Muons and magnets

Abstract:

The achiral organic radical dinitrophenyl nitronyl nitroxide crystallizes in two enan-tiomorphs, both being chiral tetragonal space groups that are mirror images of each other. Muon-spin rotation experiments have been performed to study the magnetic properties of these crystals and demonstrate that long-range magnetic order is established below a temperature of 1.10(1) K. Two oscil-latory components are detected in the muon data, which show two different temperature dependences.

Abstract:

Magnetic properties of organic radicals based on thienyl-and furyl-substituted nitronyl nitroxide (NN) and iminonitroxide (IN) were investigated by measuring the temperature dependence of the magnetization. The magnetic behavior of 2-benzo[b]thienyl NN (2-BTHNN) is interpreted in terms of the two-magnetic-dimer model, in which one dimer exhibits ferromagnetic (FM) intermolecular interaction and the other dimer shows antiferromagnetic (AFM) interaction. The existence of two dimers in 2-BTHNN is supported by crystal structure analysis. The magnetic behaviors of 2-bithienyl NN, 4-(2′-thienyl)phenyl NN (2-THPNN), 2-and 3-furyl NN, 2-benzo[b]furyl NN, and 3-benzo[b]thienyl IN are also reported. The one-dimensional alternating AFM nature observed in 2-THPNN is consistent with its crystal structure.

Abstract:

We have studied the normal and superconductive state characteristics (resistivity, Hall coefficient, heat capacity, and magnetization) of model strongly correlated electronic systems LuxZr1-xB12 with cooperative Jahn-Teller instability of the boron rigid cage and with dynamic charge stripes. It was found that these metals are s-wave dirty limit superconductors with a small mean free path of charge carriers l=5-140Å and with a Cooper pair size changing nonmonotonously in the range 450-4000 Å. The parent ZrB12 and LuB12 borides are type-I superconductors, and Zr to Lu substitution induces a type-I to type-II phase transition providing a variation of the Ginzburg-Landau-Maki parameter in the limits 0.65≤κ1,2≤6. We argue in favor of the two-band scenario of superconductivity in LuxZr1-xB12 with gap values Δ1∼14K and Δ2∼6-8K, with pairing corresponding to strong coupling limit (λe-ph∼1) in the upper band, and to weak coupling (λe-ph∼0.1-0.4) in the lower one. A pseudogap Δps-gap∼60-110K is observed in LuxZr1-xB12 above Tc. We discuss also the possibility of anisotropic single-band superconductivity with stripe-induced both pair breaking and anisotropy, and analyze the origin of a unique enhanced surface superconductivity detected in these model compounds.