The challenge of moving topological defects in quantum matter

I have really enjoyed this week at the Australasian Workshop on Emergent Quantum Matter. My UQ colleague Matt Davis is to be congratulated for putting together an excellent program. There was nice balance of cold atom and solid state talks.

Is there anything that stood out to me?
Yes. Vortices, (Josephson) phase coherence, and dimensional crossovers. Vortices kept coming up and remain a fascinating and perplexing problem.
Vortices are mesoscopic, intermediate between the microscopic (atomic) and macroscopic scales. The length scale associated with them is emergent. They have some quantum properties (quantised circulation) but obey classical equations of motion, but interact with microscopic degrees of freedom (quasi-particles and phonons).

When one has a broken symmetry vortices are novel emergent low energy excitations. They are topological defects in the order parameter. Given how much they have been studied in superfluid 4He and superconductors one would think they were pretty well understood. However, this is not the case. What is particularly poorly understood is the dynamics of these objects.

Stephen Eckel described some beautiful experiments at NIST that recently investigated  Josephson type junctions in atomic BECs. My immediate question was how was this any different from landmark experiments performed by Davis and Packard in superfluid 3He?  In those experiments the superfluid "healing" (or coherence) length is quite small and there is not a single weak link but many apertures. The origin of the coupling between these links is not clear.
It was also interesting that a key consulting role in these experiments was played by Chris Lobb, an expert on solid state Josephson junctions.

Victor Galitski and Joachim Brand both described theory motivated by recent fermionic cold atom experiments which measured a large mass (both inertial and gravitational, the two are different) for solitons in a quasi-one-dimensional superfluid. Victor discussed recent theoretical calculations based on exact solution of the dynamical Bogoliubov-de Gennes (BdG) equations.

The question of how vortices and quasi-particles interact and the dynamics of a single vortex in a Bose superfluid is highly controversial. Theoretical calculations of the mass of a vortex range from zero to infinity! A brief introduction, including key references, is in this PRL.

Dimensionality matters. Solitons and Luttinger liquid only exist in strictly one dimension. The Berezinskii-Kosterlitz-Thouless transition strictly only exists in two dimensions. However, what happens in quasi-one or quasi-two-dimensional systems is not completely clear, inspite of a lot of theoretical work. Some ultra cold atom experiments may be able to address these questions of dimensional crossover.

Grant writing tips

I have been asked to speak at a grant writing workshop for the School of Mathematics and Physics at UQ.
Here are a few preliminary thoughts.

Consider not applying.
Seriously. Consider the opportunity cost. An application requires a lot of time and energy. The chances of success are slim. Would you be better off spending the time writing a paper and waiting to apply next year? Or, would it be best to write one rather than two applications? You do have a choice.

Don't listen to me.
It is just one opinion. Some of my colleagues will give you the opposite advice. I have never been on a grant selection committee. My last 3 grant applications failed. Postmortems of failed applications are just speculation. What does and does not get funded remains a mystery to me.

Take comfort from the "randomness" of the system.
You have a chance. Don't stress the details. Recycle old unsuccessful applications. Don't take it personally when you fail.

Who is your actual audience? Write with only them in mind.
For the Australian Research Council it is probably not your international colleagues but rather the members of the College of Experts. It needs to be written in terms they can understand and be impressed by.

Why should they give YOU a grant?
I find many people sweat about the details of the research project or think if they have brilliant cutting edge project they will get funded. I doubt it. Track record, and particularly track record relevant to the proposed project is crucial.

Not all pages are equal.
Unfortunately, the application will be 60-100 pages. Don't kid yourself that reviewers will carefully read and digest every page. Some are much more important than others. You should focus on those.

The first page of the project description is the most important. Polish it.
I sometimes read this and I have no idea what the person is planning to do. I quickly lose interest.

"Contributions to the field" and "Research accomplishments" means scientific knowledge generation not career advancement or hyperactivity.

Choose your co-investigators carefully.
They may lift you up or weigh you down. I am usually skeptical of people who have "big name" co-investigators they have never actually published with before. The more investigators the larger the application, and the more material available for criticism. Junior investigators need to realise that the senior people will usually get all the credit for the grant, even if they contributed little to the application. This is the Matthew effect.

Trim the budget.
The larger the budget the greater the scrutiny. It is better to get a small grant than no grant at all. Ridiculously large requests will strain your credibility.

Moderate the hype, both about yourself and technological applications.
There are reviewers like me who will not take you seriously and be more critical of the application.

Be discerning about what publication metrics (citations, journal impact factors) to include.
Impact factors have no impact on me. I don't see the point or value of short term citations.

Writing IS hard work, even for the experienced.
See Tips in the writing struggle.
Get started early. Get feedback.
Write. Edit. Polish. Rewrite. Polish. Polish.

My criteria for research quality.

Responding to feedback from administrators in the university Research Office.
Put in the application early. They can give very helpful feedback about compliance issues, formatting, page limits. Take with "a grain of salt" advice/exhortations about selling you and the science.

I welcome comments and suggestions.
What advice and suggestions have you received that were helpful or not helpful?

Quantum computing with Majorana fermions is science fiction fantasy

Someone has to say it.
I said it publicly today. Several people told me they were glad to hear it.

Majorana fermions are fascinating from a fundamental science point of view. They are worth investigating by a few theoretical and experimental groups. However, they are the latest fashion that is taking the solid state and quantum information communities by storm. It is the latest exotica. Much of the justification for all this research investment is that Majorana fermions could be used for "fault tolerant" quantum computing.

Lets get real. Lets not kid ourselves. First, as far as I am aware, no one has even demonstrated yet that the relevant solid state "realisations" even exhibit Majorana statistics. Suppose they do. Maybe in a few years someone will have 2 qubits. Looking at the complicated nanoscale devices and fabrication needed I fail to see how on any reasonable time scale (decades?) one is going to produce say 6-8 qubits. Yet even that is just a quantum "abacus", a toy.

The cartoon is taking from Gil Kalai's blog.

An introduction to emergent quantum matter

Here are the slides for my talk, "An introduction to emergent quantum matter" that I am giving tomorrow at the Australasian Workshop on Emergent Quantum Matter.

A good discussion of some of the issues is Laughlin and Pines article The Theory of Everything and Piers Coleman's article Many-body Physics: Unfinished Revolution.

A more extensive and introductory discussion by Pines is at Physics for the 21st Century.

I welcome any comments.

Investing in soft matter

I really enjoyed my visit to the TIFR Centre for Interdisciplinary Sciences (TCIS) of the Tata Institute for Fundamental Research in Hyderabad. This is an ambitious and exciting new venture. Higher education and basic research is expanding rapidly in India, with many new IITs, IISERs, and Central Universities. These are all hiring and so it is wonderful time to be looking for a science faculty job in India.

The initial focus of hiring of the new campus of TIFR (India's premier research institution in Mumbai) has been on soft condensed matter (broadly defined) with connections in biology and chemistry. There are many good reasons for this focus. Foremost, is that there excellent Indian's working in this area. However, I see many other reasons why choosing this area is a much better idea than quantum condensed matter, ultra cold atoms, quantum information, cosmology, elementary particle physics, string theory (yuk!), ...

Other reasons why I think investing in soft matter is wise and strategic include:

• this is exciting and important inter-disciplinary research
• there are real world applications ranging from to foams to medicine to polymer turbulence drag reduction [used in oil pipelines and in fire fighter hoses]
• these types of applications are particularly important in Majority World countries
• in public outreach one can talk about flocking and do simple and impressive demonstrations such the Briggs-Rauscher oscillating chemical reaction or sand flowing through channels.
• the experimental infrastructure and start up costs are relatively small. most experiments are "table top" and at room temperature. this allows a new institution to get some momentum and "runs on the board" as quickly as possible.

Having said that I think there are significant obstacles and challenges with such interdisciplinary initiatives. These challenges are scientific, intellectual, and cultural (in the disciplinary sense).

Excellent inter-disciplinary teaching and research is a just plain hard work and slow. Getting people to put in the time and keep sticking at is difficult. It requires special individuals (both faculty and students) to build bridges, learn each others languages, respect, and persevere.

The TCIS Director, Sriram Ramaswamy is co-author of a nice RMP article, Hydrodynamics of soft active matter.

I am looking forward to seeing how this exciting new adventure develops over the years. India is leading the way.

Broken symmetry, rigidity, and dissipative structures

On monday I am giving the opening talk at the Australasian Workshop on Emergent Quantum Matter. Since it is a broad audience with a range of backgrounds I am going to give a tutorial talk, building on the UQ colloquium I gave earlier this year.  Later I will post my draft slides.

One concept I want to expand on is the concept of rigidity, associated with broken symmetry. To do this I am reading a nice article "Some general thoughts about broken symmetry," written by Phil Anderson in 1981. It is reprinted in A Career in Theoretical Physics, and here is a scanned copy of the article. It contains the figure above.

What is the connection between the "rigidity" of  solids and broken symmetry? A liquid is invariant under continuous translations and rotations. When it becomes a solid it is only invariant under discrete rotations and translations. Symmetry is broken. Unlike a liquid, a solid can "sustain/resist" a shear stress. Solids are rigid.

I also want to say something about non-equilibrium. Anderson has something critical to say about "dissipative structures" such as Bernard cells associated with self-organisation and hydrodynamic instabilities.

A dream graduation speech

While in India, I enjoyed reading Chetan Bhagat's best selling novel,  Five Point Someone:  What Not to do at IIT. The novel was made into a commercially successful movie, 3 idiots. The latter is only based loosely on the novel, and I think is better. I loved it.

In the novel, the villainous Professor Cherian undergoes a transformation. At the end of the novel he departs from his usual graduation speech and says the following profound words.

 Anyway, this is my message to all you students as you find your future. One, believe in yourself, and don’t let a GPA, performance review or promotion in a job define you. There is more to life than these things - your family, your friends, your internal desires and goals. And the grades you get in dealing with each of these areas will define you as a person.  

Two, don’t judge others too quickly. I thought my son was useless because he didnt get into IIT. I tell you, I was a useless father. It is great to get into IIT, but it is not the end of the world if you don’t. All of you should be proud to have the IIT tag, but never every judge anyone who is not from this institute - that alone can define the greatness of this institute.

This receives thunderous applause.
The only problem is then Hari, the protagonist, wakes up from his dream. He has overslept and missed his graduation.

I too dream of such graduation speeches.

How important is my graduate class cohort?

Very important. You will learn a lot from them.

Recently, when I visited TIFR-Hyderabad I was asked to meet with a group of graduate students for a question and answer session on career issues. This was actually the first time I have actually done something like that. The students had many excellent questions. Some I may later blog about. Here I will just focus on this one question.

Getting a Ph.D is not just about writing a thesis or even going to classes, doing experiments, talking to your advisor, and passing exams.
At every stage of a program you can learn an immense amount from informal interactions with your peers [your class cohort]. Each has different background, interests, expertise, strengths, and weaknesses. Talking with them and sometimes working together on joint projects can be immensely valuable. Just the art of learning to talk to each other, asking questions, and crossing specialist boundaries [theory vs. experiment, chemistry vs. physics, field theory vs. condensed matter, soft vs. hard condensed matter] can be a rewarding but slow process.
Peers can also provide significant feedback and emotional support.

For some of us this may mean taking risks, overcoming shyness, and not making comparisons.

In reality, you may actually learn more from your peers than from the formal part of the program. Some people also develop  lifetime friends and/or collaborators from their cohort.

 If I had my time over again I would have interacted a lot more with my peers. At the time I just did not realise how valuable it could be.

This is why I think you are much better off in a Ph.D program that
-is highly ranked
-has a reasonable size cohort [10 plus new students per year]
-has a significant course work component
-provides on campus shared accomodation
-encourages informal interactions including shared offices that mix up research groups
-encourages interactions across groups and departments.

Should you be concerned about nuclear weapons?

Yes!
It is amazing how since the end of the cold war the issue of the nuclear weapons stockpiles of the USA and Russia attract little political attention and concern.
In his usual inimitable style John Oliver highlights why we should be concerned.

Hyderabad talk on fluorescent protein chromophores

Today I am visiting the Chemistry department at Hyderabad Central University. My host is Susanta Mahapatra. He has done some very nice work on non-adiabatic dynamics in the excited states of organic molecules. A nice review is here. Some of this work is relevant to the puzzle of diffuse interstellar bands and is described in this PRL.

I am giving a talk "Effective Hamiltonians for excited states of fluorescent proteins and methine dyes". The slides are here. A relevant paper with Seth Olsen is here.

Tata seminar on quantum hydrogen bonds

Tomorrow I am giving a seminar at the Tata Institute for Fundamental Research (TIFR) Centre for Interdisciplinary Sciences in Hyderabad.

Here is the current version of the slides, "Effect of quantum nuclear motion on hydrogen bonding."

Most of the talk is based on this recent paper.

The personal touch

It is tempting to think that with advances in technology that we don’t really need

 -live lectures and tutorials. Students can just watch videos of the worlds best lecturers at their convenience and in the comfort of their home. Tutorials can be done online, i.e. MOOCs. 

-conferences. they can be done virtually with video conferencing.

-for students doing non-experimental Ph.Ds to ever come on campus.

-visits to collaborators. It can be done via email and Skype.

This will save lots of money and time (and carbon footprint), particularly that associated with international travel.

But, we should be nervous that the loudest proponents of these initiatives are mostly politicians, neoliberal “managers”, and commercial outfits, most of whom are not (and sometimes never have been) actual teachers or researchers.

With regard to collaborations, I continue to be surprised at how effective personal visits and informal discussions are. Even when it is not clear to me that a face to face meeting is necessary, things usually get moved forward significantly, sometimes in unanticipated directions.

I should also say that some conferences and visits are not particularly productive. But, I think it is hard to predict these things in advance. Again, often the significant events are new insights from "random" interactions.

I have been at a few conferences where some “big shot” did not personally attend but gave a talk via a video link. At worst, it was a complete waste of time. At best, it was a marginal contribution to the meeting. It never had the same engaging dynamic as when the speaker is actually physically present in the room.

I think we are blessed with these technological innovations and they do have a role to play. But, it is a limited role. Furthermore, they are most effective when there is already a strong personal relationship between highly motivated individual parties and the common project is well defined. Mostly the technologies enhance and complement old fashioned “face to face” methods.

It is interesting [weird] that I have co-authored several papers with people I have never personally met. But, it is also noteworthy that this has just been one or two papers, and there has been no long term collaboration.

I think ultimately an individuals view on these matters is determined by deep underlying (and often unquestioned) anthropological assumptions. What is the nature of humanity? Are people “economic units”, “biological machines”, “social animals”, or is there something intrinsically and profoundly relational at the heart of who we are?

Enhancement of thermal expansion by strong electronic correlations

Jure Kokalj and I just finished a paper
Enhancement of the thermal expansion of organic charge transfer salts by strong electronic correlations

Our main results concerning the electronic contribution to the thermal expansion alpha are as follows.

(i) At low temperatures strong correlations can increase the thermal expansion by as much as an order of magnitude.

(ii) A non-monotonic temperature dependence of alpha is possible.

(iii) Significant orientational dependence is possible, including the expansion having the opposite sign in different directions.

(iv) In the metallic phase the crossover from a Fermi liquid to a bad metal may be reflected in a maximum in the temperature dependence of alpha.

(v) In the Mott insulating phase a maximum in the temperature dependence of alpha can occur, at a temperature comparable to that at which a maximum also occurs in the specific heat and the magnetic susceptibility.

(vi) All of the above results are sensitive to the proximity to the Mott metal-insulator transition and the amount of frustration, reflected in the parameter values (U/t and t'/t) in the Hubbard model.

Although, we can describe many of the unusual qualitative features of experimental data for organic charge transfer salts, the overall magnitude of the thermal expansion coefficients that we calculate are
up to an order of magnitude smaller than observed. This disagreement may arise from uncertainties in how uniaxial stress changes the Hubbard model parameters, and uncertainty in the compressibilities
including not taking into account the effect of softening of the lattice associated with proximity to the Mott transition.

We welcome any comments.

Strong hydrogen bonds can be insensitive to pKa detuning

Previously I posted about the role of short strong H-bonds [sometimes called low-barrier H-bonds] in the enzyme KSI.

There is a very interesting paper
Using Unnatural Amino Acids to Probe the Energetics of Oxyanion Hole Hydrogen Bonds in the Ketosteroid Isomerase Active Site
Aditya Natarajan, Jason P. Schwans, and Daniel Herschlag

The authors report a beautifully designed and implemented experiment. The idea and results are elegantly summarised in the graphical abstract below.

 The key Tyrosine amino acid (Tyr16) is substituted with different fluorinated versions. These have different proton affinities (acidity or pKa) to the native amino acid. It is well established that the strength of a hydrogen bond is maximal when the hydrogen donor and acceptor have the same proton affinity [pKa matching].  This can be naturally understood in terms of a diabatic state model for H-bonding.

The aim of the experiment is to vary the strength of one of the key hydrogen bonds in the enzyme and see what effect it has on the activity of the enzyme. They vary the pKa from 8.35 to 9.95 and find negligible change in the activity of the enzyme. They conclude

The observed shallow dependence of activity on the pKa of the substituted Tyr residues suggests that the KSI oxyanion hole does not provide catalysis by forming an energetically exceptional pKa-matched hydrogen bond. 

I am puzzled because I have the opposite conclusion, i.e. the shallow dependence is what one expects for a strong short H-bond. For strong bonds you need to vary the pKa difference between the donor and acceptor by much more than 2. A pKa difference of 2 corresponds to “detuning” the two diabatic states by about 2.7 kcal/mol [11 kJ/mol]. Experimentally, you can also see this in the two figures below.  The first [Figure 3(d) in this paper] shows the binding energy of a series of strong H-bonds as a function of the difference in proton affinity.

 The second [Figure 2 in this paper] shows the softening of the O-H stretch as a function of the proton affinity difference [in kJ/mol].

Note that in both cases a pKa difference of 2 corresponds to a very small change in the strength of the H-bond.

This is also what one expects from the diabatic state model. For short bonds (donor acceptor distance R about 2.5 A) the diabatic coupling Delta(R) is of order 40 kcal/mol. Hence, detuning the two diabatic states by 3 kcal/mol will have negligible effect on the bond. Thus, I would not expect the pKa changes to have much effect on the enzyme activity.

Why am I skeptical about curve fitting?

It continues to amaze and frustrate me how some people will do the following.
Take experimental data for a specific quantity [e.g. resistivity vs. temperature].
Fit the data to a function from some exotic theory X involving N free parameters.
Claim that the "successful" fit "proves" that X is the correct theory.

Why am I skeptical? What would it take to convince me X is actually valid?

1. Have N < 4, remembering the elephants wiggling trunk.
2. With the same set of parameters also fit at least one, and preferably several,  other experimental observation [e.g. thermopower vs. temperature].
3. Show that the fit parameters are physically reasonable and consistent with estimates from independent determinations. Science is all about comparisons.
4. Also fit the data to the predictions of mundane theory M, and alternative exotic theory X2, and clearly show they cannot fit the data. i.e., apply the method of multiple alternative hypotheses.

Finally, there is a more profound philosophical point, the underdetermination of scientific theory. We can never be sure there are not alternative theories we have not considered.

When is curve fitting valid and useful? What does it take to convince you?