Wednesday, April 17, 2019

The emergence of social segregation

Individuals have many preferences. One is that we tend to like to associate with people who have some commonality with us. The commonality could involve hobbies, political views, language, age, wealth, ethnicity, religion, values, ... But some of us also enjoy a certain amount of diversity, at least in certain areas of life. We also have varying amounts of tolerance for difference.
A common social phenomenon is segregation: groups of people clump together in spatial regions (or internet connectivity) with those similar to them. Examples range from ethnic ghettos and teenage cliques to "echo chambers" on the internet.

In 1971 Thomas Schelling published a landmark paper in the social sciences. It surprised many because it showed how small individual preferences for similarity can lead to large scale segregation. The context of his work was how in cities in the USA racially segregated neighbourhoods emerge.

One version of Schelling's model is the following. Take a square lattice and each lattice point can be black, white or vacant. Fix the relative densities of the three quantities and begin with a random initial distribution. A person is "unhappy" if only 2 or less of their 8 neighbours (nearest and next-nearest neighbours) on the lattice are like them. [They have a 25% threshold for moving]. They then move to a nearby vacancy. After many iterations/moves to an equilibrium is reached where everyone is "happy" but there is significant segregation.

The figure is taken from here.

There are several variants of the model that Schellman presented in later papers and an influential book Micromotives and Macrobehavior, published in 1978. He received the Nobel Prize in Economics in 2005 for work in game theory.

There is a nice simulation of the model in NetLogo. For example, you can see how if you set the individual preference for similarity at 30% one ends up with a local similarity of 70%.

This can be considered to be the first agent-based model. It is fascinating that Schellman did not use a computer but rather did his ``simulation'' manually on a checkerboard!

Physicists have considered variants of Schelling's model that can be connected to more familiar lattice models from statistical mechanics, particularly the Ising model. Examples include

Ising, Schelling and self-organising segregation 
D. Stauffer and S. Solomon

Phase diagram of a Schelling segregation model
L. Gauvin, J. Vannimenus, J.-P. Nadal
This connects to classical spin-1 models such as the Blume-Capel model.

A unified framework for Schelling's model of segregation 
Tim Rogers and Alan J McKane

Competition between collective and individual dynamics 
Sébastian Grauwin, Eric Bertin, Rémi Lemoy, and Pablo Jensen

Shelling's model is a nice example of emergence in a social system. A new entity [highly segregated neighbourhoods] emerges in the whole system thatwase not anticipated based on a knowledge of the properties of the components of the system.

Friday, April 12, 2019

Should graduate students pick their own research field?

Paul Romer won the Nobel Prize in Economics in 2018. There is an interesting podcast where he is in conversation with Tyler Cowan. In it, there is the following quote that readers may love!
We subsidize graduate education through money that goes to professors, but we let the professors make the decisions about the problems they work on, and then, therefore, the things the students are trained in. I’d rather let the students be the ones who decide, “Yeah, I don’t really want to work in high-energy physics. It’s kind of dead end. I think there’s something much more exciting in condensed-matter physics.”
I mostly post this for amusement.
[I thank my economist son, for bringing it to my attention].

However, Romer does raise an interesting issue. There is a distinct contrast between the systems in the USA and Australia. In the USA faculty get grants and use them to hire graduate students. In Australia, most Ph.D. students get their own scholarship (fellowship) which pays their tuition and a living allowance (salary). They are then free to pick an advisor (supervisor) and topic, which is then approved (usually routinely) by various committees.
I am not sure this is a better system. Too often, students still tend to flock to advisors who are "famous" (but give them little time or exploit them) or those working on the latest fashionable (hyped up) topic ... On the other hand, if a student wants to work on a particular topic that is currently not "hot" there is more opportunity for that, for better or worse.

What do you think?

Tuesday, April 9, 2019

Coupling of the lattice to spin-crossover transitions

There is a very nice paper
Complete Set of Elastic Moduli of a Spin-Crossover Solid: Spin-State Dependence and Mechanical Actuation 
Mirko Mikolasek, Maria D. Manrique-Juarez, Helena J. Shepherd, Karl Ridier, Sylvain Rat, Victoria Shalabaeva, Alin-Ciprian Bas, Ines E. Collings, Fabrice Mathieu, Jean Cacheux, Thierry Leichle, Liviu Nicu, William Nicolazzi, Lionel Salmon, Gábor Molnár, and Azzedine Bousseksou

It investigates the spin-crossover in a specific compound with a suite of techniques, including x-ray diffraction, inelastic neutron scattering, and micro-electromechanical systems (MEMS).

The nice results reflect significant advances over the past few decades in neutron scattering and microfabrication.

The graph below shows the vibrational density of states of the Fe nuclei and the low-spin (LS) and high-spin (HS) states. Note how the LS modes around 50 meV (400 cm-1) soften significantly in the HS state. These modes are the Fe-N stretches in the octahedron. This softening is associated with a significant increase in entropy which helps drive the spin-crossover transition.
Note that there is a small parabolic part at low energies of a few meV.

The figure below is a "blow up" of the low energy data. 
The energy resolution is amazing! 
The graph shows the density of states divided by E^2. 
In an isotropic solid (or a powder such as used here) in the Debye model for phonons, the density of states is proportional to E^2 and the proportionality is determined by the speed of sound, v_D.

One clearly sees several things.
1. For E less than about 3 meV, the DOS is quadratic in E, as predicted by Debye.
2. The lattice softens with the spin-crossover.
3. The deviation from quadratic occurs at a smaller E for HS than LS.

The values of the speed of sound can be combined with the lattice constant to estimate the Debye frequency, which is roughly where the deviation from quadratic dependence should occur.
I have done this (since the authors don't appear to have) and one gets values of the order of a few meV, consistent with experiment.

From the speed of sound, one can also determine the Young's modulus. This can then be compared to the bulk modulus, which can be determined from MEMS. The values obtained by these different methods are consistent with one another.
Overall, the values of the bulk modulus for different spin-crossover complexes, of order 5-10 GPa, are comparable to those for organic molecular crystals.

Friday, April 5, 2019

What is condensed matter physics?

How would you answer this question if you were asked by a non-expert who likes to learn and understand new things?
For example, a smart high school student, your uncle who reads a lot of popular science, an academic colleague in sociology, an economics graduate, ...
A draft of my answer is here.
I welcome suggestions for improvements.
Feel free to try it out on people you know who might be interested.

Tuesday, April 2, 2019

Chemistry finally joins the arxiv era

The physics arXiv started way back in 1991. Yet chemists strongly resisted following suit. Indeed if you posted a preprint on the arXiv American Chemical Society (ACS) journals would not publish it.
Eight years ago, Derek Lowe, asked Why Isn't There an arXiv for chemistry?

Well, finally ACS has succumbed and set up their own chemrxiv and announced that they will consider manuscripts that have been posted on the arXiv.

I thank Ben Powell for letting me know about the promising development.

Thursday, March 28, 2019

Why is hype bad?

There is no doubt that the level of hype in science is increasing. You see it in grant applications, university press releases, introductions and conclusion in papers (especially in luxury journals), talks, ... Hype is also a broader problem in society, including in the business world and politics.

Why is hype bad for science?
Some will say something like, ``I agree that it is not good, but we have to do it to survive. Anyway, we all know what is really true and so it does not matter..."

However, I think there are many problems, particularly for the long term flourishing of science.

Waste of time
Figuring out that a ``hyped'' result or research field is actually just hype can take significant time. This is particularly true if one actually tries to reproduce a result and discover all the problems.

Mis-allocation of resources
Researchers, students, and funding agencies flock to hyped fields. However, it can take quite a while and a lot of money for the community to come to the consensus that things are not going to live up to the hype. This is compounded by the fact that people whose careers are enhanced by a hyped result or field are not going to back down too easily and are going to want to keep things going, at least until the next big thing comes along.

Obscuring or hiding problems that need to be solved to make real progress
Making real and significant progress in science is very hard. Every technique has its limitations. Every result involves some uncertainty. Turning science in the lab into a commercially viable technology may arguably be even harder. The obstacles are many. The best way to progress in science and technology is to clearly and honestly state the problems and challenges. This is one of my many concerns about functional electronic materials.

Intellectual integrity
Science is all about intellectual integrity. Losing our credibility with broader society won't be good for science or for society, particularly when it comes to developing well-informed public policy.

Tuesday, March 26, 2019

Noel Hush (1924- 2019): pioneering theoretical chemist

I was sad to hear last week that Professor Noel Hush died at age 94. Noel [also known as Prof.] was a pioneer in theoretical chemistry and chemical physics. He had a profound influence on both fields, particularly in their development in Australia.

Arguably his greatest scientific contribution was in the theory of electron transfer. Depending on where you are from this is called Hush-Marcus theory, Marcus-Hush theory, or Marcus theory. In particular, in 1958 Hush derived one of the most important equations in chemical physics, which can be used for design principles for functional electronic materials. A key concept here is the notion of diabatic states.

I had the privilege of knowing and working with Prof. Hush on and off over the past decade. As I made an adiabatic transition from condensed matter into chemical physics Prof. Hush provided a lot of encouragement, wisdom, perspective, and ideas. He strongly believed that theoretical chemists and condensed matter theorists could have mutually beneficial interactions. Together with Jeff Reimers and Laura McKemmish, we co-authored seven papers together. The last papers were published when Noel was 90 years old!

Besides his significant legacy of scientific knowledge, there is an incredible legacy of people that he taught, supervised, mentored, encouraged, and collaborated with.

There is an interesting interview of Prof. Hush about his life by Robyn Williams from 2011.