Thursday, May 5, 2016

Lineage of the Janus god metaphor in condensed matter

Five years ago, Antoine Georges and collaborators invoked the metaphor of the greek god Janus to represent the "two-faced" effects of Hund's coupling in strongly correlated metals.

I wondered where they got this idea from: was it from someones rich classical education?
In the KITP talk, I recently watched online, Antoine mentioned he got the idea from Pierre de Gennes.

In his 1991 Nobel Lecture, de Gennes said
the Janus grains, first made by C. Casagrande and M. Veyssie. The god Janus had two faces. The grains have two sides: one apolar and the other polar. Thus they have certain features in common with surfactants. But there is an interesting difference if we consider the films which they make —for instance, at a water-air interface. A dense film of a conventional surfactant is quite impermeable. On the other hand, a dense film of Janus grains always has some interstices between the grains, and allows for chemical exchange between the two sides: "the skin can breathe. " This may possibly be of some practical interest.
Here are some images of "Janus particles".

Tuesday, May 3, 2016

How do you teach students that the details DO matter?

For some reason I have only become more aware of this issue recently.
I have noticed, particularly among weaker undergraduate students, a lack of concern about details.
This is not just among beginning undergrads but even final year students.

Here are some examples, from a range of levels.

Language.
Force, energy, and power are not the same thing.
Each refers to distinct physical concepts and entities.
Similarly, temperature, heat, internal energy, and entropy...
The wave function, potential energy, and probability....

Units.
Every physical quantity has well-defined units. You need to state them and keep track of them in calculations.

Significant figures.
These need to be justified and self-consistent.

Arguments and solutions to problems.
These need to be stated in a logical order. Assumptions and approximations need to be stated and justified.

The sloppiness is particularly evident in the exam papers of weaker students. One might excuse some because of the stress and rush of the situation. But, I also encounter some of the confusion and sloppiness in private discussions and assignments. Sometimes I fear they hope they can "bluff" their way towards some partial marks.

How does one address this issue?

I really don't know. I welcome suggestions.

One can certainly "nag" students about how the details do matter and penalise them in exams and assignments for sloppiness. However, this seems to have limited effect.

Thursday, April 28, 2016

What is real scholarship?

Sometimes I bemoan the decline of scholarship in science, and in academia more broadly. About six years ago I posted about Ph.D's without scholarship, which generated a lot of comments.

This decline is reflected in a range of phenomena: hype, making hiring and promotion decisions based on metrics rather than actual scientific achievements, people writing more papers than they read, "review" articles merely listing references rather than providing critical analysis,...

But, this is all negative, it is what scholarship is not, ... what does real scholarship look like?

I think classic books give a feel for what scholarship is all about. For example, Eisenberg and Kauzmann on Water, Ashcroft and Mermin, Hewson's Kondo Problem, Coulson's Valence, and Mott's monographs. Consider the Oxford Classic Texts in the Physical Sciences.
Similarly, I am challenged by some of the monographs that some  humanities colleagues produce. (For example, Stephen Gaukroger's three volumes on science and the shaping of modernity.)
But, today I just don't see people in physics and chemistry producing books like the above.
Am I missing something?

There is certainly a subjective element. Here are a few possible ingredients to real scholarship.

1. Acknowledge the past.
Every problem, achievement, and discipline actually normally has a long history.
Even Newton said he was standing on the shoulders of giants.

2. Acknowledge and engage with the work of others and different points of view.

3. Acknowledge ambiguity, complexity, and controversy.

4. Comprehensive.
A wide range of topics are considered. The focus is not just narrow.

5. Synthesis and coherence.
A wide range of ideas, topics, and techniques are brought together.

6. Lucidity.

Do you think scholarship is declining?
What do you think are the key ingredients?

Tuesday, April 26, 2016

Low temperature physics without nuclear weapons

Liquid 3He is amazing stuff. Below temperatures of a few hundred milliKelvin it forms a model (and the original inspiration for) Landau Fermi liquid. Furthermore, below about 1 mK it forms two different superfluid states, involving Cooper pairs in a spin triplet state. This is the model case for unconventional superconductivity.

Liquid 3He is actually of great practical use since it the crucial ingredient of dilution refrigerations that allow cooling from a few Kelvin to temperatures as low milliKelvin.
But where do labs get 3He from?
Well, it is a very useful by-product of nuclear weapons production.
Currently, the scientific community (which consumes only about 1% of the supply) is experience supply problems and dramatic price increases (a 15-fold increase between 2004 and 2010).
Why is this happening?
Thankfully, we are cutting back on nuclear weapons production!

One practical way to solve this problem is to develop alternative materials for ultra-low temperature refrigeration; one possibility is by adiabatic demagnetisation. Indeed, this is the method that was first developed in the 1930s using paramagnetic salts to achieve temperatures below about 0.3 K (and was the basis of the 1949 Nobel Prize in Chemistry) and is the basis for nice undergraduate problems in thermodynamics and statistical mechanics. Simply the entropy is a function of B/T (where B is the magnetic field and T the temperature). One cools the system down in a fixed magnetic field, then adiabatic isolates it and reduces the magnetic field slowly. In the last step the entropy must not change and so the temperature must decrease. (This is shown as the red horizontal arrow in the figure below). This is also known as the magnetocaloric effect. The problem is that most paramagnetic materials are insulators and one would prefer to have a metallic material that is a good thermal conductor and can be "machined".

I learnt some of this from an interesting paper (that I actually looked at in preparing an undergraduate thermodynamics lecture about Maxwell relations).

Large magnetocaloric effect and adiabatic demagnetization refrigeration with YbPt2Sn 
Dongjin Jang, Thomas Gruner, Alexander Steppke, Keisuke Mitsumoto, Christoph Geibel and Manuel Brando

The authors mention some basic unanswered science questions about why this material is a good candidate. Specifically, why is the Kondo temperature (associated with interaction of the magnetic moments of the Yb3+ ions with the conduction electrons) and the inter-ion magnetic interactions so low? This ensures that the spins act essentially like non-interacting spins (with a large entropy) down to less than 1 K.

A key figure is below, showing the entropy versus temperature at several different magnetic fields.



Friday, April 22, 2016

KITP seminars online

A wonderful thing about the web is that now there is so much material online. A pioneer in putting all their seminars and colloquia online is the KITP at Santa Barbara. I know some people who regularly watch seminars (both old and recent). Others do not know it exist. This is a particularly valuable resource for students and those of us in distant countries.

I have to confess that until yesterday I have never actually watched a talk; just occasionally skimmed some slides. Generally I find I don't have the patience to watch talks online. I just seem to prefer to look at papers. However, yesterday I was forced to do this because at the weekly UQ condensed matter theory group meeting we watched a nice talk by Antoine Georges on Hund's metals. Although, I have read and blogged about some of the relevant papers, I really found it helpful seeing what was highlighted and going through the material at a "slow pace". Hopefully, I will do this more often.

What do you think about online talks or lectures? How often do you watch them? Are there any that you would particularly recommend?

Thursday, April 21, 2016

Cost benefit analysis of administrative policies

Administrators and senior management seem to love coming up with new policies and procedures for everything.
These are designed to make things "better".
However, a colleague recently emphasised to me that each one of these initiatives should be subject to a cost-benefit analysis. This is a point I have also heard made by my UQ law colleague, James Allen, author of a provocative essay about Australian universities.

Consider the follow examples:

* requiring grant applications to provide more information (whether reports on previous grants, details about university policies, longer project descriptions, relevance to society, ....)

* more details in course profiles

* larger committees to ensure more input, consultation, representation of diversity, accountability, and expertise

* procedures and policies to increase transparency and accountability

* broadening eligibility criteria so more people can apply for a particular grant or fellowship program.

Every one of these initiatives has benefits. 
So why might they be a bad idea?
One needs to consider the cost, particularly the opportunity cost.

Specifically, if instead of faculty spending time on these tasks what might they spend time on instead?
Mentoring graduate students and postdocs, research, preparing higher quality lectures, ....
Sometimes senior faculty simply move these tasks to junior faculty, graduate students, or junior administrative staff. However, that has a cost too. Implementing all these initiatives requires more admin staff; money that could be spent instead on hiring more faculty...

It is not just time and money. All this admin. takes mental space and sometimes reduces morale, which in the end leads to reduced productivity.


Monday, April 18, 2016

Incorporating scientist biographies into lectures

A few years ago I decided I wanted to include brief biographies of relevant great scientists in my undergraduate lectures. I posted (5 years ago!) about how I started with Landau but I lost momentum. This year I have put more effort into it. I just taught my second year undergraduate thermo class about Gibbs free energy and so I profiled Gibbs.
In solid state physics I have profiled Drude, Sommerfeld, von Laue, and Bloch.
I have found this quite enjoyable for myself and hopefully for the students. I have learnt quite a bit, just by reading the relevant Wikipedia pages. It also introduces students to the human dimension of science. For example, Drude died by suicide and so it is a good opportunity to flag mental health issues. Sommerfeld was a mentor of many great scientists. von Laue actively opposed the Deutsche Physik of the Nazis. Bloch was the first Director General of CERN.

Has anyone else experience at doing similar things? Any suggestions?