Corresponding Author

Gennady Logvenov

Max Planck Institute for Solid State Research

References

1.
Bednorz, J.G.; Mueller, K.A.
Possible high Tc superconductivity in the Ba-La-Cu-O system
2.
Ohtomo, A.; Hwang, H.Y.
A high-mobility electron gas at the LaAlO3/SrTiO3 heterointerface
3.
Reyren, N.; Thiel, S.; Caviglia, A.D.; Kourkoutis, L.F.; Hammerl, G.; Richter, C.; Schneider, C.W.; Kopp, T.; Ruetschi, A.S.; Jaccard, D.; Gabay, M.; Muller, D.A.; Mannhart, J.
Superconducting interfaces between insulating oxides

Scientific Service Group "Technology"

Digital synthesis of multicomponent oxides – We can design novel materials

Authors

F. Baiutti, F. Wrobel, Ch. Dietl, G. Christiani, and G. Logvenov

Departments

Scientific Service Group "Technology"

The unique Atomic-Layer-by-Layer Oxide Molecular Beam Epitaxy (ALL-oxide MBE) allows us to deposit atomically smooth single-crystal thin films of various complex oxides. In this report, we briefly summarize the present status of the state-of-the-art ALL-oxide MBE system and suggest that this technique could be useful in the study of the physics and the chemistry of oxides and interfaces and in the research of new materials. Some examples showing how this technique can be used to synthesize various compounds, such as stable or metastable layered structures or superlattices are presented.

Introduction 

The interest and the modern progress in the synthesis of layered complex oxide compounds with high precision have been stimulated by the discovery of high-temperature superconductivity (HTS) in layered cuprates in 1986 [1]. Beside this, one must recognize that a layered structure is common to a vast family of complex oxides that exhibit a wide range of electronic and ionic properties and a multitude of structural and electronic phases as well as a large variety of different functionalities such as electrical conductivity and magnetism (two examples are La2-xSrxNiO4 and La2-xSrxCoO4 respectively).

The concept of layered structure can be extended to the "artificial" layer that one can form at the interface between different oxides phases. Great attention has been addressed to the study of interfaces in the last decades and unexpected properties have been revealed. One of the most famous examples is given by the LaAlO3/SrTiO3 interface, where a high mobility 2D-electron gas forming at the interface between the two insulating constituents has been found responsible for extremely high electrical conductivity [2] and superconductivity [3].

Molecular Beam Epitaxy provides state-of-the art thin films of complex oxides. In the field of ALL-oxide MBE, a major development was represented by the layer-by-layer deposition scheme, called ALL-MBE, which enables an extreme control of the growth process and therefore a rational material design at various levels. By stacking molecular layers of different compounds, one can form various multilayers, even with single atomic layer and/or subatomic layer precision. Within a molecular layer, one can add or omit single atomic monolayers and thus cast novel compounds.

<strong>Fig. 1:</strong> Photo of the dual chamber ALL-oxide MBE system installed in the Max-Planck Institute for Solid State Research. Zoom Image
Fig. 1: Photo of the dual chamber ALL-oxide MBE system installed in the Max-Planck Institute for Solid State Research.

For a further progress in the atomic layer synthesis of complex oxides there is an urgent need for a synergetic cross-fertilization of the chemistry and physics approaches.  Recognizing this, researchers at the MPI-FKF have pursued a long-standing program to synthesize and investigate epitaxial metal oxide thin films and heterostructures based on the ALL-oxide MBE technique.

A dual growth chamber oxide MBE system was acquired in 2012. A photo of the MBE system is shown in Fig. 1.

Recent results

By using our ALL-oxide MBE we have deposited a broad range of heterostructures, multilayers and superlattices. Over 300 growth experiments have been performed since the MBE system has been installed in Max-Planck Institute. We have experimented with La2-xSrxCuO4, La2-xSrxNiO4, LaAlO3, LaNiO3, LaMnO3, SrMnO3, La2-xBaxCuO4, and several other complex oxides. In particular, we have deposited these layered oxides structure by using layer-by-layer deposition schemes i.e. deposition one atomic layer in the time.  In most cases such heterostructures have atomically smooth interfaces.  We also stack various combinations and sequences of stable complex oxide layers and try to create a new type of materials by chemical doping in selected layers. Here we briefly overview our recent results.

<p align="left"><strong>Fig. 2:</strong> Superconducting transition of Sr delta doped LCO superlattice with N=6 measured by using a mutual inductance setup (left panel). The sketch of the Sr delta doped LCO superlattice crystal structure (right panel)</p> Zoom Image

Fig. 2: Superconducting transition of Sr delta doped LCO superlattice with N=6 measured by using a mutual inductance setup (left panel). The sketch of the Sr delta doped LCO superlattice crystal structure (right panel)

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<p align="left"><strong>Fig. 3:</strong> X-ray &theta;&minus;2&theta; scan of a (4//2) LNO&minus;LCO superlattice on LSAT substrate.</p> Zoom Image

Fig. 3: X-ray θ−2θ scan of a (4//2) LNO−LCO superlattice on LSAT substrate.

First of all we have tested layer-by-layer capabilities of our oxide MBE by synthesis of superconducting layered superlattices via replacing a single LaO layer by complete SrO layer in La2CuO4 parent structure. This so called delta doped crystal structure is sketched in Fig.2. The structure has broken inversion symmetry and can provide necessary information related to coexisting and spatial distribution of the superconducting and antiferromagnetic layers on an atomic level, at the end open the way to synthesis of metastable artificial layered oxides.

We have grown La2-xSrxNiO4/La2CuO4, LaNiO3/LaAlO3, LaNiO3/La2CuO4 superlattices. The representative X-ray diffraction data of a (4//2) 4xLaNiO3−2xLa2CuO4 superlattice on LSAT substrate is shown in Fig. 3. This structure has artificial c-axis lattice constant c=26.9Å which is different from the lattice constants of the constituent layers and could be classified as a novel metastable oxide compound. These superlattices are currently under extensive characterization. 

In the near future we will try to stack various combinations and sequences of different layered oxides, trying to induce novel ground states in the interfaces or in the oxides blocks, or to define innovative model systems for physical and chemical investigations.

Summary

The ALL-oxide MBE technique has already been successfully applied for the growth of high quality oxide compounds and superlattices.  The ALL-oxide MBE system that has been recently installed in our facility is equipped with the state-of-art technology and a high level of automation and modularity, and has already demonstrated its capabilities in the growth of different complex oxides and heterostructures. Based on our experience in the growth processes, we aim to take advantage of this powerful tool in order to grow novel metastable compounds and superlattices with unexpected functional properties which can be used as model systems for the study of the physics and the chemistry of strongly correlated oxides and interfaces.


 
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