Interplay between multigap superconductivity and multipolar order in PrTi₂Al₂₀

Unconventional superconductivity (SC) is one of the most astonishing yet hardest problems in condensed matter physics. While exotic SCs often appear near the border of magnetic order, a new route to realize unconventional SC is orbital instability found in iron-based compounds. However, it is difficult to directly explore the orbital instability effect experimentally since the orbital degree of freedom is always accompanied by the spin degree of freedom in d-electron materials.

A cubic Pr-based rare-earth compound PrTr₂Al₂₀ (Tr = Ti, V) possesses a unique crystalline electric field ground state called cubic nonmagnetic Γ₃, where only quadrupolar and octupolar degrees of freedom are active [1-3]. Besides, heavy fermion SC is found in the multipole ordered state. Thus, this system provides an ideal platform to study purely multipole-driven physics. Indeed, PrTi₂Al₂₀ exhibits a ferroquadrupolar (FQ) order at T_Q ∼ 2.0 K, followed by a SC transition at T_c ∼ 0.2 K. Hydrostatic pressure enhances the c-f hybridization strength, resulting in a maximum T_c ~ 1 K and effective mass m^* ~110 m₀ at ~ 8 GPa which is near the boundary of the FQ order[4].

In this presentation, we will present our recent study for the SC and normal state properties of PrTi₂Al₂₀ and Pr_1-xLa_xTi₂Al₂₀ via thermodynamic and transport probes. The Two-gap behavior of SC found in undoped PrTi2Al20 vanishes rapidly with a small amount of La doping, indicating the unconventional nature of the SC.

[1] A. Sakai and S. Nakatsuji, J. Phys. Soc. Jpn., 80, 063701 (2011).

[2] A. Sakai, K. Kuga, and S. Nakatsuji, J. Phys. Soc. Jpn., 81, 083702 (2012).

[3] M. Tsujimoto et al., Phys. Rev. Lett. 113, 267001 (2014).

[4] K. Matsubayashi et al., Phys. Rev. Lett., 109, 187004 (2012).