Condensed concepts

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.