Substance before appearance

I read this amusing and instructive story about the dress sense of physicists on a Physics Today blog post by Charles Day:

The most extreme example of sartorial insouciance I've witnessed was that of James Heath, a pioneer of molecular computing (and who would probably call himself a chemist, I should point out).

One November, Heath flew from Los Angeles to Boston to give an invited talk at the Materials Research Society meeting. He showed up in the convention center wearing a brightly colored short-sleeved shirt, shorts, and, if I remember correctly, sandals. Not only had he forgotten to dress for Boston's weather, he'd also left his laptop in California.

Did those mental slips matter? Hardly. Using hastily prepared, hand-written viewgraphs, he gave one of the best talks of the meeting. Indeed, it's conceivable that in creating his viewgraphs, Heath was forced to focus more on his message than on its presentation.

Solid State Simulations on Mac OSX

I love the software and book Solid State Simulations. I have used it extensively in teaching, particularly as assignment exercises. I have always been delighted and amazed at how much I have learnt from it too! There is no substitute for visualization. Seeing is believing. I still remember the first time I saw electrons and holes going in opposite directions around their respective Fermi surfaces in a magnetic field.

However, I was alarmed a few weeks ago when I found I could not run it on my Mac. I just started installing a trial version of Parallels to see if it I could run it then. [BTW: I could never get registration for the trial version of VMware Fusion to work]. However, I just discovered that is not necessary! An OSX version has just been released. You can download it for free now.

Spin nematic fluctuations and elastic anomalies.

Previously I posted about some fascinating experimental results on anisotropic thermal expansion and elastic softening near superconducting and magnetic transitions in organic charge transfer salts.
Subsequently, I became aware that the new iron pnictide superconductors do exhibit somewhat similar phenomena. A combined theory-experimental PRL (10 co-authors!) describes shear acoustic mode spectroscopy in terms of nematic spin fluctuations.

They find in undoped BaFe2As2 that the shear modulus (C66) softens significantly as one approaches the magnetically ordered phase (which is a associated with a tetragonal-orthorhombic lattice distortion). For the optimally doped material there is a hardening of the lattice as one enters the superconducting phase.

The figure above explains the nematic order parameter and how it couples to shear lattice distortions. A key is the that the magnetic phase consists of Neel antiferromagnetic order on two separate sublattices . They are weakly coupled together and the nematic order parameter phi equals the dot product of m1 and m2, the antiferromagnetic order parameters on the two separate lattices. phi then couples directly to the shear strain.

The softening into the superconducting (SC) phase is explained by a coupling of the SC and AFM order parameters. This leads to a change in the static spin susceptibility upon entering the SC phase. This in turn effects the fluctuations in the nematic order parameter.

A couple of comments:

a. Experimental data is presented just for C66. It would be helpful to see it for longitudinal sound and for other transverse modes besides epsilon_s=epsilon_xy. These modes should not have significant coupling to superconductivity and magnetism if the nematic mode is where all the action is.

b. In other antiferromagnets lattice anomalies at magnetic transitions are explained in terms of the spin anisotropy (e.g. due to the Dzyaloshinsky-Moriya interaction) coupling to the different components of the stain tensor. This is reviewed here by Lines. Can that be ruled out in the pnictides?

10 key ideas about the electronic properties of crystals

This semester I am co-teaching PHYS4030 Condensed Matter Physics: Electronic properties of crystals. This is a fourth year undergraduate (honours) course. Since there are only 5 students enrolled the course will run as an interactive reading course, somewhat similar to a biophysics course, BIPH3001 and PHYS3170 which I ran the last two years. The text will be the classic Ashcroft and Mermin [Although I wonder if we shouldn't change to Marder].
I have an introductory overview lecture of the first half of the course which discusses 10 key ideas.

Motivating and assessing undergraduate students

In the "good old days" university students were "self-motivated", assessment was almost all based on exams and whether students showed up for tutorials and lectures or read the textbook was their own problem. However, the painful reality today is that most students will usually only do some work under a "carrot and stick" system of continuous assessment. Consequently, in many courses students can get credit for just showing up at tutorials and can get easy marks for assignments, some of which they get "help" with from their friends. This means that a student can pass a course even though they get a failing mark on the final exam. 

This semester I am co-teaching PHYS2020 Thermodynamics and Condensed Matter Physics with Joel Corney. [I have previously posted lectures I have given in this course]. Joel recently came up with a new assessment system that I think addresses some of the problems mentioned above. The details are below. I would be interested in hearing from people who have used such a system or other alternatives.


Summative Assessment  
Laboratory Reports    25%
Midsemester Exam    20%
Final Exam    55%
Total for Summative Assessment: 100%


Formative Assessment
Tutorial group work    40 points
Homework Problems    40 points
Reading Quiz    20 points
Total for Formative Assessment: 100 points

Your overall grade will primarily be determined by the total summative mark.  However, you cannot pass the course without also passing the formative component, and an excellent mark for the formative component may push you up into the next highest grade.

The assessment matrix shows the minimum summative mark (along the bottom) and formative mark (down the side) required to achieve each grade:

Assessment Matrix

85 points	2	3	4	5	6	7	7
75 points	1	2	3	4	5	6	7
65 points	1	2	3	4	5	6	7
50 points	1	2	3	4	5	6	7
45 points	1	2	2	2	2	2	2
25 points	1	2	2	2	2	2	2
0 points	1	1	1	1	1	1	1
	0%	25%	45%	50%	65%	75%	85%

Chemical bonds in a class of their own

An important concept which has featured in previous posts is that of 4 electron, 3 orbital chemical bonding. It is relevant to hydrogen bonding and methine dyes.
Here is a scan of some of the relevant pages of the beautiful book, Valency and Bonding by Weinhold and Landis.

Nanotechnology: from the fourth century A.D. to the Middle ages

Today there was an interesting Quantum science seminar by Ulrik L. Andersen (Technical University of Denmark) Quantum Plasmonics: Controlled Coupling of a Single Nitrogen-Vacancy Center to a Silver Nanowire.

A question came up about plasmons in gold  nano-particles and how the surface plasmon frequency is renormalised downwards (i.e. blue-shifted) compared to the frequency in the bulk. Gerard Milburn pointed out that this is illustrated by The Lycurgus Cup in the British Museum. Coincidentally, an article by Mark Stockman in this months Physics Today states:

The resonant properties of plasmonic metal nanoparticles are readily apparent to the naked eye because the excitations absorb and scatter light at optical frequencies. The most ancient example is the famous fourth-century CE Lycurgus cup from the British museum, whose glass looks green in reflected light but ruby red in transmitted light. Those colors are complementary, evidence that there is little optical loss inside the glass. Investigation has shown that the dichroic glass contains nanocrystals of a gold–silver alloy at a fraction of less than 1%.

Such colloidal suspensions of gold and silver have been widely used in stained glass since the Middle Ages. Transmission through a silver colloid yields yellow light and transmission through gold yields ruby red. The magnificent colored light from the stained glass of the Sainte Chapelle in Paris is assumed to be largely due to the nanoplasmonic resonances.

Unlike glass-staining metal ions such as iron, chromium, copper, and cobalt, metallic nanoparticles, which both absorb and scatter, transmit light with an intensity that strongly depends on the incident and viewing angles. The Sainte Chapelle dramatically exploits the effect: At sunset, the grazing-angle scattering of light by gold nanoparticles in the windows creates a pronounced red glow that appears to slowly move downward, while intensities of blue tints from ions of copper or cobalt remain the same. The artistic impression, probably intended, suggests a stream of blood slowly flowing downward. [See the photo below].

BTW: I can't see the stream of blood.