Dynamic phase separation as revealed by the strong resistance relaxation in epitaxially shear-strained La0.67Ca0.33MnO3/NdGaO3(0 0 1) thin-films

Strong phase competitions between the ferromagnetic metal and the charge-ordered-insulator have been induced in a wide temperature range of 10-256 K for the shear-strained La_0.67Ca_0.33MnO₃/NdGaO₃(0 0 1) films. Based on various magnetotransport measurements, the mobility of phase boundaries was inferred to change dramatically with temperature. In the high temperature range where the phase boundaries are movable, strong relaxation in resistivity was observed, while at the frozen temperatures lower than 40 K it is weakened. The resistivities tend to relax in accordance with the phase transitions driven by the temperature or magnetic field in the phase separation (PS) background. Moreover, to our surprise, while the melting fields of the insulating phase varied with film thicknesses, for a given film however, they stay unchanged when started with different phase fractions produced by the field or thermal cycling. The results show a crucial role of the inherent strain state in determining PS and phase competitions in these epitaxial thin films.     

The order of magnetic phase transition in La(Fe1−xCox)11.4Si1.6 compounds

Magnetic transitions in La(Fe_1−xCo_x)_11.4Si_1.6 compounds with x=0–0.08, have been studied by DC magnetic measurements and Mössbauer spectroscopy. The temperature dependence of the Landau coefficients has been derived by fitting the magnetization, M(μ₀H), using the Landau expansion of the magnetic free energy. For x0.02 there is a strongly first-order magnetic phase transition between ferromagnetic and paramagnetic (F–P) states in zero external field and a metamagnetic transition from paramagnetic to ferromagnetic (P–F) above T_c. Increasing the cobalt content drives the F–P transition towards second order and eliminates the metamagnetic transition.     

The simulation study for the nanometer-scale selective growth of Si islands on Si(0 0 1) windows in ultrathin SiO2 films

The nanometer-scale selective growth of Si islands on Si(0 0 1) windows in ultrathin SiO₂ films are studied using the kinetic Monte Carlo simulation. The growth of Si islands is reproduced in simulation where we assume that the migration barrier energy for Si adatom on SiO₂ film is far lower than that on the Si surface at the window.     

Adsorption of CO, CO2 and NO molecules on a BaTiO3 (0 0 1) surface

Since the development of Scanning Tunnelling Microscopy (STM) technique, considerable attention has been devoted to various molecules adsorbed on various surfaces. Also, a new concept emerged with molecules on surfaces considered as nano machines by themselves. In this context, a thorough knowledge of surfaces and adsorbed molecules at an atomic scale are thus particularly invaluable. The present work describes the first Density Functional Theory (DFT) study of adsorption of CO, CO₂ and NO molecules on a BaTiO₃ surface following a first preliminary calculation of O and O₂ adsorption on the same surface. In the previously considered work, we found that a (0 0 1) surface with BaO termination is more stable than the one with TiO₂-termination. Consequently, we extended our study to CO, CO₂ and NO molecules adsorbed on a (0 0 1) surface with BaO termination. The present calculation was performed on a (1 × 1) cell with one monolayer of adsorbed molecules. Especially, a series of cases implying CO molecules adsorbed in various geometrical configurations has been examined. The corresponding adsorption energy varies in the range of −0.17 to −0.10 eV. The adsorption energy of a CO₂ molecule directly located above an O surface atom (called O_s) is of the order of −0.18 eV. The O-C distance length is then 1.24 Å and the O-C-O and O-C-O_s angles are 134.0° and 113.0°, respectively. For NO adsorption, the most important induced structural changes are the followings: (i) the N-O bond is broken when a NO molecule is absorbed on a Ba-O_s bridge site. In that case, N and O atoms are located above an O and a Ba surface atom, respectively, whereas the O-Ba-O_s and N-O_s-Ba angles are 106.5° and 63.0°, respectively. The N-O distance is as large as 2.58 Å and the adsorption energy is as much as −2.28 eV. (ii) In the second stable position, the NO molecule has its N atom adsorbed above an O_s atom, the N-O axis being tilted toward the Ba atom. The N-O_s-Ba angle is then 41.1° while the adsorption energy is only −0.10 eV. At last, the local densities of states around C, O as well as N atoms of the considered adsorbed molecules have also been discussed.     

The atomic and electronic structure of the TiO2- and BaO-terminated BaTiO3(0 0 1) surfaces in a paraelectric phase

Relaxation and rumpling of BaTiO₃(0 0 1) surface with two different terminations have been investigated from ab initio local density approximation calculations. Large displacements of ions deviated from their crystalline sites have been obtained. These kinds of displacements lead to the formation the surface rumpling, dipole moments and electric field in the near-surface region. Band structures, density of states, bond population and electronic density redistributions have been obtained. Considerable enhancement of Ti–O chemical bond covalency nearby the surface, especially for the TiO₂ termination surface, has been found.     

Magnetic oscillations in the 2D network of compensated coupled orbits of the organic metal (BEDT-TTF) 8 Hg 4 Cl 12 (C 6 H 5 Br) 2

Interlayer magnetoresistance and magnetisation of the quasi-two dimensional organic metal (BEDT-TTF)₈Hg₄Cl₁₂(C₆H₅Br)₂ have been investigated in pulsed magnetic fields extending up to 60 T and 33 T, respectively. About fifteen fundamental frequencies, composed of linear combinations of only three basic frequencies, are observed in the oscillatory spectra of the magnetoresistance. The dependencies of the oscillation amplitude on the temperature and on the magnitude and orientation of the magnetic field are analyzed in the framework of the conventional two-dimensional Lifshitz-Kosevitch (LK) model. This model is implemented by damping factors which accounts for the magnetic breakthrough occurring between electron and hole orbits yielding conventional Shubnikov-de Haas closed orbits (model of Falicov and Stachowiak) and quantum interferometers. In particular, a quantum interferometer enclosing an area equal to the first Brillouin zone area is evidenced. The LK model consistently accounts for the temperature and magnetic field dependence of the oscillation amplitude of this interferometer. On the contrary, although this model formally accounts for almost all of the observed oscillatory components, it fails to give consistent quantitative data in most other cases. Received 4 September 2002 / Received in final form 14 November 2002 Published online 27 January 2003 RID="a" ID="a"e-mail: audouard@insa-tlse.fr RID="b" ID="b"UMR 5830: Unité Mixte de Recherche CNRS - Université Paul Sabatier - INSA de Toulouse RID="c" ID="c"UMS 5642: Unité Mixte de Service CNRS - Université Paul Sabatier - INSA de Toulouse     

Oxygen diffusion and surface exchange in La1−xSrxFe0.8Cr0.2O3−δ (x=0.2, 0.4 and 0.6)

Oxygen tracer diffusion (D*) and surface exchange rate constant (k*) have been measured, using isotopic exchange and depth profiling by secondary ion mass spectrometry (SIMS), in La_1−xSr_xFe_0.8Cr_0.2O_3−δ (x=0.2, 0.4 and 0.6). Measurements were made as a function of temperature (700–1000 °C) and oxygen partial pressure (0.21–10⁻²¹ atm) in dry oxygen, water vapour and water vapour/hydrogen/nitrogen mixtures. At high oxygen activity, D* was found to increase with increasing temperature and Sr content. The activation energies for D* in air are 2.13 eV (x=0.2), 1.53 eV (x=0.4) and 1.21 eV (x=0.6). As the oxygen activity decreases, D* increases as expected qualitatively from the increase in oxygen vacancy concentration. Under strongly reducing conditions, the measured values of D* at 1000 °C range from 10⁻⁸ cm² s⁻¹ for x=0.2 to 10⁻⁷ cm² s⁻¹ for x=0.4 and 0.6. The activation energies determined at constant H₂O/H₂ ratio are 1.21 eV (x=0.2), 1.59 eV (x=0.4) and 0.82 eV (x=0.6).

The surface exchange rate constant of oxygen for the H₂O molecule is similar in magnitude to that for the O₂ molecule and both increase with increasing Sr concentration.     

Chemical bonding and pseudogap formation in D022- and L12-structure (V, Ti)Al3

To obtain a rigorous definition of the chemical bonds in binary transition-metal aluminides, topological analyses were performed for VAl₃ and TiAl₃ in the D0₂₂ and L1₂ structures. The analyses were based on the valence charge densities calculated with the ab initio density functional theory. To better understand the formation mechanism of the pseudogap in these compounds, the band structure, the density of states (DOS) and the band decomposed charge density (BDCD) were calculated. The topological analyses reveal that the interactions between the (V, Ti) and Al atoms are all pure shared-shell interactions, the bonds are covalent and clearly have π-bond character. The study of the band structure, DOS and BDCD shows that the formation of the pseudogap is due to the crystal field energy splitting of the (V, Ti)-3d orbitals combined with the inter-unit-cell orbital interaction.     

Impact of electric currents on the insulator-metal phase transition in epitaxial thin films of La1−xAxMnO3 (A = Sr, Ca, and Ba)

The influence of dc currents with a high current density on the transport properties of epitaxial La_1−xA_xMnO₃ (A = Sr, Ca, and Ba) thin films were studied. An application of a large current could lead to a remarkable reduction in the insulator-metal phase transition peak, demonstrating a significant electroresistance effect. After removing such currents the samples could completely return to its pristine state. Our experiments reveal that such an electroresistance should be a common feature for the perovskite manganites, rather than the results caused by the self-heating or self magnetic field. It may be ascribed to the two aspects: one is the strong interaction between carrier spins and localized spins in Mn ions, the other is the percolative mechanism of phase separation.