Takahiro MURANAKA (村中 隆弘)

Abstract

We discovered a new superconductor Nb₅Si₂Al (T _c= 2.6 K), crystalizing with a W₅Si₃-type structure.
Field dependent magnetization demonstrated a type-II superconducting behavior.
The μ ₀ H _c2(T) was described with the GL model rather than the WHH theory, and it did not exceed the Pauli paramagnetic limiting field.
Critical fields μ ₀ H _c1(0) and μ ₀ H _c2(0) were estimated to be 1.65 mT and 2.31 T, respectively. 
The sharp specific heat jump C _el/γ T _c was observed at T _c, and the temperature dependence revealed a BCS-type isotropic gap within a weak-coupling regime.
Comparing other superconductors with the W₅Si₃-type structure in terms of band filling revealed that T _c obeyed an empirical relationship.

Abstract

YRh₄B₄, which exhibits high-$T _{\mathrm{c } }$ superconductivity in the RRh₄B₄ system, has two types of crystal structures, CeCo₄B₄ type (pt-type) and LuRu₄B₄ type (bct-type). Both types of structure have similar $T _{\mathrm{c } }$ (pt-type; $T _{\mathrm{c } }$ = 10.6 K, bct-type; $T _{\mathrm{c } }$ = 9.8 K), but the $\mu_0 H _{\mathrm{ { c2 } } }$ of both compounds is very different, pt-type; $\mu_0 H _{\mathrm{ { c2 } } }$ = 2.50 T and bct-type; $\mu_0 H _{\mathrm{ { c2 } } }$ = 9.85 T. From our results, one can see that the difference in $T _{\mathrm{c } }$ between pt-YRh₄B₄ and bct-YRh₄B₄ is mainly caused from the difference of the density of state at Fermi energy. The high-$T _{\mathrm{c } }$ in pt-YRh₄B₄ is derived from the high Debye temperature caused by vibrations of boron. In contrast, in bct-YRh₄B₄ it is caused by a stronger electron–phonon coupling than in pt-YRh₄B₄. Regarding the symmetry of superconductivity, it is suggested that pt-YRh₄B₄ is a clean limit superconductor and has basically a fully gapped superconducting gap, but is slightly anisotropic from the results of $C_{\textrm {el } } \propto \textrm {exp}( -\Delta / k_{\mathrm{B } } T$) below $T _{\mathrm{c } }$ and $\Delta \gamma (H) \propto H^{0.65}$. However, bct-YRh₄B₄ is a dirty limit superconductor and has an anisotropic superconducting gap from the results of $C_{\textrm {el } } \propto T^{3}$ below $T _{\mathrm{c } }$ and $\Delta \gamma (H)\propto H$.