M. Marezio

Bio: M. Marezio is an academic researcher from Centre national de la recherche scientifique. The author has contributed to research in topics: Neutron diffraction & Crystal structure. The author has an hindex of 64, co-authored 273 publications receiving 16787 citations. Previous affiliations of M. Marezio include Joseph Fourier University & Bell Labs.

Lattice effects on the magnetoresistance in doped lamno3

Abstract: A detailed study of doped LaMn${\mathrm{O}}_{3}$ with fixed carrier concentration reveals a direct relationship between the Curie temperature ${T}_{c}$ and the average ionic radius of the La site $〈{r}_{A}〉$, which is varied by substituting different rare earth ions for La. With decreasing $〈{r}_{A}〉$, magnetic order and significant magnetoresistance occur at lower temperatures with increasing thermal hysteresis, and the magnitude of the magnetoresistance increases dramatically. These results show that the notion of ``double exchange'' must be generalized to include changes in the Mn-Mn electronic hopping parameter as a result of changes in the Mn-O-Mn bond angle.

Superconductivity at 94 K in HgBa 2 Cu0 4+δ

Abstract: FOLLOWING the discovery1 of high-transition-temperature (high-Tc) superconductivity in doped La2CuO4, several families of related compounds have been discovered which have layers of CuO2 as the essential requirement for superconductivity: the highest transition temperatures so far have been found for thallium-bearing compounds2. Recently the mercury-bearing compound HgBa2Rcu2O6+δ (Hg-1212) was synthesized3 (where R is a rare-earth element), with a structure similar to the thallium-bearing superconductor TlBa2CaCu2O7 (Tl-1212), which has one T1O layer and two CuO2 layers per unit cell, and a Tc of 85 K (ref. 2). But in spite of its resemblance to Tl-1212, Hg-1212 was found not to be superconducting. Here we report the synthesis of the related compound HgBa2CuO4+δ (Hg-1201), with only one CuO2 layer per unit cell, and show that it is superconducting below 94 K. Its structure is similar to that of Tl-1201 (which has a Tc of < 10 K)4, but its transition temperature is considerably higher. The availability of a material with high Tc but only a single metal oxide (HgO) layer may be important for technological applications, as it seems that a smaller spacing between CuO2 planes leads to better superconducting properties in a magnetic field5.

Structural anomalies, oxygen ordering and superconductivity in oxygen deficient Ba2YCu3Ox

Abstract: We report the characterization of series of oxygen deficient Ba2YCu3Ox samples for 7 ≥ x ≥ 6 prepared by Zr gettered annealing at 440°C. Measurements include complete crystal structure analysis at 5 K by powder neutron diffraction, electron microscopy study of the oxygen ordering, and magnetic measurements of the superconducting transitions, with particular attention to the transition widths. The results show for the first time that the 90 K and 60 K plateaus in Tc as a function of oxygen stoichiometry are associated with plateaus in the effective valence of the plane coppers. We also correlate the disappearance of superconductivity for x

Structural effects on the magnetic and transport properties of perovskite A 1-x A' x MnO 3 (x=0.25, 0.30)

Abstract: The evolution of the structural properties of ${A}_{1\ensuremath{-}x}{A}_{x}^{\ensuremath{'}}{\mathrm{MnO}}_{3}$ was determined as a function of temperature, average $A$-site radius $〈{r}_{A}〉,$ and applied pressure for the ``optimal'' doping range $x=0.25,$ 0.30, by using high-resolution neutron powder diffraction. The metal-insulator transition, which can be induced both as a function of temperature and of $〈{r}_{A}〉,$ was found to be accompanied by significant structural changes. Both the paramagnetic charge-localized phase, which exists at high temperatures for all values of $〈{r}_{A}〉,$ and the spin-canted ferromagnetic charge-ordered phase, which is found at low temperatures for low values of $〈{r}_{A}〉,$ are characterized by large metric distortions of the ${\mathrm{MnO}}_{6}$ octahedra. These structural distortions are mainly incoherent with respect to the space-group symmetry, with a significant coherent component only at low $〈{r}_{A}〉.$ These distortions decrease abruptly at the transition into the ferromagnetic metal phase. These observations are consistent with the hypothesis that, in the insulating phases, lattice distortions of the Jahn-Teller type, in addition to spin scattering, provide a charge-localization mechanism. The evolution of the average structural parameters indicates that the variation of the electronic bandwidth is the driving force for the evolution of the insulator-to-metal transition at ${T}_{C}$ as a function of ``chemical'' and applied pressure.

Charge, orbital, and magnetic ordering in La 0.5 Ca 0.5 MnO 3 s

Abstract: The unusual magnetic properties of ${\mathrm{La}}_{0.5}$ ${\mathrm{Ca}}_{0.5}$ ${\mathrm{MnO}}_{3}$ were found to be associated with structural and magnetic ordering phenomena, resulting from the close interplay between charge, orbital, and magnetic ordering. Analysis of synchrotron x-ray and neutron powder diffraction data indicates that the anomalous and hysteretic behavior of the lattice parameters occurring between ${\mathrm{T}}_{\mathrm{C}}$ \ensuremath{\sim}225 K and ${\mathrm{T}}_{\mathrm{N}}$ \ensuremath{\sim}155 K is due to the development of a Jahn-Teller (J-T) distortion of the ${\mathrm{MnO}}_{6}$ octahedra, the ${\mathrm{d}}_{\mathrm{z}}^{2g}$ orbitals being oriented perpendicular to the orthorhombic b axis. We observed an unusual broadening of the x-ray Bragg reflections throughout this temperature region, suggesting that this process occurs in stages. Below ${\mathrm{T}}_{\mathrm{N}}$ , the development of well-defined satellite peaks in the x-ray patterns, associated with a transverse modulation with q=[1/2-\ensuremath{\varepsilon},0,0], indicates that quasicommensurate (\ensuremath{\varepsilon}\ensuremath{\sim}0) orbital ordering occurs within the a-c plane as well. The basic structural features of the charge-ordered low-temperature phase were determined from these satellite peaks. The low-temperature magnetic structure is characterized by systematic broadening of the magnetic peaks associated with the ``${\mathrm{Mn}}^{+3}$ '' magnetic sublattice. This phenomenon can be explained by the presence of magnetic domain boundaries, which break the coherence of the spin ordering on the ${\mathrm{Mn}}^{+3}$ sites while preserving the coherence of the spin ordering on the ${\mathrm{Mn}}^{+4}$ sublattice as well as the identity of the two sublattices. The striking resemblance between these structures and the structural ``charge ordering'' and ``discommensuration'' domain boundaries, which were recently observed by electron diffraction and real-space imaging, strongly suggests that these two types of structures are the same and implies that, in this system, commensurate long-range charge ordering coexists with quasicommensurate orbital ordering.

Dynamical mean-field theory of strongly correlated fermion systems and the limit of infinite dimensions

Abstract: We review the dynamical mean-field theory of strongly correlated electron systems which is based on a mapping of lattice models onto quantum impurity models subject to a self-consistency condition. This mapping is exact for models of correlated electrons in the limit of large lattice coordination (or infinite spatial dimensions). It extends the standard mean-field construction from classical statistical mechanics to quantum problems. We discuss the physical ideas underlying this theory and its mathematical derivation. Various analytic and numerical techniques that have been developed recently in order to analyze and solve the dynamical mean-field equations are reviewed and compared to each other. The method can be used for the determination of phase diagrams (by comparing the stability of various types of long-range order), and the calculation of thermodynamic properties, one-particle Green's functions, and response functions. We review in detail the recent progress in understanding the Hubbard model and the Mott metal-insulator transition within this approach, including some comparison to experiments on three-dimensional transition-metal oxides. We present an overview of the rapidly developing field of applications of this method to other systems. The present limitations of the approach, and possible extensions of the formalism are finally discussed. Computer programs for the numerical implementation of this method are also provided with this article.

Electroluminescence from single monolayers of nanocrystals in molecular organic devices

Abstract: The integration of organic and inorganic materials at the nanometre scale into hybrid optoelectronic structures enables active devices that combine the diversity of organic materials with the high-performance electronic and optical properties of inorganic nanocrystals. The optimization of such hybrid devices ultimately depends upon the precise positioning of the functionally distinct materials. Previous studies have already emphasized that this is a challenge, owing to the lack of well-developed nanometre-scale fabrication techniques. Here we demonstrate a hybrid light-emitting diode (LED) that combines the ease of processability of organic materials with the narrow-band, efficient luminescence of colloidal quantum dots (QDs). To isolate the luminescence processes from charge conduction, we fabricate a quantum-dot LED (QD-LED) that contains only a single monolayer of QDs, sandwiched between two organic thin films. This is achieved by a method that uses material phase segregation between the QD aliphatic capping groups and the aromatic organic materials. In our devices, where QDs function exclusively as lumophores, we observe a 25-fold improvement in luminescence efficiency (1.6 cd A(-1) at 2,000 cd m(-2)) over the best previous QD-LED results. The reproducibility and precision of our phase-segregation approach suggests that this technique could be widely applicable to the fabrication of other hybrid organic/inorganic devices.

Evaporated Sn‐doped In2O3 films: Basic optical properties and applications to energy‐efficient windows

Abstract: We review work on In2O3:Sn films prepared by reactive e‐beam evaporation of In2O3 with up to 9 mol % SnO2 onto heated glass. These films have excellent spectrally selective properties when the deposition rate is ∼0.2 nm/s, the substrate temperature is ≳150 °C, and the oxygen pressure is ∼5×10−4 Torr. Optimized coatings have crystallite dimensions ≳50 nm and a C‐type rare‐earth oxide structure. We cover electromagnetic properties as recorded by spectrophotometry in the 0.2–50‐μm range, by X‐band microwave reflectance, and by dc electrical measurements. Hall‐effect data are included. An increase of the Sn content is shown to have several important effects: the semiconductor band gap is shifted towards the ultraviolet, the luminous transmittance remains high, the infrared reflectance increases to a high value beyond a certain wavelength which shifts towards the visible, phonon‐induced infrared absorption bands vanish, the microwave reflectance goes up, and the dc resisitivity drops to ∼2×10−4 Ω cm. The corre...

Atomic structure of conducting nanofilaments in TiO2 resistive switching memory

Abstract: Resistance switching in metal oxides could form the basis for next-generation non-volatile memory. It has been argued that the current in the high-conductivity state of several technologically relevant oxide materials flows through localized filaments, but these filaments have been characterized only indirectly, limiting our understanding of the switching mechanism. Here, we use high-resolution transmission electron microscopy to probe directly the nanofilaments in a Pt/TiO2/Pt system during resistive switching. In situ current–voltage and low-temperature (∼130 K) conductivity measurements confirm that switching occurs by the formation and disruption of TinO2n−1 (or so-called Magneli phase) filaments. Knowledge of the composition, structure and dimensions of these filaments will provide a foundation for unravelling the full mechanism of resistance switching in oxide thin films, and help guide research into the stability and scalability of such films for applications. Nanoscale filaments with a Magneli structure are shown to be responsible for resistance switching in thin films of TiO2, and the properties of the filaments are directly observed during the switching process.