M. Cestelli Guidi

Observation of Charge-Density-Wave Excitations in Manganites

In the optical conductivity of four different manganites with commensurate charge order (CO), strong peaks appear in the meV range below the ordering temperature T_{CO}. They are similar to those reported for one-dimensional charge density waves (CDW) and are assigned to pinned phasons. The peaks and their overtones allow one to obtain, for La_{1-n/8}Ca_{n/8}MnO_{3} with n=5, 6, the electron-phonon coupling, the effective mass of the CO system, and its contribution to the dielectric constant. These results support a description of the CO in La-Ca manganites in terms of moderately weak coupling and of the CDW theory.

Vibrational properties of LaPO4 nanoparticles in mid- and far-infrared domain

Nanopowders of LaPO4 have been grown by sedimentation-micellar method. As-prepared LaPO4 nanoparticles with the average grain size of about 8 nm have a single-phase hydrated hexagonal structure. After thermal annealing at 600 and 800 °C, the average size of nanoparticles increases up to ∼35 and ∼50 nm, respectively, and the structure transforms into single-phase monoclinic. IR spectra of LaPO4 nanoparticles of different size were investigated in the wide range of wavenumbers from 130 to 5000 cm−1 in the 20–300 K temperature region. Differences between IR spectra of the bulk material and nanoparticles as well as the temperature behavior of the vibrational properties are discussed.

Staggered magnetization, critical behavior and weak ferromagnetic properties of LaMnO3 by muon spin rotation.

We present a study of a microtwinned single crystal of LaMnO3 by means of implanted muons. Two muon stopping sites are identified from the symmetry of the internal field in the ordered phase. The temperature dependence of these fields yields the behavior of the staggered magnetization from which a static critical exponent (� = 0.36(2)) is extracted and discussed. The muon spinspin relaxation rate shows a critical slowing down (contrary to preliminary findings) with a critical exponent n = 0.7(1), witnessing the Ising nature of the dynamic fluctuations. The muon precession frequencies vs. applied magnetic field reveal the saturation of the weak ferromagnetic domain structure originated by the Dzialoshinski-Moriya antisymmetric exchange.

Phonon and vibrational spectra of hydrogenated CdTe

This work presents far-infrared reflectivity spectra collected with synchrotron radiation on specially prepared CdTe monocrystals in the temperature region of 30–300K. The investigated samples were of three different types characterized by the three different levels of hydrogenation—strong, middle, and low ones. In order to interpret the experimental data, the imaginary part of the dielectric function was evaluated by means of Kramers-Kronig transformation. To determine quantitatively the influence of hydrogen atoms on the phonon and vibrational spectra of hydrogenated CdTe crystals we used the special statistical model proposed in this paper. This model takes into account the tetrahedron as the basic structural unit and distinguishes the contribution of the hydrogen-bearing tetrahedra from the contribution of the hydrogen-free ones to the crystal phonon spectra.

Phase diagram of low doping manganites

We present a study performed on several LaMnO3+x/2 compositions. Detailed analysis around x=0.09 detects phase separation between a higher transition, Tc, and a lower one TN, with coexisting fluctuating and magnetically ordered clusters (the Neel point, TN, being the onset of an inhomogeneous non-collinear antiferromagnetic order detected by neutrons). The ordered clusters give rise to an extremely fast, static Kubo–Toyabe muon depolarization, while mixed static and dynamic local fields are detected inside the fluctuating clusters. Below TN precession frequency lines are resolved, despite the huge broadening introduced by localized Mn4+ ordered moments. A phase diagram is obtained and briefly discussed in the context of electronic phase separation.

Statistical model analysis of local structure of quaternary sphalerite crystals

At the 2004 Ural International Winter School, we introduced the statistical strained tetrahedron model and discussed ternary tetrahedron structured crystals. The model allows one to interpret x-ray absorption fine structure (EXAFS) data and extract quantitative information on ion site occupation preferences and on the size and shape of each elemental constituent of the configuration tetrahedra. Here we extend the model to cover quaternary sphalerite crystal structures. We discuss the two topologically different quaternary sphalerite systems: the pseudo-balanced A1−xBxYyZ1−y (2:2 cation:anion ratio), and the unbalanced AxBx′C1−x−x′Z or AXyYy′Z1−y−y′ (3:1 or 1:3 cation:anion ratios) truly quaternary alloy systems. These structural differences cause preference values to vary with the relative contents in pseudo-quaternaries but to remain constant in truly quaternary compounds. We give equations to determine preference coefficient values from EXAFS or phonon spectra and to extract nearest-neighbor inter-ion d...

Effect of Ga substitution on the optical properties of La-Sr manganites

In a metallic manganite, the substitution of

Ultraslow polaron dynamics in low-doped manganites from (139)La NMR-NQR and muon spin rotation.

{\mathrm{Mn}}^{+3}

Far-infrared absorption of La1-xCaxMnO3-y at high pressure

by

Vibrational spectra of hydrogenated CdTe

{\mathrm{Ga}}^{+3}