We report the observation of superconductivity in infinite-layer Ca-doped LaNiO 2 (La 1− x Ca x NiO 2 ) thin films and construct their phase diagram. Unlike the metal-insulator transition in Nd- and Pr-based nickelates, the undoped and underdoped La 1− x Ca x NiO 2 thin films are entirely insulating from 300 K down to 2 K. A superconducting dome is observed at 0.15 < x < 0.3 with weakly insulating behavior at the overdoped regime. Moreover, the sign of the Hall coefficient R H changes at low temperature for samples with a higher doping level. However, distinct from the Nd- and Pr-based nickelates, the R H -sign-change temperature remains at around 35 K as the doping increases, which begs further theoretical and experimental investigation to reveal the role of the 4f orbital to the (multi)band nature of the superconducting nickelates. Our results also emphasize a notable role of lattice correlation on the multiband structures of the infinite-layer nickelates.
Electrolyte gating is widely used to induce large carrier density modulation on solid surfaces to explore various properties. Most of past works have attributed the charge modulation to electrostatic field effect. However, some recent reports have argued that the electrolyte gating effect in VO2, TiO2 and SrTiO3 originated from field-induced oxygen vacancy formation. This gives rise to a controversy about the gating mechanism, and it is therefore vital to reveal the relationship between the role of electrolyte gating and the intrinsic properties of materials. Here, we report entirely different mechanisms of electrolyte gating on two high-Tc cuprates, NdBa2Cu3O7-δ (NBCO) and Pr2-xCexCuO4 (PCCO), with different crystal structures. We show that field-induced oxygen vacancy formation in CuO chains of NBCO plays the dominant role while it is mainly an electrostatic field effect in the case of PCCO. The possible reason is that NBCO has mobile oxygen in CuO chains while PCCO does not. Our study helps clarify the controversy relating to the mechanism of electrolyte gating, leading to a better understanding of the role of oxygen electro migration which is very material specific.Keywords: electrolyte gating, cuprates, superconductors, ionic liquid, oxygen migration, electrostatic field effect 3 The electronic properties of strongly-correlated materials depend strongly on carrier density and experiments where the carrier density is modified systematically are improtant for understanding the fundamental physics behind the studied material phenomena. 1 Electric field effect induced charge modulation is preferred to chemical doping because the carrier density can be tuned quasi-continuously without introducing chemical or structural changes. [1][2][3][4][5][6][7][8][9][10] However, the electric field effect on the stronglycorrelated materials remains small in the field effect transistor (FET) devices using conventional solid dielectric layers. This is because most of the phenomena occur at carrier densities exceeding 10 14 cm -2 , which is not easy to achieve without extremely high K layers. For instance, only several Kelvin of Tc shift was realized for high-Tc cuprates using solid dielectrics. [2][3][4][5][6][7] The situation was changed recently when the technique of electrolyte gating using ionic liquids (ILs) as gate dielectrics was introduced. 11-13 Based on the principle of electrochemical capacitor, ions in the ILs are separated and accumulated on the solid surface of both electrodes under gate voltages. Meanwhile, opposite charges of equivalent density accumulate on the electrode side, creating an electric double-layer (EDL) effectively working as an interface capacitor. 14 The nanoscale separation of the EDL results in ultrahigh electric field of the order of 10 MV cm -1 with induced carrier density up to 10 15 cm -2 . This technique has been used to induce insulator-to-metal transition, 13, 15-17 superconductivity 18-22 and other electronic phase transitions. [23][24][25][26][27] Continuous superconductor...
Electro‐optic modulators are among the most important building blocks in optical communication networks. Lithium niobate, for example, has traditionally been widely used to fabricate high‐speed optical modulators due to its large Pockels effect. Another material, barium titanate, nominally has a 50 times stronger r‐parameter and would ordinarily be a more attractive material choice for such modulators or other applications. In practice, barium titanate thin films for optical waveguide devices are usually grown on magnesium oxide due to its low refractive index, allowing vertical mode confinement. However, the crystal quality is normally degraded. Here, a group of scandate‐based substrates with small lattice mismatch and low refractive index compared to that of barium titanate is identified, thus concurrently satisfying high crystal quality and vertical optical mode confinement. This work provides a platform for nonlinear on‐chip optoelectronics and can be promising for waveguide‐based optical devices such as Mach–Zehnder modulators, wavelength division multiplexing, and quantum optics‐on‐chip.
Nickel-based complex oxides have served as a playground for decades in the quest for a copper-oxide analog of the high-temperature superconductivity. They may provide clues towards understanding the mechanism and an alternative route for high-temperature superconductors. The recent discovery of superconductivity in the infinite-layer nickelate thin films has fulfilled this pursuit. However, material synthesis remains challenging, direct demonstration of perfect diamagnetism is still missing, and understanding of the role of the interface and bulk to the superconducting properties is still lacking. Here, we show high-quality Nd0.8Sr0.2NiO2 thin films with different thicknesses and demonstrate the interface and strain effects on the electrical, magnetic and optical properties. Perfect diamagnetism is achieved, confirming the occurrence of superconductivity in the films. Unlike the thick films in which the normal-state Hall-coefficient changes signs as the temperature decreases, the Hall-coefficient of films thinner than 5.5 nm remains negative, suggesting a thickness-driven band structure modification. Moreover, X-ray absorption spectroscopy reveals the Ni-O hybridization nature in doped infinite-layer nickelates, and the hybridization is enhanced as the thickness decreases. Consistent with band structure calculations on the nickelate/SrTiO3 heterostructure, the interface and strain effect induce a dominating electron-like band in the ultrathin film, thus causing the sign-change of the Hall-coefficient.
Controlling oxygen deficiencies is essential for the development of novel chemical and physical properties such as high-T c superconductivity and low-dimensional magnetic phenomena. Among reduction methods, topochemical reactions using metal hydrides (e.g., CaH2) are known as the most powerful method to obtain highly reduced oxides including Nd0.8Sr0.2NiO2 superconductor, though there are some limitations such as competition with oxyhydrides. Here we demonstrate that electrochemical protonation combined with thermal dehydration can yield highly reduced oxides: SrCoO2.5 thin films are converted to SrCoO2 by dehydration of HSrCoO2.5 at 350 °C. SrCoO2 forms square (or four-legged) spin tubes composed of tetrahedra, in contrast to the conventional infinite-layer structure. Detailed analyses suggest the importance of the destabilization of the SrCoO2.5 precursor by electrochemical protonation that can greatly alter reaction energy landscape and its gradual dehydration (H1–x SrCoO2.5–x/2) for the SrCoO2 formation. Given the applicability of electrochemical protonation to a variety of transition metal oxides, this simple process widens possibilities to explore novel functional oxides.
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