Time Reversal Symmetry Breaking Probed by Muon Spin Relaxation in Caged Superconductors Y₅M₆Sn₁₈ (M = Ru, Ir)
D. Kumar1*, C. N. Kuo1, F. Astuti2,3, M. K. Lee4, M. R. Lees5, C. S. Lue1, I. Watanabe2,3, L. J. Chang1,6
1Department of Physics, National Cheng Kung University, Tainan city, Taiwan
2RIKEN Nishina Center, Riken, Wako, Saitama 351-0198, Japan
3Department of Physics, Hokkaido University, Sapporo 060-0810, Japan
4Instrumentation center, National Cheng Kung University, Tainan city, Taiwan
5Department of Physics, University of Warwick, Coventry CV4 7AL, UK
6Advanced Science Research Center, Japan Atomic Energy Agency (JAEA), Tokai, Naka, Ibaraki 319-1195, Japan
* Presenter:D. Kumar
R₅M₆Sn₁₈ alloys, also classified as caged compounds, (R = rare earth, M = transition metal) are a family of recently discovered compounds which have attracted a lot of attention of the researchers around the world due to their fascinating characteristics [1-6]. R₅Rh₆Sn₁₈ (R = Y, Lu, Sc) alloys are found to have superconducting properties with transition temperature Tc ≈ 3 K (Y), 4 K (Lu) and 5 K (Sc) [1]. Unconventional superconductivity has been probed in Y₅Rh₆Sn₁₈ by muon-spin relaxation (μSR) measurements, in which time reversal symmetry (TRS) breaking has been observed in the superconducting state, as internal magnetic field is detected in zero-field μSR measurements [2]. TRS breaking has also been probed by μSR in Lu₅Rh₆Sn₁₈ [6]. We have prepared single crystals of Y₅Ru₆Sn₁₈ and Y₅Ir₆Sn₁₈ materials by using Sn-flux method. We have performed x-ray diffraction, dc magnetization, specific heat, resistivity and muon-spin relaxation (μSR) measurements to study the physical properties of these crystals. Crystal purity is confirmed by x-ray diffraction measurements. Y₅Ru₆Sn₁₈ and Y₅Ir₆Sn₁₈ crystallize in tetragonal type structure (space group I41/acd and Z = 8) with a = 1.3695 nm, c = 2.7539 nm and a = 1.37059 nm and c = 2.7548 nm, respectively. Y₅Ru₆Sn₁₈ and Y₅Ir₆Sn₁₈ show superconducting behavior below Tc ≈ 1.0 K and 2.1 K, respectively, as indicated in the dc magnetization and heat capacity measurements. The resistivity measurement of Y₅Ru₆Sn₁₈ indicates that the compound has typical metallic behavior above Tc. However, the resistivity of Y₅Ir₆Sn₁₈ decreases with the increase in the temperature, indicating Y₅Ir₆Sn₁₈ becomes bad metal above the superconducting temperature, similar to Y₅Rh₆Sn₁₈ [4,5]. μSR measurements of Y₅Ru₆Sn₁₈ and Y₅Ir₆Sn₁₈ have indicated that time reversal symmetry is not broken in these materials as no internal magnetic field was detected in the zero-field μSR measurements. The temperature variation of the penetration depth does not show any observable change below Tc, in ZF-μSR measurements, confirming absence of internal fields in the superconducting state. μSR and other results have been discussed in details.

¹J. P. Remeika, G. P. Espinosa, A. S. Cooper, H. Barz, Z. Fisk, L. D. Woolf, H. C. Hamaker, M. B. Maple, G. Shirane, and W. Thomlinson, Solid State Commun. 34, 923 (1980).
²A. Bhattacharyya, D. Adroja, N. Kase, A. Hillier, J. Akimitsu and A. Strydom, Scientific Reports 5, 12926 (2015).
³N. Kase, K. Inoue, H. Hayamizu, and J. Akimitsu; J. Phys. Soc. Jpn. 80, Supplement A SA112 (2011).
⁴Naoki Kase, Shunichiro Kittaka, Toshiro Sakakibara, and Jun Akimitsu, JPS Conf. Proc. 3, 015042 (2014).
⁵Z Zhang,YXu, C N Kuo, X C Hong, M X Wang, P L Cai, J K Dong, C S Lue and S Y Li, Supercond. Sci. Technol. 28, 105008 (2015).
⁶A. Bhattacharyya, D. T. Adroja, J. Quintanilla, A. D. Hillier, N. Kase, A. M. Strydom, and J. AkimitsuPhys. Rev. B 91, 060503(R) (2015).

Keywords: Superconductivity, Magnetic properties, Specific heat, Caged superconductors, Single crystals