Critical chain length and superconductivity emergence in oxygen-equalized pairs of YBa2Cu3O6.30

The oxygen-order dependent emergence of superconductivity in YBa2Cu3O6+x is studied, for the first time in a comparative way, on pair samples having the same oxygen content and thermal history, but different Cu(1)Ox chain arrangements deriving from their intercalated and deintercalated nature. Structural and electronic non-equivalence of pairs samples is detected in the critical region and found to be related, on microscopic scale, to a different average chain length, which, on being experimentally determined by nuclear quadrupole resonance (NQR), sheds new light on the concept of critical chain length for hole doping efficiency.Comment: 7 RevTex pages, 2 Postscript figures. Submitted to Phys. Rev.

Synthesis and characterization of multiferroic BiMn7_7O12_{12}

We report on the high pressure synthesis of BiMn$_7$O$_{12}$, a manganite displaying a "quadruple perovskite" structure. Structural characterization of single crystal samples shows a distorted and asymmetrical coordination around the Bi atom, due to presence of the $6s^{2}$ lone pair, resulting in non-centrosymmetric space group Im, leading to a permanent electrical dipole moment and ferroelectric properties. On the other hand, magnetic characterization reveals antiferromagnetic transitions, in agreement with the isostructural compounds, thus evidencing two intrinsic properties that make BiMn$_7$O$_{12}$ a promising multiferroic material.Comment: 4 pages, 3 figure

Singling out the effect of quenched disorder in the phase diagram of cuprates

We investigate the specific influence of structural disorder on the suppression of antiferromagnetic order and on the emergence of cuprate superconductivity. We single out pure disorder, by focusing on a series of Y$_{z}$Eu$_{1-z}$Ba$_2$Cu$_3$O$_{6+y}$ samples at fixed oxygen content $y=0.35$, in the range $0\le z\le 1$. The gradual Y/Eu isovalent substitution smoothly drives the system through the Mott-insulator to superconductor transition from a full antiferromagnet with N\'eel transition $T_N=320$ K at $z=0$ to a bulk superconductor with superconducting critical temperature $T_c=18$ K at $z=1$, YBa$_2$Cu$_3$O$_{6.35}$. The electronic properties are finely tuned by gradual lattice deformations induced by the different cationic radii of the two lanthanides, inducing a continuous change of the basal Cu(1)-O chain length, as well as a controlled amount of disorder in the active Cu(2)O$_2$ bilayers. We check that internal charge transfer from the basal to the active plane is entirely responsible for the doping of the latter and we show that superconductivity emerges with orthorhombicity. By comparing transition temperatures with those of the isoelectronic clean system we deterime the influence of pure structural disorder connected with the Y/Eu alloy.Comment: 10 pages 11 figures, submitted to Journal of Physics: Condensed Matter, Special Issue in memory of Prof. Sandro Massid

Optical study of the vibrational and dielectric properties of BiMnO3

BiMnO3 (BMO), ferromagnetic (FM) below Tc = 100 K, was believed to be also ferroelectric (FE) due to a non-centro-symmetric C2 structure, until diffraction data indicated that its space group is the centro-symmetric C2/c. Here we present infrared phonon spectra of BMO, taken on a mosaic of single crystals, which are consistent with C2/c at any T > 10 K, as well as room-temperature Raman data which strongly support this conclusion. We also find that the infrared intensity of several phonons increases steadily for decreasing T, causing the relative permittivity of BMO to vary from 18.5 at 300 K to 45 at 10 K. At variance with FE materials of displacive type, no appreciable softening has been found in the infrared phonons. Both their frequencies and intensities, moreover, appear insensitive to the FM transition at Tc

Internal-strain mediated coupling between polar Bi and magnetic Mn ions in the defect-free quadruple-perovskite BiMn3_3Mn4_4O12_{12}

By means of neutron powder diffraction, we investigated the effect of the polar Bi$^{3+}$ ion on the magnetic ordering of the Mn$^{3+}$ ions in BiMn$_3$Mn$_4$O$_{12}$, the counterpart with \textit{quadruple} perovskite structure of the \textit{simple} perovskite BiMnO$_3$. The data are consistent with a \textit{noncentrosymmetric} spacegroup $Im$ which contrasts the \textit{centrosymmetric} one $I2/m$ previously reported for the isovalent and isomorphic compound LaMn$_3$Mn$_4$O$_{12}$, which gives evidence of a Bi$^{3+}$-induced polarization of the lattice. At low temperature, the two Mn$^{3+}$ sublattices of the $A'$ and $B$ sites order antiferromagnetically (AFM) in an independent manner at 25 and 55 K, similarly to the case of LaMn$_3$Mn$_4$O$_{12}$. However, both magnetic structures of BiMn$_3$Mn$_4$O$_{12}$ radically differ from those of LaMn$_3$Mn$_4$O$_{12}$. In BiMn$_3$Mn$_4$O$_{12}$ the moments $\textbf{M}_{A'}$ of the $A'$ sites form an anti-body AFM structure, whilst the moments \textbf{M}$_{B}$ of the $B$ sites result from a large and \textit{uniform} modulation $\pm \textbf{M}_{B,b}$ along the b-axis of the moments \textbf{M}$_{B,ac}$ in the $ac$-plane. The modulation is strikingly correlated with the displacements of the Mn$^{3+}$ ions induced by the Bi$^{3+}$ ions. Our analysis unveils a strong magnetoelastic coupling between the internal strain created by the Bi$^{3+}$ ions and the moment of the Mn$^{3+}$ ions in the $B$ sites. This is ascribed to the high symmetry of the oxygen sites and to the absence of oxygen defects, two characteristics of quadruple perovskites not found in simple ones, which prevent the release of the Bi$^{3+}$-induced strain through distortions or disorder. This demonstrates the possibility of a large magnetoelectric coupling in proper ferroelectrics and suggests a novel concept of internal strain engineering for multiferroics design.Comment: 9 pages, 7 figures, 5 table

Electron localization and possible phase separation in the absence of a charge density wave in single-phase 1T-VS2_2

We report on a systematic study of the structural, magnetic and transport properties of high-purity 1T-VS$_2$ powder samples prepared under high pressure. The results differ notably from those previously obtained by de-intercalating Li from LiVS$_2$. First, no Charge Density Wave (CDW) is found by transmission electron microscopy down to 94 K. Though, \textit{ab initio} phonon calculations unveil a latent CDW instability driven by an acoustic phonon softening at the wave vector ${\bf q}_{CDW} \approx$ (0.21,0.21,0) previously reported in de-intercalated samples. A further indication of latent lattice instability is given by an anomalous expansion of the V-S bond distance at low temperature. Second, infrared optical absorption and electrical resistivity measurements give evidence of non metallic properties, consistent with the observation of no CDW phase. On the other hand, magnetic susceptibility and NMR data suggest the coexistence of localized moments with metallic carriers, in agreement with \textit{ab initio} band structure calculations. This discrepancy is reconciled by a picture of electron localization induced by disorder or electronic correlations leading to a phase separation of metallic and non-metallic domains in the nm scale. We conclude that 1T-VS$_2$ is at the verge of a CDW transition and suggest that residual electronic doping in Li de-intercalated samples stabilizes a uniform CDW phase with metallic properties.Comment: 22 pages, 10 Figures. Full resolution pictures available at http://journals.aps.org/prb/abstract/10.1103/PhysRevB.89.23512

Intrinsic electronic superconducting phases at 60 K and 90 K in double-layer YBa2_2Cu3_3O6+δ_{6+\delta}

We study superconducting transition temperature ($T_c$) of oxygen-doped double-layer high-temperature superconductors YBa$_2$Cu$_3$O$_{6+\delta}$ (0 $\le$ $\delta$ $\le$ 1) as a function of the oxygen dopant concentration ($\delta$) and planar hole-doping concentration ($P_{pl}$). We find that $T_c$, while clearly influenced by the development of the chain ordering as seen in the $T_c$ $vs.$ $\delta$ plot, lies on a universal curve originating at the critical hole concentration ($P_c$) = 1/16 in the $T_c$ $vs.$ $P_{pl}$ plot. Our analysis suggests that the universal behavior of $T_c$($P_{pl}$) can be understood in terms of the competition and collaboration of chemical-phases and electronic-phases that exist in the system. We conclude that the global superconductivity behavior of YBa$_2$Cu$_3$O$_{6+\delta}$ as a function of doping is electronically driven and dictated by pristine electronic phases at magic doping numbers that follow the hierarchical order based on $P_c$, such as 2 $\times$ $P_c$, 3 $\times$ $P_c$ and 4 $\times$ $P_c$. We find that there are at least two intrinsic electronic superconducting phases of $T_c$ = 60 K at 2 $\times$ $P_c$ = 1/8 and $T_c$ = 90 K at 3 $\times$ $P_c$ = 3/16.Comment: 4 pages, 2 figure

Synthesis and crystal structure of C2/c Ca(Co, Mg)Si2O6 pyroxenes : effect of the cationic substitution on the cell volume

A series of clinopyroxenes along the CaMgSi2O6-CaCoSi2O6 join was synthesized by quenching from melt at 1500\ub0C and subsequent annealing at 1250\ub0C (at 0.0001 GPa). This protocol proved to be the most effective to obtain homogenous, impurity free and stoichiometric pyroxenes as run products. Electron microprobe analyses in energy-dispersive mode were conducted and single-crystal X-ray diffraction data were collected on the Ca(CoxMg1-x)Si2O6 pyroxenes, with x = 0.2, 0.4, 0.5, 0.6; anisotropic structure refinements were performed. The effects of the cation substitution at the M1 site are described at the atomic level. The experimental findings of this study allowed us to extend the comparative analysis of the structural features of pyroxenes with divalent cations at the M1 and M2 sites