Condensed concepts

They are the spin-1/2 quasi-particle excitations associated with a spin liquid ground state of a quantum antiferromagnet. Perhaps the easiest way to understand them is in contrast to the low-lying excitations in an antiferromagnet with an ordered ground state. Spontaneously broken symmetry is the key concept behind understanding the nature of these excitations. Specifically, for infinite systems the ground state is usually degenerate and is not invariant under the samesymmetries as the system Hamiltonian. This family of ground states is described by an `` order parameter '' which describes the extent of the symmetry breaking. For example, quantum antiferromagnets can be described by a Heisenberg model Hamiltonian which describes a lattice of spins which interact with their nearest (and sometimes next-nearest) neigbours on the lattice. The model Hamiltonian is invariant under rotations of all the spins and under lattice translations. However, both these symmetries can be brok

A good (and painful) question to ask when evaluating research, both our own and others, is: What does the scientific community know now that we did not know when you began the research? There is a similar probing question to ask yourself before you start a project (or a new sub-project). Suppose everything goes as well as can be hoped (i.e., you are able to complete the calculation, do the measurement, get the new technique to work, or make the compound). Then will you be able to say something new? If not, is it worth even trying? The before question is a good one for both students and supervisors to contemplate. It is too easy for supervisors (including me) to say do this extra calculation (or make this extra compound and measure all its properties) without considering enough the time cost to the student or postdoc.

Hopefully, the title got your attention. This is mostly directed at people applying for postdocs. The cover letter is key. I believe most postdoc (and many faculty) applications live or die [i.e., get to the long short list] based on the quality of the cover letter. You need to specifically answer the following specific questions: Why are you interested in this specific job? Why are you interested in this specific research? Why should they hire specifically you for this specific job? Most cover letters I receive are generic. People tend to write the same letter for every job they apply for. Furthermore, the research achievements and research interests they list are usually generic. So to be specific ! Write something like: "One of the scientific questions I am most interested in is "What is the physical mechanism for XXXX in material YYY? I recently read your nice paper "blah blah" in journal YY and I have been wondering if a similar approach mi

Here are the slides of the fascinating talk that Joel Gilmore gave at the Careers session organised by the Australian Institute of Physics (Qld branch) last wednesday.

Consider a material in which there are two distinct charge carriers (e.g., electrons and protons or electrons and oxygen vacancies). A measurement of the conductivity of a sample will just yield the sum of the conductivities of the two individual charge carriers. Given that the physical conduction mechanism for the two carriers may be distinctly different (e.g., small polaron hopping vs. vacancy diffusion) the temperature, pressure, and composition dependence of the two components may be completely different. Is there a way to extract for each of the carriers the conductivity, density of carriers, and mobility? A few weeks ago I thought this was hopeless, but I was wrong. So my favourite paper for this week has the weighty title Impedance Spectroscopy as a Tool for Chemical and Electrochemical Analysis of Mixed Conductors: A Case Study of Ceria by Wei Lai and Sossina Haile from Caltech. The paper is in a journal I have never looked at before, Journal of the American Ceramic Society (n.

Understanding lattice models for strongly correlated electron systems is a major challenge. Widely studied (and still poorly understood) models include the Hubbard and Heisenberg models. But some insight can be gained from studying model Hamiltonians on small clusters such as four lattice sites. Although, such a small system is far from the thermodynamic limit, these models can illustrate some of the essential physics associated with the interplay of strong electronic correlations, frustration, and quantum fluctuations. They illustrate the quantum numbers of important low-lying quantum states, the dominant short-range correlations, and how frustration changes the competition between these states. Furthermore, understanding these small clusters is a pre -requisite for cluster extensions of dynamical mean-field theory and rotationally invariant slave boson mean-field theory which describes band selective and momentum space selective Mott transitions. See for example my earlier pos

My wife brought to my attention an article in the New York Times about the consequences of California's budget woes for the University of California system, and especially Berkeley. People interviewed include Bob Birgeneau (famous for inelastic neutron scattering studies of strongly correlated electron materials, now Chancellor at Berkeley) and Richard Mathies (pioneer in femtosecond spectroscopy, now Dean of the College of Chemistry, Berkeley)

Dr Richard Walding gave some insights into the current state of high school Physics teaching in Queensland at the AIP Careers seminar on wednesday. He was Head of Science for the past 20 years at Moreton Bay College and now tutors Senior Physics student teachers at Griffith Uni. Richard said tha the demand for physics teachers was very strong, not only here in Queensland but everywhere in the word. Teacher recruiting companies have an unmet demand for physics teachers and it would seem they can place you at countries all over the world. Richard said the starting wage for a graduate was about $51,000 rising to $72,000 after 7 years or so. He said there were 7000 Senior Physics students in Queensland in Yr 11 and 12 from 176 schools and taught by about 220 physics teachers. Most of the teachers had BSc degrees but, surprisingly enough, only a handful majored in physics at university . To a question about whether you'd have to teach other subjects beside Physics and Jun

More great quotes from Bob Laughlin , A Different Universe: Reinventing Physics from the Bottom Down “The great power of science is its ability, through brutal objectivity, to reveal to us truth we did not anticipate.” (p. xvi) ``mythologies are immensely powerful things, and sometimes we humans go to enormous lengths to see the world as we think it should be, even when the evidence says we are mistaken.’’ (p. 114) “ideologies preclude discovery. All of us see the world as we wish it were rather than as it actually is.” (p. 116). There are similarities to the cautions of Walter Kauzmann, in his Reminiscences of a Life in Protein Chemistry.

Here is a copy of the talk I gave tonight on academic careers. Some of the feedback included: * the importance of mental health issues (I will try and do a few future posts on this). * dogged perseverance is often a key component to success It would be good to get some discussion going on some of the issues I raise in the talk.

Self-esteem, stress, and depression among graduate students. Kreger DW . Wright Institute, Berkeley, CA 94704, USA. Psychological Reports 1995 Feb;76(1):345-6. In a study of 29 graduate students, self-ratings of stress correlated with low scores on self-esteem but were not related to an objective indicator of actual stress . Both self-rated stress and low self-esteem scores were related to scores on depression, with a weak interaction effect.

Tomorrow evening I am giving a talk on career advice for people who want to pursue a career in scientific research for a careers night of the local branch of the Australian Institute of Physics. Currently, points I am planning to make include: There is more to life than a research career. Be realistic and consider alternative careers. Learn to write, to get along with other people, .... plus previous career advice I have posted, especially this advice to Ph.D students.

I am getting tired of hearing talks and reading reports which state, "The first Bose-Einstein condensate was observed in 1995." I think a more accurate statement would be "The first BEC in a dilute atomic gas was observed in 1995." Many would argue that superfluid 4He is a BEC. This new phase of matter was first observed around 1930. In 1932 Fritz London proposed that this was a BEC. (BTW, this is the same London as in the Heitler-London wave function, the London penetration depth, and London dispersion forces...). But it should be noted that the case of a BEC in superfluid He is not as clear cut as in dilute atomic gases. Nevertheless, I dont think these subtleties validate ignoring 80 years of research on superfluid helium. A very useful summary of the history and the associated physical issues is contained in this nice article by Sebastian Balibar. I think that people who are supervising Ph.D students on BEC's should be familiar with these issues, make sure

Just how efficient are biomolecular systems? For a long time it was claimed that fireflies had a quantum efficiency close to 100 per cent. However, it was recently found this is not the case, the efficiency being about 40%. A nice summary of the work is in this News and Views article in Nature Photonics last year. Besides measuring the quantum efficiency Ando et al. find that there are three components to the light emission and all are pH dependent. One component is of unknown origin. Clearly there is a correlation between the colour of the emission and the protonation state of the chromophore. Only in a 2006 Nature paper was a structural basis for two different emission states proposed. I am curious as to how much quantum chemistry has been done on these issues. This may help address questions such as: What determines the quantum efficiency of emission? How are non-radiative decay channels suppressed? What role does the protein environment play?

I just went to a session in the department where 6 new staff members each had 5 minutes to introduce themselves and their research. If you have such an opportunity I would advise trying to just answer the following questions. What is the scientific question you want to answer in the next few years? Why is this important? Why are you excited about it?

Americans are known for scientific prowess but not their knowledge of geography! A colleague recently told me that he now begins all his talks overseas with a map of Australia and shows where Brisbane is. On my last trip to Europe I also did this. However, it took me a while to find the pictures I really wanted, those which I had seen on postcards. I eventually found them and will include one in all my future talks overseas. They really put the size of Australia in perspective.

Subir Sachdev has a Physics article which provides a background to recent work using tensor networks (inspired by quantum information theory) to find the ground state of quantum spin lattice models. I really like the following succinct summary of the problem: The simplest of these problems involve only the spin operators S i of electrons residing on the sites, i , of a regular lattice. Each electron can have its spin oriented either up or down, leading to a Hilbert space of 2 N states, on a lattice of N sites. On this space acts the Heisenberg Hamiltonian H = ∑ i < j J i j S i ⋅ S j , (1) where the J i j are a set of short-range exchange interactions, the strongest of which have J i j > 0 , i.e., are antiferromagnetic. We would like to map the ground-state phase diagram of H as a function of the J i j for a variety of lattices in the limit of N → ∞ . Note that we are not interested in obtaining the exact wave function of the ground state: this is a hopeless task in dim

This follows up with the earlier post about a paper, Conical Intersections, Charge Localization and Photoisomerization Pathway Selection in a Minimal Model of a Degenerate Monomethine Dye by Seth Olsen and I, which been accepted for publication in Journal of Chemical Physics. An important issue is after a organic molecule absorbs a photon what conformational change will occur. Below are several options involving bond twists. A second issue is how the charge distribution in the molecule changes upon twisting. This kind of physics is at the heart of how your eye works. When retinal absorbs a photon it undergoes a conformational change which produces a charge separation which eventually leads to an electrical signal in your brain. It is also at the heart of designing better organic solar cells. We considered a model Hamiltonian for a large class of dyes. The figure below shows contour plots for the first excited state potential energy surface for several parameter values. The lower pa

I have not read the book but found the title very helpful and challenging!

If you were going to an isolated island to do computational chemistry and you could take this year’s computers and 10-year-old algorithms or this year’s algorithms and 10-year-old computers, which would choose to take? This is a question that Donald Truhlar asks in a JACS Editorial for a Select issue of 23 papers on Molecular Modeling of Complex Chemical Systems. How would you answer the question? You can look in the article to see how most computational chemists would answer the question. The article is a very nice read to a physicist because it provides a very helpful and concise summary of historical landmarks in the computational modeling of large chemical systems. However, I disagree and am concerned with one of the opening statements in the article: Almost all modern theoretical chemistry is computational chemistry, because most of the progress that can be made with pencil and paper without a computer has been already made. Computations on complex systems are, in my o

In 1998 Max Tegmark wrote a paper with the great title, The Interpretation of Quantum Mechanics: Many worlds or many words . He suggested that the validity of different interpretations of quantum theory cannot be decided empirically, but are: “purely a matter of taste, roughly equivalent to whether or not one believes mathematical language or human language to be more fundamental.” But it is interesting that he is now writing articles about multiverses...

In 1983 Austria introduced this bank note featuring Erwin Schroedinger. Note the Psi! But where is the cat? It is interesting they also introduced a 50 Schilling note with Sigmund Freud and a 5000 Schilling note for Mozart. Is this a relative measure of their contributions to culture and society?

I am very happy that a paper, Conical Intersections, Charge Localization and Photoisomerization Pathway Selection in a Minimal Model of a Degenerate Monomethine Dye by Seth Olsen and I, has been accepted for publication in Journal of Chemical Physics. A key question concerning optically active molecules is what is dynamics of the excited states? Specifically, what are the predominant non-radiative decay mechanisms. The schematic below shows several options for the energy of the potential energy surfaces versus some configurational co-ordinate. On the left both S1 and S2 excited states decay to a conical intersection with the ground state. In contrast, on the right they have distinctly different decay paths. But how does one go beyond such schematics. It turns out that for a broad class of dyes one can justify from high level quantum chemistry calculations a description in terms of just three valence bond states (see below). The description in terms of the three diabatic states allow

I am thinking more about photophysical properties of ketocyanine dyes. I have puzzled through the basics of the molecular orbitals. Some material on this site is useful, including a visualisation of the molecular orbitals of formaldehyde. A key point is that the HOMO is a non-bonding orbital centred on the O atom and lying perpendicual to the C=O bond. Thinking in the alternative resonating valence bond picture there will be three alternative Lewis structures O || C -R | L O- | C -R | L+ O- | C -R+ | L The extent to which the lower two structures contribute will increase the C-O bond length and reduce the C-O stretch frequency. It should be possible to describe the low lying excited states in terms of a complete active space with 4 electrons in 3 orbitals (a pi* orbital on the C=O bridge, and a pi

P. W. Anderson , “ Emergence, Reductionism and the Seamless Web: When and Why Is Science Right ,” Current Science 78:6 (2000), 1. [Based on the Pagels lecture, Aspen, 1999]. Anderson suggests that emergence is the mechanism for consilience (the unifying of disparate pieces of knowledge) and reduction is the evidence for it. Theories may be under-determined, i.e., there may be many possible theories that can explain what is actually known. Hence, a successful theory may not actually correspond to what is happening. If there are only a few constraints (hypotheses, observations) that a theory must satisfy it has sometimes been the case that more than one theory can satisfy the constraints. However, as the number of constraints increases, acceptance of a theory is more likely and it becomes hard to conceive of alternative theories that could satisfy these constraints. Reduction can greatly increase the number of conditions that a theory must satisfy. For example, any alternative to quantu

It is wonderful that last year the US Postal Service issued new stamps featuring four prominent scientists. Pauling and Bardeen were indeed masters in unravelling emergent phenomenon using quantum many-body theory. Update (2016). I just discovered that in 2005 there was one for Josiah Willard Gibbs   and Feynman.

Ketocyanine dyes are of considerable interest. An example is shown above. What are the essential ingredients that determine their photophysical properties? The figure below is from a nice paper which compares essential differences between cyanines (CY), ketocyanines (KCY), and squarenes (SQ). The difference between the upper and lower panels (A and B) relates [I think] to whether one has an even (A) or odd (B) number of p-electron centres on each of the molecular units on the left and right side of the central C=O bridge. Apparently, this is following a "composite molecule" approach in a book by Fabian and Hartmann. A complementary approach to describing optical properties of these materials is within a resonating valence bond approach. There will be three dominant resonant structures, similar to those advocated by Pauling for urea. [I will try and get a picture]. Such an approach will naturally lead to two low-lying singlet excited states. This type of resonance is of gre

Following up on previous posts about the absence of energy bands and charge transport in dendrimers . For incoherent transport (equivalent to hopping or small polaron transport) the mobility and conductivity has an activated form where the activation energy is one quarter of the reorganisation energy associated with a charge moving off or on the relevant unit. But how can one distinguish such activated behaviour from what occurs in a regular semiconductor? Also measure the temperature dependence of the thermopower. The associated energy scale will be much less than the activation energy of the mobility. This is seen nicely in the figure below taken from a review by Salamon and Jaime about colossal mangetoresistance materials.

Stare at the figure below. Can you see any difference between the top and the bottom panels? [You can left click on the figure to make it larger.] Yes. There is essentially no difference. This is not boring. It is fascinating. The figure is taken from a beautiful Science paper by Seamus Davis group. It represents images from the fourier transform of STM images of the surface of an underdoped cuprate superconductor. From bottom to top the temperature increases from 0.1Tc to 1.5Tc where Tc is the superconducting transition temperature. Basically this shows that at low energies the pseudogap phase has the identical excitation spectrum as a phase-disordered version of the d-wave superconducting state.

Previously, I wrote a post about meeting Roland Wester and the prospect of doing high resolution spectroscopy experiments on trapped ions of organic chromophores. Looking at a review Roland wrote about the technique led me to this beautiful JACS paper which desribes experiments on tryptophan [a flourescent amino acid]. The spectra below show how the protonated Trp has a broad absorption line due to non-radiative decay. Adding one and two water molecules, which hydrogen bond, removes this decay channel resulting in sharp vibrational features in the spectra. The figures below show how the potential energy surface varies with twisting about the central carbon atom of the molecule. Similar experiments on methine dyes such as the green flourescent protein chromophore would be wonderful!

[Left click to enlarge and make legible.] A Ph.D is not enough! A guide to survival in science by Peter Feibelman. 15 years ago I discovered this book, read it, and wrote the enthusiastic review above. I still think it has much wise, practical and helpful advice. I try to get everyone I work with to read it.

Ilya Eremin kindly sent a link to a copy of the nice talk he gave on spin fluctuations in the iron pnictide superconductors. Reading the slides will be more helpful than my rough notes I posted earlier. Important issues addressed include: What are similarities and differences between the cuprates and pnictides? Is the key difference, the pnictides are less strongly correlated? If so, how does one describe/explain the magnetic order in the pnictides?