Crystal structure and electronic structure of CePt2In7

We report a corrected crystal structure for the CePt2In7 superconductor, refined from single crystal x-ray diffraction data. The corrected crystal structure shows a different Pt-In stacking along the c-direction in this layered material than was previously reported. In addition, all the atomic sites are fully occupied with no evidence of atom site mixing, resolving a discrepancy between the observed high resistivity ratio of the material and the atomic disorder present in the previous structural model The Ce-Pt distance and coordination is typical of that seen in all other reported Ce_nM_mIn_3n+2m compounds. Our band structure calculations based on the correct structure reveal three bands at the Fermi level that are more three dimensional than those previously proposed, and Density functional theory (DFT) calculations show that the new structure has a significantly lower energy.

Bulk properties and electronic structure of PuFeAsO

Here we present bulk property measurements and electronic structure calculations for PuFeAsO, an actinide analogue of the iron-based rare-earth superconductors RFeAsO. Magnetic susceptibility and heat capacity data suggest the occurrence of an antiferromagnetic transition at TN=50 K. No further anomalies have been observed down to 2 K, the minimum temperature that we have been able to achieve. Structural measurements indicate that PuFeAsO, with its more localized 5f electrons, bears a stronger resemblance to the RFeAsO compounds with larger R ions, than NpFeAsO does.

Negative thermal expansion and antiferromagnetism in the actinide oxypnictide NpFeAsO

A neptunium analogue of the LaFeAsO tetragonal layered compound has been synthesized and characterized by a variety of experimental techniques. The occurrence of long-range magnetic order below a critical temperature T_N = 57 K is suggested by anomalies in the temperature-dependent magnetic susceptibility, electrical resistivity, Hall coefficient, and specific heat curves. Below T_N, powder neutron diffraction measurements reveal an antiferromagnetic structure of the Np sublattice, with an ordered magnetic moment of 1.70(0.07) mu_B aligned along the crystallographic c-axis. No magnetic order has been observed on the Fe sublattice, setting an upper limit of about 0.3 mu_B for the ordered magnetic moment on the iron. High resolution x-ray powder diffraction measurements exclude the occurrence of lattice transformations down to 5 K, in sharp contrast to the observation of a tetragonal-to-orthorhombic distortion in the rare-earth analogues, which has been associated with the stabilization of a spin density wave on the iron sublattice. Instead, a significant expansion of the NpFeAsO lattice parameters is observed with decreasing temperature below T_N, corresponding to a relative volume change of about 0.2% and to an invar behavior between 5 and 20 K. First-principle electronic structure calculations based on the local-spin density plus Coulomb interaction and the local density plus Hubbard-I approximations provide results in good agreement with the experimental findings.

Superconductivity in the Heusler Family of Intermetallics

Several physical properties of the superconducting Heusler compounds, focusing on two systems (Y, Lu, Sc)Pd2Sn and APd2M, where A=Hf, Zr and M=Al, In, are summarized and compared. The analysis of the data shows the importance of the electron-phonon coupling for superconductivity in this family. We report the superconducting parameters of YPd2Sn, which has the highest Tc among all known Heusler superconductors.

Crystal fields, disorder, and antiferromagnetic short-range order in Yb0.24Sn0.76Ru

We report extensive measurements on a new compound (Yb0.24Sn0.76)Ru that crystallizes in the cubic CsCl structure. Valence band photoemission and L3 x-ray absorption show no divalent component in the 4f configuration of Yb. Inelastic neutron scattering (INS) indicates that the eight-fold degenerate J-multiplet of Yb3+ is split by the crystalline electric field (CEF) into a {Gamma}7 doublet ground state and a {Gamma}8 quartet at an excitation energy 20 meV. The magnetic susceptibility can be fit very well by this CEF scheme under the assumption that a {Gamma}6 excited state resides at 32 meV; however, the {Gamma}8/{Gamma}6 transition expected at 12 meV was not observed in the INS. The resistivity follows a Bloch- Gruneisen law shunted by a parallel resistor, as is typical of systems subject to phonon scattering with no apparent magnetic scattering. All of these properties can be understood as representing simple local moment behavior of the trivalent Yb ion. At 1 K, there is a peak in specific heat that is too broad to represent a magnetic phase transition, consistent with absence of magnetic reflections in neutron diffraction. On the other hand, this peak also is too narrow to represent the Kondo effect in the {Gamma}7 ground state doublet. On the basis of the field-dependence of the specific heat, we argue that antiferromagnetic shortrange order (possibly co-existing with Kondo physics) occurs at low temperatures. The long-range magnetic order is suppressed because the Yb site occupancy is below the percolation threshold for this disordered compound.

The magnetic properties of the Garnet and glass forms of Mn3Al2Si3O12

The magnetic susceptibilities and specific heats of the crystalline garnet and glass forms of Mn3Al2Si3O12 are reported. This allows a direct comparison of the degree of magnetic frustration of the triangle-based garnet lattice and the structurally disordered solid at the same composition for isotropic spin 5/2 Mn^2+ (3d^5). The results show that the glass phase shows more pronounced signs of magnetic frustration than the crystalline phase. Through comparison of the specific heats of Ca3Al2Si3O12 (grossular) and Mn3Al2Si3O12 (spessartine) garnets, information is provided concerning the anomalous extra specific heat in the latter material.

Physical properties of the new Uranium ternary compounds U3Bi4M3 (M=Ni, Rh)

We report the properties of two new isostructural compounds, U3Bi4Ni3 and U3Bi4Rh3. The first of these compounds is non-metallic, and the second is a nearly ferromagnetic metal, both as anticipated from their electron count relative to other U-based members of the larger 3-4-3 family. For U3Bi4Rh3, a logarithmic increase of C/T below 3 K, a resistivity proportional to T^4/3, and the recovery of Fermi-liquid behavior in both properties with applied fields greater than 3T, suggest that U3Bi4Rh3 may be a new example of a material displaying ferromagnetic quantum criticality.

The effect of Fe and Ru substitution for Ni on the thermopower in MgCNi_3

The intermetallic perovskite MgCNi3 is a superconductor with a Tc=7 K. Substitution of Fe and Ru for Ni decreases Tc monotonically as the doping concentration is increased. Here we report thermopower measurements, S(T), on MgCNi3, MgCNi3-xFex and MgCNi3-xRux. For MgCNi3, the thermopower is negative, - 12.5 mikroV/K, at 300 K. The absolute value of S decreases as x increases in MgCNi3-xFex and MgCNi3-xRux. The sign of S changes from negative to positive at low temperatures for values of x > 0.01. These data show that the carriers in MgCNi3 are electrons, and by increasing x and decreasing temperature, the participation of hole carriers clearly increases. The influence of the magnetic moments of the Fe atoms on the thermopower is not visible.