Condensed concepts

A few months ago I attended a symposium at UQ on Energy in India. The talks can be viewed on Youtube . The one by Alexie Seller is particularly inspiring. In the presentation of Chris Greig he showed a slide similar to that below with the title "Electricity affects Human well being". He did not say it, but sometimes graphs like this are used to make claims such as "the more electricity people consume the better off they will be..."  or "the only way to lift people out of poverty is to burn more coal..." Sometimes people show graphs that correlate GDP with energy consumption. But this one is better because it uses the Human Development Index , a multi-dimensional measure of human well being (as it includes life expectancy and education). Two things are very striking about the graph. First, the initial slope is very large. Second, the graph levels off quickly. A little bit of electricity makes a huge difference. If you don't have el

I wish I knew. This is something I continue to struggle with. To think clearly and creatively one needs to find "space" that is free from distractions and stresses. I find it hard to believe that one can be really productive in the midst of noise, chaos, and multiple demands. I can't. I know there are some individuals who are good at multi-tasking and even seem relish all the noise and hyper-activity. But, deep down I wonder if some are just "cranking the handle" and publishing the same paper again and again. I contend that slow science is not just enjoyable but necessary. Yet finding "mental space" is increasingly a problem because of fast pace of "modern" life. This is increased by greater demands for "productivity" and all the background noise from email, social media, and mobile phones. So how does one find the necessary "mental space"? I welcome suggestions. Here are mine. Turn off your email and/o

Kamran Behnia has published a book Fundamentals of Thermoelectricity Such a monograph is overdue. I think the topic is particularly important and interesting for several reasons. (This is illustrated by the fact that I have written almost 40 blog posts on the topic). The thermoelectric power is a transport property that presents a number of rich and outstanding puzzles. The sign, magnitude, spatial anisotropy, and temperature dependence of the thermopower can put significant constraints on theories because the thermopower is so sensitive to particle-hole asymmetry. In comparison, often it may not be too hard to cook up a theory can get a resistivity that agrees with experiment. However, the thermopower is another story. Thermoelectric materials are technologically important. Furthermore, if someone can find a material with a "Figure of merit" that is just twice that of the best current materials we could throw out all our refrigerators with moving parts! The book has

Some readers might be surprised to hear me claim this since I often highlight the problems and errors associated with calculations involving DFT. The problem is density functional approximations not the underlying theory. There are two key ideas associated with DFT. 1. A theorem. The ground state energy of an interacting electron gas is the minimum value of a unique functional of the charge density n(r) in the system. This is an exact result. The problem is that to determine the exact density and energy one needs to know the "exchange-correlation" functional. 2. An approximation. One can make a local density approximation (LDA) to the exchange-correlation functional so that the density is written in terms of a set of "orbitals" that are found by solving a set of self-consistent equations that have a mathematical structure similar to the Hartree-Fock equations for the same system. These distinct ideas are respectively associated with two different paper

A colleague once told me a story about his research field. "Ten years ago Professor X got some surprising experimental results. He then made bold claims about what this meant. Some people did not believe it. But, people then did detailed experimental and theoretical work to test his results and claims. They basically found that he was wrong but in the process they made some valuable and interesting discoveries and clarified several issues in the field. To half the people in the field he was a hero and to the other half he was a pariah ." The hero status was assigned because if he did not exist or had not made these claims, the new discoveries would not have been made (or might have been made much later). The pariah status was assigned because he did not do careful scientific work and misled people. How much credit should people get who open up new scientific directions  with “wrong” papers or with unsubstantiated speculation? Different people I talk to have quite dif

In quantum many-body physics quasi-particles are emergent entities. But, it is worth making a distinction between two cases. 1. Adiabatic continuity. As one gradually turns on the interactions the excited states of the system smoothly evolve from those in the non-interacting system. As a result the quasi-particles have the same quantum numbers and statistics as the constituent particles. The most prominent example is in Landau's Fermi liquid theory which describes elemental metals and liquid 3He. 2. Adiabatic discontinuity. The  quasi-particles do NOT have the same quantum numbers and statistics as the constituent particles. One example, is magnons (spin waves) in a spin-1/2 Heisenberg antiferromagnet. They have spin one and act like bosons. In contrast, the constituent particles are localised electron that are fermions with spin-1/2. An even more dramatic example occurs in the fractional quantum Hall effect. The constituent particles are electrons with charge -e and obey Fe

Andrew Zangwill contacted me because he is working on scientific biography of Phil Anderson. I think this is overdue. I would argue that Phil is the greatest theoretical physicist of the second half of the twentieth century. I would argue this on similar grounds to why I think Linus Pauling was the greatest theoretical chemist of the first half of the twentieth century. Crucially, their scientific legacies have extended far beyond condensed matter physics and chemistry, respectively. Specifically, Pauling did not just make essential contributions to our understanding of chemical bonding, x-ray crystallography, and quantum chemistry. His impact went far beyond chemistry. Francis Crick said Pauling was the "father of molecular biology." He proposed and elucidated alpha helices and beta sheets in proteins. Furthermore, he began the whole field of molecular medicine, by showing the molecular basis of a specific disease, sickle cell anemia. Phil Anderson has made incredibly d

In introductory science courses we try and instill in undergraduates the basic notion that any measurement has an error and you should estimate that error and report it with your measurement. Yet "Professors" who are in senior management don't do that. Today in Australia the results of the Excellence in Research Australia (ERA) ranking exercise were announced. Every research field at every university is given a score. A colleague wisely pointed out that given the ad hoc procedure involved all the rankings should include error bars. He conjectured that the error bar was about one. Hence, one cannot distinguish the difference between a 4 and 5. Yet, this is a distinction that university managers and marketing departments make a lot off. I think for almost all ranking exercises it would be quite straight forward for the compilers to calculate/estimate the uncertainty in their ranking. This is because almost all rankings are based on the average of rankings or scores pro

And why is it so elusive? Quantum many-body systems are characterised by many different energy scales (e.g. Fermi energy, Debye frequency, superconducting energy gap, Kondo temperature, ....). However, in many systems properties are "universal" in that they are determined by a single energy scale. This means that the frequency (omega) and temperature (T) dependence of a spectral function can be written in a form such as where here  T_ K is the relevant energy scale and I set hbar =1 and k_B = 1. However, what happens in the limit where the relevant energy scale T_K goes to zero, for example near a quantum critical point? Then the only energy scale present is that defined by the temperature T and we now expect a functional dependence of the form This is omega/T scaling. In one dimension the form of the scaling function is specified by conformal field theory and for quantum impurity problems (e.g. Kondo) by boundary conformal field theory. In 1989 Varma et al. sh

A first year undergraduate student who is deeply concerned about climate change asked me a number of questions by email and then came to my office to discuss them. Since I think they are excellent questions I thought I would post them here (with his permission) and give a brief version of my answers. I welcome readers to give their own answers. I am interested in and passionate about climate change. At the moment, I'm considering my uni options - wondering what I can study to best equip me to help in the great, global effort to mitigate (I'm a bit less interested in adaptation) climate change. I have a couple of questions to ask of you. 1. How would you respond to each of the following, somewhat contradictory statements:   - 'Climate change can be mitigated by developing and deploying renewable energy and energy efficiency technologies, without significantly impacting on our standard of living.'   - 'Environmental crises, including climate change, require u

At UQ we just had an interesting colloquium from Signe Riemer-Sorensen about Dark matter emission - seeing the invisible . Central to the talk was the data below. Focus on the red data around 3.6 keV. This has stimulated more than 100 theory papers! This reminds me of the faster than speed of light neutrinos and the 17 keV neutrino , 500 GeV particles seen by the Fermi gamma ray telescope,  BICEP2 "evidence" for cosmic inflation, .... The above data is discussed in detail here. I don't want to just pick on my astrophysics and high energy physics colleagues as this happens in condensed matter and chemistry too... remember cold fusion... think about periodic reports of room temperature superconductors! The painful reality is that cutting edge science is hard. One can be incredibly careful about noise, subtracting background signals, statistical analysis, sample preparation, .... but in the end there is Murphy's law .... things do go wrong .... and crap happ

I recently learned about the  Pareto principle , which according to Wikipedia "(also known as the 80–20 rule, the law of the vital few, and the principle of factor sparsity) states that, for many events, roughly 80% of the effects come from 20% of the causes." For example, if you are supervising a team of employees, 80% of your time will be spent in dealing with 20% of them, probably the mostly poorly performing or most vocal. This past year I have had a minor administrative role, as a "Research Committee" chair. Probably 80% of the time, involves co-ordinating, supporting, and assessing grant funding applications, both internal and external to the university. Most of these grant programs have success rates at the 10-20% level. Virtually none of my time is actually spent on initiatives to help improve the quality or quantity of research done by the bulk of faculty members. My experience has also made me more aware of what people in senior management appear t

UQ has just advertised for a new postdoc to work with me on a project, "The bad metallic state in quantum materials", funded by the Australian Research Council. The position is for 2 years and 9 months. Applications close on 31 January, 2016. The official advertisement and job description is here  and contains a link to a portal through which a formal application should be made. Looking at the "bad metals" label on the blog will give a flavour of some of the problems I am interested in. Looking at the "career advice" label will give some flavour of my philosophy and expectations of working together.

There is an interesting paper Quantum critical dynamics of a magnetic impurity in a semiconducting host Nagamalleswararao Dasari, Swagata Acharya, A. Taraphder, Juana Moreno, Mark Jarrell, N. S. Vidhyadhiraja The key physics of the Kondo model is the formation of a spin singlet state between the impurity spin and the spins of the electrons in the conduction band. We say, the impurity spin is “screened” by the spins in the conduction band. The "screening" electrons involved span from the Fermi energy up to some higher energy. The relevant energy scale is the Kondo temperature which depends in a non-analytic way on the density of states (DOS) at the Fermi energy, and is roughly the binding energy of the spin singlet. As the DOS goes to zero the Kondo temperature goes to zero. But, what if there is an energy gap at the Fermi energy, as in a semiconductor? One might expect that the Kondo effect disappears and the local moment is no longer screened. Specifically, is the

There is no doubt that universities and research institutions are becoming more bureaucratic. This is arguably from the increased demand for accountability and from the rise of the managerial class. This means more paperwork, more boring meetings, and more rules and regulations. How do we cope? Let me first give two extreme responses and suggest an alternative. John and Joan could be faculty, postdocs, or graduate students. 1. John is focussed on research and teaching. Afterall that is the mission of the university not all this bureacractic nonsense. Any emails from administrators are deleted. In fact he has placed a “block sender” on some. He never responds to requests to complete on line surveys, fire safety training, or annual reports. He does not attend departmental meetings. If forced to attend meetings he brings his laptop and catches up on email. Deadlines for reports, drafts of grant applications, and exam papers are missed. The only way he will complete an administrative

Today I am giving a seminar on Emergent Quantum Matter in the School of Physical Sciences at JNU . My host is Brijesh Kumar. Here are the slides. Last time I gave this talk, someone asked the tricky and controversial question "Is ferromagnetism an example of spontaneously broken symmetry?" Peierls said yes. Anderson says no. I previously discussed their exchange here. Aside. One thing I  enjoy about JNU is the very large posters that student political activists have placed on buildings. Many contain challenging quotations that are worth considering, such as this one.

There is a nice paper Plaquette-triplon analysis of a trimerized spin-1 Kagomé Heisenberg antiferromagnet  Pratyay Ghosh, Akhilesh Kumar Verma, Brijesh Kumar They consider an antiferromagnetic Heisenberg model on the Kagome lattice with three different interactions, J, J', and J'', shown below. The case J'=J and J''=0 is the regular Kagome lattice model. For J'=J''=0 one has isolated triangles for which the ground state is a singlet with an energy gap to three low-lying triplet states. [Aside: for a nice general treatment of such triangles (and tetrahedrons) see this paper] . This state is the starting point for an analysis using bosonic excitations corresponding the triplet excitations on the triangular plaquette. The calculated phase diagram is below.   One sees that turning on the inter triangle interaction J' has no effect on the quantum numbers or symmetry of the ground state. It remains a singlet, with no spontaneously brok

Today I am giving a seminar "Effect of Quantum Nuclear Motion on Hydrogen Bonding" in the Chemistry Department at IIT Delhi . My host is Charusita Chakravarty . On thursday I am giving a similar talk in a seminar  in the School of Physical Sciences at JNU (Jawaharlal Nehru University). There my host is Brijesh Kumar . Here is the current version of the slides.

Most undergraduate science curricula are essentially what they were fifty years ago. Furthermore, in Australia they are very specialised. Due to internal university politics and funding pressures departments actually design programs to discourage students from taking courses in other departments. For example, in one university that loves to promote itself as "world class" chemistry majors are not required to take any courses in physics and mathematics. How can you even do basic physical chemistry with only high school maths and physics? This specialisation is antiquated. Consider what science is like today. It is very multi-disciplinary. Furthermore, the vast majority of research, both pure and applied, involves biology or materials. Biology and medicine are becoming increasingly quantitative. Everything involves substantial use of computers and advanced instrumentation. Previously, I posted about one course every science undergraduate should take . But that is not enough,

The frequency dependence of the real part of the conductivity of a metal gives a lot of information, both qualitative and quantitatively. For example, one can extract a scattering rate and see if a Drude model is relevant. Hence, it is natural that experimentalists present “measurements” of this quantity. However, it is important to acknowledge that the conductivity is not directly measured; rather, the reflectivity or absorption of a thin film or single crystal. The real and imaginary parts of the conductivity are then extracted from a Kramers-Kronig analysis. This procedure is only stable and reliable if there is experimental data out to sufficiently high frequencies. Several experimentalists have privately told me this can be a can of worms. It is not clear how high a frequency cutoff you need and interband transitions can complicate things… Hence, one should be particularly nervous about people claiming exotica such as quantum criticality and non-Fermi liquid behaviour such

Previously I posted about how faculty members should respond to student evaluations of teaching.  There is an interesting article at Faculty Focus, that highlights the increasingly conflicting values and expectations between some faculty, students and parents. It’s Not Me, It’s You: Coping with Student Resistance by Nicola Winstanley. Nearly 20 years ago, Neil Postman warned in The End of Education that education was being replaced by “schooling,” a means whereby learning becomes deeply embedded in a capitalist structure that values knowledge only for its industrial utility. In other words, education is a means to an end—getting a job—rather than an ongoing process at the heart of culture. It’s within this context that some students have come to see education as no more than a deliverable—one that they have paid for dearly.   The fact that many students accept this paradigm is made evident every day in both their comments and behavior. For instance, some students may think

A blessing and curse is the easy and wide availability of powerful software for computational materials modelling: classical molecular dynamics, quantum chemistry, density functional theory based methods, … Although easy to use, interpreting the results, and establishing their robustness and reliability can be subtle and challenging. Any computation requires the user to make many choices from an alphabet zoo. For example, for classical molecular dynamics simulations of water there is a multitude of force fields (TIP3P, SPC/E, TIP4P-D, …). For density functional theory (DFT) one has to choose between LDA, GGA, and  different density functionals (B3LYP, PBE, ….). For plane wave approaches one must choose the energy cutoff. For quantum chemistry of molecules one has to choose the basis set (STO-3G, cc-pVDZ -Double-zeta,... ) and the level of theory for treating electron correlations (HF, MP2, CCSD, CAS-SCF, …) If one does combined quantum-classical simulations (e.g. for a chromop

The next two days I am visiting the P hysics Department at IIT Kharagpur . My host is Arghya Taraphder.  I am giving my regular talk on "Emergent quantum matter". Here is the latest versions of the slides. I have given this talk about half a dozen times now. Yet last time I gave it I realised there was a significant typo in the formula for the Hall resistance of the Fractional quantum Hall effect. It is amazing that neither I nor anyone in my audiences caught this typo before. I am not sure what that says...

Two months ago I made this claim . I made some specific suggestions as to how one could quantitatively analyse the experimental data to support the claim. The same day of my post I received an email from Zhili Xiao with a copy of a submitted manuscript that had already done exactly what I suggested. The paper has now been published: Origin of the turn-on temperature behavior in WTe2  Y. L. Wang, L. R. Thoutam, Z. L. Xiao,  J. Hu, S. Das, Z. Q. Mao, J. Wei, R. Divan, A. Luican-Mayer, G. W. Crabtree, and W. K. Kwok Below I show the relevant Kohler plot. This is consistent with the simple idea that the origin of the magnetoresistence is simply the Lorentz force, the same as in elemental metals such as copper and zinc! No exotic physics is required.

The schematic picture below shows the evolution of the spectral line of an OH stretch mode in a hydrogen bonded system as the donor acceptor distance R changes. Not only does the mode frequency significantly red shift but the spectral intensity, line width and line shape changes significantly. The figure is taken from a helpful (short) review from 1991 by S. Bratos, H. Ratajczak, P. Viot The redshift with increasing bond strength (and decreasing R) is quantitatively described and explained here.  I am currently using the same model with collaborators to describe the increase in spectral intensity (by up to two orders of magnitude). The problem of the line width and the line shape is more difficult and controversial. I discussed some related issues previously  here  and here.

Every grant and fellowship program comes with many pages of rules, regulations, guidelines, criteria, ... How carefully should you read them? What can you learn from them? I have two separate points. First, as the applicant it is your responsibility to read and to take note of these regulations. This is time consuming and boring. But it is still your responsibility . Don't expect or demand someone else to do it for you. It is not the responsibility of your supervisor, department chair, local research administrator, group secretary, or the funding agency. Don't ask questions when the answers are there in the rules if you actually read them. And don't ignore the rules. For example, if it clearly states you have to be 5 years past your Ph.D, don't apply anyway if you are 3 years past your Ph.D. This may seem inane to some readers, but it does happen. Also, just because it says you can do something does not meet it is a good idea. For example,  with regard to budget r

Today I am giving a seminar "Emergent quantum matter" in the School of Physics at Central University of Hyderabad . Here are the slides. Next tuesday I am giving a similar talk at the new Hyderabad branch of the Tata Institute for Fundamental Research. 

Tomorrow in the School of Chemistry at Central University of Hyderabad I am giving a talk, "Effect of quantum nuclear motion on hydrogen bonding". Here are the slides. My host is Susanta Mahapatra.

Last week I had a helpful discussion with Arindam Ghosh about this recent paper Conductivity noise study of the insulator-metal transition and phase coexistence in epitaxial samarium nickelate thin films  Anindita Sahoo, Sieu D Ha, Shriram Ramanathan, and Arindam Ghosh The abstract states The normalized magnitude of noise is found to be extremely large, being nearly eight orders of magnitude higher than thin films of common disordered metallic systems , and indicates electrical conduction via classical percolation in a spatially inhomogeneous medium.   The higher-order statistics of the fluctuations indicate a strong non-Gaussian component of noise close to the transition, attributing the inhomogeneity to the coexistence of the metallic and insulating phases.  The figure below shows how the non-Gaussian component [measured by the kurtosis ] increases dramatically as the temperature decreases below the metal-insulator transition. Some of the fundamental and related que

Skype and similar teleconferencing tools are great. I use them regularly to keep in touch with family while travelling and occasionally to talk to collaborators. It is much better than the phone and way better than exchanging emails. There are some techno-enthusiasts who claim we don't need to have conferences anymore because we can do it all on line and save lots of money. In companies there are those who push for tele-commuting and only have a central office with hot desks that people occasionally use. I have even heard of Australian universities who have hired faculty members from overseas solely based on a Skype interview! I think such enthusiasm is a big mistake. I have been at a few conferences where some "big shot" did not attend but gave their talk via tele-conferencing. It really wasn't the same as having them in the room. Even without technical glitches, it was not very engaging. Once I was even at a social event where a couple "attended" via S

Science education in schools in the Majority World faces many challenges including lack of resources, poorly trained teachers, and a fixation on rote learning from textbooks . Even at “good” schools students rarely ever do experiments or hands-on demonstrations. The focus is on preparing standard answers for exam questions. One recent big change in school education in the Majority World is the proliferation of low-cost private schools, even in extremely poor communities. Most of these are English medium. A recent cover story in The Economist chronicled this development. When visiting India, I enjoy reading The Hindu newspaper each day. I think the quality of journalism and the substance of the issues covered is much higher than most Western newspapers. More than once a week there is an op-ed piece or article about the problems with school education. Topics covered include the stifling of critical and creative thinking , the lack of autonomy given to teachers  by all-knowing and co

Tomorrow I am giving a seminar in the Physics Department at the Indian Institute of Science in Bangalore. The talk "Emergent states of quantum matter" is similar to the one I gave two weeks ago at JNCASR.  Then an interesting question was raised. "There are two complementary pictures of the quasi-particles in the Fractional Quantum Hall Effect: composite fermions and fractionally charged anyons. Can one explicitly show they are equivalent?" I am not sure. One can certainly show that the overlap of the relevant variational wave functions, Laughlin's and the composite fermion ones, is significant and that for small systems that the overlap of both of these wave functions with exact numerical wave function. However, that "black box" proof is not quite the same as establishing "adiabatic continuity" between the two different representations. Has anyone explicitly done that? I welcome other answers to this question.

This past northern summer I was on vacation on Lopez Island in Washington state [near my wife's hometown]. While browsing in the bookstore I came across this fantastic "coffee table" book The Elements by Theodore Gray  (co-founder of Wolfram Research). There is a page for each element with a fascinating description and beautiful photos. A sample is above. You can view all the pages on the book webpage. I bought a softcover version for US$25. I think it is important to support local bookstores, particularly given the vagaries of Amazon.  I was thinking that it would be really nice if there was also a book about molecules, since they are a lot more interesting than atoms. A week later I was in the U of Washington bookstore and I came across Molecules by Theodore Gray! He has also developed some fancy mobile phone apps. Now the hardcopy cost me US$15. This tells you something else about bookstores... I am also told that some young kids love these boo

Sometimes I give talks about science to high school students and to community groups, mostly churches. Recently I showed this one. Besides the "wow factor" I think it is valuable because it demonstrates some very basic but profound and important points about science. 1. Common sense observation and experience can be misleading. 2. Consequently, nature appears sometimes to be counter intuitive. 3. The way to discover the way things really are is by doing experiments. One can explain the historical significance of this experiment. Aristotle advocated basing science on common sense observations [heavy objects fall faster, objects that start their motion eventually slow down unless a force is applied to them, objects on earth move in a qualitatively different manner to those in the heavens, ....]. In contrast, Galileo went against this and did real experiments, dropping two balls of different mass [probably not from the leaning tower of Pisa]. This can also lead to