D. Ariosa

Structural phase transition in early growth of Bi2Sr2CaCu2O8+x films on SrTiO3 substrates

We used pulsed laser deposition, with a Bi2Sr2CaCu2O8+x target, to grow films ranging from (1/4) to 10 unit cells thick. We studied these films, and reference Bi2Sr2CaCu2O8+x single crystal samples, using angle-integrated photoemission, core level photoemission, and x-ray diffraction. The data indicate that all films exhibit a metallic-like Fermi edge in the photoemission data. More strikingly, a structural phase transition occurs at a nominal Bi2Sr2CaCu2O8+x thickness of approximately one unit cell, converting the precursor Bi2O2.33 highly coherent thin film into a Bi2Sr2CaCu2O8+x structure.

Structural properties of strained YBa2Cu3O6+x superconducting films grown by pulsed laser deposition

In YBa2Cu3O6+x compound the tetragonal to orthorhombic transition occurs around x equals 0.3, followed by a continuum variation of lattice parameters. Hence both, the structural and superconducting properties, depend upon the oxygen content in CuO chains. Conversely, the epitaxial stress, exerted by the substrate on YBCO films, modified the lattice parameters influencing the oxygen stability in the chains. The understanding of this mechanism is essential when growing epitaxial films for in- situ photoemission studies as well as for tunneling experiments, since the oxygen stability up to the top surface unit-cell is a central issue. We have studied this effect on c-axis oriented YBCO films grown by laser ablation on (001) STO single crystals. Accurate x-ray diffraction analysis of thick films (t GRT 500 angstrom) indicates the presence of two distinct layers, one strained and the other relaxed. Detailed analysis shows that the relaxed layer is as well oxidized as bulk samples, while the strained one is oxygen deficient. Furthermore, despite an oxygen content of about x equals 0.65, the strained layer is in the tetragonal phase (in bulk, the tetragonal phase exists for x < 0.3). We discuss these results in terms of competition between the chemical pressure induced by oxygen inclusion in the chains, and the uniaxial stress within the film.

Experimental electronic structure and Fermi-surface instability of the correlated 3d sulphide BaVS3: High-resolution angle-resolved photoemission spectroscopy

The correlated

Structure and vibrational properties of some PbTiO3-based ferroelectric superlattices

3d

PSEUDOGAP, QUANTUM PHASE FLUCTUATIONS AND SPECTROSCOPY OF HTCS CUPRATES

sulphide BaVS_3 exhibits an interesting coexistence of one-dimensional and three-dimensional properties. Our experiments determine the electronic band structure and shed new light on this puzzle. High-resolution angle-resolved photoemission measurements in a 4-eV wide range below the Fermi energy level uncover and investigate the coexistence of a_{; ; ; ; ; 1g}; ; ; ; ; wide-band and e_g narrow-band d-electrons that lead to the complicated electronic properties of this material. We explore the effects of strong correlations and the Fermi surface instability associated with the metal-insulator transition.The correlated 3d sulphide BaVS_3 is a most interesting compound because of the apparent coexistence of one-dimensional and three-dimensional properties. Our experiments explain this puzzle and shed new light on its electronic structure. High-resolution angle-resolved photoemission measurements in a 4eV wide range below the Fermi level explored the coexistence of weakly correlated a_1g wide-band and strongly correlated e_g narrow-band d-electrons that is responsible for the complicated behavior of this material. The most relevant result is the evidence for a_1g--e_g inter-band nesting condition.

Mixed orientation PbTiO3/BaTiO3 superlattices: X-ray diffraction and raman spectroscopy

Abstract Ferroelectric PbTiO3(PTO)/BaTiO3(BTO) superlattices have been grown on SrTiO3 buffered MgO substrates by laser ablation. An extended X-ray study has shown that the orientation of individual layers is c-domain (i.e., c-axis of the tetragonal phase perpendicular to the substrate) for BTO and a-domain for PTO, contrary to what is observed for single PTO or BTO thin films, which are both c-oriented. Raman scattering reinforces this assertion, and also defines the critical modulation period A above which stress relaxation occurs via misfit dislocations. Confined modes have been also observed, in the same way as in semiconductor superlattices. The temperature variation of the soft mode frequency is shown to depend upon A.

Dopant rearrangement and superconductivity in Bi2Sr2−xLaxCuO6 thin films under annealing

Superconducting critical temperatures Tc are calculated as a function of the doping parameter x for generic layered Cu oxide superconductors, within the framework of a 2D-XY model with quantum phase fluctuations. The model accounts for the lower doping threshold observed at x=0.05, and allows for a simple interpretation of the ‘‘pseudogap’’ in terms of condensation of pre-formed pairs. On the other hand, a novel pairing mechanism, based on the 2D confinement of carriers and on the in-plane overscreening of long-range Coulomb repulsion, is proposed. A 2–3D dimensional crossover which suppresses 2D pairing is considered in the overdoped regime

Consistent behaviour of AC susceptibility and transport properties in magnetic superconductor RuSr2GdCu2O8

Abstract We have used pulsed laser ablation to grow a series of PbTiO3/BaTiO3 (PTO/BTO) multilayers with a modulation wavelength Λ that varies between 50Å<Λ<360 Å. By modeling the x-ray diffraction patterns we have determined that the PTO layers are a-axis oriented and the BTO layers are c-axis oriented. This is in contrast to individual thin films of PTO in which the c-axis is perpendicular to the plane of the film. Raman measurements of the multilayers reinforce this x-ray model determination. We also observe that the soft mode Raman line shifts abruptly in frequency above Λ=240Å. We ascribe this to the strain relaxation in the multilayer, in which misfit dislocations appear at a critical wavelength Λc. A calculation of Λc for a-axis PTO/c-axis BTO multilayers based on elasticity theory yields a value of 260Å, in good agreement with the Raman results.

Photoemission, correlation and superconductivity: New avenues

By combining x-ray diffraction (XRD), x-ray photoemission spectroscopy (XPS) and AC susceptibility measurements we investigate the evolution of structural and superconducting properties of La-doped Bi-2201 thin films grown by pulsed laser deposition (PLD) under different annealing conditions. We find that the main effect of oxygen annealing is to improve the crystal coherence by enabling La cation migration to the Sr sites. This activates the desired hole doping. Short-time Ar annealing removes the interstitial oxygen between the BiO layers, fine adjusting the effective hole doping. The superconducting critical temperature is consequently enhanced. However, longer annealings result in phase separation and segregation of the homologous compound Bi-1201. We attribute this effect to the loss of Bi during the annealing.

XANES in Nanobiology

We report on AC susceptibility, resistivity, thermopower and measurements of sintered magnetic superconductor RuSr2GdCu2O8. The anti ferromagnetic phase transition at 133 K is seen clearly in the AC susceptibility as well as in the derivative of resistivity and thermopower. Above the antiferromagnetic transition, we have found some new evidences of the similarity between HTC compounds and the magnetic superconductor. The onset of superconductivity is observed by both transport methods at 46 K, and an explanation why it is not seen in the AC susceptibility data is given. The end of the SC transition occurs at about 24 K in all measured properties. We discuss the steps of broad SC transition, especially the maximum in susceptibility at 34 K, and compare our results with related studies reported in the literature.