Condensed concepts

A theoretical picture of the Mott insulating phase of organic charge transfer salts [such as (BEDT-TTF)2X] is that they can be described by a single-band Hubbard model on an anisotropic triangular lattice at half filling. The spin excitations can then be described by the corresponding Heisenberg model. In these models, each lattice site corresponds to a single anti-bonding orbital on a pair (dimer) of BEDT-TTF molecules. Thus the internal structure of the dimer and the corresponding two-band Hubbard model at three-quarters filling is "integrated out" leaving a one-band picture. However, there are some dielectric relaxation experiments that can be interpreted as inconsistent with the picture above. The key question is whether there is charge order within the dimer, in particular, does it have a net dipole moment? A 2010 theory paper by Hotta proposed this and an effective Hamiltonian for the Mott insulating phase where the spin on the dimer and the dipoles are coupled tog

Watching an excellent video about the invention of the transistor stimulated to me to think about other big discoveries and inventions in solid state technology. Who would have thought that huge device would become the basis of an amazing revolution (both technological, economic, and even social...)? In particular, which are the most ubiquitous ones? For which devices did both theory and experiment play a role, as they did for the transistor? I find it worthwhile to think about this for two reasons. First, this semester I am again teaching solid state physics and it is nice to motivate students with examples.  Second, there is too much hype about basic research in materials and device physics, that glosses over the formidable technical and economic obstacles, to materials and devices becoming ubiquitous.  Can history give us some insight as to what is realistic? Here is a preliminary list of some solid state devices that are ubiquitous. transistor inorganic semicond

One puzzle concerning organic charge transfer salts (such as those based on the BEDT-TTF molecule) is how the Mott metal-insulator transition can be tuned with substituting hydrogen with deuterium.  I find it particularly puzzling because the relevant hydrogen bonds are weak and so one does not expect significant isotope effects. Similar concerns are relevant to cases of isotopic polymorphism [where the actual crystal structure changes] in molecular crystals such as pyridine. I recently came across a nice example that I do understand. Hydrogen-Bond-Dynamics-Based Switching of Conductivity and Magnetism: A Phase Transition Caused by Deuterium and Electron Transfer in a Hydrogen-Bonded Purely Organic Conductor Crystal  Akira Ueda, Shota Yamada, Takayuki Isono, Hiromichi Kamo, Akiko Nakao, Reiji Kumai, Hironori Nakao, Youichi Murakami, Kaoru Yamamoto, Yutaka Nishio, and Hatsumi Mori The key to understanding how H/D substitution changes the electronic state is that there is a h

A spin liquid is a state of matter where there is no magnetic order (spontaneous breaking of spin rotational symmetry) at zero temperature. The past few decades has seen a desperate search for both real materials and Heisenberg spin models in two spatial dimensions that have this property. I have written many posts on the subject. An important question is what is a definitive experimental signature of such a system. Strong candidate materials are the Mott insulating phase of several organic charge transfer salts, which was reviewed in detail in 2011 by Ben Powell and I. One experimental signature is the temperature dependence of the specific heat. In particular, some theories predict spin liquid states with spinon excitations with a Fermi surface. This would lead to a linear term in the temperature dependence of the specific heat, as one sees in a simple metal that is a Landau Fermi liquid. This paper is one of several that claims to observe this signature. However, it is

I am on the lookout for good videos that meet roughly the following criteria: -available free online -on condensed matter or chemistry -accessible and interesting to a popular audience -represent the science in a reasonable and helpful way -lack hype This is motivated by the following experience. I knew that there were people (mostly young) who will spend endless hours watching trashy videos (whether B-grade movies or silly antics) on Youtube. However, I only recently learned that there are also people who will spend hours watching videos on serious subjects (science, politics, history, religion, ...) I am keen to find such material so I can recommend it as alternatives to the kind of thing featuring Michio Kaku or string theory propaganda from Brian Greene. This is why I recently watched Forces of Nature with Brian Cox . Unfortunately, I think only the first episode is available for free. In my search, I came across this nice history of the discovery of the transistor nar

Too often I encounter papers or people that describe some detailed study of a particular region of the parameter space of some effective model Hamiltonian (Hubbard, Holstein, Hofstadter, ...) and I am left wondering, "why are you doing this?" The most cynical answers I sometimes fear are "well, it is there" [just like Mount Everest], "no one has done this before", and "I can get a paper out of this." It is not hard to find regions of unexplored parameter space for most models. One can simply add next-nearest neighbour hopping terms, change the lattice (anisotropic, triangular, Kagome, honeycomb, ...) , or add more Coulomb or exchange interactions, or add a magnetic flux, .... The list is almost endless. But, so what? Here are a few good reasons of why exploring a particular parameter regime may be worth doing. Only one of them is sufficient. Often only one may be true. In this parameter regime: A1. There is an actual material that is

Today is the last day of my vacation and so on monday I will be back to serious (?!) blogging about science. In the mean time... On a recent long flight I enjoyed watching many episodes of Silicon Valley , a TV sitcom (?) that is a satire of start up companies. I thought it was pretty funny. [But maybe that was the long flight...] This clip is a good sample. The show reminded me of Utopia, an Australian TV satire of government bureaucracy. Both shows do well at pillorying the personalities who peddle management, marketing, money, and metrics (M^4! ) and have a disproportionate influence on the direction of things. Universities are fertile for similar satire. A starting point for ideas could be The Department , a play written in 1975 by  David Williamson , one of Australia's best-known playwrights. He had been a lecturer in mechanical engineering and social psychology for a number of years before writing the play.

I watched the first episode of  The Forces of Nature  narrated by Brian Cox, The Universe in a Snowflake . [Unfortunately, I don't think the whole episode is free online. It should be! I watched it streamed through my university library website]. The imagery and creativity are stunning. The episode focuses on shapes that occur in nature: spherical planets, human towers in Spain, hexagonal snowflakes, honeycomb beeswax, and "spherical" manatees, animals with bilateral symmetry,  ... Cox nicely discusses some of the underlying principles, including how complexity emerges from simple underlying laws. How do bees "know" that a honeycomb structure is optimal? This relates to a simple example of symmetry breaking and the much more difficult honeycomb conjecture that was only solved in 1999.

Via Peter Woit's blog I read an interesting article What Does Any of This Have To Do with Physics? Einstein and Feynman ushered me into grad school, reality ushered me out by Bob Henderson. The facts it is quite long and that I read it all on a phone (something I virtually never do) on vacation shows how interesting I found it. During his Ph.D Henderson worked on a theory of quantum gravity at the University of Rochester in the 1990s. He then left physics for Wall Street and is now a science writer. Here are a few comments. First, as often happens in discussions that come up related to Woit's blog, I take umbrage at the common assumption that "theoretical physics" is equated with  theories of elementary particles and string theory. The simplest argument against the narcissism of the proponents of this narrow view is that there are five Physical Review journals (A and E). Each contains (very roughly half) theory papers and only D is concerned with such topics.