Andrea Gauzzi

Can the structure of the Ti or V Magnéli binary oxides host superconductivity

Abstract A strategy for tailoring new superconducting systems is based on band structure calculations and the use of the empirical criteria suggested by L.F. Mattheiss. These criteria allow one to evaluate the capability of new materials to host superconductivity. Promising host structures might be found among those materials undergoing metal–insulator transitions as a function of temperature. Materials with metallic ground states can then be generated by substitution. In this article, we suggest that simple binary compounds, such as titanium or vanadium oxides belonging to the homologous series MnO2n−1 may be good candidates. They exhibit structures which can be derived from rutile by removing one oxygen atom every n M cation via a shearing mechanism, which besides creating n-octahedron-thick rutile blocks along the pseudo tetragonal c-axis, generates a non-stoichiometry with respect to the MO2 formula. Because of the mixed valence state, these materials generally exhibit metallic conductivity and undergo metal-to-insulator transitions as a function of temperature. For example, Ti4O7 undergoes a transition at 150 K, at which temperature a drop of conductivity by three orders of magnitude and one of magnetic susceptibility by a factor of three are observed. On further cooling, a second drop of conductivity by two orders of magnitude is observed at 130 K when the compound undergoes a semiconductor-to-semiconductor transition. The V counterpart seems to be a somewhat better candidate than Ti4O7. The behavior of V4O7 as function of temperature has some similarity to that of undoped La2CuO4. The vanadium Magneli phase with 50% V3+ and 50% V4+, has a metallic conductivity above 250 K, becomes a semiconductor below this temperature and orders antiferromagnetically at 40 K.

Mechanical against chemical pressure in the Y(Ba1-xSrx)2Cu3O7-d system

Abstract The structural refinement of YSr 2 Cu 3 O 6.84 allows us to analyze why the chemical pressure applied to YBa 2 Cu 3 O 6.84 , by substituting Sr for Ba, results in a decrease of T c (88 to 63 K). Even though the chemical pressure induces a volume decrease of 5.7%, the thickness along the c axis of the superconducting block increases by 3.0%. This behavior seems to be due to the incompressibility of the Y coordination polyhedron. The SrO layer becomes negatively strained since the Sr polyhedron does not reduce along with the Sr cation substitution. The calculated bond valence sum for the latter cation is 1.63 v.u. instead of the expected 2. The apical distance of the pyramid around Cu2 decreases drastically from 2.30 to 2.09 A. All these features induce an instability within YSr 2 Cu 3 O 6.84 , which makes it difficult for the extra oxygen to go in and out of the structure as it does in its Ba counterpart. This instability would also reduce the hole transfer from the CuO chains to the superconducting CuO 2 planes.

Structure and superconductivity of YSr2Cu3O7-d

Abstract Almost pure samples of YSr2Cu3O7−d have been synthesized under 3GPa and in the presence of KCIO3. The structure and oxygen stoichiometry were determined by refining the powder neutron diffraction data. The as-prepared sample was found to be tetragonal (P4/mmm space group, a =3.7855(4), c =11.386(2) A ), with d ≈ 0.2. The Tc onset measured by both resistivity and susceptibility, was found to be reproducible at 7ap;63K. When comparing these data with the corresponding ones of the Y(Ba1−xSrx)2Cu3O7−d solid solution with similar oxygen stoichiometry, the lattice parameters and most of the interatomic distances seem to scale monotonically with x, while Tc and the Cu1O1 distance seem to go through a minimum at x ≈ 0.5.

Progress in the Continuous Depostition of YBCO Coated Conductors by Thermal Co-Evaporation

We report on recent progress towards the continuous deposition of YBCO Coated Conductors (CC) by thermal co-evaporation. This is an attractive vacuum technique thanks to the simplicity, low cost and intrinsic uniformity over large areas. Recently, we published the in situ preparation route of 1 μm thick superconducting YBCO films deposited onto CeO2 buffered Ni biaxially textured tapes using a reel-to-reel system; end-to-end critical current densities Jcs at 77 K and self-field, measured by transport measurements are in the 1-2 MA/cm2 range for 1 m. long samples, with zero-resistance Tc= 87 K and transition widths DTc<3 K. In spite of the very good CC’s performances reported by a number of laboratories all over the world, several steps must be optimized in order to limit the CC production costs, in particular concerning the complexity of the CC architecture and the choice and optimization of the YBCO deposition technique. We specifically address the following critical points regarding: 1) the in situ oxidation of the YBCO layer using a novel supersonic nozzle, 2) the deposition by thermal or e-beam evaporation of thick crack-free CeO2 buffer layers and 3) the scaling-up of the deposition process using a new multichamber system.

Magnetic response of the CE structure in the doping-free system Na Mn 7 O 12

We investigated the magnetic properties of the CE structure in

Crystal and electronic structures of superconducting YSr2Cu3O6+x

\mathrm{Na}{\mathrm{Mn}}_{7}{\mathrm{O}}_{12}

Stabilization of Superconductivity in Pure and C-Intercalated 1T-TaS2 Synthesised Under High Pressure

, characterized by the absence of cation disorder and structural inhomogeneities, where an unusual

The Effect of Chemical Pressure on Tc of Layered Cuprate Superconductors

3{d}_{{x}^{2}\ensuremath{-}{y}^{2}}

Critical exponents and amplitudes of the ferromagnetic transition in La0.1Ba0.9VS3

orbital order is expected. Magnetization data show two salient features: a remanent magnetization below the magnetic ordering and a susceptibility anomaly at the charge ordering. The former feature suggests the existence of uncompensated spins in the CE structure. The latter, common to a variety of manganites, is quantitatively explained as an increase of the antiferromagnetic coupling concomitant with the Jahn-Teller distortion of both

Magnetic structure of the high-density single-valent e g Jahn-Teller system LaMn 7 O 12

{\mathrm{Mn}}^{3+}{\mathrm{O}}_{6}