Sunday, June 15, 2014

FeSe on SrTiO3: report of 100 K superconductivity

I'd heard rumors about this for a while.  I presume that the posting of this on the arxiv means that some form of this paper is in submission out there to a suitably glossy, high impact journal that requires reference citations in its abstracts.  Background:  Bulk FeSe superconducts below around 8 K at ambient pressure (see here).  Under pressure, that transition can be squeezed up beyond 35 K (see here).  The mechanism for superconductivity in this material is up for debate, as far as I know (please feel free to add a reference or two in the comments). 

These investigators have a very fancy ultrahigh vacuum system, in which they are able to grow single layer FeSe on top of SrTiO3 (with the substrate doped with niobium in this case).  This material is not stable in air, and apparently doesn't do terribly well even when coated with some protective layer.  However, these folks have a multi-probe scanning tunneling microscope system in their chamber, along with a cold stage, so that they can perform electrical measurements in situ without ever exposing their single layer to air.  They find that the electrical resistance measured in their four-point-probe configuration drops to zero below around 100 K (as high as 109 K, depending on the sample).  One subtle point that clearly worried them:  SrTiO3 is know to have a structural phase transition (the onset of ferroelasticity - see here) at around 105 K, so they wanted to be sure that what they saw wasn't somehow an artifact of that substrate effect.  (Makes me wonder what happens to superconductivity in the FeSe depending on the ferroelastic domain orientation underneath it.)  For the lay audience:  liquid nitrogen boils at ambient pressure at 77 K.  This would be the first iron-based superconductor to cross that threshold, a domain previously limited to the copper oxides.   Remember, if the bulk transition is at 8 K and the single layer case exceeds 100 K, it doesn't seem crazy to hope for some related system with an additional factor of three or four that takes us beyond room temperature.

Important caveats:  Right now, they have resistance measurements and tunneling spectroscopy measurements.  Because of the need for in situ measurement they don't have Meissner data.  It's also important to realize that the restrictions here (not air stable; only happens in single layer material when ultraclean) are not small.  At the same time, this is potentially very exciting, and hopefully it holds up well and can be the foundation for more exciting materials.

5 comments:

Anonymous said...

I know that iron based superconductors have gotten a lot of attention and generated a lot of excitement in the condensed matter community. From a technological/engineering perspective, what are some advances that these iron based superconductors offer over copper oxide based superconductors? Are these iron based superconductors more malleable for example? Is there any engineering advantage or is the excitement related to how these new class of materials can help achieve a better understanding of high-Tc superconductors (which is very important as well)?

Douglas Natelson said...

Anon, good questions. From an engineering perspective, it seems like the iron based superconductors are, if anything, more annoying than the cuprates. The are also brittle, and as an added bonus they tend not to be chemically stable in humid air. However, their very existence showed that high temperature superconductivity is actually far more generic than first appeared, and as you say they provide additional ways of testing different mechanisms for pairing. If the 100k result turns out to be right, that would undoubtedly spur much more research into the engineering of these materials and their interfaces for possible applications.

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Anonymous said...

Interesting and really big result. But really gets puzzled that in Fig.3, inset of panel a and b, the bare STO appeared to be very conducting. Is that reasonable.

Anonymous said...

Interesting and really big result. But really gets puzzled that in Fig.3, inset of panel a and b, the bare STO appeared to be very conducting. Is that reasonable.