Friday, May 22, 2015

Advice for undergrads giving research talks

At UQ all physics honours students (4th year undergrad) have to give two 15 minute talks about their year long research project. The first is a progress report at the end of the first semester and the second at the end of the project. No grades are given for these presentations but they are attended by the 3 thesis examiners [supervisor, expert, and non-expert] and so may influence the grade for the thesis.

I think these presentations are very challenging for the students and I am sometimes impressed at the quality of the talks. This is a great opportunity for students to develop and improve their communication skills. When I was an undergrad we never had opportunities like this. Most of us also had very little public speaking experience. Students today are quite different and much more confident and polished.

Here is my advice to students.

First, review general material on giving scientific talks such as Wilkins' one page or Geroch's suggestions or Mermin's. Don't think you know better.

Second, decide on your audience [your friends, other students, faculty, your research group, the examiners?]. I hate to say it but the examiners is the correct answer.
Taylor the talk accordingly.

Third, decide on your real goal [impress others, show how much work you have done, show off how much jargon you have learnt, be entertaining, talk about how great your research field is, make excuses for your lack of results....?].
Taylor the talk accordingly.

The goals of the progress seminar are simple.
Show you have a well defined and realistic project.
Show you have a clear plan.
Show you have started to make some progress.

The goal of the final seminar is simple.
Show you have achieved something concrete and worthwhile.
Anything else is subsidiary.

Be realistic about how much you can achieve in 15 minutes.
Some background is crucial but don't let it dominate your talk.
Don't spend more than a minute about why the research field is important and interesting.
Don't spend more than a minute on the history of the field.
Don't think you can explain how Shor's algorithm works, the subtleties of the quantum measurement problem, or the microscopic basis for Landau's Fermi liquid theory, ...
Most of the talk should be about what you have done and why it is significant.
Yet, if you can teach people one small thing they will be very appreciative.

Clearly distinguish between the contributions of founders of the field, those of your supervisor, and yourself.
Include relevant references on your slides.

Avoid irritants: being late, having problems with the technology, small fonts, endless jargon, hype, lavish Powerpoint animations, .....

Practise. Practise. Practise.
Consider writing out explicitly what you are going to say.

Start preparations early. Get feedback.

How you answer the questions is important.
Listen carefully. Don't cut off the questioner.
Don't bluff an answer. Saying you don't know is o.k.

Relax. The audience knows that this is a stressful experience and does not expect a perfect talk.

Thursday, May 21, 2015

A unified picture of weak chemical bonds: hydrogen, halogen, carbon...

Previously I posted about improper hydrogen bonds. These are weak hydrogen bonds that have the unusual property that in the X-H...Y system H-bonding leads to a shortening and hardening (blue shift) of the X-H bond. In contrast, for "proper" bonds, X-H lengthens and softens (red shift).

The past few years has seen a rapid increase in interest in an even broader class of weak bonds such as "halogen bonds",  denoted X-Z...Y where Z can now be not just H but a halogen (F, Cl, Br), chalcogen (O, S, Se, Te), or pnictogen (N, P, As, ..)....

There is an interesting paper that contains the helpful summary figure below
Negative hyperconjugation and red-, blue or zero-shift in X-Z---Y complexes
Jyothish Joy, Eluvathingal D. Jemmis and Kaipanchery Vidya

In trying to understand the paper I found reading the following older paper helpful
Electronic Basis of Improper Hydrogen Bonding:  A Subtle Balance of Hyperconjugation and Rehybridization
Igor V. Alabugin, Mariappan Manoharan, Scott Peabody, and Frank Weinhold

[Aside: note the senior author is Weinhold who has featured in some previous posts]

The basic idea is that there are two competing interactions. "Hyperconjugation" is Weinhold's view of proper H-bonds, via the Natural Bond Orbital donor-acceptor picture where the H-bond arises due to charge transfer from the lone pair orbital on Y to the σ* (anti-bonding) orbital associated with X-H. This lengthens and hardens X-H.
When this interaction is weak there is “X-H bond shortening” due to increase in the s-character (rehybridisation of the atomic orbital on X) and polarization of the X−H bond. This is associated with a shorter and harder X-H bond.

Bent's rule is central. It is one of the most general rules governing structure of organic molecules.
atoms tend to maximize the amount of s-character in hybrid orbitals aimed toward electropositive substituents and direct hybrid orbitals with the larger amount of p-character toward more electronegative substituents.
Increasing s-character generally leads to shorter bonds.
As the donor acceptor distance (X-Y) decreases the X-Z bond becomes more polarised and the s-character increases.
The authors note it should be possible to test predicted trends since the amount of s character in the X-Z bond can be measured from the relevant NMR coupling constant.

My question is whether this subtle competition can be captured by generalising my simple 2 diabatic state model for H-bonds to a 3 state model that includes the ionic character of the X-Z bond.

Tuesday, May 19, 2015

Measuring the viscosity of the electron fluid in a metal

Previously I posted about the theoretical issue of the viscosity of the electron fluid in strongly correlated metals. This interest is partly motivated by claims from string theory techniques [AdS-CFT] that there is a universal lower bound for the viscosity.  A recent experimental paper estimated the viscosity in the cuprates by an indirect method from ARPES data.

I only became aware recently that there is a somewhat direct way to measure the viscosity of the electron fluid in a metallic crystal. This has a long history going back to Mason and Pippard who in 1955 related the viscosity to the attenuation of sound. A more sophisticated and general theory was developed by Kahn and Allen.

The connection between shear viscosity and ultrasound attenuation can be loosely motivated as follows. In a viscous fluid the attenuation of a shear wave is given by Stokes law

where \eta is the shear viscosity of the fluid, \omega is the sound's frequency\rho is the fluid density, and V is the speed of sound in the medium.

This equation has been used to determine the shear viscosity as a function of temperature for helium three [a correlated neutral fermion fluid]. Extensive experimental data is reviewed here.

In a metal, provided the wavelength of sound is much larger than the electronic mean free path, then one is in the hydrodynamic limit, and the attenuation is given by a similar expression to that above (with appropriate indices for crystal axes), with \rho the solid density (not the electron fluid).

One can show from the Boltzmann equation that in a simple free electron model that the electronic viscosity is proportional to the scattering time, just like the conductivity. Hence, the ultrasound attenuation should scale with the conductivity.

Indirect evidence for this idea is from the data below that shows the temperature dependence of ultrasound attenuation of aluminium (taken from here).

In clean metals, such as for the data shown above, the attenuation [and viscosity] becomes very large at low temperatures, making it easier to measure.
Also, for high frequency ultrasound, one can reach the "quantum regime" where the mean free path becomes comparable to the sound wavelength. Pippard worked out a general theory describing the crossover from the hydrodynamic regime to this quantum regime.

In bad metals could one experimentally see the small viscosity, of the order of n hbar [where n is the density]? First, the small mean free path, characteristic of bad metals, means one will always be in the hydrodynamic regime. However, the small viscosity means that the sound attenuation due to the electron fluid will be small and possibly dominated by other sources of attenuation such as crystal dislocations. A rough estimate for an electron viscosity of order of n hbar and a sound frequency of 1 GHz gives an attenuation of less than 0.1 cm-1, of the order of typical sensitivity, such as in these measurements for heavy fermion compounds.

Friday, May 15, 2015

What is real scientific integrity?

According to the Oxford English Dictionary Integrity = "The quality of being honest and having strong moral principles".

When people talk about scientific integrity and misconduct they mostly have a narrow definition which means "don't make up data."

However, I think we need to consider a broader definition of integrity that relates to all communications and messages.

Scientists talk about their research in a wide range of forums:
  • private discussions
  • articles in luxury journals
  • articles in professional society journals (PRA, JCP etc)
  • grant applications and job applications
  • seminars at universities and conference presentations
  • press releases and interviews
  • public lectures and popular books
Yet it seems it has now become quite acceptable to have different messages (claims and conclusions) in different forums. This post was stimulated by a perceptive comment by Steve W on a previous post.
My finding is if you talk to the authors of luxury papers with controversial or sexy explanations, that they will be the first to admit their own skepticism regarding their explanations of the data. But somehow this skepticism is not conferred to the text, because the luxury journals like clear, concise, authoritative explanations. Most of the details get hashed out later in less prominent, but longer form journals, and these are only followed closely by those within the specific community. 
For a concrete example see a recent post by Peter Woit about the basic question, "Is string theory experimentally testable?" He highlights a significant inconsistency between the answers in a preprint, the published version in PRL, a press release, and a public talk by Amanda Peet.

Thursday, May 14, 2015

From a spin liquid to a correlated Dirac metal

There is an interesting paper
Theoretical prediction of a strongly correlated Dirac metal 
 I. I. Mazin, Harald O. Jeschke, Frank Lechermann, Hunpyo Lee, Mario Fink, Ronny Thomale, Roser Valentí

The compound Herbertsmithite ZnCu3(OH)6Clhas attracted a lot of interest because it is a Mott insulator with a layered crystal structure where the Cu2+ ions (spin 1/2) are arranged in a kagome lattice.
There is some evidence both experimentally and theoretically that the ground state is a spin liquid.
[However, inevitably there are complications such as the role of impurities and the Dzyaloshinskii-Moriya interaction].

In this paper the authors replace the Zn2+ ions with (isoelectronic) Ga3+ ions. This means that in non-interacting electron picture the bands go from half filling (n=1) to two-third filling (n=4/3). This is of particular interest because for a tight-binding model on the kagome lattice there are symmetry protected Dirac points, just like in graphene, at this band filling.

There are subtle interlayer effects because the kagome layers order ABCABC....
This changes the three-dimensional Bravais lattice from hexagonal to rhombohedral and a doped system will have a Fermi surface like that below.

However, one needs to take into account the strong interactions associated with the localised Cu orbitals that lead to a Mott insulator at half filling. The authors use a range of theoretical techniques (rotationally invariant slave bosons, functional RG, Dynamical Cluster Approximation (DMFT)), to investigate instabilities in the associated Hubbard model.
They find a subtle competition between metallicity, charge ordering, ferromagnetism, and f-wave superconductivity.

Hopefully, someone will make this compound soon!

I thank Ben Powell for bringing the paper to my attention. He and Anthony Jacko recently considered an organometallic material with a rich band structure that interpolates between honeycomb and kagome.

Tuesday, May 12, 2015

The challenging interface of science, policy, and politics

Last week I went to an interesting talk What are the effects of dredging on the Great Barrier Reef?
by Laurence McCook, at the Global Change Institute at UQ.

I went because I knew Laurence in my undergraduate days at ANU. In first year we had all the same lectures, tutorials, and labs. (I guess groups were assigned based on the alphabet.) We became friends and he introduced me to many beautiful places for bushwalking [backpacking] and cross country skiing near Canberra.

There is a piece on the Conversation that gives a brief summary of the issues associated with the report from the expert panel that Laurence and  Britta Schaffelke co-chaired. Basically, it involved a "cat herding" exercise with 17 experts from industry, government, and universities. I am always impressed by people who manage such enterprises and can produce concrete useful outcomes. I think it requires considerable patience, political skills, and leadership. 

A helpful figure is below.
Aside: it would be interesting to try and do an exercise like this for topics such as cuprate superconductors, topological quantum computing, water, glasses, quantum molecular biophysics......

So what effect does dredging have?
Specifically, which of the effects is most likely to do the greatest environmental damage?

It seems that the ongoing turbidity [cloudy water] and sedimentation associated with sediment dynamics could be the biggest problem. But, this is also one of the most poorly understood processes. 
The figure below summarises some of the complex processes involved. Modelling this presents a major challenge (and some interesting science).

A problem with these exercises where science meets policy meets politics, particularly on controversial issues, is that they can highlight uncertainty and the general public does not like that. Science is meant to be certain. People want black and white answers. "Dredging is harmless and we should not worry about it vs. Dredging is an environmental disaster and should be banned".

It is interesting that of "10 scientific ideas that scientists wish you would stop mis-using" the first is Proof.

Friday, May 8, 2015

Holon-doublon binding as the mechanism for the Mott transition

What is the mechanism of the Mott metal-insulator transition?
After 50 years this remains a debated issue.

A number of distinct mechanisms for the transition have been proposed. These include those due to Brinkman and Rice (where the quasi-particle weight in the metallic phase approaches zero as the transition is approached), Hubbard (where vanishing of the charge gap occurs when the upper and lower Hubbard bands overlap), or Dynamical Mean-Field Theory (DMFT) which combines both these features.

My collaborators and I discuss an alternative mechanism in a paper that we just finished.

Holon-Doublon Binding as the Mechanism for the Mott transition
Peter Prelovsek, Jure Kokalj, Zala Lenarcic, and Ross H. McKenzie
 We study the binding of a holon to a doublon in a half-filled Hubbard model as the mechanism of the zero-temperature metal-insulator transition. In a spin polarized system and a non-bipartite lattice a single holon-doublon (HD) pair exhibits a binding transition (e.g., on a face-centred cubic lattice), or a sharp crossover (e.g., on a triangular lattice) corresponding well to the standard Mott transition in unpolarized systems. We extend the HD-pair study towards non-polarized systems by considering more general spin background and by treating the finite HD density within a BCS-type approximation. Both approaches lead to a discontinuous transition away from the fully polarized system and give density correlations consistent with numerical results on a triangular lattice.

Two things I found (pleasantly) surprising in this study were:

-two "simple" analytical approaches (retraceable path approximation and a BCS-type variational wave function) seem to capture much of the essential physics.

-one can learn quite a lot by approaching the problem from the highly (spin) polarised limit.

We welcome comments and suggestions.