Condensed concepts

I feel this is post is a bit like extolling the value of motherhood. But it does need to be said again and again in a range of contexts. Transparency is relevant in science and universities in many different ways Provide enough information in a paper (or its supplementary material) so that others can reproduce your results. Be honest about the strengths and weaknesses of any method. Provide estimates of the uncertainty of any result. Faculty and institutions need to provide Ph.D students and postdocs with realistic information about their future job prospects within academia. [In particular, the prospect of a tenured faculty position at a research university is highly unlikely]. When people are being asked to evaluate something [a job applicant, a grant application, a commercial venture, a new technology, ...] they need to be provided enough information to make a well-informed decision. Make minutes of committee meetings, annual reports,  freely and easily available. Make salar

One of the most fundamental and profound concepts in quantum many-body theory is the emergence of low energy scales  that are much smaller than the energy scales in the "bare" Hamiltonian. For example, in a metallic phase near the Mott transition in a single band system, there is the energy scale associated with a Fermi liquid. Studies using Dynamical Mean-Field Theory (DMFT) have shown how this scale is associated with `` kinks'' in the quasi-particle dispersion relation and is related to the energy scale for spin fluctuations. The problem of the Mott transition in multi-band systems (degenerate orbitals) is fascinating and of renewed interest since the discovery of iron-based superconductors. A basic question concerns how the Mott transition is qualitatively different from in single band systems. More specifically, how does a Hund's rule coupling change things? One new concept is that of an orbital-selective Mott transition. This is where one or more of the

A colleague was recently distressed to find a significant error in a paper he had just published. There was a sign error in one calculation which effects the application of the theory to a particular class of materials. The physics and mathematics are correct but not some of the conclusions of the paper. He and his co-authors have submitted an errata to the paper. I tried to encourage my colleague that although this is disappointing it is just part of being a good scientist. I was reminded of an old post  based on a paper,  The Seven Sins in Academic Behavior in the Natural Sciences by Wilfred F. van Gunsteren. One could even defend the proposition that a scientist with a sizeable publication record in science who has not published a single corrigendum is unlikely to be a good scientist. Either he or she has done such simple work that nothing could go wrong, or he or she has committed the fifth sin in science [neglect of errors found after publication.   This is not an excuse fo

A fundamental principle of molecular biology is that structure determines properties which determine biological function . This is what drives massive scientific industry of protein structure determination. An interesting press report led me to read the following paper. Structural characterization of a novel monotreme-specific protein with antimicrobial activity from the milk of the platypus J. Newman, J. A. Sharp, A. K. Enjapoori, J. Bentley, K. R. Nicholas, T. E. Adams and T. S. Peat The reason this got my attention was that my late father would have been very interested in the paper. A major research interest of his was milk proteins and he did write several papers on the milk proteins of the echnida and platypus. These fascinating animals are unique to Australia and Papua New Guinea and are the only monotremes  (mammals that lay eggs) on the planet. My father collaborated with a biologist, Mervyn Griffiths, who was a colourful character, and was adept at finding and catchin

Every few years I write a post about problems that email creates. Such a post is here. Over the past few years, I have become aware that smartphones have reduced the effectiveness and efficiency of email. Many people now read email on a smartphone and this increases the likelihood that - they read it even faster and so are less likely to digest anything of substance - they won't open or can't read attachments properly - they are less likely to reply - if they do reply, they are more likely to have a knee-jerk response - it is a less important communication channel that SMS, Messenger, WhatsApp, ... - they are more likely to forward a message that they should think twice about forwarding - they will not take the action that the email requests. If the email message is "Shall we meet at 1pm for lunch?" then none of this matters. However, there are some email messages that consider weightier matters such a detailed discussion of a scientific question, a prop

There is an interesting preprint Viscosity Bound Violation in Viscoelastic Fermi Liquids   Matthew P. Gochan, Hua Li, Kevin S. Bedell They consider the unitary Fermi gas within the framework of Fermi liquid theory. This system undergoes a superfluid transition at a temperature of about 0.17 times T_F (the Fermi temperature). They calculate the shear viscosity as a function of temperature. (I think) the complete temperature dependence is obtained by interpolating between the low-temperature and high-temperature limits. The motivation for the study is the conjectured universal bound for the ratio of the shear viscosity to the entropy density, based on the AdS-CFT conjecture, beloved by string theorists. The authors find that the conjectured bound is violated because the viscosity can become arbitrarily small near the superfluid transition due to large scattering from superfluid fluctuations. This is because the mean free path becomes arbitrarily small, i.e. the system is similar

Two of my UQ colleagues have just finished a nice paper: Spin-state ice in geometrically frustrated spin-crossover materials  Jace Cruddas, B. J. Powell The paper brings together two fascinating topics I have written about before, spin crossover materials and spin ice . One thing that it is a little worrying and disappointing about spin ice materials is that there seem to be only two (?) of them! This paper argues that some spin crossover materials may be a new class of materials that realise ice physics (residual entropy, emergent gauge fields, monopoles, ...) Here, the Ising spin variable is the two possible spin states (High Spin and Low Spin). These materials have the potential advantage that they may be tuneable due to the creativity of synthetic chemists. The mechanism of the interaction between spins is rather unique and interesting. It is not an exchange interaction but rather and effective interaction mediated by the spin-lattice interaction, which in these compounds is

In different words,  No College Kid Needs a Water Park to Study This is a disturbing New York Times opinion piece by James V Koch, who has been president of two universities in the USA. Unfortunately, this rush to spend public money (and/or student tuition) by university managers appears to be global. In the UK, there is an article in the Financial Times Magazine University challenge: the race for money, students and status . From Swansea to Sheffield and Southampton to Strathclyde, universities are now engaged in a spending spree: renovating campuses and building lecture theatres, laboratories, libraries and halls of residence. “ What we know is that students and their parents, when they go on open days, they are impressed by shiny buildings ,” says Nick Hillman, an adviser to the universities minister David Willetts from 2010 to 2013 who now runs the Higher Education Policy Institute, a think-tank.    But as cranes dominate campus skylines, debts are mounting on vice-chancell

Several years ago I posted about Density Matrix Embedding Theory  (DMET), proposed as a (computationally cheaper) alternative to Dynamical Mean-Field Theory (DMFT). An important question is what is the relationship (if any) between the two methods? Given that the two methods are formulated in quite different ways it was not clear to me at all whether these question could be answer in any sort of definitive way. There is a very nice paper which does answers this question in a precise way, with the bonus of also giving the relationship of both methods with rotationally invariant slave bosons (RISB). Dynamical mean-field theory, density-matrix embedding theory, and rotationallyinvariant slave bosons: A unified perspective  Thomas Ayral, Tsung-Han Lee, and Gabriel Kotliar The main results are summarised in the Figure below. A result that is useful and insightful is that DMET corresponds to RISB with the quasi-particle weight set to unity (Z=1). There is then no band narrowing as

Spin crossover (SCO) materials are fascinating and raise many interesting questions. Here I want to address the underlying physics of why there is a large change in bond length (typically 10-20 per cent) when the spin state of the complex changes. Basically, it is because the ligand field splitting Delta changes significantly with bond length. The change in spin state is associated with electrons moving between from the upper d -levels on the metal ion  (e_g in an octahedral complex) to the upper levels (t_g2). What is the physical origin of this splitting? How does Delta vary with the distance between the metal (M) and the ligand L? One can answer the second question theoretically with quantum chemistry computations and experimentally by changing the ligand L, which leads to changes in bond length. The figure below is taken from the book, Ligand Field Theory and its Applications . The variation of Delta and the bond length with ligand reflects the spectrochemical