Unusual multipolar order in the unconventional superconductor CeRh2As2

• Applicant: Dr. Seunghyun Khim
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 502700722

The recently discovered superconductor CeRh2As2 promises unique opportunities to study unconventional states of matter given by the distinctive local environment of Ce. Superconductivity with Tc = 0.26 K shows an unprecedented field-induced transition between a low-field spin-singlet to a high-field spin-triplet state. This most probably is due to by the lack of local inversion symmetry at the Ce site and accompanying spin-orbit coupling. Moreover, we identified an additional transition at T0 ≈ 0.4 K that has a nearly non-magnetic nature. T0 rises with increasing in-plane magnetic fields and a new phase appears above 9 T below T0. We suppose the T0 order to originate from quadruple or higher-order moments, exceptionally allowed by a mixing of two low-lying doublets through Kondo interactions. The detailed mechanism remains, however, to be clarified. Understanding the T0 order is important because not only it is a new type of order but also it affects the type of unconventional superconductivity. The main objects of this proposal are the growth of high-quality, partially substituted single crystals and their detailed high-field characterization in order to study the nature of the T0 order. We plan to establish complete magnetic phase diagrams, which require high magnetic fields above 20 tesla. Tuning relevant parameters by chemical substitution will introduce a change in the T0 order and superconductivity that should shed light on the underlying physics. For this purpose, we will (1) optimize the synthesis of quality-improved pristine and chemically doped (Ce,La)Rh2As2, Ce(Rh,Ir)2As2, and CeRh2(As,P)2 single crystals under a horizontal temperature gradient. Using these crystals, we will (2) perform heat-capacity, magnetization, and transport measurements both at the MPI CPfS and at the Hochfeld-Magnetlabor Dresden (HLD) up to 70 tesla.

• DFG Programme: Research Grants
• Co-Investigator: Professor Dr. Joachim Wosnitza