Vinh Ta Phuoc

Synthesis of Ca0.25Cu0.75TiO3 and infrared characterization of role played by copper

Abstract Ca0.25Cu0.75TiO3 compounds with a perovskite-type structure have been reported to display the double advantage for capacitance applications to show high dielectric constant and little dependence on temperature near room temperature. Powders have been prepared by using an organic gel-assisted citrate process. The infrared reflectivity spectra obtained on sintered ceramic discs are very different from those of simple cubic perovskites. Copper indeed is expected to form hybridized bonds with oxygen, contrary to calcium. As a result, instead of remaining equidistant from calcium or copper as in simple cubic perovskite, oxygen forms shorter bonds with copper inducing a strong distortion of oxygen octahedra. Spectra have been Kramers–Kronig transformed and fitted with the factorized form of the dielectric function. Results show that the origin of high dielectric constant is little phononic but rather due to a relaxational process with the characteristic frequency in the gigahertz range. Thin films grown by PLD on (100) SrTiO3 substrate show preferential (100) orientation.

Pulsed laser-deposited VO2 thin films on Pt layers

VO2 films were deposited on Pt (111)/TiO2/SiO2/Si (001) substrates by means of a pulsed laser deposition technique. An x-ray diffraction peak at 2θ = 39.9° was deconvoluted into two pseudo-Voigt profiles of Pt (111) and VOx-originated components. The VOx diffraction peak was more obvious in a VOx/Pt (111)/Al2O3 (0001) sample, having a narrower width compared with a VO2/Al2O3 (0001) sample. Temperature-controlled Raman spectroscopy for the VOx/Pt/TiO2/SiO2/Si sample has revealed the monoclinic VO2 phase at low temperature and the structural phase transition at about 72 °C in a heating process. The electronic conductive nature at the high temperature phase was confirmed by near normal incidence infrared reflectivity measurements. Out-of-plane current-voltage characteristics showed an electric field-induced resistance switching at a voltage as low as 0.2 V for a 50 nm-thick film. A survey of present and previous results suggests an experimental law that the transition voltage of VO2 is proportional to the sq...

Identification of spin wave resonances and crystal field levels in simple chromites RCrO3 (R = Pr, Sm, Er) at low temperatures in the THz spectral region

Abstract We report on THz absorption spectroscopy combined with high magnetic fields of polycrystalline RCrO3 (R = Pr, Sm, Er) aiming understanding spin wave resonances at their low temperature magnetic phases. Our measurements show that the temperature, and the implicit anisotropies at which the Cr3+ spin reorientation at TSR takes place, are determinant on the ferromagnetic-like (FM) and the antiferromagnetic-like (AFM) spin modes being optically active. It is found that they are dependent on Rare Earth 4f moment and ion size. We also studied temperature and field dependence of crystal field levels in the same spectroscopic region. Pr3+ non-Kramers emerges at 100 K and Zeeman splits. An observed absence of spin wave resonances in PrCrO3 is attributed to Pr3+ remaining paramagnetic. In SmCrO3 near cancelation of the spin and orbital moments is proposed as the possible reason for not detecting Sm3+ ground state transitions. Here, the FM and AFM resonant modes harden when the temperature decreases and split linearly under applied fields at 5 K and below. In ErCrO3 the Er3+ Kramers doublet becomes active at about the TSR onset. Each line further experiences Zeeman splitting under magnetic fields while an spin reversal induced by a ∼2.5 T field, back to the Γ4 (Fz) from the Γ1 phase at 2 K, produces a secondary splitting. The 5 K AFM and FM excitations in ErCrO3 have a concerted frequency-intensity temperature dependence and a shoulder pointing to the Er3+ smaller ion size also disrupting the two magnetic sublattice approximation. Both resonances reduce to one when the temperature is lowered to 2 K in the Γ1 representation. Our findings have important implications on the complex interplay in the magneto-electrodynamics associated with the Rare-Earth 4f – 3d transition metal spin coupling and the structural A site instabilities in perovskite multiferroics.

OPTICAL CONDUCTIVITY OF OXIDES

Optical conductivity spectra of conducting oxides including high-TC cuprates - compared to non-superconducting nickelates in particular - deduced from infrared reflectivity spectroscopy in polarized light and their doping and temperature dependence, are discussed with emphasis on the signatures of intrinsic inhomogeneities of charge carrier concentration. The model of disordered metal is shown to be compatible with the general loss of spectral weight in the optical conductivity upon decreasing frequency, instead of a maximum conductivity at zero frequency as found within a Drude description. In high-TC multilayer cuprates, the discussion focuses upon the c-axis response and the inhomogeneity of the electronic concentration related to the conducting planes alternating with insulating sheets. The onset of stripes that are intrinsic inhomogeneities of the charge carrier distribution is discussed in the example of nickelates.