A physical picture of spinons in two dimensions

An outstanding question in frustrated quantum magnets in two dimensions is whether one can have excitations with fractional quantum numbers, i.e. spin-1/2 spinons. [See the discussion here].

It is possible to consider triplet excitations as a pair of confined (i.e. bound) spinons. This can be worked out in detail in one dimension where the spinons are domain walls. But it has also been claimed that the spinon description is unnecessary.

There is an interesting PRL by Ying Tang and Anders Sandvik, Confinement and Deconfinement of Spinons in Two Dimensions.

They consider the spinons that are present near the quantum phase transition from a Neel ordered state to a Valence Bond State (VBS). This transition is associated with "deconfined quantum critical point" which breaks a Landau paradigm that a transition between two states that break different symmetries should be first order.

They compute the intrinsic size and the confinement length of the spinons as the quantum critical point is approached. The picture that emerges of the spinons is that of Z_4 vortices, as originally proposed in a paper by Levin and Senthil [from which the figure below is taken].

Talk to high school students

Tonight I am giving a talk to a group of high school students.
They are attending the Queensland Junior Physics Olympiad (JPhO), which is organised each year by UQ Physics.

My brief was to give a basic introduction to my research. Here are the slides from my talk.
I have never given such a talk to a high school audience before.

One of the videos featured was an old BBC documentary on superfluids.

I recommended the following resources for people wanting to learn more:

David Pines article Emergent behavior in quantum matter

A video on Reductionism and emergence featuring Paul Chaikin and Piers Coleman

Bob Laughlin's book A Different Universe: reinventing physics from the bottom up.

15 tips for getting tenure

In the Chronicle of Higher Education there is an article Self-sabotage in the academic career by Robert Sternberg, the incoming president of the University of Wyoming.

Perhaps a more accurate title would be something like "15 traps to avoid for assistant professors in US universities."
But, overall it looks like good advice in broader contexts too.
I first heard about it on Doug Natelson's blog.

Nernst effect as a probe of quasi-particle coherence

There is an interesting PRL

Nernst Effect: Evidence of Local Kondo Scattering in Heavy Fermions

by Peijie Sun and Frank Steglich 

They measure the temperature dependence of the Nernst coefficient for two different heavy fermion compounds and compare them to an isostructural compound without 4f electrons.

The temperature dependence is correlated with that of the thermoelectric power. 
An important question is Nernst signal is related to the coherence temperature associated with the formation of quasi-particles associated with the Fermi liquid.
But there is a complexity associated with this identification.

They argue that Nernst effect is largely a reflection of the single-ion Kondo effect and is measuring the strong energy dependence of the scattering rate.

They suggest the low temperature minimum and the high temperature maximum should be respectively identified with the Kondo temperature for the doublet ground state of the crystal effective field
and the Kondo temperature of the Hund's rule J=5/2 state of Ce3+.

Back to buying nappies/diapers

On the weekend I did something I had not done for about 16 years. I went to the supermarket and bought a packet of disposable nappies (diapers). Why? It was all for science outreach!

This week I am doing a few demos at a kids club run by our church. Last year I did mentos and coke rockets. My glamorous assistant, my darling wife, found this video, which inspired us to increase our repertoire.



The key material is sodium polyacrylate which is a superabsorbent polymer. It is amazing that they can increase their volume by a factor of as much as 500. This is a phase transition analogous to a liquid-gas transition.

A nice follow up is to add salt which destroys the cross-linking and reduces the polymers ability to absorb water. This is shown in the video below.



A nice mean-field theory of the salt induced volume-collapse is given in this PRL from 1980.

Getting a new Macbook functional

I have had my MacBook Pro for 3.5 years. I just bought a new one. I didn't need anything faster or fancier. I was just nervous about the old one self-destructing, particularly since I am about to go on a long trip.

I am slowly getting the new one functional with the same software and settings as the old one. Overall this is going smoothly but is a bit tedious. I have switched from Apple Mail to Outlook because UQ does not support the former properly. I haven't tackled Papers yet.

Any hints on what to watch for in such a process?

Berry curvature and magnetoresistance

Tony Wright and I just finished a preprint Berry curvature determines the angle dependence of interlayer magnetoresistance oscillations

  We show that angle dependent interlayer magnetoresistance oscillations are altered due to the presence of a non-zero Berry curvature at the Fermi surface. The usual field-independent `magic angle' condition for interlayer magnetoresistance extrema, becomes field dependent due to the Berry curvature.
By two complementary methods, we obtain identical results for both bulk three-dimensional layered as well as bilayer systems. This allows a robust method to measure the Berry curvature at the Fermi surface. In particular, the topological properties of layered graphene sheets, topological insulators, Chern insulators, and topological superconductors, can be probed by angle dependent interlayer magnetoresistance oscillations. For moderate magnetic field strengths, a shift in the tilt angle of the magnetoresistance extrema of the order of degrees is feasible, well within current experimental resolution.

We welcome any comments.