Condensed concepts

Don't follow the pack! I just read the  Random Walk to Graphene , by Andre Geim. It is the lecture he gave when receiving the 2010 Nobel Prize in Physics. I should have read it long ago but was motivated to read it now because the following sentence features in Joseph Martin's "purloined letter'' argument about why condensed matter physics lacks status. Graphene has literally been before our eyes and under our noses for many centuries but was never recognized for what it really is. I learned some nice science from the lecture. Foremost, it is a great story of scientific creativity, perseverance, and serendipity. However, I want to mention a few things that highlight how the story strongly conflicts with most views about how science is currently "managed" and people operate. Geim starts by recounting his Ph.D. and early postdoc years. His Ph.D papers were cited twice, by co-authors. The subject was dead a decade before I even started my Ph.D.

A long-standing question for superconductivity in organic charge transfer salts concerns the symmetry of the superconducting order parameter. Is it unconventional (i.e. not s-wave) and if so are there nodes in the energy gap? Over the years there have been a wide range of claims, both theoretical and experimental. Most recently a combined theory-STM experiment claimed the symmetry was d + s and that there were 8 nodes on the Fermi surface. Two of my UQ colleagues recently posted a nice preprint that comes to a different conclusion. Microwave Conductivity Distinguishes Between Different d-wave States: Umklapp Scattering in Unconventional Superconductors  D. C. Cavanagh, B. J. Powell Microwave conductivity experiments can directly measure the quasiparticle scattering rate in the superconducting state. We show that this, combined with knowledge of the Fermi surface geometry, allows one to distinguish between closely related superconducting order parameters, e.g., d x 2 − y 2  and

At the fundamental level, we think of a Hamiltonian as independent of temperature. It is describing the energy of all possible states of the system in the absence of any environment. However, when one does mean-field theory (e.g. for an Ising model or BCS theory) the Hamiltonian involves temperature-dependent parameters that are determined self consistently. I have been thinking about this because one of the proposed effective minimal Hamiltonians for spin crossover compounds is an Ising model with a temperature dependent field. My immediate reaction was that this must be some sort of mean-field theory. However, I now realise that is not the case. Effective Hamiltonians can be temperature dependent without invoking any approximations. Temperature-dependent interactions can arise when one integrates out some degrees of freedom. One can see this by simply considering the case of a system with two degrees of freedom x and q. The partition function can be written as a path int

Why are string theorists celebrities who write best-selling books and popular documentaries? Why are cosmology and particle physics seen as "fundamental" and answering profound questions about "why we are here?" as they push back the frontiers of knowledge with their great intellects and imagination. In contrast, condensed matter physics gets little public attention and is not seen as exciting, "fundamental", or intellectually challenging. There is a helpful and stimulating paper Prestige Asymmetry in American Physics: Aspirations, Applications, and the Purloined Letter Effect Joseph D. Martin Why do similar scientific enterprises garner unequal public approbation? High energy physics attracted considerable attention in the late-twentieth-century United States, whereas condensed matter physics – which occupied the greater proportion of US physicists – remained little known to the public, despite its relevance to ubiquitous consumer technologies....

A basic question concerning spin crossover compounds is what are the energy difference and entropy difference between the low spin (LS) and high spin (HS) states. The relative magnitude of these two quantities determines the crossover temperature from the LS to HS state. From experiment typical values of the energy difference Delta H are of the order of 1-5 kcal/mol (4-20 kJ/mol). Entropy differences are typically about 30-60 kJ/mol/K. (See table 1 in the Kepp paper below). This relatively small difference in energy presents a challenge for computational quantum chemistry, such as calculations based on  density functional theory , because of the strong electron correlations associated with the transition metal ions, Over the past few years some authors have done nice systematic studies of a wide range of compounds with a wide range of DFT exchange-correlation (XC) functionals. Here I will focus on two papers. Benchmarking Density Functional Methods for Calculation of Sta

Unfortunately, like many universities, UQ has become a construction site in the rush to build shiny new buildings , particularly to accommodate the ever increasing expansion of senior management and nice facilities to ``enhance the student experience.'' An extra floor was added to the physics building for the Office of the Executive Dean of Science. Faculty and grad student offices are being shuffled around campus to accommodate this construction. I am now making my third move in less than eighteen months. I took this opportunity to downsize and toss a lot of old files. While filling a dumpster I saw something I thought was pretty ironic and funny.

What do condensed matter physicists study? High school students are often taught there are three states of matter: solids, liquids, and gases. However, this is misleading as there are many more states of matter. Liquid crystals, superconductors, and ferromagnets are distinct states of matter that do not fit in the high school classification. Condensed matter physics (CMP) is concerned with practically any material system that involves a large number (say at least a million) of interacting atoms or molecules. We can consider this to be a complex system because there are many different ways of arranging the constituents (atoms or molecules) of the system. What approaches and techniques do condensed matter physicists use to study and understand these systems? CMP provides a coherent intellectual framework for a multi-faceted approach to investigate and understand complex material systems. First, one can look at the material at many different scales ranging from the microscopic leve

Most of the questions are inter-related. Most have been discussed in earlier posts. How do we tune physical properties (e.g. hysteresis width) by varying chemical composition? How do we understand two-step transitions? Are they associated with spatially inhomogeneous arrangements of the spin? Are spin ice phases possible? What is the physical origin of the intermolecular interactions that lead to a first-order transition? Is it electronic (magnetic) and/or elastic? Are there long-range interactions? Are they crucial? Is there a simple way to understand the change in vibrational spectra (and thus entropy) associated with the transition? What is the role of spatial anisotropy? What is the simplest possible effective model Hamiltonian that captures the physical properties above? Can the elastic degrees of freedom be "integrated out" to give a "simple" Ising model? How do the model parameters depend on structural and chemical composition?