Unconventional superconductivity in heavy-fermion compounds

4 problems

• Order parameter of the fully gapped heavy-fermion superconductor $\mathrm{CeCu_2Si_2}$

  Contested

  $\mathrm{CeCu_2Si_2}$, 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 $\mathrm{CeRh_2As_2}$

  Contested

  In $\mathrm{CeRh_2As_2}$ 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 $\mathrm{UTe_2}$ above 400 kG

  Contested

  In $\mathrm{UTe_2}$, 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 $\mathrm{UTe_2}$

  Contested

  $\mathrm{UTe_2}$ is a heavy-fermion superconductor whose electron pairs probably have parallel spins (spin-triplet), but the exact pairing pattern is still argued over.