Friday, June 28, 2013

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].

Thursday, June 27, 2013

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.

Wednesday, June 26, 2013

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.

Tuesday, June 25, 2013

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+.

Monday, June 24, 2013

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.

Saturday, June 22, 2013

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?

Friday, June 21, 2013

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.

Thursday, June 20, 2013

Ken Wilson (1936-2013): pioneer of the renormalisation group

Ken Wilson died last saturday. He was arguably one of the most important theoretical physicists of the second half of the twentieth century. He pioneered the marriage of quantum field theory techniques with condensed matter. He developed key concepts and methods including scaling, universality, the renormalisation group, epsilon=4-d expansions, numerical solution of the Kondo problem, and lattice gauge theory.

I first heard of Wilson as a first year undergraduate when I read his 1979 Scientific American article, Problems in physics with many scales of length. I had no idea what it was all about. When I was a postdoc at Ohio State he had an office near mine. Then he was mostly interested in science education reform.

There are obituaries at Ohio State  and Cornell.
A previous post considers Wilson's comments about quantum chemistry, in a long and meandering interview about his career.

Wednesday, June 19, 2013

Students should write out their talks

I forgot this. I remember I did it for my first few talks.

Writing out the complete text of the talk may stop several things happening due to nerves
-going too fast and skipping over details
-going too slow because of going into too much detail
-getting stuck and not knowing what to say

Condensed matter rocks!

The following paper just appeared in PRL
Collective Motion of Humans in Mosh and Circle Pits at Heavy Metal Concerts 
Jesse L. Silverberg, Matthew Bierbaum, James P. Sethna, and Itai Cohen

The abstract is
Human collective behavior can vary from calm to panicked depending on social context. Using videos publicly available online, we study the highly energized collective motion of attendees at heavy metal concerts. We find these extreme social gatherings generate similarly extreme behaviors: a disordered gaslike state called a mosh pit and an ordered vortexlike state called a circle pit. Both phenomena are reproduced in flocking simulations demonstrating that human collective behavior is consistent with the predictions of simplified models.
I thank Matt Davis for bringing the paper to my attention.
There is a news story here which links to a simulation.

Monday, June 17, 2013

Quantum diesel engines?

Seth Olsen brought to my attention an interesting paper Quantum tunneling affects engine performance.

The authors consider a specific chemical reaction that occurs in diesel engines. The rate of this hydrogen abstraction reaction, shown below, is modified by quantum tunneling below the energy barrier associated with the transition state.

This reaction is one of hundreds happening in an engine but it turns out the ignition time of the diesel engine is quite sensitive to the rate at which this specific reaction occurs. The authors estimate the ignition time could vary by up to ten per cent due to tunneling.

I don't know anything about diesel engines.
But I do see a little bit of a falsification problem with the papers claim. The reaction rate also varies with small changes in the barrier height [see Table 1]. The latter is difficult to calculate accurately. One way to separate out the effect of tunneling is to do an isotope substitution. Deuterium will have less tunneling than hydrogen. But, it also has a different barrier due to the zero point correction. Similar issues come up in trying to establish the contribution of tunneling to proton transfer reactions in enzymes.

Friday, June 14, 2013

Slow email

Previously I posted about slow science, modelled on slow cooking and other "social movements". Slow down, smell the roses, and enjoy life.

Here, I am proposing a "slow email" movement.
Unfortunately, there is nothing profound and particularly meaningful about it. But that is the point.

I am trying to look at email less often and when I do to respond more slowly. Surely once a day is enough.
This is motivated by a few observations, highlighted by living in the southern hemisphere.
Each morning I get a bunch of email which was received while I was sleeping and the northern hemisphere was working. Hence, my northern colleagues should never expect to receive a response in less than 24 hours. Furthermore, when I travel to the northern hemisphere I am sleeping while the whole working day passes in Australia. And you know what, the whole world does not fall apart! Furthermore, sometimes people solve their own problems by the end of the day.

Here are a few other reasons I think slow email is valuable.

Some fraction of email is stress inducing. It includes reminders of impending deadlines, complaints from people, frustrating edicts from administrators, .....

It is more efficient.
Receiving and scanning 20-50 messages together helps put them all in perspective and prioritise action or inaction.

It decreases the likelihood of firing back a harsh response to an email you don't like. If you sleep on it, you may decide it isn't worth the bother. Just let it go.

So, will you do it? can you do it?

Tuesday, June 11, 2013

It is time to quit!

There is an interesting Freakanomics podcast about "The upside of quitting."
It is about the tension between "sunk costs" and "opportunity costs."
It points out how we often refuse to quit something [a job, project, relationship, investment] because we have invested so much time, energy, or money it [sunk cost]. But, we overlook the fact that not quitting has a significant [opportunity] cost because we could instead invest our time, energy, or money in something that will yield better returns in the future.

What does this have to do with science and academia?

Steve Levitt claims that a key to his success in economics research is that he is good quitter.
He fails fast. He has many ideas, but quickly decides which ones are not going to work out, and drops them fast. He thinks his competitors hang on longer, keeping on hoping something will come of their cherished idea that looked so promising. This wastes precious time that good have been spent on the good ideas.

What are some other areas where this is relevant to science and academia?

-specific research projects
-specific collaborators who don't pull their weight
-graduate students you are supervising and not making progress 
-applying for specific grant programs
-applying for faculty jobs
-teaching courses with consistently low enrolments

Success in science does involve a lot of perseverance, failure, and frustration.
However, never giving up on some of the above can be a mistake.
Furthermore, it is important to make a rational calculated decision, rather than just letting things drag on forever or living in denial.

Are there things you wish you had quit earlier?

Friday, June 7, 2013

Why do two fluid models work?

There is a nice preprint Isosbestic Points: Theory and Applications by M. Greger, M. Kollar, and Dieter Vollhardt.
They show just how ubiquitous isosbestic [crossing] points are in the spectra of strongly correlated systems (both materials and models).
Previously I posted that isosbestic points were A signature of a "two fluid" picture for a strongly correlated system. Look at the post for the background and to see the pictures which clearly show what isosbestic points are.

The preprint gives a detailed mathematical analysis to show why the crossing points can exist for such large parameter variations.

It is still not completely clear to me physically why these points are so robust in such complex quantum many-body systems.
I feel that there is a profound physical question that might be emphasized and discussed more. To what extent does the existence of an isosbestic point justify a "two fluid" model for a specific system? e.g. ideas promoted by David Pines and collaborators in these two papers:

The two fluid description of the Kondo lattice.

Universal behaviour and the two-component character of magnetically underdoped cuprate superconductors

Such two fluid pictures seem to involve a lot of "dephasing" of different degrees of freedom. For example, one a Hubbard model one is sort of splitting up electronic degrees of freedom into a localised spin and an itinerant charge.

I thank Ben Powell for bringing the preprint to my attention.

Aside: Ard Louis has an interesting looking paper on how in classical fluids isosbestic points in the structure factor can be used to extract particle interaction potentials.

Thursday, June 6, 2013

Green's functions are just a technique

Sometimes an abstract of a paper will say something like "we calculate the conductivity of the Hubbard model using Green's functions". Or you ask someone "How did X calculate the spin wave spectrum?" and they will answer, "X used Green's functions techniques." Experimentalists and chemists seem particularly prone to make these kind of statements.

I find this quite unsatisfactory. It is almost as bad as saying "we used a computer" or "we used pencil and paper." Green's functions are just a powerful mathematical technique to simplify calculations in quantum many-body theory. In a sense they have no real physical content.

There are really two key steps (assumptions) in any theoretical calculation involving quantum many-body theory.

1. Choice of a model Hamiltonian.
This could be anything varying from a Hubbard model to a specific density functional in a density functional theory (DFT) based calculation.

2. Choice of an approximation scheme to calculate physical observables (e.g. ground state energy, correlation functions, ...) of the model.
This could be Hartree-Fock, Dynamical Mean-Field Theory, Random Phase Approximation, Second-order perturbation theory, .....
Green's function methods often make it easier (or at least tractable) to do the mathematics of these approximations. But, the technique is not the approximation. The only general case where they can be used without approximation is for non-interacting problems.

When trying to understand a piece of theoretical work these are the two key pieces of information. Try to find them. State them clearly.

Wednesday, June 5, 2013

The key ingredient of a good colloquium?

I still remember the main idea of David Mermin's What is wrong with those talks?
It was published as a Reference Frame in Physics Today in 1992. Mermin says:
Your only goal must be to furnish ordinary physicists with some modest glimpse of what sustains your own interest in your subject.
This past week this idea really shaped the preparation of my quantum science seminar. My main goal was to give the context for my latest paper, rather than talk about the contents of the paper.

I think Mermin's comments are particularly pertinent to colloquiums. But I feel he is a bit too pessimistic about seminars and conference talks for experts.

If you read the article, let me know if you think Mermin is too pessimistic? Or is he realistic? Has Powerpoint made the problem greater or less than 20 years ago?

Monday, June 3, 2013

Slides for bad metal talk

Today I spent the whole day working on my talk at tomorrows UQ quantum science seminar.
I never looked at email once!
I worked quite hard trying to make it interesting and accessible to a broad audience.
I don't know if I will succeed.
Here is the final version of the slides.

I welcome comments.