Nodeless superconducting gap in the caged-type superconductors Y₅Rh₆Sn₁₈and Lu₅Rh₆Sn₁₈

Abstract

Low-temperature thermal conductivity measurements were performed on the single crystal of caged-type superconductors Y 5 Rh 6 Sn 18 and Lu 5 Rh 6 Sn 18 down to 80 mK. In zero magnetic field, no residual linear term T 0 k is found for either superconductor. At low field, the T 0 k of these two superconductors shows weak field dependence. These results exclude the nodal superconducting gap in Y 5 Rh 6 Sn 18 as revealed by recent specific heat measurements, and suggest that the caged-type superconductors R 5 Rh 6 Sn 18 (R = Y and Lu) may have a common nodeless superconducting gap.

Nodeless superconductivity in Lu5−xRh6Sn18+x with broken time reversal symmetry

“…Our findings of nodeless superconductivity in LRS is highly consistent with other studies of the specific heat [28], thermal conductivity [30], and μSR [24]. We find that the superfluid density is generally well described by a single-gap s-wave model, where a small deviation at low temperatures could be due to a moderate gap anisotropy or multigap superconductivity.…”

“…The temperature dependence of the resistivity [ρ(T )] of an LRS crystal is displayed in Fig. 2(a), which increases slightly with decreasing temperature, before reaching a near constant value at low temperatures, in line with previous reports [24,28,30]. A sharp superconducting transition occurs at T c = 4.08 K, where ρ(T ) drops to zero.…”

“…Resistivity measurements of all three compounds show semiconducting behavior in the normal state, before becoming superconducting below 5.0, 3.0, and 4.0 K for R = Sc, Y, and Lu, respectively [28,29]. In the case of Lu 5 Rh 6 Sn 18 , the exponentially activated behavior of the electronic specific heat, negligible residual thermal conductivity, and saturation of the superfluid density derived from transverse-field μSR, yielded clear evidence for fully gapped superconductivity [24,28,30]. On the other hand, the proposed possible TRS breaking pairing states for R 5 Rh 6 Sn 18 from the symmetry analysis corresponded to a singlet d + id state with line nodes or nonunitary triplet pairing with point nodes [24].…”

“…Using this value together with an upper critical field of μ 0 H c2 (0) = 5.2 T from Ref. [30], we estimated the zero-temperature values of the Ginzburg-Landau coherence length ξ GL (0) and penetration depth λ(0) using μ 0 H c1 = /(4πλ 2 ) ln (λ/ξ GL ) and μ 0 H c2 = /2πξ 2 GL , which yield λ(0) = 390 nm and ξ GL = [37]. Using a carrier density n = 1.86 × 10 20 cm −3 calculated from the value of λ(0), and assuming a spherical Fermi surface with S/S F ≈ 1, the mean free path is estimated to be 9.33 nm.…”

Recent progress on superconductors with time-reversal symmetry breaking

“…On the other hand, the superfluid density extracted from transverse field µSR measurements for R=Lu and Y saturates at low temperature, and is well described by a single isotropic gap 157,158 . Moreover, nodeless superconductivity was deduced for these two compounds from the absence of a residual contribution to the thermal conductivity at low temperatures 160 , while the specific heat of Sc 5 Rh 6 Sn 18 is also well described by an s-wave model 161 . However, in the case of Y 5 Rh 6 Sn 18 evidence for gap anisotropy was found from a four-fold oscillation of the specific heat coefficient in the ab-plane 162 , and a non-linear field dependence of the same quantity 163 , suggesting a possible deviation from isotropic s-wave superconductivity.…”