S.-W. Cheong

The crystal structure of superconducting La2CuO4.032 by neutron diffraction

Abstract The crystal structure of superconducting (37.5 K) La2CuO4.032 has been refined from single-crystal neutron-diffraction data at room temperature and 15 K. At both temperatures it exhibits the orthorhombic symmetry Cmca. The extra oxygen O4 atoms occupy the special positions ( 1 4 y 1 4 with y=0.243 at 15 K). They are located between two successive LaO layers and are surrounded by distorted cubes built up of two interpenetrated tetrahedra, one comprising four La atoms and the other four O1 atoms (the apical oxygen atoms of the CuO6octahedra). The O4 insertion causes a 0.75 A displacement of 4.8% of the O1 atoms towards new O3 positions (x=0.030(5), y=0.182(2), z=0.100(5)). From the refined values of the occupancy factors at 15 K, it is deduced that for each extra O4 three O1 are displaced to O3 with one short O3–O4 distance of 1.64(3) A. This value indicates the formation of a strong O-O covalent bond of peroxide type with a formal 2-valence. Since the La and Cu sublattices have been found to be fully occupied and the doping does not change the oxygen charge, the La, Cu and O sublattices have the formal valences 6+, 2+, and 8−, respectively. However, the increase in La coordination and the consequent La-O distance readjustment indicate, when compared to the undoped compound structure, that a charge transfer occurs in La2CuO4.032, with the excess positive charge going either to the La or to the O sublattice. In the latter case it would correspond to the formation of holes in the O 2p band.

Superconductivity of rare earth-barium-copper oxides

Abstract We report the superconductivity of R-Ba-Cu-O compounds for rare earths R = Sm through Ho. The Nd and Tm compounds were not observed to be superconducting. The Tc onsets for the Eu and Gd compounds are comparable to that observed for Y-Ba-Cu-O. An approximate value for the upper critical field slope of the Gd compound is given.

Superconductivity above 90 K in magnetic rare earth-barium-copper oxides

Abstract We report measurements of the superconducting and magnetic behavior of GdBa2Cu3Ox. Superconductivity occurs below Tc = 95 and 93 K, respectively, for these compounds with large Meissner effects observed. The Gd-based compound has an upper critical field slope -dH c2 d T greater than 1.2 T/K. Both compounds exhibit Curie-Weiss behavior above Tc with free-ion effective moments and small, negative temperature intercepts. In addition, the Gd-based compound orders antiferromagnetically below 2.24 K with an entropy consistent with the ordering of seven unpaired spins.

Properties of La2CuO4 and related compounds

Abstract We review the crystal chemical, electronic, magnetic and superconducting properties of La 2 CuO 4 and related planar-cuprate materials, with emphasis on single-crystal results. Although magnetism due to divalent copper ions clearly is evident in nonsuperconducting crystals, experiments do not reveal any direct relationship between localized copper spins and high temperature superconductivity. We also indicate areas where important information is lacking for our understanding of these materials.

Specific heat and anisotropic magnetic susceptibility of Pr2CuO4, Nd2CuO4 and Sm2CuO4 crystals

Abstract Specific heat and magnetic susceptibility measurements on single crystals of RE2CuO4 (RE=Pr, Nd, andSm) indicate antiferromagnetic order at TN≲1.5 K and 5.95 K for RE = NdandSm, respectively. The susceptibility of all RE2CuO4 compounds is highly anisotropic and suggests the presence of crystal field effects.

Systematics of new T*-phase superconductivity

A systematic study of the phase stability of La(Pr) 2− x − y R x Sr(Ba, Ca) y CuO 4 (R = rare earth) compounds reveals a broad composition range for T*-phase formation. Several members of this series of compounds show bulk superconducting transition temperatures up to T c ∼ 20 K. These superconducting cuprates raise further questions about the relationship between structural stability and high temperature superconductivity in oxides.

Novel phase transition in non-antiferromagnetically ordered crystals of La2CuO4

Abstract We report the discovery of a new phase diagram in the magnetic field-temperature plane for single crystals of La 2 CuO 4 that show no sign of antiferromagnetic order. The phase boundary is defined by magneto-resistance and susceptibility measurements. We discuss several possibilities that may lead to the phase diagram observed.

Electronic structure of the gold/Bi2Sr2CaCu2O8 and gold/EuBa2Cu3O7−δ interfaces as studied by photoemission spectroscopy

High‐resolution photoemission has been used to probe the electronic structure of the gold/Bi2Sr2CaCu2O8 and gold/EuBa2Cu3O7−δ interface formed by a low‐temperature (20 K) gold evaporation on cleaved high quality single crystals. We find that the metallicity of the EuBa2Cu3O7−δ substrate in the near surface region (∼5 A) is essentially destroyed by the gold deposition, while the near surface region of Bi2Sr2CaCu2O8 remains metallic. This has potentially wide ranging consequences for the applicability of the different types of superconductors in real devices.

CRYSTAL-FIELD EXCITATIONS IN ND2CUO4

Abstract Inelastic neutron scattering has been employed to study the crystal- field interaction in Nd2CuO. The energetic ordering of the crystal- field levels and the crystal-field parameters are determined from the observed transitions. J-admixture effects turn out to be important and are included in the calculations. At 1.5 K the ground-state doublet is found to be split by about 0.35 meV due to the long-range magnetic ordering of the Nd3+ ions.

Anisotropic electronic and thermal transport properties of lightly oxygen-doped La2CuO4+δ

Abstract We track the systematic variation in magnetic susceptibility (χ), thermal conductivity (κ), anisotropic resistivity (ϱ), and thermoelectric power ( S ) of crystalline La 2 CuO 4+ δ as a function of light hole doping. The level of hole doping (δ) was carefully varied by anneals under controlled oxygen-partial-pressure performed in such a way that the antiferromagnetic ordering temperature T N ranged from 250 to 310 K. χ data indicate that increased oxygen hole doping acts to reduce T N below the δ = 0 value of 320 K. Anomalies are present in κ at T n that result from Cu spin fluctuations about the magnetically ordered ground state. Anomalies are also present in ϱ and S that occur at T N for air-annealed samples wherein T N = 250 K; when more lightly hole-doped so that T N >250 K, the anomalies in ϱ and S do not occur at T N , but instead occur at T 2 CuO 4+ δ , these transport anomalies may result from phase separation involving the diffusion of non-stoichiometric, excess oxygen.