Novel multi-phase superconductor

Kondo lattice systems are intermetallic compounds based on the rare-earth elements Cerium or Ytterbium, which present very unusual properties at low temperatures. Their ground state can be continuously tuned from an ordered to a disordered state through a so-called quantum critical point, e.g. by chemical substitution or external parameters such as pressure. In the vicinity of these quantum-critical points, the resulting very strong electronic correlation effects lead to very peculiar phenomena such as, for instance, unconventional superconductivity or charge carriers with extremely large effective masses thousand times larger than that of a free electron. Often, unconventional magnetic or multipolar ordered states, which are far from being understood, can also be observed.

We recently discovered a new Kondo-lattice system, CeRh2As2, which is very close to a quantum critical point and shows unconventional superconductivity below 0.3 K. In particular, this superconductor has two-phase superconductivity characterized by a transition between a low-field even-parity and high-field odd-parity phase [1]. It is explained by asymmetric spin-orbit coupling, the so-called Rashba effect, due to the local inversion symmetry breaking on the Ce site [2]. In addition, the peculiar order below 0.4 K was proposed to be quadrupolar density-wave order in which local higher-order electronic degrees of freedom are coupled to itinerant electrons and modulate in space [3]. Moreover, the low-field superconducting state coexists with a peculiar antiferromagnetic order [4].

The purpose of this project is to clarify the origin of these peculiar properties and their relations by investigating chemically-substituted samples. We are looking for a Ph.D. student with a background in material science and solid-state physics, who is interested in the synthesis and crystal growth of exotic materials as well as in the study of their magnetic, thermodynamic, and electronic properties at very low temperatures down to the mK regime. We expect about half of the work to be devoted to the crystal growth of pure and partially substituted CeRh2As2 and the other half to the experimental investigation of its properties.

[1] S. Khim, J. F. Landaeta, J. Banda, N. Bannor, M. Brando, P. M. R. Brydon, D. Hafner, R. Küchler, R. Cardoso-Gil, U. Stockert, A. P. Mackenzie, D. F. Agterberg, C. Geibel, E. Hassinger
Field-induced transition from even to odd parity superconductivity in CeRh2As2
Science 373, 1012–1016 (2021)
[2] Alexandre Pourret and Georg Knebel
Driving multiphase superconductivity
Science 373, 962-963 (2022)
[3] D. Hafner, P. Khanenko, E.-O. Eljaouhari, R. Küchler, J. Banda, N. Bannor, T. Lühmann, J. F. Landaeta, S. Mishra, I. Sheikin, E. Hassinger, S. Khim, C. Geibel, G. Zwicknagl, and M. Brando
Possible Quadrupole Density Wave in the Superconducting Kondo Lattice CeRh2As2
Phys. Rev. X 12, 011023 (2022)
[4] S. Ogata, S. Kitagawa, K. Kinjo, K. Ishida, M. Brando, E. Hassinger, C. Geibel, and S. Khim
Parity Transition of Spin-Singlet Superconductivity Using Sublattice Degrees of Freedom
Phys. Rev. Lett. 130, 166001 (2023)

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