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Unusual superconductivity with two phases

New heavy fermion superconductor shows surprising characteristics

2021-08-29 – News from the Physics Department

Materials with multiple superconducting phases have so far rarely been observed. Scientists from the Max Planck Institute for Chemical Physics of Solids and the Technical University of Munich are now reporting on the discovery of a two-phase unconventional superconductivity in the heavy fermion superconductor CeRh\(_2\)As\(_2\). Their observations have been published in the journal Science.

Javier Landaeta, Seunghyun Khim, Elena Hassinger
Some of the study’s authors at the lab (from left to right): Javier Landaeta, Seunghyun Khim, Elena Hassinger. – Photo: J. Landaeta / MPI Dresden

Symmetry and symmetry breaking are key for the understanding of many interesting phenomena in condensed matter physics. Conventional superconductivity, i.e. superconductivity caused by phonon-electron coupling, for example, requires both time reversal and inversion symmetry, and removing one of them - like time reversal by a magnetic field - leads to the suppression of the superconducting order.

Since 2004, non-centrosymmetric superconductors became of interest whereby the lack of inversion symmetry is leading to spin-singlet and spin-triplet Cooper pair mixing in the superconducting phase. In fact, these superconductors are very robust against the application of a magnetic field due to strong spin-orbit coupling effects that arise because of the broken inversion symmetry.

Phase transition at 4 T

Scientists from the Max Planck Institute for Chemical Physics of Solids, the Technical University of Munich and international research institutions are now reporting on the measurement of a two-phase unconventional superconductivity in CeRh\(_2\)As\(_2\), a new heavy fermion superconductor. In the superconducting state, the researchers measured an internal phase transition at around 4 T when the magnetic field is applied in the direction of the crystallographic c axis. Such a phase transition at ambient pressure has so far been observed very rarely and is only known from UPt\(_3\). This exotic superconductor shows three superconducting phases at different temperatures and magnetic fields, but their nature is unclear.

‘We suggest that these observations in CeRh\(_2\)As\(_2\) result from physics different from that at play in UPt\(_3\),’ says Elena Hassinger, Professor for experimental solid-state Physics at TUM and research group leader at the Max Planck Institute for Chemical Physics of Solids in Dresden. “The key superconducting properties of CeRh\(_2\)As\(_2\) are likely a manifestation of the local inversion symmetry breaking and consequent Rashba spin-orbit coupling in an overall inversion-symmetric crystal structure.”

Manifestation of a local symmetry breaking

Although CeRh\(_2\)As\(_2\) is centrosymmetric with inversion symmetry, the material is locally non-centrosymmetry, with an inversion symmetry linking two non-centrosymmetric Ce-square lattices, each of which has a Rashba interaction. “You could think of it as a double-layer system with an interplay between an intralayer Rashba interaction and interlayer hopping, which leads to a c axis field-driven transition between two superconducting phases. In the low-field state, the superconductivity is of even parity, i.e. the sign of the wave function is the same on these two Ce layers, whereas the high-field state is of odd parity with alternating sign on the Ce layers.”

The unusual directional dependence of the critical magnetic field in CeRh\(_2\)As\(_2\) also fits into this picture: In the direction of the crystallographic c axis, the material shows an extremely high critical field (14 T) at a transition temperature of only 0.26 K, far larger than in the ground plane (1.9 T).

“Our observations that likely result from the local surrounding of Ce suggest that CeRh\(_2\)As\(_2\) will be a benchmark material to study the influence of spin-orbit coupling on electronic mechanisms for unconventional superconductors”, says Elena Hassinger.

Editing
Petra Riedel

Publication

Field-induced transition within the superconducting state of CeRh\(_2\)As\(_2\)
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

Further information

Scientists from the following institutions were involved in the study: Max Planck Institute for Chemical Physics of Solids, Dresden; University of Otago, New Zealand; University of St. Andrews, Great Britain; University of Wisconsin-Milwaukee and Technical University of Munich. In Germany, the research was supported by the Max Planck Society and the Deutsche Forschungsgemeinschaft (DFG).

Contact

Prof. Dr. Elena Hassinger
Research group Physics of Unconventional Metals und Superconductors
Max Planck Institute for Chemical Physics of Solids
Nöthnitzer Straße 40
01187 Dresden
Tel.: +49 351 4646-3229
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