CNRS Neel Institute
CNRS Neel Institute
Agency: GTR | Branch: EPSRC | Program: | Phase: Research Grant | Award Amount: 329.23K | Year: 2013
Magnetic Materials are employed in an enormous range of applications in modern society, from information storage in computers, refrigeration in security and astronomical instrumentation, biocompatible agents for use as both contrast and polarizing agents in magnetic resonance imaging (MRI) and diagnosis, and as agents for magnetic hyperthermic treatments. Academically, molecule-based magnets are also studied intensively with regard to their important fundamental chemistry and physics, since they have the potential to be exploited in nanoscale electronics devices, as beautifully demonstrated recently by the construction of single-molecule spintronic devices (spin valves and transistors). Molecule-based materials offer the great advantage of being designable and manipulable by synthetic chemistry. That is, they can be constructed atom by atom, molecule by molecule with the unparalled advantages of being soluble, monodisperse in size, shape and physical properties, and tuneable at the atomic scale. Indeed, this bottom-up research vision is not restricted to academia - IBM recently reported information storage in surface-isolated (2x6) arrays of Fe atoms at liquid He temperatures and are actively investigating spintronics and data storage with a view to the ultimate miniaturisation of such technologies. However, before any molecule or molecule-based material can have commercial application or value, the fundamental and intrinsic relationship between structure and magnetic behaviour must be understood. This requires the chemist to design and construct familes of related complexes, characterise them structurally and magnetically, and through extensive collaboration with a network of world-class condensed matter physicists and theoreticians, understand their underlying physical properties. The current proposal directly addresses these fundamental questions through the controlled aggregation and organisation of molecular magnets into designed 0-3D architectures in the solid state. Specifically it applies the fundamental principles underpinning supramolecular chemistry to assemble single-molecule magnets into novel topologies by taking advantage of simple coordination-driven self-assembly processes. We will employ molecular magnets as building blocks for the formation of supramolecular assemblies and coordination polymers in which the spin dynamics of the molecular building blocks are modulated through the attachment of, and interaction with, other paramagnetic moieties. In order to achieve this we will: design and build a range of metalloligands, ranging from simple isotropic molecules to more complex and exotic anisotropic molecules and attach them to pre-made SMMs; construct hybrid magnetic materials from SMMs and cyanometalate building blocks; design and synthesise dual-functioning ligands which are capable of directing the formation of SMMs and simultaneously linking them into higher order (O-3D) materials; and characterise all materials, structurally and magnetically, through a battery of techniques.
Trambly De Laissardiere G.,Cergy-Pontoise University |
Mayou D.,CNRS Neel Institute
Physical Review Letters | Year: 2013
We propose a unified description of transport in graphene with adsorbates that fully takes into account localization effects and loss of electronic coherence due to inelastic processes. We focus in particular on the role of the scattering properties of the adsorbates and analyze in detail cases with resonant or nonresonant scattering. For both models, we identify several regimes of conduction, depending on the value of the Fermi energy. Sufficiently far from the Dirac energy and at sufficiently small concentrations, the semiclassical theory can be a good approximation. Near the Dirac energy, we identify different quantum regimes, where the conductivity presents universal behaviors. © 2013 American Physical Society.
Ciuchi S.,CNR Institute for Complex Systems |
Fratini S.,CNRS Neel Institute
Physical Review Letters | Year: 2011
The consequences of several microscopic interactions on the photoemission spectra of crystalline organic semiconductors are studied theoretically. It is argued that their relative roles can be disentangled by analyzing both their temperature and their momentum-energy dependence. Our analysis shows that the polaronic thermal band narrowing, which is the foundation of most theories of electrical transport in organic semiconductors, is inconsistent in the range of microscopic parameters appropriate for these materials. An alternative scenario is proposed to explain the experimental trends. © 2011 American Physical Society.
Paul I.,CNRS Neel Institute
Physical Review Letters | Year: 2011
We examine the relevance of magnetoelastic coupling to describe the complex magnetic and structural behavior of the different classes of the iron superconductors. We model the system as a two-dimensional metal whose magnetic excitations interact with the distortions of the underlying square lattice. Going beyond the mean field, we find that quantum fluctuation effects can explain two unusual features of these materials that have attracted considerable attention: first, why iron telluride orders magnetically at a non-nesting wave vector (π/2,π/2) and not at the nesting wave vector (π,0) as in the iron arsenides, even though the nominal band structures of both these systems are similar, and second, why the (π,0) magnetic transition in the iron arsenides is often preceded by an orthorhombic structural transition. These are robust properties of the model, independent of microscopic details, and they emphasize the importance of the magnetoelastic interaction. © 2011 American Physical Society.
Arenal R.,ONERA |
Blase X.,CNRS Neel Institute |
Advances in Physics | Year: 2010
We present in this review a joint experimental and theoretical overview of the synthesis techniques and properties of boron-nitride (BN) and boron-carbonitride (BCN) nanotubes. While their tubular structure is similar to that of their carbon analogues, we show that their electronic properties are significantly different. BN tubes are wide band gap insulators while BCN systems can be semiconductors with a band gap in the visible range.
Lacroix C.,CNRS Neel Institute
Journal of the Physical Society of Japan | Year: 2010
While consequences of frustration of magnetic interactions are much studied in localized spin systems, less studies have been performed on frustrated metallic systems. In this review I show that several effects due to strong frustration have also been observed in some metallic correlated systems compounds containing rare-earth or transition metal magnetic atoms. There are also effects, specific to frustrated metallic systems, which were theoretically predicted but not yet observed. This paper will review some of these aspects, concerning either the magnetically ordered systems, as the existence of mixed magnetic structures, or the anomalous Hall effect in chiral magnetic structures, or in non ordered systems, as the heavy fermion behavior or the possibility of superconductivity. The last part is devoted to the metal- insulator transition in these systems. © 2010 The Physical Society of Japan.
Allain A.,CNRS Neel Institute |
Han Z.,CNRS Neel Institute |
Bouchiat V.,CNRS Neel Institute
Nature Materials | Year: 2012
Graphene is a sturdy and chemically inert material exhibiting an exposed two-dimensional electron gas of high mobility. These combined properties enable the design of graphene composites, based either on covalent or non-covalent coupling of adsorbates, or on stacked and multilayered heterostructures. These systems have shown tunable electronic properties such as bandgap engineering, reversible metal-insulating transition or supramolecular spintronics. Tunable superconductivity is expected as well, but experimental realization is lacking. Here, we show experiments based on metal-graphene hybrid composites, enabling the tunable proximity coupling of an array of superconducting nanoparticles of tin onto a macroscopic graphene sheet. This material allows full electrical control of the superconductivity down to a strongly insulating state at low temperature. The observed gate control of superconductivity results from the combination of a proximity-induced superconductivity generated by the metallic nanoparticle array with the two-dimensional and tunable metallicity of graphene. The resulting hybrid material behaves, as a whole, like a granular superconductor showing universal transition threshold and localization of Cooper pairs in the insulating phase. This experiment sheds light on the emergence of superconductivity in inhomogeneous superconductors, and more generally, it demonstrates the potential of graphene as a versatile building block for the realization of superconducting materials. © 2012 Macmillan Publishers Limited. All rights reserved.
Ralko A.,CNRS Neel Institute
Physical Review B - Condensed Matter and Materials Physics | Year: 2011
The phase diagram of the XXZ spin-1/2 magnet, equivalent to hard-core bosons, under a staggered magnetic field and on the Cairo pentagonal lattice is computed at zero and finite temperature by using a cluster mean-field theory and a stochastic series expansion quantum Monte Carlo. The complex connectivity and the frustration lead to unconventional phases such as a 1/3 ferrimagnetic plateau stabilized by quantum fluctuations as well as a 5/12 topological phase induced by a local ice-rule constraint. We also report the presence of a ferrimagnetic superfluid and its thermal melting. Finally, we discuss the ferro- and antiferro- magnetic (hopping sign) cases. © 2011 American Physical Society.
Klein H.,CNRS Neel Institute
Acta Crystallographica Section A: Foundations of Crystallography | Year: 2011
The structure solutions of the two phases Mn 2O 3 and PbMnO 2.75 by precession electron diffraction (PED) are presented. In the powder samples used these structures could not be solved by X-ray diffraction because Mn 2O 3 is a minority phase in an MnO 2 powder and the complex structure of PbMnO 2.75 leads to severe peak overlap in the powder diffraction pattern. The influence of different parameters on the structure solution is studied, i.e. the number of reflections measured, precession angle, resolution limit and Lorentz-type correction. It is shown that the number of reflections is the most important parameter for successful structure solution from PED data. © 2011 International Union of Crystallography.
Monceau P.,CNRS Neel Institute
Advances in Physics | Year: 2012
This article reviews the static and dynamic properties of spontaneous superstructures formed by electrons. Representations of such electronic crystals are charge density waves (CDW) and spin density waves in inorganic as well as organic low-dimensional materials. A special attention is paid to the collective effects in pinning and sliding of these superstructures, and the glassy properties at low temperature. Charge order and charge disproportionation which occur in organic materials resulting from correlation effects are analysed. Experiments under magnetic field, and more specifically field-induced CDWs are discussed. Properties of meso-and nanostructures of CDWs are also reviewed. © 2012 Copyright Taylor and Francis Group, LLC.