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.

Since then over 30 heavy fermion superconductors were found (in materials based on Ce, U), with a critical temperature up to 2.3 K (in CeCoIn₅).

Material	TC (K)	comments	original reference
CeCu2Si2	0.7	first unconventional superconductor
CeCoIn5	2.3	highest TC of all Ce-based heavy fermions
CePt3Si	0.75	first heavy-fermion superconductor with non-centrosymmetric crystal structure
CeIn3	0.2	superconducting only at high pressures
UBe13	0.85	p-wave superconductor
UPt3	0.48	several distinct superconducting phases
URu2Si2	1.3	mysterious 'hidden-order phase' below 17 K
UPd2Al3	2.0	antiferromagnetic below 14 K
UNi2Al3	1.1	antiferromagnetic below 5 K

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.Template:Citation needed

From specific heat measurements $\mathrm{\Delta }C/C(T_{\mathrm{C}})$ one knows that the Cooper pairs in the superconducting state are also formed by the heavy quasiparticles. In contrast to normal superconductors it cannot be described by BCS theory. Due to the large effective mass, 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.Template:Citation needed

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