Out of plane effect on the superconductivity of Sr2-xBaxCuO3+d with Tc up to 98K

We comment on the paper published by W.B. Gao, Q.Q. Liu, L.X. Yang, Y.Yu, F.Y. Li, C.Q. Jin and S. Uchida in Phys. Rev. B 80, 094523 (2009) and give alternate explanations for the enhanced superconductivity. The enhanced onset Tc of 98K observed upon substituting Ba for Sr is attributed to optimal oxygen ordering rather than to the increase in volume. Comparison with La2CuO(4+x) samples suggest that the effect of disorder is overestimated

Overdoped Cuprates With High Temperature Superconducting Transitions

Evidence for High Tc cuprate superconductivity is found in a region of the phase diagram where non-superconducting Fermi liquid metals are expected. Cu valences estimated independently from both x-ray absorption near-edge structure (XANES) and bond valence sum (BVS) measurements are > 2.3 for structures in the homologous series (Cu0.75Mo0.25)Sr2(Y,Ce)sCu2O5+2s+{\delta} with s = 1, 2, 3, and 4. The s = 1 member, (Cu0.75Mo0.25)Sr2YCu2O7+{\delta}, 0 \leq {\delta} \leq 0.5, is structurally related to YBa2Cu3O7 in which 25% of the basal Cu cations [i.e. those in the chain layer] are replaced by Mo, and the Ba cations are replaced by Sr. After oxidation under high pressure the s = 1 member becomes superconducting with Tc = 88K. The Cu valence is estimated to be ~2.5, well beyond the ~2.3 value for which other High-Tc cuprates are considered to be overdoped Fermi liquids. The increase in valence is attributed to the additional 0.5 oxygen ions added per chain upon oxidation. The record short apical oxygen distance, at odds with current theory, suggests the possibility of a new pairing mechanism but further experiments are urgently needed to obtain more direct evidence. From the structural point of view the members with s \geq 2 are considered to be equivalent to single-layer cuprates. All have Tc ~ 56 K which is significantly higher than expected because they also have higher than expected Cu valences. The XANES-determined valences normalized to give values in the CuO2 layers are 2.24, 2.25, and 2.26 for s = 2, 3, and 4, while the BVS values determined for the valence in the CuO2 layer alone are 2.31-2.34 for the s = 2 and 3 members. No evidence for periodic ordering has been detected by electron diffraction and high resolution imaging studies. The possibility that the charge reservoir layers are able to screen long range coulomb interactions and thus enhance Tc is discussed

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

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

High-Tc Superconducting Cuprates, (Ce,Y)sO2s-2Sr2(Cu2.75Mo0.25 )O6+[delta] : Tc-increase with apical Cu-O decrease at constant Cu-O planar distance

Evidence for high-Tc cuprate superconductivity is found in a region of the phase diagram where non-superconducting Fermi liquid metals are expected. Cu valences estimated independently from both XANES measurements and bond valence sum calculations are greater than 2.25 and are in close agreement with each other for structures of the homologous series given in the title with s = 1, 2, 3, 4 and 5. Two questions arise from the present perspective: 1) Is all the action in the CuO2 layers? 2) Is there superconductivity beyond the usual dome? The record short apical oxygen distance found in the homologous series especially in the s = 1 member, at odds with the current theory, suggests the possibility of a new pairing mechanism. The apical Cu-O distance in the s = 1 member decreases upon oxygenation from 2.29 to 2.15 dot A while the Cu valence increases to 2.45 dot A.Peer reviewe

Commensurate structural modulation in the charge- and orbitally-ordered phase of the quadruple perovskite (NaMn3_3)Mn4_4O12_{12}

By means of synchrotron x-ray and electron diffraction, we studied the structural changes at the charge order transition $T_{CO}$=176 K in the mixed-valence quadruple perovskite (NaMn$_3$)Mn$_4$O$_{12}$. Below $T_{CO}$ we find satellite peaks indicating a commensurate structural modulation with the same propagation vector q =(1/2,0,-1/2) of the CE magnetic order that appears at low temperature, similarly to the case of simple perovskites like La$_{0.5}$Ca$_{0.5}$MnO$_3$. In the present case, the modulated structure together with the observation of a large entropy change at $T_{CO}$ gives evidence of a rare case of full Mn$^{3+}$/Mn$^{4+}$ charge and orbital order consistent with the Goodenough-Kanamori model.Comment: Accepted for publication in Phys. Rev. B Rapid Communication

In Vitro Preformed Biofilms of Bacillus safensis Inhibit the Adhesion and Subsequent Development of Listeria monocytogenes on Stainless-Steel Surfaces

Listeria monocytogenes continues to be one of the most important public health challenges for the meat sector. Many attempts have been made to establish the most efficient cleaning and disinfection protocols, but there is still the need for the sector to develop plans with different lines of action. In this regard, an interesting strategy could be based on the control of this type of foodborne pathogen through the resident microbiota naturally established on the surfaces. A potential inhibitor, Bacillus safensis, was found in a previous study that screened the interaction between the resident microbiota and L. monocytogenes in an Iberian pig processing plant. The aim of the present study was to evaluate the effect of preformed biofilms of Bacillus safensis on the adhesion and implantation of 22 strains of L. monocytogenes. Mature preformed B. safensis biofilms can inhibit adhesion and the biofilm formation of multiple L. monocytogenes strains, eliminating the pathogen by a currently unidentified mechanism. Due to the non-enterotoxigenic properties of B. safensis, its presence on certain meat industry surfaces should be favored and it could represent a new way to fight against the persistence of L. monocytogenes in accordance with other bacterial inhibitors and hygiene operations

A novel heavy-fermion state in CaCu_3Ru_4O12

We have measured susceptibility, specific heat, resistivity, and thermopower of CaCu$_3$Ti$_{4-x}$Ru$_x$O$_{12}$ and CaCu$_{3-y}$Mn$_y$Ru$_4$O$_{12}$, and have found that CaCu$_3$Ru$_4$O$_{12}$ can be regarded as a heavy-fermion oxide in d-electron systems. The Kondo temperature is near 200 K, and the susceptibility (1.4$\times10^{-3}$ emu/Cu mol) and the electron specific heat coefficient (28 mJ/Cu molK$^2$) are moderately enhanced. The resistivity is proportional to $T^2$ at low temperatures, and satisfies the Kadowaki-Woods relation. The heavy-fermion state comes from the interaction between the localized moment of Cu 3d and the conduction electron of Ru 4d. An insulator-metal transition occurs between $x=1.5$ and 4 in CaCu$_3$Ti$_{4-x}$Ru$_x$O$_{12}$, which can be regarded as a transition from magnetic insulator to heavy-fermion metal.Comment: 4 pages, 5 figures, submitted to J. Phys. Soc. Jp

Effect of Sr substitution on superconductivity in Hg2(Ba1-ySry)2YCu2O8-d (part2): bond valence sum approach of the hole distribution

The effects of Sr substitution on superconductivity, and more particulary the changes induced in the hole doping mechanism, were investigated in Hg2(Ba1-ySry)2YCu2O8-d by a "bond valence sum" analysis with Sr content from y = 0.0 to y = 1.0. A comparison with CuBa2YCu2O7-d and Cu2Ba2YCu2O8 systems suggests a possible explanation of the Tc enhancement from 0 K for y = 0.0 to 42 K for y = 1.0. The charge distribution among atoms of the unit cell was determined from the refined structure, for y = 0.0 to 1.0. It shows a charge transfer to the superconducting CuO2 plane via two doping channels pi(1) and pi(2), i.e. through O2(apical)-Cu and Ba/Sr-O1 bonds respectively.Comment: 13 pages, 5 figures, accepted for publication in Journal of Physics: Condensed Matte