Naoki Kase

Superconducting State of the Binary Boride Ru7B3 with the Noncentrosymmetric Crystal Structure

Single crystals of Ru 7 B 3 were grown by the Czochralski puling method in a tetra-arc furnace. The compound crystallizes in a Th 7 Fe 3 -type structure (hexagonal, space group: P 6 3 m c ), which has no inversion symmetry along the c -axis. Measurements in magnetic fields indicate type-II superconductivity with upper critical fields of H c2 (0) ≃17.2 kOe for H ∥[001] and 15.8 kOe for H ∥[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 144 A, 3110 A, and 21.6 for H ∥[100], and 138 A, 3520 A, and 25.5 for H ∥[001], respectively. Specific heat measurements reveal that Ru 7 B 3 is a weak-coupling superconductor with an isotropic superconducting gap.

Unconventional superconductivity in Y 5 Rh 6 Sn 18 probed by muon spin relaxation

Conventional superconductors are robust diamagnets that expel magnetic fields through the Meissner effect. It would therefore be unexpected if a superconducting ground state would support spontaneous magnetics fields. Such broken time-reversal symmetry states have been suggested for the high—temperature superconductors, but their identification remains experimentally controversial. We present magnetization, heat capacity, zero field and transverse field muon spin relaxation experiments on the recently discovered caged type superconductor Y5Rh6Sn18 (u2009TC=u20093.0u2009K). The electronic heat capacity of Y5Rh6Sn18 shows a T3 dependence below Tc indicating an anisotropic superconducting gap with a point node. This result is in sharp contrast to that observed in the isostructural Lu5Rh6Sn18 which is a strong coupling s—wave superconductor. The temperature dependence of the deduced superfluid in density Y5Rh6Sn18 is consistent with a BCS s—wave gap function, while the zero-field muon spin relaxation measurements strongly evidences unconventional superconductivity through a spontaneous appearance of an internal magnetic field below the superconducting transition temperature, signifying that the superconducting state is categorized by the broken time-reversal symmetry.

Highly Anisotropic Gap Function in a Nonmagnetic Superconductor Y5Rh6Sn18

The low temperature specific heat of R 5 Rh 6 Sn 18 (R = Y, Lu) was measured under various magnetic fields ( H ) up to 8 T. In a zero field, electronic specific heat of Y 5 Rh 6 Sn 18 shows a T 3 dependence, indicating an anisotropic superconducting gap, although that of Lu 5 Rh 6 Sn 18 shows an exponential dependence. In addition, the coefficient of the T -linear term γ( H ) of Y 5 Rh 6 Sn 18 in specific heat in the mixed state is found to obey a square root dependence. This is another evidence that Y 5 Rh 6 Sn 18 has the anisotropic superconducting gap. On the other hand, the γ( H ) of Lu 5 Rh 6 Sn 18 shows linear dependence, which means the isotropic superconducting gap.

Thermodynamic Properties of the Non-centrosymmetric Type-I Superconductor Rh2Ga9 and Ir2Ga9

We report low-temperature specific heat measurements on the single-crystalline non-centrosymmetric superconductors T 2 Ga 9 (T = Rh, Ir) with T c = 1.95 and 2.25 K. Electronic contribution on the specific heat, C el , in each compound is well described by the BCS scheme, suggesting that both Rh 2 Ga 9 and Ir 2 Ga 9 fully open an isotropic gap in the superconducting state. The values of Δ C /γ T c and 2Δ(0)/ k B T c indicate that both compounds are categorized as a weak coupling superconductor. The value of κ obtained from specific heat as a function of magnetic field strongly suggestes that T 2 Ga 9 is a type-I superconductor.

Superconducting Properties of Ca3T4Sn13 (T = Co, Rh, and Ir)

We present the results of the superconducting properties of R 3 T 4 Sn 13 (R = La, Sr, and Ca; T = Co, Rh, and Ir) with quasi skutterudite structure by means of magnetic susceptibility, electrical resistivity, and specific heat measurements. It is shown that the superconductivity is of conventional s-wave type and lies in the extremely strong-coupling regime. Electron–phonon coupling constants λ ep were estimated to be almost 1 for R 3 T 4 Sn 13 , respectively. The result can be categorized as a strong-coupling superconductor; this categorization is in good agreement with the results of specific heat measurements. The λ ep and the electronic density of states at the Fermi level ( N ( E F )) increases with the superconducting transition temperature. However, superconducting transition temperature of Co-compound cannot be explained by the difference of N ( E F ). From these results, we concluded that T c of R 3 T 4 Sn 13 can be described by the strength of the λ ep .

Coexistence of Superconductivity and Magnetism in the Tm-Based Reentrant Superconductor Tm5Rh6Sn18

We report here the magnetic properties of the thulium-based reentrant superconductor Tm 5 Rh 6 Sn 18 ( T c =2.2 K), which were determined by magnetization and µSR measurements. The spontaneous oscillation signal in the µSR spectra gradually develops under zero magnetic field below 13 K. The development of a quasi-static local magnetic field is also observed even below T c , indicating the coexistence of magnetism and superconductivity at zero magnetic field. From the analysis of longitudinal µSR measurements, magnetic ordering can be considered to be not truly long range but superparamagnetic.

Superconducting State in the Ternary Germanide Y2AlGe3 with Related ThCr2Si2-Type Structure

Superconductivity in a ternary germanide, Y 2 AlGe 3 , was revealed at the transition temperature of 4.5 K by electrical resistivity and magnetic susceptibility measurements. The magnetization versus magnetic field ( M – H ) curve indicated that Y 2 AlGe 3 was a type-II superconductor. The lower critical field H c1 (0) and the upper critical field H c2 (0) were estimated to be 1.66(8) mT and 1.26(15) T. The coherence length ξ(0) GL , penetration depth λ(0) GL , and Ginzburg–Landau (GL) parameter κ(0) obtained from the critical fields were 16.2(5) nm, 631(16) nm, and 38(3), respectively. Specific heat measurements revealed that Y 2 AlGe 3 was a weak-coupling BCS superconductor with an isotropic superconducting gap. The electron–phonon mass enhancement parameter λ γ was calculated to be 0.13, which was in good agreement with the weak-coupling limit inferred from the specific heat Δ C e /γ T c and 2Δ(0)/ k B T c .