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Spin and Orbital Physics : Research of Advanced New Oxides

Final Report Summary - SOPRANO (Spin and Orbital Physics : Research of Advanced New Oxides)

Within SOPRANO, 34 fellows (27 ESRs and 7 ERs) have finally been hired. They have been working on a large number of scientific topics in the field of transition metal oxides. In this family of materials, the strong interplay between spin, lattice and charges can generate interesting physical properties with potential applications. Among them, superconductivity, multiferroïcity, thermoelectricity and magneto-caloric effect are a few examples of these potentialities. To understand and optimize these properties, it is essential to perform a detailed analysis of the relationship between the crystallographic structure, the spin, valence and orbital filling of the transition metal oxides in these structures and the macroscopic properties that can be obtained depending on the structure. This was the major goal of SOPRANO: to reinforce the exchanges between the spectroscopic analysis (X-Ray absorption spectroscopy, photoemission, Raman spectroscopy…) and the macroscopic properties (magnetism, resistivity, dielectric constant, thermopower…), in materials synthesized by a large variety of techniques (to be able to stabilize different valence and spin states), and in different forms (as crystals, ceramics or thin films), these materials being characterized by X-Ray, neutron diffraction and High Resolution Electron Microscopy.

As initially planned, the fellows have benefited from different experimental techniques within all the laboratories or at large scale facilities. Various materials have been synthesized (layered materials deriving from NaxCoO2, rutiles, hollandites, perovskites, Ruddelsden – Popper phases, oxypnictides, selenides) and investigated for different properties all coming from the complex interplay between spin, charge and orbital inside these materials : insulator to metal transition, ordering phenomena and possible charge ordering, high dielectric constant, thermoelectric and magneto-caloric properties, materials for batteries, superconductivity… Some fellows have also developped theoretical calculations (band structure and local electronic structure calculations) on these materials. All the results have been presented at different conferences, and published (or under publication). One patent has been deposited about hydrothermal synthesis. The complete list of publication can be found on http://www.soprano.cnrs.fr.

Considering the very large number of fellows and topics, in the following are given some examples of the major results obtained within SOPRANO, but this list can not be exhaustive. Some materials, never synthesized so far despite many years of attempt have been stabilized by hydrothermal techniques (AgCoO2 in Timisoara), and combination of different synthesis techniques has been very powerful to prepare samples and then investigate their properties (VO2, NaxCoO2 in Bordeaux with Timisoara or comparison of hollandite systems synthesized by solid state reaction and microwave in Caen). Electrochemistry has also been used to modify doping as in K2Cr8O16 (Madrid) and thus the electronic properties. Thin films growth by Molecular Beam Epitaxy or Pulsed Laser Deposition has been used both to prepare model systems (EuO in Koeln) for spectroscopic investigation, or to prepare artificially layered materials (Ruddelsden Popper in Liverpool, YBCO/LCMO superlattices in Stuttgart), or materials with potential industrial applications such as high kappa materials (Caen with IPDiA). Physicists (Stuttgart) have benefited from the synthesis skills within the network (Liverpool) to obtain new samples and make a detailed investigation of their superconducting properties.
From a more physical point of view, work has been performed both theoretically and experimentally on hollandite systems, layered materials and perovskites to probe the spin and valence states of the transition metal oxides and / or the relationship with the properties : band structure calculations to probe insulator to metal transition and magnetic transitions in K2Cr8O16 (Koeln), ordering phenomena in ordered or disordered perovskites (Madrid and Bordeaux for the materials and macroscopic properties, together with Koeln for the spectroscopic investigation). The cobalt spin and valence states have been probed in different environments, bringing important new information on the debated intermediate spin state of cobalt (Koeln). In Koeln and Stuttgart, the fellows have benefited from a state of the art equipment for spectroscopic investigations and the very recent developments (like Resonant Inelastic X Ray Scattering, with Stuttgart) have been presented and used by some fellows working on superconductivity.

The network has been very active and lively due to the many exchanges between the fellows. Even if they have worked on a large variety of topics, the fellows all share the common scientific language built from the experimental techniques and from a common theoretical background for these topics (relationship between the spin and orbital state and the environment of the transition metal cation, importance of the electronic correlations, interplay between structural, electronic and magnetic properties, interactions between carriers and phonons...). This common language and large spectrum of properties, techniques and materials, has been a fundamental part of their training and will play an important role in their future career.

Apart from the laboratory training, the fellows have received extra training during the semestrial seminars where scientific staff, visiting scientists and external researchers have presented talks and tutorials. They have followed some Complementary Skills workshops with an industrial partner, IPDiA, and for research management in Liverpool, at the Center for Lifelong Learning. They have also taken in charge all the organization (scientific and administrative) of two of these semestrial meetings in Timisoara and Stuttgart. Most of the fellows are now defending or preparing the defense of their PhD. Among them, some have already been hired as scientific staff. Most of them are now interested in pursuing their career as post-doctoral fellows, and hope to later apply for academic positions.