Condensed concepts

The controversy about the plagiarised Ph.D of the President of Hungary, Pal Schmitt, is making for "interesting" reading. In 1992 he received a Ph.D for a 200 page thesis that contains 17 pages directly translated from a German book. The rest seems to largely be a translation of work by a Bulgarian sports writer. A committee from the university reviewed the case and wrote a 1100+ page report (!) and concluded that he should keep his degree. The supervisors and examiners were to blame! However, following widespread criticism, the university just announced that they would revoke the degree. Following the resignation of the German Defence minister for another plagiarised doctorate it seems that the academic backgrounds of prominent politicians are getting more scrutiny. This raises an interesting question. Which of the following is more likely to be true? there has been a lot of plagiarism in the social sciences and humanities but it is only being detected in the case of thes

Previously I have posted about a diverse set of layered strongly correlated electron materials which exhibit a non-classical magnetoresistance. Specifically, the magnetoresistance can be largest (smallest) when the current and magnetic field are parallel (perpendicular). Michael Smith has just published a nice paper Anomalous interlayer magnetoresistance in bilayer crystals which considers a simple concrete model that exhibits such behaviour. The key physics is that in alternating layers the conducting chains are perpendicular to each other. This does occur in some organic charge transfer salts, as mentioned in footnote 1 of the paper.

Picking an advisor/supervisor is one of the most important decisions that budding young science students must make at the final year undergraduate and Ph.D level. You should pick the advisor rather than the topic . A colleague once said to me, "Students are very good at picking bad supervisors." An important aspect to the choice is whether is better to work with an energetic young faculty member (lecturer/Assistant Professor) (a " young turk ") or a well established faculty member (Professor) (an " old fart "). Here are a few random thoughts on the relative merits of each. Bear in mind these are just generalisations and ultimately you will be working with (or for) an individual human being not some abstract concept or social classification. There are always exceptions. Young turks offer you energy and enthusiasm. They have a lot riding on your success and may have significant time to invest in you. They may be working in some exciting new area or tec

A challenge to quantum chemistry is to describe many of the empirical correlations that experimentalists have painstakingly catalogued. For example, an earlier post  discusses a correlation between the rate of a photoisomerisation reaction and the electron withdrawing ability of a substitutent. An important empirical rule for organic dye molecules is the Dewar-Knott rule which relates the frequency (wavelength) of maximum light absorption [the colour of the dye] to the electron withdrawing (or donating) ability of a substituent. Seth Olsen recently published a nice paper  which gives a high level quantum chemistry justification of the Dewar-Knott rule for a family of  diarylmethane dyes , including Michler's hydrol blue. The graph below shows how the excitation energy varies with a parameter characterising the composition of the ground state many-body wavefunction, and which varies with the substituent X.

Enzymes are amazing. Today I went to an interesting chemistry seminar today by Ian Dance , Nitrogenase reduces N2 to NH3 and CO to hydrocarbons. What chemistry is used? It was also a David Craig lecture and was a model seminar for a general audience. A major industrial process is the fixation of nitrogen to from ammonia. N2 + 3H2 -> 2 NH3. This is done via the Haber-Bosch process and requires pressures of 1000 atm and high temperatures of 450 degrees C with iron or ruthenium as catalysts. However, nature does this at room temperature and pressure via  nitrogenase enzymes. A surprising recent discovery was that vanadium nitrogenase can also reduce carbon monoxide to small hydrocarbons. Dance used an interesting dance (!) metaphor during the talk. You need a stage [key part of the enzyme], centre stage [the active sites], dancers [the intermediate states], and a choreography [reaction mechanisms]. The stage for nitrogenase is shown below. the FeMo-co, which can be view

An interesting question [which has featured in many of my blog posts] concerns defining the coherence temperature at which quasi-particles in a Fermi liquid are "destroyed". There are several possible experimental signatures of this: absence of a sharp dispersing peak in the one electron spectral density (as seen in ARPES) collapse of the Drude peak in the optical conductivity collapse of the linear in T term in the specific heat capacity a peak in the thermopower versus temperature deviations of the resistivity temperature dependence from T^2 the resistivity becoming larger than the Mott-Ioffe-Regel limit . I tended to think that these would all occur at the approximately the same temperature. ( A Nature paper also makes a similar assumption.) However, Jure Kokalj pointed out to me that DMFT gives quite different temperatures for the different signatures above. For example, in this PRB the thermopower and specific heat have a peak at a temperature at which ther

I was pleased to learn this week that my paper on hydrogen bonding has been accepted for publication in Chemical Physics Letters . I believe this may be one of the most significant papers I have written. Time will tell... The paper has an "interesting" history. Six months ago I sent the paper with the title  Unified description of hydrogen bonding and proton transfer by a two-state effective Hamiltonian  to Physical Review Letters . I chose PRL because I thought the paper was significant and it approached the problem from a physicist's point of view, putting simplicity and physical insight before chemical detail. However, based on one brief referee report , an Adjunct Associate Editor rejected the paper. I wrote a rebuttal and resubmitted it. You may find the rebuttal  interesting reading as it highlights what I consider to be some fundamental issues about what is good and novel science, particularly of complex chemical systems. I was optimistic because I thought my r

This is the latest in a series of posts cataloguing how there are wide range of strongly correlated metals which exhibit magnetoresistance which is qualitatively different from the semi-classical orbital magnetoresistance seen in most metals due to the Lorentz force. For the latter the magnetoresistance is maximal (zero) for the current and magnetic field perpendicular (parallel) to each other. The data above is taken from a paper reporting measurements on the quasi-one-dimensional metal Li0.9Mo6O17, which also exhibits other non-Fermi liquid properties. The graph shows the magnetic field dependence (in Teslan) of the relative change in the interlayer resistance with the field and current parallel to one another. The different curves correspond to different temperatures, increasing from 3 K to 50 K, from top to bottom.

I just noticed that I have now made more than 1000 posts on this blog! It is hard to believe. It all started three years ago when I heard a talk about blogging at the I2CAM annual meeting by Clifford Johnson. The blog now gets about 3,000 page views per week. However, I still wish it generated more comments and discussion. I fear I am just not controversial enough.

Some students see the instructor/teacher/lecturer as their ally. Someone whose role is to help them learn and understand. Other students see the teacher as their adversary. Someone who is assessing them, critiquing them, making them feel dumb, forcing them to work, ... Someone they have to do battle with to get the marks and grade they think they deserve... I am not sure how to get students to see me as their ally rather than their adversary. I think a lot of it boils down to my attitude and their attitude . I can change my attitude but there are some students whose attitude may not change, regardless of what I do. Also, the attitude of a class can be swayed, for better or for worse, by a few influential students. I think using a formative/summative assessment mix can help. This enables students to get feedback in a less threatening way and with lower stakes. I welcome other ideas.

There is an interesting PRB Conductivity of hard core bosons: a paradigm of a bad metal by Netanel Lindner and Assa Auerbach. They calculate the frequency and temperature dependence of a model for hard core bosons on a square lattice at half filling. There is only a single energy scale J, the intersite boson hopping energy, in the Hamiltonian. At low temperatures (below the Kosterlitz-Thouless transition temperature ~ J) the system is a superfluid. At T > 2J the model is in a metallic phase with a resistivity which increases approximately linearly with temperature and has values larger than the Mott-Ioffe-Regel limit (~ the quantum of resistance). There is a broad "Drude peak" with a width which is much larger than J and a spectral weight which decreases with increasing temperature. The model also approximately obeys Homes scaling law which relates the superfluid density, the superfluid transition temperature Tc, and the conductivity at Tc, for cuprate superconductors

If you want some new (and old) ideas to improve your teaching it is worth looking at McKeachie's teaching tips: strategies, research, and theory for college and university teachers.  The book is now in the 13th edition.

Much of condensed matter physics is concerned with finding the relevant order parameter for new phases of matter. Indeed this is a good way to win a Nobel Prize! This is much of what was done by Ginzburg, Neel, Leggett, de Gennes,.... A fundamental and controversial question is whether one can a priori predict new order parameters. Historically, the progression has always been: Experimental discovery of a new phase of matter. Proposal of an order parameter and a phenomenological (Ginzburg-Landau) theory to explain a range of experiments. Proposal of an effective Hamiltonian which has a ground state with the desired spontaneous symmetry breaking and associated order parameter. Justification of the effective Hamiltonian from so-called "ab initio" electronic structure calculations starting with Schrodinger's equation and the actual chemical composition of specific materials.  The grand challenge is to invert this process, even just one step. Laughlin and Pines seem t

It seems to me many authors (particularly non-experts) are rather sloppy when they refer to electronic structure calculations based on Density Functional Theory (DFT). They say things like "our results are in good agreement with DFT calculations" or "we calculate the excited state energies using DFT". What don't I like about this? DFT is an exact theory,.... provided one has the exact exchange-correlation functional and one can solve the non-linear variational functional equation the density must satisfy.... Hence, DFT should always agree with experiment! But in reality, pure DFT is practically useless. One cannot do either of the above and so much use approximate functionals such as those based on the Local Density Approximation (LDA) or Generalised Gradient Approximation (GGA). These then lead to Kohn-Sham type equations which are straight-forward to solve. Hence, it seems to me it is preferable to say, "our experimental results are comparable to

Many readers may be unaware of an embarrassing period in the history of physical chemistry and quantum chemistry [and Science, Nature, and DARPA]. (I only know about it because I grew up hearing my father talk about it). For a few heady years around 1970, many scientists sincerely believed that there was a stable polymer form of water. This was even "supported" by quantum chemistry calculations by Allen and Kollman (Princeton University)  published in Science. Eventually, it turned out that the original experimental evidence for polywater resulted from impurities. This sad episode is recounted in detail in the book Polywater by Felix Franks. There is a very nice review by David Eisenberg, entitled "A Scientific Gold Rush"  (published in Science!) of the book. It is worth reading, including the discussion of how the fact that the existence of polywater would have violated the laws of thermodynamics did not deter many true believers... Here is a quote from the rev

Two years ago Seth Olsen and I published a paper , pointing out that in the Green Fluorescent Protein there should be a dark excited state [i.e. a state which does not contribute significantly to the one-photon absorption cross section] which should have a large two photon absorption (TPA) cross section . This has a natural explanation in terms of a valence bond description of the three lowest lying singlet states of the chromophore. We were very pleased that last year there was a long article in Nature methods, Two-photon absorption properties of fluorescent proteins , by Drobizhev et al.. It contains the data below. The peak on the left (around 650 nm) is due to TPA from the dark excited state. The authors were unaware of our work, but cite it in their latest paper on vibronic effects in two-photon absorption.

CNR Rao has published an auto-biography Climbing the Limitless Ladder: A Life in Chemistry . There are nice reviews in the Hindu newspaper and Chemistry World.  It is impressive how he continues to be so passionate about doing science, in spite of many successes and the burden of significant administrative and advising responsibilities.

Today we had a nice colloquium by Chris Vale about recent experiments from his group testing Tan's universal relations for ultracold high density (k_F a much greater than 1) atomic Fermi gases with large scattering lengths a. This regime corresponds to the centre of the figure below, taken from a Physics Today article by Carlos Sa de Melo There is an interesting short Physics article by Eric Braaten which puts Tan's theory in a broader historical context.

On your CV you need to clearly distinguish real publications, preprints, and works "in preparation". When reviewing CV's I personally think the latter are just irritating, count for nothing, and should be left off your CV.

For the fourth semester I am teaching a course with a blog where students must write weekly on the blog about what they are learning. They also must write some comments on other student posts. I think this is a very valuable experience for the students and for me. It keeps them engaged and forces them to think about what they are learning. I find it very valuable to read their posts because it gives me a good measure of their level of understanding and what topics or concepts they are struggling with. I am usually struck by how much they still need to learn and understand, and how things that seem obvious to me are not to them. It is a good reminder how about 25 years ago I struggled to master the subject too. I need to slow down my teaching and explain everything carefully. You can check out the blog here.

I have been working through a really nice paper Topological Berry phase and semiclassical quantization of cyclotron orbits for two dimensional electrons in coupled band models by J. N. Fuchs, F. Piéchon, M. O. Goerbig and G. Montambaux The Appendix below I found particularly illuminating

Below is a schematic picture of hydrogen bonding between a donor D and an acceptor A. An interesting and important question is how the potential energy of the system varies with the angle phi which measures deviations from linearity. A key property of H-bonds is that they are highly directional.  This leads to the four-fold co-ordination of water in liquid and solid phases. There is a vibrational mode associated with this co-ordinate phi. [In water this rotation is connected to the librational mode ]. Previously I posted about an empirical correlation showing how this mode hardens with the increasing strength of the H-bond (decreasing R above). This is the opposite trend to the D-H stretch frequency (associated with the r co-ordinate above) which softens with decreasing R. I was very pleased when I discovered that the model effective Hamiltonian I proposed for H-bonding can describe this correlation (see the figure below) without introducing any new parameters. This is discus

I have been teaching the Drude model for the umpteenth time, but have a new slant I am emphasizing to the students. It only involves a single new parameter , the scattering time. Yet it is able to capture the details of a wide range of experiments on elemental metals. I think it is important for students to appreciate the importance of considering the simplest possible models, with as few free parameters as possible, and see what they can and cannot explain. Only then one should move to more advanced models which are closer to the "truth".

Chelsea football club announced on the weekend that they are sacking their manager Andre Villas-Boas (AVB) after less than 12 months on the job. Why is this relevant? In the midst of all the analysis one point was clear. One of the problems was that AVB was only 34 years old and was managing players who were only two years younger than him. Sometimes this is a problem in academia. I have known of very successful young faculty members who have struggled to gain the respect needed for a good working relationship from students or postdocs who are older than them. This is a complex issue (because people are complex!). There are counter examples and every case is different but it is something to be cautious about. If you are a young faculty member think twice before you hire an older person and if you are an older student make sure you really do respect your potential advisor before you sign on.

It contains no actual experimental data. Just schematic representations of data or functional relationships. Or, it contains "artist renditions" of data. Below is an example from Atkins' Physical Chemistry . The best texts show actual data. Two of my favourite texts are: Biological Physics by Phil Nelson and An Introduction to Thermal Physics by Daniel Schroeder. It is also interesting and disturbing that looking through the Australian edition of  a popular introductory microeconomics text  co-authored by Robert E. Frank [of Economic Naturalist fame] I could only find schematic graphs and no actual data. Why does this matter? After all, the figures look so much nicer and cleaner if they are just schematic. It is easier for students to understand them. However, I think it is very important that students learn: Real science is messy. A theory is only as good as the experimental evidence for it. How to read, understand, and critique experimental data. Below

At the cake meeting this week I gave a talk on Nozieres' classic 1974 paper, A "Fermi liquid" description of the Kondo Problem at low temperatures . He gives a very elegant (but hard to follow) argument as to why the Wilson ratio should have the universal value 2, independent of the strength of the Kondo coupling J. There is actually a clearer restatement of Nozieres' argument in a review article by Piers Coleman (see section 2.6). [I thank Ben Powell for pointing this out]. Hewson's book (Section 5.1) also has an equivalent argument but I found that even harder to follow. But, I did like the connection to Friedel's sum rule and the emphasis that the charge compressibility on the impurity site is zero. Key assumptions (and physical insights) required in the argument seem to be: A Fermi liquid fixed point (J=infinity). An analytic dependence of the phase shift on energy. The Kondo singlet acts as a spinless, elastic scattering centre with phase shif

The field is particularly demanding due to the need to assess, integrate and synthesize large amounts of information from a diverse range of experimental, theoretical, and computational studies that use a plethora of techniques to study a multitude of properties, models, and materials. This requires significant amounts of time and experience to evaluate and make appropriate judgement calls about the relative importance and probable validity of specific pieces of information. Inadequately informed judgements can lead to wasted time due to pursuing directions that ultimately turn out to be at best irrelevant and at worst wrong. Furthermore, an intimate knowledge of both the chemical and physical properties of solids is a necessary ingredient for the development of realistic models of different classes of complex materials. A key judgement has to be made about how much detail is necessary. Or, is the field a mine field?