Condensed concepts

Today I am giving a talk to scientists [mostly postdocs and grad students] at the Black Mountain laboratories of CSIRO [Australia's national industrial labs]. Here are the slides. On the personal side there is something "strange" about the location of this talk. It is less than one kilometre from where I grew up and was an undergrad. Back then I never even thought about these issues.

Today I am giving a talk on bad metals at the 2013 Gordon Godfrey Workshop on Strong Electron Correlations and Spins in Sydney. Here is the current version of the  slides  for the talk. One question I keep getting asked is, "Are these dirty systems?" NO! They are very clean. The bad metal arises purely from electron-electron interactions. The main results in the talk are in a recent  PRL , written with Jure Kokalj. The organic charge transfer salts and the relevant Hubbard model are discussed extensively in  a review , written with Ben Powell. However, I stress that this bad metal physics is present in a wide range of strongly correlated electron materials. The organics just provide a nice tuneable system to study. A recent review of the Finite Temperature Lanczos Method is by Peter Prelovsek and Janez Bonca.

Here "sloppy" science is good science! How do effective theories emerge? What is the minimum number of variables and parameters needed to describe some emergent phenomena? Is there a "blind"/"automated" procedure for determining what the relevant variables and parameters are? There is an interesting paper in Science Parameter Space Compression Underlies Emergent Theories and Predictive Models Benjamin B. Machta, Ricky Chachra, Mark K. Transtrum, James P. Sethna These issues are not just relevant in physics, but also in systems biology. The authors state: important predictions largely depend only on a few “stiff” combinations of parameters, followed by a sequence of geometrically less important “sloppy” ones... This recurring characteristic, termed “sloppiness,” naturally arises in models describing collective data (not chosen to probe individual system components) and has implications similar to those of the renormalization group (RG) and conti

I am not talking about commuting operators in quantum mechanics. When considering a job offer, or the relative merits of multiple job offers [a luxury] rarely does one hear discussion of the daily commute associated with the job. Consider the following two options. A. The prestigious institution is in a large city and due to the high cost of housing you will have to commute for greater than an hour. Furthermore, this commute involves driving in heavy traffic or taking and waiting for crowded public transport. B. A less prestigious institution offers you on campus [or near campus] housing so you can walk 5-15 minutes to work each day. The difference is considerable. Option A will waste more than 10 hours of each week and increase your stress and reduce your energy. In light of that you may end up being more productive and successful at B. It is interesting that I realise the options often aren't that simple. Furthermore, you may not have a choice. Also, time is not the

Today I am giving a seminar in the Physics Department at the Indian Institute of Science in Bangalore. Here is the current version of the  slides. The main results in the talk are in a recent  PRL , written with Jure Kokalj. The organic charge transfer salts and the relevant Hubbard model are discussed extensively in  a review , written with Ben Powell.

There is a general view that great nations have great universities. This motivates significant public and private investment [both financial and political] in universities. Unfortunately, these days much of the focus is on universities promoting economic growth. However, I think equally important are the contributions that universities can make to culture, political stability, and positive social change. Aside: Much of this discussion assumes a causality: strong universities produce strong nations. However, I think caution is in order here. Sometimes it may be correlation not causality. For example, wealthy nations use their wealth to build excellent universities. The main purpose of this post is to make two bold claims. For neither claim do I have empirical evidence. But, I think they are worth discussing. First some nomenclature. In every country the quality of institutions decays with ranking. In different countries that decay rate is different. Roughly the rate decreases fr

The point here is a basic one. But, it is important to keep in mind. One might tend to think that in quantum many-body theory the hardest problems are strong coupling ones. Let g denote some dimensionless coupling constant where g=0 corresponds to non-interacting particles. Obviously for large g perturbation theory is most unreliable and progress will be difficult. However, in some problems one can treat 1/g as a perturbative parameter and make progress. But this does require the infinite coupling limit be tractable. Here are a few examples where strong coupling is actually tractable [but certainly non-trivial] The Hubbard model at half filling. For U much larger than t, the ground state is a Mott insulator. There is a charge gap and the low-lying excitations are spin excitations that are described by an antiferromagnetic Heisenberg model. Except for the case of frustration, i.e. on a non-bipartite lattice, the system is well understood. BEC-BCS crossover in ultracold fermionic

Recently, I posted about a laundry detergent I bought in India that features "Vibrating molecules" (TM) and wryly commented that the marketing of some universities is not much better. I saw that this week, again in India. I read that  a former Australian cricket captain,  Adam Gilchrist , [an even bigger celebrity in India than in Australia], was in Bangalore as a "Brand name Ambassador" for a particular Australian university. The university annually offers one   Bradman scholarship to an Indian student for which it pays 50 per cent of the tuition for an undergraduate degree. [Unfortunately, the amount of money spent on the business class airfares associated with the launch of this scholarship probably exceeded the annual value of the scholarship]. Some measure of Gilchrist's integrity is that at the same event sponsored by the university he said he supported the introduction of legalised betting on sports in India . Many in Australia think such betting has

There is a nice review article Low-barrier hydrogen bonds in proteins by M.V. Hosur, R. Chitra, Samarth Hegde, R.R. Choudhury, Amit Das, and R.V. Hosur Most hydrogen bonds in proteins are weak, as characterised by a donor-acceptor distance larger than 2.8 Angstroms, and interaction energies of a few kcal/mol (~0.1 eV~3 k_B T). However, there are some bonds that are much shorter. In particular, Cleland proposed in 1993 that for some enzymes that there are H-bonds that are sufficiently short (R ~ 2.4-2.5 A) that the energy barrier for proton transfer from the donor to acceptor is sufficiently small that it is comparable to the zero-point energy for the donor-H stretch vibration. These are called low-barrier hydrogen bonds. This proposal remains controversial. For example, Ariel Warshel says they have no functional role. The authors perform extensive analysis of crystal structure databases, for both proteins and small molecules, in order to identify the relative abundance of short b

Tomorrow I am giving the  theory colloquium at the Tata Institute for Fundamental Research in Mumbai . My host is Kedar Damle . Here are the slides for " Frustrated Mott insulators: from quantum spin liquids to superconductors ". A related review article was co-authored with Ben Powell.

Previously I have argued that Science is broken and raised the question,  Have journals become redundant and counterproductive? . Reading these earlier posts is recommended to better understand this post. Some of the problems that need to be addressed are: journals are wasting a lot of time and money rubbish and mediocrity is getting published, sometimes in "high impact" journals honorary authorship leads to long author lists and misplaced credit increasing emphasis on "sexy" speculative results metrics are taking priority over rigorous evaluation negative results or confirmatory studies don't get published lack of transparency of the refereeing and editorial process ..... These problems are serious and need to be addressed by the scientific community. It is better we address them before "solutions" be imposed on us by politicians and administrators. It is always much easier to identify problems than to provide constructive and realisti

This follows an earlier post Emergence of dynamical particle-hole asymmetry  concerning the new approach of Sriram Shastry to doped Mott insulators, describing them as an extremely correlated Fermi liquid, and characterised by two self energies. There is a nice preprint Extremely correlated Fermi liquid theory meets Dynamical mean-field theory: Analytical insights into the doping-driven Mott transition Rok Zitko, D. Hansen, Edward Perepelitsky, Jerne Mravlje, Antoine Georges, Sriram Shastry The part ice-hole asymmetry means that electron-like quasi-particles have much longer lifetimes than hole-like quasi-particles. This means that the imaginary part of the (single-particle Dyson) self energy Sigma(omega) is asymmetric about omega=0, the chemical potential. Here are two particularly note-worthy figures from the paper. The first shows how the asymmetry at omega=0 is associated with the quasi-particle band (QPB) at the top of the lower Hubbard band (LHB). This band emerges as th

This week I bought this laundry detergent in India Note how at the bottom it features the TradeMark, " Vibrating molecules ", giving it Power! It is interesting/amusing to read the Wikipedia page for Surf Excel . It is full of similar marketing nonsense. Scientists might mock this, but some university marketing campaigns are comparable.

There is a very thoughtful article in Angewandte Chemie The Seven Sins in Academic Behavior in the Natural Sciences by Wilfred F. van Gunsteren It is worth reading slowly and in full. He highlights the negative influence of "high impact" journals and discusses many of the same issues as the recent cover story in the Economist. He has some nice examples of each of seven sins. But, there was one paragraph that really stood out. Administrative officials at universities and other academic institutions should refrain from issuing detailed regulations that may stifle the creativity and adventurism on which research depends. They should rather foster discussion about basic principles and appropriate behavior, and judge their staff and applicants for jobs based on their curiosity-driven urge to do research, understand, and share their knowledge rather than on superficial aspects of academic research such as counting papers or citations or considering a person’s grant income

Here are a couple of things I find surprising about the electronic transport properties of materials. 1. One cannot simply have materials, particularly metals, that have any value imaginable for a transport coefficient. For example, one cannot make the conductance or the thermopower as large as one wishes by designing some fantastic material. 2. Quantum mechanics determines what these fundamental limits are. Furthermore, the limiting values of transport coefficients are often set in terms of fundamental constants [Planck's constant, Boltzmann's constant, charge on an electron]. The fact that this is profound is indicated by the fact that this was not appreciated until about 25 years ago. A nice clean example is the case of a  quantum point contact with N channels. The conductance must be N times the quantum of conductance, 2e^2/h. This result was proposed by Rolf Landauer  in 1957 but many people did not believe it until the first experimental confirmation in 1988. Th