Superconducting Ground State of the Topological Superconducting Candidates Ti₃X (X = Ir, Sb)

Superconductivity in topological materials is reported to host various unconventional properties. Due to the limited number of topological superconducting materials, the pairing mechanism is not clearly understood. Recent studies suggest that bulk superconductors can possess a topologically nontrivial band structure [1]. Among these, A15 superconductors, a well-known family of metal-based compounds, are promising candidates to realize topological superconductivity. Reports suggest that the standard crystal structure symmetry of A15 compounds, along with spin-orbital coupling (SOC), creates a gapped crossing near the Fermi level. The unnoticed topological surface states near the Fermi surface of the A15 compounds were unveiled in recent theoretical calculations, indicating the potential candidacy to host topological superconductivity [2, 3]. The topological bulk band structures of these compounds can be characterized by nontrivial Z2 invariants, with topological surface bands appearing near the Fermi surface as dictated by the bulk boundary correspondence. Though the surface state of the A15 - Ti₃X (X = Ir, Sb) compound has been studied theoretically, to the best of our knowledge, none of the early works had a follow-up regarding the microscopic investigation of their superconducting properties. We investigated the superconducting ground state of topological superconducting material candidates Ti₃Ir and Ti₃Sb, coupled with magnetization, heat capacity, and muon spin rotation and relaxation measurements [4].<br/>Magnetization measurements suggest that both the compounds are type-II superconductors with bulk transition temperatures of 4.3 and 6.7 K, respectively. The upper critical field for Ti₃Ir is higher than the Pauli limit indicating a possible unconventional pairing mechanism in the superconducting ground state. Muon spin rotation and relaxation (μSR) measurement in transverse field (TF) mode was done to reveal the field distribution across the sample and find temperature dependence of magnetic penetration depth to a high degree of accuracy. However, specific-heat and TF-μSR measurements rule out any possibility of a nodal or anisotropic superconducting gap but reveal a conventional isotropic s-wave gap structure in Ti₃X (X = Ir, Sb). Zero-field μSR measurement is an excellent means to detect spontaneous magnetization that can be associated with spin-triplet superconductivity [5-7]. The measured zero-field-μSR spectra below and above superconducting transition T_C were best fitted by the static Kubo-Toyabe function multiplied by an exponential decay [8]. There is no change in asymmetry spectra for the samples at temperatures above and below Tc. This suggests the absence of a spontaneous magnetic field below the superconducting transition temperature, categorizing Ti₃X (X = Ir, Sb) compounds into time-reversal-symmetry-preserved topological superconducting candidates [4]. In future, it is vital to study more A15 superconductors to understand the role of the nontrivial band topology and topological surface states in the superconducting ground state.<br/>References:<br/>[1] S. Yonezawa, AAPPS Bull. 26, 3 (2016).<br/>[2] M. Kim, et al., Phys. Rev. B 99, 224506 (2019).<br/>[3] E. Derunova, et al., Sci. Adv. 5, eaav8575 (2019).<br/>[4] M. Mandal, et al., Phys. Rev. B 103, 054501 (2021).<br/>[5] G. M. Luke, et al., Nature (London) 394, 558 (1998).<br/>[6] Y. Maeno, et al., J. Phys. Soc. Jpn. 81, 011009 (2012).<br/>[7] G. M. Luke, et al., Phys. Rev. Lett. 71, 1466 (1993).<br/>[8] R. S. Hayano, et al., Phys. Rev. B 20, 850 (1979).