Condensed concepts

Previously, I have highlighted how one of the organising principles for understanding quantum nuclear effects in hydrogen bonding is that of competing quantum effects. This idea features in this talk and this recent  review about water. Basically, as the strength of the hydrogen bond in an X-H...Y systems increases, the zero point energy associated with the X-H stretch (bending) vibrational modes increases (decreases). The effect manifests in a wide range of isotope effects where hydrogen is replaced with deuterium. The relative magnitude of these competing effects changes with the bond strength, and so the sign of the isotope effects can be positive or negative. This week I learned of another nice example of competing quantum effects in the paper. Why Does Argon Bind to Deuterium? Isotope Effects and Structures of Ar·H 5O 2 + Complexes Laura R. McCunn, Joseph R. Roscioli, Ben M. Elliott, Mark A. Johnson, and Anne B. McCoy The figure below shows the ground state geometry

There is an interesting blog post Yes the equations are correct about how the latest Ghostbusters movie used props from the MIT physics department. The Big Bang Theory is also known for using real science and equations, as I have noted before. This week Anne McCoy, who is a Deputy Editor of the Journal of Physical Chemistry A,  told me about another prop, shown below, a Festschrift issue for Sheldon Cooper. The story of how this came about is described here.  It is not totally accurate, because these Festschrifts are normally for people on their 60th birthday and Sheldon is a string theorist, at least until recently , not a Physical Chemist.  Below are a couple of screen shots from the show where you can see the prop in the background in Sheldon's office.

I am on sabbatical this semester. The first week I visiting the Chemistry Department at the University of Washington. My host is Anne McCoy who has done a lot of nice work on the theory of the infrared spectroscopy of H-bonds. [Some of her work featured in this post a while back]. Here is the current version of slides for a seminar I am giving tomorrow. It is largely based on this paper and this preprint.

Science is a verb not just a noun. This idea is expanded upon in detail in the introduction to Phil Nelson's beautiful new textbook Physical Models of Living Systems. “Science is not just a pile of facts for you to memorize. Certainly you need to know many facts, and this book will supply some as background to the case studies. But you also need skills . Skills cannot be gained just by reading through this (or any) book. Instead you'll need to work through at least some of the exercises, both those at the ends of chapters and others sprinkled throughout the text. Specifically, this book emphasises    Model construction skills :   It's important to find an appropriate level of description and then write formulas that make sense at that level. (Is randomness likely to be an essential feature of this system? Does the proposed model check out at the level of dimensional analysis?) When reading others' work, too, it's important to be able to grasp what assumptio

In clean elemental metals the mean free path is much larger than the lattice constant and the Fermi wavelength. This means that an electron (or quasi-particle) has a well defined wavelength and momentum (wave vector) between collisions which change its momentum. Thus, quasi-particles are a well defined entity. However, consider that limit where the scattering becomes so strong that the mean-free path becomes comparable to the Fermi wavelength (or lattice constant). Then clearly the idea of a quasi-particle with a well define wave vector and a mean free path does not make sense. The resistivity (in a Boltzmann-Bloch) picture is inversely proportional to kF l. Waving ones hands one can argue that in a metal there is a maximum value for the resistivity. This is known as the Mott-Ioffe-Regel (MIR) limit. Waving one hands  some more, one might argue that as the temperature increases (and inelastic scattering increases) towards the MIR limit the resistivity might saturate or eve

The New York Times has a good article So Many Research Scientists, So Few Openings as Professors . I am happy these issues are getting more attention in the media. However, I feel that it fits with a common unquestioned narrative: “The purpose of science Ph.Ds is to train people to be faculty members at research universities. However, it turns out the Ph.D production rate is vastly greater (an order of magnitude or more?) than the job vacancy rate. Therefore, Ph.D production rates should be reduced.” My perspective is somewhat different. I think having a lot of science Ph.Ds is arguably good for society. Here is what needs to change in the three relevant communities. Politicians, funding agencies, and university administrators and marketers. Stop lying or being deluded. There is no shortage of science Ph.Ds. Tenured jobs in academia are very limited. Most Ph.Ds have very little chance of getting one. The job market is pretty much like it has been for the past 40 years. D

Following up on my previous post , below are some selected comments I got from students in my thermodynamics class last semester.  A couple (in bold) comment on how the lectures help understand the reading. But, there is an interesting follow up. Since the students now have their grades I received the student evaluations. Some complained strongly that the lectures were poor/useless because they just repeated what was in the reading. Others complained that I did not answer all the questions they raised in the reading.  I am sure I can do better, but this just highlights to me that it is impossible to keep all students happy.  For some you go too fast, some too slow.  For some you give too much detail, others not enough detail.  For some you follow the book too closely, for others not closely enough.  For some you repeat things too much, for others not enough. I would really like to go through the derivations and maths in class. It's so much harder to read than it

I finally read most of an interesting Colloquium article in Reviews of Modern Physics Statistical mechanics of money, wealth, and income  Victor M. Yakovenko and J. Barkley Rosser, Jr. [I mentioned the review 2 years ago in a post about the science of economic inequality ]. It reviews the history and concept of econophysics , pointing out how some of the founders of statistical mechanics actually had a vision for its application to economics and sociology. Most of the review is about analogues with statistical mechanics that use the notion of money as a conserved quantity that is exchanged by individuals, leading to Boltzmann type distributions for wealth and income. I found the article a nice accessible introduction to the field. What is impressive is that the simple exponential distribution (Boltzmann) does describe empirical data over two orders of magnitude. Furthermore, the analysis gives some insight into economic inequality. This is summarised in the following sentences

Physicists tend to think of universality (the details don't matter)  as a physical phenomena. However, in the early days of this blog I posted about how I became aware that the central limit theorem in mathematics is an example of universality. Terence Tao has a nice article intended for a popular audience , “E pluribus unum: from complexity, universality”  that gives several examples of universality in mathematics and physics. The figures are particularly nice. Some of the examples given include the law of large numbers, the central limit theorem, and  Benford's law .  "A histogram of the first two digits of accounts payable data of a major software company, together with the Benford's law prediction. (Source: Journal of Accountancy)" The nicest part of the article is the discussion of the connections between random matrix theory, energy level spacings in nuclear physics, and the spacing of prime numbers and zeros of the Riemann zeta function . Univer

A major achievement of ultra cold atoms was to simulate the Bose Hubbard model 14 years ago. Progress towards the fermion Hubbard model (beloved in the strongly correlated electron community) has been slower.  Cooling fermionic atoms is much more difficult. There is a recent PRL which gives some indication of where things are at. Observation of 2D Fermionic Mott Insulators of K40 with Single-Site Resolution  Lawrence W. Cheuk, Matthew A. Nichols, Katherine R. Lawrence, Melih Okan, Hao Zhang, and Martin W. Zwierlein The figure below shows the measured local magnetic moment as a function of temperature. The upper curve is at half filling and the bottom for a strongly doped system. The solid lines are theoretical curves obtained from high temperature series expansions. The temperature is scaled by the hopping integral t in the Hubbard model. The authors note they have reached entropies as low as k_B. Although this is significant progress it should be noted that these a

Fifty years ago in most universities there were generally only three academic activities for undergraduates: lectures, labs, and end of year exams. Students did not have to show up for lectures [which mostly consisted of transcribing the lecturers notes from the board]. Students were considered adults and held responsible for their actions. They were more or less left to study on their own. If they failed exams that was their fault. Times are now very different, for better or for worse. There are a host of additional activities  and assessments to enhance learning,  keep students engaged, and motivate them to work : tutorials , weekly problem sets, class blogs,  randomly timed in class quizzes, mid-semester tests, on-line reading quizzes , clickers, ... In some courses students even get marks for just showing up at lectures and/or tutorials. I give one example, that is not unusual. At one Ivy League university in the freshman chemistry course, attendance is monitored by requiring

In Telluride Francesco Paesani gave a nice talk about his work on metal-organic-frameworks , including those that exhibit spin crossover. The underlying physics is fascinating as there is a subtle interplay of electronic, magnetic, and lattice effects. The crossover/transition is actually driven by changes in the vibrational entropy. Spin "crossovers" involve a transition such as that shown above, from a S=2 state (HS=high spin) to a S=0 state (LS=low spin) often seen in Fe(2+) compounds.This transition occurs as the temperature is decreased, and sometimes is not a crossover, but a first order transition with hysteresis. Spin crossover in metal-organic frameworks (MOFs) are of such great interest because the transition temperature can  be comparable to room temperature vary significantly as the MOF (which is very porous) adsorbs "guest" molecules such as water. These properties have significant technological potential but also involve some fascinating

On tuesday the Telluride Science Research Center hosted a nice public lecture , "Clouds in a bowl of soup," by Graham Feingold , an atmospheric scientist. He emphasised how the atmosphere is a complex system that exhibits emergent phenomena, particularly pattern formation and synchronisation. He discussed how one sees these phenomena in other systems, such as soup ( Rayleigh-Benard convection cells ) and fire flies. Aside: in a similar vein there is a nice Physics Today, Quick Study, The universe in a cup of coffee by John Wettlaufer. Emergent behaviour results from simple rules. The four rules for clouds are 1. Drops form on aerosols (suspended particles) and grow by vapour diffusion. 2. Drop coalescence generates rain. (Aerosols can influence rain). 3. Drops fall and evaporate. 4. Continuity of air flow. Some of the work he described is in this paper , which includes the figure below. Some key physics relevant to climate change is that clouds generally re