Tuesday, March 30, 2021

Life transitions

Sometimes I never get around to writing or finishing planned blog posts. Last month something changed, but nothing changed for me. None of this is covid-related.

Three years ago I negotiated with my university a "transition to retirement" contract. These seem to be designed by the accountants to incentivize "highly paid old farts" to retire (regardless of whether they have anything to contribute) and make the university "financially sustainable". I got to go half-time for three years with no teaching and administrative responsibilities.  (BTW. I actually love teaching. I just don't enjoy it or see the point when it becomes bureaucratic and/or students are disengaged.) Pretty strong incentive!

I did this for a multitude of reasons: mental health, other opportunities and priorities, an unwillingness to take on administrative roles that seem to be mostly implementing dubious management decisions, and general concerns about where Australian universities are heading...(money, management, marketing, and metrics)...  Taking long service leave helped clarify things. Since then the wisdom of decision for me personally has only been confirmed, particularly with covid and some family health issues. I could not imagine that I would have coped with having to teach online with two weeks notice. I admire those of you who have done it... And then there is all these other things in the background such as an opinion piece in The Wall Street Journal  and associated submissions to a recent parliamentary inquiry...

At end of February, the three years came to an end and I officially "retired" and became an Emeritus Professor. My wife says I have not "retired" but just changed to new responsibilities and income streams.  I agree.

I am 60. I consider myself very blessed and privileged that I am able to do this. Not everyone has this freedom. I had about 25 years in Australian universities and most of it was as "research faculty" and I was generously funded and so got to work with many excellent postdocs.

In the short term very little has changed (besides the paychecks). I still have an office, am finishing Condensed Matter Physics: A Very Short Introduction, and collaborating with Ben Powell's group on spin-crossover materials, and trying to write more blog posts. I think some of this work is the nicest I have been involved with. I also work half-time as a consultant for a Christian NGO on a project that combines some of my passions and concerns: science, theology, Jesus, and the Majority World. Again I feel privileged to have that opportunity.

I am hardly "a man of leisure," contrary to what one of my wife's friends said last week.

So what about this blog? I have no immediate plans to change anything. I would like to post more often but always seem too "busy" or spend too much time polishing posts...

Thursday, March 25, 2021

Isotope effects in spin crossover materials

A range of isotope substitution experiments have been performed on spin-crossover materials.
Just like for other systems such as superconductors their interpretation is subtle.

The first studies are reviewed in Section 2.3.5 of this review article.
Isotopic exchange was investigated for a tris(picolylamine)iron(II) system which exhibits a two-step spin transition. Results are shown in the figure below. Significant changes in the spin-state transition curve were observed only when the isotopic substitution (H/D and 14N/15N) was made for atoms directly involved in the hydrogen-bonding network that connects the spin-crossover molecules. For example, with C2H5OD/ND2 the crossover temperature was shifted to higher temperatures by about 15 K and the middle step was no longer present.

I would not have expected such a large effect given the chemical complexity of these systems and that the H atoms are not immediately bonded to the iron atoms which undergo the spin-state transition.

I now mention two other studies. They are particularly helpful because they also measured how the enthalpy and entropy change associated with the spin-state transtion changed with isotopic substitution.

Weber et al. studied the iron(II) spin-crossover complex [FeL1(HIm)2] and the deuterium-substituted [FeL1(DIm)2] where Him is (not a man but) imidazole. Both exhibit a single-step transition with hysteresis. H/D exchange decreased both the transition temperature and the hysteresis width by a few K. Deuteration decreased the value of the enthalpy and entropy differences between the low spin and high spin states (determined from differential scanning calorimetry) by about twenty and ten percent, respectively. (See Table 2 in the paper). They estimated an interaction parameter J = 560 K, indicating strong intermolecular interactions, which they attributed to a hydrogen bond. They reference some earlier studies showing how the magnitude of the ligand field in a transition metal complex can be modified by hydrogen bonds involving the complex.

Very recently, Jornet-Mollá et al. studied the iron(ii) salt [Fe(bpp)2](isonicNO)2·HisonicNO·5H2O, which with decreasing temperature undergoes a transition at 162 K. There is a width of about 5 K, associated with hysteresis. With deuteration, the transition temperature decreases to 155 K, the width increases to 7 K, and the enthalpy and entropy differences both increase by about fifteen percent. “Annealing the compound at lower temperatures results in a 100% LS phase that differs from the initial HS phase in the formation of a hydrogen bond (HB) between two water molecules (O4W and O5W) of crystallisation. Neutron crystallography experiments have also evidenced a proton displacement inside a short strong hydrogen bond (SSHB) between two isonicNO anions.”
I am particularly interested in this because of previous work I have done on strong hydrogen bonds.

Again I am surprised at the magnitude of these effects because the zero-point energy associated with the relevant H atoms is only a small fraction of the total zero-point energy and the entropy contribution from vibrations.

I now start a preliminary discussion of how these experiments might be interpreted in terms of an Ising model picture, such as in a recent preprint. The Hamiltonian is

where the pseudospin sigma=+1/-1 corresponds to high spin and low spin states.

The crossover temperature is independent of the Ising interactions J's and given by

Our results in Appendix A of the preprint imply that there should be no dynamical isotope effects on the J’s, i.e., provided other parameters such as structural details and bond lengths do not change with isotope substitution.

This does not rule out changes in the crossover temperature. Both the enthalpy and entropy differences can change with isotope substitution (as is observed). The former due to changes in zero-point energies, and the latter due to changes in the vibrational contribution to the entropy change.

Friday, March 19, 2021

Interpretation of isotope effects can be subtle

Isotopic substitution has provided significant insights into molecular and solid-state physics. This involves the substitution of particular atoms in a compound by the same chemical element with a different nuclear mass (i.e. a nuclear isotope). An example is hydrogen/deuterium substitution which has shown the significant role that quantum nuclear motion can play in hydrogen bonding, particularly in strong hydrogen bonds. Of particular relevance to the discussion below is that isotopic substitution does not only change vibrational frequencies but can also change bond lengths.

A key piece of evidence on the road to the BCS theory of superconductivity in 1957 was the observation of an isotope effect. In 1950 a shift in the transition temperature of mercury was observed, suggesting that superconductivity resulted from electron-phonon interactions, as argued by Frohlich that same year. In particular, the magnitude of the shift was consistent with theoretical work by Herbert Frohlich. (Whether he predicted or postdicted the observed effect is a matter of debate, as discussed by Jorge Hirsch.) BCS theory gives that $\Delta T_c/T_c = - {1/over 2} \Delta M /M$, which arises from the fact that phonon frequencies scale with $1/\sqrt{M}$, consistent with the mercury experiments.

However, in the 1960s there were many observations of “anomalous” isotope effects, particularly in transition metals, that were inconsistent with the prefactor in this equation. These anomalies were resolved by going beyond the BCS theory and allowing for strong-coupling effects. Following the discovery of cuprate superconductors in 1986, isotope effects were observed in some cuprates. However, the consensus now is that these observations do not support an electron-phonon mechanism for superconductivity but rather are due to structural changes due to the isotope substitution. For example, isotopic substitution changes the zero-point energy, and that can alter the unit cell volume and the hopping parameter t in a Hubbard model.

This illustrates that there are subtleties in interpreting isotope experiments. This is because there are both structural and dynamical isotope effects. Changes in isotope can lead to changes in structure, such as bond lengths or lattice constants, and even in changes in crystal symmetry. These structural changes arise because the equilibrium structure of the system is that which minimises the total energy of the system. The contribution to this energy from the zero-point energy of the atomic vibrations changes with isotope substitution and with bond lengths. Dynamical effects are those that involve exchange of phonons such as in superconductivity.

I am not sure how to sharpen this structural/dynamical or static/dynamical distinction. Or is it secondary and primary effects?

In the next post, I will discuss observations of isotope effects in spin-crossover materials and how that relates to recenttheoretical work with my collaborators.

Wednesday, March 17, 2021

Condensed matter physics in 250 words

How would you define condensed matter physics? In 250 words how might you motivate someone to want to know more. For Condensed Matter Physics: A Very Short Introduction,  I need to write a brief blurb (about 250 words) that will be used for marketing.

Here is my first attempt. What do you think?
There are many more states of matter than just solid, liquid, and gas. Examples include liquid crystal, magnet, glass, and superconductor. New states are continually being discovered leading to a stream of Nobel Prizes. Some states, such as superfluid and superconductor, exhibit the weirdness normally associated with the quantum physics of single atoms, such as Schrodinger's cat. Condensed matter physics seeks to understand how states of matter and their distinct physical properties emerge from the atoms that a material is composed of. Materials and states studied by condensed matter physicists are central to modern technology. Examples include superconductors in hospital MRI machines, magnetic multilayers in computer memories, LEDs in solid-state lights, crystalline silicon in computer chips, and liquid crystals in digital displays.

Condensed matter physics is not just defined by the objects that it studies (states of matter in materials), but rather by a particular approach to the study of these objects. It addresses fundamental questions and produces unifying concepts to describe a wide range of phenomena in materials that are chemically and structurally diverse.

A system composed of many interacting parts can have properties that the parts do not have. Water is wet, but a single water molecule is not. Your brain is conscious, but a single neuron is not. Such emergent phenomena are central to condensed matter physics and also occur in many fields, from biology to computer science to sociology, leading to rich intellectual connections. When do quantitative differences become qualitative differences? Can simple mathematical models describe rich and complex behaviour? What is the relationship between the particular and the universal? Condensed matter physics is all about these big questions.

If you gave the book to your aunt would this motivate her to start reading?

If your colleague in engineering was browsing in a book store and read this on the cover would they buy the book?

I welcome suggestions.

BTW. In case you were wondering, the book is not about to come out. I just need to submit the marketing plan to OUP.

Monday, March 15, 2021

Radioactive science for the masses

My wife and I watched the movie, Radioactive, based on the life of Marie Sklodowska Curie. She was an amazing scientist who showed incredible perseverance, particularly in the face of discrimination by the scientific establishment in France at the beginning of the twentieth century, and attacks in the media because of her gender, nationality, and personal life.

The movie is good entertainment and creative, maybe a bit too creative at times. But they be a matter of personal taste. There were many things that I learnt, some substantial and others just interesting trivia, particularly after reading more on Wikipedia. Here are a few.

Curie did not only discover radioactivity, the elements radium and polonium (named after her native Poland), but also was the founder of nuclear medicine.

One can easily forget that more than a century ago, chemistry labs were very basic and that producing pure samples was a tedious process. For example, a tonne of pitchblende (uranium oxide ore) had to be crushed and processed to produce just one-tenth of a gram of radium chloride.

Back then the boundaries between physics and chemistry were fuzzy. Marie received a Nobel Prize in both. I wonder whether the legacy of joint chemistry-physics institutes in France was essential to de Gennes's work in soft matter.

Both Curie's started out in solid-state physics. Pierre Curie and his older brother discovered piezoelectricity. Pierre discovered the transition temperature for ferromagnetism (Curie temperature), and the Curie law for the temperature dependence of the magnetic susceptibility in a paramagnet. He followed his wife into nuclear research.

After her husband's tragic death, she had an affair with Paul Langevin and was vilified in the press.

A strong critique of the movie is by Geraldine McGinty. On the one hand, I agree with her concerns. On the other hand, I am just happy that "Hollywood" is exposing people to this extraordinary woman and her science. Maybe my hopes and expectations are just too low.

This relates to an ongoing debate about to what extent movies based on historical figures have to be historically accurate in every detail. The Crown (which I love) has brought these debates to the fore. Simon Jenkins states The Crown's fake history is as corrosive as fake news. Recent movies about scientists also raise the question to what extent the science must be absolutely accurate.

What do you think?

I welcome other comments on the movie, particularly from women.

Thursday, March 11, 2021

PhD students and postdocs need to learn soft skills

Most Ph.D. students and postdocs will end up employed outside academia and doing work that is not related to their current research. For this reason alone it is important to learn a broad range of skills beyond what is needed to publish that paper in a luxury journal that their supervisor craves.

Furthermore, for faculty to survive, let alone flourish, in today's university (corporate) environment soft skills are very important.

David Sholl (a frequent commenter on this blog) has just published a relevant book. Here is the publisher blurb.

Long-term success in scientific research requires skills that go well beyond technical prowess. Success and Creativity in Scientific Research: Amaze Your Friends and Surprise Yourself is based on a popular series of lectures the author has given to PhD students, postdoctoral researchers, and faculty at the Georgia Institute of Technology. Both entertaining and thought-provoking, this essential work supports advanced students and early career professionals across a variety of technical disciplines to thrive as successful and innovative researchers.

If you read it please post some thoughts in the comments below.

Friday, March 5, 2021

Management is not leadership

Being in a management position is neither a necessary nor a sufficient condition for academic leadership.

Senior managers at Australian universities sometimes wax lyrical about how they are in leadership. When it comes to promotion decisions, they also judge junior academics on whether they show "leadership".  This seems to be equated with the size of one's research group and the number of one's citations. The rise of this fixation on "leadership" in universities was highlighted by a commenter on a recent post.

This misunderstanding is another example of how university management does not actually consider what their own academics in the university may actually know. Leadership is a well-researched topic. If managers talked to faculty in business and history, they might be told something along the following lines.

The cartoon is from here.

Real leadership is characterised by influence. It leads to change. Real leaders can motivate people to change their views and their lives. This is of substance, unlike "change management" which seems to me to be a euphemism for sacking people, changing lines of reporting, and renaming (rebranding) the names of departments and courses.

Real leadership is not about occupying a powerful position that you use to exert control over people. The authority that real leaders have is intellectual or moral authority, not legal authority.

Previously I posted about how humility and listening to others has been found to be a key ingredient of leadership, rather than self-promotion and defensiveness.

Consider Einstein working in the patent office, Douglas Hofstadter unemployed and living with his parents while writing Godel, Escher, Bach, the obscure virus club, Nelson Mandela in prison on Robben Island, and Gandhi on a hunger strike. None held formal positions of authority or commanded large salaries, budgets, or staff. But they were leaders. They influenced people.

Being in a management position or holding a political office does not mean you are a leader. Gorbachev and Brezhnev both held the same position (for 6 and 18 years respectively). Who was the real leader?

My postdoctoral mentor, the late John Wilkins, never held a management position, but he sure was a leader. He was influential for the good of others and for condensed matter physics.

I like the following text

Within minutes they were bickering over who of them would end up the greatest. But Jesus intervened: “Kings like to throw their weight around and people in authority like to give themselves fancy titles. It’s not going to be that way with you. Let the senior among you become like the junior; let the leader act the part of the servant."

Tuesday, March 2, 2021

Nobel prizes and condensed matter physics

In Condensed Matter Physics: A Very Short Introduction I would like to include an Appendix with a list of all the Nobel Prizes related to condensed matter physics. My list includes 31 in Physics and 6 in Chemistry. I acknowledge that a few in my list are debatable. Some of the Chemistry prizes were to people who trained in condensed matter physics but eventually worked in chemistry departments. A few of the physics prizes are closer to electronic engineering than condensed matter. On the one hand, Nambu was not a condensed matter physicist, but he took Anderson's ideas about broken symmetry in superconductivity and applied them to particle physics.

I want to include the list because I find it pretty amazing and it illustrates just how CMP consistently seems to turn up surprising new discoveries. Many of these prizes are mentioned in the text of the VSI.

In writing the book, and particularly the chapter on topological quantum matter, I found that nobelprize.org is very helpful. The popular descriptions for specific prizes contain helpful language, metaphors, images, and anecdotes. The technical descriptions give very nice introductions/summaries of the discoveries associated with the prize. Beginning Ph.D. students may find them very helpful.