Frank Steglich: Superconductivity of heavy electrons: New Insights into the Enigma

Frank Steglich (MPI CPfS, Dresden) Pictures to be added.

Frank Steglich discussed some new insights into CeCu2Si2. CeCu2Si2 is the first heavy fermion superconductor to be discovered. Thirty years later, this material continues to provide new clues into the nature of the glue involved in heavy electron superconductivity. In the last 5 years or so, it has become apparant that the phase diagrams of heavy electron materials are frequently controlled by a quantum critical point - and in this respect, CeCu2Si2 is no different.

The ground state of CeCu2Si2 is highly sensitive to the method of preparation, and there are two limiting types of sample - "A" and "S". The former are antiferromagnetic, whereas S samples are superconducting. The S-type samples have a slight excess of Cu.

"A" type samples have the property that the application of a pressure drives T_N down to zero, where they pass through a quantum critical point arpimd 0.7GPa. In the vicinity of the QCP, the A type material becomes an S-type material, and develops heavy electron superconductivity.

In the vicinity of this QCP, A-type CeCu2Si2 appears to behave as a classic quantum spin density wave, or "Millis-Hertz" quantum critical point. In particular,

• resistivity rho(T) ~ T^1.5
• Cv/T = gamma(T) = constant - b T^ 0.5

(See Gegenwart et al, PRL, 1501, 1998, Sparn et al, Rev. High Press Sci Technol. 7, 431 (1998)). These features are hall marks of a quantum critical point associated with the formation of a spin density wave.
Frank showed us recent data that has confirmed this hypothesis - showing a clear incommensurate magnetic peak in the elastic neutron scattering around

Q=(0.226,0.226, 1.467).

(See Stockert et al, PRL 92, 136401 (2004) ). Frank asked the question - do these soft magnetic fluctuations provide the glue, at the QCP, that drives the superconductivity?

He then showed us some hot new data taken by Oliver Stockert and collaborators at the MPICPFS in which a "resonance" has been observed in the inelastic neutron scattering
results, in the superconducting phase. As one cools below Tc, this resonance develops at the incommensurate Q vector of the antiferromagnet, and the energy of the resonance grows as the temperature is reduced, maxing out at 0.22meV. This energy is closely related to the gap of this superconductor, and indicates that 2Delta/ k T_C ~ 4.3, around the right value for a weak coupling sc.

Frank Steglich argued that these results support the idea that the incommensurate magnetic fluctuations provide the glue that drives the pairing in S-type CeCu2Si2. In this respect,
this material is quite close to UPd2Al3, which develops superconductivity around 2K. This material has an acoustic crystal field excitation around Q=(0,0,1/2) which can be directly related to an anomaly in the conductance. In UPd2Al3, a Macmillan-Rowell-Eliashberg analysis was able to relate the neutron data with dI/dV, providing support for the idea that here too, the superconducting glue is provided by a lightly damped, acoustic excitation.
(See Sato et al, Nature 410, 340 (2001). )

However! This is not the whole story, for it turns out that one can apply chemical pressure to CeCu2Si2, by alloying with Germanium, to form CeCu_2(Si_1-x Ge_x)_2. When one does so, one finds that a second "island of superconductivity" develops. at an effective pressure beyond the quantum critical point. (See Yuan et al., Science 202, 2104 (2003) , Yuan et al, PRL 96, 047008 (2006) ). This second island of superconductivity is believed to be connected with a valence instability. Various groups have suggested that here, the soft excitation responsible for the glue, is a soft valence fluctuation.



These results lead Frank Steglich to conclude that more than one pairing mechanism
is operating in heavy electron systems - one has signs that pairing is produced by

• valence fluctuations
• antiferromagnetic fluctuations
• ferromagnetic fluctuations
• quadrupole fluctuations (skudderudite compound)

Frank ended with a note of caution - remarking that there also appear to be heavy electron systems such as the CeCoIn5 and YbRhIn5 that do not fit so naturally into a critical spin, or valence fluctuation picture. These "hard" quantum critical systems remain an open challenge, he remarked.

Remarks and questions from the blogger:

1. Can we really be sure that there is such a diverse set of pairing mechanisms? If so - what is the unifying thread between them?
   
2. What has happened to the Kondo effect in systems like CeCu2Si2 - how can a density wave form without this effecting the screening of the ions - in which case - can we really be so sure that a density wave scenario works in this case?
3. If CeCu2Si2 has a magnetic and a mixed valence pairing mechanism - then why is it that the superconducting domes for these two mechanisms merge together? It would seem quite unlikely that two completely different pairing mechanisms would produce share a common superconducting ground state.