Condensed concepts

Scientists giving public lectures face a formidable challenge. It needs to be both interesting, exciting, and accessible to a broad audience. Hopefully, the speaker can communicate something not just about science but also about how science is done. Yesterday I attended a very nice public lecture at UQ. Professor Ullrich Steiner from Cambridge spoke on How Nature Makes Materials . It nicely bridged physics, chemistry, and biology. His work on photonic structures in nature is described here. The plant seed shown on the right is particularly amazing. Steiner and colleagues found a fifty year old one in a museum in Cambridge. One thing, among others, that I appreciated was the lack of hype and the sober assessment of what his work in biomimetics has achieved. Sometimes it has provided some insight into how biological systems make and utilise specific materials. Some of the biomimetic materials and structures his group has made have some of the desirable features. But, most ar

Late last year I gave a talk in Canberra for a group of scientists at CSIRO [Australia's government industrial research organisation]. This got a lot of positive feedback, and the associated blog post got a lot of page views. Several people told me the slide below is very helpful and so I post it here. A group at CSIRO at Dutton Park [just over the river from UQ] has asked me to come and speak next month on the issue. I would rather be getting science speaking invitations, but if this is helpful to others I am happy to do it.

I believe that one of the most important concepts in quantum many-body physics is that of quasi-particles and the associated incoherent excitations. Often the one particle spectral function can be written in the form. where the first term is a well-defined peak associated with quasi-particles and total spectral weight Z_k.  The second term describes incoherent excitations, i.e., it has a weak dependence on the momentum k and as a function of omega is a broad distribution, in contrast to the sharp quasi-particle peak. Futhermore, due to a sum rule [conservation of particle number] the total spectral weight of the incoherent part is 1-Z_k. I think this equation is one of the most profound and important results in quantum many-body theory. Some of this is illustrated in the figure below taken from a Nature Physics commentary by Nandini Trivedi. The above equation and concepts have come to the fore over the past two decades due to wide studies of strongly correlate

I love simple models. There is a very nice paper Correlated double-proton transfer. I. Theory Zorka Smedarchina, Willem Siebrand, and Antonio Fernández-Ramos It considers an incredibly simple potential energy surface to describe double proton transfer. x_1 is the (dimensionless) position of one proton relative to the middle of its donor and acceptor. x_2 is the corresponding position for the second proton. The first term describes a quartic potential with an energy barrier for transfer of the proton between the donor and acceptor. The dimensionless parameter G describes the extent of correlation or coupling between the two hydrogen bonds. The coupling term is chosen to have the important property that it is symmetric in the two co-ordinates but sensitive to their sign. This is an important difference to earlier [rather nice] work by Benderskii et al.  who considered competition between two dimensional quantum tunneling paths [instantons] associated with concerted and seque

It great to try new things in teaching. We desperately need to when we are honest about how little many students actually learn, particularly with traditional modes of delivery. Technology also makes possible all sorts of things. People will often promote innovations; but sometimes a few years later it is found that they don't work as well as they did or were hoped to. Yet I suspect that sometimes, because of disappointment or embarrassment, the proponents are a bit coy about making known regressions. So in the interest of transparency and to promote discussion here are a couple of mine. They both relate to a course PHYS4030: Condensed Matter Physics that I have taught on of off for the past ten years. It is a final year undergraduate course that basically covers approximately half the material in Ashcroft and Mermin. A few years ago I introduced three innovations. Both seemed to work for a while. Formative and summative assessment following the example of another undergrad

I read an interesting PRL High-Energy Anomaly in the Band Dispersion of the Ruthenate Superconductor H. Iwasawa, Y. Yoshida, I. Hase, K. Shimada, H. Namatame, M. Taniguchi, and Y. Aiura They perform ARPES [Angle Resolved Photoemission Spectroscopy] on strontium ruthenate [Sr2RuO4]. Some of the main results are shown below [the vertical scale is energy]. The key issue is understanding how the measured quasi-particle dispersion (left panel) differs from the band structure calculated from LDA [Local Density Approximation of Density Functional Theory (DFT)]. Where the two curves cross is the "high energy anomaly". This is very much related to "kinks" and "waterfalls" in the cuprates, as I discussed in an earlier post. The spectrum is compared to a very simple model self energy (right panel) that is consistent with Fermi liquid theory and includes a "cut off" energy scale associated with the underlying interactions [bosons?, magnons?, elect

I would say an ionic Hubbard model on the triangular lattice. About a decade ago sodium cobaltate [NaxCoO2] was the "flavour of the month" when it came to strongly correlated electron materials. And then along came the iron pnictide superconductors.... The cobalt ions within a layer of the crystal structure form a triangular lattice and the sodium ions donate electrons to conducting layers. Hence, it is natural to consider a doped Hubbard model on a triangular lattice as the simplest possible effective Hamiltonian for these materials. This led to numerous studies of this model. Today most studies of this model will  also claim relevance to sodium cobaltates. I disagree. The sodium ions play a larger role that cannot be neglected. They actually modify the intra-layer electronic structure. Specifically, they spatially order in a manner dependent on the doping level. This is unlike the case of the cuprates where the atoms between layers [and dopants] are merely spacers

I  recently reviewed a bunch of grant applications from several different countries and so I thought it might be interesting to compare the approach of different funding agencies and offer some general comments. First, I don't review everything I am asked to. It just takes too much time. But I do try to be a good citizen. I do make a particular effort in two cases: when I really like the work of the person or when I think the proposal is shoddy and should not be funded, but may have a chance because of luck/politics/hype. Lately I am receiving a lot more proposals. I fear this may be because of my increased profile due to this blog. Getting international expert reviews for funding agencies is an increasing challenge. Yet it is absolutely crucial to making sure that money is allocated in the best manner, particularly given low success rates, and the level of complexity and specialisation of proposals. This is particularly important for small countries, such as Australia, as the

I previously posted about how double proton transfer is a concrete example of a chemical reaction that can occur via either a concerted or sequential process? Precisely defining this question and answering it is a subtle issue. There is a nice classification of types of potential energy surfaces for double proton transfer, summarised on the website of Antonio Fernandez-Ramos . It is based on a very simple model potential energy surface described here  and compared to surfaces from computational quantum chemistry [at the DFT level] here  [source of the figures below]. There are three qualitatively different potential energy surfaces, depending on the strength of the coupling of the motion of the two protons. (1) One transition state and two minima, as in the formic acid dimer; (2) Two equivalent transition states, one maxima and two minima, as in the pyrazole dimer; (3) Four transition states, one maxima and four minima, as in porphine. I am pretty happy because I a

I was asked to comment on the blog post  Get a PhD—but leave academia as soon as you graduate  by Allison Schrager. She completed a Ph.D in economics at Columbia University and now works in the financial industry. It is worth reading and pondering. Overall, I like the post for a number of reasons, but would add some qualifiers. Schrager nicely highlights The reality, painful to many, that only a very small fraction of Ph.D's will end up as tenured faculty. Adjunct teaching positions [part-time faculty and short-term contracts] are a career dead end. The value of a good Ph.D, both in terms of the educational value and the enjoyment that it can provide. The unfortunate fact that some faculty have the view that industry offers second-rate careers and soft intellectual challenges. Yet her experience shows this is far from the case. Most Ph.D programs prepare people poorly for looking for jobs outside academia. The transition from academia to industry can be difficult and pain

An important discovery of the past decade is that the Berry phase and associated geometric curvature may play a role in the electronic properties of many solids. [A nice review is here ]. The curvature enters the semi-classical equations for the electron dynamics in a magnetic field. This gives rise to some forms of the anomalous Hall effect. One might also expect the curvature to be readily manifested in orbital magnetoresistive properties. However, for subtle reasons this turns out not to be the case. Tony Wright and I just published a paper Signatures of the Berry curvature in the frequency dependent interlayer magnetoresistance in tilted magnetic fields The abstract is below with the result that I found the most surprising [and discouraging] in bold. We show that in a layered metal, the angle dependent, finite frequency, interlayer magnetoresistance is altered due to the presence of a non-zero Berry curvature at the Fermi surface. At zero frequency, we find a conservation

I welcome discussion on this point. I don't think it is as sensitive or as important a topic as the author order on papers. With regard to paper authorship my general rule is that the person who does the bulk of the work, including actually writing the paper should be the first author. Doug Natelson has a good post on co-authorship , that I largely agree with. My only difference is that I am not really convinced that good practice prevails in the majority of circumstances. I fear there are increasing numbers of co-authors, particularly senior ones, with marginal contributions. But, what about conference talks and posters? Many of these are based on work that is already or about to be published. Should the author order be identical as the associated papers? I am not sure it should necessarily be. My tentative view is that the person who writes and submits the abstract and actually prepares and presents the post/talk should be the first author. Perhaps they should also highlig

I recently heard a talk by someone working on cold atoms who kept saying again and again that these were strongly correlated systems. I may have missed it but the justification was never clear. This got me wondering, what criteria would I use as a signature of strong correlations? Here is my tentative answer motivated by strongly correlated electron materials. A key signature of strong correlations is a significant redistribution of spectral weight [i.e., the many-body eigenvalue spectrum] compared to the corresponding non-interacting electron problem. Common phenomena associated with this redistribution are the emergence of new low-energy scales  [e.g. Kondo temperature] large renormalisation of quasi-particle energies [heavy fermions] separation of the energy scales for spin and charge excitations incoherent spectral features [Hubbard "bands"]  breakdown of quasi-particle approximations [bad metals] This redistribution is usually poorly [never?] described by w

Please bear with me. I just don't get it. Universities used to have teaching committees and lecture rooms. Now they have committees, rooms, policies, and vice Presidents for "Teaching and Learning". "Teaching and learning" seems to be a tautology to me. If students aren't learning then you are not teaching. Teaching is not giving lectures, setting assignments, and marking exams. Teaching only happens when someone learns something. I think replacing "teaching" by "teaching and learning" has noble goals. It is trying to make this important point that many traditional "teaching" methods are ineffective and don't result in students learning much. Teaching and learning have to go together. You can't have one without the other. But, that is also why I think this redundant nomenclature may degenerate into a silly marketing exercise. 

Most papers about organic photovoltaics are full of discussion about HOMO's and LUMO's, their relative energies and spatial extents.   In the early days of this blog,  I asked Am I HOMO- and LUMO-phobic? Molecular orbitals are beautiful intuitive concepts that are extremely valuable for qualitative understanding. However, they do not exist , i.e., there is no way to measure one, even in principle. Furthermore, for typical organic molecules used in organic photonics and electronics the one-electron energies associated with these orbitals usually do not give reliable estimates of physically observable energies [associated with true many-body states] such as the ionisation energy, electron affinity, optical energy gap.... I was pleased to see that the above issues are nicely explained and quantified in a recent paper Reassessing the use of one-electron energetics in the design and characterization of organic photovoltaics Brett M. Savoie, Nicholas E. Jackson, Tobin J. Mar

Someone from the company D-wave is coming to UQ [not the physics department] next week to give a seminar. Their claim of producing the first commercial quantum computer has met with considerable skepticism. This has been led by Scott Aaronson, on his blog. It contains some nice detailed and thoughtful discussion of the relevant scientific issues. But here is the key concern, that I fully agree with, As I’ve said many times, I’d support even the experiments that D-Wave was doing, if D-Wave and its supporters would only call them for what they were: experiments.  Forays into the unknown.  Attempts to find out what happens when a particular speculative approach is thrown at NP-hard optimization problems.  It’s only when people obfuscate the results of those experiments, in order to claim something as “commercially useful” that quite obviously isn’t yet, that they leave the realm of science, and indeed walk straight into the eager jaws of skeptics ... I think this reflects larger pr

Yesterday I read an interesting paper Enol Tautomers of Watson−Crick Base Pair Models Are Metastable Because of Nuclear Quantum Effects Alejandro Pérez, Mark Tuckerman, Harold Hjalmarson, and Anatole von Lilienfeld A key to the double helix structure of DNA and its ability to provide reliable stable storage of genetic information is hydrogen bonding between base pairs [G-C and A-T]. However, it is possible to switch around the positions of the protons on each of the base pairs, producing different tautomers of T, A, C, and G].  This is an example of double proton transfer. This could lead to problems with correctly storing genetic information.  An important question concerns just how rare this is. For example, what is the free energy of these tautomers relative to the Watson-Crick ones? Over the past two decades a number of classical molecular dynamics simulations, using potentials derived from quantum chemistry suggested that the tautomers of DNA could be a problem.

If you want people to look at your paper you need to spend significant time coming up with an engaging title and abstract. If you want people to keep reading and engage with the scientific content you need to produce clear figures and write effective figure captions. This is not easy. Here are a few suggestions that bear in mind the following reality. Most people will look at the figures to decide whether or not they think the paper is worth reading. Some will do just that. Thus, the main messages need to be contained in the figures and they need to be quickly ascertained. 1. Try to begin the caption with a short title sentence that summarises the main point of the figure. Why are you including the figure in the paper? Hopefully, not because "I took lots of data" or "I did lots of calculations." What do we really learn from the figure? For example "Resistivity exceeds Mott-Ioffe-Regel limit at high temperatures", rather than "Resistivity ver