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We report the phase diagram of Nd1-xSrxNiO2 infinite layer thin films grown on SrTiO3. 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 ~1/x dependence observed for cuprates. This suggests the presence of both electron-and hole-like bands, consistent with band structure calculations.

The recent observation of superconductivity in Nd0.8Sr0.2NiO2 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 SrTiO3 (001) substrates, and the subsequent topotactic reduction.We find that to achieve single-crystalline, single-phase superconducting Nd0.8Sr0.2NiO2, it is essential that the precursor perovskite Nd0.8Sr0.2NiO3 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 SrTiO3 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 Nd0.8Sr0.2NiO2 (001) epitaxial thin films up to ~ 10 nm. ________________________ a kyuho@stanford.edu b denverli@stanford.edu

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 Nd0.8Sr0.2NiO2, epitaxially stabilized on SrTiO3 substrates via topotactic reduction from the perovskite precursor phase. Here, we present the synthesis and properties of PrNiO2 thin films on SrTiO3. Upon doping in Pr0.8Sr0.2NiO2, we observe superconductivity with a transition temperature of 7–12 K and robust critical current density at 2 K of 334 kA/cm2. These findings indicate that superconductivity in the infinite layer nickelates is relatively insensitive to the details of the rare earth 4f configuration. Furthermore, they motivate the exploration of a broader family of compounds based on two-dimensional NiO2 planes, which will enable systematic investigation of the superconducting and normal state properties and their underlying mechanisms.

It has led to considerable theoretical focus on this distinction. [20,21] Across the lanthanide series, systematic studies of the infinite layer nickelates using density-functional theory (DFT)-based approaches have generally shown a smooth evolution of the electronic structure, with little qualitative dissimilarity. [22,23] While these studies treat the 4f electrons as part of the lanthanide core, others have investigated 4f-orbital hybridization explicitly to consider their potential role. [20,21] In parallel with these theoretical efforts, significant advances have been made in the understanding and optimization of thin film synthesis of the Nd/Pr based systems. [12,17] Important aspects include improving the crystallinity of the strained perovskite precursor phase, as well as the subsequent oxygen deintercalation reaction. As a result, the magnitude of the normal state resistivity for optimized samples is significantly lower than in several prior reports, [24][25][26][27] consistent with these advances in materials quality. Furthermore, a key observation is that the low-temperature normal state resistivity in Nd/Pr based infinite-layer nickelate films must be below the Mott-Ioffe-Regel limit to exhibit a superconducting transition. [13,18] Given the conceptual importance of the presence or absence of superconductivity in (La,Sr) NiO 2 , and recent materials improvements, we have revisited the synthesis of La 1−x Sr x NiO 2 thin films epitaxially stabilized on SrTiO 3 substrates and the study of its phase diagram. UponThe occurrence of unconventional superconductivity in cuprates has long motivated the search for manifestations in other layered transition metal oxides. Recently, superconductivity is found in infinite-layer nickelate (Nd,Sr) NiO 2 and (Pr,Sr)NiO 2 thin films, formed by topotactic reduction from the perovskite precursor phase. A topic of much current interest is whether rare-earth moments are essential for superconductivity in this system. In this study, it is found 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, the unexpected indication of a superconducting ground state in undoped LaNiO 2 is observed, 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.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.202104083.

A defining feature of emergent phenomena in complex oxides is the competition and cooperation between ground states. In manganites, the balance between metallic and insulating phases can be tuned by the lattice; extending the range of lattice control would enhance the ability to access other phases. We stabilized uniform extreme tensile strain in nanoscale La0.7Ca0.3MnO3 membranes, exceeding 8% uniaxially and 5% biaxially. Uniaxial and biaxial strain suppresses the ferromagnetic metal at distinctly different strain values, inducing an insulator that can be extinguished by a magnetic field. Electronic structure calculations indicate that the insulator consists of charge-ordered Mn4+ and Mn3+ with staggered strain-enhanced Jahn-Teller distortions within the plane. This highly tunable strained membrane approach provides a broad opportunity to design and manipulate correlated electron states.

Advances in complex oxide heteroepitaxy have highlighted the enormous potential of utilizing strain engineering via lattice mismatch to control ferroelectricity in thin-film heterostructures. This approach, however, lacks the ability to produce large and continuously variable strain states, thus limiting the potential for designing and tuning the desired properties of ferroelectric films. Here, we observe and explore dynamic strain-induced ferroelectricity in SrTiO 3 by laminating freestanding oxide films onto a stretchable polymer substrate. Using a combination of scanning probe microscopy, optical second harmonic generation measurements, and atomistic modeling, we demonstrate robust room-temperature ferroelectricity in SrTiO 3 with 2.0% uniaxial tensile strain, corroborated by the notable features of 180°ferroelectric domains and an extrapolated transition temperature of 400 K. Our work reveals the enormous potential of employing oxide membranes to create and enhance ferroelectricity in environmentally benign lead-free oxides, which hold great promise for applications ranging from non-volatile memories and microwave electronics.

We report the phase diagram of infinite layer Pr 1-x Sr x NiO 2 thin films synthesized via topotactic reduction from the perovskite precursor phase using CaH 2 . Based on the electrical transport properties, we find a doping-dependent superconducting dome extending between x = 0.12 and 0.28, with a maximum superconducting transition temperature T c of 14 K at x = 0.18, bounded by weakly insulating behavior on both sides. In contrast to the narrower dome observed in Nd 1-x Sr x NiO 2 , a local T c suppression near x = 0.2 was not observed for the Pr 1-x Sr x NiO 2 system. Normal state Hall effect measurements indicate mixed carrier contributions of both electrons and holes, and show a sign change in the Hall coefficient as functions of temperature and x, quite similar to that in Nd 1-x Sr x NiO 2 . Also similar is the observation of a minimum in the normal state resistivity associated with the superconducting compositions. These findings indicate an infinite layer nickelate phase diagram that is relatively insensitive to the rare-earth element, but suggest that disorder arising from the variations of the ionic radii on the rare-earth site affects the superconducting dome. High-T c cuprates are unusual materials systems, marked by a rather complex phase diagram enriched with intertwined electronic orders [1,2]. Despite exhibiting a generic doping-dependent phase diagram with common features, which many consider a hallmark of the physics of the CuO 2 planes [1], cuprates of various crystal structures [3-7] show rather diverse properties, manifested by different configurations of Cu-O stacking layers [8-10], a remarkably wide range of transition