Wednesday, May 23, 2018

Metrics and mental health

I never thought I would write a post linking the two issues in the title.

I have been working on my talk on mental health for the School of Maths and Physics colloquium on friday. Here is the current version of my slides. I welcome any comments.

In my preparation I have become aware of a few more resources. A recent issue of Nature includes several articles, including:

An Editorial, What to do to improve postgraduate mental health.
Four researchers write from their own experience, How to handle the dark days of depression
A collection of resources.

On the one hand, it is wonderful that Nature is highlighting the issue. On the other hand, it would be nice if they reflected how Nature Publishing Group might actually be part of the problem, as they mindlessly promote metrics and their journals. It is a case of corporate "well-washing."
The link between metrics and mental health is brought out in a report to the Higher Education Funding Council for England.

The Metric Tide, Report of the Independent Review of the Role of Metrics in Research Assessment and Management, July 2015. The preface states:
Too often, poorly designed evaluation criteria are dominating minds, distorting behaviour and determining careers. At their worst, metrics can contribute to what Rowan Williams, the former Archbishop of Canterbury, calls a “new barbarity” in our universities. 
The tragic case of Stefan Grimm, whose suicide in September 2014 led Imperial College to launch a review of its use of performance metrics, is a jolting reminder that what’s at stake in these debates is more than just the design of effective management systems. 
Metrics hold real power: they are constitutive of values, identities and livelihoods. 

Monday, May 14, 2018

Conducting metallic-organic frameworks

Thanks to the ingenuity of synthetic chemists metallic-organic frameworks (MOFs) represent a fascinating class of materials with many potential technological applications.
Previously, I have posted about spin-crossover, self-diffusion of small hydrocarbons, and the lack of reproducibility of CO2 absorption measurements in these materials.

At the last condensed matter theory group meeting we had an open discussion about this JACS paper.
Metallic Conductivity in a Two-Dimensional Cobalt Dithiolene Metal−Organic Framework 
Andrew J. Clough, Jonathan M. Skelton, Courtney A. Downes, Ashley A. de la Rosa, Joseph W. Yoo, Aron Walsh, Brent C. Melot, and Smaranda C. Marinescu

The basic molecular unit is shown below. These molecules stack on top of one another, producing a layered crystal structure. DFT calculations suggest that the largest molecular overlap (and conductivity) is in the stacking direction.
Within the layers the MOF has the structure of a honeycomb lattice.

The authors measured the resistivity of several different samples as a function of temperature. The results are shown below. The distances correspond to the size of the compressed powder pellets.

Based on the observation that the resistivity is a non-monotonic function of temperature they suggest that as the temperature decreases there is a transition from an insulator to a metal. Since there is no hysteresis they rule out a first-order phase transition, as is observed in vanadium oxide, VO2.
They claim that the material is an insulator about about 150 K, based on fitting the resistivity versus temperature to an activated form, deducing an energy gap of about 100 meV. However, one should note the following.

1. It is very difficult to accurately measure the resistivity of materials, particularly anisotropic ones. Some people spend their whole career focussing on doing this well.

2. Measurements on powder pellets will contain a mixture of the effects of the crystal anisotropy, random grain directions, intergrain conductivity, and contact resistances. This is reflected in how sample dependent the results are above.

3. The measured resistivity is orders of magnitude larger than the Mott-Ioffe-Regel limit. suggesting the samples are very "dirty" or one is not measuring the intrinsic conductivity or this is a very bad metal due to electron correlations.

4. It is debatable whether one can deduce activated behaviour from only an order of magnitude variation in resistance, due to the narrow temperature range considered.

The temperature dependence of the magnetic susceptibility is shown below, and taken from the Supplementary material.

The authors fit this to a sum of several terms, including a constant term and a Curie-Weiss term. The latter gives a magnetic moment associated with S=1/2, as expected for the cobalt ions, and an antiferromagnetic exchange interaction J ~ 100 K. This is what you expect if the system is a Mott insulator or a very bad metal, close to a Mott transition.

Again, there a few questions one should be concerned about.

1. How does this relate to the claim of a metal at low temperatures?

2. The problem of curve fitting. Can one really separate out the different contributions?

3. Are the low moments due to magnetic impurities?

The published DFT-based calculations suggest the material should be a metal because the bands are partially full. Electron correlations could change that. The band structure is quasi-one-dimensional with the most conducting direction perpendicular to the plane of the molecules.

All these questions highlight to me the problem of multi-disciplinary papers. Should you believe physical measurements published by chemists? Should you believe chemical compositions claimed by physicists? Should you believe theoretical calculations performed by experimentalists? We need each other and due diligence, caution, and cross-checking.

Having these discussions in group meetings is important, particularly for students to see they should not automatically believe what they read in "high impact" journals?

An important next step is to come up with a well-justified effective lattice Hamiltonian.

Thursday, May 10, 2018

David Pines (1924-2018): quantum-many body theorist

I learnt today that David Pines died last week. He was a pioneer in quantum many-body theory, applying it not just to solid state physics but also to nuclear physics and astrophysics (neutron stars).  Furthermore, he was a great advocate of the importance of emergence in science, writing a classic paper, "The Theory of Everything," together with Bob Laughlin.

David also left a legacy of creative new institutions, being a co-founder of the Sante Fe Institute and the International Institute for Complex Adaptive Matter (I2CAM). Indeed, starting this blog was stimulated by David Pines and I2CAM. In January 2009, I attended the annual meeting of I2CAM in Cambridge and David organised a session on public outreach, including a presentation on blogging.

Piers Coleman, a current co-Director of I2CAM, has written a nice obituary which gives more details about David's contributions, both scientific and institutional.

Friday, May 4, 2018

Metric madness outside the university

Universities are going off the rails because of the blind use of metrics. Unfortunately, this reflects what is also happening in wider society, due to the rise of neoliberalism.
Australia has recently been rocked by scandals involving large banks, leading to the resignation of CEO's, Board chairs, lawyers, ...

This stimulated the following column, by Ross Gittins, the Economics Editor of the Sydney Morning Herald. It is worth reading in full, but I reproduce a few choice extracts.
Banks' misbehaviour shows power of KPIs
... though the financial services industry must surely be the most egregious instance of the misuse of performance indicators and performance pay, let’s not forget “metrics” is one of the great curses of modern times.
It’s about computers, of course. They’ve made it much easier and cheaper to measure, record and look up the various dimensions of a big organisation’s performance, as well as generating far more measurable data about many dimensions of that performance.
Which gave someone the bright idea that all this measurement could be used as an easy and simple way to manage big organisations and motivate people to improve their performance. Setting people targets for particular aspects of their performance does that. And attaching the achievement of those targets to monetary rewards hyper-charges them. Hence all the slogans about “what gets measured gets done” and “anything that can be measured can be improved”. 
Thus have metrics been used to attempt to improve the performance of almost all the major institutions in our lives: not just big businesses, but primary, secondary and higher education, medicine and hospitals, policing, the public service – the Tax Office and Centrelink, for instance. Trouble is, whenever we discover new and exciting ways of minimising mental effort, we run a great risk that, while we’re giving our brains a breather, the show will run off the rails in some unexpected way. .... 
I’ve long harboured doubts about the metric mania, but it’s all laid out in a new book, The Tyranny of Metrics, by Jerry Muller, a history professor at the Catholic University of America, in Washington DC....

Wednesday, May 2, 2018

Two things every Ph.D thesis should contain

I find that too often both of the elements below are missing or are superficial in a thesis. Yet doing them is important training for a student and just plain old good science.

Suggestions for Future Directions
A good research project will raise new questions and challenges.
Most projects do not get as far as the advisor and student would have liked.
The student should write about:
What would do if I had more time?
What should the next Ph.D. student do?
This can be a separate section or chapter at the end of the thesis.

A sober assessment of the strengths and weaknesses of the approach taken and the methods used.
In these current times of hype, the fierce competition for funding and publication in luxury journals mean that many are reluctant to admit any weakness or the value of possible alternatives.
During question time for a colloquium, the speaker was asked, "What are the weaknesses of your approach compared to the alternatives?"
"I can't think of any. It is better in every way."
However, the only way we can improve things is by considering the weaknesses of what we are doing.

Another reason why both of these may be neglected because doing them well is just plain old hard work.

I think these features are equally important in undergraduate and Masters theses as well.

What do you think?
Are there other essentials?

Friday, April 27, 2018

Relating frustrated spin models and flat bands in tight-binding models

What kind of theory paper to I enjoy?
Here are some personal tastes
- "simple" enough I can understand it
- physical insight
- some analytical results
- some pretty pictures that illuminate

This week I read the following paper which I consider nicely meets these criteria.

Band touching from real-space topology in frustrated hopping models
Doron L. Bergman, Congjun Wu, and Leon Balents

The quantum spin antiferromagnetic Heisenberg model on the kagome lattice attracts a lot of attention because it may have a spin liquid ground state, for spin-1/2 and spin 1. This is arguably driven by the large spin frustration. A reflection of this frustration is that the classical model has a non-zero entropy at zero temperature due to a manifold of degenerate states. For this reason, the kagome lattice is sometimes said to be "maximally frustrated". This is in contrast to the triangular lattice for which their is a unique classical ground state and the spin-1/2 model exhibits long-range order.

The kagome lattice is also of interest because of the band structure for the tight-binding model has a flat band, i.e. it is dispersionless. This means that in the presence of interactions the electrons in this band may be strongly correlated and susceptible to instability to new states of matter.

The question arises as to whether there is any connection between these two properties of models on a particular "frustrated" lattice: flat bands and a manifold of degenerate classical ground states.

The purpose of this paper is to show that for a whole class of lattices, in two and three dimensions, that there is an close relationship between these properties.
It turns out that a key feature is that the flat bands touch a dispersive band at one point in k-space.

My interest was stimulated by the work of some of my UQ colleagues on a class of organometallic compounds that exhibit a kagomene lattice (that interpolates between kagome and honeycomb (graphene). The associated band structure (taken from this paper) is shown below.

The abstract states:
We demonstrate that this band touching is related to states which exhibit nontrivial topology in real-space. Specifically, these states have support [i.e. non-zero values] on one-dimensional loops which wind around the entire system 􏰀with periodic boundary conditions􏰁. A counting argument is given that determines, in each case, whether there is band touching or none, in precise correspondence to the result of straightforward diagonalization. When they are present, the topological structure protects the band touchings in the sense that they can only be removed by perturbations, which also split the degeneracy of the flat band.
I know illustrate this with the kagome lattice.

It has a three site basis (mu=1,2,3) and so there are three bands. If q is the Bloch wave vector, the Bloch states for the flat band can be written

One of these plaquette states is shown on the left below. 
A key point is that there is constructive interference between these plaquette states. Thus, one can take superpositions of them. On the right is the superposition of three neighbouring plaquette states.

A whole line of plaquette states can lead to visualising something with nontrivial topology.

The authors then show how similar physics occurs in other two- and three-dimensional lattice models. The one below is the dice lattice.
Finally, they show that the corresponding Hubbard model leads to a Heisenberg model in the classical limit does have macroscopic degeneracy.

I thank Ben Powell for bringing the paper to my attention.

Tuesday, April 24, 2018

What needs to be said about mental health issues in universities?

On friday I am giving the UQ Physics Department colloquium on mental health issues for scientists. The talk may be similar to one I gave a few years ago.

I will update my talk incorporating some recent reading and the articles below.

A recent Editorial in Nature declared
Time to talk about why so many postgrads have poor mental health 
An outpouring on Twitter highlights the acute pressures on young scientists.

[I thank Tanglaw Roman for bringing the editorial to my attention. I never look at luxury journals unless someone refers me to a specific article.]

The Editorial was in response to the Twitter response to an article in a baby Nature
Evidence for a mental health crisis in graduate education

Poisonous science: the dark side of the lab 
The bullying and subsequent suicide of a talented Ivy League scientist exposes ugly truths about the cruelty and dysfunction at the heart of academic science

Mindfulness won't fix bad management
It also conveniently shifts the burden of wellbeing from the employer causing stress to the employee trying to deal with it. Worse, it allows what you might call "well-washing": employers who cloak themselves in a veneer of caring for their workers while hurting them with bad management practices. 
Five tips to get a good nights sleep

However, I would like some feedback and suggestions from readers.

What do you think needs to be said?

Update. The colloquium was postponed to avoid a scheduling conflict and to make it accessible to a broader audience. Thus, there is still time to send in your suggestions.