Institut für Festkörperphysik



Unconventional Superconductors

A small number of ferro- or antiferromagnetically ordered heavy fermion compounds exhibit a supercondcuting instability, if magnetic ordering is driven towards zero (compare also „Quantum critical compounds“). Superconductivity in such a scenario is, presumably, mediated by spin fluctuations rather than by the common BCS-like electron-phonon coupling. Moreover, the Cooper pairs carrying the supercurrent are constituted by heavy quasiparticles formed in most of the cases by a Kondo type interaction. The reduced Fermi velocity of these charge carriers with large effective masses causes an incredibly high upper critical field in relation to the respective transition temperature. In general, the superconducting order parameter (OP) in such materials exhibits line or point nodes, instead of the fully gapped OP in the classical BCS picture. As a consequence, many physical properties (e.g., specific heat, thermal conductivity, ultrasonic attenuation ...) exhibit power laws instead of exponential dependencies as it is the case for BCS superconductors.

In almost all previous studies on superconductors, it was assumed that the crystal has an inversion center, which makes it possible to separately consider the even (spin-singlet) and odd (spin-triplet) components of the superconducting OP. Recently, we have discovered a novel Ce compound (CePt3Si) which does not possess inversion symmetry in its crystal structure, but exhibits simultaneously long range magnetic order at 2.2 K and superconductivity below 0.75 K. The absence of inversion symmetry and an extraordinarily large upper critical field creates a number of constraints for the superconducting OP.

Currently, the following issues are discussed with respect to CePt3Si:
a specific mixing of spin-singlet and spin-triplet components; a helical modulation of the order parameter; linear dpendencies of the temperature dependent penetration depth and thermal conductivity; paramagnetic supercurrent; coexistence of supercondcutivity and magnetic order; splitting of the Fermi surface due to SO coupling.

Low temperature specific heat
of CePt3Si

Fig.1: Temperature dependent specific heat Cp/T of CePt3Si; the dashed line is a T3 extrapolation of Cp(T) at 0 T. The light and the dark shaded areas represent the Sommerfeld values γs and γn associated with electrons condensing into Cooper pairs, and normal state electrons, respectively.

Contact: E. Bauer, H. Michor, G. Hilscher