Condensed concepts

Developing reliable numerical methods that can give meaningful and useful results for lattice models [e.g. the Hubbard, Heisenberg, and t-J models] of strongly correlated electrons is a challenging and tedious task. An important outcome is when complementary methods (including analytical methods) give the same result! There is a nice Physical Review E article by Marcos Rigol , Tyler Bryant, and Rajiv Singh which considers the application of a new numerical linked cluster algorithm (NLC) method to the t-J model. To put nicely things in context they state In spite of its simplicity, understanding finite-temperature thermodynamic properties of the t-J model has proven to be a very challenging task. Quantum Monte Carlo simulations suffer from severe sign problems, which become a major difficulty at low temperatures. The two general approaches that have been commonly used to study this model are [exact diagonalisation] ED and [high temperature expansions] HTE. ED studies in which one f

No. Businesses (small and large) are more likely saviours. My January edition of Physics Today arrived in the snail mail this week! Nevertheless, it was still worth reading. The most interesting article were interviews with Steve Koonin , a former BP Chief Scientist and DOE Undersecretary for science and Ellen Williams , the current BP Chief Scientist. Two quotes from the Koonin interview particularly struck me many of the people reading   Physics Today   are in the academic world, and if they want to really change energy, I would strongly recommend six months or a year out in the private sector, whether in a big company or a small startup. It really is a very different mindset than what a basic [academic] researcher has....   For some of our biggest problems, whether energy or other big problems in society, the technology is in many ways the easy part. The rate-limiting steps for many of our problems are societal: How people behave, what incentives there are, etc. I think

The first key to preparing a good talk is taking into account the backgrounds and interests of your audience. This preparation begins with the title and abstract. You need to motivate people to come! You should NOT give the same talk to experimentalists as to theorists, or the same talk at a specialised conference and a departmental colloquium. This may seem obvious but it is amazing how much it happens. It actually does require significant work, experience, and discipline to give relevant, appropriate, and enjoyable talks. Next week I am giving the UQ Quantum science seminar. It is attended by physicists, mostly theorists, working in condensed matter, BECs, quantum information, and quantum optics. Below is the abstract I prepared. A quantum physicist's view of hydrogen bonding Hydrogen bonding plays a central and diverse role in chemistry and biology. It is key to the unique properties of water, the double helix structure of DNA, and the unique folding of proteins. Yet it

In Bangalore recently it was nice to meet Shobhana Narasimhan . Following discussions about the challenge of teaching science in the developing world she sent me an interesting and helpful paper she wrote  Training the Future Scientist: Making the Transition from 'Knowledge’ to 'Synthesis’ . After describing the context of Indian education [which utilises copious amounts of rote learning], she reviews Bloom's taxonomy of learning objectives  [knowledge, comprehension, application, analysis, evaluation, synthesis], and then discusses specific initiatives she has taken when teaching Introductory and Advanced Condensed Matter Physics to graduate students. Although geared to the Indian context many of the ideas are relevant and adaptable to other contexts. The paper showcases the importance of establishing where students are at, what their needs are, and adapting our teaching accordingly.

On his website Joel Moore has a copy of the slides from a colloquium   New topologically ordered phases of condensed matter that he gave in 2009. I found this a helpful introduction/review. Sometimes talk slides are more helpful than review articles. A picture can be worth a thousand words...

Chemists are very good at coming up with new concepts and organising principles which can describe a wide range of chemical trends. Two examples are electronegativity and chemical hardness.  The latter provides a basis to understand the principle of hard and soft acid and bases  (HSAB): hard acids prefer to co-ordinate (bond) with hard bases while soft acids prefer to co-ordinate with soft acids. Can one provide a quantitative measure of "chemical hardness"? A 1983 JACS paper by Robert Parr and Ralph Pearson suggested that for a specific atom or molecule it could be defined as the second derivative of the ground state energy with respect to the number of electrons N or the discrete version  where I is the ionisation energy and A the electron affinity. For comparison, the electronegativity is I+A, and the first derivative of E with respect to N is the chemical potential. Aside: Physicists can relate eta to the charge compressibility. At the end of their article the

In Science last week there was a Perspective Cuprates Get Orders to Charge by John Tranquada about a paper  that finds long-range charge order in YBCO in the same doping region at which quantum oscillations are observed. The Perspective is worth reading because it gives a nice overview of the issues. On the other hand, I find it disappointing that Tranquada takes a view, similar  to other experimentalists, that little progress has been made in theory: Although there are many theoretical proposals to describe this unusual “normal” state, no new simplifying paradigm has yet appeared. I disagree. I think "plain vanilla" RVB theory does provide the simplifying paradigm. I acknowledge that not everyone agrees and there are still outstanding questions. But not all the theories out there are equal. There have varying amounts of supporting evidence and internal consistency. My somewhat ill-informed and prejudicial view is that stripes, charge ordering, and quantum oscillati

Here are a few hints about writing papers and grant applications. First, just do it! Start writing. Worry about all the details on the second and third rewrite. Second, review every single sentence. Is each sentence actually true? Any claim needs to be backed up with evidence (e.g., a reference). Never offer undefendable ground . In my view, far too many papers contain assertions that are debatable. For example, "this experimental data proves theory A is correct" should be replaced with "this experimental data is consistent with theory A". Third, reconsider qualitative language and value judgements. You may write "the agreement between my theory and experiment is excellent" but others might think it is not particularly impressive. Perhaps it is better to be more circumspect and write "the theory and experiment are compared in Figure X" and let the reader make their own judgement. Fourth, avoid vagueness. Be concrete, precise, specific, an

There is a really nice preprint Power-law dependence of the optical conductivity observed in the quantum spin-liquid compound κ-(BEDT-TTF)2Cu2(CN)3 by Sebastian Elsasser, Dan Wu, Martin Dressel, John A. Schlueter The simplest possible picture of charge excitations in a Mott insulator is that it is similar to a band insulator but the origin of the gap is from correlations. Then one expects that 1. There is a non-zero charge gap. 2. The same energy gap occurs in the dc conductivity (activation energy) and the optical conductivity. 3. At finite temperature the subgap absorption in the optical conductivity arises due to thermal excitations across the energy gap. Hence, the subgap absorption decreases with decreasing temperature. Indeed such behaviour is observed in the organic Mott insulator kappa-(BEDT-TTF)2Cu(NCN)2Cl which has an antiferromagnetic ground state [denoted kappa-Cl below]. However, this preprint reports qualitatively different behaviour in the title compound [deno

Undergraduate students from Western countries commit plagiarism. This has led to highly effective commercial software to detect it such as Turnitin. Recently I have encountered several incidents of plagiarism from postgraduate students from non-Western countries. It is easy to be shocked and morally indignant about this, until one learns some of the cultural background and common educational practices in some countries. The following is helpful: T. A. Abinandanan, an engineer at the Indian Institute of Science, says that the practice of duplicating information is deeply ingrained in Indian education. “Right from school our students are encouraged to take material from books and websites and use it in their charts and lab notebooks. In written answers, verbatim reproduction from textbooks is even rewarded with higher marks ,” he told Nature. “Thus, when students start doing research, they have so much of this attitude to unlearn. This unfortunate case provides us yet another opport

Seth Olsen and I just finished a paper A two-state model of twisted intramolecular charge transfer in monomethine dyes . It presents a simple characterisation the ground and excited state potential energy surfaces associated with bond twisting of the chromophore for the green fluorescent protein. This is based on a 2 x 2 matrix Hamiltonian acting on diabatic states which have a simple valence bond interpretation.

A publon is a "quantum" of publication. It is the least publishable unit of research. I have found this to be a very helpful concept in doing and supervising research and deciding what and when to publish. The basic idea is that once you have a result that is publishable [a new equation or measurement or graph or numerical result] for a specific research project one should start to prepare a paper around it. Large projects can be broken down into publons. I first really took on board the concept after reading Peter Feibelman's A Ph.D is not enough.  The advantages are -research and publication can much more manageable -it avoids procrastination, particularly by perfectionists who always want to get one more result... -it placates the "bean counters" who expect quick results and large numbers of papers -papers should have a single message and not several. publons prevent that single message being diluted or distracted from -it can be helpful in buil

Today I had a nice visit to the Physics Department at Indian Institute of Science. My host was Subroto Mukherjee . I gave a talk Interlayer magnetoresistance as a probe of Fermi surface anisotropies in overdoped cuprates. It was nice to get so many questions during the talk. The more recent results are in a  PRB written with Jure Kokalj and Nigel Hussey.

Tomorrow I am visiting the Solid State and Structural Chemistry Unit at the Indian Institute of Science in Bangalore. My host in Ramasesha. I will give a talk [ slides ] about effective Hamiltonians for the excited electronic states of methine dyes (including the chromophore for the Green Fluorescent Protein). All of this work was done largely by  Seth Olsen . A good introduction and overview is our recent J. Chem. Phys. paper.

It is amazing to me how much the Higgs boson has featured in the press and entered the public consciousness lately. I fear this is partly because CERN has such a large and effective press office. This was particularly brought home to me when my wife sent me the above cartoon which one of her friends had posted on Facebook. On the one hand, I think it is good that physics is getting all this publicity. On the other hand I think the scientific significance of this discovery is being blown out of proportion, driven by the large budgets involved....

Over the past year I have encountered the following annoying and sometimes embarrassing problem with Mail on my Mac. It seems that sometimes (apparently randomly to me) it strips attachments from incoming emails. A first I thought the sender had forgotten to attach the file, as sometimes happens. I asked them to resend it. Sometimes the attachment comes through on a second attempt. Other times it does not. The size does not seem to be relevant. I thought the university system might be stripping the attachment and so I looked on the UQ Webmail and found that it was there in the original email there but not in Mail. I welcome suggestions. I looked online and found other people with this problem, but no clear solutions.

At the cake meeting [weekly condensed matter theory group meeting] we took three weeks to go through the review article Inelastic light scattering from correlated electrons by Devereaux and Hackl. I found it very helpful. Here are a few things I learnt. Polarisation dependence It is sensitive to anisotropy in Fermi surface properties via different light polarisations. This is because the relevant matrix elements depend on the polarisation. In principle this can be used to detect unconventional superconductivity and even distinguish d-xy and d_x2-y2. This polarisation dependence also shows signs of anisotropies in the metallic state of cuprates. Background electronic continuum In a simple metal the scattering off electron-hole pairs should cutoff at a wavevector of about v_F q where v_F is the Fermi velocity and q is the change in wavevector of the light. This is a relatively small energy. However, in strongly correlated materials such as the cuprates this extends to much

Everyone, and particularly junior people, need to accept the painful reality that within any department and university they only have a finite and limited amount of political capital and goodwill. So, conserve it like a precious commodity. Only cash it in for things you really need: e.g., getting tenure, adequate lab and office space, a strong letter of support for an important grant application, a reasonable teaching load, promotion, .... Don't squander your capital and goodwill with minor gripes/requests/demands about lab space, small salary increments, not getting exactly the teaching you want, minor irritations about administrative procedures,... Middle (and senior) management may quickly get tired of these minor concerns and will then be much less sympathetic and willing to address major concerns.

There is a nice article by Frank Close,  Higgs boson: beginning of the end or end of the beginning. It gives a succinct discussion of the background to the recent discovery and what it does and does not mean for elementary particle physics. A couple of points I found helpful. Reports in the media that "the Higgs boson is responsible for mass" are misleading. First, almost all the mass in our bodies comes from neutrons and protons, and more than 99% of their mass comes from the binding energy of quarks (due to quantum chromodynamics) and not from the mass of isolated quarks. The latter are hypothesized to get their mass from the Higgs field. [Aside: the boson and the field are not the same thing. The boson is one specific excitation of the field]. Secondly, the CERN experiments do not establish that leptons and quarks obtain their mass from the Higgs field. Rather, only that the gauge bosons in electro-weak theory obtain their mass from the Higgs field. Further exp

Previously, I posted about Stanford's criteria for tenure in chemistry. It is striking that so little consideration is given to the metrics (grant dollars, number of publications, number of graduate students, number of citations, h-index,...) that lesser institutions use to judge staff. In this light, it was particularly interesting to read the extract below of an interesting interview with Ken Wilson about his career. PoS [Physics of Scale] So by the time you come to Cornell, people don't really know what you're working on. I mean on the surface, there are many things that you're doing, but deep down, there's a commitment to try to explore how far can you go with quantum field theory. KGW The people at Cornell had more of an interest to know what I was doing than people at Harvard, because they were going to have to make a decision... Of course one of the things that happened was, as you may or may not be aware, is that they gave me tenure after only two y

In simple metals the temperature and magnetic field dependence of the magnetoresistance is encoded in some function of the product omega_c*tau. omega_c is the cyclotron frequency which is proportional to the magnetic field B and independent of temperature. tau is the scattering time, which is temperature dependent and field independent, and should have the same temperature dependence as 1/rho where rho(B=0) is the zero field resistivity. These observations lead to Kohler's rule which is obeyed by simple metals. A plot of the ratio of the rho(B)/rho(B=0) versus B/rho(B=0) should be independent of temperature. In 1995 Ong's group observed significant violations of Kohler's rule in the underdoped and overdoped cuprates. In 1998 I pointed out that in one mysterious organic metal there were also significant violations.  The paper also has an extensive discussion of reasons why Kohler's rule can fail. Recently, Jure Kokalj, Nigel Hussey and I wrote a paper which showe