Condensed-matter physics gets its hands dirty

Here is the original version of my leader for Nature Materials, which I want to put up here to acknowledge the insightful input from Bob Cava and Bertram Batlogg - N Mat's leader style doesn't permit direct quotes, so I had to paraphrase their words.

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Is condensed-matter physics becoming more materials-oriented? Or is this just a new wrinkle in an old tradition?

Condensed-matter physics is becoming increasingly oriented towards materials science and engineering. That’s the conclusion reached by two Harvard physicists, Michael Shulman and Marc Warner, after analyzing the statistics of abstracts for the main annual (March) meeting of the American Physical Society since 2007. They enumerated key words used in abstracts to identify trends over the past eight years, and say that during this time the words that are increasing in popularity are often ones associated with specific types of material system, such as “layer”, “thin”, “organic”, “oxide” and indeed “material”. In contrast, words or (word fragments) with generally declining popularity include “superconduct” and “flux” (as well as, oddly, “science”).

What should we make of this? Probably not too much. As the authors are the first to point out, the analysis is preliminary and its timespan limited. It would be good to see it extended over a longer period and expanded to include, say, words in the abstracts of publications in Physical Review Letters, not to mention paying more attention to soft matter rather than primarily solid-state. The present results also paint a slightly confusing picture, taken at face value: condensed-matter physics (CMP) as a whole has been expanding if one judges from the gradual rise in the total number of abstracts submitted to CMP sessions of the March meeting, yet the “condensed matter” section of the preprint server arxiv has made up a shrinking proportion of the total during that time. There are various possible explanations for the discrepancy.

All the same, if it is qualitatively true that CMP has become more materials-focused, it’s worth asking why. Are established researchers in the field are altering the direction of their work away from abstract theoretical questions – what is the origin of high-temperature superconductivity, to take one obvious former preoccupation of theorists – and towards applications of particular materials systems? Or does that reflect a change in the interests of young researchers entering the field? Robert Cava of Princeton University doubts that it’s merely the latter, since old hands enjoy fresh challenges: “For old-timers like me, new areas are a way to use your stored knowledge to have insights that the youngsters miss.”

It is tempting to infer that researchers are just following the money: in this increasingly goal-oriented scientific climate, there may be better funding prospects for a project that can promise concrete applications at the end of the line. But might not the trend instead reflect the internal dynamics of the research community, so that funding follows areas deemed “hot” for other reasons? It’s almost sure to be a bit of both, as the example of graphene shows: there are high hopes for applications in electronics and composites, but much of the interest has come from the fundamental physics that this one-dimensional system seems to offer. More data on the dynamics and trends of funding priorities might help to separate cause and effect.

In any event, Bertram Batlogg at ETH in Zurich says that practical applications of the materials it studies has always been “in the best tradition of CMP”. Given the enormous contributions that the field has made to society – underpinning the technologies of smart phones and solar energy, say – it’s only natural that researchers should have an eye on ensuring that this tradition continues.

Shulman and Warner found that, in comparison to subjects such as atomic, molecular and optical physics, CMP changes fast: the statistics of key words are more volatile. Cava agrees that this is a feature of the field. “Occasionally, say once every 5-10 years, a subject comes up that is so new that many people work on it, because physicists are intrinsically enthusiastic and interested in new science.” He cites the case of pnictide superconductivity, which enjoyed its greatest popularity just before the period of this analysis. Superconductivity is now seeing another little surge of interest owing to topological superconductors.

“I believe that all fields have a natural life cycle”, says Cava. “They naturally go up in activity and then back down as people have had a chance to see what they can contribute and then move on to other new areas.” Shulman and Warner wonder if this cycle is shorter in CMP than elsewhere, perhaps because it can be stimulated by the discovery of a new material system (carbon nanotubes, say, or magnetic multilayers) but also because it can be hard to get at the high-lying fruits for many of these systems owing to the complexities of the many-body interactions they present – that, at least, seems to be what has kept a general theory of high-temperature superconductivity out of reach. What’s more, high-temperature superconductivity showed that there is a very low entry barrier for studying exciting new materials if they are relatively easy to synthesize: any lab well equipped with instrumentation can quickly and easily switch direction and still hope to make a useful contribution.

Might there also be a life cycle for CMP as a whole? The APS division was created only in 1978, from what was formerly the Division of Solid State Physics. Yet Shulman and Warner wonder if it still presents the kind of exciting challenges of 20-30 years ago. No one would claim, however, that the most demanding questions are all answered: perhaps some of them will need to await new techniques or new theoretical methods better able to accommodate complexity. And like chemistry, to some extent CMP creates its own subject: our inventiveness (or serendipity) with new materials systems prompts new questions. As Cava says, “to explore the complexity of the physics people have to think about and perform experiments on real materials. Each material has a different balance of the competing forces that give rise to the complexity of condensed matter physics, so each new material is an opportunity to learn new physics.”