Condensed-matter physicists knew that CeRh2As2 was an unconventional superconductor, but they didn’t appreciate just how unconventional it was until an international team of researchers took a closer look at how it behaves in high magnetic fields. According to the latest findings from researchers at the Max Planck Institute for Chemical Physics of Solids (MPI CPfS) in Dresden, Germany and colleagues, CeRh2As2 is one of only a few materials to boast an odd-parity superconducting state – that is, one that is stable to magnetic fields applied in certain directions.
Superconductivity usually exists in two forms. The first is easily destroyed by the presence of a magnetic field and is said to have even parity – that is, the wavefunction of the superconducting state is symmetric with respect to an inversion point. The second is stable in magnetic fields applied in certain directions and has odd parity – that is, its wavefunction is asymmetric.
In materials with odd-parity superconductivity, the critical field at which superconductivity disappears should exhibit a characteristic angle dependence, explains study leader Elena Hassinger. Odd-parity superconductivity is, however, rare and only a few materials (including UPt3 and the ferromagnetic superconductors UCoGe, URhGe, UGe2 and UTe2) are known to have it – and none of them display the expected angle dependence.
Low- and high field states
CeRh2As2 is a so-called heavy fermion compound that was recently found to exhibit two superconducting states: a low-field state and a higher-field one that appears when a magnetic field of 4 T is applied along a certain direction of the material’s crystal structure (the c-axis). In their work, Hassinger and colleagues showed that the angle dependence in CeRh2As2 is precisely that expected of the odd-parity state.
They obtained this result by measuring the material’s AC magnetic susceptibility, magnetic torque and specific heat as a function of temperature, applied magnetic field and different directions of the magnetic field with respect to the crystal axes. In these experiments, ultralow temperatures below 1 K and high magnetic fields of up to 36 T were required.
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“We compared the resulting phase diagrams with a model by our international collaborators that contains both the even- as well as the odd-parity superconducting states,” Hassinger tells Physics World. “The model we used is simple yet powerful since it is based on the symmetry of the crystal and relies on a very limited number of parameters. It can reproduce our experimental results almost perfectly.
“Researchers have been looking for odd-parity superconductivity for a long time and CeRh2As2 allows us to investigate the properties of this type of superconductivity – namely the electron pairing mechanisms at play [that are] responsible for how superconductivity evolves,” she adds. “The material is also important for further understanding the interplay of superconductivity with the other ordered states that have been previously observed – for example, a quadrupole density-wave state and an antiferromagnetic state that appear at a similar temperature to the superconductivity.”
Full details of the research are reported in PRX.
