Laura Greene (Illinois): Point contact spectroscopy of strongly-correlated electron materials

Laura Greene was asked by the organizers to give an overview of her approach for developing new families of superconducting materials.

This research is a part of a much larger program to develop better practical superconductors (higher Jc and Hc2) that is being funded by the DOE as an Energy Frontier Research Center (EFRC). This program started about one year ago, so only some general guiding principles were presented by Laura, with a few specific examples of systems that they have started work on. The six strategies for their search for new SCs are to look for materials with:

1) Reduced Dimensionality
2) Transition Metal and other large U ions
3) Light atoms
4) Tunability
5) Charged and Multivalent ions
6) Low dielectric constant

In addition one would hope to combine some of these criteria with systems that have competing phases (as has been the case for cuprates and Fe-based SCs). I will not go into much detail on Laura’s discussion of the positives and negatives of each of these six strategies, since many of these have been discussed in the literature in the past. As was pointed out by Paul Canfield, and agreed to by Laura, the particular list of strategies is not so important. What is important, if such a search is to be successful, is that many groups go ahead and start synthesizing new materials based upon their own ideas of where to search. As history has shown, most discoveries of new superconductors have been based upon empirical methods with a lot of serendipity.

In agreement with Paul, Laura mentioned that there are now five other groups in the USA that are being supported by the Air Force to search for new superconductors. This is unprecedented support for such an activity in the USA. In addition, the Air Force and ICAM would like to encourage collaboration in this endeavor with scientists from other countries. Laura and Rick Greene have formed a working group of international scientists interested in such a collaboration (first meeting was at the SCES conference in July). However, it will take time and more discussions to see if, and how, such collaboration can be actually be made to work.

One strategy that was extensively discussed was #2 above, where the Illinois group feels that having a parent compound that is a Mott insulator that can be doped is a very promising approach to higher Tc materials. Questions about this were raised by many in the audience. In particular, Meigan Aronson asked if being near any metal-insulator phase boundary might not be a good approach. Laura agreed.

Some specific materials for the EFRC search were mentioned by Laura. These are:

1) Doping of BiOCuS, a system on the verge of a FM instability
2) Doping of Fe2La2O3E2 (E= S, Se), a material known to be a Mott insulator (although no evidence for this was presented).

In addition the Illinois group is trying to make artificial quantum materials using Jim Eckstein’s layer by layer MBE method. The exact systems to be tried were not specified.

The last 5 minutes of Laura’s talk were a very quick summary of her point contact spectroscopy (PCS) studies of correlated SCs, namely CoInCe5 and Fe (Se,Te). Her very interesting and nice work on the 115 material is published and the Fe (Se, Te) study is on the arXives. In 115, this work shows that the pairing symmetry is d-wave. However, this technique cannot easily tell about an s+- order parameter, as strongly suggested by other experiments in the Fe-based SCs . However, she is thinking about how to do this with PCS.

[Blogged by Rick Greene (no relation)]