Condensed concepts

Nitrogen fluoride (NF) seems like a very simple molecule and you would think it would very well understood, particularly as it is small enough that it is accessible to high level quantum chemistry calculations. However, the molecule exhibits some subtle properties that present a theoretical challenge. There is limited experimental data because the molecule is only found as an intermediate in some chemical reactions. Just like oxygen (O2), to which it is isoelectronic, the ground state is a triplet due to Hund's rule, as discussed for O2  here. I just read a nice paper A Valence Bond Study of the Low-Lying States of the NF Molecule  Peifeng Su, Wei Wu, Sason Shaik, and Philippe C. Hiberty Given that F is more electronegative than N one might expect the ground state to have a large electric dipole moment and this to increase as the molecule is stretched. However, the ground state has only a small moment, it has the opposite direction to that expected from the electronegativ

How do you convince politicians to fund basic research? On The West Wing there is an episode Dead Irish Writers in which many things are happening simultaneously. One is that Sam Seaborn [White House Deputy Communications Director] is meeting with a Princeton Physics Professor, Dr. Millgate who is dying of cancer but trying to secure funding for the Superconducting Super Collider . It features the following excellent dialogue. Sam Seaborn: Okay. I said I'd do this, but it's likely he's gonna say this is an unaffordable luxury.  Millgate: We're losing the race for discovery, Sam. For discovery. Tonight, it's just me and you.  Sam Seaborn: That doesn't really sound like enough.  Millgate: No. Then in a later scene Senator Enlow: If only we could only say what benefit this thing has, but no one's been able to do that.  Dr. Millgate: That's because great achievement has no road map. The X-ray's pretty good. So is penicillin. Neither wer

John Nash and his wife, Alicia, tragically died in a car crash on the weekend. There is a  New York Times obituary  He was a brilliant young pure mathematician who laid foundations for game theory in a 27 page Ph.D thesis. Nash became widely known outside academia through the movie A Beautiful Mind , that tells his life story, focusing on his struggle with severe mental illness. It is based on an excellent biography  by Sylvia Nasar. It is less sanitised than the Hollywood version. Tragically Nash's life also illustrates the importance of mental health issues in academia, and so I mention him in talks I give about mental health for scientists. I have a strange personal connection with Nash. When I was a graduate student at Princeton I often saw a middle aged man reading Scientific American in the Maths/Physics library. He was often there and I wondered why he was there. Didn't he have a job? Yet I don't remember ever asking anyone about him. Also, sometimes in the Physi

At UQ all physics honours students (4th year undergrad) have to give two 15 minute talks about their year long research project. The first is a progress report at the end of the first semester and the second at the end of the project. No grades are given for these presentations but they are attended by the 3 thesis examiners [supervisor, expert, and non-expert] and so may influence the grade for the thesis. I think these presentations are very challenging for the students and I am sometimes impressed at the quality of the talks. This is a great opportunity for students to develop and improve their communication skills. When I was an undergrad we never had opportunities like this. Most of us also had very little public speaking experience. Students today are quite different and much more confident and polished. Here is my advice to students. First, review general material on giving scientific talks such as Garland's Advice to Beginning Physics Speakers or  Wilkins' one pa

Previously I posted about  improper hydrogen bonds.  These are weak hydrogen bonds that have the unusual property that in the X-H...Y system H-bonding leads to a shortening and hardening (blue shift) of the X-H bond. In contrast, for "proper" bonds, X-H lengthens and softens (red shift). The past few years has seen a rapid increase in interest in an even broader class of weak bonds such as " halogen bonds ",  denoted X-Z...Y where Z can now be not just H but a halogen (F, Cl, Br), chalcogen (O, S, Se, Te), or pnictogen (N, P, As, ..).... There is an interesting paper that contains the helpful summary figure below Negative hyperconjugation and red-, blue or zero-shift in X-Z---Y complexe s Jyothish Joy, Eluvathingal D. Jemmis and Kaipanchery Vidya In trying to understand the paper I found reading the following older paper helpful Electronic Basis of Improper Hydrogen Bonding:  A Subtle Balance of Hyperconjugation and Rehybridization Igor V. Alabugin, Mar

Previously I posted about the theoretical issue of the viscosity of the electron fluid in strongly correlated metals. This interest is partly motivated by claims from string theory techniques [AdS-CFT] that there is a universal lower bound for the viscosity.   A recent experimental paper estimated the viscosity in the cuprates by an indirect method from ARPES data. I only became aware recently that there is a somewhat direct way to measure the viscosity of the electron fluid in a metallic crystal. This has a long history going back to Mason and  Pippard  who in 1955 related the viscosity to the attenuation of sound. A more sophisticated and general theory was developed by Kahn and Allen. The connection between shear viscosity and ultrasound attenuation can be loosely motivated as follows. In a viscous fluid the attenuation of a shear wave is given by Stokes law whe re   is the shear  viscosity  of the fluid,   is the sound's  frequency ,   is the fluid  density , and   is

According to the Oxford English Dictionary Integrity = "The quality of being honest and having strong moral principles". When people talk about scientific integrity and misconduct they mostly have a narrow definition which means "don't make up data." However, I think we need to consider a broader definition of integrity that relates to all communications and messages. Scientists talk about their research in a wide range of forums: private discussions articles in luxury journals articles in professional society journals (PRA, JCP etc) grant applications and job applications seminars at universities and conference presentations press releases and interviews public lectures and popular books Yet it seems it has now become quite acceptable to have different messages (claims and conclusions) in different forums. This post was stimulated by a perceptive comment by Steve W on a previous post . My finding is if you talk to the authors of luxury papers

There is an interesting paper Theoretical prediction of a strongly correlated Dirac metal   I. I. Mazin, Harald O. Jeschke, Frank Lechermann, Hunpyo Lee, Mario Fink, Ronny Thomale, Roser Valentí The compound Herbertsmithite ZnCu 3 (OH) 6 Cl 2  has attracted a lot of interest because it is a Mott insulator with a layered crystal structure where the Cu2+ ions (spin 1/2) are arranged in a kagome lattice. There is some evidence both experimentally and theoretically that the ground state is a spin liquid. [However, inevitably there are complications such as the role of impurities and the  Dzyaloshinskii-Moriya  in teraction ]. In this paper the authors replace the Zn2+ ions with (isoelectronic) Ga3+ ions. This means that in non-interacting electron picture the bands go from half filling (n=1) to two-third filling (n=4/3). This is of particular interest because for a tight-binding model on the kagome lattice there are symmetry protected Dirac points, just like in graphene, at this b

Last week I went to an interesting talk What are the effects of dredging on the Great Barrier Reef? by Laurence McCook , at the Global Change Institute at UQ. I went because I knew Laurence in my undergraduate days at ANU. In first year we had all the same lectures, tutorials, and labs. (I guess groups were assigned based on the alphabet.) We became friends and he introduced me to many beautiful places for bushwalking [backpacking] and cross country skiing near Canberra. There is a piece on the Conversation that gives a brief summary of the issues associated with the report from the expert panel that Laurence and  Britta Schaffelke co-chaired. Basically, it involved a "cat herding" exercise with 17 experts from industry, government, and universities. I am always impressed by people who manage such enterprises and can produce concrete useful outcomes. I think it requires considerable patience, political skills, and leadership.  A helpful figure is below. As

What is the mechanism of the Mott metal-insulator transition? After 50 years this remains a debated issue. A number of distinct mechanisms for the transition have been proposed. These include those due to Brinkman and Rice (where the quasi-particle weight in the metallic phase approaches zero as the transition is approached), Hubbard (where vanishing of the charge gap occurs when the upper and lower Hubbard bands overlap), or Dynamical Mean-Field Theory (DMFT) which combines both these features. My collaborators and I discuss an alternative mechanism in a paper that we just finished. Holon-Doublon Binding as the Mechanism for the Mott transition Peter Prelovsek, Jure Kokalj, Zala Lenarcic, and Ross H. McKenzie   We study the binding of a holon to a doublon in a half-filled Hubbard model as the mechanism of the zero-temperature metal-insulator transition. In a spin polarized system and a non-bipartite lattice a single holon-doublon (HD) pair exhibits a binding transition (e.

Last friday we had a great colloquium at UQ from Carl Caves on High-impact-factor syndrome: What, why, and what to do. Much of the talk followed Carl's article  The High-impact-factor syndrome  on The Back Page of the American Physical Society News. I posted  about it before. Here are a few new things that emerged. Reinhardt Werner had a nice piece in Nature,  The focus on bibliometrics makes papers less useful . The comments and his responses are worth reading. Last week Nature published The Leiden Manifesto for Research Metrics. Nature Publishing Group has launched the Nature Index to rate individuals, departments, institutions, and countries. They claim it is a "global indicator of high quality research". It is based on only 68 journals , including many NPG journals! For example, the only APS journals included are PRL and the Rapid Communications parts of PRA, PRB, and PRD. Journal of Chemical Physics is not included. There are no mathematics journals. I fi

Two weeks ago it was nice to have Meera Parish visit UQ and give a colloquium Fermions in Flatland.  She recently moved to Monash University from University College London. One important point she made was the comparison of the two figures below, showing a colour intensity plot of the one fermion spectral function A(E,k) for a two-dimensional Fermi gas near the unitary limit (BCS-BEC crossover). The bottom figure is experimental data from a Nature paper,  Observation of a pairing pseudogap in a two-dimensional Fermi gas . It makes much of the possible connection to the pseudogap seen in cuprate superconductors. The top figure is from a theory paper Pair Correlations in the Two-Dimensional Fermi Gas  Vudtiwat Ngampruetikorn, Jesper Levinsen, and Meera M. Parish Therefore, our results suggest that the observed pairing gap [Nature paper] effectively arises from two-body physics and does not correspond to a pseudogap regime . This view is further supported by the fact

We all have limited time, energy, and money. We all have priorities even if we can't clearly state them or don't publicly state them. Setting priorities is a challenge not just for individuals but also for departments, institutions, and research fields. I think rarely does this happen well. When priorities are not clearly stated, whether from individuals to institutions, "stake holders" are left trying to guess and speculate what the priorities really are. Publicly stated priorities too often look like a " dog's breakfast ": a mishmash of wish lists from competing interests, or a laundry list... I feel this sometimes applies to lists of "research strengths" or "research priorities" that  Australian universities come up with every few years. Every few years departments in Australian universities are extensively reviewed, leading to a list of 20-30 specific recommendations, that the department chair is then held accountable t