Ⅲ-2. Atomic & Solid State Physics (Muon)

RIKEN Accel. Prog. Rep. 51 (2018)µCaged-type superconductor Sc5 Ru6 Sn18 probed by µSR
Dinesh Kumar,∗1 C. N. Kuo,∗1 F. Astuti,∗2,∗3 M. K. Lee,∗1 C. S. Lue,∗1 I. Watanabe,∗2,∗3 and L. J. Chang∗1,∗4Superconductors with caged-type structures have attracted considerable attention among researchers because of their fascinating characteristics, such as heavy
fermion superconductivity and exciton-mediated superconductivity. These compounds crystallize in thetetragonal structure having space group I4 1 /acd andconsist of three-dimensional skeletons that surroundlarge atomic cages, in which small atoms are situated. One such type of caged compound is R5 Rh6 Sn18
(R = Sc, Y, Lu), which exhibits superconductivity at 5 K (Sc), 3 K (Y) and 4 K (Lu),1) whereR occupies sites of diﬀerent symmetry.2) Superconducting gap structures and ZF-µSR have been studied in Lu5 Rh6 Sn18 and Y5 Rh6 Sn18 ,3,4) where thetime-reversal-symmetry-broken phenomenon has been
observed below Tc in both compounds. However,Lu5 Rh6 Sn18 is a strong-coupling s-wave superconductor with isotropic gap; while Y5 Rh6 Sn18 reveals ananisotropic superconducting gap with a point node.
These experimental
results in sharp contrast motivate≈
us to study the time-reversal-symmetry-broken mechanism in the R5 M6 Sn18 (R = Sc, Y, Lu; M = Co, Rh,Ru, and Ir) family. Sc5 Ru6 Sn18 is the ﬁrst compound
3for our studies.
µ Sc5 Ru6 Sn18 has Tc ≈ 3.5 K. The T
dependence of electronic heat capacity (Ce ) below Tc
at zero ﬁeld clearly indicates that Sc5 Ru6 Sn18 has ananisotropic superconducting gap with a point node.
In the ZF-µSR measurement, the sample temperature was changed in the ZF conditionµ from approxiµ TC ). The sammately 30 K (> TC ) down to 1.6 K (<ple was cooled down using a helium-ﬂow cryostat in theλHe exchange gas, maintaining good temperature homogeneity. The time dependence of A(t), which is called
the µSR time spectrum, was measured. Figure 1(a)shows the µSR time spectrum measured at varioustemperatures crossing Tc . Time spectra were analyzedby using the function A(T ) = A0 GKT × exp(−λt),subtracting the background µsignals coming fromtheλsample holder. Here, A0 is the initial asymmetry at
t = 0. GKT is the static Gaussian Kubo-Toyabe function to describe static internal ﬁelds, which come fromsurrounding nuclear moments, and are randomly distributed at a muon site. This term
can be treated asλa temperature-independent parameter from the viewpoint of the µSR time window.5) λ is the muon-spin depolarization
rate, which is considered related to the dy＊
namicspinﬂuctuationof surrounding electronic spins＊＊aroundthe muon. The solid lines are the best-ﬁt re＊
sults
obtained by using this analytical function. Fig∗1∗2∗3∗4

Department of Physics, National Cheng Kung University
RIKEN Nishina Center
Department of Physics, Hokkaido University
ASRC., JAEA

- 201 -µFig. 1. (a) ZF-µSR
time spectra of Sc5 Ru6 Sn18 measured
at various temperatures. The solid lines are the best-fit
results obtained by using the analysis function, A(T ) =λ KT ×exp(−λt). (b) Temperature dependence of theA0 Gmuon-spin relaxation rate obtained from the analysis by
λ A0 GKT × exp(−λt).
using the function A(T ) =ure 1(b) shows the ﬁtting of time spectra.
λ increasesµwith decreasing temperature, which was obtainedfromλtemperatures above Tc . This would mean that someλelectronic spins remain in the sample, causing thismuon-spin depolarization behavior.
An important observation in this ZF-µSR measurement is the temperature-independent behavior of λ below approximately 10 K, even crossing Tc . No changein λ µmeans that the time spectrum does not change
any more below 10 Kµ even in the superconducting
phase. In the case of the conventional superconducting
state with the BCS type s-wave Cooper pair symmetry,
there is no change in the time spectrum and we do notsee any eﬀect on the ZF-µSR data from the superconducting electronic state. On the other hand, the ZFµSR time spectrum is modiﬁed by the appearance of aspontaneous internal magnetic ﬁeld below Tc when thetime reversal symmetry (TRS) is broken. Such a casecan occur in the case of the p-wave Cooper pair symmetry, as has been proved in the case of Sr2 RuO4 .6)
As shown in Fig. 1(b), the time spectrum does notshow any changes in its shape within statistical errors; we can conclude that the TRS breaking of thesuperconducting state is unlikely in this system, eventhough the appearance of point-node structures on thesuperconducting gap state is suggested from the heat
capacity measurement. This result would restrict thediscussion of the model of the superconducting mechanism by excluding the possibility of the formation of
the p-wave symmetry.
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