Condensed concepts

Earlier this year Jure Kokalj brought to my attention an interesting PRL Quantum Critical Transport near the Mott Transition by H. Terletska, J. Vučičević, Darko Tanasković, and Vlad Dobrosavljević. My interest in this paper increased this week when Vlad emailed me to tell me about a recent talk at KITP by Kazushi Kanoda. The right side of the slide below [click on it to see it larger] shows a scaling analysis of the temperature and pressure dependence of the resistivity of the organic charge transfer salt kappa-(ET)2(CN)3 near the pressure driven Mott transition. This scaling analysis is based on the theory in the PRL. The left side shows the Dynamical Mean-Field Theory (DMFT) results [for a Hubbard model at half filling] in the PRL. The top shows the scaling of the resistivity curves and the bottom the T vs. U phase diagram where the yellow region is the "quantum critical" region above the Mott transition. It is striking that the experimental curves involve sc

Physics Today has a fascinating review by Naomi Oreskes of the book The Hockey Stick and the Climate Wars by Michael Mann. In the Print edition the review is entitled, "A call to throw caution to the wind". I suggest reading the one page review in full with two questions in mind: 1. How would you personally cope with the level of public scrutiny (and attack) that Mann was subjected to following publication of his 1998 Nature paper? e.g., having emails and grant applications subject to subpoenas from politicians. The Wikipedia page on Mann  describes the many investigations he has been subjected to. 2. Is it really possible for non-scientists to understand and realistically evaluate the evidence for and against scientific hypotheses concerning complex systems? Oreskes seems to claim not in this political context and suggests Mann should not have been so cautious with his public pronouncements about ambiguities and subtleties in the data. If people post their ans

We now have significance experimental evidence which should end any debate about whether the superconductivity in organic charge transfer salts is unconventional. There is a nice preprint In-plane superfluid density and microwave conductivity of the organic superconductor κ-(BEDT-TTF)2Cu[N(CN)2]Br: evidence for d-wave pairing S. Milbradt, A. A. Bardin, C. J. S. Truncik, W. A. Huttema, P. L. Burn, S.-C. Lo, B. J. Powell, D. M. Broun They provide definitive measurements of the temperature dependence of the superfluid density and of the quasi-particle scattering rate. What is distinctly new and exciting about these very careful and precise measurements is the absolute determination of these quantities. The results also highlight some of the similarities of the organics with the cuprates: a d-wave superconductor "derived" from an antiferromagnetic Mott insulator and that quasi-particle scattering is largely due to electron-electron interactions. Indeed, the key temperatu

There is a video clearly explaining the case for Open Access to research journals. It is illustrated by Ph.D comics.  I thank Tony Wright for bringing it to my attention. A previous post The insatiable greed of commercial journals discussed some of relevant political background in the USA. To curtail the influence the role of commercial journals, I think the least we can do is aim to mostly publish in journals run by professional societies such as American Physical Society and the American Chemical Society. I also think research groups should be putting their "raw" research data sets online so others can analyse them independently. I welcome comments.

Many authors like to make statements such as "our calculated value of 1.23 for X is in excellent agreement with the experimental value of 2 for X". In contrast, this week I read the modest statement "the DMFT value of 0.66 is in mediocre agreement with the Brinkman-Rice value of 0.47" in this review [below eqn. (266)]. I think the quality of agreement in such comparisons is in the eye of the beholder. Different people can have quite different standards. Furthermore, the level of agreement one might hope for depends strongly on the context and many other factors. e.g., how many free parameters there are, how difficult it is to calculate or measure the relevant quantity, and how sensitive the calculated quantity is to the level of theory. Let me illustrate with two statements a hypothetical author might write in the future: 1. "my new formulation of renormalisation in quantum electrodynamics (QED) leads to value for the g-factor of the electron that a

In the cuprate high-Tc superconductors it is now well established experimentally that the superconductivity has d_x^2-y^2 symmetry with nodes in the energy gap along the Brillouin zone diagonals. This is what is predicted by spin fluctuation and RVB theories. In the organic charge transfer salts the experimental situation is not as clear. There are clear theoretical predictions of unconventional superconductivity with nodes in the gap. Hence, I was interested to read the paper: Location of gap nodes in the organic superconductors kappa-(ET)2Cu(NCS)2 and k appa-(ET)2Cu[N(CN)2]Br determined by magnetocalorimetry L. Malone, O.J. Taylor, J.A. Schlueter, and A. Carrington This experiment is difficult [measuring the specific heat at low temperatures while rotating a magnetic field within the layers] and its interpretation is subtle. The conclusion is that the nodes are as shown below. This is where they are predicted to be by both spin fluctuation and RVB theory. (A nice article by B

Physics Today recently published an excellent commentary Too Many authors, two few creators  by Philip Wyatt. It is worth reading in full. He compares the number of authors per paper in 8 major journals in 1965 and 2011. The number of single authors has plummeted while the number of papers with 4+ authors has dramatically increased. In contrast, the number of authors of patent applications has remained steady. Wyatt argues that this reflects a decrease in creativity. This is all caused by the pressure for everyone to have large numbers of publications and citations to survive in science. He asserts that many authors have not made significant scientific contributions to a paper and so should only be in the acknowledgements section. He sings the praises of single author papers, as I do. Why has not the number of authors on patent applications increased? If a listed inventor, or “innovator,” did not actually contribute to the invention, the issued patent will be void if such de

Like most scientists I don't think I have a very balanced or healthy or disciplined diet. It has changed over the years, both with seniority and the internet. I tend not to work on highly fashionable topics and so don't feel I have to look at the arXiv each day or the Table of Contents of journals. I hope (and expect) that postdocs or collaborators will bring to my attention particularly noteworthy recent papers or articles. I greatly appreciate it when they do. Colleagues also kindly send me some their recent work that they think may be of interest. I appreciate this and unlike some do not begrudge it as shameless self-promotion. On the other hand I don't appreciate being sent stuff which is of marginal or no relevance. A lot of stuff I need I find with Google Scholar using carefully chosen word searches. About 15 years ago, I believe I once heard Doug Stone claim that one thing he learnt from Patrick Lee was that he did not need to keep up with journals. Why? If the

This week we am part of new reading group which aims to work through Hewson's beautiful book The Kondo Problem to Heavy Fermions . Why the choice of this topic? Is it only of interest and relevance to people working on the Kondo physics and/or heavy fermion compounds? I would say definitely no! The Kondo problem represents a very important paradigm in quantum many-body physics. Perhaps the other main (well established and accepted) paradigms are Fermi liquid theory, BCS theory of superconductivity, Mott insulators,  spontaneous symmetry breaking quantum antiferromagnets (Heisenberg models and Anderson superexchange), Fractional Quantum Hall effect Asides: 1. Perhaps Mott insulators should not be on the list because there is no well-accepted theory beyond the "zeroth-order approximation". 2. Two other paradigms that I believe will eventually be accepted are Dynamical Mean-Field Theory and the RVB theory for superconductivity in proximity to a Mott insulator

A week ago I gave a talk "The Higgs Boson: scientific reality vs. media hype" at the Centre for Science, Religion, and Society at Emmanuel College at UQ. You can see the slides on my other blog.

Sometimes I get asked, "Why do you do it?" "How much time does it take?" It does require significant time. I estimate I spend an average of 30-60 minutes per post and I try to make one post per weekday. But, why do it? 1. It is fun. Why? Probably because I like learning and understanding things. If I have to write something concrete about a paper or an idea then I am forced to understand it better and more carefully think through the basis of my opinions. 2. It saves time. -On career advice I don't have to keep repeating myself to students and postdocs. I can just refer them to posts on the relevant topics. -I (or a reader) may detect mistakes in my thinking, that might have gone undetected for a period, wasting time. -blog=Web log=diary=note book. It is a good source of notes. Searching old posts I can find information and ideas I have already forgotten about or might be lost otherwise. -hopefully it is a useful source of information about me

There is a really nice preprint How bad metals turn good: spectroscopic signatures of resilient quasiparticles by Xiaoyu Deng, Jernej Mravlje, Rok Zitko, Michel Ferrero, Gabriel Kotliar, and Antoine Georges They study a doped Hubbard model using Dynamical Mean-Field Theory (DMFT). Although the ground state is a Fermi liquid this is only a good description at very low temperatures. In particular, the quadratic temperature dependence (characteristic of a Fermi liquid) only occurs below a temperature of about 0.05 delta D [where delta=doping and D=band width]. But, well-defined quasi-particles still exist all the way up to the "bad metal" region at which the mean-free path is comparable to the lattice constant. I found this resilience of quasi-particles somewhat surprising [I am not quite sure why] and interesting. Furthermore, in this intermediate temperature regime there is large entropy and significant local magnetic moments, and Kelvin's formula gives a good descr

Tony Wright and I just finished a paper Quantum oscillations and Berry's phase in topological insulator surface   states with broken particle-hole symmetry Quantum oscillations [e.g., Shubnikov de Haas] can be used to determine properties of the Fermi surface of metals by varying the magnitude and orientation of an external magnetic field. Topological insulator surface states are an unusual mix of normal and Dirac fermions. Unlike in graphene and simple metals, Berry's geometric phase in topological insulator surface states is not necessarily quantised. We show that reliably extracting this geometric phase from the phase offset associated with the quantum oscillations is subtle. This is especially so in the presence of a Dirac gap such as that associated with the Zeeman splitting or interlayer tunneling. We develop a semi-classical theory for general mixed normal-Dirac systems in the presence of a gap, and in doing so clarify the role of topology and broken particle-hole

For the cuprates an outstanding question concerns whether there is a connection between the pseudogap and superconducting states. The same question arises in organic charge transfer salts, although it has received little attention. For the organics evidence for the pseudogap is rather indirect, mostly coming from NMR, which shows a reduction in the Knight shift and the relaxation rate below about 50 K. There is no direct evidence of nodes in the pseudogap. A recent PRB presents evidence from ultrasound measurements that there is an intimate connection between the two. Symmetry-imposed signatures at the pseudogap crossover in κ-(BEDT-TTF)2X organic superconductors by Mario Poirier, Maxime Dion, and David Fournier The figure below shows the temperature dependence of different elastic moduli near the temperature at which the pseudogap opens. A post from last year considered in detail an earlier paper by the same group [plus their theory colleague A.M. Tremblay] that considered t

I am not working on the new iron pnictide superconductors and so am only casually following the field. Here is one recent paper that got my attention. Nodal versus Nodeless Behaviors of the Order Parameters of LiFeP and LiFeAs Superconductors from Magnetic Penetration-Depth Measurements K. Hashimoto, S. Kasahara, R. Katsumata, Y. Mizukami, M. Yamashita, H. Ikeda, T. Terashima, A. Carrington, Y. Matsuda, and T. Shibauchi The temperature dependence of the London penetration depth [which determines the superfluid density] is strikingly different for LiFeP and LiFeAs. This suggests that for the former there are nodes in the superconducting gap. Furthermore, they correlate the presence of nodes in the superconducting energy gap with  the height of the pnictogen atoms (Pn=As or P) above the plane of the Fe atoms in the crystal structure.

A few previous posts about BECs  in dilute atomic gases show that I am at times skeptical about some of the claims made by members of the cold atom community. Those posts also generated some interesting and worthwhile comments. Yesterday I endured an irritating seminar about realising spin-orbit coupling, topological superconductors, and Majorana fermions in cold atom systems. It was claimed that all of the problems and ambiguities with observing these effects in condensed matter systems could be solved in cold atom systems. I wish this were true. However, it seemed to me that the complexities and challenges associated with the speakers proposed cold atom realisation was just as great. The speaker made the fundamental mistake, never offer undefendable ground. I mention this because I have heard several cold atom talks (and reviewed grant applications) like this. Basically, there is a lot of hubris and hype. There is also ignorance of the existence of standard condensed matter tech

I really like the award of the 2012 Physics Nobel Prize to Serge Haroche and David Wineland. ( I predicted  entanglement but not these individuals). Their studies have made theoretical ideas about quantum decoherence and entanglement actually testable in the lab. A nice summary of the scientific background is here. One thing the award highlights to me is the high value (return on investment) of table-top science . I would guess that Haroche and Wineland's annual research budgets would be less than one million dollars.  This is to be contrasted to How much does it cost to find a Higgs boson? (about 10 billion dollars). This award also highlights the incredible and enviable track record of NIST, who have received 4 Nobels in the past 15 years (Bill Phillips, 1997; Eric Cornell, 2001; John Hall, 2005; Phillips, 2012). The former 3 recently testified to the US congress that NIST management style was key to their success. Table top science is relatively cheap. It is the most cos

Understanding the origin of the pseudogap state in cuprate superconductors (and organic charge transfer salts) remains a challenge. It has now been shown that within a doped Hubbard or t-J model one can produce a pseudogap like state when the model is treated at the level of the Dynamical Cluster Approximation. The DCA is a generalisation of Dynamical Mean-Field Theory (DMFT) to small clusters. Although this is a significant advance it is still somewhat at the level of a "black box". One would like to know the essential physics. Jaime Merino and Olle Gunnarsson have a preprint which shows how a two-site two-orbital model can capture some of the essential physics. This model is motivated by DCA calculations on the smallest four site cluster. The two orbitals correspond to (0,pi) and (pi,0) in the first Brillouin zone. Each orbital couples to an independent bath. As U increases there is a transition from the cluster orbitals forming a Kondo singlet with the bath states to

Last night it was announced that John Gurdon shared the 2012 Nobel Prize in Physiology or Medicine. The Guardian website report had this interesting quote from him. Probably the worst stage in an academic career is when starting on one's own as a new assistant professor, with a hefty load of new lectures to be prepared, the need to acquire research support for an independent program, the wish to attract students to form a group, and no one except oneself to do the lab work with which to attract students and research support. I was very fortunate to be joined, within two years of starting at this level, by two outstanding students, Christopher Graham and Ron Laskey. "From Nuclear Transfer to Nuclear Reprogramming: The Reversal of Cell Differentiation" - J.B. Gurdon, Annual Review of Cell and Developmental Biology Vol. 22: 1-22 (Volume publication date November 2006)

Does anyone have any questions so far? If I am giving a seminar I try to ask the audience this at several points in the talk, particularly if there have been no questions so far. Even if no one has a question is shows them you are open to them, and will keep the audience more engaged because they may make more of an effort to think of questions.

Understanding the Hall effect in strongly correlated electron materials is a challenge. In simple metals, the Hall coefficient R_H is, to a good approximation, equal to the inverse of the charge carrier density, and so is weakly dependent on temperature. Furthermore, it has the same sign as the charge carriers. In contrast, in strongly correlated metals, R_H can vary significantly with temperature, including changing sign, and its magnitude and signh can be significantly different from the charge carrier density estimated from band structure calculations or alternative experimental measurements such as the Drude weight in the optical conductivity. I have written several previous posts about this issue. A post on the cuprates illustrates the problem. There is a really nice paper Hall effect in quasi-one-dimensional metals in the presence of anisotropic scattering by Nicholas Wakeham and Nigel Hussey. They show that due to the highly anisotropic band structure in a quasi-one-dime

Consider including a Table in your next paper. They can be very useful. For example, they can compactly summarise previous work (experimental and/or theoretical) relevant to your paper. To me Tables highlight an important aspect of science that I think is increasingly devalued or ignored: comparison. Everyone whats to talk about their own results but is less inclined to critically compare their results to earlier work and put it in that context. It may be painful to admit it but the Table may actually be more useful to the community than the main results in your paper. I believe Jim Brooks once told me that this was an extremely valuable lesson he learnt from his thesis advisor Russell Donnelly. To make this concrete I include below a Table from one of my papers with Jaime Merino.

This is a follow up on a previous post which discussed the strange fact that in quantum many-body theory certain limits (e.g., zero frequency, and the long wavelength) do not necessarily commute. Today I learnt that in 1960 Kohn and Luttinger pointed out that the following definitions of the ground state energy are not necessarily the same: 1. The lowest eigenvalue of the Hamiltonian in the thermodynamic limit. 2. The zero temperature limit of the free energy of an infinite system. This observation was based on an examination of Goldstone's perturbation theory, which is based on 1. Ward and Luttinger later showed that for spatially isotropic band structures and interactions the two are equivalent. Metzner and Vollhardt [where I learned all this] claim that for the Hubbard model, to second order in U, the two are equivalent. Aside: Kohn was very productive in the early 1960's! Another recent post referred to his seminal paper on insulators.

Both things we desperately want to believe. We tell them to our colleagues. The first may also be told by department chairs to try and persuade someone to teach a course. The second we may tell prospective students. 1. Teaching this course for the first time won't really take that much time. I won't be a perfectionist. I will cut corners. After all, I have someone else's notes (and assignments, exams, ...). It is a really good textbook.... 2. My student can complete their Ph.D in the minimum amount of time [3 years in Australia]. They will work really hard. They are smart. They will cut corners. I will supervise them closely. The project is quite straight-forward. I won't repeat past mistakes... Face it. Neither is true. Or do you believe them? Why or why not?

At the Journal Club for Condensed Matter Patrick Lee has a helpful commentary on a recent preprint Non-Fermi liquid d-wave metal phase of strongly interacting electrons Hong-Chen Jiang, Matthew S. Block, Ryan V. Mishmash, James R. Garrison, D. N. Sheng, Olexei I. Motrunich, Matthew P. A. Fisher The preprint presents numerical evidence that a t-J model on a ladder with a particular kind of  ring exchange has an exotic ground state. This state is particularly interesting for two reasons. First, it falls outside the Luttinger liquid paradigm which is the one-dimensional version of Landau's Fermi liquid theory. Second, this exotic state is well described by a variational wave function based on the  "parton" construction [a generalisation of slave bosons] where electron operators are replaced by a product of three fermion operators. The million dollar question remains: is any of this relevant to two dimensions? The authors state, "estimating the strength

I find it is rare that I read a paper that I think is highly original and creative. One I read recently is Water Modeled As an Intermediate Element between Carbon and Silicon by Valeria Molinero and Emily Moore They model water molecules as single atoms! It is just like a spherical cow. And it works.. The abstract of the paper is beautifully written and very informative and so I reproduce it here. Water and silicon are chemically dissimilar substances with common physical properties. Their liquids display a temperature of maximum density, increased diffusivity on compression, and they form tetrahedral crystals and tetrahedral amorphous phases. The common feature to water, silicon, and carbon is the formation of tetrahedrally coordinated units. We exploit these similarities to develop a coarse-grained model of water ( mW) [monatomic Water] that is essentially an atom with tetrahedrality intermediate between carbon and silicon. mW mimics the hydrogen-bonded structure of water th