Eric Bauer (Los Alamos National Laboratory): Understanding anisotropy to develop superconductors by design

In the afternooon session, two of the worlds leading intermetallic synthesis experts: Eric Bauer from the condensed matter group at Los Alamos National Laboratories and Yuri Green from the Max Planck Institute for Chemical Physics, Dresden,  presented two contrasting views of the challenges of material design.

Eric Bauer posed the question - how can you make superconductors by design? One of the strategies adopted by the LANL group is to try to exploit and understand anisotropies of intermetallic crystals. He showed how in the family of heavy electron 115 materials, the superconducting transition temperature Tc scales with the c/a ratio of the tetragonal unit cell - a feature that traditionally was interpreted as a consequence of Tc becoming larger in more 2dimensional metallic environment. He aslo showed  how it Tc scales with the characteristic scale of the spin fluctuations.

As an example, he discussed PuCoGa5.  This system has the highest Tc=18.5K of the 115 superconductors - but how might one make it larger?  He described three possibilities:

• Make the system more two dimensional by trying PuCo2Ga7 (he didn't say what happens).
• Try to increase the spin fluctuations by going to more magnetic PuCo5Ga2
• Seek still higher bandwidth by trying to discover d-electron analogs of this material.

Eric discussed how heavy fermion materials were an ideal platform for studying new kinds of material behavior in metals, because they are

• highly tunable (low energy scales)
• there are a wide variety of ground-states
• they can be made with very high purity.

He showed how one could explore material space - varying the structure with a fixed f-ion and also changing from 4f to 5f.

4f:       CeIn3 -   Ce2MIn8 ->   CeMIn5 ->  CeM2In17    (attempt to make it more 2D)
              |                                                       (eg CePt2In7 - an AFM that becomes SC with pressure)
              |
             \/
5f    NpPd5Al2

He showed us a fascinating new compound CePd1.5Al3.5 with an incredible unit cell that they are studying.

As a last topic, Eric discussed how anisotropies in spin fluctuations appear to be closely correlated with transition temperature.











There followed about 20 minutes of discussion.  One of the topics discussed, was how we might
increase the amount of serendipitious discovery.  Yuri Grin pointed out that in the drugs industry, typically 20,000 people are required to bring one new drug to the point where it can be tested.
Zlatko Tesanovic has pointed out that its not just Tc, but getting a better material for superconductivity thats important - if only we could get the iron-based superconductors up to the same Tc as the cuprate materials - because iron based sc are much more friendly for making contacts and wires. This would be a multi-billion industry if we could only find the material.

A long discussion ensued about how it would certainly be cost-effective to society if we could only encourage funding of materials exploration that would permit us to find these exciting new materials.