Condensed concepts

Today I gave my PHYS4030 Condensed Matter Physics students a Pre-test , a 50 minute exam to test whether they have some of the background knowledge and skills I consider necessary for the course. I am pretty happy with the test I came up with as it tested some basic skills as well as knowledge. But I am sure it can be improved. I welcome comments on it and examples of alternatives. I marked the exams and gave them back to the students. I did not keep a record of the marks and they don't count at all for the final grade. In case some students felt they did poorly they can take the exam home, do some review, and then submit a new set of answers I will mark. Most of the marks were in the 50-85% range, which suggests to me the test was set at about the right level. Although, of course, I really wish they were all scoring above 80%. A few random thoughts: 1. I think this exercise also has value as a bit of a reality check and wake-up call at the beginning of the semester.

The new semester has started here and I am helping teaching PHYS4030 Condensed Matter Physics. We basically cover some fraction of Ashcroft and Mermin. For some students, particularly those with weak backgrounds, this is a difficult course. Some find it much harder than other courses. Why? I can think of a few reasons of why CMP can be more demanding than other courses. It requires a working knowledge of basic thermodynamics, statistical mechanics, kinetic theory, electromagnetism, and quantum mechanics. Weak students do o.k. in some subjects but poorly in others. Weak students do not remember much from previous courses and struggle to apply what they do learn in new contexts. Hence, CMP really exposes some of these weaknesses. Furthermore, one has to understand how to integrate all this knowledge. There is an emphasis on orders of magnitude estimates  making approximations relating theory and experiment On the other hand, for these same reasons motivated and well prepared

The Journal of Chemical Physics website has a fascinating podcast with Roald Hoffmann, Neil Ashcroft, and Vanessa Labet talking about a series of 4 papers they have just published about "molecular" hydrogen under pressure. They illustrate some very rich and subtle physics and chemistry. It highlights the importance of both physical and chemical insight, simple models, and how there are still these old problems waiting to be solved.

There is an interesting article  by Ben Goldacre in the Guardian newspaper which nicely summarises research on the following questions: Will asking a question in the title get your paper cited more? No. But it will be downloaded more! What is the evidence that having your paper mentioned in the New York Times will increase  its citation rate?

Jure Kokalj, Nigel Hussey, and I have just completed a paper, Transport properties of the metallic state of overdoped cuprate superconductors from an anisotropic marginal Fermi liquid model . We show how a relatively simple model self-energy [considered earlier in this PRL ] gives a nice quantitative description of a wide range of experimental results on Tl2201 including intra-layer resistivity, frequency-dependent conductivity, and the Hall resistance. No new parameters are introduced beyond those needed to describe angle-dependent magnetoresistance experiments from Nigel's group. One thing I found striking was just how sensitive the Hall conductivity is to anisotropies in the Fermi surface and the scattering rate [a point emphasized by Ong  with his beautiful geometric interpretation]. We also show that our model self-energy successfully describes both the resistivity (with a significant linear in temperature T dependence) and the Hall angle ( ~T^2) without invoking exotic n

The observed sensitivity of strongly correlated metals to laboratory magnetic fields of the order of 5-50 Tesla presents a significant theoretical puzzle and challenge. There have been very few calculations on lattice models such as the Hubbard model in a magnetic field. The few calculations that have been done only see very small perturbative effects on the scale of laboratory fields. They require huge magnetic fields of the order of a thousand Tesla for any significant effect, such as a change in ground state. In terms of coupling of the magnetic field to the orbital degrees of freedom, most studies have been on ladder models (e.g. this PRB ), at zero temperature, and only see significant effects at extremely high fields, of the order of thousands of Tesla, when there is a quantum of magnetic flux through a single lattice plaquette. The smaller field scale of the upper critical field for superconductivity corresponds to the longer length scale of a superconducting coherence length.

Different metrics claiming to measure research impact, such as the h-index, are receiving increasing prominence in grant and job applications. I have written before that I think they have some merit as a blunt instrument to filter applications, particularly for people at later career stages. However, I am noticing an increasingly silly trend to cite a whole range of metrics, where I think one or two  (probably the h-index and m-index=h-index/no. of years since Ph.D) would suffice. I have seen not just a paragraph, but a whole page! of analysis citing all sorts of metrics [e.g. comparing an authors citation rate for a particular journal to the impact journal factor, no's of papers with more than 50 citations, citation rate relative to others in the field, .... the list goes on and on...]. Don't people have better things to do with their time? In the end it becomes like university rankings. Every university seems to cite the one in which they rank the highest.

Following my recent post An end to a frustrating materials search , Radu Coldea brought to my attention a 2011 PRL  which reports thermodynamic studies of the analogous Cu compound, Ba3CuSb2O9. The authors suggest this material is also described by the antiferromagnetic Heisenberg model on the isotropic triangular lattice. They estimate the exchange interaction J ~ 32 K, but observe no magnetic ordering down to 0.2 K. This leads the authors to suggest the compound has a spin liquid ground state. It should be stressed that this absence of magnetic ordering is inconsistent with the material being described by a model with purely nearest neighbour coupling. They observe a large Schottky anomaly in the specific heat and suggest that 5% of the Cu2+ ions are on Sb sites. It is not clear to me -what affect these large impurity contributions have to the subtractions which are performed to get the pure lattice magnetic contribution to the susceptibility and specific heat. -whether the di

The Weekend Australian newspaper had an interesting article about the success of Asian high schools [particularly Shanghai, Hong Kong, Korea, and Singapore]. The article is largely based on a recent report from the Grattan Institute , an independent Australian think tank. One thing the report emphasizes is the value of teachers getting feedback from their colleagues (and other expert classroom observers) about their teaching. At UQ we are increasingly being required/encouraged/expected to do this.

The Heisenberg model for anti-ferromagnetically coupled spins on an isotropic triangular lattice is one of the most widely studied (and mentioned) lattice models in quantum many-body physics. This is in spite of the absence of any actual materials described by it! This incredible theoretical interest was stimulated by Anderson's 1987 Science paper on an RVB theory for cuprate superconductors. He invoked his earlier work with Fazekas suggesting that the model had a spin liquid ground state. This turned out to be incorrect: the model does exhibit magnetic ordering and spontaneously broken symmetry, just like conventional antiferromagnets. However, the materials drought may be over. There is a recent PRL,  Experimental Realization of a Spin-1/2 Triangular-Lattice Heisenberg Antiferromagnet by Yutaka Shirata, Hidekazu Tanaka, Akira Matsuo, and Koichi Kindo They report magnetisation measurements on Ba3CoSb2O9, and argue that the magnetic Co2+ ion has a spin 1/2 [Kramer's doubl

The graph below shows the temperature dependence of the Nernst signal measured in the normal metallic state of a family of electron doped cuprates Pr_{2-x}Ce_xCuO_4. It is taken from a 2007 PRB by Li and Greene. A few noteworthy features -the signal is proportional to B and so not due to superconducting fluctuations -the signal is proportional to temperature at low temperatures but has a non-monotonic temperature dependence -the magnitude of the linear temperature dependence is an order of magnitude smaller than predicted by the simple quasi-particle theory of Behnia. The authors consider how the data can be explained by a two-band with both electrons and holes, but point out such a model is inconsistent with the single hole Fermi surface seen in ARPES. It would be interesting to re-consider this data in light of the recent experiments on these materials which showed a linear temperature dependence of resistivity (and thus a quasi-particle scattering rate) with a magnitude prop

There is an interesting editorial by Roald Hoffmann, What, Another Nobel Prize in Chemistry to a Nonchemist ? in the latest issue in Angewandte Chemie International.  Hoffmann, thoughtfully argues that chemists should not be upset [some are] that the chemistry prize seems to be increasingly awarded to people from outside chemistry departments [esp. biochemistry and molecular biology, but also physics and materials science]. He also asks the interesting question: will a chemist ever win the Nobel Prize in physics? He argues that Bednorz and Muller who discovered superconductivity in cuprate compounds might be considered chemists. I don't buy that. Their education, employment, and publications were clearly in the physics. I welcome possible answers to Hoffmann's question. My answer might be: in principle, yes; but in practice no. I think this may be partly because of the arrogant reductionism of influential parts of the physics community. Possible areas impacting physics

Today's Australian newspaper has an article on page 3, `Campaign' targets depression guru Ian Hickie . Hickie is a psychiatrist who is the Australian governments Mental Health Commissioner and has influenced significant changes in government policy. My limited perspective is that many of these changes are positive. They have helped reduce social stigmas associated with mental health problems and made treatments more accessible. However, a major beneficiary of these changes have been drug companies . Furthermore, there are questions about whether anti-depressants are being over-prescribed and cognitive therapies are being under-valued, both to the detriment of patients best interests. Hickie is being criticised because he published a paper in The Lancet commending a drug produced by a company that he had financial and grant ties to. The methodology and conclusions of the paper are being criticised. I believe that when there are large amounts of money (company profits a

In an early post I argued against experimentalists feeling a compulsion to present a theoretical "explanation" for their data. I should have also expressed concern about how in a similar vein many theory papers waffle on about how the calculations presented are relevant to specific experiments. Reading through More and Different , I found an interesting Physics Today column that Phil Anderson wrote in 1990, Solid State Experimentalists: Theory should be on tap, not on top. He argues for re-instatement of Cornelius Gorter's rule that theory and experiment should be published in separate papers. It is quite possible that they will both not be correct. Much more serious is the distortion of priorities, of communication and of the refereeing process that occurs when excessive weight is given to theoretical interpretation. We don't want to lose sight of the fundamental fact that the most important experimental results are precisely those that do not have a theoretic

Bigger is not always better! Previously I posted about Pauling points in Quantum chemistry: as one increases the size or sophistication of a calculation [e.g. through a larger basis set, a higher order or perturbation theory, or a larger active space] one does not necessarily get closer to the correct answer. Sometimes the quality of results degrade with increasing the number of degrees of freedom. My colleague, Seth Olsen has just published a nice paper in J. Phys. Chem. A,  A Four-Electron, Three-Orbital Model for the Low-Energy Electronic Structure of Cationic Diarylmethanes: Notes on a ‘Pauling Point’ . For the specific case of the dye Michler's Hydrol Blue he systematically increases the size of the active space and observes the effect on the two lowest lying excited singlet states. One has to increase the active space from 3 orbitals to 15 to before one recovers the same transition difference densities. This means increasing the number of Slater determinants by 6 orders

The issue of scientists disgruntlement with commercial publishers has made it into The Economist. The excellent article The Price of Information  describes a campaign started by mathematicians to boycott Elsevier journals. I thank Seth Olsen for bringing the article to my attention.

There is a very interesting preprint Dynamical particle-hole asymmetry in the cuprate superconductors  by Sriram Shastry. Most theories of the metallic state (including Fermi liquid and marginal Fermi liquid theories) assume/assert/embody that adding electrons or holes to the ground state requires the same amount of energy. In a similar vein quasi-particle peaks in spectral functions should be symmetric. There are two important exceptions to theories which assert dynamical particle-hole symmetry: The hidden Fermi liquid theory of Anderson The extremely correlated Fermi liquid theory of Shastry Shastry points out that there is already some experimental evidence for asymmetry. The figure below [taken from a Science paper by Pasupathy et al.  about a different issue] shows the differential conductivity [roughly proportional to the local density of states] from an STM measurement on an overdoped sample of BSCCO. The relevant point is that the background has a significant slope. If

Physicists might think Crystal Violet is just some obscure dye, which has been used at times to treat sexually transmitted diseases. The dye is not only of industrial and historical significance but still presents some fundamental scientific questions. In vacuum the molecule has three possible Lewis structures (valence bond structures). One  is shown above. The others involve the positive charge on one of the two other nitrogen atoms. The molecule should have D_3h symmetry and the ground state should be non-degenerate and the first excited state two-fold degenerate. However, in 1942 G.N. Lewis  pointed out that the optical absorption spectrum showed a shoulder and suggested that at room temperature two structural isomers may be accessible. A fascinating article,  Crystal violet’s shoulder  by Scott Lovell,  Brian J. Marquardt and Bart Kahr, reviews how the question of whether the shoulder arose from two "ground" states or two close excited states was still not resolved

There is a nice JACS paper  Population Branching in the Conical Intersection of the Retinal Chromophore Revealed by Multipulse Ultrafast Optical Spectroscopy by Goran Zgrablić, Anna Maria Novello, and Fulvio Parmigiani. It looks at how the polarity of a solvent changes the outcome of the photo-isomerisation (trans-cis) reaction of a retinal molecule. A similar reaction is responsible for vision: it seems that the protein environment is key to the speed and selectivity of the reaction. This experimental study finds that as one increases the polarity of the solvent (as measured by the static dielectric constant) the branching ratio for the blue to green/purple curves in the figure above increases. This is argued to be broadly consistent with a theoretical study published earlier this year,  Dynamical Friction Effects on the Photoisomerization of a Model Protonated Schiff Base in Solution, by João Pedro Malhado, Riccardo Spezia, and James T. Hynes. A key insight seems to be the

The two faces of chemistry in the developing world is an interesting Commentary in Nature Chemistry by the legendary C.N.R. Rao. While acknowledging the great advances that India, China, and Brazil have made the past few decades he stresses the incredible challenge to the least developed countries. There is one statement in the article I disagree with. He states: I do not know of a single institution in India that is comparable to any of the best institutions in the advanced countries.   I think that, at least in condensed matter physics and chemistry (physical, theoretical, materials), I would rank the Indian Institute of Science (IISc) in Bangalore, ahead of most Australian universities. Some of my Indian colleagues tell me this strength is largely a legacy of C.N.R. Rao. So perhaps he is being modest about his contributions.  Unfortunately though this favourable comparison also exposes the weakness of Australian universities.... 

I am rather confused by a Nature News and Views Why the temperature is so high? by Jan Zaanen. A key claim in the article is that in quantum systems there is a fundamental limit to the "dissipation time"  h/(2 pi k_B T). ( An earlier post on this article gives more background). Here are some of Zaanen's claims: In fact, according to the laws of quantum physics, it is impossible for any form of matter to dissipate more than these metals do.... the laws of quantum physics forbid the dissipation time to be any shorter at a given temperature than it is in the high-temperature superconductors. If the timescale were shorter, the motions in the superfluid would become purely quantum mechanical, like motion at zero temperature, and energy could not be turned into heat. In analogy with gravity, this timescale could be called the 'Planck scale' of dissipation (or 'planckian dissipation').  It is not clear if these statements concern dissipation of the energy

For the past ten years I have been involved in teaching a course "Thermodynamics and Condensed Matter Physics" to second year physics majors. In the past I have posted some of lectures which cover the second half of the course, applying thermo to condensed matter. I did not design the course but inherited it. [For department historical/political/personal reasons the condensed matter part was actually originally crystallography!] I quite like the course because I believe (unlike many people) that students should learn and master macroscopic thermodynamics before they learn statistical mechanics. Entropy should be introduced as a measure of the relative accessibility of the equilibrium (macroscopic) states in an adiabatically isolated system (i.e. irreversibility) rather than as related to the possible number of microstates in an system. However, finding a textbook is not easy. Students find Callen , the one originally used for the course, too difficult, and it is very expe

" Is this [publication in high profile journals], then, an efficient way to marshal evidence in support of grant proposals, appointments, promotions or fellowships? Of course not - it is madness.  The time is overdue to abolish journals and reorganise the way we do business" David Mermin, Publishing in Computopia, Physics Today, May 1991 [This was written a few months before the arXiv started. See also the followup column, What's wrong in Computopia? in April 1992 which was after the arXiv started] Given the way the world is today, rather than one hundred years ago, if we wanted to design the most effective way to promote good science would we come up with the current system of scientific journals? Keep in mind the following: Journals consume vast amounts of resources including - as much as 60 per cent of the total budget of many university libraries   [see Academic publishers make Murdoch look like a socialist ]. - the time of referees and editorial board m