Rudolph Marcus (1923-2026): theoretical chemical physicist

Rudolph Marcus died last week. He was 102. There is a nice obituary in The New York Times. He was best known for his theory of electron transfer, for which he was the sole recipient of the Nobel Prize in Chemistry in 1992.

Although the theory was proposed for electron transfer in a polar solvent it applies to a wide range of other systems, where two quantum states are coupled to one another and to an environment. One example is for Forster transfer of excitons between molecules, which is central to photosynthesis.

I will give a physics perspective, based on a talk I gave in Slovenia back in 2013. Slides are here.

Basically, Marcus proposed an effective Hamiltonian for two diabatic states and calculated a transition rate in a limit that is relevant to most chemical contexts.

The Hamiltonian can be viewed as the spin-boson model, in which a two-level system is coupled to a bath of harmonic oscillators. 

[The dense book by Weiss on Quantum Dissipative Systems makes the connection explicit in detail. A more accessible treatment may be chapter 16 in the book Chemical Dynamics in Condensed Phases by Nitzan.]

The Hamiltonian is 

This defines a spectral density, which is important for quantum decoherence, but not so much here, except it defines a timescale that determines the classical limit, which Marcus assumed.

This can be used to define a quantity central to Marcus' theory, the reorganisation energy.

The transition rate between the two quantum states is given by

I consider this to be one of the most important equations in chemical physics, particularly for the understanding and design of functional materials.

Aside. Much of this is equivalent to Holstein's 1959 treatment of incoherent polaron transport (see Mahan, Many-body physics).

A key experiment by John R. Miller, Lidia T. Calcaterra and Gerhard L. Closs in 1984 showed how good the theory was and how its predictions were counter-intuitive. The authors considered a family of molecules that allowed them to tune epsilon, the energy difference between the two electronic states. [On the graph below epsilon = - Delta G].

The vertical scale is the reaction rate on a logarithmic scale. It varies by four orders of magnitude.

What is surprising? As stated at the Nobel prize award ceremony:

"The quadratic equation predicts that electron transfer reactions will occur more slowly the larger the driving force of the reaction is. This phenomenon received its own name, “the inverted region.” To a chemist, the phenomenon is just as unexpected as when a skier finds himself gliding more slowly down a slope the steeper it is."

The theory implies an important design principle for functional materials: if optimising functionality means maximising the reaction rate, then tune the energy difference epsilon to equal the reorganisation energy E_R.

The theory illustrates two important aspects of emergence: effective theories and universality. Many different systems can be described by the same theory. The environment may involve many degrees of freedom and its coupling to the system is characterised by many parameters (the M_alpha above). However, only one parameter matters, the reorganisation energy.

For Australians, there is some ambivalence about the way Marcus' name is often solely associated with electron transfer theory. We often refer to it as Marcus-Hush or Hush-Marcus theory because Noel Hush did similar work around the same time. Some of the history is recounted here by Ian Rae and Jeff Reimers. There are also subtle debates about whether the electron transfer is adiabatic or non-adiabatic.

My only personal interaction with Marcus was in 2011 when I visited the chemistry department at Caltech. Marcus kindly took me to lunch at the faculty club, along with his research group. Then he was 84 years old. He kept publishing papers until he died.

Rodney Baxter (1940-2025): Mathematical Physicist

I recently learnt that Rodney Baxter died earlier this year. He was adept at finding exact solutions to two-dimensional lattice models in statistical mechanics. He had a remarkably low public profile. But, during my lifetime, he was one of the Australian-based researchers who made the most significant and unique contributions to physics, broadly defined. Evidence of this is the list of international awards he received.

On Baxter's scientific achievements, see the obituary from the ANU, and earlier testimonials from Barry McCoy in 2000, and by Vladimir Bahzanov, on the award of the Henri Poincaré Prize to Baxter in 2021.

Exact solutions of "toy models" are important in understanding emergent phenomena. Before Onsager found an exact solution to the two-dimensional Ising model in 1944, there was debate about whether statistical mechanics could describe phase transitions and the associated discontinuities and singularities in thermodynamic quantities. 

Exact solutions provide benchmarks for approximation schemes and computational methods. They have also guided and elucidated key developments such as scaling, universality, the renormalisation group and conformal field theory.

Exact solutions guided Haldane's development of the Luttinger liquid and our understanding of the Kondo problem.

I mention the specific significance of a few of Baxter's solutions. His Exact solution of the eight-vertex model in 1972 gave continuously varying critical exponents that depended on the interaction strength in the model. This surprised many because it seemed to be against the hypothesis of the universality of critical exponents. This was later reconciled in terms of connections to the Berezinskii-Kosterlitz-Thouless transition (BKT) phase transition, which was discovered at the same time. I am not sure who explicitly resolved this.

It might be argued that Baxter independently discovered the BKT transition. For example, consider the abstract of a 1973 paper, Spontaneous staggered polarization of the F-model

"The “order parameter” of the two-dimensional F-model, namely the spontaneous staggered polarization P0, is derived exactly. At the critical temperature P0 has an essential singularity, both P0 and all its derivatives with respect to temperature vanishing."

Following earlier work by Lieb, Baxter explored the connection of two-dimensional classical models with one-dimensional quantum lattice models. For example, the solution of the XYZ quantum spin chain is related to the Eight-vertex model. Central to this is the Yang-Baxter equation. Alexander B. Zamolodchikov connected this to integrable quantum field theories in 1+1 dimensions. [Aside: the Yang is C.N. Yang, of Yang-Mills and Yang-Lee fame, who died last week.]

Baxter's work had completely unanticipated consequences beyond physics. Mathematicians discovered profound connections between his exact solutions and the theory of knots, number theory, and elliptic functions. It also stimulated the development of quantum groups.

I give two personal anecdotes on my own interactions with Baxter. I was an undergraduate at the ANU from 1979 to 1982. This meant I was completely separated from the half of the university known as the Institute for Advanced Studies (IAS), where Baxter worked. Faculty in the IAS there did no teaching, did not have to apply for external grants, and had considerable academic freedom. Most Ph.D. students were in the IAS. By today's standards, the IAS was a cushy deal, particularly if faculty did not get involved in internal politics. As an undergraduate, I really enjoyed my courses on thermodynamics, statistical mechanics, and pure mathematics. My honours supervisor, Hans Buchdahl, suggested that I talk to Baxter about possibly doing a Ph.D. with him. I found him quiet, unassuming, and unambitious. He had only supervised a few students. He wisely cautioned me that Ph.D. students might not be involved in finding exact solutions but might just be comparing exact results to series expansions.

In 1987, when I was a graduate student at Princeton, Baxter visited, hosted by Elliot Lieb, and gave a Mathematical Physics Seminar. This visit was just after he received the Dannie Heinemann Prize for Mathematical Physics from the American Physical Society. These seminars generally had a small audience, mostly people in the Mathematical Physics group. However, for Baxter, many string theorists (Witten, Callen, Gross, Harvey, ...) attended. They had a lot of questions for Baxter. But, from my vague recollection, he struggled to answer them, partly because he wasn't familiar with the language of quantum field theory. 

I was told that he got nice job offers from the USA. He could have earned more money and achieved a higher status. For personal reasons, he turned down the offer of a Royal Society Research Professorship at Cambridge.  But he seemed content puttering away in Australia. He just loved solving models and enjoyed family life down under.

Baxter wrote a short autobiography, An Accidental Academic. He began his career and made his big discoveries in a different era in Australian universities. The ANU had generous and guaranteed funding. Staff had the freedom to pursue curiosity-driven research on difficult problems that might take years to solve. There was little concern with the obsessions of today: money, metrics, management, and marketing. It is wonderful that Baxter was able to do what he did. It is striking that he says he retired early so he would not have to start making grant applications!

Autobiography of John Goodenough (1922-2023)

 John Goodenough was an amazing scientist. He made important contributions to our understanding of strongly correlated electron materials, magnetism, solid state chemistry, and materials science and engineering. He developed materials that are widely used in computer RAMs and rechargeable lithium batteries. He kept working in the laboratory and writing papers into his early 90s. Goodenough was awarded the Nobel Prize in Chemistry in 2019. Here is his Nobel Lecture, including text, slides, and video.

In 2008 he published Witness to Grace, a brief autobiography that chronicles his personal, scientific, and spiritual journeys. It is a fascinating story. The book is now out of print and the publisher is out of business. I have scanned a copy. You can download it here. I thank David Purdy for bringing to my attention the need to preserve the book.

Tom McLeish (1962-2023): natural philosopher

I was very sad to hear last month that Tom McLeish died of cancer. He was an extraordinary person and scientist. Tom can been characterised as a polymath or a "renaissance man".

Tom's career is briefly sketched in an obituary from the University of York, where for the last few years he held a position, created for him, Professor of Natural Philosophy in the physics department.

A small measure of Tom's influence on me is that there are eight posts on this blog about his work and another seven posts on my soli deo gloria blog.

Tom was best known in the scientific community for his work on the theory of soft matter, for which he was elected a Fellow of the Royal Society. I highly recommend his Very Short Introduction on the subject. But the influence and recognition of his intellectual contributions go far beyond his work on soft matter. For example, after the publication of The Poetry and Music of Science: Comparing Creativity in Science and Art by Oxford University Press in 2019, the following year the journal Interdisciplinary Science Reviews devoted a whole issue to seven different reviews of the book, with a response from Tom. 

In 2015, Tom visited the University of Queensland for two days. During this time he gave three different seminars, including in the School of Chemical Engineering, and the Centre for the Study of Science, Religion and Society in Emmanuel College. I wasn't game to also ask him to give a seminar in the physics department, although now I wish I had.

One of my fond memories of Tom was being with him in Cyprus at a small interdisciplinary meeting on the science of human flourishing, sponsored by CERN (Rolf Heuer) and the Templeton World Charity Foundation (Andrew Briggs). Tom was so excited that he was able to race off and go scuba diving during an afternoon break to a particularly choice spot. To me, Tom lived with the passion, excitement, and wonder of a little kid, as he encountered the world of nature and ideas. There was no jadedness, no cynicism, BS, no self-promotion, no exclusivity, just excitement about life in all its richness.

I love the moments in this video, during his recent Boyle lecture, where he talks about the physics of rubber, entropy, emergence, and the importance of choosing appropriate scales to investigate phenomena.

My condolences to Tom's family, friends, and colleagues. He died much too young and will be sorely missed.

Anthony Jacko (1985-2022): condensed matter theorist

I was very sad when last week I learned of the death of Anthony Jacko, a former member of the Condensed Matter Theory group at UQ. He was only 36 years old, having been diagnosed with stage 4 cancer at the end of last year.

Jacko's funeral was this week. Family and friends spoke warmly of his intelligence, humour, faithfulness, passion for life, and endearing quirkiness. There were both tears and laughs.

I will say something here about his scientific contributions, though at times like this what we achieve professionally does not really seem that important.

I first met Jacko as an undergraduate at UQ when he took an advanced undergraduate condensed physics course with me in 2006. That year he did an undergraduate honours (fourth year) project with Ben Powell and John Fjaerestad, on the Kadowaki-Woods ratio. This work eventually led to a Nature Physics paper, that I discussed in this blog post.

In 2007 I was quite happy when Jacko decided to do a Ph.D. with me and Ben Powell. We tried to come up with simple effective Hamiltonians for organometallic complexes that are used in organic LEDs and solar cells. Although we made some progress, I think the questions we tried to address have still not been answered definitively. The most progress has subsequently been made by Ben Powell.

For a postdoc, Jacko moved to Frankfurt to work with Roser Valenti and Harald Jeschke (now at Okayama University). I was really impressed how Jacko learned how to do reliable DFT-based electronic structure calculations and to use Wannier orbitals to extract tight-binding model parameters. Jacko brought this expertise back to Ben Powell's group at UQ, where he worked from 2013 to 2018.

During that time Jacko co-authored a string of really nice papers that inspired me to write multiple blog posts, such as those below. Looking back over that work I see how careful, solid, and systematic it is. Basically, good science, that we do not see enough of these days.

The broad issue is as follows. Understanding strong electron correlations in complex molecular materials requires effective Hamiltonians that are a realistic representation of the essential physics and chemistry. Sometimes next-nearest-neighbour interactions and subtleties in crystal structure really do matter. Other times they do not. The methods used by Jacko provided a robust way of doing this.

How robust are your tight-binding model parameters?

Common challenges with constructing diabatic states and tight-binding models

Quantum spin liquid on the hyper-honeycomb lattice

Springy stringy molecular crystals

Faculty hope that former students will come to their funeral. We also hope that we won't have to attend the funeral of any of our students. It is very sad.

An endowment is being created at The University of Queensland, to fund an undergraduate physics prize that will be awarded each year in honour of Jacko.

My condolences to Jacko's partner, Alana, and to family and friends.

Sir John Houghton (1931-2020): climate scientist

I was sorry to hear that Sir John Houghton died on April 15, from complications associated with coronavirus. There is a nice obituary in The New York Times.
He was appointed to an array of distinguished and influential positions, including Professor of Physics at Oxford, Director of Rutherford Appleton Lab, Director of the UK Meteorological Office, and most significantly, lead editor of the first three reports of United Nationas Intergovernmental Panel on Climate Change (IPCC).
I highly recommend his autobiography, In the Eye of the Storm, which I blogged about a few years ago, highlighting his integrity and influence.

Phil Anderson (1923-2020): theoretical physicist extraordinaire

Phil Anderson died two weeks ago. There have been many obituaries, including at The New York Times, Not Even Wrong (Peter Woit), and Nanoscale Views (Doug Natelson). Few would argue that he was the greatest condensed matter theorist of the second half of the twentieth century. I would go further and suggest that he and Ken Wilson were the greatest theoretical physicists of the second half of the twentieth century. Anderson's scientific legacy extends far beyond condensed matter physics.

More than sixty posts on this blog include ``P.W. Anderson'' in the label. There is no doubt that Anderson is the largest intellectual influence on this blog.

Phil Anderson made incredibly diverse and valuable contributions to condensed matter physics (anti-ferromagnetism, localisation, weak localisation, magnetic impurities in metals, Kondo problem, poor mans scaling, superfluid 3He, spin liquids, RVB theory of superconductivity... ).

It is noteworthy that Anderson applied scaling to condensed matter before Wilson. In the late 1960s he wrote a series of papers on ``poor man's scaling" for the Kondo problem.

I can think of several significant and profound influences of Phil beyond condensed matter physics.

1. Codifying and elucidating the concept of emergence (and the limitations of reductionism) in all of science, in More is Different in 1972.
[Although it should be acknowledged that the word ``emergence'' does not appear in the article and that Michael Polanyi developed similar ideas about emergence earlier.]

2. Nambu referenced several papers by Anderson about superconductivity in his seminal papers on the mass of elementary particles and symmetry breaking.

3. Laying the groundwork for the Higgs boson in 1963 by connecting spontaneous gauge symmetry breaking and mass. 

4. Elucidating spin glasses in a way that was key to John Hopfield's development of a particular neural network and to the notion of a "rugged landscape", relevant in protein folding and evolution. Anderson described these connections nicely in two pages in Physics Today in 1990.

Phil had a significant influence on my own job/career trajectory. For my Princeton Ph.D. I worked with Jim Sauls on superfluid 3He, which Phil supported financially. He was on the committee for my Ph.D. thesis defense in 1988. In 1993, towards the end of a postdoc, my job prospects were extremely slim. Phil told me that he had been asked to review an application I made for a five-year research fellowship back in Australia. My success was probably based on a positive review from Phil. I regret that during my time as a graduate student I did not have the confidence to interact much with him. However, from about 1995 to 2002, I made a visit to Princeton practically every year and had some nice discussions with him. It was also fascinating to see the close personal and scientific relationship that Phil and N.P. Ong had; it was clearly mutually very beneficial.
One cryptic comment: ``look at the metal-insulator-metal tunneling theory from the 1960s" [I found Mahan has a nice discussion] set me on the right path to do the calculations in this paper, about angle-dependent-magnetoresistance oscillations in layered metals.

I highly recommend the Anderson anthologies (reprint collections), listed below in order of increasing technical difficulty.

More and Different: notes from a thoughtful curmudgeon.
It is a collection of essays on wide-ranging subjects: personal reminiscences, history, philosophy, sociology, science wars, ...
Some of these have been published before but many have not.

A Career in Theoretical Physics
Something amazing about this collection of papers is what is not in it; e.g. his papers on superfluid 3He with Brinkman, or on charge ordering and antiferromagnetism in ferrites.

Basic Notions of Condensed Matter Physics

Andrew Zangwill is working on a scientific biography of Phil Anderson. I am looking forward to reading.

John Wilkins (1936-2019): condensed matter leader

I was sad to hear last week of the death of John Wilkins. He was a mentor to a whole generation of condensed matter physicists and a generous servant, both individuals and institutions. This obituary and memories from some colleagues gives a nice description of his many contributions.

I was privileged to do a postdoc with Wilkins at Ohio State University in the early 1990s. He had a significant influence on me, both scientifically and professionally. Much of the practical advice I write on this blog relating to jobs, writing, and giving talks, I learned from Wilkins. Even ten years after I worked with him I would still occasionally phone him for advice, particularly with negotiating and deciding on job offers.

Real leadership does not involve having a position, but rather having influence. Servant leaders are not concerned with advancing their own interests, but rather those of others in their community. They do this by investing in people and institutions. Wilkins did this in many ways. He invested heavily in his own graduate students and postdocs. He advised and mentored countless other students, postdocs, and young faculty, for whom he had no formal responsibility or anything to gain from their success. He was proud of the fact that he never held an administrative position in a university. Nevertheless, his influence was far greater than most department chairs and deans. He served the American Physical Society in countless ways, particularly their publishing activities and the Division of Condensed Matter Physics. He wrote innumerable reference letters, referee reports, and grant reviews.

Reflecting on Wilkins, I was reminded of these recent words of David Brooks, written in a different context.

I had a feeling of going back in time. Why did it feel so strange? It was because I was looking at people who are not self-centered. They’ve dedicated themselves to the organization that formed them, and which they serve.

A few other basic but important things I learned from Wilkins:
Write clearly. Rewrite. Talk to people. Theory should relate to real materials and real experiments. Defining the problem clearly can be an important contribution. A concrete calculation on a concrete model is valuable.

Wilkins did have significant scientific achievements, but they tend to get dwarfed in comparison to his influence over people. Perhaps, the most significant relate to the Kondo problem. This began with his student Krishnamurthy, who used Wilson's numerical renormalisation group to understand all the different regimes of the Anderson single impurity model. Later with his students Dan Cox and Gene Bickers, Wilkins applied slave boson techniques to describe a wide range of experimental properties of valence fluctuation associated with magnetic impurities in metals.

In classic Wilkins style, he convened a group of distinguished theorists to meet in Los Alamos one summer to write a definitive early review article on heavy fermions.

Wilkins was larger than life. He laughed a lot and was a tease. He could also be intimidating. Before his groups' annual pilgrimage to the APS March meeting, everyone had to give a practice talk to the group and Wilkins. A fellow postdoc confided to me that each year he was more nervous about giving the practice talk than the real talk! One time, Wilkins got frustrated that too many of us had small fonts on our overhead transparencies. He made us all chant together: ``22 point type is the smallest! 22 point type is the smallest! ...."  again and again until we got the point.

It was well known that Wilkins did not like his picture taken. On his department web page he put a picture of another John Wilkins, one of the founders of the Royal Society. However, my wife did not know his aversion. In 1992? Kevin Ingersent hosted a group Thanksgiving dinner at his house. Later to my shock, I discovered my wife took the photo below. ``What?! You took a photo of Wilkins?!"

Wilkins was a great role model as a scientist, a faculty member, and a servant of a professional community.

J.R. Schrieffer (1931-2019): quantum many-body theorist

Bob Schrieffer died last month, as reported in a New York Times obituary.

Obviously, Schrieffer's biggest scientific contribution was coming up with the variational wave-function for the BCS theory of superconductivity. BCS theory was an incredible intellectual achievement on many levels. Many great theoretical physicists had failed to crack the problem. The elegance of the theory was manifest in the fact that it was analytically tractable, yet could give a quantitative description of diverse physical properties in a wide range of materials. BCS also showed the power of using quantum-field-theory techniques in solid state theory. This was a very new thing in the late 50s. Then there was the following cross-fertilisation with nuclear physics and particle physics (e.g. Nambu).

Another significant contribution was the two-page paper from 1966 that used a unitary transformation to connect the Kondo model Hamiltonian to that of the Anderson single impurity model. In particular, it gave a physical foundation for the Kondo model, which at the time was considered somewhat ad hoc.
John Wilkins wrote a nice commentary on the background history and significance of the Schrieffer-Wolff transformation.

The SW transformation is an example of a general strategy of finding an effective Hamiltonian for a reduced Hilbert space. This can also be done via quasi-degenerate perturbation theory. In different words, when one ``integrates out'' the charge degrees of freedom in the Anderson model one ends up with the Kondo model.

There is also the Su-Schrieffer-Heeger model, that is related to Heeger's Nobel Prize in Chemistry. However, although this spawned a whole industry (that I worked in as a postdoc with Wilkins) its originality and significance is arguably not comparable to BCS and SW.

Because of when he was born, like many of the pioneers of quantum many-body theory, Schrieffer may have been born for success?

I am somewhat (scientifically) descended from Schrieffer because I did a postdoc with John Wilkins, who was one of Schrieffer's first PhD students. My main interaction with Schrieffer was during 1995-2000. Each year I would visit my collaborator, Jim Brooks, at the National High Magnetic Field Laboratory, and would have some helpful discussions with Schrieffer. During one of those visits, I stumbled across a compendium of reprints from a Japanese lab. [This was back in the days when some people snail-mailed out such things to colleagues]. It had been sent to Schrieffer and contained a copy of a paper by Kino and Fukuyama on a Hubbard model for organic charge transfer salts. That was the starting point for my work on that topic.

Noel Hush (1924- 2019): pioneering theoretical chemist

I was sad to hear last week that Professor Noel Hush died at age 94. Noel [also known as Prof.] was a pioneer in theoretical chemistry and chemical physics. He had a profound influence on both fields, particularly in their development in Australia.

Arguably his greatest scientific contribution was in the theory of electron transfer. Depending on where you are from this is called Hush-Marcus theory, Marcus-Hush theory, or Marcus theory. In particular, in 1958 Hush derived one of the most important equations in chemical physics, which can be used for design principles for functional electronic materials. A key concept here is the notion of diabatic states.

I had the privilege of knowing and working with Prof. Hush on and off over the past decade. As I made an adiabatic transition from condensed matter into chemical physics Prof. Hush provided a lot of encouragement, wisdom, perspective, and ideas. He strongly believed that theoretical chemists and condensed matter theorists could have mutually beneficial interactions. Together with Jeff Reimers and Laura McKemmish, we co-authored seven papers together. The last papers were published when Noel was 90 years old!

Besides his significant legacy of scientific knowledge, there is an incredible legacy of people that he taught, supervised, mentored, encouraged, and collaborated with.

There is an interesting interview of Prof. Hush about his life by Robyn Williams from 2011.

David Pines (1924-2018): quantum-many body theorist

I learnt today that David Pines died last week. He was a pioneer in quantum many-body theory, applying it not just to solid state physics but also to nuclear physics and astrophysics (neutron stars).  Furthermore, he was a great advocate of the importance of emergence in science, writing a classic paper, "The Theory of Everything," together with Bob Laughlin.

David also left a legacy of creative new institutions, being a co-founder of the Sante Fe Institute and the International Institute for Complex Adaptive Matter (I2CAM). Indeed, starting this blog was stimulated by David Pines and I2CAM. In January 2009, I attended the annual meeting of I2CAM in Cambridge and David organised a session on public outreach, including a presentation on blogging.

Piers Coleman, a current co-Director of I2CAM, has written a nice obituary which gives more details about David's contributions, both scientific and institutional.

Seth Olsen (1975-2018): theoretical chemist

I was very sad to learn last week of the tragic death of Seth Olsen in an accident. He was a former collaborator and colleague at UQ.

Seth was an outstanding and energetic scientist who easily crossed discipline boundaries, especially between chemistry, physics, and molecular biology.

Much of what I know about computational quantum chemistry, fluorescent proteins, conical intersections, and diabatic states, I learnt from Seth. He played a significant role in this blog. A search revealed that his name is mentioned in more than 70 posts. Many posts were stimulated by his work, his questions, or his suggestions. He often wrote comments, covering a wide range of topics. I found his interest helpful and stimulating.

Seth grew up in the USA. He was a physics major at the College of William and Mary. In 2004 he completed a Ph.D in in Biophysics and Computational Biology at The University of Illinois at Urbana-Champaign. His thesis was entitled, ` Electronic Excited States of Green Fluorescent Protein Chromophore Models,'' and his advisor was Todd Martínez, now at Stanford.

I first met Seth in 2005 when he was a postdoc with Sean Smith at the Centre for Computational Molecular Science at University of Queensland. During that time he met Louise Kettle, a Ph.D student in chemistry, who he later married.

I was very happy when in 2008 I was able to persuade Seth to join my group as a Research Fellow. He helped my group expand from condensed matter into chemical physics.  In 2010 I was pleased when Seth was awarded a 5-year Australian Research Fellowship. We continued to collaborate, although in many ways I was the junior author.

A significant contribution of Seth was to use high-level quantum chemistry calculations to show that the low-lying excited electronic states of the chromophore molecule in the green fluorescent protein has a natural description in terms of the resonant colour theory of organic dyes developed in the middle of the twentieth century by Brooker, Platt, and Moffitt. In different words, he used quantum chemistry to justify and parametrise a simple effective Hamiltonian for a complex system. Furthermore, he provided a rigorous quantum chemical justification for the colour theory description of a very wide class of organic dyes based on the methine motif. These results provide chemical and physical insight, an understanding of trends, elucidate design principles, and make modeling in condensed environments such as proteins, solvents, and glasses much more feasible.

I had great respect for Seth's integrity, both personal and scientific. He carefully checked calculations and arguments, would not rush to publish, and would not indulge in hype. Much of my skepticism and caution about computational materials science I gained from Seth's critiques.

Seth had his priorities right, putting family first.
My kids thought Seth was pretty cool, particularly when he came to a group social at our house with a backpack that contained a home brew beer set up!

My sincere condolences to Louise and their three young children.

Don't know what else to say. This is the saddest blog post I have had to write.

Ahmed Zewail (1946-2016): father of femtosecond chemistry

The New York Times has an obituary for Ahmed Zewail who died this week. He received the Nobel Prize in Chemistry for work that used ultrafast lasers to probe the dynamics of chemical reactions and the associated potential energy surfaces. This is all standard today. However, before Zewail, many reaction mechanisms and the associated surfaces were just theoretical constructs and conjectures.
I often use the picture below from one of his papers, which I posted about years ago.

I also posted about a nice article about the future of chemical physics and a Nature column about the importance of basic science and how to cultivate it. His wisdom needs to be heeded.

The NYT obituary points out how after the Nobel, Zewail took on an admirable challenge that was greater than anything he had tackled in science: the promotion of scientific research and education in the Arab world, and particularly in his native Egypt. I really hope he will have a significant legacy there. In this vein, Margaret Warner has a nice tribute to Zewail on the PBS site.

Stunning and creative microscope images where science meets fashion

The New York Times has a nice obituary Michael W. Davidson, a Success in Microscopes and Neckwear, Dies at 65

I did not know Davidson personally but I did benefit from his art. I visited the National High Magnetic Field Lab at Florida state several times in the 1990s. In appreciation my host Jim Brooks gave me a few of the neckties [which I still wear, on the rare event I actually wear a tie!] and a series of prints of images of  Australian products such as that of Vegemite below. I still have these prints on display in my office.

The website Molecular expressions contains not just a gallery of many beautiful microscope images but also more technical discussions about microscopy.

One thing I did not know about Davidson that I learnt from the obituary was the important role he played in the work for which the Chemistry Nobel Prize of 2014 was awarded.

Yoichuro Nambu (1921-2015): spontaneously broken symmetry in particle physics

Yoichuro Nambu died earlier this month, and there was an obituary in the New York Times yesterday. He shared the Nobel Prize in Physics in 2008, and is best known for this paper

Dynamical Model of Elementary Particles Based on an Analogy with Superconductivity. I 
 Y. Nambu and G. Jona-Lasinio

I reproduce the abstract below because it really does summarise the work and is a nice example of a beautifully written abstract.

It is suggested that the nucleon mass arises largely as a self-energy of some primary fermion field through the same mechanism as the appearance of energy gap in the theory of superconductivity. The idea can be put into a mathematical formulation utilizing a generalized Hartree-Fock approximation which regards real nucleons as quasi-particle excitations. We consider a simplified model of nonlinear four-fermion interaction which allows a γ5-gauge group. An interesting consequence of the symmetry is that there arise automatically pseudoscalar zero-mass bound states of nucleon-antinucleon pair which may be regarded as an idealized pion. In addition, massive bound states of nucleon number zero and two are predicted in a simple approximation.
 The theory contains two parameters which can be explicitly related to observed nucleon mass and the pion-nucleon coupling constant. Some paradoxical aspects of the theory in connection with the γ5 transformation are discussed in detail.

I offer a few minor contextual comments, in order of decreasing significance.

1. Nambu's work is a very nice example of the cross-fertilisation between solid state physics and elementary particle physics. Before Nambu's paper it went mostly one way: solid state theorists used field theoretical techniques. However, Nambu showed how significant new insights in particle physics could be obtained from solid state analogues.

2. Before Nambu there was a lot of concern about the fact that BCS theory was not gauge invariant. He clarified this to the point that these objections were considered dealt with. However, I still get confused about this because of subtle issues about the Goldstone boson [associated with the broken U(1) gauge symmetry of electromagnetism] being "renormalised" by the Coulomb interaction leading to gapped plasmons. Even today there is still debate about whether there is a spontaneously broken symmetry or whether superconductors are topologically ordered, as advocated here.

3. One elegant and technical aspect of this paper was that he introduced the Nambu matrices for describing superconductivity. These and the associated Lie algebras naturally generalise to more complicated situations such field theories and superfluid 3He where the order parameter has 3 spin and 3 orbital degrees of freedom. I found this approach incredibly useful when I did my Ph.D thesis on order parameter collective modes in superfluid 3He-B. Some of this is described here.

4. Was Nambu at the right place at the right time?
 In a previous post, Born for success in quantum many-body theory, I noted how more than half of the founders of the application of field theory techniques to solid state physics were born between 1923 and 1926. Nambu was born in 1921.

John Nash (1928 - 2015): a founder of game theory

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 Physics building there were strange scribblings all over the chalk boards.

One day in 1994 I got a shock when I received a copy of The Princeton Alumni Weekly. There on the cover was a photo of the man I often saw in the library with the headline "John Nash wins Nobel Prize in Economics". Finally I found out who he was and learnt his story.

Can you imagine a university president like this?

He gives away his whole salary to a not-for-profit.
He is reluctant to go into administration because he has to give up teaching.
He serves as president at the same institution for 35 years.
He increases the endowment from $9 million to $350 million.
When he takes over the university is mostly known for football. When he leaves it is a major research university.
He does not embrace the football program.
He introduces enrolment of female students.
He stands up to the president of the country over civil rights.
He is a major leader of campus opposition to a controversial war.
He changes the governance of the university so that it is no longer controlled by the sectarian founders, although he is one of them.
He speaks out often about issues of justice, human rights, racism, and poverty.
A survey of his peers identify him as the most effective college president in the country.

Is this fantasy? Could such a person actually exist?
Previously I posted about a dream graduation speech.
But, this president is real.

Theodore Hesburgh, President of Notre Dame University in the USA, from 1952-1987.
He died last week, aged 97.
The New York Times obituary is worth reading.

Jim Brooks (1944-2014): pioneer in high magnetic fields

I was saddened to hear of the recent sudden death of Jim Brooks. He is the experimentalist who arguably has had the biggest impact on me scientifically and my career.

Jim grew up in Los Alamos in an extended family of physicists. He did a Ph.D at U. Oregon with Russell Donnelly as an advisor, working on low temperature physics.
I believe he may have been the first person to put a dilution fridge in a high field [30 tesla] magnet, while working at Boston University and the Bitter Magnet Lab at MIT. This was significant following the discovery of the fractional quantum Hall effect by Tsui and Stormer. After a sabbatical at Princeton with Paul Chaikin [involving the discovery of a quantum Hall state in the field induced spin density wave of a Bechgaard salt] he began to work almost exclusively on organic charge transfer salts. He made many studies that mapped out their rich phase diagrams [as a function of temperature, pressure, uniaxial stress, magnetic field, and chemical substitution] and "fermiology". The latter involved using high magnetic fields and low temperatures to use quantum oscillations [Shubnikov de Haas and de Haas van Alphen] and angle-dependent magnetoresistance oscillations [AMRO] to map out Fermi surfaces.

I first met Brooks in 1994 at a conference in Korea, just after I had moved to University of New South Wales. Later that year he came to UNSW to use the pulsed magnetic field lab, set up by Bob Clark, to perform a series or experiments on organic charge transfer salts, in fields up to 50 tesla. This led to us writing about half a dozen papers together. From 1995 to 2002 he hosted an (approximately) annual visit I made to the Florida magnetic lab. I benefited greatly from these visits.

The most significant scientific thing Brooks did for me was introduce me to organic charge transfer salts and to AMRO. This led directly to some of my best scientific work, such as a review on organics and showing that a 3-dimensional Fermi surface is not necessary for AMRO. My positive experience from talking (a lot) to Brooks heavily flavours the thoughts in my post on listening to experimentalists.

Several times Brooks wrote letters of reference for me that I think were probably very important in my survival/success in science.

Brooks was fun to work with and to be around. He was a bit of a clown. He really did not take himself very seriously, despite his professional stature. The first day he came into the lab at UNSW he arrived on roller blades with all his shirt buttons undone. I remember on one visit to Florida he had dinner with my family, when my kids were very young.  Brooks came out of the bathroom with strings of toilet paper stuffed into his nose! The kids loved it.

On the National High Magnetic Field Laboratory web site there are some nice tributes from a range of people. Brooks biggest legacy is probably the many young people he mentored and supported.

Arthur Wightman (1922-2013): Doyen of Mathematical Physics and Gentleman Scientist

I just learned that Arthur Wightman died earlier this year. He is probably best known for axioms of quantum field theory, super-selection rules, and a famous book, PCT, Spin, Statistics and all that.
Arguably, Wightman's greatest legacy is being the advisor and mentor to a selection of Princeton Ph.D students who went on to distinguished careers, mostly in mathematical physics.
There are some nice testimonials on the Princeton Physics web site. Reading them it struck me that Wightman would have measured rather poorly on today's common metrics [grant money, numbers of publications, journal impact factors, numbers of Ph.D students, citations]; yet, he had an incredible scientific impact!

Wightman was extremely helpful and generous to me when I was a beginning graduate student at Princeton in the mid 1980s. In particular, I had a paper from my undergraduate thesis that I was trying to publish. He gave me great encouragement, some helpful feedback, and arranged for it to be published in the Journal of Mathematical Physics. To quote the beautiful testimonial of John Preskill, "Though I did not sufficiently appreciate it at the time, Arthur was incredibly generous with his time."

The following observations by Jurg Frohlich are particularly poignant:

As Arthur Jaffe said, the disappearance of Arthur Wightman marks the end of an era. I fear it may also mark the gradual disappearance of an attitude and style among scientists that I associate directly with people like Arthur Wightman and Res Jost: 

Focus on the central problems of your field – even if they may not be doable immediately  

– generously share your time, insights and ideas with others, especially with young colleagues, 

generously support the careers of young scientists, 

maintain unerring intellectual honesty and integrity – in short, try to be a gentleman scientist!

More than his scientific oeuvre, I view the latter qualities as Arthur Wightman’s central legacy for which he will be remembered, and which, in a time when they are endangered, we should cherish!

Ken Wilson (1936-2013): pioneer of the renormalisation group

Ken Wilson died last saturday. He was arguably one of the most important theoretical physicists of the second half of the twentieth century. He pioneered the marriage of quantum field theory techniques with condensed matter. He developed key concepts and methods including scaling, universality, the renormalisation group, epsilon=4-d expansions, numerical solution of the Kondo problem, and lattice gauge theory.

I first heard of Wilson as a first year undergraduate when I read his 1979 Scientific American article, Problems in physics with many scales of length. I had no idea what it was all about. When I was a postdoc at Ohio State he had an office near mine. Then he was mostly interested in science education reform.

There are obituaries at Ohio State  and Cornell.
A previous post considers Wilson's comments about quantum chemistry, in a long and meandering interview about his career.