Condensed concepts

A few months ago I attended a symposium at UQ on Energy in India. The talks can be viewed on Youtube . The one by Alexie Seller is particularly inspiring. In the presentation of Chris Greig he showed a slide similar to that below with the title "Electricity affects Human well being". He did not say it, but sometimes graphs like this are used to make claims such as "the more electricity people consume the better off they will be..."  or "the only way to lift people out of poverty is to burn more coal..." Sometimes people show graphs that correlate GDP with energy consumption. But this one is better because it uses the Human Development Index , a multi-dimensional measure of human well being (as it includes life expectancy and education). Two things are very striking about the graph. First, the initial slope is very large. Second, the graph levels off quickly. A little bit of electricity makes a huge difference. If you don't have el

I wish I knew. This is something I continue to struggle with. To think clearly and creatively one needs to find "space" that is free from distractions and stresses. I find it hard to believe that one can be really productive in the midst of noise, chaos, and multiple demands. I can't. I know there are some individuals who are good at multi-tasking and even seem relish all the noise and hyper-activity. But, deep down I wonder if some are just "cranking the handle" and publishing the same paper again and again. I contend that slow science is not just enjoyable but necessary. Yet finding "mental space" is increasingly a problem because of fast pace of "modern" life. This is increased by greater demands for "productivity" and all the background noise from email, social media, and mobile phones. So how does one find the necessary "mental space"? I welcome suggestions. Here are mine. Turn off your email and/o

Kamran Behnia has published a book Fundamentals of Thermoelectricity Such a monograph is overdue. I think the topic is particularly important and interesting for several reasons. (This is illustrated by the fact that I have written almost 40 blog posts on the topic). The thermoelectric power is a transport property that presents a number of rich and outstanding puzzles. The sign, magnitude, spatial anisotropy, and temperature dependence of the thermopower can put significant constraints on theories because the thermopower is so sensitive to particle-hole asymmetry. In comparison, often it may not be too hard to cook up a theory can get a resistivity that agrees with experiment. However, the thermopower is another story. Thermoelectric materials are technologically important. Furthermore, if someone can find a material with a "Figure of merit" that is just twice that of the best current materials we could throw out all our refrigerators with moving parts! The book has

Some readers might be surprised to hear me claim this since I often highlight the problems and errors associated with calculations involving DFT. The problem is density functional approximations not the underlying theory. There are two key ideas associated with DFT. 1. A theorem. The ground state energy of an interacting electron gas is the minimum value of a unique functional of the charge density n(r) in the system. This is an exact result. The problem is that to determine the exact density and energy one needs to know the "exchange-correlation" functional. 2. An approximation. One can make a local density approximation (LDA) to the exchange-correlation functional so that the density is written in terms of a set of "orbitals" that are found by solving a set of self-consistent equations that have a mathematical structure similar to the Hartree-Fock equations for the same system. These distinct ideas are respectively associated with two different paper

A colleague once told me a story about his research field. "Ten years ago Professor X got some surprising experimental results. He then made bold claims about what this meant. Some people did not believe it. But, people then did detailed experimental and theoretical work to test his results and claims. They basically found that he was wrong but in the process they made some valuable and interesting discoveries and clarified several issues in the field. To half the people in the field he was a hero and to the other half he was a pariah ." The hero status was assigned because if he did not exist or had not made these claims, the new discoveries would not have been made (or might have been made much later). The pariah status was assigned because he did not do careful scientific work and misled people. How much credit should people get who open up new scientific directions  with “wrong” papers or with unsubstantiated speculation? Different people I talk to have quite dif

In quantum many-body physics quasi-particles are emergent entities. But, it is worth making a distinction between two cases. 1. Adiabatic continuity. As one gradually turns on the interactions the excited states of the system smoothly evolve from those in the non-interacting system. As a result the quasi-particles have the same quantum numbers and statistics as the constituent particles. The most prominent example is in Landau's Fermi liquid theory which describes elemental metals and liquid 3He. 2. Adiabatic discontinuity. The  quasi-particles do NOT have the same quantum numbers and statistics as the constituent particles. One example, is magnons (spin waves) in a spin-1/2 Heisenberg antiferromagnet. They have spin one and act like bosons. In contrast, the constituent particles are localised electron that are fermions with spin-1/2. An even more dramatic example occurs in the fractional quantum Hall effect. The constituent particles are electrons with charge -e and obey Fe

Andrew Zangwill contacted me because he is working on scientific biography of Phil Anderson. I think this is overdue. I would argue that Phil is the greatest theoretical physicist of the second half of the twentieth century. I would argue this on similar grounds to why I think Linus Pauling was the greatest theoretical chemist of the first half of the twentieth century. Crucially, their scientific legacies have extended far beyond condensed matter physics and chemistry, respectively. Specifically, Pauling did not just make essential contributions to our understanding of chemical bonding, x-ray crystallography, and quantum chemistry. His impact went far beyond chemistry. Francis Crick said Pauling was the "father of molecular biology." He proposed and elucidated alpha helices and beta sheets in proteins. Furthermore, he began the whole field of molecular medicine, by showing the molecular basis of a specific disease, sickle cell anemia. Phil Anderson has made incredibly d

In introductory science courses we try and instill in undergraduates the basic notion that any measurement has an error and you should estimate that error and report it with your measurement. Yet "Professors" who are in senior management don't do that. Today in Australia the results of the Excellence in Research Australia (ERA) ranking exercise were announced. Every research field at every university is given a score. A colleague wisely pointed out that given the ad hoc procedure involved all the rankings should include error bars. He conjectured that the error bar was about one. Hence, one cannot distinguish the difference between a 4 and 5. Yet, this is a distinction that university managers and marketing departments make a lot off. I think for almost all ranking exercises it would be quite straight forward for the compilers to calculate/estimate the uncertainty in their ranking. This is because almost all rankings are based on the average of rankings or scores pro

And why is it so elusive? Quantum many-body systems are characterised by many different energy scales (e.g. Fermi energy, Debye frequency, superconducting energy gap, Kondo temperature, ....). However, in many systems properties are "universal" in that they are determined by a single energy scale. This means that the frequency (omega) and temperature (T) dependence of a spectral function can be written in a form such as where here  T_ K is the relevant energy scale and I set hbar =1 and k_B = 1. However, what happens in the limit where the relevant energy scale T_K goes to zero, for example near a quantum critical point? Then the only energy scale present is that defined by the temperature T and we now expect a functional dependence of the form This is omega/T scaling. In one dimension the form of the scaling function is specified by conformal field theory and for quantum impurity problems (e.g. Kondo) by boundary conformal field theory. In 1989 Varma et al. sh