Condensed concepts

Over the years I have moved into new established research areas with mixed success. Sometimes this has been a move from one sub-field of condensed matter theory to another. Other times it has been to try and cross disciplines, e.g. into theoretical chemistry. I have also watched with interest as others have tried to break into communities I have been a part of. Here is my list of suggestions as to things that may increase your chances of success. 1. Listen. What do the well established experts in the field say? What are they working on? What do they think are the important questions? What are the key concepts and landmarks in the field? Bear in mind the values and culture may be quite different from your own field. Note: this is the economist Paul Krugman's first research tip. 2. Be humble. Most fields have a long history and have been pioneered by some very smart and hard working people. That doesn't mean that the field isn't populated by some mediocre people o

As the strength of hydrogen bonds varies there are significant qualitative changes in the effective Born-Oppenheimer potential that the proton moves in [double well, low barrier double well, single well]. These changes will lead to qualitative differences in the ground state probability distribution for the proton. How does one measure this probability distribution? Over the past decade this has become possible with deep inelastic neutron scattering. Calculation of the probability distribution functions using path integral techniques with potentials from Density Functional Theory (DFT) is considered in Tunneling and delocalization effects in hydrogen bonded systems: A study in position and momentum space by Joseph Morrone, Lin Lin, and Roberto Car They consider three different situations, O-H...O, shown below, and corresponding to average proton donor-acceptor (oxygen-oxygen) distances of 2.53, 2.45, and 2.31 Angstroms respectively (top to bottom). They also correspond to three

For a long time I have been meaning to write this post. It was finally stimulated by seeing that the Electronic Structure Theory community has started an initiative to improve the Wikipedia articles relevant to their community. I think most of the articles relevant to theoretical condensed matter physics and chemistry are quite poor or non-existent. We need to take responsibility for that. I think the simplest way forward may be for us to encourage our graduate students to write and update articles for Wikipedia. Many draft literature reviews for undergrad, Masters, and Ph.D theses could be used as starting points. For that matter some of my blog posts, could be too. It could be quite educational for students to post material and hopefully see it revised.

A recent post considered the problem of deconstructing the kinks that occur in energy versus wavevector dispersion relations in some strongly correlated electron systems. I have now read two of the related references to the recent PRL I discussed. A nice earlier  PRL  discusses how kinks occur in the temperature dependence of the specific heat. Furthermore, the authors use an obscure formula from the classic AGD to show that these kinks are related to the kinks in the self energy. Emergent Collective Modes and Kinks in Electronic Dispersions Carsten Raas, Patrick Grete, and Götz S Uhrig They calculate the self energy and the frequency dependent local spin susceptibility for the Hubbard model at half filling using Dynamical Mean-Field Theory (DMFT). The latter quantity is shown below. As the Mott insulator is approached a low-energy peak develops in the spin susceptibility. This reflects the large spin fluctuations in the metallic phase as the electrons become more localised.

I am very proud to have my first paper published in the Journal of Chemical Education! Moreover, I believe it concerns a very important topic Connecting Resources for Tertiary Chemical Education with Scientists and Students in Developing Countries The paper was written with Ross Jansen-van Vuuren (UQ) and Malcolm Buchanan (St. John's University, Tanzania) The abstract is The ability of developing countries to provide a sound tertiary chemical education is a key ingredient to the improvement of living standards and economic development within these countries. However, teaching undergraduate experimental chemistry and building research capacity in institutions based within these countries involves formidable challenges. These are not just a lack of funding and skilled teachers and technicians, but also take the form of cultural and language barriers. In the past three decades a diverse range of initiatives have aimed to address the situation. This article provides a summary

Here is a random collection of a few of the things I learnt last week in Korea. Yuji Matsuda described experimental work on the iron pnictides which shows evidence [via a diverging effective mass] for a Quantum critical point hidden beneath the superconducting dome. Jan Zaanen gave an interesting talk about AdS/CFT correspondence techniques from string theory. I am slowly becoming less skeptical about this surreal enterprise. Some concrete results that could conceivably relevant to experiment are being produced. Zaanen mentioned work by Gary Horowitz and Jorge Santos that produces a frequency-dependent conductivity that has some similarities to what is observed in the cuprates. [But, one needs to consider the alternative explanation ]. I was disappointed that Zaanen ignored the work described in this post , claiming that interlayer "incoherence" in the cuprates is a mystery. Henri Alloul recently posted on the arXiv What is the simplest model which captures the basi

I doubt it. They are just pragmatists. Today we had an interesting Quantum Science Seminar by Peter Riggs , author of  Quantum Causality: Conceptual Issues in the Causal Theory of Quantum Mechanics . Other names for the causal theory are  Bohmian mechanics, De Broglie's pilot-wave theory , ontological quantum theory... The talk provided a nice overview of the theory and different objections to it. Riggs is an advocate of the theory, claiming it "solves" the measurement problem. He does not seem concerned that relativistic and spin versions of the theory seem a rare species or intractable. I was intrigued that Riggs invoked physical chemists, such as Robert E. Wyatt, in a manner that suggested that they were advocates of the theory. Wyatt has had considerable success in applying the mathematics of Bohmian mechanics to solve quantum dynamics problems in chemistry. He is the author of Quantum dynamics with trajectories . However, my impression is that Wyatt and his c

It amazes and frustrates me that this basic point is so often neglected. Comparisons are central to science in two respects. Control variables. As you vary just ONE parameter [pressure, electron-phonon coupling, temperature, isotope, Planck's constant] how do the results of the experiment or calculation vary. People seem to often vary more than one parameter. Or, they don't vary any. For example, they do some complicated calculation and get "an answer", i.e. a number, but don't investigate or report how that answer depends on the parameters or approximations in their calculation. Comparison with earlier work. Many people don't seem to feel a need or obligation to report how their results compare to those of others who did related experiments or simulations. Earlier I made the case for why Tables are wonderful  in papers for these reasons. I also considered the astonishing case of a widely downloaded paper about the theory of hydrogen bonding that t

There are a whole range of dynamical processes found in biomolecular systems that one would like to simulate and understand. Examples include: charge separation at the photosynthetic reaction centre proton transfer in an enzyme photo-isomerisation of a fluorescent protein exciton migration in photosynthetic systems. These are particularly interesting and challenging because they lie at the quantum-classical boundary. There is a subtle interaction between a quantum subsystem [e.g. the ground and excited electronic states of a chromophore] and the environment [the surrounding protein and water] whose dynamics are largely classical. [image from here ] Due to recent advances in computational power and new algorithms [some based on conceptual advances] it is now possible to perform simulations with considerable atomistic detail. This is an example of "multi-scale" modeling. However, it is important to bear in mind the many ingredients and many approximations emp

At the workshop this week Luca de' Medici gave a nice talk, based on work described in a preprint  Selective Mottness as a key to iron superconductors  with Gianluca Giovannetti, Massimo Capone They argue that the best way to understand the iron superconductors is in terms of Hund's rule coupling and the total orbital filling. Thus the "parent compounds" which are antiferromagnetic should be viewed as being at 0.2 doping because they have 5 electrons in 3 degenerate iron orbitals. If one airbrushes out the antiferromagnetic phase one is left with the phase diagram below. This has a rather striking resemblance to the cuprate phase diagram (!), particularly when the pseudogap phase can be viewed as a "momentum selective" Mott insulator.

Bruce Alberts, Editor in Chief of Science, has a powerful editorial Impact Factor Distortions.  Here is the beginning and the end. This Editorial coincides with the release of the  San Francisco declaration on research Assessment  (DORA), the outcome of a gathering of concerned scientists at the December 2012 meeting of the American Society for Cell Biology.  To correct distortions in the evaluation of scientific research, DORA aims to stop the use of the "journal impact factor" in judging an individual scientist's work. The  Declaration  states that the impact factor must not be used as "a surrogate measure of the quality of individual research articles, to assess an individual scientist's contributions, or in hiring, promotion, or funding decisions." DORA also provides a list of specific actions, targeted at improving the way scientific publications are assessed, to be taken by funding agencies, institutions, publishers, researchers, and the organizati

This is a very subtle question. I learnt a lot from a nice talk that Steve Kivelson gave on the subject. In a single component fluid [e.g. water or a gas] a hydrodynamic approach works because one has local conversation of energy and momentum. Then ALL transport properties are determined by just three quantities: the shear viscosity (eta), the second viscosity (zeta), and the thermal conductivity (kappa). However, the electron fluid in a solid can exchange energy and momentum with the lattice and impurities. Hence, hydrodynamics is not relevant. What about temperature ranges where electron-electron scattering dominates? Well then one has resistivity as a result of Umklapp scattering, which means that there is momentum exchange with the lattice. [I never understand all the subtle details of that]. Hence, one still does not have conditions under which hydrodynamics may apply. So when might hydrodynamics apply? Kivelson suggests it may be relevant in 2DEGS [2-Dimensional Electron

At the conference today Changyoung Kim gave a nice talk about how the "Rashba effect" [coupling of momentum and spin] on the surface of a semiconductor has a different physical origin to what is usually claimed. In particular, the energy scale for the relativistic Zeeman splitting proposed by Rashba gives a band splitting that is six orders of magnitude smaller than what is actually observed! Here is the abstract of the associated PRL : We propose that the existence of local orbital angular momentum (OAM) on the surfaces of high-Z materials plays a crucial role in the formation of Rashba-type surface band splitting. Local OAM state in a Bloch wave function produces an asymmetric charge distribution (electric dipole). The surface-normal electric field then aligns the electric dipole and results in chiral OAM states and the relevant Rashba-type splitting. Therefore, the band splitting originates from electric dipole interaction, not from the relativistic Zeeman splitting as

There is a nice review Strong correlations from Hund's coupling  by Antoine Georges, Luca de' Medici, and Jernej Mravlje. Section 6 describes the main physical point of this review: Generally, the Hund’s rule coupling has a conflicting effect on the physics of the solid-state. On the one hand, it increases the critical U above which a Mott insulator is formed (Section 3); on the other hand, it reduces the temperature and the energy scale below which a Fermi liquid is formed, leading to a (bad) metallic regime in which quasiparticle coherence is suppressed (Section 4). In particular, this means a new organising principle for strong correlated electron systems: one can have a bad metal and be a long way from a Mott insulator. "Mottness" is not the only ingredient for strong correlations. A previous post describing some of this work includes  Hund's rule coupling in multi-band metals .

This week I am at a meeting Bad metal behavior and Mott quantum criticality  being held at POSTECH in Korea. Here is the current version of the slides for my talk, "Organic charge transfer salts: model bad metals near a Mott transition."  The main results in the talk are in a recent PRL , written with Jure Kokalj. Aside: Unfortunately, I am missing the beginning of the meeting. I got stuck in Tokyo because of a delayed flight from Denver. On this leg of my trip it seems flying in and out of Denver has been a comedy of errors and delays.

A key and profound property of quantum-many body systems is the emergence of new low energy scales that are much less than the underlying bare interactions. Classic cases are the Kondo temperature and the superconducting transition temperature in BCS theory. How the relevant low energy scales emerge in strongly correlated electron systems is an outstanding problem. There is a very nice PRL Poor Man’s Understanding of Kinks Originating from Strong Electronic Correlations K. Held, R. Peters, and A. Toschi A decade ago there was a Nature paper [where else?!] that made a big deal about the fact that they saw a kink in the quasi-particle dispersion [energy versus momentum] of a cuprate material measured by ARPES and claimed this was due to electron-phonon coupling. Hence, (!) electron-phonon coupling must be important in the cuprates. I found this claim troubling because it seemed to be based on the classic fallacy of not distinguishing sufficient and necessary conditions. Just becau

Although hydrogen bonded systems (D-H....A) are chemically diverse and have a wide range of physical properties one observes simple empirical correlations between physical properties and two key variables: the donor-acceptor (D-A) distance R and the difference in proton affinity between the donor and acceptor. That is the main starting point of my paper on the subject. I am continually learning of more empirical correlations that need to be explained.  The vibrational frequency for motion of the donor D-H relative to the acceptor A can be measured by infra-red spectroscopy. No particular correlations are observed with R. However, this is because of a diversity of donor and acceptor masses. If instead one uses the frequency and masses to calculate the force constant [the curvature of the hydrogen bond potential] one observes a correlation.  The Figure below is taken from a 1974 review article by Novak. I believe this correlation is a key ingredient to understanding the s

The Berry (geometric) phase is a significant quantum effect which is associated with conical intersections on excited state potential energy surfaces in molecular photophysics. An observable consequence is that if a wavepacket is split in two and the two parts traverse opposite sides of the conical intersection (CI) then they will interfere destructively when they meet again on the other side of the CI. [An earlier post considers this effect]. An important question is: what happens to this quantum interference in the presence of decoherence due to the environment? This question is considered in a nice paper Quantum-classical description of environmental effects on electronic dynamics at conical intersections Aaron Kelly and Raymond Kapral To answer the above question they calculate the probability density on the other side of the CI as a function of the distance from the maximum interference point. This is done for a range of different environment [harmonic oscillator bath] pa

I am not sure that Sheldon Cooper should be considered as a useful source of advice, either personal or scientific [he is a string theorist!]. Nevertheless, the clip below shows he does understand the importance of "down time", particularly for introverts. This is something that it took me too many years to learn. I think the need for this quiet time is particularly true at busy meetings or during times of intense research.

At the workshop a couple of talks have brought home to me two important organising principles for understanding and describing quantum effects involving hydrogen bonding in condensed phases. These are relevant to a wide range of problems from properties of bulk water to proton transfer in enzymes. 1. Competing quantum effects. In hydrogen bonding as the donor acceptor distance decreases the hydrogen bonding gets stronger. This decreases the frequency of the O-H stretch on the donor and increases the frequency of the O-H bend. [This can be seen in Figures 5 and 6 in my CPL ]. Consequently, their are two competing contributions to the zero-point energy. These reduce the total quantum effect and also make it more challenging to calculate accurately. This idea was highlighted in a 2007 JCP by Scott Habershon, Tom Markland, and David Manolopoulos and a 2012 PNAS by Tom Markland and Bruce Berne. On tuesday, Tom Markland highlighted how these competing quantum effects were important

Singlet fission is the process where a excited spin singlet state of an organic molecular complex can decay into two spin triplet states on spatially separated molecules. This process has received considerable attention recently because of the possibility of using it to increase the efficiency of organic photovoltaic cells. Several previous posts  considered some of the fascinating science involved, including the reverse process of triplet-triplet annihilation. There are a beautiful series of papers by Timothy Berkelbach, Mark Hybertsen, and David Reichman Microscopic theory of singlet exciton fission. I. General formulation Microscopic theory of singlet exciton fission. II. Application to pentacene dimers and the role of super exchange To me these papers provide a quality benchmark for theoretical work in the field and highlight the limitations and confusion of some of the previous work on the subject. Why do I like the papers? The authors use diabatic states as a star

Besides the great science, a wonderful thing about Telluride meetings is the opportunity to do some hiking in the Rocky Mountains. This is my third time here and slowly I am finding some nice hikes that are directly accessible from the town. I arrived one day early to try and get over jetlag. Yesterday I did the hike described here.  It took about 5 hours at a leisurely place. I think Eric Bittner made me aware of this hike. A few of my photos are above. But, today I am a bit sore from all the downhill... Disclaimer: this is for experienced fit hikers. Make sure you have adequate clothing, water, sun screen, food, map, ..... Also beware of altitude sickness.

Next week I attending a Telluride Science Research Center workshop on Quantum effects in condensed phase systems.  Here is the current version of  the slides for my talk, "Effect of quantum nuclear motion on hydrogen bonding." The empirical potential energy curves used are based on the simple two-state model presented in this paper. Comments welcome.

Should members of senior management at universities hold the title Professor if they did not become a Professor via a normal academic career? This question is examined for some specific cases in  a fascinating blog post by UQ economics Professor Paul Fritjers.

There is a paper in Science Engineering Coherence Among Excited States in Synthetic Heterodimer Systems Dugan Hayes, Graham B. Griffin, Gregory S. Engel Six years ago Engel was first author of a Nature paper claiming that photosynthetic systems used quantum computing to maximise efficiency. The claims of this paper are more modest. The abstract begins: The design principles that support persistent electronic coherence in biological light-harvesting systems are obscured by the complexity of such systems. Some electronic coherences in these systems survive for hundreds of femtoseconds at physiological temperatures, suggesting that coherent dynamics MAY play a role in photosynthetic energy transfer. Coherent effects MAY increase energy transfer efficiency relative to strictly incoherent transfer mechanisms.  The key data is in the Figure below. It shows a Fourier transform of the "cross peaks in the two-dimensional spectra". The three boxes correspond to the di

Previously, I asked the question : are there any condensed phase systems in chemistry where quantum effects [e.g. tunneling, interference, entanglement] are enhanced compared to the gas phase? Let me clarify. Suppose we take a molecular system X and consider the magnitude of some quantum effect Y in the gas phase. We then put X in some condensed phase environment [e.g., solvent, protein, or glass] and measure or calculate Y. It is quite possible that Y increases due to what I would call "physically trivial" effects, e.g. a change in the geometry of X which makes Y larger. For example, the polarity of a solvent can decrease the donor-acceptor distance for proton transfer in a molecule and thus increase quantum tunnelling. To me a physically "non-trivial" effect is where the environment enhances the quantum effect for the same reference system X [e.g., one uses the same geometry of the molecule in the gas and condensed phases]. I am not sure this ever happens. Ge

There is a recent  article  in The Chronicle of Higher Education about what role, if any, "loyalty" should play in giving permanent jobs to faculty on "casual contracts" [referred to as adjuncts in the article]. The article and the comments highlight, at least to me, how the notion of "loyalty" is an ill-defined and highly emotionally charged concept. Surviving and succeeding in science [and academia in general] requires building, maintaining, and preserving a complex array of personal relationships. The notion of loyalty can have a significant effect, both for good and bad, on these relationships. The problem is that different people may have very different expectations about where loyalty should lie and what it means. Loyalty can affect information flow, response to criticism, and the sharing of resources. Here are some situations where loyalty may play out: A student takes a postdoc with a competitor of his advisor. A graduate student is taking