Inhomogeneous superconductivity in LuxZr1−xB12 dodecaborides with dynamic charge stripes

Abstract

We have studied the normal and superconductive state characteristics (resistivity, Hall coefficient, heat capacity and magnetization) of model strongly correlated electronic systems Lu x Zr 1-x B 12 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 -140 Å and with a Cooper pair size changing nonmonotonously in the range 450 -4000 Å. The parent ZrB 12 and LuB 12 borides are type-I superconductors, and Zr to Lu substitution induces a type-I -type-II phase transition providing a variation of the Ginzburg-Landau-Maki parameter in the limits 0.65   1,2  6. We argue in favor of the two-band scenario of superconductivity in Lu x Zr 1-x B 12 with gap values Δ 1 ~ 14 K and Δ 2 ~ 6 -8 K, with pairing corresponding to strong coupling limit ( e-ph ~ 1) in the upper band, and to weak coupling ( e-ph ~ 0.1 -0.4) in the lower one. A pseudogap Δ ps-gap ~ 60 -110 K is observed in Lu x Zr 1-x B 12 above T c . We discuss also the possibility of anisotropic single-band superconductivity with stripe-induced both pair-breaking and anisotropy, and analyze the origin of unique enhanced surface superconductivity detected in these model compounds.

Evidence of symmetry lowering in antiferromagnetic metal TmB12 with dynamic charge stripes

“…Rare earth dodecaborides RB 12 demonstrate a vast diversity of magnetic and transport properties including ferro-and antiferromagnetic ground states, metal-insulator transition and intermediate valence (see, for review [1][2][3]) and superconductivity [4,5]. These compounds have a relatively simple and highly symmetrical UB 12 -type fcc crystal structure [6], but various simultaneously active interactions in these strongly correlated electron systems make their quantitative theoretical description a difficult task.…”