Condensed concepts

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 quant

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) g

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

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 ultracol

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

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

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

Yes. 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.

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

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

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 guide :  Writing 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. 

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

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 availabl

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 phys

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)

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

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.

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?

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 quas

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 pr

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   T c ) 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&quo

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-ab

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 o