Condensed concepts

Ken Wilson is very well known for developing the renormalisation group and applying it to critical phenomena and the Kondo problem. There is a long and interesting (but somewhat meandering) interview with him about his career. Amongst various choice tid-bits there is the exchange below about his father, E. Bright Wilson , a pioneer in quantum chemistry. PoS Did your father use computers? Did you know of his models for getting infrared spectra? Did things like that play a role in your thinking? KGW What I remember from discussions with my father was that he used get very wrought up about computational quantum chemists. Garbage in, garbage out. That set me up to spend some time at Ohio State studying quantum chemistry. I had done a little bit towards the end of my stay at Cornell, but took it more seriously while I was at Ohio State. And so then I had to find out from my father who were the good people. And he knew them, he had a list of them. PoS And who did he say were the g

Most common hydrogen bonds involve an interaction of the form X-H...Y where the donor X and acceptor Y are highly electronegative atoms such as O, N, and F. Signatures of H bond formation include lengthening of the X-H bond, softening of the X-H stretch frequency, and increase in the X-H stretch IR intensity. For strong H bonds these effects on the X-H bond are substantial, of the order of 10-100 per cent. I presented a unified picture of this in a recent paper. However, over the past 15 years it has been discovered that there are a class of (very weak) bonds, best described as improper H bonds which are distinctly different. Generally, the donor X is not particularly electronegative (e.g. a carbon atom) and bond formation results in contraction of the X-H bond (by a few milliAngstroms) hardening of the X-H stretch frequency (by less than one per cent) decrease in the X-H stretch IR intensity There is a nice 2007 JACS paper  Red-, Blue-, or No-Shift in Hydrogen Bonds:  A Unifi

This week at the Quantum Science seminar Ben Powell gave a tutorial about the Higgs boson, highlighting its conceptual origin in condensed matter physics. The talk followed some of Section 12.6 of Piers Coleman's nice book Introduction to Many Body Physics ( free online ). It is a nice clear and helpful discussion. One of the key ideas first emphasized by Phil Anderson in 1963 was that a massless gauge field can aquire a mass in the presence of a coupling to a spontaneously broken field. A concrete realisation of this occurs in superconductors. In the Meissner effect a superconductor thicker than the penetration depth expels magnetic fields. This is like the photon acquires a mass. In the electro-weak theory of Weinberg-Salam there is a combined U(1) x SU(2) gauge symmetry. Due to coupling to the Higgs field (whose symmetry is spontaneously broken) one gauge field remains massless (the photon) and the other three become massive. These massive particles are the W+, W-, and Z b

I found this sad story interesting and disturbing because of what it reveals about journal impact factors, university rankings, self-citations, Elsevier, scientific crack pots, lawsuits.... More of the weird history is here  and here .

In an article The Nature of the Chemical Bond - 1992 , Linus Pauling makes the following fascinating statement: The concept of quantum mechanical resonance and the theorem that in quantum mechanics the actual structure of a system has a lower energy than any other structure have turned out to be especially important in chemistry. The energy could be calculated for an assumed wave function for a molecule. Any change that lowered the energy indicated some addition to the picture of the chemical bond. The polarization of bond orbitals and the partial ionic character of bonds were discovered in this way. The minimum-energy theorem led to the formulation of the electronegativity scale . Modern chemistry and molecular biology are the products of quantum mechanics. Chemistry has been changed by quantum mechanics even more than physics. In 1936 in the Preface to The Nature of the Chemical Bond  he also emphasized how quantum physics led to new chemical concepts.

On monday night I heard Paul Davies give an interesting public lecture The origin and the end of the Universe in Brisbane. One small aspect that I was particularly interesting was the discussion of the second law of thermodynamics in gravitational systems. He emphasized the following puzzle (in my words). For an isolated system the entropy can never decrease. In some sense this means that the "order" cannot increase. However, a long time ago matter in the universe was relatively uniform, and now it is not just ordered into galaxies and stars, but even biological life! The key to resolving this is to realise that in a gravitational system the second law looks different. Uniform "disordered" states do not have low entropy. If you start with a fairly uniform system this is not an equilibrium state. The natural tendency of the system is to evolve to a non-uniform state with clumps of matter. Hence, the "clumpy" state [with a sun which transfers energy to

APS News has a fascinating article A Cold-War Folly?  by Nina Byers. She describes a course entitled Nuclear Power: Power plants and weapons of war that she teaches UCLA undergraduates. A couple of things I found particularly interesting. First, the graph below showing the dramatic variation in nuclear weapon stockpiles with time. Second, the diverse views that  physicists (esp. famous ones) had about the use of nuclear weapons, both against Japan, and after the war. Third, it was a good reminder that our current undergraduates were actually born after the end of the cold war! A great strength of the US college system compared to Australia and Europe is the flexibility of the curriculum and broad general education requirements that allow and encourage such courses.

On Nanoscale views, Doug Natelson has an excellent post  Exotic (quasi)particles, and why experimental physics is challenging . He points out recent theoretical work showing that a zero-bias peak in the I-V characteristic seen in recent experiments is not conclusive evidence for Majorana particles. 

Electronegativity is a simple and profound concept for understanding and giving a semi-quantitative description of broad classes of chemical bonds. It reflects the brilliant intuition of Linus Pauling who first introduced it in 1932. The bold idea is to assign a single number to each element of the periodic table; the relative value of the number between two elements then determines the magnitude of the polarity (charge distribution) in a chemical bond between two different elements. I have been reviewing the concept because it is a key element to understanding hydrogen bonds, the recent IUPAC definition stating: the hydrogen bond is an attractive interaction between a hydrogen atom from a molecule or a molecular fragment X-H in which X is more electronegative than H , ... Unlike earlier definitions it does not require that the acceptor be more electronegative than H, only that for X-H...Y-Z the acceptor is an electron-rich region such as, but not limited to, a lone pair in Y o

"whether the candidate's research has changed the community's view of chemistry in a positive way". This is the main point of a nice Editorial, Assessing Academic Researchers  that just appeared in Angewandte Chemie by Richard Zare. He points out that this is the main criteria that is used to decide whether or not Assistant Professors in Chemistry at Stanford get tenure. We do not look into how much funding the candidate has brought to the university in the form of grants. We do not count the number of published papers; we also do not rank publications according to authorship order. We do not use some elaborate algorithm that weighs publications in journals according to the impact factor of the journal. We seldom discuss h-index metrics, which aim to measure the impact of a researcher’s publications. We simply ask outside experts, as well as our tenured faculty members, whether a candidate has significantly changed how we understand chemistry. I thank Seth Ols

In Telluride I heard David Coker give a fascinating talk about his recent work on quantum decoherence in photosynthetic proteins, reported in a recent paper  with Jeremy Moix, Jianlan Wu, Pengfei Huo, and Jianshu Cao. A 2007 Nature paper claimed to report evidence for quantum coherence of electronic excitations for a few hundred femtoseconds at liquid nitrogen temperatures in the FMO complex. This led to a flurry of theoretical activity [and various silly (at least to me) and grandiose claims about "quantum biology" and "green quantum computers"]. [Aside: it is often overlooked that the claimed coherence is just between 2 chromophores not all seven in the complex]. However, these experiments were not done on the complete photosynthetic complex, but a subset with 7 chromophores. Recent structural studies have shown that there is actually an 8th chromophore. Surely, such small details don't matter... But, they do. Coker's group has shown that this extra

There is a somewhat amusing column Our Political Black Hole  in the New York Times by Gail Collins. It considers the fictional response of US presidential candidates to the discovery of the Higgs boson. I found it somewhat amusing, but in someways it is a bit painfully close to the truth. This is a bit how I feel when I watch the classic BBC political comedy, Yes Minister . Perhaps Gail Collins use of physics metaphor was inspired by David Javerbaum's earlier NYT column  A Quantum Theory of Mitt Romney.

There is an interesting Editorial in the Journal of Chemical Education Science Education for Global Sustainability: What Is Necessary for Teaching, Learning, and Assessment Strategies? by Uri Zoller. I agree with many of his concerns and sentiments. Clearly he has thought about the relevant issues and worked hard at trying to implement them. I particularly like the sample exam questions he uses to illustrate his goals. However, I feel that some of his proposed solutions are unrealistic (e.g., no text books) and unhelpful to students, particularly for beginning undergraduates. I think (and my anecdotal observations are) that multi-disciplinary courses are either too hard (or too superficial) for such students. Students need to learn basic chemistry, physics, and biology before they can  cope with integrating these ideas in biophysics, materials science, or environmental policy. I thank Ross Jansen-van Vuuren for bringing the article to my attention.

Previously at UQ we have run a few successful book reading groups for postdocs and graduate students. We have worked through parts of Electronic Correlations in Molecules, and Solids by Peter Fulde Advanced Solid State Physics by Phillip Philips A Chemists Guide to Valence Bond Theory by Shaik and Hiberty A postdoc, Tony Wright, and I are considering starting a new group based on a book on quantum many-body theory. We want a book that makes a strong connection to experiment. I welcome suggestions. Here is my current suggestions in order of roughly decreasing preference The Kondo Problem to Heavy Fermions  by Alex Hewson. Although focussed on the Kondo problem, it covers techniques and concepts that are more broadly applicable including Fermi liquid theory, scaling, and slave bosons. It does connect strongly to experiment. The e-book is available through the library. An introduction to Many-Body Theory by Piers Coleman. This is particularly clear, emphasizes key concep

Recently I encountered a new metric, the "5 year h-index" which was being used to evaluate someone's research performance. Explicity, one counts the number h of papers published in the last 5 years that have been cited more than h times. Perhaps one might argue that this is a good metric for deciding whether to give someone a grant now. Afterall, just because 10 or 20 years ago they published highly cited papers does not mean that right now they are at the cutting edge. However, I do not agree. I think this is a highly unreliable metric because there is significant noise. Except for a few rare exceptional papers, citations within a few years of publication will be low (1-10?). Hence, comparing two people with 5 year h-indexes, say one with a 6 and another with a 10, I would contend is meaningless. Two of the most cited papers in Physical Review journals are the EPR (Einstein Podolsky Rosen) paper and Steven Weinberg's electro-weak interaction paper.  The latter a

The great appeal of computational quantum chemistry is that it aims to be  ab initio. One simply calculates the properties of molecules from Schrodinger's equation and Coulomb's law. However, the painful reality is that many methods have to include parameters (or make choices about approximations) that are determined by comparison with experiment. Indeed there is now a whole industry of people who tweak parameters in density functionals in order to get better agreement with experiment. Although I am not enthusiastic about this I can live with it as long as people are transparent about what they are doing. A recent JACS paper  Mechanism for Singlet Fission in Pentacene and Tetracene: From Single Exciton to Two Triplets from Martin Head-Gordon's group is not as transparent as it could be about whether it is ab initio . It states The originally proposed mechanism for SF [Singlet Fission] is based on model Hamiltonians that couple of monomer states between adjacent molecul

Demonstrations to school students can easily degenerate into the following format. First, one does something spectacular such as the Coke-Mentos fountain or the barrel crush. Second, one tells the student how it works. I confess I have often done this. However, this is actually terrible because it reinforces the misconception that science is a noun not a verb. It teaches nothing about the scientific method. This week my wife and I demonstrated the Coke-Mentos fountain to a group of kids at a holiday club that my church was running. In order to promote critical thinking we did some comparative measurements. The fountain was done for diet Coke, Solo (a lemon drink), and generic brand (Coles) Cola. We also compared Mentos bought in the USA (on my recent trip) and in Australia. It turned out that the former is much more effective. I later learnt that we had been scooped in this important scientific discovery. It had already been published on YouTube! I also discovered there is so

At the Journal Club for Condensed Matter there is a very nice and clear commentary Identifying a spin liquid on Kagome lattice by quantum entanglement by Ashvin Vishwanath. I learnt a lot from reading it. It reviews two recent preprints which use numerical methods (density matrix renormalisation group = DMRG) to establish topological order in the ground state of the Heisenberg spin-1/2 model on the Kagome lattice. An earlier post considered earlier DMRG evidence that was suggestive of a spin liquid, with an energy gap, but did not establish topological order.

One thing I really enjoyed and appreciated about the workshop this week has been the emphasis on developing "simple" models to describe systems that are structurally and chemically complex. Here "simple" means that there are a just a few degrees of freedom and a few parameters in the model. "Complex" means there are many degrees of freedom. Even when people are doing very large and demanding molecular dynamics simulations of solvated proteins the goal has been to understand the essential physics and chemistry of what is going on. Here are a few examples. Phil Geissler considered a  simple model for force generation in cellular processes, showing how Actin filament curvature biases branching direction . Frank Brown  considered an analytical model that could be used for  Interpreting neutron spin echo experiments on lipid bilayer membranes  without introducing a "fudge factor" for the value of the solvent viscosity that experimentalists ha