Thursday, April 16, 2015

Teaching enhances research enhances teaching

This is the main point of a nice article by Roald Hoffmann, that I posted about in the early days of this blog.

I experienced this a few weeks ago. I have been working on a paper with my postdoc Nandan Pakhira about the viscosity of strongly correlated fermion fluids, focussing on the Hubbard model. A basic issue I got quite confused about is the relation between the momentum, Bloch wave vector, and velocity of an electron in a Bloch state. Yet, I when I taught this to my solid state physics class I was reminded of the correct result.

Wednesday, April 15, 2015

You really should turn off your email occasionally

I should too.
Last week I was on vacation at Bribie Island with my family.
[Aside: this is the location of my profile picture you can see to the right].

We stayed in a house with, thankfully, no internet access.
I don't have a phone.
I could have gone to the local library or to McDonalds to access the internet.
But, why spoil a good holiday?
I set up my email with an "out of office" auto-reply. When I came back to work 2 days ago I went through the 140 messages in about 20 minutes.
This was incredibly efficient.
Most were deleted. About a dozen were about science or some admin. tasks requiring action.
I set up one delayed post on my blog during my absence.

I survived. My colleagues survived. My students survived. My friends survived. My collaborators survived. Bureaucrats survived.
Most things can wait.

I benefited from not having the distraction or of thinking about things I would have to do when I got back.

I find a greater challenge is turning off email for a few hours during the day. I need to keep working on it.

Monday, April 13, 2015

Are American universities Crystal Cathedrals?

The documentary The Ivory Tower is worth watching.
It gives a broad balanced overview of the major challenges facing undergraduate education in the USA. The particular focus in on the sky-rocketing cost of tuition and student debt [now greater than $1 trillion]. It is alarming that since the 1970’s tuition has increased at a greater rate than any other “commodity”, even health care!

The documentary highlights that major contributions to the increasing cost are increasing number of administrators, many of excessive salaries, and fancy buildings [gyms, swimming pools, cafes, luxury apartments, …. all with lots of glass and open space] designed to lure students.
Rankings and status have played a perverse role leading to an unsustainable “arms race”.

But it is not all about affordability; attention is given to the other two vertices of the iron triangle [although that term is not used]: quality and access.
The hype and limited potential of MOOC’s is discussed.
They are no substitute for personal interaction, illustrated by “office hours” for the massive CS150 course at Harvard.

Andrew Delbanco features several times, emphasing the importance of quality, the intrinsic value of education [not just its monetary or utilitarian value], its role in personal transformation, specifically a broad liberal arts education, particularly in a  healthy democracy. Yet, he is no naive idealist, discussing the extreme pressures, contradictions, and instabilities of the current system.

The fascinating story of The Cooper Union is interspersed through the documentary, featuring the 60 day student occupation of the President's office.
It has now started charging tuition, contrary to the stated wishes of its founder, and its mission statement.
This dramatic change has been forced by financial mismanagement.
Cooper took out hundreds of millions dollar in loans to pay for a very fancy new building and to invest in hedge funds that lost a lot of money in the 2008 crash. The president is paid $700K per year plus a free multi-story apartment in New York City; just to lead an institution with 1000 students!

I am not sure “The Ivory Tower” is the appropriate title or metaphor.
To me  it may be The Crystal Cathedral: ostentatious, expensive, full of debt, and a perversion of what it should be?

I thank Stewart Gill for lending me a copy of the DVD. It is a pity the documentary is not available for free. But, that is a sign of how big the problem is. People are willing to pay money to watch a documentary about a depressing subject.

Tuesday, April 7, 2015

Superfluid helium on prime time TV

Unfortunately, The Big Bang Theory TV show features little physics these days. It is often just like "Friends" except some of the characters happen to work at Caltech. However, a recent episode, "The Troll Manisfestation" centred on superfluid helium and also mentions co-authorship, the arXiv, and physics blogs. There is some commentary on the physics in the episode by string theorist, Lubos Motl.

Saturday, April 4, 2015

Effective tutorials, II.

I think one of the weakest aspects of my teaching is running tutorials. In Australia, for most upper level undergraduate courses there is a weekly one hour tutorial [problem solving session] that is run by the lecturer.

Mostly I have run these tutorials according to a traditional format. There are a set of problems that the students are meant to attempt before the session. At the tutorial I then work through the solutions on the board. There are many problems with this approach. Students often don't attempt the problems beforehand because they are not assessed. It is just like a lecture. Students are hesitant to ask questions and just write down what you write on the board. It is somewhat boring. I am not sure the students get much out of it.

Previously, I posted about a different approach that my colleague Joel Corney introduced for a large second year class we were co-teaching. I thought this was quite effective. But, it also required TA's (grad. student tutors) to help.

For PHYS4030 [a solid state physics class with 15 fourth year undergrads] I finally did something I have wanted to do for a long time. Each week I have assigned two students in the class to run the tutorial. They can opt out if they want. They are meant to attempt them beforehand. They then stand at the board and do what they can. Other students offer suggestions and ask questions. I only speak up when essential.

I think it is going well. The students seem more engaged.  Furthermore, it is very helpful for me to see what they find difficult or are confused about; sometimes things that I think are basic and gloss over too quickly. On the other hand, I think you do need a critical mass of motivated and engaged students. Unfortunately, not every class has this.

I welcome other ideas.

Wednesday, April 1, 2015

What does it mean to "observe" a Fermi surface?

The primary point of this post is to raise a philosophical question, "What is definitive experimental evidence for the existence of quasi-particles and a Fermi surface in a metal?"
Specifically, if one sees quantum oscillations, such as Shubnikov de Haas, or maps out the Fermi surface using Angle Dependent MagnetoResistance, has one "seen" the Fermi surface?

The secondary point is an unfortunate one. It provides another concrete example of the perverse influence of luxury journals, particularly the Nature Publishing Group, on science.
People make silly unjustified claims to get published.

At first I was excited when I saw the Nature Communications paper
Quasiparticles and Fermi liquid behaviour in an organic metal 
 T. Kiss, A. Chainani, H.M. Yamamoto, T. Miyazaki, T. Akimoto, T. Shimojima, K. Ishizaka, S. Watanabe, C.-T. Chen, A. Fukaya, R. Kato, S. Shin

It reports Angle Resolved PhotoEmission Spectroscopy (ARPES) measurements on an organic metal. For the last 20 years ARPES has been a workhorse for studying cuprate superconductors. However, organics seem to have been beyond its reach, partly because the crystals can be easily damaged by the high intensity X-rays used. When I give talks about organics people often ask about ARPES measurements. So, I thought perhaps finally the time had come.
The authors of the paper are to be commended for taking on this challenging task.

The abstract of the paper states

Many organic metals display exotic properties such as superconductivity, spin-charge separation and so on and have been described as quasi-one-dimensional Luttinger liquids. However, a genuine Fermi liquid behaviour with quasiparticles and Fermi surfaces have not been reported to date for any organic metal. Here, we report the experimental Fermi surface and band structure of an organic metal (BEDT-TTF)3Br(pBIB) obtained using angle-resolved photoelectron spectroscopy, and show its consistency with first-principles band structure calculations. Our results reveal a quasiparticle renormalization at low energy scales (effective mass m*=1.9 me) and ω2 dependence of the imaginary part of the self energy, limited by a kink at ~50 meV arising from coupling to molecular vibrations. The study unambiguously proves that (BEDT-TTF)3Br(pBIB) is a quasi-2D organic Fermi liquid with a Fermi surface consistent with Shubnikov-de Haas results.

Then I looked at the actual data in the paper. Some is shown below.
It is rather noisy!
The lower figure shows the deduced Fermi surface on top of an ARPES intensity map.

Based on the quality of the data, I don't think it is appropriate to state "The study unambiguously proves that (BEDT-TTF)3Br(pBIB) is a quasi-2D organic Fermi liquid with a Fermi surface".

What do you think?

Prior to this paper there were Shubnikov de Haas measurements on the same material and Angle-Dependent MagnetoResistance, reported here. The data is shown below.

This is clean and impressive. Indeed the beating in the SdH and the peaks in ADMR at 90 degrees reflect that there is actually a coherent three-dimensional Fermi surface, a warped cylinder.

From the ADMR one can map out the intra-layer Fermi surface, using some theory, which assumes Fermi liquid quasi-particles. The result is below. The area is consistent with the frequency of SdH oscillations.

The authors neglect to mention this, even though they reference the paper that contains this figure. Furthermore, they make the extraordinary claim,

the present result constitutes the only case of an experimentally measured k-resolved Fermi surface of an organic metal. 

I am gobsmacked because in 1996 a book was published
Fermi Surfaces of Low-Dimensional Organic Metals and Superconductors by Joachim Wosnitza.
It contains multiple pictures of "experimentally measured k-resolved Fermi surfaces".

If you asked me "Does this material have a Fermi surface?" I would say, purely based on the ADMR that I was pretty confident it did.

Putting aside all the noisy data and hype, there is an important philosophical and scientific question,
"Is ARPES really a more fundamental measurement of or robust evidence for a Fermi surface than ADMR and SdH?"
There are some subtle issues here, as discussed here. For example, with a marginal Fermi liquid one can still get SdH.

Monday, March 30, 2015

Balancing democracy, transparency, and efficiency

I tend to avoid being on committees. However, this year I became chair of one, leading to this reflection. How do you find a balance between democracy and autocracy, between transparency and secrecy, and between efficiency and wasting a peoples precious time?

Over the years I have noticed committees can tend to one of two extremes.

1. Some are very democratic and transparent. All business is discussed in great detail. Votes are held about many things. Between meetings, committee members are emailed about the latest "urgent" matter, asked their opinion, and sometimes asked to vote to approve some small action.
The problem is this takes a lot of time. It would be quicker and more efficient if on these small matters that the chair or a subgroup simply made a unilateral decision.

2. Some committees are secretive and merely "rubber stamp" a bunch of decisions that have already been made by the chair or a select subgroup of members. This is efficient, particularly with regard to the small matters. However, it is problematic when this happens for weightier matters for which committee members could have given useful input and/or have a significant stake in the outcome.

Is this a fair and useful characterisation?

Humorous aside: When I was a teenager, I remember seeing the satirical movie The Rise and Rise of Michael Rimmer. If I recall correctly the main character becomes a dictator by a devious strategy. First, he makes every citizen vote on every piece of government legislation. They quickly get sick of this and so pass all power and authority to him.

I am not sure what the appropriate balance is between the extremes. The compromise I made so far for my current committee, is that I have a shared folder in which I put all my documents relating to the committee. That way members who want to can see what I am dealing with. However, between meetings I try to make unilateral decisions on small matters that I think they would agree with but don't want to be bothered with.

Having written this, I realised that similar issues actually arise in scientific collaborations, particularly international ones, where the collaborators rarely meet in person. From my experience with many different collaborators, I have noticed there is a challenge to find a balance between two extremes.

1. A collaborator is constantly asking others for approval of or suggestions on next steps [should I do this extra measurement or calculation?] and/or changes to a draft manuscript. For small initiatives or changes this can be very inefficient, particularly when there are a large number of co-authors. On the other hand, for large changes clear communication and discussion is important.

2. A collaborator communicates little and sometimes some of the co-authors see a draft manuscript that contains large sections (including methods and results) that had never been discussed before. This is problematic if one could have been asked earlier and had the opportunity to make useful comments before some major parts of the project were embarked upon. The horse has already bolted.

Again I am not sure what the balance is. Some depends on personalities, tastes in working styles, and respective expertise. But, one practical option is to have a shared Dropbox folder containing progress reports, detailed results, and draft manuscripts. Then, all the collaborators can peruse these as frequently as they want.  

I welcome suggestions or experiences.