Condensed concepts

I think it is crucial that any research group have a group meeting at a designated time each week. It surprises me that some groups do not do this or how some people hate their group meetings to think they are a waste of time. A typical group consists of one to a few faculty members and postdocs, grad. students, and possibly some undergrads working in the group. It is important that these weekly meetings are informal, relaxed and inclusive. They should encourage learning, interaction and feedback. What might happen at the meeting? Here are a few things that we do in the Condensed Matter Theory group at UQ [senior faculty are Ben Powell and I]. These meetings are compulsory and they are in addition to a weekly meeting between each individual in the group and their supervisor. Each week a group member is assigned to bring a cake or a packet of cookies/biscuits to share with the group. Group members provide their own drinks. A group member gives a talk on the white board abou

This post concerns a basic but bold claim about the effects of a protein or solvent environment on chemical structure and reactivity. I am not clear on how original or how radical or controversial the claim is. In some sense, I think it is largely consistent with what Ariel Warshel has being saying for a long time. [See here for example]. I would be interested to hear feedback on the claim. Consider some chemical reaction A + B to C + D One can consider the reaction is the gas phase [i.e. without an environment] or in a solvent [polar or non-polar] or in a protein environment. The relative stability of the reactants and the products, the rate of the reaction, and the reaction mechanism [i.e. the reaction co-ordinate and transition state geometry] can vary significantly and dramatically. This is what is amazing about enzymes : you can increase the rate of a reaction by a factor of a billion. So what is the most basic hypothesis about the effect of the environment? It can do

Conical intersections between potential energy surfaces get a lot of attention in the theoretical chemistry of electronic excited states (photochemistry) of molecules, particularly with regard as a mechanism for ultrafast (i.e. sub picosecond) non-radiative decay. The surfaces are functions of the spatial coordinates R=(x1,x2, ....) of the nuclei in the molecule. In the past decade the (hard) condensed matter physics community has become obsessed(?) with Dirac cones [graphene, topological insulators, Weyl semimetals, ...]. They occur in the electronic band structure [one-electron spectrum] when two energy bands cross. Here the system has spatial periodicity and the k's are Bloch quantum numbers. I want to highlight some similarities between conical intersections (CIs) and Dirac cones (DCs) but also highlight some important differences. First the similarities. A. Both CIs and DCs give rise to rich (topological) quantum physics associated with a geometric phase and the ass

At every institution occasionally an absolutely brilliant young student comes on the scene. A number of colleagues have pointed out to me that it seems that often these students are in a "race". They [or their parents] have some goal like: Get an undergraduate degree before most people graduate from high school. Be the youngest person ever to get a Ph.D from university X. Become a faculty member before they are 23. Be the youngest person to ever to get tenure at university Y. Sheldon Cooper from the Big Bang Theory embodies this. He endlessly reminds his friends that he graduated from college when he was 14 and was the youngest person at the time to receive the Stevenson award. This big rush is a mistake on several grounds. Overall it reduces enjoyment, deep learning , and substantial achievement . It also increases stress. Furthermore, promising students slow down as they go up the academic ladder. In most countries, the clock really does start clicking at the Ph

Weyl and Dirac semimetals are getting quite a bit of attention. Part of this interest is because of the possible solid state realisation of the chiral anomaly from quantum field theory.  One proposed signature is negative longitudinal magnetoresistance.  A different, arguably more definitive, signature is in the following paper. Quantum oscillations from surface Fermi arcs in Weyl and Dirac semimetals  Andrew C. Potter, Itamar Kimchi, and Ashvin Vishwanath In a thin slab of material there are "Fermi arc" states on the top and bottom surfaces. When a magnetic field is applied perpendicular to the slab, there are unusual closed orbits (shown below) where an electron can move around the arc on the  top surface, tunnel via a bulk chiral state to the bottom surface, move around the arc on the bottom surface, and then tunnel back to the top surface. The resulting Shubnikov de Haas oscillations have some unique signatures such as the periodicity and the dependence of the ph

Previously, I stated   that " The only value I see in Impact Factors is helping librarians compile draft lists of journals to cancel subscriptions to in order to save money." Now I don't even think Impact factors are good for that! This was brought home last week when the UQ library announced that because of the declining Australian dollar that it has to save A$1M and so would be cancelling some journal subscriptions. A proposed "cull" list has been circulated. The exact selection criteria used are not clearly defined but impact factor is stated as one. Guess what? More Mathematics journals are slated for cancellation than in any other field!  This is hardly surprising because the average number of citations per article in Mathematics is one third of that in Physics and Chemistry.  Thus, IF's for maths journals will be typically smaller by a factor of three. Also on the cancellation list is the American Journal of Physics and The Physics Teacher.  It

At the level of the Born-Oppenheimer approximation replacing hydrogen with deuterium in a molecule or crystal should not change anything. The "chemical forces" responsible for all types of bonding, and encoded in a potential energy surface, remain the same. However, in reality changes can occur such as geometric isotope effects. This is because the zero-point energy associated with hydrogen bonds changes. The essential physics is described here. In molecular crystals one can see not just small quantitative changes, such as changes in bond lengths of the order of a few hundredths of an Angstrom, but actual changes of the geometric arrangements of the molecules in the crystal. This " isotopic polymorphism " is nicely reviewed in a recent article by Klaus Merz and Anna Kupka. A specific example is pyridine . The H and D polymorphs are shown below and taken from here.  Note, the hydrogen bonds involved are relatively weak C-H...N bonds. Why does this sensitiv

A common feature of bad metals is that at relatively low temperatures [of the order of the coherence temperature which is much less than the non-interacting Fermi temperature] they have an entropy per electron of the order of Boltzmann's constant, k_B. This is more characteristic of a classical than a quantum system. For localised non-interacting spins the entropy is ln(2) k_B. In contrast, in a Fermi liquid such as an elemental metal, the electronic entropy is of order k_B T/T_F where T is the temperature and T_F is the Fermi temperature (10,000s K in an elemental metal). I don't think this bad metal property of the large electronic entropy is emphasised enough, although it was highlighted here. I illustrate this below with two sets of experimental data. The first set is  measurements on YBCO , with x related to the doping, small x corresponding to the under doped regime and x=1 approximately optimal doping. In the metallic state, the entropy increases approximately lin

Cherry picking data is not just done by scientific "denialists" but also some "respected" theorists who are seeking support for their scientific theory. I recently realised that some experimentalists cherry pick theories to describe their experimental data. I heard a talk by a theorist who reported having several disturbing conversations along the following lines. Experimentalist : We fitted our data to your theory. Theorist : But the theory is not valid or relevant in the parameter regime of your experiment. Experimentalist : We don't care. The theory fits the data.

Recently, several new approaches have been suggested to experimentally measure the viscosity of the electron fluid in a metallic crystal. Previously, I posted about how ultrasound attenuation can be used to indirectly measure the viscosity. However, that method is arguably not sensitive enough for the small viscosities [of the order  n hbar, where n is the electron density] that are of particular relevance to possible quantum limits to the viscosity. Forcella, Zaanen, Valentinis, and van Der Marel considered electromagnetic properties of viscous charged fluids, finding new possible signatures due to the viscosity such as negative refractive index, a frequency dependent peak in the reflection coefficient, and a strong frequency dependence of the phase. However, they note that these effects may be difficult to observe for viscosities of the order of the quantum limit, n hbar. Tomadin, Vignale, and Polini considered a two-dimensional electron fluid in a Corbino disk device in the p

I recently received my student evaluations for teaching last semester. I was pleased to see that the scores were very high and students made many positive comments about my teaching and the course. I would like to think this is due to my brilliant performance. But it is not. This years positive results are in contrast to several years ago when students in the same course were so unhappy that they met with the head of department to complain about me and the course. That year many students failed. This year more than half the class got the highest grade possible. What brought about this dramatic change? What did I do? Actually, virtually nothing! The course content and difficulty is the same. The assignments and exams are basically identical, as they have been for the past decade. I did minor fine tuning to my lectures, as I always do, and to the assessment mix. Students also do a pre-test to check prior knowledge and the tutorials are more student led. The real significant chan

I have received a lot of helpful feedback on a recent paper about shear viscosity in strongly interacting quantum fermion fluids. As a result I have learnt some interesting things that I will post about. Here is the first one. The shear viscosity can be written in terms of a Kubo formula which is an unequal time correlation function of the stress energy tensor. In a general fluid there are two terms in the stress energy tensor: one associated with the kinetic energy and the second with the interparticle interaction. In dense classical liquids the term in the Kubo formula due to the interaction term dominates and are associated with the  Einstein-Stokes relation where the viscosity is inversely proportional to the particle self-diffusion constant. In contrast, in dilute gases and fluids the kinetic term dominates and the shear viscosity scales with the diffusion constant and scattering time. The crossover from the dilute to the dense case in a classical fluid is discussed here.

When I was recently visiting my mother-in-law in Anacortes, Washington she took my wife and I to a meeting of the local chapter of Transition , an international grass roots movement responding to climate change. First, an employee of a local not-for-profit Sustainable Connections  spoke briefly about home energy efficiency audits that they organise. Then there was an interesting talk from a local climate change researcher, Roger Fuller , that focussed on the potential impact of climate change on surrounding Skagit County. It is somewhat unique because much the water flowing through the county comes from glacial snow melt in the nearby Cascade mountains. Increased temperatures will mean a greater rain/snow ratio, and greater river flow in the winter and less in the spring. This could have significant effects on the frequency of extreme flooding events. I think these local initiatives are particularly important beyond the immediate concrete [but modest] energy savings and reduced CO

For assessing grant applications in Australia, and some other countries, one criteria is "research environment". This means different things to different people. Unfortunately, I too often see both applicants and assessors/referees using this criteria in an unhelpful and/or meaningless way. When does the environment of the proposed research project matter? Here are a few ways, listed in order of decreasing importance. Access to crucial equipment, materials, and infrastructure. For example, if the project involves femtosecond laser spectroscopy, then there is little point if the researchers do not have access to the relevant lasers, probably at their own institution. Similarly, neutron scattering requires relevant beam time on a user facility. Experimental studies of strongly correlated electron materials require access to high quality single crystals of the relevant materials. Large scale computational chemistry requires access to the relevant supercomputing facilities

Previously I have posted about the fascinating challenge of understanding singlet fission [and the inverse process of triplet-triplet annihilation] in large organic molecules.  A key feature to understand is how fission can occur in less than 100 femtoseconds, suggestive of a conical intersection between excited state potential energy surfaces. In Telluride Nandini Ananth gave a nice talk about work described in the paper The Low-Lying Electronic States of Pentacene and Their Roles in Singlet Fission  Tao Zeng,  Roald Hoffmann , and Nandini Ananth Diabatic states provide a natural and powerful approach to understanding what is going on. The authors perform high level quantum chemistry calculations to describe the relevant electronic excited states. They claim that for a pair of pentacene molecules one needs to include at least six diabatic states. Their dominant electronic configuration is shown in the schematic below. We find that only one of the two charge-transfer state