Condensed concepts

There is no doubt that the level of hype in science is increasing. You see it in grant applications, university press releases, introductions and conclusion in papers (especially in luxury journals), talks, ... Hype is also a broader problem in society, including in the business world and politics. Why is hype bad for science? Some will say something like, ``I agree that it is not good, but we have to do it to survive. Anyway, we all know what is really true and so it does not matter..." However, I think there are many problems, particularly for the long term flourishing of science. Waste of time Figuring out that a ``hyped'' result or research field is actually just hype can take significant time. This is particularly true if one actually tries to reproduce a result and discover all the problems. Mis-allocation of resources Researchers, students, and funding agencies flock to hyped fields. However, it can take quite a while and a lot of money for the community

I was sad to hear last week that Professor  Noel Hush died at age 94. Noel [also known as Prof.] was a pioneer in theoretical chemistry and chemical physics. He had a profound influence on both fields, particularly in their development in Australia. Arguably his greatest scientific contribution was in the theory of electron transfer. Depending on where you are from this is called Hush-Marcus theory, Marcus-Hush theory, or Marcus theory. In particular, in 1958 Hush derived one of the most important equations in chemical physics , which can be used for design principles for functional electronic materials.  A key concept here is the notion of diabatic states. I had the privilege of knowing and working with Prof. Hush on and off over the past decade. As I made an adiabatic transition from condensed matter into chemical physics Prof. Hush provided a lot of encouragement, wisdom, perspective, and ideas. He strongly believed that theoretical chemists and condensed matter theorists could

Emergent phenomena occur in social systems. For example, self-organisation, power laws, networks, aggregation/segregation, political polarisation, political revolutions... Can lessons from condensed matter physics help at all in understanding and modeling of social systems? Can analogies from social systems help non-scientists understand some of the basic ideas in condensed matter? In two months I am giving a  seminar in a new UQ multi-disciplinary seminar series, Futures of International Order . In preparation, I am slowly engaging with relevant literature, particularly the work of Scott Page , including his course on Model Thinking at  Coursera . The NetLogo software is helpful for exploring a range of simple models. However, before plunging in here are a few tentative thoughts of ideas that might connect with condensed matter, in the vein of reviews such as Physics and financial economics (1776–2014): puzzles, Ising and agent-based models  Didier Sornette Statistical physic

Even though I have not posted about it for a while, mental health continues to be on my radar. I monitor my own mental health carefully and generally things are going well. Tragically, I still meet many in academia struggling with the issue. It is also in the news because of the recent death by suicide of  Princeton economist, Alan Krueger.  A few months ago, Stanford theoretical physicist, Shoucheng Zhang , also died by suicide. The Chronicle of Higher Education has an article about how Krueger's death is prompting conversations about how the culture of academia can be unconducive to mental health. Last week there was an excellent New York Times Opinion piece by Lisa Pryor Mental Illness Isn’t All in Your Head  A “formulation” gathers the biological, psychological and social factors that lead to a mental illness — and offers clues to the way out of suffering.

Alejandro Mezio and I just posted a preprint Orbital-selective bad metals due to Hund’s rule and orbital anisotropy: a finite-temperature slave-spin treatment of the two-band Hubbard model The central result is shown in the Figure below. It shows the phase diagram of the metallic phase as a function of temperature and the Hund's rule interaction J in a system with two bands of differing bandwidth. Uc1 ~ W1 is the critical interaction for a Mott insulator in a one band system with bandwidth W1. The system is a Hund's metal in that the strong correlations arise from J and not from proximity to a Mott insulating phase (note that U=0.5Uc1). In the orbital-selective bad metal , one of the bands is a coherent Fermi liquid (with well-defined Fermi surface) and the second (narrower) band is a bad metal. Two things that I find particularly interesting are the following. Stability of the bad metal and the orbital-selective bad metal are enhanced by increasing J and/or by inc

Over the past two decades, a powerful new technique has been developed to determine quasi-particle properties in strongly correlated electron systems, based on STM (scanning tunneling microscope) measurements. Quasi-particle interference (QPI) has proved to be particularly useful for studying cuprates (e.g. in revealing the d-wave pairing) and now for iron-based superconductors. The basic physics is as follows. One measures the changes in the local tunneling density of states N(r,E), associated with a single impurity that scatters quasi-particles with a change in momentum q. Then the Fourier transform of this change is The text above is taken from a nice paper I maging orbital-selective quasiparticles in the Hund’s metal state of FeSe  A. Kostin, P.O. Sprau, A. Kreisel, Yi Xue Chong, A.E. Böhmer, P.C. Canfield, P.J. Hirschfeld, B.M. Andersen and J.C. Séamus Davis They show theoretically that the intensity of the interference pattern is quite sensitive to the quasi-particle we

Last year Ben Powell wrote a Perspective for Science , The Expanding Materials Multiverse . It begins with a nice statement about why quantum condensed matter is so interesting, exciting, and challenging. High-energy physicists are limited to studying a single vacuum and its  excitations, the particles of the standard model. For condensed-matter physicists, every new phase of matter brings a new “‘vacuum.” Remarkably, the low-energy excitations of these new vacua can be very different from the individual electrons, protons, and neutrons that constitute the material. The materials multiverse contains universes where the particle-like excitations carry only a fraction of the elementary electronic charge, are magnetic monopoles, or are their own antiparticles. None of these properties have ever been observed in the particles found in free space. Often, emergent gauge fields accompany these “fractionalized” particles, just as electromagnetic gauge fields accompany charged particles. On

I am slowly working towards writing a Condensed Matter Physics: Very Short Introduction. But first I am trying to clarify my audience and goals. Some earlier posts have helped me clarify this. My intended audience is probably not you! Rather it is a person who wants to get the flavour of what CMP is actually about.  Examples might include a smart final year high school who wants to study science at university, or a first-year chemistry undergraduate, or an economics graduate, or a sociology professor, ... My goal is to show that CMP is intellectually exciting, intellectually challenging, and intellectually important. The VSI format is 8-10 chapters and 30-35 thousand words. It is meant to be written in the style of an engaging essay not a technical paper. My plan is to basically have one clear and specific idea that I want to communicate in each chapter. I am thinking that in order to increase interest and comprehension that for each chapter I will aim to include. An easily

What are some of the most important concepts in condensed matter physics? In a recent comment on this blog Gautam Menon suggested that one of them is that of generalised rigidity, i.e. the elasticity of order parameters associated with broken symmetry phases.  A while ago I wrote a post trying to introduce Phil Anderson's discussion of the concept. Thinking about this made me appreciate just how important and useful the concept is. Basically, generalised rigidity quantifies how the free energy of a system varies when introduces spatial variations in the order parameter. These variations can result from boundary conditions, fluctuations, or topological defects. Depending on the type of broken symmetry there are just a few parameters, maybe only one, involved in defining the rigidity. One is looking at "linear" response and so symmetry determines how many different terms one can write down that are second order in a gradient operator. A concrete example is the F