Condensed concepts

How and when do Majorana fermions arise in a quantum many-body system? What are their experimental signatures? For some reason I find Kitaev's discussion somewhat ad-hoc. I do find the following more helpful and illuminating. Perhaps, it is just because it deals with things I am more familiar with. Start with the transverse field Ising model in one dimension. It describes interacting localised spin-1/2 particles. J is the nearest-neighbour ferromagnetic interaction. h is the transverse magnetic field. At J=h it undergoes a quantum phase transition from a ferromagnetic phase to a paramagnetic phase. One performs a Jordan-Wigner transformation which maps the spin-1/2 operators onto spinless fermion operators. This is a non-local transformation. The Hamiltonian then becomes quadratic in the fermion operators and so is analytically soluble via a Bogoliubov transformation. This means the "quasi-particles" are spinless fermions. [For the details see this article which

This is a classic scene from the Bollywood movie 3 Idiots.

First, I am no expert. I have probably supervised less than a dozen undergraduate projects in my whole career. What should be the primary goals? Hopefully, the student will learn some science (including some combination of concepts, theory, and techniques) learn something about how research is done (searching and reading the literature, trying different things, asking good questions, making mistakes, brainstorming, ...) experience some of the joys and frustration of doing science (including feeling dumb ). get to personally interact with a range of scientists (faculty, postdocs, grad students) The dominant goals should not be: use the student as slave labour get a publication keep the student happy recruit the student to do a Ph.D in the same group Projects I don't like include ones which are highly technical [the students learns a lot of jargon or advanced techniques but does not know the basics, or context] mostly use prepackaged software (e.g. for comput

Last week I heard Seth Olsen give a nice talk about his recent paper Why Bindschedler's Green is redder than Michler's Hydrol Blue It addresses the important and subtle question of what happens in methine dye molecules when the central carbon atom is replaced by a nitrogen atom: I was going to write a summary. But, the abstract of the paper is beautifully written, summarising the main results. And so, here it is. We offer a new physical interpretation of the color shift between diarylmethane dyes and their azomethine analogues. We use an isolobal analogy between state-averaged complete active space self consistent field solutions for corresponding methines and azomethines to show that the shift contains a significant contribution from configuration interaction between a methine-like ππ* excitation and an nπ* excitation out of the azomethine lone pair. The latter does not exist in the corresponding methine systems. This picture is qualitatively inconsistent with tradit

I would have thought it was Einstein, or some other physicist, around 1905. However, it was actually the distinguished chemist G.N. Lewis , as late as 1926! I learnt this in a nice article from "This Month in Physics History" in the APS News. It also discusses Lewis' possible suicide due to depression. On a lighter side, this reminds me of a silly achievement on my own: getting the term "squashon" into the scientific literature (see this paper from my Ph.D).

I think when a grant application has a section F15.5 there is a problem! My latest application is running at 76 pages. Only about 8 pages is actually about science. The rest is administrative details, publication lists, statistics, budgets, justifications, and "bragging" about how great all the Investigators and their institutions are. Every year more information is required and the applications get longer. The problem may be that every year or so a new administrator decides it would be "helpful" to request an additional piece of information. But, adding just 7 per cent per year doubles the application length every decade.... Is this really necessary? Not only does it take a lot of time to prepare, but it also takes a lot of time to review. Actually, the painful reality is that most reviewers (including me, sorry) don't read much of the "fluff" but just focus in on a few key pieces of information: the science proposed, what the Investigators have

The cartoon is from Saturday Morning Breakfast Cereal. I thank my son Luke for bringing it to my attention. This does raise an important issue. To what extent should students be encouraged or allowed to use Mathematica and Matlab? It seems to me there needs to be a balance: between learning to use a powerful tool an understanding how it works. For example, I think it is very important that students learn to sketch graphs of simple functions. This provides intuitive understanding and a way of checking that the computer is giving a reasonable answer. Perhaps it is no different from pocket calculators. Here is Ben Powell's comment on this post. It took me a while to get it!

I deem it of more value to find out some truth about however light a matter than to engage in long disputes about the greatest questions without achieving any truth. Galileo I encountered this quotation in a Physics Today obituary for an Italian particle physicist Massimilla Baldo Ceolin. [I often read the obituaries, even of people I have never heard of before. I usually learn some interesting physics and something about what it takes to be an influential scientist].

I doubt it. There is an interesting paper Similarity of Scattering Rates in Metals Showing T-Linear Resistivity J.A.N. Bruin, H. Sakai, R.S. Perry, A.P. Mackenzie The central result is the figure below. The graph shows the magnitude of the estimated scattering rate per Kelvin versus the Fermi velocity for a wide range of materials. The line alpha=1 corresponds to a value of k_B/hbar, comparable to what one gets from a simple dimensional analysis or the "minimum viscosity limit" of "quantum hydrodynamic fluids" described by some theoretical models connected to string theory. What worries me about this graph? It is that elemental metals [copper, silver, aluminum, paladium, ...] lie on the same curve. As far as I am aware, they are not strongly correlated. They are nowhere near a quantum critical point. The resistivity is due to electron-phonon scattering. So given that they "accidentally" lie on this "universal" curve suggests to me t

"Table top" science is great. But how about "table top" nuclear technology? On the Back Page of the APS News there is an important debate about The Benefits and Risks of Laser Isotope Separation . The science is really interesting, but the prospect of being able to make enriched uranium with a "table top" facility is scary. I felt Mark Raizen's view that "this will only work for light atoms so we don't have to worry about complex atoms like uranium" is a little naive given the history of science.

Only people who have made a "significant scientific contribution to the content of the paper". In particular, getting a grant, employing someone, or being a lab director does not justify co-authorship. I have observed that the issue cuts both ways with regard to seniority. It is not just senior people demanding to be co-authors. Sometimes it is junior people putting a senior person as co-author to try and "curry favour" with them or in the hope that it will increase the likelihood of publication. Here are two frank and helpful articles in Nature Materials which discuss some of the relevant issues. Authorship without authorization (2004) Authorship matters (2008) Like a lot of things, problems may be avoided if there are open discussions before employment or a collaboration begins. Sometimes co-authorship may be a "grey issue". What constitutes a "significant contribution" can be highly subjective.  But I fear there are too many cases o

There is a nice paper  Hydrogen Motion in Proton Sponge Cations: A Theoretical Study by Yevhen Horbatenko and Sergei Vyboishchikov Proton sponge is a trade name for a particular compound which is a strong base, i.e. it likes to bond to protons. The proton sponge compounds are of particular interest to me because they work by the proton forming a strong hydrogen bond between two nitrogen atoms. In this paper the authors first use quantum chemistry to calculate the adiabatic potential energy surface for the ground state as a function of the proton position. They then calculate the proton vibrational wavefunction and energy. The three potential energy curves above correspond to the three distinct cases of hydrogen bonding: i) strong hydrogen bond, ii) low barrier hydrogen bond, iii) weak hydrogen bond. [They arise naturally in my simple model of H-bonds.] I found two results of the authors particularly interesting. First, the shape of the potential energy curve [and s

In applications for grants, jobs, tenure, and promotion one is asked to list a range of accomplishments: research, collaborations, teaching, community service, ... There is real danger here that you produce a long list of activities and this can really dilute the impact of your actual significant accomplishments on the reader/reviewer. Hence, I think it is best to not list everything but highlight a few accomplishments and give some specific details of why they are significant. On a related matter I think that universities are putting increasing pressure on faculty to be involved in a diverse range of activities so they can produce such lists. I noticed this particularly in a couple of tenure/promotion cases I recently reviewed. I was really impressed by how much the applicants had done but I wondered if they had focussed more on just a few of the activities whether everyone would have been better off.

Today I am giving a seminar in the School of Physical Sciences at J awaharlal Nehru University in Delhi. "Spin frustration in organic Mott insulators: from quantum spin liquids to superconductors." Slides are  here . The talk material is covered in great detail in a  review article , written with Ben Powell. My host is Brijesh Kumar . Aside: JNU is the best university for the humanities in India. It is a hot-bed of student and faculty political activism. Yesterday there was a strike. One of the student demands was that Ph.D scholarships be extended from 5 to 7 years!  Being on campus reminded me a bit of an Australian campus when I was an undergraduate. One of my Indian friends told me many faculty [and consequently students] believe that science and rationality are imperialistic Western social constructs, just like their counterparts in the West! I think the West is guilty of many imperialistic atrocities; but science and rationality are not among them.

Before I gave my talk in Bangalore about proton transfer in enzymes I reviewed some of the recent literature since I have not worked on the problem for a few years. I found a very nice paper in Nature Chemistry Taking Ockham's razor to enzyme dynamics and catalysis by David R. Glowacki, Jeremy N. Harvey, and Adrian J. Mulholland They consider a simple transition state model for the anomalous kinetic isotope effects that have been observed in several enzymes. These anomalies have previously been claimed to be evidence for quantum tunneling, breakdown of transition state theory, and require a new paradigm for enzyme catalysis. The key feature of their model is the assumption that there are two transition states, not one, being associated with two possible conformations of the enzyme-substrate complex. They end the paper quoting a 1991 Nature paper by Jeremy Knowles Enzyme catalysis: Not different, just better. Another nice reference removing the almost mystical interpr

No. Breadth of experience is important; both for your development as a scientist and to demonstrate your versatility to potential employers. It should also be fun and interesting to work on something different. However, a complete change of research field is not a good idea because the learning curve is so great meaning it is unlikely you will produce your first postdoc paper in a timely manner. ( I recommend 6 months; others one 100 days ). It is a good if you can use some of the expertise, experience, and/or techniques you have developed in your Ph.D during your postdoc. So on balance, here is my suggestion. Most projects involve applying a specific  technique (experimental, computational, or analytical) to a specific  system   (e.g., a class of materials or model Hamiltonians). A great situation is if you either: use your Ph.D technique on a new system or apply a new technique to your Ph.D system. For example, if you did a Ph.D using neutron scattering to study transitio

Friends in India recommended my family watch 3 Idiots . It is the most commercially successful Bollywood movie ever. And it is about university education in India! The movie tackles important issues such as rote learning vs. understanding learning for passion vs. career success parental pressure mental health and suicide abuse of authority by faculty  Some of it is funny. Some of it is very sad.

Last Friday I had a nice meeting at IISc Bangalore with Subroto Mukerjee . One thing he emphasized to me is that if you see evidence of surface states with a Dirac cone (e.g. in ARPES or quantum oscillations) it is not unambiguous evidence that you have a topological insulator. That requires seeing an odd number of Dirac cones.