Friday, May 31, 2013

Sharing my ignorance and confusion

There are things that we all wish we knew or we keep forgetting or we should understand but don't. Furthermore, there are things that I read about, think I understand, but then when pressed I don't think that I can explain to others.
Some of these things are so basic it is embarrassing.

Tony Wright had the nice idea that at the UQ condensed matter theory group meeting each member should have a turn saying a topic they don't understand and feel they should. Speakers will be in order of decreasing seniority. Hopefully, then the junior people will not feel so bad. I guess the ultimate goal is to help one another understand these things. It is also to create a culture where people are comfortable asking basic questions.

So here is a list for me in order of increasing profoundness:
  • Why are metals shiny?
  • How do p-n junctions and transistors work?
  • What is the origin of hysteresis in ferromagnets?
  • Why do extremal areas of the Fermi surface determine the frequency of quantum oscillations?
  • How does a Fano resonance between a bound state in the continuum lead to an asymmetric spectral lineshape?
  • Why does the Kondo effect lead to unitary scattering and a phase shift of pi/2?
  • How do you derive Hund's rules?
  • How do you derive Goldstone's theorem?
  • What is the chiral anomaly in quantum field theory? How is it related to edge states and topological order in condensed matter?
I know that I can look up a book and find the "answer". In some cases I have but then I have forgotten the answer. In other cases, I can mouth the words (say the mantra) but somehow there are things I am just not comfortable with, particularly at a deeper level. Sometimes I am comfortable with the maths but not the physics. Other times it is the opposite. Then the urgency of other tasks takes over ....

What are some questions on your list?

Thursday, May 30, 2013

Quantum effects in condensed phase chemistry

I am looking forward to attending a workshop on Quantum effects in condensed phase systems at the Telluride Science Research Centre in July.
I thank Scott Habershon and Tom Markland for organising what looks like a great meeting. I don't normally do the crazy thing of flying to USA for just one week, but I think this meeting should be worth it.

Much of chemistry is "classical" in the sense that it can be described by semi-classical dynamics of the nuclear degrees of freedom moving on potential energy surfaces that can be calculated in the Born-Oppenheimer approximation.
But, there are important exceptions.

I list below some of the quantum nuclear effects that need to be considered. They are listed roughly in the order of increasing exoticness and decreasing frequency of attention they receive.
  • zero-point energy
  • tunneling
  • non-adiabatic, breakdown of the Born-Oppenheimer approximation
  • interference
  • entanglement (of nuclear and electronic degrees of freedom)
  • geometric (Berry) phases
  • collective coherent effects
These effects can all be present in small molecules in the gas phase.
A key question is how are the above effects modified in a condensed phase environment (e.g. a solvent, glass, or protein)?
Generally, interaction with the many degrees of freedom of the environment will decohere the small molecule degrees of freedom and reduce the quantum effects.

Here are some big questions.
Are there any instances where the environment can 
-enhance any of above quantum effects?
-lead to qualitatively new effects (e.g. associated with collective degrees of freedom) that are absent in the gas phase?

Wednesday, May 29, 2013

Write your abstract for your audience

Next week I am giving the Quantum science seminar at UQ. This is attended by people with diverse interests and backgrounds: cold atoms, condensed matter, quantum information, and quantum optics.

Hence, I have written a talk abstract that is hopefully attractive and interesting enough to motivate people to come to the talk.

Comments welcome.

When good metals turn bad: from organic superconductors to ultracold atomic gases

Key properties usually associated with metals are that they are shiny and excellent conductors of electricity and heat. Hence, one might think that the best strategy to find a good superconductor (e.g. one that works at room temperature) is to study good metals. Actually, the opposite is true. The past few decades have shown that the most interesting and important metals are "bad metals". They often occur in proximity to a Mott insulating phase. Bad metals are characterised by a large electrical resistance of the order of the quantum of resistance h/e^2 and their theoretical description is an outstanding problem.

I will discuss the distinct experimental and theoretical signatures of bad metals [1]. They occur in a wide range of correlated electron materials, including high-Tc cuprate superconductors, heavy fermion compounds, and superconducting organic charge transfer salts [2]. A key feature is that with increasing temperature good metals turn bad, at a temperature corresponding to the loss of quantum coherence.

The simplest possible effective Hamiltonian for organic charge transfer salts is a one-band Hubbard model on an anisotropic triangular lattice at half-filling [2]. The model exhibits a transition from a Mott insulator to a bad metal as the interaction (U/t) is reduced or the frustration (t'/t) is varied.
A recent numerical study of the model [3] showed that near the Mott insulator the calculated quantum coherence temperature was much less than the non-interacting Fermi temperature, consistent with experiment. Furthermore, the bad metal is characterised by a small charge compressibility, a large spin susceptibility, and fluctuating local magnetic moments.

Finally, I will discuss the connection with the viscosity of perfect fluids, including experiments on ultracold atomic gases, and calculations based on string theory techniques!

[1] J. Merino and R.H. McKenzie, Phys. Rev. B 61, 7996 (2000).
[2] B.J. Powell and R.H. McKenzie, Rep. Prog. Phys. 74, 056501 (2011).
[3] J. Kokalj and R.H. McKenzie, Phys. Rev. Lett. 110, 206402 (2013).

From cold atoms to quark-gluon plasmas

In 2007 Gordon Baym gave a fascinating talk New States of Quantum Matter which is nicely summarised in a short conference paper. You can watch a 2010 version of the talk here.

Baym discusses similarities of the physics associated with cold atomic gases and quark-gluon plasmas. These similarities occur in spite of the fact that the relevant energy scales in the two systems differ by more than 20 orders of magnitude!

For example, the phase diagram below shows the different phases of a many-body quark system as a function of temperature and chemical potential.
Increasing the chemical potential corresponds to increasing the density. [Remember that for a non-interacting Fermi gas the Fermi energy increases with density].
Note that at "low" temperatures there is a continuous cross-over from a hadronic superconductor [roughly a BEC of paired quarks] to a quark superconductor. Baym points out that some level this is analogous to the BEC-BCS crossover that occurs in ultracold Fermi gases as one tunes the interaction from repulsion to attraction (e.g. via a Feshbach resonance). However, like all analogies this is not perfect. The quark system involves three different "colours" of fermion with different masses, whereas the cold gas one involves two with identical mass. An interesting challenge for the cold atom community is to design the corresponding three fermion system. This has been discussed by Rapp, Zarand, Honerkamp, and Hofstetter  (see the associated Nature Physics News and Views by Frank Wilczek). 

Tuesday, May 28, 2013

Interview questions for faculty positions

From my experience, some of the most common questions are listed below. In Commonwealth countries [e.g., Australia, UK] these are usually asked by a formal interview panel. In North America they are usually asked in informal meetings with individuals. I don't know how it works in Europe.

Why are you interested in this position?

What do you think is your most significant research achievement?

What are your scientific goals for the next 5 years? 10 years?

How will you obtain funding for your research?

Who do you think you might collaborate with at this university?

What is your philosophy of teaching?

What courses would you like to teach here?

What is your philosophy of supervision of postgraduate research students?

How do you think you could be involved in university service and community outreach?

Given the common occurrence of these questions I suggest you write out your answers beforehand and keep the piece of paper in your pocket.
It may be helpful to think of your three main selling points and try to integrate them into your answers.

There is a longer list of good questions here.

What questions should you ask? 

Ones that show you know something about the department and institution, that you are interested in coming there, and that you want to be successful together.

At this stage, your goal is NOT to gather information to help you decide whether or not you would accept an offer. Your goal is to get them to make you an offer. Later you can ask hard and demanding questions about salary, start up funds, teaching loads, departmental and institutional politics, .....

Here I disagree significantly with some of the websites that show up when you Google "faculty interview questions", e.g. this one at Dartmouth College. I think the questions listed are more the type of questions I would ask if I actually got an offer. I think asking too many of these questions may irritate people and backfire. I have seen this happen.

Perhaps, my concern is a cultural difference (Australian vs. American). But I find many of those questions as too aggressive and a bit too direct for me. Some come across as rather naive and unlikely to get an honest answer. For example, if you ask a Department chair "Is this department united or divided?" I am skeptical that you will get an accurate answer.

Finally, my post-doctoral mentor John Wilkins has sage advice.

Please share your experience, either being interviewed or doing the interviewing.

Monday, May 27, 2013

Universal scaling relations for exotic superconductors

There is an interesting paper
Do organic and other exotic superconductors fail universal scaling relations?
S. V. Dordevic,  D. N. Basov, and C. C. Homes

It has been found previously that a wide range of superconductors obey certain scaling relations involving their superfluid density. These are a generalisation of a scaling between the superfluid density and transition temperature Tc that Uemura originally found for underdoped cuprates. They have been particularly promoted by these authors. But, in a 2005 PRL Frances Pratt and Steve Blundell argued that molecular superconductors did not obey them. In 2004 Ben Powell and I pointed out that a number organic transfer salts with relatively low Tc had much lower superfluid densities than the Uemura relation.

In this paper the authors stress how tricky it is to measure the superfluid density and the corresponding conductivity at the transition temperature. It is important to measure these quantities on the same sample with the same technique. [Their preferred technique is the microwave surface impedance]. They show that when this is done for a range of organics that the data then do lie on the universal curve shown below.

A few comments.
  1. It remains to be shown whether the low-Tc organics of particular concern to Ben and I lie on the curve.
  2. It is a worry to me that the elemental superconductors also lie on the universal curve. This suggests a non-exotic explanation. The authors flag this issue too. 
  3. The issues here highlight the pro's and con's of listening to experimentalists, as I discussed recently.
I thank Ben Powell for bringing the paper and a helpful discussion about it.

Friday, May 24, 2013

What is quantum matter?

It may depend on who you ask.
It is interesting that even twenty years ago the phrase "quantum matter" was rarely used.
Now we have

Department of Quantum Matter, Hiroshima University 

Quantum Matter Institute, University of British Columbia 

 Shoenberg Laboratory for Quantum Matter, University of Cambridge 

 So, what is quantum matter?
To some it is any material system (solid, liquid, or gas) where the quantum statistics of the constituent particles significantly affect the properties of the system. One could argue on some level this is any state of matter! After all, the Pauli exclusion principle is key to chemistry!

The above departments are largely concerned with studying what used to be called "strongly correlated electron systems". Hence, one also often sees the phrase "correlated quantum matter". I think David Pines and Piers Coleman may be two of the people who have most promoted the phrase. Coleman and Andy Schofield use the phrase "quantum matter" repeatedly in their 2005 Nature review Quantum criticality. Pines has a nice tutorial article Emergent behavior in quantum matter.
Does anyone have a better etymology?

To me the key idea is that there are states of matter [quantum many-body systems] with emergent macroscopic properties that are intrinsically quantum mechanical. Superconductivity is the classic example, being described by a macroscopic quantum mechanical wave function. Furthermore, there may not be broken symmetries. Instead, the many-body states of quantum matter may require concepts such as topological order, the most common examples being found in fractional quantum Hall effect and topological insulators. In some sense different metallic states: bad metals, "quantum critical metals", and the "strange metal" in the cuprates are all quantum matter.

The notion of quantum matter is useful as a unifying concept for describing many of the common themes of interest in two culturally distinct research communities: those studying ultracold atomic gases and correlated electron materials.

There is also a puzzling somewhat philosophical question:
Is quantum matter itself emergent or does quantum matter have emergent properties?

Should you be concerned about Massive On-Line Courses (MOOC)?

MOOCs are all the rage in some circles, particularly amongst politicians and university administrators. On Doug Natelson's blog he has a helpful post which links to two thoughtful, critical and challenging articles. I think the social justice issues raised by the Philosophy Department at San Jose State University are particularly pertinent.

I agree with Doug's point:
I do think it's worth thinking hard about the purpose of MOOCs.  Are they about idealistically providing access to fantastic educational opportunities at very low cost to the student for millions of potential pupils who have an internet connection?  Are they about cynically slashing the operating costs of universities by restructuring the educational experience and potentially eliminating large numbers of faculty jobs?  These are not mutually exclusive.

Thursday, May 23, 2013

Ice X is quantum

Solid water (ice) is amazing having more than ten distinct phases. Ice X exists above pressures of about 70 GPa. It is of particular interest because it represents a case of strong hydrogen bonding where protons are equidistant between oxygen atoms.

There is a nice Nature paper from 1998 Tunnelling and zero-point motion in high pressure ice by Benoit, Marx, and Parrinello. The figure below is of particular interest to me. It shows the OH bond length as a function of the O-O distance (bottom scale) in the crystal. The latter can be tuned continuously with pressure (top scale).
The non-solid points are from a classical calculation at two different temperatures. The solid points are when one takes into account the full quantum dynamics of the protons, thus taking into account the effects of tunneling and zero point motion.
The proton becomes equidistant between the two oxygen atoms (solid line) for pressures larger than about 70 kbar, consistent with experiment.

The figure above is similar to a figure in my recent paper about hydrogen bonding, and is consistent with comments I made there about the importance of zero-point motion.

I thank Christiaan Bekker [who is doing an undergraduate research project with me] for bringing the paper to my attention.

Wednesday, May 22, 2013

Listen to experimentalists (sometimes)

Over the years I have benefited greatly from my interactions with experimentalists. These interactions have varied from informal discussions, listening to talks, and reading papers.
These interactions have led me to work on interesting and important problems and helped make my theoretical work sharper and more relevant to experiment.

But on reflection, I regret I have also wasted significant amounts of time, energy, and money because I have listened (too much) to experimentalists.

So is there a key to getting the benefits without the liabilities?

I think the key is to listen to the "broad brush strokes" and not get distracted or hung up on the details.

Experimentalists can teach us
  • what measurable quantities we should aim to calculate [e.g. thermopower vs. temperature, interlayer magnetoresistance vs. magnetic field direction]
  • what specific systems or materials are of particular interest
  • what is actually known about specific systems or materials [e.g. the shape of the Fermi surface]
  • orders of magnitude and reasonable parameter ranges for experimental observables
  • we should not believe every published experimental result. Things can go wrong. If you visit a lab and see how complicated and delicate some of the apparatus are you may wonder why there aren't more spurious results!
Theory papers often contain things like a graph of ground state energy vs. a variational parameter for a trial wave function. That may be of some theoretical relevance or importance. But, it is not something an experimentalist is going to be interested in. It is not something they can measure!

Theorists will sometimes be not that concerned with the actual magnitude of energy, temperature, or field scales. Instead they just work with some model parameter, e.g. t and U in a Hubbard model. But, experimentalists really want to know how this translates to temperature or magnetic field. Earlier I have posted about this issue for the case of magnetic fields in Hubbard model calculations.

So when should theorists ignore experimentalists? When do I wish I had ignored them?

I think my mistake has been to sometimes get caught up in the puzzle of specific results of individual experimentalists.

Suppose an experimentalist comes to you and says "We have these interesting/weird results on this exotic material using our fancy new fangled measuring technique. Maybe you can help us explain them." I recommend listening politely and not getting involved. Wait until another group has independently observed similar results on a different sample with a different technique.

I also think that sometimes experimentalists get hung up on small discrepancies between theory and experiment and try to get us to explain them. There are important historical cases where this has been important. But, I think often the benefits to theorists can be marginal.

I have just focussed on the scientific benefits from the fruitful interaction of theorists with experimentalists. I think there can also be some significant career benefits (and liabilities) because generally experimentalists have more money and clout than theorists and so being liked by them can be a significant career booster. But, that is another story....

I welcome comments and peoples own experiences.
I would also be interested to hear an experimentalists view on listening (not too much) to theorists!

Tuesday, May 21, 2013

Shameless spam from ACS publications

Occasionally I scan my Junk Mail folder because sometimes useful email does turn up. In amongst all the spam from Linked In, ResearchGate, administrators, Nigerian widows, conferences and journals I have never heard of... there was an email from American Chemical Society (ACS) Publications.

In the world of networked science, it isn't enough to be published — you have to be found.
Optimizing your papers for search isn't a skill taught in grad school. Ensure your research gets found by the right people. Download your free guideWriting Scientific Manuscripts for the Digital Age. Your research is important. Help it get the impact it deserves.

Guide Highlights:
  • Optimizing your keywords and visuals for search
  • The abstract: importance of your mini-manuscript
  • Selecting the optimal journal to publish your research
  • Broadening your reach with social media
  • Measuring the influence of your article
This sounded great. I also thought it might be good blog fodder. 
First the catch: you can't directly access the article. When you click on the link there is a request for your email address and name.

They then send you an email giving you a link to the actual article.

I must say I was very disappointed. 

The "article" is really just a shameless marketing blurb for ACS journals and social media.
I found no real useful tips beyond the obvious: include keywords that may show up in searches in your title and abstract.

Friday, May 17, 2013

Enumerating classical and quantum Hall effects

The helpful figure below is taken from The Complete Quantum Hall Trio, a recent Perspective in Science by Seongshik Oh.

Thursday, May 16, 2013

Most decisions are binary: yes or no?

I find reminding myself of this fact. It makes decisions a lot easier.
It may also help in influencing decision makers.

These days I have to make many decisions:
Should this paper be published in PRL?
Should this person get a grant?
Should this person get tenure?
Should I interview this person for a postdoc?
Should this student be allowed to continue their Ph.D?

Making these decisions can consume large amounts of time and energy.
However, I have found it is important and somewhat liberating to sharpen the decision down to a simple yes/no question. It is easy to get distracted from this.

For example when reviewing a grant application it is easy to get distracted by secondary questions:
Does this young applicant deserve to get a grant?
Is the applicants last paper valid, important and significant?
How much should I let citations influence my decision?
Is the budget reasonable and realistic?

But the real question is more like:
Given the competition, the funds available, and the goals and criteria of the funding agency should I recommend the person get a grant?
The answer to that can often be decided very quickly. I don't have to research all the scientific subtleties behind the proposed project or wade through the budget details or fluff about impact factors and committee service....
Furthermore, once I have the yes/no answer I don't see the need to write a long and detailed report justifying it. The decision makers (at the next stage) reading my report are mostly interested in the yes/no not the subtleties behind it.

I don't deny that the answers to secondary questions influence the yes/no answer to the primary question. But, I have found it is easy to get to distracted by them.

I also find this focus on the binary character of decisions helps in trying to influence the decisions that affect me. It particularly affects how much time I spend on "preparing my case." 

Suppose I want to get a travel grant which has a 70 per cent success rate. All I care about is whether I get the grant. Yes/no. Whether my proposal is highly ranked or is appreciated because it contains a beautiful literature review is really irrelevant.

Suppose I want an editor at PRL to accept my paper after some critical referee reports. I don't really care if the editor thinks I have given a particularly cogent response to all of the criticisms (e.g. three different counter arguments for each criticism). I just want the editor to be convinced that it is o.k. to publish the paper, even if she/he has misgivings.

I welcome your thoughts. Am I ruthless and superficial? Is this a helpful idea?

Wednesday, May 15, 2013

Mott physics with spin-orbit coupling

There is a very nice and helpful review article
Correlated quantum phenomena in the strong spin-orbit regime
William Witczak-Krempa, Gang Chen, Yong Baek Kim, Leon Balents

Just a few things I learnt from quickly skimming it.

There are many outstanding and basic questions concerning the phase diagram of even the simplest possible two-orbital Hubbard model with spin-orbit coupling. There a many possible new phases waiting to be discovered [both experimentally and theoretically] or to be shown to not actually exist because their theoretical proposal is based on uncontrolled approximations. The figure below is a possible schematic phase diagram.
Much of the interesting physics requires spin-orbit coupling energies of the order of hundreds of meV, i.e. comparable to electronic band energy scales. Hence, this is irrelevant to many materials.
But the spin-orbit coupling can be quite strong in 5d transition metals. Iridates (iridium oxides) may be model compounds to realise this physics.

Table I provides a nice summary of the properties of the plethora of different new phases that have been proposed including axion insulator, Weyl semi-metal, fractional Chern insulator, ...

Na2IrO3 was originally proposed to be a realisation of the Heisenberg-Kitaev model and thus to have a possible spin liquid ground state. However, neutron scattering shows it has an unanticipated  magnetic ground state: "a zig-zag state with four-sublattice structure." This has led to new proposals as to the relevant effective spin Hamiltonian.

The review has only limited discussion of the role of Hunds rule.

It is repeatedly stated that spin-orbit coupling leads to entanglement of spin and orbital degrees of freedom. But the exact nature of this quantum entanglement is not clearly stated or calculated. For the case of entanglement arising via Hund's rule (not spin-orbit coupling) this is nicely discussed by Oles here.

The key thing that the spin-orbital coupling/entanglement does is remove (or at reduce) the coupling of the orbital degeneracies to the lattice which normally produces the Jahn-Teller effect and orbital ordering.

Sr2IrO4  is an approximate homolog of the parent material of the high-Tc cuprates, La2CuO4. It is a Jeff = 1/2 antiferromagnetic insulator and has an exchange constant J ∼1000 K comparable to that of the cuprates. This has led to a strong push to dope the material in the search of cuprate-type physics [high-Tc superconductivity, pseudogap, strange metal]. This has not occurred yet.

Both Sr2IrO4 and Sr3Ir2O7 have illustrated the power of the rapidly evolving technique of Resonant Inelastic X-ray Scattering (RIXS). It seems to be particularly suited for 5d compounds, and has been used to map out the full spin wave dispersion in both compounds.

I thank Tony Wright for bringing the review to my attention.

Tuesday, May 14, 2013

Bad metal conference talk abstract

In July I am going to a great conference Bad metal behaviour and Mott quantum criticality.
Here is the abstract that I submitted for my talk.
(Double click to see a large version)

Monday, May 13, 2013

Validating a diabatic state picture for strong hydrogen bonds

In a recent paper I argued that a diabatic state picture can give a nice description of hydrogen bonding, spanning from strong symmetric bonds to weak asymmetric bonds.

A key implication/prediction of this picture is the existence of a "twin state" to the ground electronic state. The excited (ground) state is an antisymmetric (symmetric) combination of the two diabatic states.
This state should be in the UV region. I suggested it has a large photo-absorption cross-section and should lead to photo-dissociation of the complex.
The excited state should also be "visible" in quantum chemistry calculations, but may be mixed with other excited states (Rydberg states).

Recently I came across a paper (published in the Ukrainian Journal of Physics!) which supports this view for the H5O2+ complex (Zundel cation)
Pseudo Jahn−Teller Origin of the Proton Tunneling in Zundel Cation Containing Water Clusters
Geru I., Gorinchoy N., Balan I.

The Zundel cation consists of a proton shared by two water molecules and is believed to be important for understanding proton solvation (i.e. acids!) and transport in water. It is a classic example of a strong symmetric hydrogen bond.

The authors fit the ground and excited state potential energy surfaces to a two state effective Hamiltonian (pseudo Jahn-Teller), reminiscent of the one used in my paper (and many other peoples too!).

The matrix element coupling the two diabatic states, Delta = 7 eV, showing a very strong coupling.

The calculated energies for the lowest lying electronic states are shown below
The quantum chemistry calculations are
the electronic correlation was taken into account via the configuration interaction (CI) with single and double excitations (CISD). In calculating the potential energy curves, the full valence CI was employed. The active space of CI ...included six occupied and five lower unoccupied molecular orbitals (more than 40000 configurations with S = 0).
n.b. The low-lying excitations 1E and 3E are essentially intra-molecular excitations within one of the two water molecules.
In contrast, the 1B excitations are collective excitations of the whole complex.
The twin state of interest is the 1B2 state.
It is higher in the UV than I expected; but it is there.

Friday, May 10, 2013

Molecular oxygen is paramagnetic

This video allows one to see the effect of Hund's rule coupling with the naked eye!

Molecular nitrogen is a spin singlet (and so diamagnetic) and molecular oxygen is a spin triplet (and so paramagnetic).

The figure below taken from Atkins' Physical Chemistry (Figure 10.33 of the ninth edition) illustrates the comparative molecular orbital electronic structure. The key difference is that the two valence electrons in oxygen are in two degenerate pi_g orbitals. Hund's rule coupling then causes the ground state to be a spin triplet. There is a large Curie paramagnetism associated with that.
A nice discussion of the relevant two-site two-orbital Hubbard model is here.

Thursday, May 9, 2013

Do you read attachments to emails?

I rarely do. Maybe I am alone on this.

The same applies to hyperlinks within emails too.

Obviously, if the email is from a collaborator and the attachment concerns their latest results then I do open the attachment and read it carefully.

However, emails from administrators, conference and seminar organisers, and other random matters are only read if the subject looks important.
The main message needs to be in the body of the text otherwise it just does not get my attention.

What about you?

Wednesday, May 8, 2013

Long live Fermi liquid theory!

There is a very nice preprint
Hidden Fermi Liquid, Scattering Rate Saturation and Nernst Effect: a DMFT Perspective
by Wenhu Xu, Kristjan Haule, and Gabriel Kotliar

I think it is original and important. I wish I had written it!

They consider the metallic phase of a two-dimensional Hubbard model at (close to optimal) hole doping 0.15 away from the Mott insulator, within Dynamical Mean Field Theory (DMFT). 

The surprising result (to me) is that one can talk about quasi-particles (i.e. poles in the one electron Green's function) up to much high temperatures than one might expect (specifically, far beyond the temperature T_FL, below which the scattering rate has a quadratic temperature dependence).
One just has to allow the quasi-particle weight Z to be temperature dependent, as shown in the Figure below.
This leads to a temperature dependent band structure.
Furthermore, most of the transport properties calculated within DMFT are quantitatively described by a quasi-particle approximation and Sommerfeld expansion, even into the bad metal region. The graph below shows the temperature dependence of the thermopower. Note the change of sign.
A few comments:

1. The authors suggest there may be a connection to Nigel Hussey's phenomenology of the cuprates, particularly with regard to saturation of the scattering rate at high temperatures.
These ideas are developed more in a recent PRB by Jure Kokalj, Nigel and I.
[But as the authors point out DMFT cannot capture the anisotropy observed in the cuprates].

2. I am not sure about calling this a "Hidden Fermi liquid" since that terminology is associated with a specific idea of Phil Anderson which I discussed here. It is not clear to me that these "Fermi liquids" are the same thing. In particular, Anderson's seems much more exotic.

3. The emergence of the different temperature scales and "strange metal" behaviour confirms my prejudice (and Anderson's) that the AdS/CFT approach is not relevant.

4. Minor quibble: It would be helpful to have units on the axes in Figure 2. I can see that the thermopower S is in units of k_B/e but have no idea about the Nernst signal. This would help in comparing the magnitude to experimental values for the cuprates.

I welcome more comments on this work.

Tuesday, May 7, 2013

Should we have "work in progress" seminars?

I was talking to a historian colleague yesterday and he introduced me to an interesting idea. In the research centre (about a dozen faculty and postdocs) he directs they have a fortnightly (I think) "work in progress" seminar. The format is (something like) as follows.

One individual presents a 10 minute paper (circulated beforehand) describing a project they are currently working on. Everyone present (faculty, postdocs, students, visitors) then discusses the project for more than an hour! 
They discuss strengths, weaknesses, and possible directions for the project.
Afterwards everyone goes out for a meal.

The fact that the feedback is appreciated is testified by the fact that there is a backlog of people (including from outside the centre) who are waiting to give presentations.

They also do this with all their grant applications.

Would this work for science?

I think we need to do something more like this.
Generally, the only time I see people discuss work in progress is within research groups. Then everyone present is more or less "committed" to the topic and to the approach.
At larger formal seminars people tend to only present "completed" research and there is virtually no time for real discussion or feedback.

Why don't we do this?
I fear some of the reasons might be:
-fragile egos
-senior people don't want to see their pet projects cut down even before they begin
-people are scared of getting "scooped" [Aside: I think this is sometimes an egotistical delusion].
-people don't want others suggesting ideas and collaborations and expecting to be co-authors.

What do you think?

Monday, May 6, 2013

Anderson's critique of AdS/CFT

Phil Anderson has an important letter to Physics Today, Strange connections to strange metals which criticises the Quick Study, "From black holes to strange metals," by Hong Liu. The "thoughtful curmudgeon" begins
It is one of many quasi-journalistic discussions I have seen of results using the AdS/CFT (anti–de Sitter/conformal field theory) correspondence from quantum gravitation theory ostensibly to solve condensed-matter physics problems such as the “strange metal” in the cuprate (high Tc) superconducting metals. 
I think the following criticism is particularly important:
The strange-metal region of the cuprate phase diagram exhibits not only a linear dependence on temperature of the conductivity relaxation rate, which is generally taken by string theorists as the characteristic symptom identifying a strange metal and is the only feature they discuss.
There are many other properties of the strange metal.
Anderson lists the "Drude tail" in the infra-red region of the frequency dependent conductivity, the temperature dependence of the Hall angle, and power-law dependence of the various spectra. Any decent theory must also explain all these other features.

Anderson then claims that this phenomenology is all described by his hidden Fermi liquid theory, culminating in a 2011 PRL he wrote with Phil Casey.

I disagree on this point. A PRL by Jure Kokalj and I showed this theory could not explain the doping and temperature dependence of the scattering rate deduced from angle-dependent magnetoresistance measurements in the overdoped region (i.e. dopings larger than that of the strange metal phase).
A PRB by Jure, Nigel Hussey, and I shows how a model self energy can describe the frequency, temperature, and doping dependence of the properties of metal at overdoping. However, this does not give a microscopic explanation or describe the strange metal.

Anderson also mentions a connection of his theory to the chiral anomaly [Jackiw-Rebbi] in field theory.
[I think this is the "parity anomaly" in Haldane's famous "Quantum Hall effect without Landau levels" paper].
I don't follow this connection at all.

Anderson concludes by suggesting that most condensed matter systems are not relativistic and conformal, particularly due to the presence of the lattice.

I did not find Hong Liu's response to Anderson's letter particularly convincing. To me it just highlights all of the assumptions [and wishful thinking?] implicit in the AdS/CFT approach.
What do you think?

It should also be pointed out that even if AdS/CFT is an appropriate effective field theory for the strange metal, the important question will remain: How does that effective theory emerge from the underlying microscopic Hamiltonian?

Friday, May 3, 2013

Some ultra-cold atom experiments I would like to see

I have been having some stimulating interactions with my Australian cold atom colleagues, including Matt Davis, Chris Vale, Andy Martin, and Kris Helmerson.

As I see it ultracold atomic gases and solid-state materials have complementary strengths and weaknesses for investigating emergent quantum many-body phenomena. Solid state materials are much easier to bring to spatially uniform thermal equilibrium, achieve temperatures much less than characteristic temperatures (such as the Fermi temperature), and perform high precision thermometry. On the other hand it is hard to drive solid state systems far from equilibrium, to investigate non-equilibrium phenomena such as turbulent charge flows, and the time scales for relaxation to equilibrium are often too fast to be observed.

In contrast, ultra-cold atomic gases make it is much easier to access non-equilibrium states, and image them and their time evolution. The two platforms are also complementary in the access they provide to tune-ability, control and design. Solid state systems can be tuned considerably by temperature, pressure, magnetic field, electric field, and chemical substitution. However, sometimes it is hard to know how these variations produce changes in the underlying microscopic interactions between the constituent particles. In contrast, some of the underlying interatomic interactions in ultracold atom systems can be readily tuned from weak to strong in a precise and the known manner. However, a major challenge remains to expand the repertoire of possible tune able interactions, particularly to include some of the more common interactions found in solid state systems (e.g., the coupling of orbital motion of fermions to a magnetic field and the Heisenberg antiferromagnetic spin interaction in Mott insulators).

Here a few experiments that I would particularly like to see done and may be "relatively straight-forward", i.e, feasible in the next few years. Of particular interest would be observing these phenomena in fermionic atom systems in which one can tune the strength of the interactions, observe the BEC-BCS crossover, and universal behaviour associated with scattering close to unitarity.

Probing Thermoelectric transport with cold atoms
Quantum oscillations in ultracold Fermi gases: Realizations with rotating gases or artificial gauge fields
Charles Grenier, Corinna Kollath, Antoine Georges

The "Higgs boson"!
Visibility of the amplitude (Higgs) mode in condensed matter
Daniel Podolsky, Assa Auerbach, and Daniel P. Arovas

For bosonic systems there is a recent experimental paper from Immanuel Bloch's group
The ‘Higgs’ amplitude mode at the two-dimensional superfluid/Mott insulator transition

This then connects to
Conductivity of hard core bosons: A paradigm of a bad metal
by Lindner and Auerbach
An earlier post discussed this paper, suggesting calculation of the thermopower.

Observation of an d-wave pseudogaps. For the s-wave case see
Observation of a pairing pseudogap in a two dimensional Fermi gas.

Thursday, May 2, 2013

Do you ever reveal your identity as a referee?

I don't, even when I write positive reviews.
If someone asks me, "Did you review my paper/grant?" I tend to say, "I never say one way or the other."

I think it is tempting to tell colleagues:
"I reviewed X's grant and it was a load of rubbish."
"I reviewed that paper for Nature and rejected it."
"I reviewed that paper for PRL and thought it was brilliant."
"I really liked your grant application and gave it a 5."

I try not to do this.
Even if you don't directly tell someone of a negative review it can get back to them.
On the other hand, I would like to tell people of positive reviews to encourage them.
But, I really don't want them to think I am asking them to return the favour. "I scratched your back. Now you, scratch mine.".

If I say neither yes/no when asked, it keeps people guessing.

So, do you ever reveal your identity?
If someone asks you?
Do you ever offer the information to others?