Condensed concepts

I tend to avoid being on committees. However, this year I became chair of one, leading to this reflection. How do you find a balance between democracy and autocracy, between transparency and secrecy, and between efficiency and wasting a peoples precious time? Over the years I have noticed committees can tend to one of two extremes. 1. Some are very democratic and transparent. All business is discussed in great detail. Votes are held about many things. Between meetings, committee members are emailed about the latest "urgent" matter, asked their opinion, and sometimes asked to vote to approve some small action. The problem is this takes a lot of time. It would be quicker and more efficient if on these small matters that the chair or a subgroup simply made a unilateral decision. 2. Some committees are secretive and merely "rubber stamp" a bunch of decisions that have already been made by the chair or a select subgroup of members. This is efficient, particularly

Last October I enjoyed attending a meeting,  Water: the most anomalous liquid  at NORDITA. One of the goals of the workshop was to produce a review article, co-authored by about a dozen working groups, each covering a specific aspect of water. I was in the group on "Nuclear quantum effects in water", led by Tom Markland. I was worried that this goal was a bit too ambitious. After all, I am into modest goals! However, it is all coming together, a great credit to the organisers. Our group is now finalising our "chapter". An important and difficult task is to write something concrete and useful about future challenges and directions. Here I give a few of my own biased tentative thoughts. Comments and suggestions would be very welcome. Over the past decade there have been several significant advances that are relevant to understanding nuclear quantum effects in water. It was only by writing this summary that I realised just how tangible and significant these advanc

Carl Caves has a helpful two pager, tips for solving physics homework problems . It nicely emphasises the importance of drawing a clear diagram, dimensional analysis, thinking before you calculate, and checking the answer. He also discusses moving from homework problems to "real world" problems, e.g. research. Then, just formulating the problem is crucial. I wonder if the goals of some Ph.D projects might be revised if the supervisor and/or student simply combined dimensional analysis with a realistic order of magnitude estimate. Just doing the exercise might also significantly increase the students understanding of the underlying physics.

This past week I taught my condensed matter class about crystal structures and their determination by X-rays. This can be a little dry and old. Here, are few things I do to try and make things more interesting and relevant. I emphasise that many of these developments go beyond what was known or anticipated when Ashcroft and Mermin was written. Furthermore, significant challenges remain. Discuss whether the first X-ray crystallography experiment  the most important experiment in condensed matter, ever ? Take crystal structure "ball and stick" models to the lectures. Give a whole lecture on quasi-crystals. Use the bravais program in Solid State Simulations  to illustrate basic ideas. For example, the equivalence of each reciprocal lattice vector to an X-ray diffraction peak, to a family of lattice planes in real space, and to a Miller indice. Show a crystal structure for a high-Tc cuprate superconductor and an organic charge transfer salt. Emphasize the large number

Is it ever a "bad metal"? Liquid 3He mostly gets attention because at low temperatures it is a Fermi liquid [indeed it was the inspiration for Landau's theory] and because it becomes a superfluid [with all sorts of broken symmetries]. How strong are the interactions? How "renormalised" are the quasi-particles? The effective mass of the quasi-particles [as deduced from the specific heat] is about 3 times the bare mass at 0 bar pressure and increases to 6 times at 33 bar, when it becomes solid. The compressibility is also renormalised and decreases significantly with increasing pressure, as shown below. This led Anderson and Brinkman to propose that 3He was an "almost localised" Fermi liquid. Thirty years ago,  Dieter Vollhardt worked this idea out in detail , considering how these properties might be described by a lattice gas mode with a Hubbard Hamiltonian. The system is at half filling with U increasing with pressure, and the solidification

The Professor Is In: The Essential Guide To Turning Your Ph.D. Into a Job by Karen Kelsky. The author left a tenured position at a research university and now has an excellent blog and runs a career advice consulting business, for people in academia, both those who want to stay and those who want to (or have to) leave. Navigating the Path to Industry: A Hiring Manager's Advice for Academics Looking for a Job in Industry  by M.R. Nelson Has anyone read either book? I welcome comments.

When I started teaching sometimes I would arrive at the lecture to find that I left behind something I needed (chalk!, laser pointer, computer connector, textbook, notes, ...). Do I go back to my office and get it and start the lecture late, or do without? Sometimes these are things that should be in the room but are not. (e.g. chalk, erasers, or markers). Even worse was to discover I was missing something in the middle of the lecture! I eventually came up with a simple solution. Have a separate bag in which I store absolutely everything I need or may need (white board markers, eraser, Mac adapters, text, clicker receiver, course profile, ....) When I leave for the lecture I don't have to remember or find all these things. For things like Mac adapters I have an extra one just for the bag. It is a simple thing but it does reduce anxiety and problems.

The first observation is that each mathematician is a special case, and in general mathematicians tend to behave like “fermions” i.e. avoid working in areas which are too trendy whereas physicists behave a lot more like “bosons” which coalesce in large packs and are often “over-selling” their doings , an attitude which mathematicians despise. Alain Connes, Advice to beginning mathematicians I learnt this quote today, courtesy of Elena Ostrovskaya, who gave todays Physics Colloquium at UQ .

On Friday Robert Mann gave a very nice colloquium at UQ, The Black Hole at the Beginning of Time.  The video is below. The (end of) the talk is based on the recent paper Out of the white hole: a holographic origin for the Big Bang  Razieh Pourhasan, Niayesh Afshordi, and Robert B. Mann The key idea is to consider our universe as the 4-dimensional boundary (brane or hologram) of a 5-dimensional space-time in which there is a black hole. In our universe one then has not just 4D gravity and matter, but also induced gravity and an effective fluid from the 5D "bulk". (For better or worse) this work was recently featured on the cover of Scientific American. Robert covered a massive amount of material moving through special relativity, general relativity, black holes, big bang, cosmology, recent results from the Planck satellite,  inflation, the multiverse,... and finally his alternative model. I took several pages of notes. He went overtime. I think this was one of the

I was recently asked to complete an online questionnaire about my experiences with Physical Review B: both as an author, reader, and a referee. Many readers probably also did. I felt the survey was fishing for the conclusion, "PRB has increased in quality lately." I was a bit ambivalent in my responses. However, on further reflection I feel I now agree with this conclusion. But, I was disappointed I did not get to make the following point that I think is very important. With the rise of High Impact Factor Syndrome , luxury journals, hype, and fashions, I think the scientific importance and stature of PRB has increased significantly in the past 20 years.  It provides an avenue to publish solid detailed honest reliable careful non-sexy research without the need to indulge in hype, speculation, or hiding important details. This is the research that will have real scientific impact. Making generalisations is difficult and dangerous because my experience is limited to reading,

This helpful table appears in a review article Studying Two-Dimensional Systems with the Density Matrix Renormalization Group  E.M. Stoudenmire and Steven R. White The review also shows that comparisons of the 2D DMRG with methods such as PEPS and MERA (heavily promoted by quantum information theorists) imply that 2D DMRG performs significantly better. I thank Seyed Saadatmand for bringing the table to my attention.

Previously, I wrote about my concern  that little attention and publicity is given these days to issues of nuclear security and proliferation. Hence, it was good to see the cover (and a lead editorial ) of The Economist this past week. On related matters there is an interesting article [and cover story] in the February Physics Today,  Pakistan’s nuclear Taj Mahal  by Stuart W. Leslie Inspired by the promise of Atoms for Peace , the Pakistan Institute of Nuclear Science and Technology eventually succumbed to the demands of the country’s nuclear weapons program.  One thing I learnt was the central role that Abdus Salam played. I found the following rather disturbing. Salam, t hough still the director of the ICTP, organized the theoretical-physics group that performed the sophisticated calculations for the bomb, and he personally asked his former student and protégé Riazuddin to head it. Riazuddin, then teaching at the University of Islamabad (now Quaid-i-Azam University), to

One on the many disturbing things I find about science today is people claiming that because a particular theory agrees with a particular experiment that the theory must be valid. Little consideration is given to the possibility that the agreement may just be an accident. The "correct" theory may actually be quite different. They may be getting the "right" answer for the "wrong" reasons. I am never sure if the people who make these kind claims are sincere, naive, and/or just engaging in marketing. Students need to be taught to be more critical. I am currently teaching an advanced undergraduate course on solid state physics, PHYS4030.  It follows Ashcroft and Mermin closely. I have just taught the Drude and Sommerfeld model. Drude provides a nice example of getting the "right" answer for the "wrong" reasons. In both models the thermal conductivity is given by the following expression from kinetic theory where c_p is the specifi

Yesterday, Matthew Woolley gave an interesting Quantum science seminar at UQ about some of his recent work on Photon assisted tunnelling with non-classical light. I just want to focus on one point that was deeply imbedded in the talk. It is a idea that is profound and central to the physics of quantum electronic circuits. The idea is so old now its profoundness and brilliance may be lost on a new generation. The idea and result is easiest for me to explain in terms of the figure below which describes a superconducting (Josephson junction) qubit connected to an electrical circuit. It is taken from this review. One can quantise the electromagnetic field and consider a spin-boson model to describe decoherence and dissipation of the qubit. This is associated with a spectral density that is proportional to frequency with a dimensionless pre factor alpha, which for this circuit is given by where R_V is the electrical resistance of the circuit, R_K is the quantum of resistance, an

There is a helpful and challenging article The Physics of Physics Colloquia by James Kakalios on The Back Page of the APS News. It is based around old notes Suggestions for giving talks by Robert Geroch. Both Kakalios and Geroch are worth reading in full, but here are a few random things that stood out to me. [Things I need to keep working on]. "What is the key take-away point that you want to impress on everyone when they leave your talk?" Divide the talk up, centred around 3 or 4 key messages. "Figures are easier to understand than words." "You have been staring at these data and plots for years, but many in the audience have not." Don't include more than five non-trivial equations. "It is almost always a disaster to run over time". Much of this may seem "common sense". However, as management guru Steven Covey said, "Common sense is not common practise." Preparing and giving a good talk requires a lot

Last week Peter Woit kindly recommended my blog to his readers . This immediately doubled the number of daily page views for several days thereafter! Peter also drew attention to my recent paper with Nandan Pakhira that shows that the charge diffusion constant in a bad metal violates a conjectured lower bound. This bound was conjectured, partly on the basis of arguments from string theory techniques [holographic duality, AdS-CFT]. Our calculations were all based on a Dynamical Mean-Field Theory (DMFT) treatment of the Hubbard model. One commenter "Bernd" wrote The violation of the holographic duality bound is based on DMFT calculations, which is a bit like string theory for strongly correlated fermions in the sense that it is somtimes sold as “the only game in town”. Nobody knows how accurate these methods really are. For background, "the only game in town" refers to a common argument of string theorists that string theory must be correct because there are n

This week classes started for the first semester of the year at UQ. Campus is swarming with students. I live about three kilometres from campus, but the buses don't even stop by the time they pass near my home because they are already full. I have to find other ways to get to campus. There are no spare seats in the library. The lines at the food outlets are very long. But, this overcrowding is not the discouraging thing . It is that two weeks from now the buses won't be full. In about six weeks they will be half full. I will have no trouble getting a seat. Why? After a few weeks a noticeable fraction of students have decided it is not worth attending lectures. Previously I made a simple and concrete proposal  to address the issue.

There is an interesting paper Quantum criticality of Mott transition in organic materials  Tetsuya Furukawa, Kazuya Miyagawa, Hiromi Taniguchi, Reizo Kato, Kazushi Kanoda Some of the results were  flagged several years ago by Kanoda in a talk at KITP. Key to the analysis is theoretical concepts developed in three papers based on Dynamical Mean-Field Theory (DMFT) calculations Quantum Critical Transport near the Mott Transition by H. Terletska, J. Vučičević, Darko Tanasković, and Vlad Dobrosavljević Finite-temperature crossover and the quantum Widom line near the Mott transition  J. Vučičević, H. Terletska, D. Tanasković, and V. Dobrosavljević Bad-metal behavior reveals Mott quantum criticality in doped Hubbard models J. Vučičević, D. Tanasković, M. J. Rozenberg, and V. Dobrosavljević The experimental authors consider three different organic charge transfer salts that undergo a metal-insulator transition as a function of pressure, with a critical point at a finite temp