Wednesday, August 15, 2007

Greg Stewart : ``Short review of non centro symmetric superconducting compounds/recent results on CePt3Si''


Greg Stewart presented an experimental study of the non centro-symmetric superconductor CePt3Si. The main issue here is to understand the pairing state when the superconductor is lacking an inversion center. Parity is no longer a symmetry of the Cooper pairs, so that the pairing state is expected to be a mixture of even parity singlets and odd parity triplet pairs.

Superconductivity in CePt3Si develops inside an antiferromagnetic phase with Tc=0.75 K and TN= 2.2K. The extrapolation to zero temperature of the specific heat coefficient inside the supercondcuting phase seems to suggests the presence of nodes in the gap. The large enough value of the specific heat coeficient in the normal state (C/T ~ 160 mJ.mol.K^(-2)) shows that the compound has some heavy Fermi liquid character. Then Greg showed a cleaner compound by the group of Takeushi, which shows a bigger normal state specific heat coefficient (\gamma_n ~ 335 mJ mol K^(-2) ) as well as a bigger residual specific heat coefficient in the superconducting phase. The overall decrease of the thermodynamic coefficients with disorder reamains mysterious. Greg then compared CePt3Si with various othernon centro-symmetric superconductors recently sudied, such as Li2 Pt3B. It appears that in some compounds the specific heat coefficient has large values but in some others it has small values,!
so that not all of them can be considered as heavy fermions. Moreover Greg says that some of the compounds have residual specific heat coefficient in the superconducting phase while some others don?t, so that there is no systematic evidence for nodes.

The discussion became lively about what one can expect for such sueprconductors. It was asked by Chandra varma whether one could find at least one of these superconductors which is not at the same time antiferromagnet. Greg's answer was that there is not any. Then the discussion went on the fact that one theoretically expects triplet superconductivity, at least for part of the pairing. To show this experimentally is very difficult, Greg said.

11 comments:

greg stewart said...

I believe I said in answer to Chandra's question that, at this time, all the known *heavy Fermion* non-centrosymmetric superconductors, which include CePt3Si and the pair of pressure induced superconductors CeRhSi3 and CeIrSi3 (i. e. only three systems), are coexistent antiferromagnets and superconductors. There are certainly non-centro symmetric superconductors (like the recently discovered Al3Mg2) which are lower-gamma systems that are not antiferromagnets. Also, as a minor point, the normal state gamma for Bauer's sample of CePt3Si is 390 mJ/molK2, not 160.

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