Superconductivity in the YIr2Si2 and LaIr2Si2 Polymorphs

We report on existence of superconductivity in YIr2Si2 and LaIr2Si2 compounds in relation to crystal structure. The two compounds crystallize in two structural polymorphs, both tetragonal. The high temperature polymorph (HTP) adopts the CaBe2Ge2-structure type (space group P4/nmm) while the low temperature polymorph (LTP) is of the ThCr2Si2 type (I4/mmm). By studying polycrystals prepared by arc melting we have observed that the rapidly cooled samples retain the HTP even at room temperature (RT) and below. Annealing such samples at 900C followed by slow cooling to RT provides the LTP. Both, the HTP and LTP were subsequently studied with respect to magnetism and superconductivity by electrical resistivity, magnetization, AC susceptibility and specific heat measurements. The HTP and LTP of both compounds respectively, behave as Pauli paramagnets. Superconductivity has been found exclusively in the HTP of both compounds below Tsc (= 2.52 K in YIr2Si2 and 1.24 K in LaIr2Si2). The relations of magnetism and superconductivity with the electronic and crystal structure are discussed with comparing experimental data with the results of first principles electronic structure calculations

A Method for Generation of Bone Marrow-Derived Macrophages from Cryopreserved Mouse Bone Marrow Cells

The broad use of transgenic and gene-targeted mice has established bone marrow-derived macrophages (BMDM) as important mammalian host cells for investigation of the macrophages biology. Over the last decade, extensive research has been done to determine how to freeze and store viable hematopoietic human cells; however, there is no information regarding generation of BMDM from frozen murine bone marrow (BM) cells. Here, we establish a highly efficient protocol to freeze murine BM cells and further generate BMDM. Cryopreserved murine BM cells maintain their potential for BMDM differentiation for more than 6 years. We compared BMDM obtained from fresh and frozen BM cells and found that both are similarly able to trigger the expression of CD80 and CD86 in response to LPS or infection with the intracellular bacteria Legionella pneumophila. Additionally, BMDM obtained from fresh or frozen BM cells equally restrict or support the intracellular multiplication of pathogens such as L. pneumophila and the protozoan parasite Leishmania (L.) amazonensis. Although further investigation are required to support the use of the method for generation of dendritic cells, preliminary experiments indicate that bone marrow-derived dendritic cells can also be generated from cryopreserved BM cells. Overall, the method described and validated herein represents a technical advance as it allows ready and easy generation of BMDM from a stock of frozen BM cells

YbAgGa₂: Synthesis, crystal structure and magnetic behaviour

A novel intermetallic compound with two crystallographic modifications, alpha- and beta-YbAgGa2, was prepared by argon high frequency melting from ingots of the elements in tantalum crucibles with homogenization heat treatments at 600 degrees C (beta-YbAgGa2) and 400 degrees C (alpha-YbAgGa2). The crystal structure of beta-YbAgGa2 has been refined from powder diffraction data obtained using a Ge(111) primary monochromator and a linear position-sensitive detector. The sample was measured in transmission mode with Cu K alpha(1) radiation in the range 10 degrees<2 theta<130 degrees, Delta theta=0.02 degrees: a=6.9653(3) Angstrom, b=4.3362(2) Angstrom, c=10.3386(4) Angstrom, V=311.85(4) Angstrom(3). Full profile refinement led to the residual values R(I)=0.061 and R(P)=0.081 (for 303 reflections), space group Pnma, oP16; 4 Yb in 4(c) x 1/4 z, x=0.2241(2), z=0.82774(9), 4 Ga1=0.926(6)Ga + 0.074(6)Ag in 4(c), x=0.2950(3), z=0.5369(2), 4 Ga2 in 4(c), x=0.0751(3), z=0.1497(2), 4 Ag=0.951(6)Ag + 0.049(6) Ca in 4(c), x=0.4595(2), z=0.0883(1). Coordination numbers are 16 for Yb, 10 for Ga and 12 for Ag. Shortest interatomic distances were revealed for the following: Yb-Ga, 3.014 Angstrom; Yb-Ag, 3.183 Angstrom; Yb-Yb, 3.832 Angstrom; Ga-Ga, 2.622 Angstrom; Ga-Ag, 2.720 Angstrom; Ag-Ag, 3.024 Angstrom. The structure of beta-YbAgGa2 is a distorted variant of the ScRhSi2 type. Magnetic susceptibility measurements (4.8-900 K) revealed a rather low effective paramagnetic moment for beta-YbAgGa2 (0.5 mu(B) (Yb atom)(-1), Theta(p)=6.5 K) whereas a tripositive ground state is inferred for alpha-YbAgGa2 (about 4.5 mu(B) (Yb atom)(-1), Theta(p)=-71 K)