Condensed concepts

According to blogspot here are the six most popular posts from this blog over the past year Effective "Hamiltonians" for the stock market Thirty years ago in Princeton Relating non-Fermi liquid transport properties to thermodynamics  Mental health talk in Canberra What simple plotting software would you recommend? A political metaphor for the correlated electron community Thanks to my readers, particularly to those who write comments. Best wishes for the New Year!

Previously I posted about a laundry detergent that had "Vibrating molecules" [TradeMark]. Here is a product my son recently bought that benefits from "Invisible science" [TradeMark]. I welcome guesses as to what the product.

The American Physical Society has prepared a draft statement calling on all universities "to provide all physics and astronomy majors with significant research experiences". The statement is worth reading because of the claims and documentations about some of the benefits of such experiences. In particularly, such experiences can better prepare students for a broad range of career options. I agree. However, I add some caveats. I think there are two dangers that one should not ignore. First, departments need to be diligent that students are not just used as "cheap labour" for some faculty research. Earlier I posted about What makes a good undergraduate research project? , which attracted several particularly insightful comments. Second, such undergraduate research experiences are not a substitute for an advanced undergraduate laboratory.  APS News recently ran a passionate article, Is there a future for the Advanced Lab?  by Jonathan Reichert. It is very tempt

Just because you read something in a chemistry textbook does not mean you should believe it. Basic [pun!] questions about what happens to H+ ions in acids remain outstanding. Is the relevant unit H3O+, H5O2+ [Zundel cation], H9O4+ [Eigen cation], or something else? There is a fascinating Accounts of Chemical Research Myths about the Proton. The Nature of H+ in Condensed Media Christopher A. Reed Here are a few highlights. H+ must be solvated and is nearly always di-ordinate, i.e. "bonded" to two units. H3O+ is a rarely seen. "In contrast to the typical asymmetric H-bond found in proteins (N–H···O) or ice (O–H···O), the short, strong, low-barrier (SSLB) H-bonds found in proton disolvates, such as H(OEt2)2+ and H5O2+, deserve much wider recognition.'' This is particularly interesting because quantum nuclear effects are important in these SSLBs. A poorly understood feature of the IR spectra of proton disolvates in condensed phases is that IR bands assoc

A good principle in science is "extra-ordinary claims require extra-ordinary evidence", i.e. the more exotic and unexpected the claimed new phenomena the greater the evidence needs to be before it should be taken seriously. A classic case is the recent CERN experiment claiming to show that neutrinos could travel faster than the speed of light. Surely it wasn't too surprising when it was found that the problem was one of detector calibration. Nevertheless, that did not stop many theorists from writing papers on the subject. Another case, are claims of "quantum biology". About a decade ago some people tried to get me interested in some anomalous experimental results concerning elastic scattering of neutrons off condensed phases of matter. They claimed to have evidence for quantum entanglement between protons on different molecules for very short time scales and [in later papers] to detect the effects of decoherence on this entanglement. An example is this PRL  w

No. Someone can write a brilliant paper and yet poorly reference previous work. On the other hand, one can write a mediocre or wrong paper and reference previous work in a meticulous manner. But, I have to confess I find I sometimes do judge a paper by the quality of the referencing. I find there is often a correlation between the quality of the referencing and the quality of the science. Perhaps this correlation should not be surprising since both reflect on how meticulous is the scholarship of the authors. If I am sent a paper to referee I often find the following happens. I desperately search the abstract and the figures to find something new and interesting. If I don't I find that sub-consciously I start to scan the references. This sometimes tells me a lot. Here are some of the warning signs I have noticed over the years. Lack of chronological diversity. Most fields have progressed over many decades. Yet some papers will only reference papers from the last few year

A size-able amount of time and energy has been spent by the "hard condensed matter" community over the past quarter century studying unconventional superconductors. A nice and recent review is by Mike Norman.  In the absence of spin-orbit coupling spin is a good quantum number and the Cooper pairs must either be in a spin singlet or a spin triplet state. Furthermore, in a crystal with inversion symmetry spin singlets (triplets) are associated with even (odd) parity. Actually, pinning down the symmetry of the Cooper pairs from experiment turns out to be extremely tricky. In the cuprates the "smoking gun" experiments that showed they were really d-wave used cleverly constructed Josephson junctions, that allowed one to detect the phase of the order parameter and show that it changed sign as one moved around the Fermi surface. How can one show that the pairing is spin triplet? Perhaps the simplest way is to show that they have an upper critical magnetic field that

Roald Hoffmann and Sason Shaik are two of my favourite theoretical chemists. They have featured in a number of my blog posts. I particularly appreciate their concern with using computations to elucidate chemical concepts. In Angewandte Chemie there is a fascinating article, One Molecule, Two Atoms, Three Views, Four Bonds that is written as a three-way dialogue including Henry Rzepa. The simple (but profound) scientific question they address concerns how to describe the chemical bonding in the molecule C2 [i.e. a diatomic molecule of carbon]. In particular, does it involve a quadruple bond? The answer seems to be yes, based on a full CI [configuration interaction] calculation that is then projected down to a Valence Bond wave function. The dialogue is very engaging and the banter back and forth includes interesting digressions such the role of Rzepa's chemistry blog , learning from undergraduates, the relative merits of molecular orbital theory and valence bond theory, the

This week three excellent articles have been brought my attention that highlight current problems with science and academia. The first two are in the Guardian newspaper. How journals like Nature, Cell and Science are damaging science The incentives offered by top journals distort science, just as big bonuses distort banking Randy Schekman, a winner of the 2013 Nobel prize for medicine. Peter Higgs: I wouldn't be productive enough for today's academic system Physicist doubts work like Higgs boson identification achievable now as academics are expected to 'keep churning out papers' How academia resembles a drug gang  is a blog post by Alexandre Afonso , a lecturer in Political Economy at Kings College London. He takes off from the fascinating chapter in Freakanomics, "Why drug dealers still live with their moms." It is because they all hope they are going to make the big time and eventually become head of the drug gang. Academia has a similar hierarchic

When I recently gave a talk on bad metals in Sydney at the Gordon Godfrey Conference, Andrey Chubukov and Janez Bonca asked some nice questions that stimulated this post. The main question that the talk is trying to address is: what is the origin of the low temperature coherence scale T_coh associated with the crossover from a bad metal to a Fermi liquid? In particular,  T_coh is much less than the Fermi temperature of for non-interacting band structure of the relevant Hubbard model [on an anisotropic triangular lattice at half filling]. Here is the key figure from the talk [and the PRL written with Jure Kokalj]. It shows the temperature dependence of the specific heat for different values of U/t for a triangular lattice t'=t. Below T_coh, the specific heat becomes approximately linear in temperature. For U=6t, which is near the Mott insulator transition, T_coh ~t/20. Thus, we see the emergence of the low energy scale. Note that well into the Mott phase [U=12t] there i

This past year I have been surprised and encouraged that this blog has a wide readership. However, I have also learnt that I don't want it to become too popular. A few months ago, when I was visiting Columbia University I met with Peter Woit. He writes a very popular blog, Not Even Wrong , that has become well known, partly because of his strong criticism of string theory. It is a really nice scientific blog, mostly focusing on elementary particle physics and mathematics. The comments generate some substantial scientific discussion. However, it turns out that the popularity is a real curse.   A crowd will attract a bigger crowd. The comments sections attracts two undesirable audiences. The first are non-scientists who have their own "theory of everything" that they wish to promote. The second "audience" are robots that leave "comments" containing links to dubious commercial websites. Peter has to spend a substantial amount of time each day monitorin

Understanding the unique properties of water remains one of the outstanding challenges in science today. Most discussions and computer simulations of pure water [and its interactions with biomolecules] treat the nuclei as classical. Furthermore, the hydrogen bonds are classified as weak. Increasingly, these simple pictures are being questioned. Water is quantum! There is a nice PNAS paper  Nuclear quantum effects and hydrogen bond fluctuations in water Michele Ceriotti, Jerome Cuny, Michele Parrinello, and David Manolopoulos The authors perform path integral molecular dynamics simulations where the nuclei are treated quantum mechanically, moving on potential energy surfaces that are calculated "on the fly" from density functional theory based methods using the Generalised Gradient Approximation. A key technical advance is using an approximation for the path integrals (PI) based on a mapping to a Generalised Langevin Equation [GLE] [PI+GLE=PIGLET!]. In the figure below

On the arXiv, Andre-Marie Tremblay has posted a nice tutorial review  Strongly correlated superconductivity . It is based on some summer school lectures and will be particularly valueable to students. I think it is particularly clearly and nicely highlights some key concepts.  For example, the figure below highlights a fundamental difference between a Mott-Hubbard insulator and a band insulator [or semiconductor]. There is also two clear messages that should not be missed. A minority of people might disagree. 1. For both the cuprates and large classes of organic charge transfer salts the relevant effective Hamiltonians are "simple" one-band Hubbard models. They can capture the essential details of the phase diagrams, particularly the competition between superconductivity, Mott insulator, and antiferromagnetisim. 2. Cluster Dynamical Mean-Field Theory (CDMFT) captures the essential physics of these Hubbard models. I agree completely. Tremblay does mention some

Sometimes when I am at a conference or in a seminar I find that I have absolutely no idea what the speaker is talking about. It is not just that I don't understand the finer technical details. I struggle to see the context, motivation, and background. The words are just jargon and the pictures are just wiggles and the equations random symbols. What is being measured or what is being calculated? Why? Is there a simple physical picture here? How is this related to other work? A senior experimental colleague I spoke to encouraged me to post this. He thought that his similar befuddlement was because he wasn't a theorist. There are three audiences for this message. 1. Me. I need to work harder at making my talks accessible and clear. 2. Other speakers. You need to work harder at making your talks accessible and clear. 3. Students. If you are also struggling don't assume that you are stupid and don't belong in science. It is probably because the speaker is doing a po