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?

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?

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). 

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.

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.

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?