Condensed concepts

Oh, how I love simplicity and physical insight! There is a beautiful Physics Viewpoint  Untangling the Orbitals in Iron-Based Superconductors by Daniel Podolsky. It describes a Phys. Rev. X paper by Jiangping Hu and Ningning Hao. They show that a key to understanding the electronic structure of the iron pnictide and chalcogenide superconductors is the S4 symmetry of the lattice of Fe ions. They are able to explain why the iron chalcogenides do not have holelike Fermi surface pockets whereas the iron pnictides do. Furthermore, by means of a simple gauge transformation associated with the S4 symmetry they map the s-wave superconducting order parameter to the d-wave order parameter associated with the C_4v symmetry of the Cu ions in the cuprates.

Antoines Georges has a nice article  Thinking locally: Reflections on Dynamical Mean-Field Theory from a high-temperature/high-energy perspective. The article is a short review dedicated to Dieter Vollhardt on his 60th birthday. He discusses recent work which compares DMFT to high temperature series expansions for the Hubbard model. They agree for temperatures above about t, the hopping integral. This supports intuition that with increasing temperature physics becomes more local and so DMFT should be more reliable. He also mentions cold atom experiments which are getting towards low enough temperatures that they could potentially be used to test DMFT predictions.

How do single nucleons and associated excitations couple to collective degrees of freedom such as rotations and shape deformations? Is there a Born-Oppenheimer approximation in nuclear physics? What is the origin of non-spherical nuclei and the associated symmetry breaking? Is the notion of a Jahn-Teller effect and conical intersections relevant? There issues go back to classic ideas in theoretical nuclear physics for which Aage Bohr, Mottelson, and Rainwater were awarded the Nobel Prize in Physics in 1975.  This is discussed in an earlier post. There is also a classic paper by Hill and Wheeler  which does include a discussion of conical intersections [I thank Seth Olsen for bringing it to my attention]. The relevant physics is elegantly discussed in a nice review article The Nuclear Collective Motion by Witold Nazarewicz. Here is an extract He then goes on to discuss how the deformations of nuclei can be understood in terms of the Jahn-Teller effect. The figure below

There is an interesting JACS communication Backbone Modification of Retinal Induces Protein-like Excited State Dynamics in Solution  from a group at Oxford. Here is some of the abstract: The drastically different reactivity of the retinal chromophore in solution compared to the protein environment is poorly understood. Here, we show that the addition of a methyl group to the C=C backbone of all-trans retinal protonated Schiff base accelerates the electronic decay in solution making it comparable to the proton pump bacteriorhodopsin. Contrary to the notion that reaction speed and efficiency are linked, we observe a concomitant 50% reduction in the isomerization yield. The results are of particular interest because most previous attempts to modify the chromophore and/or solvent have not led to much change in the photo-isomerisation rate. A point I am not clear on, concerns the significance of the statement:   "Contrary to   the notion that reaction speed and efficiency a

In the past week I saw two articles highlighting changes in Australian universities. The Weekend Australian ran a piece Mass education changes face of universities  by Julie Hare. [There is also a video summary  but it lacks some of the important statistics and graphs in the article.] While the total population of Australia has increased by a factor of less than than three since the 1960's the university student population has increased by a factor of 26! But, increasingly, graduates work in areas unrelated to their studies. Only half of law graduates will work as lawyers. Just 2 per cent of teaching graduates in NSW each year are directly offered a position in the state school system. About 4750 students are enrolled in journalism schools, but with 100 entry-level positions in mainstream media, many will struggle to get off the starting block.   Corkindale says: "Does the community, including school-leavers and their parents, need to be made more aware that going to st

Last two days I have read through a nice paper  Modeling the Nonradiative Decay Rate of Electronically Excited Thioflavin T by Yuval Erez, Yu-Hui Liu, Nadav Amdursky, and Dan Huppert The relevant molecule [chromophore] is shown below. It is of particular interest because its fluorescence intensity increases significantly when bound to amyloid fibrils which are associated with Parkinson's, type II diabetes, and Alzheimer's disease. The key photophysics is associated with twisting about the central carbon-carbon bond. The experimental results that need to be explained are -the non-radiative life time increased linearly with the solvent viscosity over 3 orders of magnitude -the fluorescence intensity decreases with time on the scales of tens of picoseconds The calculated [via TDFT = Time-Dependent Density Functional Theory] dependence of the ground and excited state energies as a function of the twist angle is shown below. As the twist angle increases from zero to 9

It is interesting to see how much undergraduate teaching has changed in just the past decade. Here is a case study for just one course I have been involved in. In 2001 when I came to UQ I first helped teach a second year undergraduate course Thermodynamics and Condensed Matter Physics. Back then any use of Powerpoint or online resources was a novelty. There was no Blackboard or YouTube. Videos were shown by taking a videocassette to a central projection unit on campus and booking for them to show the video in the lecture at the requested time. Students expected lecture notes and these were photocopied and handed out. The course comprised 3 lectures and 1 tutorial per week. All the assessment was exams: one mid semester and a final. There were 20+ students enrolled. What about now? There are 2 lectures and 1 tutorial per week. In addition students do 3 labs lasting 3 hours. The assessment has expanded substantially . Students now complete    23 online reading quizzes    10 p

What is the structure of small clusters of water molecules? The latest issue of Science has a fascinating article  Structures of Cage, Prism, and Book Isomers of Water Hexamer from Broadband Rotational Spectroscopy . The associated Perspective by Saykally and Wales is particularly helpful. In particular, for six molecules there are three alternative structures which are very close in energy, denoted the prism, cage, and book, in the figure above. Quantum chemistry calculations give different energetic orderings depending on the level of theory used. Furthermore, the zero point motion of the atoms is important. The high resolution spectroscopy in the paper suggests that the cage structure is the most stable, but only by an energy of about 1 kJ/mol (~10 meV). Contrary to what one may expect, not all the bond lengths are equal.

Following the example of some of my colleagues this semester I have started doing pre-lecture reading quizzes for my second year undergraduate course on Thermodynamics and Condensed Matter. Here is how it works. A reading on the subject of the lecture (usually a Section from the textbook by Schroeder) is assigned. A brief quiz of 2-4 questions is placed on Blackboard. These can be multiple choice and/or brief essay. The aim is to "force/encourage" students to engage with the text , think about the material, and be better prepared for the lecture. Reading the quiz results before the lecture provides some useful feedback on students levels of understanding and misconceptions. The occasional question, "What don't you understand in the reading?" provides useful feedback to the lecturer who can try and address these in the actual lecture. The marks/grades for the quiz contribute a small amount to the formative assessment. This seems to be enough to motivate the

Last night my wife and I went to watch the movie Footnote. It chronicles the tensions and competition between a father and son who do research in the same obscure field of Talmudic studies. The movie was made in Israel, is in Hebrew with English subtitles, and received an awarded for best script at the Cannes film festival. The movie captures some of the silly ways of academia: pedantry, exclusion, ambition, stubborn idealism, ... I found some of it quite humorous but some of it was a bit too close to the truth to be funny.

There is an interesting article Emergent Physics the Mesoscale: Report from the special Kavli session at the 2012 APS March meeting by Sam Bader on The Back Page of the May edition of the American Physical Society News. It sounds like there was a fascinating and contrasting series of talks by Bob Laughlin, Bill Phillips, Angela Belcher, Bill Bialek, and George Whitesides. Apparently Phillips "gave short shrift of the concept of emergence, discarding it mercilessly." I would be interested to see copies of the talks. Has anyone seen them online? Overall, this latest focus on the "meso" seems to be driven by hopes of a new burst of funding like what happened with nanotechnology in 2000. [See this brief piece in Science ] In the end I think that initiative was a big disappointment scientifically. I feel the whole field was hijacked by people who just relabelled whatever they were doing as nanoscience or nanotechnology. To me it should have been all about control

At the cake meeting I gave an informal talk on how a two-site Hubbard-Holstein model can illuminate some basic and important concepts in the photo-isomerisation of simple molecules such as ethylene. A previous post discusses how the two-site Hubbard models illustrates many basic concepts in quantum chemistry and many-body theory. Here are a few of the key references and ideas I drew upon in my talk. A PRA from 2000 (and largely uncited) by Aalberts et al. Quantum coherent dynamics of molecules: A simple scenario for ultrafast photoisomerization 1. It points out that photoisomerisation only occurs if there are "steric" interactions. i.e. the sigma bonds (not included in the Hubbard model) do not favour a planar arrangement for the molecule. Thus, the ground state is only planar due to the delocalisation energy associated with the pi electrons included in the Hubbard model. 2. This then leads to a twist angle (phi) dependence of the energies of three singlet states si

There can be significant career value is writing a single author paper, particularly for graduate students and postdocs. It clearly shows that one has become independent , and is not completely dependent on senior people for ideas, guidance, techniques, ... Hence, one is ready for a faculty position. I try to encourage my postdocs, particularly senior ones to do this, occasionally suggesting my name should not be on a paper because I have not made a significant contribution. Unfortunately, there are some senior people who will not allow or will strongly discourage this. They believe that if they pay the salary or provide the lab that entitles their name to be on every paper produced by someone in their group (even sometimes associated junior faculty). Hence, negotiating single authorship can be a difficult and sensitive subject. Thus, it can also be potential career killer... For experimentalists single author papers may be difficult, except for review articles. For theorists it

Finding conical intersections between potential energy surfaces is key to understanding photochemistry, particular for ultrafast non-adiabatic reactions. An earlier post  pointed out how often these conical intersections occur at a molecular geometry where there is a local triangular symmetry. This leads naturally to an effective Hamiltonian which has a C_3 (or higher) symmetry and the degenerate eigenstates are in the two-dimensional E representation. However, there is more to the story... There is a nice recent review Electronically excited states and photodynamics: a continuing challenge  by Plasser, Barabatti, Aquino, and Lischka. They present a Table of common motifs for "primitive conical intersections". These do not have the "hidden" triangular symmetry discussed above. The authors also suggest there are three distinct excited state pathways, summarised schematically in the diagram below. They respectively occur in the molecules shown:

Calculating the rate of a chemical reaction in a condensed phase environment is a highly non-trivial problem. It is easy to make a hand-waving argument that the rate is proportional to an Arrhenius factor associated with the activation energy. But, getting the prefactor is a rather subtle problem. There is a seminal 1990 review Reaction rate theory: 50 years after Kramers by Hanggi, Talkner, and Borkovec. One of the main results is the Kramers turnover described below. For "strong" friction the rate is controlled by "spatial diffusion" and given by where gamma is the friction which can depend on frequency (i.e. be non-Markovian). This rate decreases monotonically with increasing friction. For "weak" friction the rate is controlled by "energy diffusion" and increases monotonically with increasing friction. Interpolating between the two regimes is difficult. But, the important point is there should be a Kramers turnover in the prefacto

I believe every research group should have one timetabled meeting each week. If not, people tend to get disconnected and drift. Students can get isolated and lose motivation. Attendance should be compulsory. Effective meetings are enjoyable and productive. People learn new things, particularly what is relevant to their own research. Furthermore, informal interactions associated with these meetings can lead to breakthroughs, both minor and major. What should be the format of the meeting? A balance between structure and informality seems to be key. Talks on the whiteboard are preferable to power point presentations. Here are a few ideas on meeting content: A group members talks about what they are currently working on. This provides feedback and accountability in a friendly environment. People learn more about what other group members are working on. Hopefully this leads to other conversations. A group member talks about an important paper by someone else. It can be an old "c

In the cuprate superconductors there is a value of the doping at which the superconducting transition temperature is a maximum (optimal doping). Coincidentally (?) this also seems to the doping at which the metallic phase is most non-Fermi liquid like. Some theories (especially due to Varma) try and connect these two phenomena via a quantum critical point below the superconducting dome. An earlier post discusses how the entropy is maximal and the thermopower changes sign near optimal doping. A cluster DMFT (Dynamical Mean-Field Theory) calculation by Kristian Haule reproduces the correlation between high-Tc and anomalous metallic properties. The figure below shows the Matsubara frequency dependence of the imaginary part of the self energy (at wave vector (0,pi) = anti-nodal region) (top) and the anomalous self energy (related to the superconducting pairing) for different dopings. In a simple Fermi liquid the slope of the upper curve at low frequencies is related to the quasi-partic

Today I gave a  lecture on first-order phase transitions to undergraduates. Again I find the students love the videos I show.  I mostly use videos from the Video Encyclopedia of Physics Demonstrations , which I got my dept. to buy a decade ago. However, now you can find virtually anything you need on YouTube! For example, here is a nice one of regelation of ice which demonstrates that the solid-liquid phase boundary has a negative slope.