JCJC SIMI 8 - JCJC - SIMI 8 - Chimie du solide, colloïdes, physicochimie

Geometrically confined doping in complex oxide superlattices – GeCoDo

New functional materials: an approach by geometrically confined doping

The creation of a confined conducting zone in insulating thin films, i.e. geometrically confined doping, allows to synthesize materials with emergent and functional properties . The GeCoDo project is dedicated to the synthesis and the characterization of this type of new materials.

Two-dimensional physics in doped Mott Insulators

Mott insulators are a group of materials which do not conduct electricity because of a strong repulsion between the charges in the system. If these insulators are doped with supplementary charges, for example by the formation of a solid solution, functional properties can be observed: superconductivity, magnetism or thermoelectric behavior among others. Doped Mott insulators obtained by solid solutions are the subject of a strong research activity because of the large number of systems and the multitude of their functional properties.<br />The GeCoDo project takes part in this research, but we have chosen to use a less common doping technique: the geometrically confined doping. The solid solution technique, where the doping ions are distributed homogeneously in the system, is replaced by the introduction of some atomic layers of the doping material. With this approach, the doping atoms and thus the doped charges are confined geometrically in a two dimensional zone of the Mott insulator. This confinement leads to emergent properties which are observed neither in the solid solutions nor in the parent materials. The GeCoDo project is dedicated to the synthesis and the characterization of this kind of systems.<br />

Our experimental approach for the creation of geometrically confined doping zones is the preparation of the samples by Pulsed Laser Deposition. This technique, used for the growth of thin films, allows to synthesize complex oxides, like those used in this project, of a thickness of only some atomic layers on a substrate. By ablating a pellet of the Mott insulator and another one of the doping material, we can prepare superlattices, in which the two materials are piled up in a periodic way in the growth direction. The position of the doping atoms in the Mott insulator can therefore be perfectly controlled.
Furthermore, this project brings together theoreticians and experimentalists in order to have a strong interaction between these two research directions. Systems with geometrically confined doping are poorly studied both in the theoretical and the experimental aspects. This approach is thus particularly important for a project like this one, aiming to make the link between the observed properties and the physical effects at their origin.

Three different systems have been synthesized and characterized in the framework of the GeCoDo project: LaVO3/SrVO3, PrVO3/SrVO3 and LaCrO3/SrCrO3. An emergent room temperature magnetic phase has been observed in the first system, and we have been able to identify its origin in bond length distortions in the vicinity of the doped regions. All three systems show a very low resistivity related to the reduction of the dimensionality of the charge carriers due to the geometrically confined doping. The values of the resistivity are situated among the lowest observed in complex oxide conductors and the solid solutions of the investigated systems.

The in-depth analysis of the experimentally realized systems will be carried on, as well as the synthesis and the characterization of other systems with geometrically confined doping.

The scientific production of the GeCoDo project has been concentrated on communications at international conferences, as for example an invited presentation on the latin-american workshop on magnetism in Buenos Aires, and articles in scientific journals. An important moment of this project has been the organization of a workshop dedicated to the theoretical aspects of geometrically confined doping, gathering international specialists during four days in Caen.

Doped Mott insulators build a prosperous research area since several decades for materials science and the underlying chemistry and physics. This is due to the multitude of discovered functional materials (e.g. high Tc superconductors and Colossal Magnetoresistance manganites) and to other intriguing phenomena related to the metal-to-insulator transition upon doping. One of the most interesting parts in the phase diagram is typically the region in the vicinity of the metal-insulator transition, as for a small number of charges in the system, these strongly correlated electrons exhibit additional orders, fluctuations and other effects leading to exotic phases and interesting properties.
Recently, we have evidenced a new ferromagnetic phase of the doped Mott insulator (La,Sr)VO3, which was neither observed in the solid solution, nor in the parent materials. This ferromagnetic phase, with a Curie temperature above 700K, emerged in samples with a superlattice structure LaVO3 (6 unit cells) /SrVO3 (1 unit cell) [U. Lüders et al, Phys. Rev. B 80, 241102R (2009)], and was shown to be generated by the layered structure of the material due to the synthesis in the form of thin films of the parent materials. The theoretical description of the system emphasizes the role of the geometrical confinement of the dopants in two dimensional regions of LaVO3, introducing therefore the dimensionality of the doping as another key parameter in the description of doped Mott insulators. It has to be pointed out that this ferromagnetic phase is not accessible by classical doping. Indeed, in solid solutions, the dopants are distributed homogeneously in the matrix leading to a three dimensional character of the charge carriers. Thus, geometrically confined doping (GCD) is a very powerful approach to a more complete exploration of the phase diagrams of doped Mott insulators.
Apart from these new phases to be explored, the 2D character of the dopants leads to interesting electronic properties comparable to those of polar discontinuous oxide interfaces. 2D electron gases or liquids in oxides may have an important impact on the post-Si electronics, and the understanding of the intrinsic properties of these systems is only possible in high-quality and well-controlled systems.
Within this context, we ask for funding to conduct an extensive study on the interface chemistry and the resulting properties of GCD systems of t2g Mott insulators in the superlattice geometry. The objective of this project is establishing the synthesis conditions, the structural and electronic phase diagrams, the nature and properties of the new phases by varying the involved materials, the composition and the period of the GCD superlattices. This approach marks a new area of solid state chemistry: often, the thin film form of a material is used to investigate the influence of the structure on the properties or to create new properties, but it was not used to engineer intentionally the bandwidth of electrons to access new phases with potentially interesting properties. Ordered phases with high critical temperatures are expected.in analogy to the GCD (La,Sr)VO3 system.
The remarkable combination at the CRISMAT of techniques for the growth of high-quality oxide superlattices, the characterisation of their structure, magnetic and transport properties, as well as theoretical techniques assures the feasibility of this project in one laboratory only. The coordinator and the participants of this project are recognized experts in the field of complex oxides, so that apart from the technical prerequisites, also the scientific knowledge is present to succeed in this leading edge and complex project.



Project coordination

Ulrike LÜDERS (CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE DELEGATION REGIONALE NORMANDIE) – ulrike.luders@ensicaen.fr

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

CNRS - CRISMAT CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE DELEGATION REGIONALE NORMANDIE

Help of the ANR 185,006 euros
Beginning and duration of the scientific project: October 2011 - 36 Months

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