Condensed concepts

I think one of the most interesting ideas to emerge in the theory of correlated electron materials over the past five years is that of a Hund's metal , particularly how bad metal behaviour is enhanced by the presence of the Hund's rule coupling associated with degenerate d-orbitals and multiple bands.  This is relevant to many transition metal compounds. I recently found the following paper quite helpful. It builds on important earlier work by Luca de Medici. Electronic correlations in Hund metals L. Fanfarillo and E. Bascones A couple of key ideas. In a single band Hubbard model as one approaches the Mott insulator the probability of double occupancy decreases and so does the local charge fluctuations. This reduces the quasi-particle weight Z, which is the overlap of the ground state with a non-interacting Fermi sea. A Hund's metal is different. Hund's coupling polarizes the spin locally. The small  Z  in a Hund metal is due to the small overlap between the n

Recently I had a discussion with a friend who just started a new faculty position. There are both positives and negatives with his new situation. We discussed how it is easy for a few negatives to weigh upon us and we lose perspective of the bigger picture. This is particularly true for those of us who are more introspective and melancholy, and possibly perfectionists. No job is perfect. There will always be frustrations and disappointments. Some of these issues are addressed in my post, Should I change jobs? I also think some people loose perspective about positives. They have a success [whether finishing a Ph.D., getting a grant, or a paper in a luxury journal] and they start thinking they are entitled to all sorts of things [whether, a permanent job, large amounts of funding, a promotion, ....]. How can we keep things in perspective? Talk to others about your situation. Write a list of both positives and negatives. Factor in how your personality distorts your objectivity.

The past few years has seen Hollywood make movies about famous scientists and mathematicians, including A Beautiful Mind (John Nash), The Theory of Everything (Stephen Hawking), The Imitation Game (Alan Turing), and now The Man Who Knew Infinity ( Ramanujan ). The latter is to be released April 29 in the USA and May 5 in Australia. Are there others? This post is not about the important issue whether this is a good thing, particularly when you consider all the creative license taken, and whether the movies capture the science in an appropriate way. First, what kind of scientist is an appropriate candidate for such a movie? I think their life must have some significant components of romance, scandal, tragedy, and redemption . The list above does include substantial ingredients of most of these. Pure scientific heroism and brilliance just does not cut it. Second, who might be some candidates from condensed matter physics or theoretical chemistry? Feynman was one that came

I am currently teaching my solid state class the basics of crystal structures. For any simple material a basic question is: can you construct a simple analytical model that can quantitatively predict ( actually postdict ) the following? the most stable crystal structure (e.g. FCC vs. BCC) the lattice constant  the binding energy of the crystal  the bulk modulus (i.e. compressibility) ? Sometimes people make a big deal about the fact that computations based on Density Functional Theory approximations (with the "right" functional!) do reasonably well at post-dicting the above. However, it is important to acknowledge that * there are very simple analytical models that do well too * the relative energy differences between different structures are very small and may be quite sensitive to the choice of approximation. Previously, I have posted about the challenge of crystal structure prediction for organic molecules. In past years I gave a lecture about the predicti

When considering applicants for postdoc and junior faculty positions it is natural to focus on the track record of the applicant, particularly journal publications (number, quality, citations, ...). However, I think one really needs to take into account who are the advisor(s) of the candidate. It is important to remember that you are hiring the student not their advisor. Consider two extreme candidates and their advisors. Joan's advisor Susan is at a "highly ranked" university and runs a large well oiled machine that graduates students on time and produces a steady stream of publications, some in luxury journals. Each student project makes extensive use of hardware and software from previous generations of students. Most of the papers have long author lists. Susan tends to work on somewhat interesting but safe problems that are guaranteed to produce papers in a timely manner. Joan met rarely one-to-one with Susan, but was actually mostly supervised by postdocs. The