Superconductivity in ZrB12 and LuB12 with Various Boron Isotopes

Abstract

In BCS-type superconductors Zr N B 12 (T C ≈ 6 K) and Lu N B 12 (T C ≈ 0.42 K) heat capacity C(T ) and magnetization measurements have been carried out on high quality single crystals with various boron isotopes (with N = 10, 11, and with natural composition of 10 B and 11 B). Parameters of the superconducting and normal states have been deduced from this study, allowing comparison between these two dodecaborides. It was shown that ZrB 12 is a type-II superconductor in which the Ginzburg-Landau parameter varies in the range κ = 0.8-1.12. A detailed analysis of specific heat in the normal state of R 10 B 12 , R 11 B 12 and R nat B 12 (where R = Zr, Lu) has revealed three Einstein type vibration modes of Zr 4+ -ions in ZrB 12 with characteristic energies θ E1 (Zr N B 12 ) ≈ 200 K and θ E2,3 (Zr N B 12 ) ≈ 450 K, N. Sluchanko ( ) •

Lattice instability and enhancement of superconductivity in YB6

“…[44]. It is also in agreement with the data of Θ D ≈ 1160-1190 K obtained in [2,45,48] for the analog non-magnetic higher boride − lutetium dodecaboride (LuB 12 ), and comparable to Θ D ≈ 1250-1370 K deduced for β-boron in X-ray diffraction studies [48]. Along with Einstein component, which leads to maximum on (C − γT − C D )/T 3 vs. T curves near 20 K [see Fig.14(a)], we observed two additional features on these dependence − one near 10 K and another below 4 K. The separation of these low-temperature contributions was made in the same manner as it was done for LaB 6 in [43,44], where the heat capacity below 20 K was associated with two additive two-level components attributed to vibrations of rare earth ions in the vicinity of boron vacancies (see two-level systems TLS 1 and TLS 2 in Fig.14(a), and also [49]).…”

“…This supports the expected s-type superconductivity in YB 6 . In the analysis of normal state heat capacity contributions of YB 6 we used the approach similar to that employed earlier [2], [41]- [45] in studies of higher borides of rare earth elements. In addition to the electronic component C el = γT with γ ≈ 3.8 mJ/(mol K 2 ) and the Debye C D contribution which originates from the rigid covalent framework of boron atoms…”

Charge transport inHoxLu1−xB12: Separating positive and negative magnetoresistance in metals with magnetic ions

“…As promising materials for the study of negative MR effect we have chosen the fcc metallic substitutional solid solutions Ho x Lu 1−x B 12 with Ho magnetic ions embedded in a rigid covalent boron cage of the dodecaboride lattice. Comprehensive investigations of high-quality single crystals of LuB 12 with various boron isotope compositions allowed recently finding a new disordered "cage-glass" phase at liquid nitrogen temperatures [38][39][40]. It was shown [39,40] 12 with magnetic rare-earth ions, there is also Lu to Ho substitutional disorder which interferes with the random displacements (static disorder) of R sites in the metallic cage-glass phase.…”

Investigation of the properties of superconducting plates with a thickness of the order of the coherence length ξ in the framework of the Ginzburg-Landau theory

“…However, for the penetration depth of magnetic field λ, there is an empirical formula describing the behavior of the parameter λ in the entire temperature range: (8) For actual superconducting structures (especially, high temperature superconductors), the temperature dependences of the magnetic field penetration depth λ and the coherence length ξ can differ from those described above [32], and, in some cases, the GL parameter k, which in the theory is considered to be temperature independent, can change with variations in the temperature [33,34]. In our work, we use tem perature dependences of λ and ξ in the form of expres sions (7).…”