M. K. Haas

Thin Film Magnesium Boride Superconductor with Very High Critical Current Density and Enhanced Irreversibility Field

Larbalestier †§ N. Rogado*, K.A. Regan*, M.A. Hayward*, T. He*, J.S. Slusky*, K. Inumaru*, M.K. Haas* and R.J. Cava* † Department of Materials Science and Engineering, Univer-sity of Wisconsin, 1509 University Avenue, Madison, WI 53706 USA § Applied Superconductivity Center, University of Wisconsin, 1500 Engineering Drive, Madison, WI 53706 USA * Department of Chemistry and Princeton Materials Institute, Princeton University, Princeton, NJ 08544 USA

Loss of superconductivity with the addition of Al to MgB2 and a structural transition in Mg1-x|[thinsp]|AlxB2

The basic magnetic and electronic properties of most binary compounds have been well known for decades. The recent discovery of superconductivity at 39 K in the simple binary ceramic compound magnesium diboride, MgB2, was therefore surprising. Indeed, this material has been known and structurally characterized since the mid 1950s (ref. 2), and is readily available from chemical suppliers (it is commonly used as a starting material for chemical metathesis reactions). Here we show that the addition of electrons to MgB2, through partial substitution of Al for Mg, results in the loss of superconductivity. Associated with the Al substitution is a subtle but distinct structural transition, reflected in the partial collapse of the spacing between boron layers near an Al content of 10 per cent. This indicates that superconducting MgB2 is poised very near a structural instability at slightly higher electron concentrations.

Giant anharmonicity and nonlinear electron-phonon coupling in MgB2: a combined first-principles calculation and neutron scattering study.

First-principles calculations of the electronic band structure and lattice dynamics for the new superconductor MgB (2) are carried out and found to be in excellent agreement with our inelastic neutron scattering measurements. The numerical results reveal that the E(2g) in-plane boron phonons near the zone center are very anharmonic and strongly coupled to the planar B sigma bands near the Fermi level. This giant anharmonicity and nonlinear electron-phonon coupling is key to quantitatively explaining the observed high T(c) and boron isotope effect in MgB (2).

Static disorder from lone-pair electrons in Bi2−xMxRu2O7−y(M=Cu,Co;x=0,0.4) pyrochlores

Abstract The crystal structures of Bi2Ru2O7 and Bi 1.6 M 0.4 Ru 2 O 7 (M= Cu,Co ) pyrochlores are determined by Rietveld refinement of neutron powder diffraction data collected between 300 and 12 K . An appreciable oxygen non-stoichiometry, with no vacancy ordering, is found only in undoped Bi2Ru2O7. In all three compounds, static displacive disorder of both the Bi and O′ sites is observed. This type of disorder has not been reported previously for Bi2Ru2O7, and is proposed to be a common feature of A2B2O7 pyrochlores having a lone electronic pair on the A-site cation. The electronic properties of Bi2Ru2O7 are discussed in terms of calculated electronic band structure, local ruthenium coordination, and the static bismuth displacement.

Static disorder from lone-pair electrons in Bi{sub 2-x}M{sub x}Ru{sub 2}O{sub 7-y} (M = Cu, Co; x = 0, 0.4) pyrochlores.

Abstract The crystal structures of Bi2Ru2O7 and Bi 1.6 M 0.4 Ru 2 O 7 (M= Cu,Co ) pyrochlores are determined by Rietveld refinement of neutron powder diffraction data collected between 300 and 12 K . An appreciable oxygen non-stoichiometry, with no vacancy ordering, is found only in undoped Bi2Ru2O7. In all three compounds, static displacive disorder of both the Bi and O′ sites is observed. This type of disorder has not been reported previously for Bi2Ru2O7, and is proposed to be a common feature of A2B2O7 pyrochlores having a lone electronic pair on the A-site cation. The electronic properties of Bi2Ru2O7 are discussed in terms of calculated electronic band structure, local ruthenium coordination, and the static bismuth displacement.

The suppression of superconductivity in MgCNi3 by Ni-site doping

The effects of partial substitution of Cu and Co for Ni in the intermetallic perovskite superconductor MgCNi3 are reported. Calculation of the expected electronic density of states suggests that electron (Cu) and hole (Co) doping should have different effects. For MgCNi3-xCux, solubility of Cu is limited to approximately 3% (x = 0.1), and Tc decreases systematically from 7K to 6K. For MgCNi3-xCox, solubility of Co is much more extensive, but bulk superconductivity disappears for Co doping of 1% (x = 0.03). No signature of long range magnetic ordering is observed in the magnetic susceptibility of the Co doped material.

The complex superstructure in Mg1-xAlxB2 at x 0.5

Abstract Electron diffraction and high resolution microscopy have been performed on Mg 1− x Al x B 2 with x ≅0.5. This composition displays a superstructure with a repeat period of exactly 2 c along the c -axis and about 10 nm in the a–b plane. The superstructure results in ring-shaped superreflections in the diffraction pattern. Irradiation by a strong electron beam results in a loss of the superstructure and a decrease of about 1% in the c lattice parameter. In situ heating and cooling on the other hand showed that the superstructure is stable from 100 to 700 K. Possible origins for the superstructure are proposed.

β-Cu3V2O8: magnetic ordering in a spin-1/2 Kagomé-staircase lattice

The spin-1/2 Cu2+ ions in β-Cu3V2O8 occupy the sites of a Kagome-staircase lattice, an anisotropic variant of the Kagome net: buckled layers and imbedded plaquettes of three edge-shared CuO4 squares break the ideal Kagome symmetry. Susceptibility and heat capacity measurements show the onset of short-range ordering at approximately 75 K, and a magnetic phase transition with the characteristics of antiferromagnetism at ~ 29 K. Comparison to the Curie–Weiss theta (θCW = −135 K) indicates that the geometric frustration is largely relieved by the anisotropy.

Temperature dependence of the structural parameters of the non-oxide perovskite superconductor MgCNi3

Abstract We report the structural parameters of superconducting MgC x Ni 3 ( x =0.96, T c =7.3 K) as a function of temperature, from 2 to 295 K, determined by neutron powder diffraction profile refinement. The compound has the perovskite structure over the whole temperature range, with symmetry Pm 3 m and a=3.81221(5) A at 295 K: no structural or long range magnetic ordering transitions were observed. The lattice parameter a and the Debye–Waller factors for the individual atoms decrease smoothly with decreasing temperature. There are no unusual changes of the structural parameters near T c .

Robust paramagnetism in Bi{sub 2-x}M{sub x}Ru{sub 2}O{sub 7} (M = Mn,Fe,Co,Ni,Cu) pyrochlore.

We report magnetic susceptibility, resistivity, and Seebeck coefficients for Bi 2 - x M x Ru 2 O 7 pyrochlore. The solid solution exists up to x=0.5 for M=Cu,Ni,Co and up to x=0.1 for M=Fe,Mn. The doped materials do not exhibit ferromagnetism or any localized ruthenium moment behavior. Instead we find the Ru-O and Bi-O sublattices to be essentially independent, with any magnetism resulting from the unpaired first-row transition metal dopant spins. Cobalt substitution for bismuth results in localized Co 2 + and low-temperature spin-glass transitions in several cases. Nickel moments on the pyrochlore lattice display properties intermediate to localized and itinerant. Finally, copper doping results in an enhancement of the Pauli metallic density of states.