interfaces. The extension of this technique to cryogenic temperatures opens the door to new experiments across many fields including materials physics, energy storage and conversion, and biology. Such experiments, however, often face signal limitations due to sample sensitivity or the need for rapid data acquisition under less stable cryogenic conditions. Compared to traditional indirect detection systems such as charge coupled devices (CCDs), direct electron detectors (DEDs) offer improved detective quantum efficiencies, narrower point spread functions, and superior signal-to-noise ratios. Here, we compare the performance of a Gatan K2 Summit DED to an UltraScan 1000 CCD for use in signal-limited atomic-resolution STEM-EELS experiments. Due to its improved point spread function, the DED's energy resolution remains comparable to that of the CCD at a 5 times lower dispersion, providing simultaneous access to a much broader total energy range without sacrificing spectral resolution. More importantly, the benefits of direct detection enable a variety of low-signal experiments, including atomic-resolution mapping of minor and high energy edges such as the La-M$_{2,3}$ edge at 1123 eV and the Bi-M$_{4,5}$ edge at 2580 eV. For rapid acquisitions at 400 spectra per second, elemental maps recorded with the DED show an up to 40 percent increase in atomic lattice fringe contrast compared to those acquired with the CCD. Taking advantage of these performance improvements and the fast readout of the K2 DED, we use direct detection STEM-EELS to demonstrate atomic-resolution elemental mapping at cryogenic temperatures.

cond-mat.mtrl-sci↗

arXiv2020

A superconducting praseodymium nickelate with infinite layer structure

A variety of nickel oxide compounds have long been studied for their manifestation of various correlated electron phenomena. Recently, superconductivity was observed in nanoscale infinite layer nickelate thin films of Nd$_{0.8}$Sr$_{0.2}$NiO$_2$, epitaxially stabilized on SrTiO$_3$ substrates via topotactic reduction from the perovskite precursor phase. Here we present the synthesis and properties of PrNiO$_2$ thin films on SrTiO$_3$. Upon doping in Pr$_{0.8}$Sr$_{0.2}$NiO$_2$, we observe superconductivity with a transition temperature of 7-12 K, and robust critical current density at 2 K of 334 kA/cm$^2$. These findings indicate that superconductivity in the infinite layer nickelates is relatively insensitive to the details of the rare earth 4$f$ configuration. Furthermore, they motivate the exploration of a broader family of compounds based on two-dimensional NiO$_2$ planes, which will enable systematic investigation of the superconducting and normal state properties and their underlying mechanisms.

cond-mat.supr-con↗

arXiv2020

Superconducting Dome in Nd$_{1-x}$Sr$_x$NiO$_2$ Infinite Layer Films

We report the phase diagram of Nd$_{1-x}$Sr$_x$NiO$_2$ infinite layer thin films grown on SrTiO$_3$. A superconducting dome spanning $0.125 < x < 0.25$ is found, remarkably similar to cuprates, albeit over a narrower doping window. However, while cuprate superconductivity is bounded by an insulator for underdoping and a metal for overdoping, here we observe weakly insulating behavior on either side of the dome. Furthermore, the normal state Hall coefficient is always small and proximate to a continuous zero crossing in doping and in temperature, in contrast to the $\sim 1/x$ dependence observed for cuprates. This suggests the presence of both electron- and hole-like bands, consistent with band structure calculations.

cond-mat.supr-con↗

arXiv2020

Strain-stabilized superconductivity

Superconductivity is among the most fascinating and well-studied quantum states of matter. Despite over 100 years of research, a detailed understanding of how features of the normal-state electronic structure determine superconducting properties has remained elusive. For instance, the ability to deterministically enhance the superconducting transition temperature by design, rather than by serendipity, has been a long sought-after goal in condensed matter physics and materials science, but achieving this objective may require new tools, techniques and approaches. Here, we report the first instance of the transmutation of a normal metal into a superconductor through the application of epitaxial strain. We demonstrate that synthesizing RuO$_{2}$ thin films on (110)-oriented TiO$_{2}$ substrates enhances the density of states near the Fermi level, which stabilizes superconductivity under strain, and suggests that a promising strategy to create new transition-metal superconductors is to apply judiciously chosen anisotropic strains that redistribute carriers within the low-energy manifold of $d$ orbitals.

cond-mat.supr-con↗

arXiv2020

Aspects of the Synthesis of Thin Film Superconducting Infinite-Layer Nickelates

The recent observation of superconductivity in Nd$_{0.8}$Sr$_{0.2}$NiO$_{2}$ calls for further investigation and optimization of the synthesis of this metastable infinite-layer nickelate structure. Here, we present our current understanding of important aspects of the growth of the parent perovskite compound via pulsed laser deposition on SrTiO$_{3}$ (001) substrates, and the subsequent topotactic reduction. We find that to achieve single-crystalline, single-phase superconducting Nd$_{0.8}$Sr$_{0.2}$NiO$_{2}$, it is essential that the precursor perovskite Nd$_{0.8}$Sr$_{0.2}$NiO$_{3}$ thin film is stabilized with high crystallinity and no impurity phases; in particular, a Ruddlesden-Popper-type secondary phase is often observed. We have further investigated the evolution of the soft-chemistry topotactic reduction conditions to realize full transformation to the infinite-layer structure with no film decomposition or formation of other phases. We find that capping the nickelate film with a subsequent SrTiO$_{3}$ layer provides an epitaxial template to the top region of the nickelate film, much like the substrate. Thus, for currently optimized growth conditions, we can stabilize superconducting single-phase Nd$_{0.8}$Sr$_{0.2}$NiO$_{2}$ (001) epitaxial thin films up to ~ 10 nm.

cond-mat.mtrl-sci↗

arXiv2020

Doping evolution of the Mott-Hubbard landscape in infinite-layer nickelates

The recent observation of superconductivity in Nd$_{0.8}$Sr$_{0.2}$NiO$_2$ has raised fundamental questions about the hierarchy of the underlying electronic structure. Calculations suggest that this system falls in the Mott-Hubbard regime, rather than the charge-transfer configuration of other nickel oxides and the superconducting cuprates. Here, we use state-of-the-art, locally-resolved electron energy loss spectroscopy to directly probe the Mott-Hubbard character of Nd$_{1-x}$Sr$_x$NiO$_2$. Upon doping, we observe emergent hybridization reminiscent of the Zhang-Rice singlet via the oxygen-projected states, modification of the Nd 5$d$ states, and the systematic evolution of Ni 3$d$ hybridization and filling. These results clearly evidence the multiband nature of this system and the distinct electronic landscape for infinite-layer nickelates despite their formal similarity to the cuprates.

cond-mat.supr-con↗

arXiv2020

Atomic-resolution cryo-STEM across continuously variable temperature

Atomic-resolution cryogenic scanning transmission electron microscopy (cryo-STEM) has provided a path to probing the microscopic nature of select low-temperature phases in quantum materials. Expanding cryo-STEM techniques to broadly tunable temperatures will give access to the rich temperature-dependent phase diagrams of these materials. With existing cryo-holders, however, variations in sample temperature significantly disrupt the thermal equilibrium of the system, resulting in large-scale sample drift. The ability to tune temperature without negatively impacting the overall instrument stability is crucial, particularly for high-resolution experiments. Here, we test a new side-entry continuously variable temperature dual-tilt cryo-holder which integrates liquid nitrogen cooling with a 6-pin MEMS sample heater to overcome some of these experimental challenges. We measure consistently low drift rates of 0.3-0.4 Angstrom/s and demonstrate atomic-resolution cryo-STEM imaging across a continuously variable temperature range from ~100 K to well above room temperature. We conduct additional drift stability measurements across several commercial sample stages and discuss implications for further developments of ultra-stable, flexible cryo-stages.

physics.app-ph↗

arXiv2018

A high-energy density antiferroelectric made by interfacial electrostatic engineering

Dielectric capacitors hold a tremendous advantage for energy storage due to their fast charge/discharge times and stability in comparison to batteries and supercapacitors. A key limitation to today's dielectric capacitors, however, is the low storage capacity of conventional dielectric materials. To mitigate this issue, antiferroelectric materials have been proposed, but relatively few families of antiferroelectric materials have been identified to date. Here, we propose a new design strategy for the construction of lead-free antiferroelectric materials using interfacial electrostatic engineering. We begin with a ferroelectric material with one of the highest known bulk polarizations, BiFeO3. We show that by confining atomically-precise thin layers of BiFeO3 in a dielectric matrix that we can induce a metastable antiferroelectric structure. Application of an electric field reversibly switches between this new phase and a ferroelectric state, in addition, tuning of the dielectric layer causes coexistence of the ferroelectric and antiferroelectric states. Precise engineering of the structure generates an antiferroelectric phase with energy storage comparable to that of the best lead-based materials. The use of electrostatic confinement provides a new pathway for the design of engineered antiferroelectric materials with large and potentially coupled responses.

cond-mat.mtrl-sci↗