Condensed concepts

Tomorrow I am giving a talk on the absence of quantum limits to the shear viscosity at the Telluride workshop on Condensed Phase Dynamics . Here are the slides . The main results are in this paper.

This week I am in Telluride at the bi-annual workshop on Condensed Phase Dynamics.  I really enjoyed the talks today. A common topic was that of non-equilibrium thermodynamics, particularly in nanoscale systems. Abe Nitzan began his talk mentioning a recent PRL, Quantum Thermodynamics: A Nonequilibrium Green’s Function Approach , which unfortunately, is not valid because the expressions it gives do not give the correct result in the equilibrium limit. This is shown in Quantum thermodynamics of the driven resonant level model   Anton Bruch, Mark Thomas, Silvia Viola Kusminskiy, Felix von Oppen, and Abraham Nitzan What is striking to me about both papers is that they consider a non-interacting model, i.e. the Hamiltonian is quadratic in fermion operators and exactly soluble. This shows just how far we are from any sort of theory of a realistic system, i.e. one with interactions and which is not integrable. Phil Geissler  gave a nice introduction to different theorems for fluct

This past semester I taught part of two undergraduate courses: thermodynamics for second year, and solid state physics for fourth years. I was particularly struck by two related things. 1. Taylor expansions kept coming up in many contexts: approximate forms for the Gibbs free energy (e.g. G vs. pressure is approximately a straight line with slope equal to the volume), Ginzburg-Landau theory, Sommerfeld expansion, linear response theory, perturbation theory, and solving many specific problems (e.g. where one dimensionless parameter is very small). 2. Many students really struggled with the idea and/or its application. They have all done mathematics courses where they have covered the topic but understanding and using it in a physics course eludes them. Physics is all about approximations , both in model building and in applying specific theories to specific problems. Taylor expansions is one of the most useful and powerful methods for doing this. But, it is not just about a mathem

Words, labels, and definitions mean something. They can colour a debate or idea from the start. A while back I changed one post label for this blog from "Developing world" to "Majority world" because I think the latter is more accurate and makes a statement. I also recently considered changing "career advice" to "job advice". Why might it matter? What is the difference? Why care? I am increasingly concerned by the notion of an "academic career". First, most people who aspire to an "academic career" actually don't get to have one. University marketing departments, funding agencies, and politicians don't want to face this painful reality. Furthermore, the young, idealistic and uncritical either don't realise this or don't want to believe it. Increasingly, positions in academia, whether Ph.D, postdoc, mid-career fellowships, or temporary faculty, are terminal. They don't lead to another position i

Bill Parson kindly gave me a copy of the new edition of his book, Modern Optical SpectroscopyWith Exercises and Examples from Biophysics and Biochemistry It is a excellent book that covers a range of topics that are of increasing importance and interest to a range of people. I am not sure I am aware of any other books with similar scope. Two particular audiences will benefit from engaging with the material. 1. Biochemists and biophysics who have a weak background in quantum theory and need to understand how it underpins many spectroscopic tools that are now widely used to describe and understand biomolecules. 2. Quantum physicists who are interested in the relevance (and irrelevance!) of quantum theory to biomolecular systems. For some it could be a reality check of the complexities and subtleties involved and the long and rich history associated with the subject. I highly recommend it. I have learnt a lot from it, some it quite basic stuff I should have known.

I just posted a preprint Effect of hydrogen bonding on infrared absorption intensity  Bijyalaxmi Athokpam, Sai G. Ramesh, Ross H. McKenzie We consider how the infrared intensity of an O-H stretch in a hydrogen bonded complex varies as the strength of the H-bond varies from weak to strong. We obtain trends for the fundamental and overtone transitions as a function of donor-acceptor distance R, which is a common measure of H-bond strength. Our calculations use a simple two-diabatic state model that permits symmetric and asymmetric bonds, i.e. where the proton affinity of the donor and acceptor are equal and unequal, respectively. The dipole moment function uses a Mecke form for the free OH dipole moment, associated with the diabatic states. The transition dipole moment is calculated using one-dimensional vibrational eigenstates associated with the H-atom transfer coordinate on the ground state adiabatic surface of our model. Over 20-fold intensity enhancements for the fundamental

There is an interesting article in the latest issue of Nature Chemistry, Challenges and opportunities for chemistry in Africa , by Berhanu Abegaz , the Executive Director of the African Academy of Sciences. He begins by discussing how metrics don't really give an accurate picture of what is going on, before discussing how natural products chemistry is a significant area of interest. But then he moves to the formidable challenges of science and education in such an under-resourced continent.... It was encouraging to me personally that Abegaz spends several paragraphs summarising a paper I co-authored three years ago with Ross van Vuuren and Malcolm Buchanan.  He has far greater expertise and experience than us and so it was nice to see that our modest contribution was valuable. As an aside, this brought home to me again the issue of the important distinction between token and substantial citations.  Most citations I receive are token and so it is really encouraging to have a

Tomorrow I am giving the weekly Quantum sciences seminar at UQ. Here are my  slides . The title and abstract below are written to try an attract a general audience. I welcome any comments. TITLE: Absence of a quantum limit to the shear viscosity of strongly interacting fermion systems ABSTRACT: Are there fundamental limits to how small the shear viscosity of a macroscopic fluid can be? Could Planck’s constant and the Heisenberg uncertainty principle determine that lower bound? In 2005 mathematical techniques from string theory and black hole physics (!) were used to conjecture a lower bound for the ratio of the shear viscosity to the entropy of all fluids. From both theory and experiment, this bound appears to be respected in ultracold atoms and the quark-gluon plasma. However, we have shown that this bound is strongly violated in the "bad metal" regime that occurs near a Mott insulator, and described by a Hubbard model [1]. I will give a basic introduction to s

Previously I posted about learning how to critically read experimental papers.  A theory paper may claim "We can understand property X of material Y by studying effective Hamiltonian A with approximation B and calculating property C." Again it is as simple as ABC. 1. Effective Hamiltonian A may not be appropriate for material Y. The effective Hamiltonian could be a Hubbard model or something more "ab initio" or a classical force field in molecular dynamics. It could be the model itself of the parameters in the model that are not appropriate. An important question is if you change the parameters or the model slightly how much do the results change. Another question, is what justification is there for using A? Sometimes there are very solid and careful justifications. Other times there is just folklore. 2. Approximation B may be unreliable, at least in the relevant parameter regime. Once one has defined an interesting Hamiltonian calculating a measurable o

John Oliver has an impressive ability to combine humour with cutting political and social commentary, and thorough and substantial background research, in a very engaging manner that is accessible to a broad audience. Recently he tackled the important issue of how the media poorly engages with recent scientific research; but the problem is not just the media but university press offices, some journals, and scientists themselves. If you cannot view the link above (e.g. because you are in Australia) try to watch it here.

A hot area of research is that of functional electronic materials. The goal is to find new materials that can be used in new devices, ranging from solar cells to biosensors to catalysts to transistors. Let me first concede some positive points. Overall this is an important and exciting area of research which involves some interesting science and significant potential technological benefits. There are some excellent people working in this challenging field and doing careful and valuable work. History shows that going from a university lab to mass produced technology can take a long time. Who would have ever thought you could go from the first transistor to computer chips? Or from the first giant magnetoresistance materials to current computer memories? Good science is hard. However, I wonder if I am the only one who is underwhelmed by the average work in this field. In a "typical" experiment someone might do something like the following. They get some large comp

Tomorrow I am giving a summary lecture for the end of an undergraduate course PHYS2020 Thermodynamics and Condensed Matter . I taught the second half of the course, which has featured in some earlier posts. Here are the slides where I attempt to summarise 20 key ideas/results/concepts in the course. My approach to the key ideas in thermodynamics is heavily influenced by Hans Buchdahl , my ANU undergraduate lecturer (and honours thesis supervisor) and his (dense) Twenty Lectures on Thermodynamics. A similar axiomatic macroscopic approach which starts with the second law has more recently been championed by Elliot Lieb and Jacob Yngvason , and described in a nice Physics Today article.