Condensed concepts

This is the first week of classes for the beginning of the academic year. In preparation for a busy semester, I took last week off work (my last four posts were automated) and visited my son in Canberra (where I grew up) and spent some time hiking in  one of my favourite places , Kosciusko National Park. One photo is below. This reminded me of the importance of vacations and  down time , of the  therapeutic value of the nature drug , and of  turning off your email occasionally. Above Lake Albina on the main range.

In the condensed matter theory group at UQ we regularly run reading groups, where we work through a book, review article, or some lecture notes. This is particularly important as our PhD students don't take any courses. Currently we are working through some nice lecture notes on the Quantum Hall effect , written by David Tong.  They are very accessible and clear, particularly in putting the QHE in the context of topology, edge states, Berry's phase, Chern insulators, TKNN, ... On his website he also has lectures on a wide range of topics from kinetic theory to string theory.

Since my previous post about the search for a Weyl semimetal in pyrochlore iridates (such as R2Ir2O7, where R=rare earth) read two more interesting papers on the subject. Metal-Insulator Transition and Topological Properties of Pyrochlore Iridates  Hongbin Zhang, Kristjan Haule, and David Vanderbilt Using a careful DMFT+DFT study they are able to reproduce experimental trends across the series, R=Y, Eu, Sm, Nd, Pr, Bi. They show that when the self energy due to interactions is included that the band structure is topologically trivial, contrary to the 2010 proposal based on DFT+U. They also find that the quasi-particle weight is quite small (about 0.1 for R=Sm, Nd and 0.2 for Pr). This goes some way towards explaining the fact that the infrared conductivity gives an extremely small Drude weight (about 0.05 electrons per unit cell), a puzzle I highlighted in my first post. Field-induced quantum metal–insulator transition in the pyrochlore iridate Nd2Ir2O7  Zhaoming Tian, Yos

My department has just advertised a faculty position.  I will be interested to see how many applicants want to escape Trumpland for sunny Queensland [which BTW has excellent gun control and national health care...].

Two key ideas concerning unconventional superconductors are the following. 1. s-wave and p-wave pairing (in momentum space) are associated with spin singlet and spin triplet pairing, respectively. This can be shown with minimal assumptions (no spin-orbit coupling and spatial inversion symmetry). 2. If superconductivity is seen in proximity to an ordered phase (e.g. ferromagnetism or antiferromagnetism) with a quantum critical point (QCP) then the pairing can be "mediated" by low energy fluctuations (e.g. magnons) associated with the ordering. 3. Non-fermi liquid behaviour may be seen in the quantum critical region about the QCP. However, an interesting paper shows that neither of the above is necessarily true. Superconductivity from Emerging Magnetic Moments  Shintaro Hoshino and Philipp Werner They find spin triplet superconductivity with s-wave symmetry. This arises because there is more than one orbital per site and due to the Hund's rule coupling spin tri

A while back I was in a discussion about "What is the culture of the university? What would a sociologist or anthropologist say?" I thought about this quite a while and came to the conclusion that most universities (particularly research universities in the Western world) do not have a single culture, but rather four distinct subcultures. First, let me make an observation about modern cosmopolitan cities: New York, Brisbane, Bangalore, Paris, London, ... Within each city, there can co-exist several distinct social groups and subcultures, e.g. African-American, Jewish, homeless, business elite, Muslim, WASPs, Hispanic, ... Culture is not just about what kind of restaurants they eat at. It concerns values. Although they may occupy the same physical space (and to a certain extent the same political and economic space), the values of these communities are often distinctly different . If you don't think this I suggest you talk to someone from one community who has marri

In the metallic state of many strongly correlated electron materials, Fermi liquid properties are only observed at relatively low temperatures, at a scale (the coherence temperature T_coh) that can be orders of magnitude less than the Fermi temperature that is estimated from the relevant electronic band structure. Above T_coh one observes a "bad metal" and the absence of quasi-particles. These features are nicely captured by Dynamical Mean-Field Theory (DMFT). An interesting question is whether this low-temperature scale can be captured in simpler theories. Alejandro Mezio and I just finished a paper Low quasi-particle coherence temperature in the one band Hubbard model: a slave-boson approach The phase diagram at half filling is shown below. Note how near the Mott insulator T_coh is orders of magnitude smaller than W/2, the scale of the Fermi temperature for U=0. It is also much smaller than this scale multiplied by Z, the band renormalisation due to intera

Dynamical Mean-Field Theory (DMFT) has given many insights into the Mott metal-insulator transition in strongly correlated electron materials. In the metallic phase, DMFT nicely describes the interplay between the quasi-particles associated with Fermi liquid behaviour and the Hubbard bands that also exist in the insulating phase. DMFT gives a first-order phase transition and captures the emergence of bad metallic behaviour and the associated transfer of spectral weight. On the down side DMFT is computationally expensive, particularly close to the Mott transition, as it requires solution of a self-consistent Anderson impurity problem. [If Quantum Monte Carlo is used one also has to do a tricky imaginary time continuation]. When married with atomistic electronic structure calculations (such as based on (Density Functional Theory) DFT-based approaches) DMFT becomes even more expensive. Sometimes I also feel DMFT can be a bit of a "black box." Slave boson mean-field theory (S

I think it is worth noting that there are many distinct goals for public outreach activities concerning science. These include the following: Show that science is fun, cool, and beautiful. Teach about science, both with regard to how it is done and what we know from it. Recruit students to study science, possibly at a particular institution. Lobby for increased funding for science. Enhance the public visibility of a specific institution (lab, university). Defend scientific knowledge as reliable.  This is particularly true of areas which have become politicised (partisan) such as climate change, childhood vaccinations, and evolutionary biology, and for which there are significant enterprises promoting "denial", "skepticism", or "alternative" views. First, given these distinct goals, I think one needs to design activities that are tailored to a specific goal . Previously, I have discussed the problem of doing demonstrations for school kids tha

A referee for a recent paper, entitled "Effect of hydrogen bonding on infrared absorption intensity" , suggested that we should add "theory" to the title since the chosen title could equally be about an experimental paper. In the end, we declined but did make the abstract clearer that the paper was purely theoretical. I thought this is an interesting issue, that I had not thought about explicitly before. If you look at titles of papers it is true that it is sometimes not clear whether the paper is theoretical, experimental, or joint theory and experiment. This is particularly true with theory papers with titles such as "Property X of material ABC" or experimental papers with titles such as "Strong electron correlations in materials class Y". To experts who working are on the same topic or who know the authors it may be obvious. But to others, it may not be so obvious. Does it matter? Surely if the abstract makes it clear then it is o.k.? [A