Superconducting state of the binary boride Ru ₇B ₃ with the noncentrosymmetric crystal structure

Single crystals of Ru ₇B ₃ were grown by the Czochralski puling method in a tetra-arc furnace. The compound crystallizes in a Th ₇Fe ₃-type structure (hexagonal, space group: P6 ₃, mc), which has no inversion symmetry along the oaxis. Measurements in magnetic fields indicate type-II superconductivity with upper critical fields of H _c2(0) ≃ 17.2kOe for H &par [001] and 15.8kOe for H &par [100], respectively. The coherence length ξ(0) _GL, penetration depth λ(0)GL, and Ginzburg-Landau (GL) parameter κ(0) obtained from these values are 144Å, 3U0Å, and 21.6 for H ∥ [100], and 138 Å, 3520Å, and 25.5 for H &par [001], respectively. Specific heat measurements reveal that Ru ₇B ₃ is a weak-coupling superconductor with an isotropic superconducting gap.