Transport and thermodynamic properties of a sintered pellet of the newly discovered M gB2 superconductor have been measured to determine the characteristic critical magnetic fields and critical current densities. Both resistive transition and magnetization data give similar values of the upper critical field, Hc2, with magnetization data giving dHc2/dT = 0.44 T /K at the transition temperature of Tc = 40.2 K. Close to the transition temperature, magnetization curves are thermodynamically reversible, but at low temperatures the trapped flux can be on the order of one Tesla. The value of dHc/dT at Tc is estimated to be about 12 mT /K, a value similar to classical superconductors like Sn. Hence, the Ginsburg-Landau parameter κ ∼ 26. Estimates of the critical supercurrent density, Jc, using hysteresis loops and the Bean model give critical current densities on the order of 10 5 A/cm 2 . Hence the supercurrent coupling through the grain boundaries is comparable to intermetallics like N b3Sn.74.25. Bt, 74.25.Fy, 74.25.Ha, 74.60.Ge, 74.60.Jg
M gB2 becomes superconducting just below 40 K. Whereas porous polycrystalline samples of M gB2 can be synthesized from boron powders, in this letter we demonstrate that dense wires of M gB2 can be prepared by exposing boron filaments to M g vapor. The resulting wires have a diameter of 160 µm, are better than 80% dense and manifest the full χ = −1/4π shielding in the superconducting state. Temperaturedependent resistivity measurements indicate that M gB2 is a highly conducting metal in the normal state with ρ(40 K) = 0.38 µOhm-cm. Using this value, an electronic mean free path, l ≈ 600Å can be estimated, indicating that M gB2 wires are well within the clean limit. Tc, Hc2(T ), and Jc data indicate that M gB2 manifests comparable or better superconducting properties in dense wire form than it manifests as a sintered pellet.74.70. Ad, 74.25.Fy, 74.25.Ha, 74.60.Jg
Neutron diffraction and magnetic susceptibility studies of (Li 1Ϫ3x Fe x )NiPO 4 (xϭ0 and xϭ0.033) compounds reveal remarkable differences between the magnetic properties of pure LiNiPO 4 and those of its lightly iron-doped derivative. The spin system associated with the Ni 2ϩ ions (Sϭ1) in the pure compound undergoes a collinear antiferromagnetic ordering at T N ϭ19.1Ϯ0.5 K, with the characteristics of weakly coupled twodimensional ͑2D͒ Ising square planes. By contrast, randomly intercalated iron spins ͑in Li sites͒ between Ni 2ϩ planes comprise a spin-glass-like subsystem which, despite their minute amount, drives the antiferromagnetic transition to higher temperatures T N ϭ25.2Ϯ0.5 K, and significantly modifies the critical behavior of the 2D Ni 2ϩ system. It is argued that the doped compound can serve as a model system for studying the randomly coupled planar Ising model. ͓S0163-1829͑99͒15425-0͔
Two polytypes of potassium rare-earth-metal hexaselenodiphosphates(IV), K(RE)P(2)Se(6) (RE = Y, La, Ce, Pr, Gd), have been synthesized from the stoichiometric reaction of RE, P, Se, and K(2)Se(4) at 750 degrees C. Both single-crystal and powder X-ray diffraction analyses showed that the structures of these polytypes vary with the size of the rare earth metals. For the smaller rare-earth metals, Y and Gd, K(RE)P(2)Se(6) crystallized in the orthorhombic space group P2(1)2(1)2(1). The yttrium compound was studied by single-crystal X-ray diffraction with the cell parameters a = 6.7366(5) Å, b = 7.4286(6) Å, c = 21.603(2) Å, and Z = 4. This structure type comprises a layered, square network of yttrium atoms that are bound to four distinct [P(2)Se(6)](4)(-) units through selenium bonding. Each [P(2)Se(6)](4)(-) unit possesses a Se atom that is not bound to any Y atom but is pointing out into the interlayer spacing, into an environment of potassium cations. For larger rare-earth metals, La, Ce, and Pr, K(RE)P(2)Se(6) crystallized in a second, monoclinic polytype, the structure of which has been published. Both of these two different polytypes can be related to each other and several other isoelectronic chalcophosphate structures based on a Parthé valence electron concentration analysis. These structures include Ag(4)P(2)S(6), K(2)FeP(2)S(6), and the hexagonal M(II)PS(3) structure types. The magnetic susceptibilities of the title compounds have been studied, and the behavior can been explained based on a simple set of unpaired f-electrons. The diffuse reflectance spectroscopy also showed that these yellow plates are moderately wide band gap ( approximately 2.75 eV) semiconductors.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.