Condensed concepts

I read this amusing and instructive story about the dress sense of physicists on a Physics Today blog post by Charles Day: The most extreme example of sartorial insouciance I've witnessed was that of James Heath, a pioneer of molecular computing (and who would probably call himself a chemist, I should point out). One November, Heath flew from Los Angeles to Boston to give an invited talk at the Materials Research Society meeting. He showed up in the convention center wearing a brightly colored short-sleeved shirt, shorts, and, if I remember correctly, sandals. Not only had he forgotten to dress for Boston's weather, he'd also left his laptop in California. Did those mental slips matter? Hardly. Using hastily prepared, hand-written viewgraphs, he gave one of the best talks of the meeting. Indeed, it's conceivable that in creating his viewgraphs, Heath was forced to focus more on his message than on its presentation.

I love the software and book Solid State Simulations . I have used it extensively in teaching, particularly as assignment exercises. I have always been delighted and amazed at how much I have learnt from it too! There is no substitute for visualization. Seeing is believing. I still remember the first time I saw electrons and holes going in opposite directions around their respective Fermi surfaces in a magnetic field. However, I was alarmed a few weeks ago when I found I could not run it on my Mac. I just started installing a trial version of Parallels to see if it I could run it then. [BTW: I could never get registration for the trial version of VMware Fusion to work]. However, I just discovered that is not necessary! An OSX version has just been released. You can download it for free now.

Previously I posted about some fascinating experimental results on anisotropic thermal expansion and elastic softening near superconducting and magnetic transitions in organic charge transfer salts. Subsequently, I became aware that the new iron pnictide superconductors do exhibit somewhat similar phenomena. A combined theory-experimental PRL (10 co-authors!) describes shear acoustic mode spectroscopy in terms of nematic spin fluctuations . They find in undoped BaFe2As2 that the shear modulus (C66) softens significantly as one approaches the magnetically ordered phase (which is a associated with a tetragonal-orthorhombic lattice distortion). For the optimally doped material there is a hardening of the lattice as one enters the superconducting phase. The figure above explains the nematic order parameter and how it couples to shear lattice distortions. A key is the that the magnetic phase consists of Neel antiferromagnetic order on two separate sublattices . They are weakly coupled

This semester I am co-teaching PHYS4030 Condensed Matter Physics : Electronic properties of crystals. This is a fourth year undergraduate (honours) course. Since there are only 5 students enrolled the course will run as an interactive reading course, somewhat similar to a biophysics course, BIPH3001 and PHYS3170 which I ran the last two years. The text will be the classic Ashcroft and Mermin [Although I wonder if we shouldn't change to Marder ]. I have an introductory overview lecture of the first half of the course which discusses 10 key ideas.

In the "good old days" university students were "self-motivated", assessment was almost all based on exams and whether students showed up for tutorials and lectures or read the textbook was their own problem. However, the painful reality today is that most students will usually only do some work under a "carrot and stick" system of continuous assessment. Consequently, in many courses students can get credit for just showing up at tutorials and can get easy marks for assignments, some of which they get "help" with from their friends. This means that a student can pass a course even though they get a failing mark on the final exam.  This semester I am co-teaching PHYS2020 Thermodynamics and Condensed Matter Physics with Joel Corney . [I have previously posted lectures I have given in this course]. Joel recently came up with a new assessment system that I think addresses some of the problems mentioned above. The details are below. I would be inte

An important concept which has featured in previous posts is that of 4 electron, 3 orbital chemical bonding. It is relevant to hydrogen bonding and methine dyes . Here is a  scan  of some of the relevant pages of the beautiful book, Valency and Bonding by Weinhold and Landis.

Today there was an interesting Quantum science seminar by  Ulrik L. Andersen (Technical University of Denmark) Quantum Plasmonics: Controlled Coupling of a Single Nitrogen-Vacancy Center to a Silver Nanowire. A question came up about plasmons in gold  nano-particles and how the surface plasmon frequency is renormalised downwards (i.e. blue-shifted) compared to the frequency in the bulk. Gerard Milburn pointed out that this is illustrated by The Lycurgus Cup in the British Museum.  Coincidentally, an article by Mark Stockman in this months Physics Today states: The resonant properties of plasmonic metal nanoparticles are readily apparent to the naked eye because the excitations absorb and scatter light at optical frequencies. The most ancient example is the famous fourth-century CE Lycurgus cup from the British museum, whose glass looks green in reflected light but ruby red in transmitted light. Those colors are complementary, evidence that there is little optical loss inside the g

Previously, I posted some notes about an empirical valence bond model for hydrogen bonding. I have been thinking about whether it is possible to justify such a "simple" picture [and effective Hamiltonian] from high-level quantum chemical calculations. A helpful paper I have been looking at is a 2001 J. Chem. Phys. by Kowal, Roszak, and Leszczynski. They perform complete active space - self-consistent field (CASSCF) calculations on the ground and excited states of the water dimer, (H4O2) and H3O2- and H2O52+. The equilibrium geometry of the latter is pictured below. The three systems correspond to weak, intermediate, and strong hydrogen bonds respectively. This morning I read slowly read through the paper again. They have 12 electrons in an active space of 7-9 (I think) orbitals, which are localised on the individual atoms. Below I reproduce the potential energy curves as the O-H bond length with increased with the distance between the oxygen atoms at the equilibrium value.

This month's issue of Physics Today has a nice article Facilitating science in developing countries about a US based non-profit group Seeding Labs. It was started in 2002 by a few Harvard graduate students to send "out-dated" lab equipment to universities in the developing world.

Thermoelectric materials  are of significant technological interest and present some fundamental scientific questions. An extremely useful concept is the dimensionless thermoelectric figure of merit . The optimum material with have a high electrical conductivity and thermoelectric power (Seebeck coefficient), but also a low thermal conductivity. This has led to the notion of a Phonon Glass Electron Crystal (PGEC): a material which has the low thermal conductivity characteristic of a glass and the high electrical conductivity characteristic of a crystal. How might one achieve this? In simple kinetic theory [with well-define acoustic phonon quasi-particles] the thermal conductivity is proportional to the phonon velocity and the phonon mean-free path. In glasses there is so much structural disorder the concept of phonon quasi-particles and a mean-free path is ill defined. In the quasi-particle picture one could reduce the thermal conductivity either by decreasing the phonon mean-free p

A landmark contribution of Linus Pauling was to show the intimate connection between molecular structure and the spatial arrangement of electronic wavefunctions. This is exemplified by the concept of orbital hybridisation.  The figure above shows how by taking different linear combinations of s and p orbitals one can produce orbitals with different directionality. However, as is often the case in quantum chemistry, it turns out not to be quite so simple. I was surprised to learn recently about the notion of imperfect orbital following. Specifically, the direction of the orbitals is not always the same as that of the nuclei, particularly, for non-equilibrium geometries. This can be seen for ammonia as it undergoes the umbrella inversion (the mode associated with the MASER). This phenomenon  of orbital following in ammonia was elucidated by Cohan and Coulson in 1956.  The figures below are from a JACS paper by Foster and Weinhold. [There is also a nice discussion in the book by Weinho

It is amazing and encouraging to me how there are still outstanding problems in science and mathematics that are so simple that statement of the problem can be understood by high school students. An example which has attracted a lot of interest in the past year is the problem of finding the optimum packing efficiency of tetrahedra. There is a nice New York Times article  and a Wolfram blog post about this.

Ahmed Zewail has a thoughtful piece The future of Chemical Physics  which concludes: If chemical physicists look ahead with intellectual curiosity to examine the fundamentals of nature, unswayed by fad or funding , I believe that the discipline will be here to stay. Here are a few other extracts which resonate with me: opportunities in this century are even more exciting than, and as significant as, in the past, provided that we do not restrict our vision to orthodox boundaries and keep in perspective the core objective of chemical physics....  providing new tools and defining new concepts, but with the lens being focused on significant questions in emerging areas of molecular complexity which span the gamut of applications at the frontiers of chemistry and biology. Breakthroughs will continue to emerge when applications of visualization methods extend into systems of thousands of atoms and cells, and when the pertinent concepts are generalized with the help of “simpl

The past few weeks I have been puzzling through the implications of some really nice experimental results on superconductivity in organic charge transfer salts. A group at Sherbrooke measured the speed of sound as a function of temperature for different polarisations. The Figure below, taken from their PRB , shows how anisotropic the elastic response is for the material  κ-(BEDT-TTF)2Cu[N(CN)2]Br  . The sound is always propagating perpendicular to the layers, which lies in direction of the b axis of the crystal. C22 is the elastic constant for longitudinal sound. C44 has polarisation parallel to the c direction in the crystal which is the same direction as the t' (diagonal) hopping in the relevant Hubbard model [see picture below and this review ]. C66 has polarisation in the a direction. A few thoughts  A really helpful succinct summary of elasticity theory and sound velocity anomalies at Tc is found in Section II of this PRB , also from Sherbrooke [I believe there is an importa

O.K. Now I have got your attention, my real point is that just because university paperwork and bureaucracy are often a waste of time does not mean they are always a waste of time. There are several exercises at UQ that I think are particularly valuable and I encourage people to make the most of. They are Ph.D confirmation documents, seminars, and interviews. This is a "hoop" that students have to jump through after one year of enrolment in order to be allowed to proceed with their Ph.D. Annual staff performance appraisals. Some people think these are just another tedious exercise. But, I think both exercises are of great value to all concerned, and are particularly important in cases where performance is poor. Then documentation, due process, and clear communication are important. With regard to much of the other admin and paperwork, just grit your teeth, tick and complete the boxes, turn it in, and humour the administrators....

Many fundamental processes in biology such as signalling and energy conversion make use of the transport and storage of protons within proteins. One widely studied example occurs in vision; in the membrane protein bacteriorhodopsin. Absorption of a photon leads to isomerisation of the retinal molecule which eventually leads to transport of a protein across the cell membrane, resulting in an electrical signal in your nerves. Hence, a key question concerns the location of the protons at different stages of the process. Generally, high-resolution crystal structures (from X-rays) do not reveal the location of protons. Over the past decade it has become more appreciated that the location of water molecules inside the protein can provide a key functional role due to their ability to form hydrogen bonds which allow take up and release of protons. In 2006 a Nature paper  by Garczarek and Gerwert used infra-red spectroscopy to investigate the proton dynamics and argued that the water molecules

Sorry! nLA= n-letter acronym If you want to attract an audience to attend your talk, read your paper, and/or like your grant application, I would encourage you not to use acronym's (e.g., EPR, DFT, SA-CAS-SCF, DMFT, NOON, ADMR, etc.). The chances are most of the people not working on exactly the same topic will have no idea what you are talking about. If so, they will quickly lose interest and possibly resent being immediately excluded.

To physicists details don't matter. To chemists they do and to biologists they are a matter of life and death. The key question in understanding biomolecular structure, property, and function is: which details really do matter. The Figure above is taken from a fascinating PNAS paper which I will post about later on. But this post is just to point out an important point illustrated by the Figure. It shows the location of different amino acids (Glu194, Ser193, etc..) inside the protein bacteriorhodopsin. Each different colour corresponds to the location determined and published by a different experimental group. To a physicist the differences look pretty minor. However, it turns out the exact distance between the oxygen atoms at the end of the Glu194 and Glu204 residues turn out to be crucial for understanding where proton involved in the protein-pump function of the protein is stored. A second more general point is that the different structures illustrate that one should always t

What are the really key ingredients to important research breakthroughs? Is the involvement of commercial interests helpful, neutral, or a hindrance? The following letter appeared in the Sydney Morning Herald last month. I read it when it was reprinted in The Week.  This was in the context of an article Doctors criticise leukemia drug study  concerning potential conflicts of interest. Drug companies are not a cure-all Stephen Mulligan ( Letters, January 26 ) says: "Collaboration between leading clinicians and industry, under appropriate guidelines, is not just desirable but essential. Without it there will be no progress in clinical research, and the textbooks for treatment for 2050 and beyond could be written now." Really? Two of the greatest medical advances in the past 50 years, both identified primarily by Australians, were the discoveries of the causal relationship of some types of papilloma viruses to cancer of the cervix and that of Helicobacter pylori to peptic ulce

As I have posted before I always like papers by Roald Hoffmann for their clarity and chemical intuition. The latest one I have read is  Bonding in the trihalides (X3–), mixed trihalides (X2Y–) and hydrogen bihalides (X2H–). The connection between hypervalent, electron-rich three-center, donor–acceptor and strong hydrogen bonding It seems the concept of four electron-three orbital bonding has wide chemical applications. I also noticed that the Figure below is reminiscent of the "backbonding" which occurs in organo-metallic complexes.

In 1995 Phil Anderson organised a colloquium: "Physics: the opening to complexity" for the USA National Academy of Sciences. His introduction is worth reading. I reproduce below an extract because it gives such a clear discussion of emergence. At this frontier [of complexity], the watchword is not reductionism but emergence. Emergent complex phenomena are by no means in violation of the microscopic laws, but they do not appear as logicaly consequent on these laws. That this is the case can be illustrated by two examples which show that a complex phenomenon can follow laws independently of the detailed substrate in which it is expressed. (i) The "Bardeen-Cooper-Schrieffer (BCS)" phenomenon of broken gauge symmetry in dense Fermion liquids has at least three expressions: electrons in metals, of course, where it is called "superconductivity"; 3He atoms, which become a superfluid when liquid 3He is cooled below 1-3 mK; and nucleons both in ordinary nuclei (th

I just read a really helpful paper Optical symmetries and anisotropic transport in high-Tc transport by Tom Devereaux. Although the paper is a PRB it reads more like a short review. It is very clear and provides lots of summary expressions. The main purpose of the paper is to show the intimate connection between low-frequency electronic Raman scattering and charge conductivity. [n.b. this Raman scattering is NOT scattering off phonons but electron-hole pairs]. Furthermore, intra-layer and inter-layer conductivity can be related to Raman scattering of different symmetries [A1g, B1g, B2g]. The figure below shows this correlation. I was particularly interested in the paper for several reasons: It is possible to define scattering rates associated with the electronic scattering and to relate these to scattering on the Fermi surface in the nodal and anti-nodal directions. This allows a comparison to be made with the temperature dependence predicted by "cold spot", "hot spot

Being a referee is somewhat of a thankless task. It means being a good citizen and putting the interests of the scientific community before your own. Furthermore, if you are conscientious and provide a helpful report in a timely manner, the journal editors will "reward" you by sending you more and more papers to referee. Postdocs can easily sink a lot of time into refereeing papers, to little personal gain. Someone once told me a helpful rule for being a good scientific citizen: you should referee approximately the same number of papers per year as you submit [normalised to the number of co-authors]. I encourage my postdocs to follow this guideline. Some good reasons to referee a paper: You have some constructive feedback that will improve the paper. The paper has serious flaws. It is good experience to learn how referee's may respond to your papers. Some good reasons to not referee a paper: You are too busy and can't provide a report in a timely manner. Y