Wednesday, April 21, 2021

Implicit versus explicit beliefs

 How can we design a room-temperature superconductor? How can a government stimulate economic growth? How can an NGO help reduce domestic violence? Why do communities become segregated on racial lines? How can I improve my mental health?

These important questions may seem unrelated. However, I propose that often there is a common issue about the strategies that people (whether individuals, professions, NGOs, funding agencies, governments, ...) propose to find answers or when definite answers are proposed.

Many strategies and answers involve a heavy dose of implicit beliefs. These are assumptions that are never stated. They may be elements of a worldview, which according to one definition, is

a commitment, a fundamental orientation of the heart, that can be expressed as a story or in a set of presuppositions (assumptions which may be true, partially true, or entirely false) which we hold (consciously or subconsciously, consistently or inconsistently) about the basic construction of reality, and that provides the foundation on which we live and move and have our being.

 James W. Sire, The Universe Next Door: A basic worldview catalog

These implicit beliefs may relate to values and morality. But I want to focus more on implicit beliefs that are related to academic disciplines such as philosophy of science, psychology, political science, theology, economics, anthropology,  sociology, ...  Most of us have never studied these disciplines and some of us may be skeptical about some of them. But, my point is that everyone has implicit ideas about what is true with regard to the objects these disciplines study. Everyone has a philosophy of science. Everyone has ideas about how minds work and how to change societies. It is just that these beliefs are rarely stated. 

Why does this matter? If implicit beliefs are never stated, they can never be tested, evaluated, critiqued, refined, or rejected. I believe that implicit beliefs are too often based on intuition, prejudice, common sense, or culture (social pressure to conform to accepted wisdom). This is not necessarily bad. Sometimes intuition, common sense, and culture are helpful and correct. We could not survive in life if we did not have them. We simply don't have the time, energy, and resources to constantly question and validate everything. On the other hand, if there is a vacuum, it will get filled with something. A major lesson from scientific history is that intuition, prejudice, common sense are sometimes wrong.

I now give three concrete examples of implicit beliefs. They cover computational materials science, public policy, and social activism.

Understanding materials using computers

Amongst others, there are two things, we would like more computational power to be able to do. One is to do reliable ab initio calculations of the properties of complex molecules and solids, from proteins to crystals with unit cells containing large numbers of atoms.  Another is to do exact diagonalisation (or some alternative reliable method) of many-body Hamiltonians, such as the Hubbard model, on large enough lattices that finite-size effects are minimal or can be reliably accounted for.

Over the past decade, there has been a lot of hype about how quantum computers and/or machine learning techniques will solve these problems and thus initiate a new era of materials understanding, discovery, and design with significant technological and economic benefits. My problem is that these claims usually seem to have the implicit belief that the only obstacle to progress is one of computational power. This fallacy has recently been deconstructed and critiqued in detail in three beautiful essays by Roald Hoffmann and Jean-Paul Malrieu, Simulation vs. Understanding: A Tension, in Quantum Chemistry and Beyond.

Public policy

National economies around the world have been battered by the covid-19 pandemic. In response, governments of prosperous countries are spending big on stimulus packages. This involves taking on massive amounts of debt and significant government intervention in "free" market economies. Will these initiatives achieved their goals, particularly in the long term? Could they actually make things worse? Responses from pundits, both for and against, are laden with implicit beliefs. Unfortunately, economists cannot agree on the answer to the basic question, "Does government stimulus spending actually produce economic growth?" This issue is nicely discussed in a pre-pandemic podcast at Econtalk. 

NGOs and social activism

Many NGOs are about change. They aim to build a better world, addressing problems such as domestic violence, poverty, climate change, corruption, racism,... They aim to promote education, human rights, good governance, democracy, health, transparency, .. I love NGOs. I support many: philosophically, financially, and practically. To survive most NGOs have to raise funds, whether from many small donors or large philanthropies. This requires a well-honed pitch that aims to inspire potential donors to give. Furthermore, the whole operation of most NGOs is laden with implicit beliefs, whether those of the founders, staff or donors.

Consider a hypothetical NGO whose goal is to reduce the number of murders in a country. I chose this example because it may at first appear less controversial and contentious than some. Almost everyone thinks murder is wrong (always) and societies should stop reduce it. But why do murders occur? Revenge, passion, drugs, alcohol, money, politics, racism, ... Will making the purchase of guns difficult reduce the murder rate? Gun lobbyists will claim "Guns don't kill people. Criminals do! Law-abiding citizens need guns for self-defense." (cringe). There are many other alternative strategies: increasing penalties (longer jail terms or even the death penalty), the number of police, weapons for police, community policing, drug rehabilitation, breaking up gangs, ... Wow! It's complicated. My main point is that the hypothetical NGO will probably have an implicit belief that one particular strategy is the best one. Furthermore, if you identify and question this belief reasonable debate may not follow, but it may even be claimed that you don't care about stopping murder.

Some NGOs and philanthropies have become mindful of these issues. In response to a grant application that I helped an NGO write we were asked what our "theory of change" was? I discovered that there is a whole associated "industry. According to (!)

One organisation which began to focus on these issues was the US-based Aspen Institute and its Roundtable on Community Change. ... [leading] to the publication in 1995 of New Approaches to Evaluating Comprehensive Community Initiatives. In that book, Carol Weiss, ... hypothesized that a key reason complex programs are so difficult to evaluate is that the assumptions that inspire them are poorly articulated. She argued that stakeholders of complex community initiatives typically are unclear about how the change process will unfold and therefore give little attention to the early and mid-term changes that need to happen in order for a longer term goal to be reached.  
This led to software designed to help organisations plan initiatives, with a particular emphasis on teasing out assumptions embedded in plans. A related method is construction of a logframe matrix [logical framework].

All models are wrong but some are useful. I first learnt this aphorism from Scott Page, in his wonderful course Model Thinking at Coursera.  Models help us think more clearly. Simple quantitative models, such as agent-based models, in the social sciences, have the value that their assumptions can be clearly stated, and then the consequences of these assumptions can be investigated in a rigorous manner.

What do you think? Are there examples that you think involve implicit beliefs that need to be stated explicitly?

Thursday, April 15, 2021

Fifty years ago: three big discoveries in condensed matter

For the marketing plan for my Very Short Introduction, I was recently asked whether there were any significant anniversaries happening in condensed matter physics (and associated conferences). This is not something I normally think about.

I realised that fifty years ago there were three big discoveries. All eventually led to Nobel Prizes. Each discovery had a profound effect on the formation of condensed matter as a distinct discipline built around a few unifying concepts. At the time the discoveries and ideas appeared quite independent, but there are deep connections between them.

Renormalisation group and critical phenomena

In 1971 Ken Wilson published two papers  [PRB 4, 3174, and PRB 4, 3184] laying the foundations, followed by two PRLs in 1972, including one with the provocative title, Critical Exponents in 3.99 Dimensions

Wilson received the Nobel Prize in 1982. This work had many implications and applications. 

Explained universality in critical phenomena.

Highlighted how spatial dimensionality changes physics.

Illustrates why effective Hamiltonians work (so well).

Showed the power of quantum field theory techniques.

Defined concepts of scaling and fixed points.

Superfluidity in liquid 3He

In 1972,  Osheroff, Richardson, and Lee reported new phase transitions in liquid/solid 3He. Tony Leggett identified these transitions as due a superfluid phases and also identified the order parameters. The experimentalists shared the Nobel Prize in 1996 and Leggett in 2003. The discovery was significant for many reasons, beyond just being a new state of matter.

It provided a rich example of a state of matter with multiple broken symmetries. The order parameter has eighteen components, which can be viewed as a combined superfluid, ferromagnet, and liquid crystal.

The rich order parameter led to an exploration of diverse topological defects, from superfluid vortices with magnetic cores to boojums. This highlighted the concepts of broken symmetry, rigidity, and topological defects.

This was the first example of an unconventional fermionic superfluid. Specifically, it could be described by BCS theory, but not with s-wave pairing nor with the pairing mechanism of the electron-phonon interaction in elemental superconductors. This showed the adaptability of BCS theory. It laid the groundwork for understanding unconventional superconductivity in heavy fermions, organics, and cuprates.

Berezinskii-Kosterlitz-Thouless phase transitions

In Berezinskii published papers in 1970 and 1971, and Kosterlitz and Thouless published papers in 1972 and 1973. This work was significant for reasons including the following.

It showed states of matter and phase transitions were qualitatively different in two and three dimensions.

New concepts such as topological order, quasi-long-range order, essential singularities, and defect-mediated phase transitions were introduced.

Like that of Wilson, this work highlighted universality. There were connections between superfluids, superconductors, and XY magnets.

Scaling equations provided insight.

Kosterlitz and Thouless were awarded the Nobel Prize in 2016

We should celebrate!

Wow! Quite the Golden Jubilee!

Does anyone know of any conferences, events, or books that are planned to mark these anniversaries?

Monday, April 12, 2021

Time management and stress reduction

I am not the greatest manager of my time. I am easily distracted and too often ruled by the tyranny of the urgent. I let the good become the enemy of the excellent. I look at my email too often...

Here are just a few points that I do find helpful to keep in mind and act on. They not only lead to better use of time but also reduce stress. I struggle with all of them.

1. It can wait.

We live in an urgent world with many people and tasks demanding immediate attention. There are some very rigid deadlines, such as for most grant applications. However, there are many other tasks such as submitting a paper, checking an experiment, replying to an email, ... that can wait for another today. It is time to log off, literally and mentally and relax. The world will not fall apart if you wait another day, week, or even month.

2. Delegate

Do I personally need to do this task or take on this responsibility? Is there someone else who is able and available to do it instead? Might they actually do it better than me? Even if they might not do it as well, would it be better that they do it anyway and free me up to do more important things?

Having said this, I am slow learn and have become aware that there are some cautions needed in delegating. 

First, suppose I delegate to a person of lower "authority" than me, but who I have full confidence in. Others may not think they have the appropriate authority and so may be reluctant to act on or support what my delegate is.

Second, delegating tasks is no good if the person does not have the time, energy, and resources to complete the tasks. I may also need to provide the necessary resources and help them to see how they might delegate some tasks too.

3. Before embarking on a task, large or small, be clear on what your goal is.

This reduces the chance of getting distracted. Here is a concrete example. I often want to look for a paper on a specific question I have. Yet, I find that an hour later I am looking at my fourth paper because I got distracted by something I found interesting... and I have forgotten my initial question.

Some earlier thoughts on time management are here.

I welcome other suggestions.

Thursday, April 1, 2021

Where might condensed matter physics be heading?

Will there be big new discoveries? Will old problems be solved?  

I have finished my draft of, "An endless frontier" the last chapter of Condensed Matter Physics: A Very Short Introduction.

I aim to give a balanced perspective that is optimistic but realistic. Have I? Obviously, this is highly subjective.

I am interested in general feedback, particularly on whether your aunt or uncle or an eager undergraduate would find this interesting and engaging.

Besides your own research area :), are there particular topics that you think are ripe for exploration?

Perhaps, a cartoon about predicting the future. Maybe one of these two?

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.