Federico Baiutti

High-temperature superconductivity in space-charge regions of lanthanum cuprate induced by two-dimensional doping.

The exploitation of interface effects turned out to be a powerful tool for generating exciting material properties. Such properties include magnetism, electronic and ionic transport and even superconductivity. Here, instead of using conventional homogeneous doping to enhance the hole concentration in lanthanum cuprate and achieve superconductivity, we replace single LaO planes with SrO dopant planes using atomic-layer-by-layer molecular beam epitaxy (two-dimensional doping). Electron spectroscopy and microscopy, conductivity measurements and zinc tomography reveal such negatively charged interfaces to induce layer-dependent superconductivity (Tc up to 35 K) in the space-charge zone at the side of the planes facing the substrate, where the strontium (Sr) profile is abrupt. Owing to the growth conditions, the other side exhibits instead a Sr redistribution resulting in superconductivity due to conventional doping. The present study represents a successful example of two-dimensional doping of superconducting oxide systems and demonstrates its power in this field.

Towards precise defect control in layered oxide structures by using oxide molecular beam epitaxy.

Summary In this paper we present the atomic-layer-by-layer oxide molecular beam epitaxy (ALL-oxide MBE) which has been recently installed in the Max-Planck Institute for Solid State Research and we report on its present status, providing some examples that demonstrate its successful application in the synthesis of different layered oxides, with particular reference to superconducting La2CuO4 and insulator-to-metal La2− xSrxNiO4. We briefly review the ALL-oxide MBE technique and its unique capabilities in the deposition of atomically smooth single-crystal thin films of various complex oxides, artificial compounds and heterostructures, introducing our goal of pursuing a deep investigation of such systems with particular emphasis on structural defects, with the aim of tailoring their functional properties by precise defects control.

Atomic-Scale Quantitative Analysis of Lattice Distortions at Interfaces of Two-Dimensionally Sr-Doped La2CuO4 Superlattices

Using spherical aberration corrected high-resolution and analytical scanning transmission electron microscopy, we have quantitatively studied the lattice distortion and the redistribution of charges in two-dimensionally strontium (Sr)-doped La2CuO4 superlattices, in which single LaO planes are periodically replaced by SrO planes. As shown previously, such structures show Tc up to 35 K as a consequence of local charge accumulation on both sides of the nominal SrO planes position. This is caused by two distinct mechanisms of doping: heterogeneous doping at the downward side of the interface (space–charge effect) and “classical” homogeneous doping at the upward side. The comparative chemical and atomic-structural analyses reveal an interrelation between local CuO6 octahedron distortions, hole spatial distribution, and chemical composition. In particular we observe an anomalous expansion of the apical oxygen–oxygen distance in the heterogeneously doped (space–charge) region, and a substantial shrinkage of the apical oxygen–oxygen distance in the homogeneously doped region. Such findings are interpreted in terms of different Jahn–Teller effects occurring at the two interface sides (downward and upward).

Dopant size effects on novel functionalities: High-temperature interfacial superconductivity

Among the range of complex interactions, especially at the interfaces of epitaxial oxide systems, contributing to the occurrence of intriguing effects, a predominant role is played by the local structural parameters. In this study, oxide molecular beam epitaxy grown lanthanum cuprate-based bilayers (consisting of a metallic (M) and an insulating phase (I)), in which high-temperature superconductivity arises as a consequence of interface effects, are considered. With the aim of assessing the role of the dopant size on local crystal structure and chemistry, and on the interface functionalities, different dopants (Ca2+, Sr2+ and, Ba2+) are employed in the M-phase, and the M–I bilayers are investigated by complementary techniques, including spherical-aberration-corrected scanning transmission electron microscopy. A series of exciting outcomes are found: (i) the average out-of-plane lattice parameter of the bilayers is linearly dependent on the dopant ion size, (ii) each dopant redistributes at the interface with a characteristic diffusion length, and (iii) the superconductivity properties are highly dependent on the dopant of choice. Hence, this study highlights the profound impact of the dopant size and related interface chemistry on the functionalities of superconducting oxide systems.

Oxide molecular beam epitaxy of complex oxide heterointerfaces

Oxide molecular beam epitaxy (MBE) is a unique technique for the synthesis of high-quality oxide thin films and heterostructures devoted to fundamental studies and to the fabrication of nanosized devices. In recent times, such a growth method has been customized for the realization of different multicomponent oxide materials. In the present article, the most important aspects related to oxide MBE are discussed, including design, control of the growth process, different deposition schemes, advantages, and challenges of the method. A focus will be put on the synthesis of oxide heterointerfaces by using atomic layer-by-layer (ALL)-oxide MBE, an advanced deposition method which allows for designing materials down to the atomic layer level. Several successful examples of oxide heterostructures and heterointerfaces, which have been fabricated by oxide MBE, are presented, demonstrating the power of the method.

High-Temperature Thermoelectricity in LaNiO3–La2CuO4 Heterostructures

Transition metal oxides exhibit a high potential for application in the field of electronic devices, energy storage, and energy conversion. The ability of building these types of materials by atomic layer-by-layer techniques provides a possibility to design novel systems with favored functionalities. In this study, by means of the atomic layer-by-layer oxide molecular beam epitaxy technique, we designed oxide heterostructures consisting of tetragonal K2NiF4-type insulating La2CuO4 (LCO) and perovskite-type conductive metallic LaNiO3 (LNO) layers with different thicknesses to assess the heterostructure-thermoelectric property-relationship at high temperatures. We observed that the transport properties depend on the constituent layer thickness, interface intermixing, and oxygen-exchange dynamics in the LCO layers, which occurs at high temperatures. As the thickness of the individual layers was reduced, the electrical conductivity decreased and the sign of the Seebeck coefficient changed, revealing the contribution of the individual layers where possible interfacial contributions cannot be ruled out. High-resolution scanning transmission electron microscopy investigations showed that a substitutional solid solution of La2(CuNi)O4 was formed when the thickness of the constituent layers was decreased.

Atomic-Scale Quantitative and Analytical STEM Investigation of Sr-δ-Doped La 2 CuO 4 Multilayers

Superconductivity in copper oxides arises when a parent insulator compound is doped beyond some critical concentration [1]. In the case of La2CuO4 (LCO), high-Tc superconductivity is obtained either by substituting La with Sr or by inserting interstitial O [2]. At internal interfaces, the enhancement of the superconducting critical temperature is influenced by the interfacial structure [3]. Recently, by using atomic layer-by-layer oxide molecular beam epitaxy (MBE), we have fabricated Sr-δ-doped LCO multilayered structures on LaSrAlO4 (LSAO) substrate, in which some atomic planes of LaO were intentionally substituted by SrO. By varying the spacing between the LCO and SrO layers high-Tc superconductivity (~ 40 K) was obtained [4]. Here we lay emphasis on the detailed and quantitative STEM analysis.

Direct Observation of Asymmetric Sr Diffusion in Sr-δ-Doped La 2 CuO 4

Superconductivity in copper oxides arises when a parent insulator compound is doped beyond some critical concentration [1]. In the case of La2CuO4 (LCO), high-Tc superconductivity is obtained either by substituting La 3+ with Sr 2+ or by inserting interstitial O 2[2]. Latest advance of the unique capabilities of layer-by-layer oxide molecular beam epitaxy (MBE) allows controlling the structural composition down to the single atomic layer. Recently, we have fabricated Sr--doped La2CuO4 multilayered structures in which some atomic layers of LaO have been fully substituted by SrO layers. By varying the spacing between the LCO and SrO layers high-Tc superconductivity (~ 40 K) has been obtained.