Condensed concepts

Beginning in 2007 luxury journals published some experimental papers making claims that quantum coherence was essential to photosynthesis. This was followed by a lot of theoretical papers claiming support. I was skeptical about these claims and in the first few years of this blog wrote several posts highlighting problems with the experiments, theory, interpretation, and hype. Here is a recent paper that repeats one of the first experiments. Nature does not rely on long-lived electronic quantum coherence for photosynthetic energy transfer Hong-Guang Duan, Valentyn I. Prokhorenko, Richard Cogdell, Khuram Ashraf, Amy L. Stevens, Michael Thorwart, R. J. Dwayne Miller During the first steps of photosynthesis, the energy of impinging solar photons is transformed into electronic excitation energy of the light-harvesting biomolecular complexes. The subsequent energy transfer to the reaction center is understood in terms of exciton quasiparticles which move on a grid of biomolecular sit

The common narrative in physics is that the limitations of reductionism, the importance of emergence, and the stratification of scientific fields and concepts were first highlighted in 1972,  by P.W. Anderson in a classic article, "More is Different" published in Science . Anderson nicely used broken symmetry as an example of an organising principle that occurs at one strata and as a result of the thermodynamic limit. The article was based on lectures Anderson gave in 1967. The article actually does not seem to contain the word "emergence". He talks about new properties "arising". I recently learned how similar ideas about emergence and the stratification of fields was enunciated earlier by Michael Polanyi , in  The Tacit Dimension , published in 1966, based on his 1962 Terry Lectures at Yale. The book contains a chapter entitled "Emergence". Here is a quote: you cannot derive a vocabulary from phonetics; you cannot derive the grammar o

The New York Times has an interesting Op-ed. piece  Quit Social Media. Your Career May Depend on It , by Cal Newport , a faculty member in computer science at Georgetown University. When I saw the headline I thought the point was going to be an important one that has been made many times before; people sometimes post stupid stuff on social media and get fired as a result. Don't do it! However, that is not his point. Rather, he says social media is bad for two reasons: 1. It is a distraction that prevents deep thinking and sustained  "deep" work. Because you are constantly looking at your phone, tablet, or laptop or posting on it, you don't have the long periods of "quiet" time that are needed for substantial achievement. 2. Real substantial contributions will get noticed and recognised without you constantly "tweeting" or posting about what you are doing or have done. Cut back on the self-promotion. Overall, I agree. When I discussed th

The spin-1 antiferromagnetic Heisenberg chain provides a nice example of emergence in a quantum many-body system. Specifically, there are three distinct phenomena that emerge that were difficult to anticipate: the energy gap conjectured by Haldane, topological order, and the edge excitations with spin-1/2. An interesting question is whether anyone could have ever predicted these from just knowing the atomic and crystal structure of a specific material. I suspect Laughlin and Pines would say no. To understand the emergent properties one needs to derive effective Hamiltonians at several different length and energy scales. I have tried to capture this in the diagram below. In the vertical direction, the length scales get longer and the energy scales get smaller. It is interesting that one can get the Haldane gap from the non-linear sigma model. However, it coarse grains too much and won't give the topological order or the edge excitations. It seems to me that the profund

It has dubious origins. Some people are very impressed that I have a Wikipedia page. I find this a bit embarrassing because there are many scientists, more distinguished than I, who do not have pages. When people tell me how impressed they are I tell them the story. Almost ten years ago some enthusiasts of "quantum biology" invited me to contribute a chapter to a book on the subject. The chapter I wrote, together with two students, was different from most of the other chapters because we focussed on realistic models and estimates for quantum decoherence in biomolecules. (Some of the material is here. ) This leads one to be very skeptical about the whole notion that quantum coherence can play a significant role in biomolecular function, let alone biological processes. Most other authors are true believers. I believe that to promote the book one of the editors had one of his Ph.D. students [who appeared to also do a some of the grunt work of the book editing] create a

One of the key predictions of the  diabatic state picture of hydrogen bonding  is that there should be an excited electronic state ( a twin state ) which is the "anti-bonding" combination of the two diabatic states associated with the ground state H-bond. Recently, I posted about  a possible identification of this state in malonaldehyde. The following recent paper is relevant. Symmetry breaking in the axial symmetrical configurations of enolic propanedial, propanedithial, and propanediselenal: pseudo Jahn–Teller effect versus the resonance-assisted hydrogen bond theory Elahe Jalali, Davood Nori-Shargh The key figure is below. The lowest B2 state is the twin state. In the diabatic state picture, Delta is half of the off-diagonal matrix element that couples the two diabatic states. Similar diagrams occur when O is replaced with S or Se. The paper does not discuss twin states, but interprets everything in terms of the framework of the   ( A 1  +  B 2 ) ⊗  b 2  p

Sometimes when I speak about science to church groups I show the old (1977) video Powers of Ten which nicely illustrates the immense scale of the universe and orders of magnitude. I often wished there was a more polished modern version. Yesterday it was pointed out to me there is,  Cosmic Eye . The phone app can be purchased here  for $1.

In 2011 it was proposed that pyrochlore iridates (such as Y2Ir2O7) could exhibit the properties of a Weyl semi-metal, the three-dimensional analog of the Dirac cone found in graphene. Since the sociology of condensed matter research is driven by exotica this paper stimulated numerous theoretical and experimental studies. However, as often is the case, things turn out to be more complicated and it seems unlikely that these materials  exhibit a Weyl semi-metal. This past week I have read several nice papers that address the issue. Variation of optical conductivity spectra in the course of bandwidth-controlled metal-insulator transitions in pyrochlore iridates K. Ueda, J. Fujioka, and Y. Tokura There is a very nice phase diagram which shows systematic trends as a function of the ionic radius of the rare earth element R=Y, Dy, Gd, ... Most of the materials are antiferromagnetic insulators. The colour shading describes the low energy spectral weight in the optical conductivi

I am very disturbed at how I encounter people, particularly young people, who work ridiculously long hours. Furthermore, it worries me that some are deluded about what they might achieve by doing this. Due to a variety of cultural pressures I think Ph.D. students from the Majority World are particularly prone to this. First let's not debate exactly how many hours is too many or exceptions to the generalisations below. At the end I will give some caveats. Here are some reasons why very long hours are not a good idea. Something may snap. And, when it does it will be very costly. It may be your mental or physical health, or your spouse, or your children, ... Don't think it won't happen. It does. Long hours may be making you quite inefficient and unproductive. You become tired and can't think as clearly and so make more mistakes, have less ideas, and find it harder to prioritise. It is a myth that long hours is mostly what you need to do to survive or prosper

Through a nice blog post by Anshul Kogar, I became aware of a beautiful Physics Today Reference Frame (just 2 pages!) from 1998 by Frank Wilczek Why are there Analogies between Condensed Matter and Particle Theory? It is worth reading in full and slowly. But here a few of the profound ideas that I found new and stimulating. A central result of Newton's Principia was "to prove the theorem that the gravitational force exerted by a spherically symmetric body is the same as that due to an ideal point of equal total mass at the body's center. This theorem provides quite a rigorous and precise example of how macroscopic bodies can be replaced by microscopic ones , without altering the consequent behavior. "  More generally, we find that nowhere in the equations of classical mechanics [or electromagnetism] is there any quantity that fixes a definite scale of distance . Only with quantum mechanics do fundamental length scales appear: the Planck length, Compton

It is strange that I have never done this. Furthermore, I don't know anyone who does. Why do this? First, it is helpful for me to think about and decide what my goals actually are, particular relating to the big picture. Second, it will be helpful for students to know. Too often they are guessing. Even worst, I fear that most just assume that my goals are theirs. Then they get frustrated if/when they discover their goals and/or values  are different. So here are some goals I could think of. They are listed in order of decreasing importance to me. To help you learn to THINK. To inspire you to learn. To help you see this is a beautiful subject. To help you learn skills that are useful in other endeavors (including outside physics). The help you put this subject in the context of others. To help you learn the technical details of the subject. To be your ally not your adversary  . My goals are NOT the following. (Listed in no particular order). To make you happy

Time has a direction. Macroscopic processes are irreversible. Mixing is a simple example. The second law of thermodynamics encodes universal property of nature. Yet the microscopic laws of nature [Newton's equations or Schrodinger's equation] are time reversal invariant. There is no arrow of time in these equations. So, where does macroscopic irreversibility come from? It is helpful to think of irreversibility [broken time-reversal symmetry] as an emergent property. It only exists in the thermodynamic limit. Strictly speaking for a finite number of particles there is a " recurrence time " [whereby the system can return to close to its initial state]. However, for even as few as a thousand particles this becomes much longer than any experimental time scale. There is a nice analogy to spontaneously broken symmetry in phase transitions.  Strictly speaking for a finite number of particles there is no broken symmetry as the system can tunnel backwards and forwards

I welcome comments on this preprint. Quantum critical local spin dynamics near the Mott metal-insulator transition in infinite dimensions Nagamalleswararao Dasari, N. S. Vidhyadhiraja, Mark Jarrell, and Ross H. McKenzie Finding microscopic models for metallic states that exhibit quantum critical properties such as $\omega/T$ scaling is a major theoretical challenge. We calculate the local dynamical spin susceptibility $\chi(T,\omega)$ for a Hubbard model at half filling using Dynamical Mean-Field Theory, which is exact in infinite dimensions. Qualitatively distinct behavior is found in the different regions of the phase diagram: Mott insulator, Fermi liquid metal, bad metal, and a quantum critical region above the finite temperature critical point. The signature of the latter is $\omega/T$ scaling where $T$ is the temperature. Our results are consistent with previous results showing scaling of the dc electrical conductivity and are relevant to experiments on organic charge transfe

Today I am giving the first talk in a session on Computational materials science at the  4th International Conference on Advances in Materials and Materials Processing . Here are the slides for my talk "The role of simple models and concepts in computational materials science". I will be referring the audience to the article such as those mentioned here , here and here t hat give a critical assessment of computer simulations and stress the importance of concepts. I welcome comments, particularly as I think the talk could be stronger and clearer.

Like everything in India, higher education is incredibly diverse, both in quality, resources, and culture. These statistics give some of the flavour. There are about 800 universities. A significant distinction is between state and central universities. The former are funded and controlled by state governments. The latter (and IITs, IISERs, IISc, TIFR...)  are funded and controlled by the central (i.e. national/federal) government. Broadly, the quality, resources, and autonomy (i.e. freedom from political interference) of the latter is much greater. On my many trips to India I have only visited these centrally funded institutes and universities. This afternoon I looking forward to visiting the Physics Department of Vidyasagar University . It is funded by the West Bengal state government, and was started in 1981. It is named in honour of Ishwar Chandra Vidyasagar, a significant social reformer from the 19th century. I am giving my talk on "Emergent Quantum Matter". Here a

Today I am giving a seminar, "Effect of quantum nuclear motion on hydrogen bonding" in the Chemistry Department at IIT Kharagpur . My host is Srabani Taraphder . Here are the slides . The talk is mostly based on this paper.

Today I am giving a seminar in the Physics Department at Indian Institute of Technology (IIT) Kharagpur , "Frustrated organic Mott insulators: from quantum spin liquids to superconductors." Slides are  here . Due to the recent Nobel Prize to Haldane, I included one slide about quantum spin liquids in one dimension. The talk material is covered in great detail in a  review article , written together with Ben Powell .