Condensed concepts

Today I gave a lecture to a Solid State Physics class on " Magnetic quantum oscillations and mapping out the Fermi surface ." I basically follow chapter 14 of Ashcroft and Mermin. The central result is Onsager 's 1952 equation that the period of the magnetic oscillations is related to extremal areas of the Fermi surface perpendicular to the magnetic field. [Aside: This is an amazing result because it only involves fundamental constants and so the interpretation of the experiments is not "theory laden", a rare thing in condensed matter]. There is one point I struggle to explain: why extremal areas? Ashcroft and Mermin have a figure to justify this. It is lost on me, no matter how many times I read it and stare at the pictures. Does anyone know a clear and convincing way to demonstrate this? The only way I know how to get this result of extremal areas is to do a very fancy calculation (Lifshitz-Kosevich) which evaluates the magnetisation (or thermodyn

I write posts with titles such as "The Dirac cone in graphene is emergent " and "Holes are emergent quasi-particles?" What do I mean? To me emergent properties and phenomena have the following distinguishing characteristics. 1. They are collective phenomena resulting from the interactions between the constituent particles of the system and occur at different length/energy/time scales. For example, superconductivity results from interactions between the electrons and ions in a solid and involves energy (temperature) scales much less than the underlying interaction energies. 2. They are qualitatively different from the properties of the constituent particles. For example, individual gold atoms in a metallic crystal are not "shiny". One cannot speak about superfluidity of individual (or small groups of) atoms. 3. The property is difficult ( or almost impossible ) to anticipate or predict from a knowledge of the microscopic constituents and the asso

The Sunday New York Times magazine has a fascinating and disturbing article The Mind of a Con Man about Diederik Stapel , former Dean of Behavioural and Social Sciences, at Tilburg University in the Netherlands. He had a stellar academic career which was based on fabricating experimental data. The article is rather long but worth reading. Here are a few of the extracts I found particularly pertinent: Stapel did not deny that his deceit was driven by ambition. But it was more complicated than that, he told me. He insisted that he loved social psychology but had been frustrated by the messiness of experimental data, which rarely led to clear conclusions . His lifelong obsession with elegance and order, he said, led him to concoct sexy results that journals found attractive.   In his early years of research — when he supposedly collected real experimental data — Stapel wrote papers laying out complicated and messy relationships between multiple variables. He soon realized that jou

Recently I was asked by a university to evaluate an individual for tenure and promotion. The process was fascinating and I found refreshing. The university send me copies of a selection of the individuals papers and a copy of a short CV. I was asked to review the scientific merit of the papers and thus comment on the suitability of the individual for tenure. There was no discussion of grant money received, numbers of Ph.D students graduated, number of publications, citation metrics, university "service", public outreach, journal impact factors, speaking invitations, ..... I found this refreshing, since it was in striking contrast to the values and emphasis of most institutions, which are very concerned with these other criteria, that I consider are secondary.

I learned today of two impressive endorsements of Fritz London as a great theoretical physicist. First, after John Bardeen got his second Nobel Prize he used the money to endow the Fritz London lectures at Duke University. Second, in 2005 Phil Anderson wrote an essay in Nature, Thinking big which lauded London for having the vision that quantum theory was correct on all length scales, including the macroscopic, as manifested in superconductivity and superfluidity. Furthermore, Anderson argues this allows a "common sense" understanding of the quantum measurement problem. London's vision is contrasted with that of Bohr and Einstein, of whom the "thoughtful curmudgeon" says In reading about these [Einstein-Bohr] debates I have the sensation of being a small boy who spots not one, but two undressed emperors. Niels Bohr’s ‘complementarity principle’ — that there are two incompatible but equally correct ways of looking at things — was merely a way of using

When I first taught Solid State Physics [following Ashcroft and Mermin] I would introduce holes as the absence of an electron. I then discuss the effective mass of "electrons" and holes near the bottom and top of bands, respectively. I would not introduce the concept of a quasi-particle until several weeks later when I discussed electron-electron interactions and Landau's Fermi liquid theory. Now I do it differently. I explain how holes are an example of a quasi-particle with a positive charge and an effective mass [which can be significantly larger or smaller than the free electron mass]. This is a nice "simple" example of emergence . When you put interacting particles ["non-interacting" electrons interacting with a nuclei in a periodic lattice] together new entities emerge which has properties that are qualitatively different from the constituent particles. [Aside: it is interesting that the only "interactions" between the electrons

In 2007 an advisory committee for the USA Department of Energy published a report Directing matter and energy: 5 challenges for science and the imagination . They decided a "grand challenge" must be scientifically deep and demanding be clear and well defined be relevant to the broad portfolio of basic energy sciences, promise real dividends in devices or methods that can significantly improve the quality of life and help provide a secure energy future for the US. Here are their five grand challenges: 1. Control material processes at the level of electrons 2. Design and perfect atom- and energy-efficient syntheses of new forms of matter with tailored properties. 3. Understand and control the remarkable properties of matter that emerge from complex correlations of atomic and electronic constituents. 4. Master energy and information on the nanoscale to create new technologies with capabilities rivaling those of living things 5. Characterize and control m

Otherwise it may lead to economic ruin, or at least bad government policy! Well not for scientists. Earlier I posted that  MIstakes happen  in research, describing one of my recent ones. Paul Krugman has a nice New York Times op-ed piece The Excel Depression  which describes a flawed paper by two Harvard economists that led to a widely quoted claim that once government debt exceeds 90 per cent of Gross Domestic Product (GDP) economic growth declines sharply. One of the mistakes the authors made was an Excel spread sheet coding error that meant they left five countries (including Australia!) out of their analysis. You can read a more detailed discussion here.

I find it mildly irritating when speakers at conferences or seminar series say something like, "I am not going to explain all this background material or my earlier results because last year I talked about that." There are several reasons I don't like this. 1. It assumes that everyone present was there last time. 2. It assumes that those present on the last occasion understood and remember the relevant material! [To me that is fantasy.] 3. It seems to be shutting off certain topics for questions and discussion. Obviously, one can't cover everything and must leave out material. Everyone knows and accepts that. But, I think it is better not to deal with the problem in this way.

Until a few years ago the shear viscosity of a metal was not a topic of interest. However, that has changed, largely stimulated by some calculations based on string theory techniques! The history is described here. In particular, these calculations suggest that for a quantum critical metal the ratio of the shear viscosity to the entropy density has a universal lower bound, hbar/(4 pi k_B). Calculations for graphene suggest the ratio is close to the lower bound leading it to be dubbed "a nearly perfect fluid". Recent experiments on fermionic cold atoms find the ratio is several times the universal minimum. The quark-gluon plasma is close to the minimum. Somehow this "minimum viscosity" (which in simple kinetic theory scales with the relaxation time) is related to a minimum conductivity, and thus the somewhat elusive and poorly defined Mott-Ioffe-Regel limit, which bad metals comfortably violate. The exact relationship between viscosity [a hydrodynamic conce

Why do so many physicists end up working on Wall Street? Is it an accident that one of the most successful hedge funds ever, was founded by James Simons, known to many of us from Chern-Simons theory? What unique insights might physicists bring to modelling financial markets? Lots of fields use mathematical models to understand the world. But physicists have a particular way of thinking about approximation and idealization. To make progress on interesting problems, physicists always have to make assumptions and approximations. They work with the zero temperature limit, or the thermodynamic limit, or the mean field approximation.  They are trained to think these approximations through–to justify their assumptions with physical arguments. Most importantly, physicists are taught how to think about what happens when their assumptions fail. They are taught to calculate, or at least estimate, the second order corrections to their first order equations.  This is from a fascinating article

The nature of the gapless excitations in a quantum many-body system is an emergent property, including underlying order. A simple "classical" example is that the sound waves in crystals result from the breaking of continuous translational symmetry. More profound examples are the Goldstone modes associated with spontaneously broken symmetry and the edge states associated with topological order in fractional quantum Hall states.  Furthermore, the presence and gapless character of these excitations are particularly robust against perturbations and variations in microscopic details . This property is dubbed by Laughlin and Pines, a quantum protectorate. A key property of graphene is the Dirac cone that describes the elementary electronic excitations. This can be "derived" from considering the band structure of a tight-binding model on the honeycomb lattice [a hexagonal lattice with two identical atoms per unit cell]. Hence, one might think that the Dirac cone is "

My family has been enjoying watching the comedy TV series 3rd Rock from the sun  from the 1990s. We thank Jure Kokalj for introducing us to it. The main character Dick Solomon is a physics professor so occasionally there is some physics. Unfortunately, some of the "physics" is just mumbo-jumbo. This is unlike The Big Bang Theory which has accurate physics. Nevertheless, this clip has some pretty classic physics lines.

I often contrast elemental metals to strongly correlated electron materials such as cuprates, organic charge transfer salts, and heavy fermion compounds. However, this is not strictly correct. Some of the lanthanide and actinide elements are strongly correlated metals. This is most clearly demonstrated in the case of cerium and plutonium which undergo isostructural phase transitions involving large volume increases. This is particularly nicely illustrated in the figure below, taken from a beautiful 2001 Nature news and views by Bob Albers, An expanding view of plutonium.  Electronic structure methods based on Density Functional Theory (DFT) completely fail here. A nice explanation of the figure was given by Savrasov, Kotliar, and Abrahams , in terms of strong electronic correlations that can be captured by Dynamical Mean-Field Theory (DMFT). In particular the expansion from the alpha to the delta phase is associated with a delocalised-localised transition of the f electrons. The l

Last friday I attended a very nice physics colloquium by  Mandyam Srinivasan on "Honeybees as a Model for the Study of Visually Guided Flight, Navigation, and Biologically Inspired Robotics" Here are two fascinating things that really stood out to me. The picture below [taken from this review ] gives a schematic of the experiment showing that image flow is the key property that bees balance to navigate obstacles. Building on this led to insights as to how bees make smooth landings. Analysis of the landing trajectories revealed that the flight speed of the bee decreases steadily as she approaches the surface. In fact, both the forward speed and the descent speed are approximately proportional to the height above the surface ( Fig. 8 ), indicating that the bee is holding the angular velocity of the image of the surface approximately constant as the surface is approached . This strategy automatically ensures that both the forward and the descent speeds are close to zero

The last post was about spam. Unfortunately, this one is too. It turns out there is a dark side to open access journals. This is covered in a fascinating and disturbing New York Times article Scientific articles accepted (Personal checks too) . Think twice before you receive an invitation to submit an article, speak at a conference, or serve on an editorial board, from an organisation you have not had prior dealings with. I get a lot of this spam and it almost all automatically goes to my Junk mail folder. But, I did not realise just how bad the problem is, particularly for those who are duped.

Eric Bittner suggested I post about the following problem: Pretty much every day I receive email solicitations from potential post-doc applicants (I presume you do as well).  Most of these look as if they are on some sort of fishing expedition since it's clear from their description of their research interests and expertise, they have not even bothered looking at any of my papers, read my web-site, or have any idea of what I might be working on.  As rude as it seems, my response is to give them the same amount of time they spent looking into my research before hitting the delete button on my email browser.   I'm thinking of making a link on my  group page for potential post-doc applicants with a list of things--which would include a brief research outline in addition to their CV--that postdoc and grad students should send to potential advisors. Does anyone who reads your forum have a better suggestion? What's the best (and polite) way to filter robo-postdocs?  I probabl

After drowning in paperwork I asked Do grant applications ever get shorter? I was interested to learn that several researchers from down the river at QUT recently got a letter published in Nature,  Funding: Australia's grant system wastes time . They focussed on the National Health and Medical Research Council claiming: Australian scientists spent around 550 years' worth of research time writing applications for project grants in 2012, many of which were more than 100 pages long.   Yet 400 years' worth of that valuable research time was wasted because only around 20 per cent of applications were successful.   Applicants spent an average of 34 days writing preparing their proposals, at a combined estimated salary cost of $66 million. Unfortunately, I doubt this is that different from most countries. I review applications from overseas which look just as bulky, full of frivolous information, and with comparable low success rates.

A straight-forward measurement for any superconductor is how the transition temperature varies with pressure (and thus volume). Calculating this variation is not easy. For example, in a simple BCS superconductor one would have to calculate how the phonon spectrum, electron phonon-coupling constant, and density of states vary with pressure. Presumably this can be done with some electronic structure method such as based on density functional theory (DFT). But, how about in a strongly correlated electron system?  One would have to first find how the parameters in some Hubbard type model varied with pressure. Then one has the extremely tricky issue of calculating Tc as a function of the Hubbard model parameters. It turns out that the volume dependence of Tc in organics is dramatically larger than in cuprates and elemental superconductors. I was recently drawn to the paragraph below from this paper by a football team (11 authors!) Comparative thermal-expansion study of β″-(ET)2SF5CH2C

Yesterday I was looking up the phone number of a local business in the White Pages for Brisbane. I leafed past Q and was encouraged to see how many local businesses involved quantum technologies. Names I saw included Quantum quartz Quantum mechanical Quantum chemicals Quantum trading distribution services Quantum racing industries Quantum reading learning vision.... Sorry I did not post this on the first day of the month....

I have quite a few conversations with students and postdocs who are floundering because of (what they perceive as) inadequate supervision and feedback. What should they do? What should I tell them? First, it is hard for me to discern how much of the problem lies with with the supervisee and how much with the supervisor. This is particularly so when I don't really know the parties involved or the particular research area. I am also reluctant to get involved in problems possibly created by other faculty colleagues. It is hard to know how to move forward. But, here are few basic things that I sometimes tell people and I think may help. 1. Take responsibility and take action. The clock is ticking. Don't wait for someone else to do something. 2. Write. Putting things down on paper can help. This is whether it is the details of a stalled calculation, a draft of a paper or thesis chapter, or a new research idea. This can sometimes clarify your thinking to the point that mor