Condensed concepts

On tuesday I had nice discussion with Raghu Mahajan, Maissam Barkeshli, and Sean Hartnoll about their recent preprint  Non-Fermi liquids and the Wiedemann-Franz law . Aside: I generally find that discussing a paper with the authors before/after I have read it greatly increases my understanding. Here are a few things that became clearer to me. In this paper "almost conserved quantities" means quantities for which the relaxation time is very long. Thus in a Fermi liquid the quasi-particles have very long lifetimes and so one can think of the quasi-particle number for every wave-vector near the Fermi surface as being "almost conserved". This means there are many conserved quantities. However, they consider a system in which there is a Drude peak in the frequency dependent conductivity but fermionic quasi-particles are poorly defined due to large scattering. Optimally doped cuprates might be an example of a real material with this property. I thought that one dime

Hopefully only the Fermi liquid, and not the talk content, is marginal! On thursday I am giving a seminar  for the Stanford Institute for Materials and Energy Sciences . Here is the current version of the slides: Overdoped cuprates are anisotropic marginal Fermi liquids. The most recent results in the talk are described in great detail in a  PRB  written with Jure Kokalj and Nigel Hussey.

Previously I posted how a negative classical magnetoresistance with an unusual angular dependence would provide a "smoking gun" for the chiral anomaly. Another one is described Probing the chiral anomaly with nonlocal transport in Weyl semimetals Sid Parameswaran, Tarun Grover, D.A. Abanin, D.A. Pesin, and Ashvin Vishwanath The figure below nicely describes the proposed experimental setup and signature. I thank Sid for describing these results to me.

Not all citations are equal. This highlights how they are an imperfect measure of scientific impact. Broadly they might be classified as being of two types. 1. Token. "There are many papers on this topic, including Jones et al." 2. Substantial. "We use the results [method, equation, material, or concept] of Jones et al. to obtain new results". There is a big difference. For token citations, the existence of the citing paper is really independent of the cited paper, i.e., it would not really matter whether or not the cited paper existed. In contrast, for substantial citations,  parts [or even the whole] of the citing paper would not exist if the cited paper did not exist. Unfortunately, too many of the citations I receive are token rather than substantial. I find this discouraging and embarrassing. Even worse, a few times I have been cited as "McKenzie has shown X to be true" when my paper actually showed X to be false! Thus, it is very satis

Hopefully the talk isn't bad, just the metals! On Friday I am giving a seminar in the Physics Department at Berkeley. Here is the current version of the  slides  for my talk. 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.

Bill Miller has a very nice Perspective: Quantum or Classical coherence?  in the Journal of Chemical Physics. I thank him for explaining some of it to me today. He clearly defines what he considers to be a truly quantum effect in chemical dynamics. It is particularly interesting because by his definition Rabi oscillations are not quantum. They are just like two coupled classical harmonic oscillators. He starts with a Feynman path integral representation of some time-dependent correlation function and considers the semi-classical (SC) limit. The correlation function can be written as an initial value representation (IVR). If one linearises the paths (LSC) one obtains classical Wigner functions and one cannot capture quantum interference effects [e.g. double slit interference which involves paths with more than infinitesimal separation]. Tao and Miller  considered the semi-classical path-integral representation of the spin-boson model. They use the Meyer-Miller-Stock-Thoss repres

It is amazing since a common science project for school children is to make blue crystals of copper sulphate [CuSO4.5H2O]! [Although I was surprised and disappointed when my son just told me he never did it]. Perhaps, one may not have to look so hard for quantum materials. The first X-ray crystallography experiment [by von Laue] was also performed on copper sulphate pentahydrate. It turns out that the Cu2+ ions (spin-1/2) form chains that are very weakly coupled to one another and so are effectively one-dimensional antiferromagnetic Heisenberg chains above the three-dimensional Neel ordering temperature of about 100 mK. [Caveat: strictly speaking half of the Cu2+ ions form chains; the other half are essentially isolated and non-interacting]. Minor caveat: the relevant intrachain exchange interaction J ~ 0.25 meV and so one only sees the spinons for temperatures of order a Kelvin. I first learned all this in the introduction of this Nature Physics paper.

It is not unusual in papers to see graphs in which the vertical scale is given in "arbitrary units".  The most common occurrence of this may be experimental measurements of some spectrum, for example, a graph of the absorbance versus frequency (photon energy) of a solution of a specific molecule. However, some theoretical papers do this too. There are several reasons why authors may do this. Laziness. It can be hard work and confusing to work out the actual physical units for some theoretical calculations. Complexity.  For experiments it can be extremely difficult to normalise and calibrate some detectors. Uncertainty and embarrassment.  Parameters such as detector efficiency, sample thickness, geometric corrections, solution concentration can involve large uncertainties so the horizontal scale may be unknown by as much as an order of magnitude. But I think these uncertainties should be reported because they present a challenge for improvement. I realise that t

Maybe I am reviewing too many CVs with inflated claims. This Dilbert struck a chord.

There is a nice preprint Chiral Anomaly and Classical Negative Magnetoresistance of Weyl Metals by Dam  Son [a string theorist!] and Boris Spivak This clearly shows a distinct experimental signature of the chiral anomaly associated with Weyl metals. [Aside: I think that Weyl should not get his name on this for the same reasons discussed in this post. ] The key physics is summarised in the Figure below. The Dirac cones [another misnomer?] associated with the chiral anomaly must come in pairs. Consider the case where the magnetic field and electric field are parallel [and in the z-direction]. The magnetic field induces an anomalous charge current that destroys charge at one Dirac point and creates it at the other. This leads to an anomalous current that is proportional to the square of the magnetic field strength. Thus, the resistance decreases with increasing magnetic field, i.e. (classical) negative magnetoresistance. This is in distinct contrast to traditional classical

I just learned that Arthur Wightman died earlier this year. He is probably best known for axioms of quantum field theory, super-selection rules , and a famous book, PCT, Spin, Statistics and all that. Arguably, Wightman's greatest legacy is being the advisor and mentor to a selection of Princeton Ph.D students who went on to distinguished careers, mostly in mathematical physics. There are some nice testimonials on the Princeton Physics web site. Reading them it struck me that Wightman would have measured rather poorly on today's common metrics [grant money, numbers of publications, journal impact factors, numbers of Ph.D students, citations]; yet, he had an incredible scientific impact! Wightman was extremely helpful and generous to me when I was a beginning graduate student at Princeton in the mid 1980s. In particular, I had a paper from my undergraduate thesis that I was trying to publish. He gave me great encouragement, some helpful feedback, and arranged for it to be p

When I was an undergraduate I don't think Course Profiles [detailed descriptions of course content, assessment, policies, ...] even existed. If I recall correctly there was a Course handbook which contained a paragraph about each course. Sometimes on the first day of class the lecturer might hand out a one page sheet with some more details about the course. Times have certainly changed. Now at University of Queensland [and I presume at most other universities] the course profile can be 15 plus pages. Here is an example from a course I have taught. It contains very detailed descriptions of not just course content, learning methods, and assessment but how these map onto "graduate attributes." Profiles have to include details about university policies about plagiarism, student appeals, disabilities, library resources, ....  Hence, it is not surprising that students often don't read the details, including the ones that really matter (e.g., what topics will be cover

In considering the electronic properties of the metallic phase of solids one almost always assumes spatial homogeneity of the the underlying electron fluid. This is convenient and powerful. But, that does not mean that it is always correct! Occasionally one might entertain the possibility of some sort of charge order and symmetry breaking such as a charge density wave or stripes. Over the past two decades it has been found that the two-dimensional electron gases (2DEGs) that occur at interfaces in semiconductor heterostructures exhibit a metal-insulator transition that has confounded definitive theoretical understanding. Furthermore, there has been considerable debate about the relative importance and interplay of disorder (due to impurities) and strong electronic correlations. A fundamental challenge is to describe the dependence of the resistance on the temperature, density, and magnetic field (parallel to the 2DEG). Furthermore, double layer systems exhibit extremely large and

In a previous post  What is wrong with this textbook?  I made the disturbing observation that a popular introductory economics textbook did not contain any real data. My son [an economics major] and I recently read  Poor Economics: a radical rethinking of the way to fight global poverty  by two MIT econom ists,  by  Abhijit V. Banerjee  and  Esther Duflo . The book is all based on real data. The book has a very impressive website . For each chapter it has the associated data and figures, and interactive tools to work with the data. [n.b. how this transparency contrasts to the two Harvard economists who were reluctant to release the Excel spreadsheet they used in a controversial study about government debt ]. The authors address questions such as Why do the poor remain poor? Are they trapped? What is the most effective way to help them? Why do many well-intentioned aid programs fail? The emphasis is on finding strategies that have actually been proven to work, rather than pr

Today I visited the Linus Pauling Archives  at Oregon State University. [I was actually on vacation in Corvallis visiting my sister-in-law but I just had to take a visit]. They have assembled a lot of fascinating material online , which is worth perusing. For example, how a funding agency convinced him to start working on proteins , and the details of correspondence about his (erroneous) ideas about quasi-crystals. But, in the actual library there is a small display featuring Pauling's last blackboard, his desk, some molecular models, some calculators, his two Nobel Prize medals, and his signature beret. Pauling is definitely one of my scientific heroes. He made multiple landmark contributions. Most of us would be happy to do just one of the things he is known for. He was truly the master of multi-disciplinarity. He brought quantum physics to chemistry, structural chemistry to biology, and molecular biology to medicine . But he had "clay feet", failing to see

Physics World recently ran a special report about Physics in India . The government is investing heavily in science through a range of worthwhile initiatives. But, the challenges are substantial. Here are a few things that stood out to me. The Tata Institute for Fundamental Research (TIFR) admits about 100 graduate students every year. More than 10,000 take the entrance exam which is held in about 20 cities! Although India has hundreds of universities and research institutes it seems that only the IITs and a handful of universities give an undergraduate education that is adequate preparation for a Ph.D. TIFR is opening a new campus in Hyderabad . This involves hiring 250 faculty members in the next 12 years! This will be larger both in space and faculty numbers than Mumbai. This is in addition to the International Center for Theoretical Sciences being established by TIFR in Bangalore. Students who do decide to pursue science Ph.D's have often resisted considerable cultur

Yesterday we had another cake meeting where people "shared their ignorance".  Mine concerned: what is the relationship between the chiral (parity) anomaly in quantum field theory, edge states, and topological insulators? I learnt something in the week from this PRL  [more string theorists writing about Fermi liquids!], which at least taught me what the relevant "anomaly" equation is for the "violation" of charge conservation. Ben Powell pointed out that there is a relevant blog post Monopoles passing through flatland by John Preskill. His beautiful post certainly provides the background information I need, in a particularly lucid form. Having read it once I now need to digest it properly. Otherwise, I will still be "ignorant.".

I recently went and did some science demonstrations to several grade 3 classes at the local primary school [elementary school in the USA]. I mostly did it because a friend from church, who is a teacher at the school, asked my wife if we could do it. The students were studying a unit on heat transfer. Here are a few random observations from the experience. The kids think scientists are like rock stars! You are so cool! I was asked to wear the white coat and wild hair and so obliged. The teachers really appreciate it. I was told I could not do just one or two classes because it would not be fair for some of the teachers and students to miss out! I had to do all! These kids have had an incredible "diet" of computer games and special effects in movies. But, seeing something simple LIVE such as the baking soda rocket or coke can crush really wows them. They are not at all jaded, unlike the kids in this scene from Big Bang Theory ! The kids love it if they can help. Even

Recently I was at a chemistry workshop in the US and a physics one in Korea. It was really encouraging for me to hear from a range of people that they read this blog and find it stimulating. I particularly appreciated that my readers range in seniority and are from groups whose work I respect (and would like to influence). Sometimes I have worried if the 500 hits per day are coming from non-scientists searching for random topics on Google. This must be the case for "Who is following who?" which is actually about orbital hybridisation, but has received 1000+ page views! But, now I know I am reaching my target audience. So, thanks to those who took the initiative to talk to me.