Extracting data from published figures

A key aspect of science is comparing your results with those of earlier work. This may seem basic but people do not do it as much as they should. For example, theorists should be comparing their results to actual experimental data.

But this post is just concerned with the mundane practicalities. In the "old days" one would ask experimentalists to send an electronic version of the data. Now it is possible to just extract the data from figures in papers.

A relatively easy way is using DataThief. Last week I downloaded it and found within 2 hours I had figured out how to use it and could produce graphs with my theoretical curves compared to the experimental data.

I welcome ideas on alternatives.

Can more energy research help save the planet?

There is an interesting (and depressing and challenging) Perspective, The Gratzel cell: Where next?, by Laurence Peter in Journal of Physical Chemistry Letters. Here is the beginning of the abstract:

Twenty years after O’Regan and Grtzel’s seminal Nature paper entitled “A Low-Cost, High-Efficiency Solar-Cell Based on Dye-Sensitized Colloidal TiO₂ Films”, dye-sensitized solar cells (DSCs) and analogous devices have become a major topic of research, with over 1000 papers published in 2010. Although much more is now known about the physical and chemical processes taking place during operation of the DSC, the exponential increase in research effort during this period has not been matched by large increases in efficiency. 

The paper gives a nice summary of some of the key scientific challenges. I thank Seth Olsen for bringing the paper to my attention.

Hund's rule coupling in multi-band metals

There is a really nice preprint, Janus-faced influence of the Hund’s rule coupling in strongly correlated materials, by Luca de’ Medici, Jernej Mravlje, and Antoine Georges.

The figure below is a helpful summary of the main results. The colour shading shows the quasi-particle weight Z in the metallic phase as a function of U/D and band filling for a system with 3 degenerate bands for a fixed value of J=0.15U. [The relevant multi-band Hubbard model is solved at the level of Dynamical Mean-Field Theory (DMFT)]. The cases n=2 and 4 are particularly interesting because there is a large range of U/D for which one has a metallic phase with small Z. The authors characterise this as a bad metal (i.e. which occurs above some relatively low coherence temperature T*).

One minor comment. The authors mention just a few signatures of bad metals [large resistivity above the Mott-Ioffe-Regel limit and large poorly screened local moment]. Others include no Drude peak in the optical conductivity, and a large thermopower ~k_B/e [see this earlier post].

So what is this about Janus faced? I had no idea. But I found out that Janus was a Roman God of transitions. He looked to the past and future and was often depicted in statues as "two-faced". The point here is that Hund's rule can either enhance or reduce the correlations, depending on the filling.

I thank Jaime Merino for bringing the paper to my attention and for some helpful discussions about it.

Detecting small electric fields in membranes

A lot of molecular biophysics is determined by classical electrostatics. But actually measuring and simulating the relevant electric fields is a very difficult exercise.

Seth Olsen brought to my attention a nice paper

Direct Measurement of the Membrane Dipole Field in Bicelles Using Vibrational Stark Effect Spectroscopy

These results should provide significant constraints on molecular dynamics simulations on such systems.

Also, this seems a more straightforward approach to investigating ion channels than a recent proposal to use quantum decoherence of NV diamond centres to detect the very small magnetic fields associated with the electrical currents in ion channels. But, I am probably missing something.

A classic paper on a theory of the hydrogen bond

Charles Coulson is one of my scientific heroes. I love his book Valence, which help shape the development of quantum chemistry. With Danielsson he published two often cited papers on a valence bond theory of the hydrogen bond in 1954. But they are in a now defunct journal Akiv fur Fysik, published by the Swedish Academy of Sciences from 1949 to 1974. I could not find the article online by managing to get a copy from the Warehouse of the UQ library. So I post a copy here in case others would like to read this classic paper. Just the conclusion is worth reading.

One minor comment. The authors consider a basis of 3 valence bond states. Two of these involve the ionic and covalent components of an O-H bond in the donor molecule. Hence, it may be possible to combine these both into a single diabatic state describing the full O-H bond. I think this is essentially what is done in Warshel's empirical valence bond theory of proton transfer reactions.

Trivial aside: Danielsson's address is Swedish Cement and Concrete Research Institute! But, the article says they did the work at Kings College London.

Deconstructing electron-doped cuprates

Last week there was an interesting paper in Nature Link between spin fluctuations and electron pairing in copper oxide superconductors 
Most high-Tc cuprate superconductors are hole doped. However, the past decade has seen studies of electron doped cuprates which have some similarities but also some significant qualitative differences. Thus there is electron-hole asymmetry.
In particular there appears to be no pseudogap in the electron-doped materials and they are not as strongly correlated.

The authors measured the temperature and doping dependence of the intralayer resistivity and deduced the phase diagram below.

Specifically, they found that for dopings x less than approx. 0.17 they could fit the resistivity to a linear in T form over 3 decades of temperature. As x decreased the co-efficient of proportionality increased roughly proportional to Tc.
For x larger than 0.17 there is no superconductivity and the resistivity could be fit to a quadratic T dependence, characteristic of Fermi liquid theory. The coefficient of proportionality becomes larger as x=0.17 is approached.

The authors note similar behaviour is seen in the Bechgaard salt (TMTSF)2PF6.

Later I will compare the above behaviour to what happens in the hole-doped cuprates, reported in a 2009 Science paper.

If one sticks with a one band Hubbard or t-J model and a band structure with next-nearest neighbour hopping t' [which produces electron-hole asymmetry] can one reproduce the key aspects of the electron-hole asymmetry in the phase diagram?
Maybe. But if one looks at the underlying electronic structure the electron and hole doped materials are different, according to this recent Nature Physics paper.

I thank Nigel Hussey for bringing the paper to my attention.

Not the humanities!

This is an amusing clip about "physics hubris" and fund raising on The Big Bang Theory.