Unconventional superconductivity in heavy-fermion compounds

4 problems

• Order parameter of the fully gapped heavy-fermion superconductor CeCu2Si2

  Contested

  CeCu2Si2, the first heavy-fermion superconductor (1979), was long believed to have a gap vanishing along lines on the Fermi surface, but measurements since 2014 show a full gap, which a simple d-wave state cannot produce.

• Nature of the two superconducting phases of CeRh2As2

  Contested

  In CeRh2As2 a magnetic field along one crystal axis switches the superconductor from one state to another, possibly from even-parity to odd-parity pairing, made possible because each cerium layer lacks a center of inversion although the whole crystal has one.

• Mechanism of field-reentrant superconductivity in UTe2 above 400 kG

  Contested

  In UTe2, superconductivity reappears in magnetic fields of about 400 to 650 kG when the field is tilted at a particular angle, fields far above those that destroy ordinary superconductors.

• Symmetry of the superconducting order parameter of UTe2

  Contested

  UTe2 is a heavy-fermion superconductor whose electron pairs probably have parallel spins (spin-triplet), but the exact pairing pattern is still argued over.