Heavy fermion superconductor

Heavy fermion superconductors are a type of unconventional superconductor.

The first heavy fermion superconductor, CeCu₂Si₂, was discovered by Frank Steglich in 1978.^[1]

Since then over 30 heavy fermion superconductors were found (in materials based on Ce, U, but also other lanthanide and actinide elements), with a critical temperature up to 18.5 K (in PuCoGa₅).

Heavy fermion materials are intermetallic compounds, containing rare earth or actinide elements. The f-electrons of these atoms hybridize with the normal conduction electrons leading to quasiparticles with an enhanced effective mass.^{[citation needed]}

From specific heat measurements ${\displaystyle \Delta C/C(T_{\rm {C}})}$ one knows that the Cooper pairs in the superconducting state are also formed by the heavy quasiparticles.^[12] In contrast to normal superconductors it cannot be described by BCS theory. Due to the large effective mass,^[13] the Fermi velocity is reduced and comparable to the inverse Debye frequency. This leads to the failing of the picture of electrons polarizing the lattice as an attractive force.^{[citation needed]}

Some heavy fermion superconductors are candidate materials for the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase.^[14] In particular there has been evidence that CeCoIn₅ close to the critical field is in an FFLO state.^[15]