F. Licci

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.

Thermochemistry of YBa2Cu3-xMxOy (M = Ni, Zn)

Abstract The partial molar enthalpy of incorporation of oxygen (Δ ox H ) and enthalpy of formation (Δ f H ) have been measured via high-temperature reaction calorimetry for YBa 2 Cu 3− x M x O y (where M=Ni, Zn) in the composition range 0.0 x ox H and Δ f H values were nearly independent of dopant for both Ni and Zn, with enthalpies of oxidation in the range −86 to −110 kJ/(mol 1 2 O 2 ) and enthalpies of formation in the range −110 to −118 kJ/mol. The independence of thermodynamic behavior from superconducting properties is consistent with an electronic mechanism for T c suppression in Zn-doped YBCO superconductors. Oxygen contents, lattice parameters, and T c measurements are also reported.

Chemical pressure for optimizing Tc in a given superconducting system

Abstract The application of external pressure to cuprate superconductors induces increases in T c . The same cannot be stated for the chemical pressure, applied through substituting large cations with smaller ones, for example, Sr for Ba. Both types of pressure induce T c increases in only one system among the cuprates, namely La 2− x (Ba 1− y Sr y ) x CuO 4 . For this system, T c increases either for increasing y or for increasing the external pressure. By studying the crystal structure and the superconducting properties of several samples in the system Y(Ba 1− x Sr x ) 2 Cu 3 O w as a function of x and w , we show that in this system the substitution of Sr for Ba reduces the unit cell volume as does the external pressure. However, it also reduces the strain of the Ba/Sr layers that seem to be needed for optimizing the charge transfer between the chain and planar Cu cations. The strain of the Ba layer seems to scale well with T c , at least in the YBCO system.

Effects of oxygen stoichiometry on the copper NMR and NQR in YBa2Cu3O7−y

Abstract We present Cu NMR and NQR measurements performed in the normal phase on YBa 2 Cu 3 O 7− y samples of different oxygen content. An attribution of the signals to the inequivalent copper sites in the lattice, and the charge distribution around these ions are discussed.

Relation between Tc and London penetration depth in YBa2(Cu1−xZnx)3O7 by μSR

Abstract We report on NQR and μSR experiments on YBa 2 (Cu 1− x Zn x ) 3 O 7 aimed at investigating the role of carrier effective mass in determining T c . Our findings indicate Zn localization in Cu planes and a nonlinear dependence of T c on the ratio m ∗ c / m ∗ ab .

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.

Phase separation in antiferromagnetic YBa2Cu3O\it 6+x

We present a detailed study of the ZF‐μSR precession signals in AF YBa2Cu3O6+x for a series of 0.06\,\leq\,x\,\leq\,0.39 ceramic samples. Careful analysis of the temperature and oxygen content dependence shows a complex behaviour of the magnetic field at the muon site. The two main novelties are that a) the muon does not diffuse freely above 250 K, as previously stated, and that b) mobile holes phase separate, inducing a finite size reduction of the AF order parameter.

Muon spin rotation evidence for a glassy superconducting state in YBa2Cu3O7-y

Abstract We report on muon spin rotation (μSR) measurements of the internal magnetization in polycrystalline samples of the high T c superconductor YBa 2 Cu 3 O 7- y . The mean value as well as the mean square deviation of the internal magnetic field seen by the muons undergo large variations with temperature. This effect shows a marked dependence on the magnetic history of the sample, which is what is expected of a glassy superconducting state.

NMR-NQR evidence on the independence of Tc of the spin-fluctuations properties in YBa2Cu4O8 and YBa2Cu3O7−δ

Abstract 89 Y NMR shifts Δ K and 63 Cu NQR spin lattice relaxation rate T −1 1 in YBCO-family superconductors with different transition transition temperatures T c are used to analyze the possible role of the Cu 2+ magnetic correlations and spin dynamics in driving the superconducting transition. The changes in T c are obtained by varying the oxygen stoichiometry or by Ca doping in YBa 2 Cu 4 O 8 or by Zn for Cu substitutions in YBa 2 Cu 3 O 7−δ . From 89 Y ΔK and 63 Cu(2) NQR T 1 no sizeable modifications either in the statio (spin susceptibility) or in the dynamic ( q -integrated low frequency dynamical susceptability) properties of Cu 2+ spins in the CuO 2 plane are inferred and therefore it is argued that the superconducting transition is not directly related to them.

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.