Thursday, April 19, 2018

Laughing at your life in science

I watched Ph.D Movie 2: Still in Grad School, which is based on the legendary Ph.D comics, written by Jorge Cham.



It is worth watching as it is quite funny. On the other hand, some of the caricatures are getting a little too close to reality....

While on the funny side of science, my wife and I have been enjoying watching Young Sheldon. I am a Big fan of The Big Bang Theory, but was not sure whether this new show would be as good. This was partly influenced by a moderately negative review in the New York Times (albeit based on one episode). I disagree as I think that both shows do have an interesting cast of characters.
On the other hand, Young Sheldon does not have as much science as TBBT, at least for the first six episodes that I have seen. Here Schrodinger's cat gets discussed.


Saturday, April 14, 2018

Junior faculty position in Experimental Condensed Matter available at UQ

The physics department at UQ has just advertised for a new faculty member in experimental condensed matter. The advert is here.

There is also a junior faculty position available in astrophysics.

Aside. The picture is of Lake McKenzie (no relation) on Fraser Island, which I just visited on mid-semester break.

Wednesday, April 11, 2018

Physics, Politics, Pride, and Moral Failure

Which scientist had the greatest political influence of all time? the greatest influence on government policy?
Oppenheimer? Any suggestions?


Not Schrodinger, but arguably Frederick Lindemann.
Until last week I had barely heard of Lindemann. I knew of the Lindemann criterion for estimating the melting temperature of a solid. At Oxford, I had been in the Lindemann building (the front of the Clarendon lab) many times, but had not bothered to find out who Lindemann was.

Last week I listened to a fascinating podcast by Malcolm Gladwell, The Prime Minister and the Prof, that recounts Lindemann's long relationship with Winston Churchill. The podcast draws heavily on two sources (interestingly both written by physicists). The first source is three lectures that C.P. Snow [of two cultures fame] gave at Harvard in 1960, and published as Science and Government.
The second source is a book, Churchill's Secret War: The British Empire and the Ravaging of India during World War II by Madhusree Mukerjee.

Lindemann was a lifelong friend and advisor of Churchill and his main scientific advisor. It was largely his advice that led Churchill to implement policies that literally led to the deaths of millions.

One disaster, considered by Mukerjee, was the Bengal famine of 1943, which resulted because the British refused to send food supplies to their colony in India, even though they had bountiful supplies and shipping at the time.
The second disaster, considered by Snow, was "strategic bombing" of German civilians. Lindemann argued the bombing would break morale, even though the data he had actually supported the opposite view.

I highly recommend the podcast. It is stimulating and disturbing. I thank my daughter for bringing it to my attention. It was great listening in the car on a recent holiday together.

What other scientists can you think of who have exerted such great political influence, for better or for worse?

Wednesday, April 4, 2018

What do you call a mixture of a bad metal and a good metal?

It is fun to come up with clever names for new physical phenomena: quark, big bang, Janus, slepton,  chromodynamics, inflation, squashon, ...
There is an amusing article by David Mermin about how he managed to get boojum  accepted as a scientific term.
Can you think of others?

What is a good synonym for something that has both good and bad qualities?
A curate's egg?

I was wondering about this because of thinking about a metal that is a mixture of a good metal and a bad metal. This is relevant close to an orbital-selective Mott transition. There it may be possible to have multiple Fermi liquids (associated with multiple bands) at low temperatures with different coherence temperatures. For example, this does occur in strontium ruthenate.  As a result, when the temperature is increased one can enter a state in which one of the bands has coherent quasi-particles (and a well-defined Fermi surface) and another does not, i.e. it is a bad metal.

A relevant paper is
Observation of Temperature-Induced Crossover to an Orbital-Selective Mott Phase in AxFe2-ySe2 (A 1⁄4 K, Rb) Superconductors 
M. Yi, D. H. Lu, R. Yu, S. C. Riggs, J.-H. Chu, B. Lv, Z. K. Liu, M. Lu, Y.-T. Cui, M. Hashimoto, S.-K. Mo, Z. Hussain, C. W. Chu, I. R. Fisher, Q. Si, and Z.-X. Shen

They present ARPES data, including that below, that shows how the spectral intensity changes as the temperature increases. The blue and red curves are identified with different d-orbital bands.



Being cautious, I am a bit wary about how clear the data is to support the conclusions. Nevertheless, ...
The authors also present a slave-spin theory calculation for a five-band Hubbard-Kanamori model that is consistent with the experimental data.

I thank Alejandro Mezio for helpful discussions about this topic.

Saturday, March 31, 2018

Why does transparency matter?

I feel this is post is a bit like extolling the value of motherhood. But it does need to be said again and again in a range of contexts. Transparency is relevant in science and universities in many different ways
  • Provide enough information in a paper (or its supplementary material) so that others can reproduce your results.
  • Be honest about the strengths and weaknesses of any method.
  • Provide estimates of the uncertainty of any result.
  • Faculty and institutions need to provide Ph.D students and postdocs with realistic information about their future job prospects within academia. [In particular, the prospect of a tenured faculty position at a research university is highly unlikely].
  • When people are being asked to evaluate something [a job applicant, a grant application, a commercial venture, a new technology, ...] they need to be provided enough information to make a well-informed decision.
  • Make minutes of committee meetings, annual reports,  freely and easily available.
  • Make salaries and benefits of senior management publically available.
  • If someone is affected by a decision they should be informed of the basis of that decision.
My view is it always better to provide too much information rather than little. Due to the existence of the internet it is very easy to make information available (either publically or in a password protected site) for those who are interested.

I am increasingly concerned how individuals and institutions hide behind excuses such as intellectual property, commercial-in-confidence issues, legal action, personnel matters, non-disclosure agreements, internal budgetary matters, .... to justify a lack of transparency.
I am not saying that there is no role for these considerations, just that they are invoked way too often.

When people and institutions are not transparent, it is natural for others to
  • suspect something is being hidden [corruption, mismanagement, ...]
  • lose confidence, respect or trust in the non-transparent parties
What do you think?
Are there areas of science and universities that justify greater transparency?

Wednesday, March 28, 2018

Low energy scales near the orbital-selective Mott transition

One of the most fundamental and profound concepts in quantum many-body theory is the emergence of low energy scales that are much smaller than the energy scales in the "bare" Hamiltonian.
For example, in a metallic phase near the Mott transition in a single band system, there is the energy scale associated with a Fermi liquid. Studies using Dynamical Mean-Field Theory (DMFT) have shown how this scale is associated with ``kinks'' in the quasi-particle dispersion relation and is related to the energy scale for spin fluctuations.

The problem of the Mott transition in multi-band systems (degenerate orbitals) is fascinating and of renewed interest since the discovery of iron-based superconductors. A basic question concerns how the Mott transition is qualitatively different from in single band systems. More specifically, how does a Hund's rule coupling change things?

One new concept is that of an orbital-selective Mott transition. This is where one or more of the bands remains metallic but others become Mott insulators. This concept was originally introduced to explain the intriguing properties of Ca_xSr_2-xRuO4 with x ~ 0.5: it is metallic but has localised spin-1/2 magnetic moments.
[For a critical discussion see the  nice review Strong correlations from Hund's coupling by Antoine Georges, Luca de' Medici, and Jernej Mravlje.]

One might expect that near this transition there are separate low energy scales associated with each of the bands and that these scales are quite different for the bands that become insulator.
However, this is not the case.

There is a nice paper
Emergence of a Common Energy Scale Close to the Orbital-Selective Mott Transition 
Markus Greger, Marcus Kollar, and Dieter Vollhardt

They use DMFT to study a two-band Hubbard model with different bandwidths. They calculate the one-electron spectral functions, the electronic self energy, and the dynamical spin susceptibilities.

The left panel below shows the spectral functions for the two bands. Note how for one the quasi-particle peak width is much smaller than the other.
The right panel (top) shows the energy dependence of the real part of the self-energy for the two bands. Surprisingly, the kink occurs at the same energy.
Furthermore, the bottom of the right panel shows that this peak corresponds to the peak in the dynamical spin susceptibility for both bands.


The figure below shows that "If the Hund’s rule coupling is sufficiently strong, one common energy scale emerges which characterizes both the location of kinks in the self-energy and extrema of the diagonal spin susceptibilities."



The authors then give a physical explanation of this energy scale from a two-impurity Kondo model.

Monday, March 26, 2018

Good scientists will have published errata

A colleague was recently distressed to find a significant error in a paper he had just published. There was a sign error in one calculation which effects the application of the theory to a particular class of materials. The physics and mathematics are correct but not some of the conclusions of the paper. He and his co-authors have submitted an errata to the paper.

I tried to encourage my colleague that although this is disappointing it is just part of being a good scientist. I was reminded of an old post based on a paper, The Seven Sins in Academic Behavior in the Natural Sciences by Wilfred F. van Gunsteren.
One could even defend the proposition that a scientist with a sizeable publication record in science who has not published a single corrigendum is unlikely to be a good scientist. Either he or she has done such simple work that nothing could go wrong, or he or she has committed the fifth sin in science [neglect of errors found after publication.  
This is not an excuse for the sloppy work which is becoming more and more common in science due to the rush to publish the most surprising and spectacular results.
Results do need to be carefully checked and double checked.
But we have to face the fact that mistakes will happen... just like car crashes, burnt toast, stubbed toes, catching colds, ...  Precautions should be taken, but inevitably they will fail sometimes...
The amount of checking should be in proportion to the importance and the surprise of the results.

I think my most significant errors were several in my first paper on hydrogen bonding. I pointed out the errors on this blog and in the second paper I wrote on hydrogen bonding. There was a fortuitous cancellation of errors that meant the conclusions of the first paper were still valid. Arguably, I should also publish an errata on the first paper.

I also think it is important that in public all co-authors take joint responsibility for errors. In particular, it is quite dubious for senior co-authors to shift the blame to junior co-authors. Many senior people are only too happy to take credit. They also have to accept liability.

What do you think about the errata criteria for a good scientist?

I also welcome your own stories about erratum.