## Friday, March 21, 2008

### A new (apparently unconventional) family of superconductors

I heard about this at the March Meeting, and now it looks like things are picking up steam. There are a number of papers that have started appearing on the arxiv (1, 2, 3, 4, 5, 6, 7 update 8, 9, 10) about a new high temperature superconductor based on the parent compound LaOFeAs. This material has FeAs planes rather reminiscent of the CuO planes in the copper oxide superconductors. It's quite unusual to have an iron-based superconductor, since ferromagnetic correlations are usually associated with killing ordinary superconductivity. More exciting is the fact that this is not directly related to the cuprates and when doped with electrons (by replacing some of the oxygen with fluorine) it has a clear superconducting transition at 28 K. There are indications already that this is an unconventional superconductor, and third-hand rumors suggest that higher Tc values are on the way. It'll be interesting to see where this leads!

CarlBrannen said...

The last arXiv link led to an "author only" thingy which I guess is associated with papers that are not yet released. And Physics World was for members only, it would be nice to link to something else.

Condensed matter is very cool and this subject is clearly going to be interesting. I don't have anything else to say, but thanks for keeping up the hard work.

CarlBrannen said...

It seems possible that the 0.7 anomaly is related to the difficulty in analyzing these superconductors given that the problem arises in materials that are almost non conducting. The 0803.1288 paper even suggested "spin scattering" as the explanation as an explanation for violating Kohler's law which certainly is reminiscent of the 0.7 anomaly.

Douglas Natelson said...

Carl - the last arxiv link was a typo on my part. It's fixed now. Registration at Physics World is free, so unless you're really opposed to that sort of thing (and I know that some people are), you may want to consider creating an account. Their reporting is generally quite good.

Unknown said...

Doug -- Here are some preprints from ORNL and NRL on LaFeAsO. Please note that the chemistry does not support the LaOFeAs nomenclature (see D. J. Singh and M.-H. Du, 0803.0429).

http://xxx.lanl.gov/abs/0803.0429
http://xxx.lanl.gov/abs/0803.2528
http://xxx.lanl.gov/abs/0803.2740

Regards,

-David Mandrus

Anonymous said...

We live in an exciting time for CMP as there are numerous hot fields that have yet to mature: HTSC, nanotubes, graphene, exotic oxides, molecular physics, biophysics, AMO-CMP, among others. I wonder what the APS meetings were like pre-1987. Were the band structure DFT sessions standing room only back then?

Douglas Natelson said...

David - I've now added those to the list - thanks. I'd arrived at the original list by doing a quick look on the arxiv using its search function looking for "FeAs". What we see here is that the arxiv search function is lame. Indeed, if you search generally for FeAs you get the seven papers I'd originally listed. If you use the advanced form to search for "FeAs" in titles, you only get five out of those seven, despite the fact that all ten of the papers have "FeAs" as part of the title. Clearly the arxiv needs a bigger budget so that they can develop an improved search tool.

mkg - Well, the 1980s did see things like the invention of the scanning tunneling microscope and the atomic force microscope; the experimental observations of all your favorite mesoscopic effects (weak localization, universal conductance fluctuations, Aharonov-Bohm oscillations); the development of many and various Coulomb blockade/tunneling devices (single-electron transistors, Josephson junction arrays); the rise of molecular beam epitaxy and the observation of the fractional quantum Hall effect; etc. I think it's safe to say that they were pretty busy then, too. High Tc was something special, though.

Anonymous said...

Isn't it disconcerting that, in the face of exciting progress like the one we are witnessing, research on high-Tc superconductivity in the US has almost ground to a halt ? That it is frequently branded as a "dead field" (to my amazement, I heard in conversation a number of colleagues at the March meeting say things like "High-Tc is old stuff", "never went anywhere" etc.).
That young condensed matter theorists are actively discouraged from pursuing research in that field ?
That it is almost impossible to get funding ?
I still remember the keynote speech given by nobel laureate P. W. Anderson in Santa Fe, at the conference on recent progress in many-body theories, essentially asking the question "Since when do physicists stop working on a problem because it is too difficult ?.

Schlupp said...

okham, as for young physicists being discouraged from high-T_C: One problem the field does have is that one can too easily piss off someone. Which is not necessarily something a young scientist wants to do.

Anonymous said...

Schlupp: indeed, high-Tc superconductivity is a competitive field, full of excellent scientists with big egos (and chips on their shoulders), and yes, one can easily make enemies.
It is disappointing, though, to see promising, talented young theorists stay away from an outstanding, important scientific issue for fear of "pissing off" some influential senior scientist.
And you know what ? I still think that taking risks is a better strategy, career-wise, than sticking to safer, less controversial but also less important problems, less likely to lead to some significant contribution. Just my opinion, of course.

Anonymous said...

As a grad student in physics, I would not go into HTS because of the limited availabily of jobs. As far as Physics Today Job List, there is no faculty opening for superconductivity. Not so sure I would be happy in the industry doing superconductivity.

Anonymous said...

Arl, the likelihood of landing a faculty job in condensed matter physics in North America is very slim anyway, these days. Frankly, I doubt if it has much to do with one's field of research. The main problem, especially in condensed matter theory, is that there are very few jobs.
HTS may not be the "hottest" field that there is, but then which one is ? As for industry, I do not think that they care either way, when they do hire physicists it is not to make them do basic physics (or, physics at all), HTS or anything else.

Douglas Natelson said...

While it's true that high Tc is an area with lots of big egos and sharp elbows, that's true of several other parts of physics as well. Indeed, anyone who has tried to break into a personally new research area has encountered some level of resistance from the "established" community.

One reason why people don't rush en masse into high Tc anymore is the perception that it's hard to make a distinctive intellectual contribution, not necessarily that it's too hard. It's a twenty-year-old problem - what are you going to do that hasn't been tried yet? You need to have a new angle or a new tool. Most of what I wrote here still applies.

The funding problem is tricky to address. High Tc was oversold pretty badly (though to be fair, the potential uses of a room temperature superconductor are truly transformative, provided that it has a decent critical current and critical field and can be made into wires). You can't reasonably expect agencies to fund this stuff at a very high level for twenty years continuously. I do think that the pendulum has swung too far, though, particularly in the case of materials development overall. This issue is addressed very well in this freely available article from Physics Today.

Anonymous said...

It's a twenty-year-old problem - what are you going to do that hasn't been tried yet?

Me, personally, nothing, but a talented, creative 26-yr old who may bring completely new ideas ? I mean, if we take this away from our science, I am not sure what is left. How old was the problem of superconductivity when BCS started working on it ?

High Tc was oversold pretty badly
Absolutely true. Is it a good reason not to fund it anymore ? And more generally, is the fact that a problem has stayed unsolved for a long time a good reason not to fund research on it anymore ? Can you imagine if medical researchers did the same ?

Douglas Natelson said...

Okham - hey, I agree with you. I was just laying out reasons why people aren't flocking to it in droves. I definitely hope that there are young, creative people out there with new ideas on how to approach this problem. As far as BCS goes, clearly Bardeen thought he had new ideas and new techniques to bear on a forty-year-old outstanding problem.

As for funding, as you know it's an unfortunate fact of life that nothing succeeds like success, particularly when real $resources are increasingly scarce. It's much easier for a program officer to make the case to their higher-ups (and for agencies to make their case to Congress, in the US) if there is some real, well-defined, easy-for-laypeople-to-understand accomplishment. When it's public money, it's not too surprising that people get tired of claims that don't materialize. For example, the US fusion program is absolutely a worthwhile investment, but after fifty years of fusion being thirty years away, it's clear that Congress has lost a lot of its enthusiasm. Anonymous said... Well, see, Doug, sometimes I find myself wondering whether the reason why physics is no longer as attractive to students as it used to be, is because they feel that it is no longer the place to do challenging, fundamental science, of the type that can "change the world", intellectually as well as technologically. I am afraid that this editorial written ten years ago hit the nail on the head. Anonymous said... Now, okham, Pentagon's 2008 budget happens to be 520 billion$. This figure completely excludes the spending in iraq and afghanistan. If you add them up, we are talking about roughly 650 billion $just for military expenditures. That is more than the rest of the world combined. I am not going to question here whether it is justified or not let alone play political games but NSF's annual 2008 budget is only and only 5 billion$. In these circumstances, no grad student in any field can realistically expect to find a decent faculty job and even if he does he will not have any money to carry out research. Like, you pointed out High Tc is obviously dead but what is alive really? Funding is incredibly tough right now for anyone. Two weeks ago, Science magazine reported that the success rate of NIH grant applications is below %20 now and that is all time low. Yet, it is projected to keep dropping. In these circumstances, I would not advice anyone to go to grad school in hard sciences, let alone postdoc. Let's be realistic.

Anonymous said...

More preprints today:

arXiv:0803.3236

arXiv:0803.3286

arXiv:0803.3325

arXiv:0803.3426