You are too polite!

The blogosphere is not known for restrained, thoughtful, and respectful responses to posts. However, the readers of this blog certainly are! Overall the blog has been much more successful and useful (both to me and others) than I anticipated, with one big exception. It generates very few comments and virtually no online discussion. Furthermore, occasionally I get an email from a reader who carefully, thoughtfully, and respectfully points out an error or disagreement. I really appreciate these emails and usually suggest that the sender post the contents of their email as a comment on the blog, since it will be helpful to other readers. Usually I am wrong or the person has a different point of view which I think should be heard.

So, do not be shy!

Please post more comments, especially pointing out errors, ignorance, and different points of view.

A basic question about novel energy materials

A fundamental scientific question of technological importance concerning oxides of transition metals and rare earths is:

When an oxygen atom is removed from a bulk crystal of the oxide where do the two excess electrons go?

Elvis Shoko, Michael Smith, and I recently finished a review article which answers this question for the case of cerium oxide.

The approach we took was to consider high resolution crystal structures of

Ce11O20 and Ce7O12 and see how they could be viewed as ordered arrays of oxygen vacancies in an underlying CeO2 crystal. The charge distribution in the local environments of the O vacancies can then be deduced from the bond valence model.

An important finding we make is that the results are incompatible with the widely accepted standard picture of charge localization on two cerium ions next to the vacancy. Instead, we found that the charge distributes itself predominantly in the second coordination shell of cerium ions. Furthermore, one excess electron can be delocalised over more than one cerium ion.

Our conclusions concerning the charge distribution near oxygen vacancies are significant for several reasons.

First, they contradict many (but not all) atomistic simulations based on density functional theory.

Second, the actual charge distribution around the defect has important implications for the other questions we posed at the beginning of the review. For example,

1. the charge distribution has a significant effect on the relative stability of surface and subsurface vacancies.

2. the charge around oxygen surface and subsurface vacancies is not simply localised on Ce ions next to the vacancy this could change our understanding of the catalytic activity of these surfaces since it has been claimed or assumed that it is associated with Ce3+ ions at the surface.

3. the charge distribution around the vacancies has implications for the relative importance of electronic and ionic conduction, a subject we have discussed in this preprint.

Correlation does not imply causality

I think this mantra is one that is not chanted enough!

I feel I read too many papers concerning material properties that say something like "we measured both properties A and B on material 1 and material 2. Properties A and B were correlated (e.g. they both increased). Therefore A causes B."

We should do better. We should teach our students to do better.

What is more likely is that there is some other property C which causes both A and B to change. For a concrete example of this, consider the oxygen isotope effect in cuprate superconductors. The fact that Tc and other superconducting properties such as the penetration depth varies with the oxygen isotope does not imply that superconductivity is caused by electron-phonon coupling.

Perhaps, a point of even greater concern should be that this mantra is overlooked in many public discussions of social issues.

Should a major goal of high school science education be to help students learn why they should believe this mantra?

A really good book which I recommend is not a "science" book but Freakonomics. I thank my colleague, Ben Powell for giving my 15-year old son a copy. After he read it, I did. In the book, the authors several times emphasise this point in their analysis of "economic" data and show how the data can be analysed in a way to tease out true causality.

PC vs. Mac

So at the end of last year I did take the plunge, gave up on my 3 year old Dell PC laptop and bought a MacBook Pro 13 inch. I have no regrets. What helped tip me over the edge to do it was I must have talked to almost ten people who told me that had made the change in the past few years and had no regrets. A common comment was that they don't seem to slow down with use the way that PC's do.

My only recommendation is that if you do it, do NOT do it when you have important deadlines, e.g, about to go on a trip, write a grant, teaching a new class. I deliberately did it at the end of the year when I had no pressing deadlines. For the first month it was amazing how many little bugs and problems I had getting things to work (e.g. internet was o.k. at work but not home, one printer would work but not another, ...).

A good result of the change is that it forced me to update various software.

In fairness, I think some of the things that impress me, I may have also found if I had replaced my 3 year old PC with the latest.

I love the battery which can easily last 6 hours.

A mark of academic freedom?

Are you free?

"The first duty of free people is to say NO."

Jacques Ellul (1912-1994)

Professor of Law

University of Bordeaux

Besides the freedom I have found that this is one of the keys to good time management and stress reduction.

Are quasi-particles real?

Everyone would agree that electrons and protons are "real". But what about holes? magnons? rotons? quasi-particles in a Fermi liquid? I would say yes. It is interesting to find that this question has received some attention in the philosophical literature. One paper I found by Axel Gelfert, "Manipulative Success and the Unreal", who argues that quasi-particles counter a reality criterion due to Hacking, "If you can spray them, they exist." i.e., Gelfert claims that quasi-particles do not exist even though it is possible to manipulate them.

However, I agree with Falkenberg's counter-argument and I like her stock market analogy in the following:

quasi-particles are as real as a share value at the stock exchange. The share value is also due to a collective effect ...,namely to the the collective behavior of all investors. It is also possible to `spray' the share value in Hacking's sense, that is, to manipulate its quotation by purchase or sale for purposes of speculation. Its free fall can make an economy crash, its dramatic rise may make some markets flourish. And the crash as well as the flourishing may be local, i.e., they may only affect some local markets. But would we conclude that the share value does not exist, on the sole grounds that it is a collective effect? Obviously, share values as well as quasi-particles have another ontological status than, say, Pegasus. Pegasus does not exist in the real world but only in the tales of antique mythology. But quasi-particles exist in real crystals, as share values exist in real economies and markets. Indeed, both concepts have a well-defined operational meaning, even though their cause cannot be singled out by experiments or econometric studies.

Brigitte Falkenburg, Particle Metaphysics: a critical account of sub-atomic reality (Springer 2007), p. 245.

Evidence for climate change

I increasingly get asked by non-scientist friends "Do scientists really believe in climate change?". Consequently, I am trying to be better informed. I found this Summary for Policymakers from the Intergovernmental Panel on Climate Change (IPCC) helpful. The Figure above is taken from that report.

I also liked this presentation by Sir John Houghton because it gave a succinct summary of some of the history.