A unified picture of iron and cuprate superconductors

Oh, how I love simplicity and physical insight!

There is a beautiful Physics Viewpoint Untangling the Orbitals in Iron-Based Superconductors by Daniel Podolsky.

It describes a Phys. Rev. X paper by Jiangping Hu and Ningning Hao.
They show that a key to understanding the electronic structure of the iron pnictide and chalcogenide superconductors is the S4 symmetry of the lattice of Fe ions.

They are able to explain why the iron chalcogenides do not have holelike Fermi surface pockets whereas the iron pnictides do.

Furthermore, by means of a simple gauge transformation associated with the S4 symmetry they map the s-wave superconducting order parameter to the d-wave order parameter associated with the C_4v symmetry of the Cu ions in the cuprates.

Strong correlations lead to local physics at relatively low temperatures

Antoines Georges has a nice article Thinking locally: Reflections on Dynamical Mean-Field Theory from a high-temperature/high-energy perspective.
The article is a short review dedicated to Dieter Vollhardt on his 60th birthday.

He discusses recent work which compares DMFT to high temperature series expansions for the Hubbard model. They agree for temperatures above about t, the hopping integral. This supports intuition that with increasing temperature physics becomes more local and so DMFT should be more reliable.

He also mentions cold atom experiments which are getting towards low enough temperatures that they could potentially be used to test DMFT predictions.

Born-Oppenheimer in nuclear physics

How do single nucleons and associated excitations couple to collective degrees of freedom such as rotations and shape deformations?
Is there a Born-Oppenheimer approximation in nuclear physics?
What is the origin of non-spherical nuclei and the associated symmetry breaking?
Is the notion of a Jahn-Teller effect and conical intersections relevant?

There issues go back to classic ideas in theoretical nuclear physics for which Aage Bohr, Mottelson, and Rainwater were awarded the Nobel Prize in Physics in 1975. This is discussed in an earlier post.
There is also a classic paper by Hill and Wheeler which does include a discussion of conical intersections [I thank Seth Olsen for bringing it to my attention].

The relevant physics is elegantly discussed in a nice review article The Nuclear Collective Motion by Witold Nazarewicz. Here is an extract

He then goes on to discuss how the deformations of nuclei can be understood in terms of the Jahn-Teller effect.

The figure below is a microscopic calculation from a density functional method of the energy as a function of the nuclear deformation of different Nd isotopes. As the mass number A=N+Z increases there is a transition from a spherical nuclei to an axially deformed one.

This figure is taken from a recent RMP Quantum phase transitions in the shape of atomic nuclei. 

Things I am still looking for discussions are 

1. using diabatic states

2. roles of conical intersections, particularly in dynamics

3. breakdown of Born-Oppenheimer.

Are reaction speed and efficiency linked?

There is an interesting JACS communication
Backbone Modification of Retinal Induces Protein-like Excited State Dynamics in Solution from a group at Oxford.
Here is some of the abstract:

The drastically different reactivity of the retinal chromophore in solution compared to the protein environment is poorly understood. Here, we show that the addition of a methyl group to the C=C backbone of all-trans retinal protonated Schiff base accelerates the electronic decay in solution making it comparable to the proton pump bacteriorhodopsin.
Contrary to the notion that reaction speed and efficiency are linked, we observe a concomitant 50% reduction in the isomerization yield.

The results are of particular interest because most previous attempts to modify the chromophore and/or solvent have not led to much change in the photo-isomerisation rate.

A point I am not clear on, concerns the significance of the statement:
  "Contrary to the notion that reaction speed and efficiency are linked, we     observe a concomitant 50% reduction in the isomerization yield."

The authors point out that "a common feature of highly efficient light-induced biological processes" is "the correlation between ultra-fast dynamics and high reaction efficiency."
One some level this is not surprising: if one process is much faster than all the other competing processes then that process will have a high quantum yield. However, if we change a variable and increase the speed of a process we should not necessarily expect the quantum yield to increase dramatically since the speed of other competing processes may also increase.
Or am I missing something?

I thank Seth Olsen for bringing this paper to my attention.

Changing face of Australian universities

In the past week I saw two articles highlighting changes in Australian universities. The Weekend Australian ran a piece Mass education changes face of universities by Julie Hare.
[There is also a video summary but it lacks some of the important statistics and graphs in the article.]

While the total population of Australia has increased by a factor of less than than three since the 1960's the university student population has increased by a factor of 26!

But, increasingly, graduates work in areas unrelated to their studies. Only half of law graduates will work as lawyers. Just 2 per cent of teaching graduates in NSW each year are directly offered a position in the state school system. About 4750 students are enrolled in journalism schools, but with 100 entry-level positions in mainstream media, many will struggle to get off the starting block. 

Corkindale says: "Does the community, including school-leavers and their parents, need to be made more aware that going to study at a university for a lot of students is, it seems, for 'education's sake', not training for a particular job, profession or career?" 

And should universities and government be more upfront about career prospects, particularly in narrow vocational fields? Or is it a case of caveat emptor?

This illustrates to me that degrees such as Arts and Science should be more valued. You might as well do something interesting and which will develop basic thinking, writing, and analytical skills.

The Sydney Morning Herald published Lonely students play varsity blues by Adele Horin, which points out how isolated and relationally disengaged many students are.
For science, this highlights to me the importance of running tutorials which force students to work together in small groups and the value of student clubs such as the legendary student physics club PAIN at UQ.

Effect of a solvent on excited state dynamics

Last two days I have read through a nice paper Modeling the Nonradiative Decay Rate of Electronically Excited Thioflavin T
by Yuval Erez, Yu-Hui Liu, Nadav Amdursky, and Dan Huppert

The relevant molecule [chromophore] is shown below. It is of particular interest because its fluorescence intensity increases significantly when bound to amyloid fibrils which are associated with Parkinson's, type II diabetes, and Alzheimer's disease.
The key photophysics is associated with twisting about the central carbon-carbon bond.

The experimental results that need to be explained are
-the non-radiative life time increased linearly with the solvent viscosity over 3 orders of magnitude
-the fluorescence intensity decreases with time on the scales of tens of picoseconds

The calculated [via TDFT = Time-Dependent Density Functional Theory] dependence of the ground and excited state energies as a function of the twist angle is shown below.

As the twist angle increases from zero to 90 degrees the transition dipole moment between the ground and excited states decreases by 2 orders of magnitude.

Much the above can be qualitatively understood in terms of a two-site Hubbard model where the two sites correspond to two orbitals localised on the two opposite sides of the molecule.

Excitation from S0 to S1 at 30 degrees then leads to downward movement on the S1 potential energy surface towards the minimum at 90 degrees [referred to as the TICT =Twisted Intra-molecular Charge Transfer] state.
Hence, with increasing time the fluorescence intensity decreases due to decreasing oscillator strength for the S1-S0 transition. However, the twisting motion of the large rings is opposed by friction (viscosity) arising from the solvent.

The paper applies a theory due to van der Meer, Zhang, and Glasbeek that I discussed in an earlier post, to give a quantitative description of the experiment. It is assumed that the viscosity is sufficiently large that the twisting motion is classical and over-damped and so can be described by a Smoluchowski diffusion equation. This is solved to given emission line shapes as a function of time. For times larger than 3 picosecond a diffusion constant of D=0.1/psec gives results consistent with experiment. This value is nicely consistent with that predicted by combining the Einstein relation [fluctuation-dissipation relation]
D= k_B T/friction
with a Stokes formula relating the viscosity to the friction for a rotating disc with the size of a phenyl ring.

A few open issues

• it would be nice to see a plot showing the calculated relationship between the non-radiative lifetime and the solvent viscosity with a comparison of the correlations seen experimentally.

• describing the short time dynamics (greater than 3 psec) requires a larger diffusion constant (smaller friction) consistent with the idea of a frequency dependent friction due to the finite relaxation time of the solvent.

• I assume the classical dissipative dynamics is justified because the timescales of interest are much larger than the relevant thermal time ~ hbar/k_B T ~ 20 fsec.

• A very broad "line shape function characteristic of the Frank-Condon factor" is used. The width is 3300 cm-1. It is not clear what the physical origin of this large width is.

• The line shape function is taken to be a log normal distribution. This form is often used to empirically describe emission lines, going back to a 1969 paper by Siano and Metzler. However, as far as I am aware, there is still no clear theoretical justification for this form.

Assessment creep

It is interesting to see how much undergraduate teaching has changed in just the past decade. Here is a case study for just one course I have been involved in.

In 2001 when I came to UQ I first helped teach a second year undergraduate course Thermodynamics and Condensed Matter Physics. Back then any use of Powerpoint or online resources was a novelty. There was no Blackboard or YouTube. Videos were shown by taking a videocassette to a central projection unit on campus and booking for them to show the video in the lecture at the requested time.
Students expected lecture notes and these were photocopied and handed out.
The course comprised 3 lectures and 1 tutorial per week.
All the assessment was exams: one mid semester and a final.
There were 20+ students enrolled.

What about now?
There are 2 lectures and 1 tutorial per week. In addition students do 3 labs lasting 3 hours.
The assessment has expanded substantially. Students now complete
   23 online reading quizzes
   10 problems sets
   12 tutorial problem sets, done during the tutorial in teams of 3 students
   3 lab reports
   mid-semester exam
   final exam

There are now 50+ students enrolled.
There are no lecture notes. Powerpoint slides are uploaded onto Blackboard after the lectures.
Faculty only grade the online quizzes and the exams.
But setting all this assessment is a significant expansion in workload.
Part time tutors and lab demonstators do the rest of the grading.

Times they are a changing...