The mechanism of superconductivity in cuprates remains one of the big challenges of condensed matter physics. High-Tc cuprates crystallize into a layered perovskite structure featuring copper oxygen octahedral coordination. Due to the Jahn Teller effect in combination with the strong static Coulomb interaction, the octahedra in high-Tc cuprates are elongated along the c axis, leading to a 3dx2-y2 orbital at the top of the band structure wherein the doped holes reside. This scenario gives rise to 2D characteristics in high-Tc cuprates that favor d-wave pairing symmetry. Here, we report superconductivity in a cuprate Ba2CuO4-y, wherein the local octahedron is in a very exceptional compressed version. The Ba2CuO4-y compound was synthesized at high pressure at high temperatures and shows bulk superconductivity with critical temperature (Tc) above 70 K at ambient conditions. This superconducting transition temperature is more than 30 K higher than the Tc for the isostructural counterparts based on classical La2CuO4. X-ray absorption measurements indicate the heavily doped nature of the Ba2CuO4-y superconductor. In compressed octahedron, the 3d3z2-r2 orbital will be lifted above the 3dx2-y2 orbital, leading to significant 3D nature in addition to the conventional 3dx2-y2 orbital. This work sheds important light on advancing our comprehensive understanding of the superconducting mechanism of high Tc in cuprate materials.

PbMO3 (M = 3d transition metals) family shows systematic variations in charge distribution and intriguing physical properties due to its delicate energy balance between Pb 6s and transition metal 3d orbitals. However, the detailed structure and physical properties of PbFeO3 remain unclear. Herein, we reveal that PbFeO3 crystallizes into an unusual 2ap × 6ap × 2ap orthorhombic perovskite super unit cell with space group Cmcm. The distinctive crystal construction and valence distribution of Pb2+0.5Pb4+0.5FeO3 lead to a long range charge ordering of the -A-B-B- type of the layers with two different oxidation states of Pb (Pb2+ and Pb4+) in them. A weak ferromagnetic transition with canted antiferromagnetic spins along the a-axis is found to occur at 600 K. In addition, decreasing the temperature causes a spin reorientation transition towards a collinear antiferromagnetic structure with spin moments along the b-axis near 418 K. Our theoretical investigations reveal that the peculiar charge ordering of Pb generates two Fe3+ magnetic sublattices with competing anisotropic energies, giving rise to the spin reorientation at such a high critical temperature.

The simple ABO3 and A-site-ordered AA′3B4O12 perovskites represent two types of classical perovskite functional materials. There are well-known simple perovskites with ferroelectric properties, while there is still no report of ferroelectricity due to symmetry breaking transition in A-site-ordered quadruple perovskites. Here we report the high pressure synthesis of an A-site-ordered perovskite PbHg3Ti4O12, the only known quadruple perovskite that transforms from high-temperature centrosymmetric paraelectric phase to low-temperature non-centrosymmetric ferroelectric phase. The coordination chemistry of Hg2+ is changed from square planar as in typical A-site-ordered quadruple perovskite to a rare stereo type with 8 ligands in PbHg3Ti4O12. Thus PbHg3Ti4O12 appears to be a combinatory link from simple ABO3 perovskites to A-site-ordered AA′3Ti4O12 perovskites, sharing both displacive ferroelectricity with former and structure coordination with latter. This is the only example so far showing ferroelectricity due to symmetry breaking phase transition in AA′3B4O12-type A-site-ordered perovskites, and opens a direction to search for ferroelectric materials.

The local structure of the highly “overdoped” 95 K superconductor Sr2CuO3.3determined by Cu K X-ray absorption fine structure (XAFS) at 62 K in magnetically oriented samples shows that 1) the magnetization is perpendicular to thecaxis; 2) at these levels of precision the Cu sublattice is tetragonal in agreement with the crystal structure; the O sublattice has 3) continuous -Cu-O- chains that orient perpendicular to an applied magnetic field; 4) approximately half-filled -Cu-O- chains that orient parallel to this field; 5) a substantial number of apical O vacancies; 6) O ions at some apical positions with expanded Cu-O distances; and 7) interstitial positions that imply highly displaced Sr ions. These results contradict the universally accepted features of cuprates that require intact CuO2planes, magnetization along thecaxis, and a termination of the superconductivity when the excess charge on the CuO2Cu ions exceeds 0.27. These radical differences in charge and structure demonstrate that this compound constitutes a separate class of Cu-O–based superconductors in which the superconductivity originates in a different, more complicated structural unit than CuO2planes while retaining exceptionally high transition temperatures.

The structural, ferroelectric and magnetic properties of bulk perovskite Fe -doped BaTiO 3 (BFTO) prepared by standard solid-state reaction have been investigated. X-ray diffraction (XRD) identifies the tetragonal structure of BFTO samples. Rietveld refinements of XRD data indicates that the doping ions led to ab-plane expansion and out-of-ab-plane shrinkage of the BFTO phases. X-ray photoelectron spectroscopy (XPS) measurements for the prepared samples reveals that Fe 3+ and Fe 4+ ions replaces Ti 4+ ions in the crystal lattice to form single-phase BFTO solids. The results of the temperature-dependent dielectric properties and magnetic hysteresis loops for the BFTO solids show simultaneously the ferroelectric order and ferromagnetic order at room temperature. The doping of magnetic element Fe brings about ferromagnetic order for the samples, and the measured magnetic moment for each Fe atom increases from 0.70 μB to 1.55 μB in BFTO samples. The origin of ferromagnetism of the BFTO samples should be attributed to the double exchange interactions of Fe 3+– O 2– Fe 4+ ions.

The report on the superconductivity of the two-legged spin ladders BaFe 2 S 3 and BaFe 2 Se 3 has established 123-type iron chalcogenides as a novel subgroup in the iron-based superconductor family and has stimulated the continuous exploration of other iron-based materials with new structures and potentially novel properties. In this paper, we report the systematic study of a new quasi-one-dimensional (1D) iron-based compound, Ba 9 Fe 3 Te 15 , including its synthesis and magnetic properties. The highpressure synthesized Ba 9 Fe 3 Te 15 crystallized in a hexagonal structure that mainly consisted of face-sharing FeTe 6 octahedral chains running along the c axis, with a lattice constant of a = 10.23668 Å; this led to weak interchain coupling and an enhanced onedimensionality. The systematic static and dynamic magnetic properties were comprehensively studied experimentally. The dc magnetic susceptibility showed typical 1D antiferromagnetic characteristics, with a T max at 190 K followed by a spin glass (SG) state with freezing at T f ≈ 6.0 K, which were also unambiguously proved by ac susceptibility measurements. Additionally, X-ray magnetic circular dichroism (XMCD) experiments revealed an unexpected orbital moment for Fe 2+ , i.e., 0.84 μ B per Fe in Ba 9 Fe 3 Te 15 . The transport property is electrically insulating, with a thermal activation gap of 0.32 eV. These features mark Ba 9 Fe 3 Te 15 as an alternative type of iron-based compound, providing a diverse candidate for high-pressure studies in order to pursue some emerging physics.

A new compound with one-dimensional spin chains, Ba9Co3Se15, was synthesized under high pressure and high temperature conditions and systematically characterized via structural, transport and magnetic measurements. Ba9Co3Se15 crystallizes in a hexagonal structure with the space group P-6c2 (No. 188) and lattice constants of a = b = 9.6765 Å and c = 18.9562 Å. The structure consists of trimeric face-sharing octahedral CoSe6 chains, which are arranged in a triangular lattice in the ab-plane and separated by Ba atoms. The distance of the nearest neighbor of CoSe6 chains is very large, given by the lattice constant a = 9.6765 Å. The Weiss temperature Tθ associated with the intra-chain coupling strength is about −346 K. However, no long-range magnetic order but a spin glass transition at ∼ 3 K has been observed. Our results indicate that the spin glass behavior in Ba9Co3Se15 mainly arises from the magnetic frustration due to the geometrically frustrated triangular lattice.

Abstract:Rapid urbanization has occurred in northwestern China, threatening the sustainability of its fragile dryland ecosystems. A lack of precise urban land-cover information has limited our understanding on the urbanization in the dryland. Here, we examined urban land-cover changes from 2000 to 2014 in 21 major cities that comprise over 50% of the developed land in arid China, using Landsat Enhanced Thematic Mapper Plus and Operational Land Imager data, and a hybrid classification method. The 15-m resolution urban land-cover products (including impervious surfaces, vegetation, bare soil, and water bodies) had an overall accuracy of 90.37%. Based on these new land use products, we found the urbanization in arid China was characterized by the dramatic expansion of impervious surface (+13.23%) and reduction of bare soil (−13.41%), while the proportions of vegetation (+0.27%) and water (−0.10%) remained stable. The observed dynamic equilibrium of vegetated ratio implies an increasing harmonization of urbanization and greening, which was particularly important for the sustainability of fragile urban ecosystems in arid regions. From an economic perspective, gross domestic product and population were significantly correlated with impervious surfaces, and oasis cities displayed a stronger ability to attract new residents than desert cities.