Condensed concepts

Making progress in science requires significant focus and discipline. In some sense you need to put in your 10,000 hours.  People can distracted by all sorts of non-scientific pursuits: Facebook, romance, family dramas, hobbies, partying, .... But this post is about scientific distractions. I illustrate this will two extreme caricatures. John really wants to understand his Ph.D project in experimental solid state physics at a deep level. He thinks the quantum measurement problem is really interesting and so he is reading a lot of papers about that. The software he needs for his experiment is functional but he does not like some of the way it interfaces with other software and so he is rewriting it all. In one month he is giving a talk at a conference and so he is not going in the lab for the next month because he wants to give a really nice talk. Whenever his advisor gives him a paper to look at he not only reads it but some of the background references. He spends a lot of time talki

Sharon Hammes-Schiffer gave an interesting talk in Telluride last week about coupled electron-proton transfer. [A couple of my earlier posts on this fascinating subject are here and here ]. Here are a few things that stood out. There are a lot more people working on this problem now than twenty years ago. This is because of possible solar energy applications. Diabatic states are the key to understanding. There are four relevant states. Simply the proton can be on the donor or acceptor. The electron can be on the donor or the acceptor. Whether the process is concerted or sequential depends on the relative energy of these four states. A key question is whether the process is adiabatic or non-adiabatic. What are the key experimental signatures of each? One contrast is coupled electron-proton transfer (EPT) and hydrogen atom transfer (HAT). The two cases are nicely embodied respectively in the model systems HAT - benzyl/toluene EPT - phenoxly/phenol The theoretical details

I am skeptical of the grand and speculative claims of "quantum biology".  There is a nice paper in PNAS which systematically considers the specific claim that smell is based on sensing the vibrational frequencies of particular molecules, and rebuts it from both theoretical and experimental points of view. Implausibility of the vibrational theory of olfaction Eric Block, Seogjoo Jang, Hiroaki Matsunami, Sivakumar Sekharan, Bérénice Dethier, Mehmed Z. Ertem, Sivaji Gundala, Yi Pan, Shengju Li, Zhen Li, Stephene N. Lodge, Mehmet Ozbil, Huihong Jiang, Sonia F. Penalba, Victor S. Batista, and Hanyi Zhuang. I thank Suggy Jang for bringing the paper to my attention.

Last week I was at Stanford and my collaborators and I finished a paper Spin triplet superconductivity in a weak-coupling Hubbard model for the quasi-one-dimensional compound Li0.9Mo6O17 Weejee Cho, Christian Platt, Ross H. McKenzie, and Srinivas Raghu The purple bronze Li_0.9Mo_6O_17 is of interest due to its quasi-one-dimensional electronic structure and the possible Luttinger liquid behavior resulting from it. For sufficiently low temperatures, it is a superconductor with a pairing symmetry that is still to be determined.  To shed light on this issue, we analyze a minimal Hubbard model for this material involving four Molybdenum orbitals per unit cell near quarter filling, using asymptotically exact perturbative renormalization group methods. We find that spin triplet odd-parity superconductivity is the dominant instability. Approximate nesting properties of the two quasi-one-dimensional Fermi surfaces enhance certain second-order processes, which play crucial roles in determini

Tomorrow I am giving a talk, "Competing quantum effects in hydrogen bonding: geometric isotope effects and isotope fractionation" at the meeting on Quantum effects in condensed phase systems Here is the current version of the slides. The talk is largely based on these two papers Effect of quantum nuclear motion on hydrogen bonding Isotopic fractionation in proteins as a measure of hydrogen bond length

I am currently in Telluride for a meeting on Quantum effects in condensed phase systems . Two years ago I attended a similar meeting and in preparing it has been helpful to re-read several posts I wrote stimulated by that meeting. In my first post , I listed possible quantum effects [zero-point motion, tunnelling, geometric phases, entanglement, ...] and pointed how generally one expects a condensed phase environment [protein, glass, solvent] for a molecular system will tend to reduce these quantum effects by decoherence. I then asked two big questions. Are there any instances where the environment can A. enhance quantum effects? B. lead to qualitatively new effects (e.g. associated with collective degrees of freedom) that are absent in the gas phase? I clarified what I meant by a trivial vs. non-trivial enhancement of a quantum effect, from a physics point of view. An example of a "trivial" enhancement is where the environment changes the molecular geometry to enha

Yoichuro Nambu died earlier this month, and there was an obituary in the New York Times yesterday. He shared the Nobel Prize in Physics in 2008, and is best known for this paper Dynamical Model of Elementary Particles Based on an Analogy with Superconductivity. I   Y. Nambu and G. Jona-Lasinio I reproduce the abstract below because it really does summarise the work and is a nice example of a beautifully written abstract. It is suggested that the nucleon mass arises largely as a self-energy of some primary fermion field through the same mechanism as the appearance of energy gap in the theory of superconductivity. The idea can be put into a mathematical formulation utilizing a generalized Hartree-Fock approximation which regards real nucleons as quasi-particle excitations. We consider a simplified model of nonlinear four-fermion interaction which allows a γ5-gauge group. An interesting consequence of the symmetry is that there arise automatically pseudoscalar zero-mass bound sta

I wish to highlight some common issues that occur in the construction, justification and parametrisation of effective Hamiltonians in both theoretical chemistry and solid state physics. The basic issue is one needs to keep in mind that just because one gets the energy eigenvalues of a quantum system "correct" does not mean that one necessarily has the correct wave function. Previously, I posted  how sometimes a variational wave function can give  a good ground state energy but be qualitatively incorrect. For molecular systems a powerful approach to understanding the potential energy surfaces of the ground state and the lowest lying electronic states is to construct a Hamiltonian matrix based on a few diabatic states. For crystals in which the electronic degrees of freedom are strongly correlated a powerful approach is to construct a Hubbard model where the non-interacting band structure is described by a tight-binding model. The latter describes hopping of electrons bet

If you applying for a grant or a fellowship that involves competing with people from other disciplines consider the suggestions below. Most of these applications are reviewed and decided by committees that comprise people from a range of disciplines, e.g. from marine biology to theoretical physics. Remember your audience They are mostly not in your field and they are busy and jaded. They will look over [n..b I did not say read] your application very quickly. Count the cost of applying These applications are actually much more work than field focused [e.g. condensed matter theory] ones. Don't kid yourself you can just cut and paste material from other applications. Don't kid your self that your "excellent" "track record" and "outstanding" "research project" will carry the day. Inevitably, you will be competing with a few individuals who will spend a lot of time carefully crafting a compelling application that does things like those b

Physical Chemistry Chemical Physics  has just published a series of four articles by Jeff Reimers, Laura McKemmish, Noel Hush , and myself. A unified diabatic description for electron transfer reactions, isomerization reactions, proton transfer reactions, and aromaticity" Non-adiabatic effects in thermochemistry, spectroscopy and kinetics: the general importance of all three Born-Oppenheimer breakdown corrections Electron-vibration entanglement in the Born-Oppenheimer description of chemical reactions and spectroscopy Bond angle variations in XH3 [X=N,P,As,Sb,Bi]: the critical role of Rydberg orbitals exposed using a diabatic state model It took a number of years to finish these papers. I am certainly the junior co-author and I commend my co-authors for all their hard work and perseverance. The four papers have two common related themes, that are hopefully not lost in all the technical detail. 1. Diabatic states provide a powerful scheme, both conceptually and quant

I really think this is the wrong question. I increasingly get irritated by grant reviewers passing judgement on people, particularly junior scientists, because their "publication rate" is "not high enough" or "good but not outstanding". First, we should be concerned with the quality of the publications; i.e. the significance, originality, technical difficulty, and reliability of the scientific content. Quality is more important than quantity. A major problem with science today is not that people are not publishing enough papers! Rather a problem is the low quality of what is produced. If you look at Einstein, Feynman, Onsager, .... their publication rates were not very high. Furthermore, even lesser mortals such as Mermin, Hohenberg, Haldane, ... have publication rates, particularly early in their career, that might be rated as "good but not outstanding" by some grant assessors today. Second, given most papers are multi-author one should b

There is a very nice article in the new journal, ACS Central Science Short Hydrogen Bonds and Proton Delocalization in Green Fluorescent Protein (GFP)  Luke M. Oltrogge and Steven G. Boxer This is an impressive piece of work spanning from molecular biology to chemistry to quantum physics. There is also a commentary on the paper by Judith Klinman, placing it in the context of the controversial issue of low-barrier hydrogen bonds in enzymes. An extensive study was made of mutants of the Green Fluorescent Protein with a short hydrogen bond between the chromophore and the amino acid Asp148. The donor-acceptor bond length estimated from X-ray structures was 2.4 +/- 0.2 Angstroms. This is in the range of low-barrier H-bonds. What is particularly new here is that through ingenious molecular biology techniques [nonsense suppression] the acidity [ pK_a = measure of tendency to give up protons ] of the chromophore was systematically varied by 3.5 units through halogen substitutions.

Then sign up for a Google Scholar account! With one click people will not just see your publications but also how many times they have been cited. More problematic is that they will also see the values of different metrics such as your h-index and ten year h-index. Recently, I encouraged someone on the job market to delete their account. Why? Unfortunately, there are people who will look at job and funding candidates and quickly dismiss them  if their metrics fall below certain threshold values. No consideration is given to scientific content, quality of publications, difficulty or popularity of the research field, career or personal history, .... People inevitably make unhealthy and unrealistic comparisons to "stars" and more senior people. I have seen this happen. I detest this and so I do not have my own account. Furthermore, I do not look at peoples pages just for the sake of it. I particularly think making comparisons with colleagues is very unhelpful. So her

Physics World [Magazine for the Institute of Physics (UK) = British equivalent of Physics Today] has a feature Web life that covers different web sites. The June 2015 issue features this blog! Besides the publicity, I was really happy because I felt the article nicely captured what I am on about. I was not interviewed and only heard about it from a Commenter, Peter Morgan. One mild amusement was that I was classified as "a chemical physicist". I would certainly classify myself as a "condensed matter physicist" who sometimes tries to do chemical physics. So I took this as a compliment!