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Winiarski, M. J.

Publications and source records attributed to Winiarski, M. J..

8 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Ferromagnetism in Pr-rich binary Pr 7 Ru 3 intermetallic compound

We present the synthesis and experimental characterization of the binary intermetallic compound Pr 7 Ru 3 . The polycrystalline sample was prepared by arc melting pure Pr and Ru, followed by homogenization at 500 °C and 600 °C for 48 and 89 h, respectively. Powder x-ray diffraction confirms that Pr 7 Ru 3 crystallizes in an orthorhombic crystal structure (Pnma, space group no. 62) with the lattice parameters: a = 7.3606(7) Å, b = 23.120(1) Å and c = 6.5959(5) Å. Magnetization, resistivity, and heat capacity measurements reveal a ferromagnetic transition in Pr 7 Ru 3 with the Curie temperature T C ~ 24.5 K. The bulk transition is confirmed by a large λ-shape anomaly observed in the specific heat measurement. The magnetic susceptibility above the transition obeys the modified Curie-Weiss law with a positive Curie-Weiss temperature $\ominus$ CW = 30(1) K and an effective magnetic moment of 3.39(1) μ B /Pr. Resistivity data for Pr 7 Ru 3 reveals metallic-like behavior with a clear anomaly at the transition temperature which is smeared by an applied magnetic field. We also synthesized high-quality nonmagnetic analog La 7 Ru 3 for which superconducting transition is observed with T c = 1.95 K, in agreement with the literature.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Dirac fermions and possible weak antilocalization in LaCuSb 2

Layered heavy-metal square-lattice compounds have recently emerged as potential Dirac fermion materials due to bonding within those sublattices. We report quantum transport and spectroscopic data on the layered Sb square-lattice material LaCuSb 2 . Linearly dispersing band crossings, necessary to generate Dirac fermions, are experimentally observed in the electronic band structure observed using angle-resolved photoemission spectroscopy, along with a quasi-two-dimensional Fermi surface. Weak antilocalization that arises from two-dimensional transport is observed in the magnetoresistance, as well as regions of linear dependence, both of which are indicative of topologically nontrivial effects. Measurements of the Shubnikov–de Haas quantum oscillations show low effective mass electrons on the order of 0.065m e , further confirming the presence of Dirac fermions in this material.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Superconductivity in the intermetallic compound Zr 5 Al 4

Polycrystalline Zr 5 Al 4 was synthesized using the arc-melting method. Powder X-ray diffraction confirms the previously reported crystal structure of the Ti 5 Ga 4 -type ( P 6 3 / mcm ) with lattice parameters: a = 8.4312(6) Å, and c = 5.7752(8) Å. Electrical resistivity and low-temperature magnetic susceptibility measurements indicate that Zr 5 Al 4 exhibits superconducting behavior below 2K. The normalized heat capacity jump at T c = 1.82 K, Δ C/γT c = 1.50, confirms the bulk superconductivity. The Sommerfeld coefficient γ = 28.5 mJ mol -1 K -2 and the Debye temperature Θ D = 320 K were obtained by fitting the low-temperature heat capacity data. The electron-phonon coupling strength λ el-ph = 0.49 and estimated upper critical field μ 0 H c2 (0) = 1.09 T (dirty limit) suggest that Zr 5 Al 4 is a weakly coupled type-II superconductor. Finally, first-principles calculations show the presence of a van Hove singularity near the Fermi energy.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Crystal structure and physical properties of new Ca{sub 2}TGe{sub 3} (T = Pd and Pt) germanides

The crystallographic, electronic transport and thermal properties of Ca{sub 2}PdGe{sub 3} and Ca{sub 2}PtGe{sub 3} are reported. The compounds crystalize in an ordered variant of the AlB{sub 2} crystal structure, in space group P6/mmm, with the lattice parameters a = 8.4876(4) Å/8.4503(5) Å and c = 4.1911(3) Å/4.2302(3) Å for Ca{sub 2}PdGe{sub 3} and Ca{sub 2}PtGe{sub 3}, respectively. The resistivity data exhibit metallic behavior with residual-resistivity-ratios (RRR) of 13 for Ca{sub 2}PdGe{sub 3} and 6.5 for Ca{sub 2}PtGe{sub 3}. No superconducting transition is observed down to 0.4 K. Specific heat studies reveal similar values of the Debye temperatures and Sommerfeld coefficients: Θ{sub D} = 298 K, γ = 4.1 mJ mol{sup −1} K{sup −2} and Θ{sub D} = 305 K, γ = 3.2 mJ mol{sup −1} K{sup −2} for Ca{sub 2}PdGe{sub 3} and Ca{sub 2}PtGe{sub 3}, respectively. The low value of γ is in agreement with the electronic structure calculations.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Physical properties and electronic structure of La 3 Co and La 3 Ni intermetallic superconductors

La 3 Co and La 3 Ni are reported superconductors with transition temperatures of 4.5 and 6 K, respectively. In this work, we reinvestigate the physical properties of these two intermetallic compounds with magnetic susceptibility χ, specific heat C p and electrical resistivity ρ measurements down to 1.9 K. Although bulk superconductivity is confirmed in La 3 Co, as observed previously, only a trace of it is found in La3Ni, indicating that the superconductivity in La 3 Ni originates from an impurity phase. Superconducting state parameters for La 3 Co, including lower and upper critical fields and the superconducting gap, are estimated. Results of the theoretical calculations of the electronic structure for both materials are also presented, and comparison of the Fermi level location in La 3 Co versus La 3 Ni explains its larger superconducting T c . A major discrepancy between band structure calculations and the experimentally measured Sommerfeld coefficient is found. The measured electronic density of states is about 2.5 times larger than the theoretical value for La 3 Co. This effect cannot be explained by the electron-phonon interaction alone. Renormalization of γ, as well as an ~T 2 behavior of the resistivity, suggests the presence of spin fluctuations in both systems.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Superconductivity in CaBi 2

We observed superconductivity with critical temperature T c = 2.0 K in self-flux-grown single crystals of CaBi 2 . This material adopts the ZrSi 2 structure type with lattice parameters a = 4.696(1) Å, b = 17.081(2) Å and c = 4.611(1) Å. The crystals of CaBi 2 were studied by means of magnetic susceptibility, specific heat and electrical resistivity measurements. The heat capacity jump at T c is ΔC/γT c = 1.41, confirming bulk superconductivity; the Sommerfeld coefficient γ = 4.1 mJ mol -1 K -2 and the Debye temperature Θ D = 157 K. The electron–phonon coupling strength is λ el–ph = 0.59, and the thermodynamic critical field H c is low, between 111 and 124 Oe CaBi 2 is a moderate coupling type-I superconductor. Our results of electronic structure calculations are reported and charge densities, electronic bands, densities of states and Fermi surfaces are discussed, focusing on the effects of spin–orbit coupling and electronic property anisotropy. Furthermore, we find a mixed quasi-2D + 3D character in the electronic structure, which reflects the layered crystal structure of the material.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

The electronic structure of homogeneous ferromagnetic (Ga, Mn)N epitaxial films

X-ray Absorption Fine Structure (XAFS) techniques, namely, X-ray Near Edge Structure (XANES), Extended XAFS (EXAFS), and Anomalous X-ray Diffraction (AXRD) were used to investigate the local atomic and electronic structure of (Ga, Mn)N magnetic layers with Mn concentrations of up to 10% grown by Molecular Beam Epitaxy. The XANES and AXRD analysis prove the Mn incorporation on substitutional GaN lattice sites. EXAFS results indicate the good quality of the structure under examination, although 0.5 nitride atom vacancies were found. The Wien2k code was applied to interpret the XANES spectra quantitatively, i.e., to determine the electronic structure of the Mn atoms. It was shown that accounting for the core-hole effect is necessary to reconstruct effectively the XANES spectra. Conducted charge density analysis based on DFT calculations identified the valency of Mn atom to be of 2.4+.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗