Condensed concepts

 My economist son often listens to the podcast, Conversations with Tyler Cohen. We recently listened to a conversation with Esther Duflo , who shared the 2019 Nobel Prize in Economics. Like most episodes it covers a wide range of territory, from development economics to Indian classical music to parenting. I highly recommend it. Perhaps, the bit that was most striking for me was the following. What advice do you give to your talented undergraduates that differs from the advice your colleagues would give them?    I give almost all of them the advice to take some time off, in particular if they have any interest in development, which is generally the reason why they come to see me in the first place. But even if they don’t really, to spend a year or two in a developing country, working on a project. Not necessarily inner city. Any project spending time in the field.    It’s only through this exposure that you can learn how wrong most of your intuitions are and preconceptions are.

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

Last night I heard a model public lecture about science. The School of Mathematics and Physics at UQ hosted a public lecture at the Queensland State Library. Holly Krieger , a pure mathematician at Cambridge, spoke on the Mathematics of Life. This is part of a biannual lecture series endowed by Kurt Mahler. The lecture was amazing, both in content and presentation. It was engaging for high school students, and stimulating for experts. I wish I had a video or a copy of the slides. Krieger is well known to some through her Numberphile videos on YouTube.  Here are a few things I learned in the lecture. Mathematics is the language of relationships and patterns. We forget how even the concept of numbers is abstract. The notion of functions is even more so. An underlying theme of the lecture was that of emergence : a simple rule describing the interactions between the components of a system lead to collective behaviour (complexity) of the whole system. Examples were given from biol

Following the fall of the Berlin Wall, one incredible revelation was the expansive role of the secret police, vast network of informers, and level of personal surveillance. This was underscored to me in movies such as The Lives of Others , novels such as The Day of the Lie , and a seminar I attended about human rights abuses in Syria. The survival of totalitarian regimes is facilitated by the regime creating a culture of fear at every level of society and institutions, from factories to families. You do not dare to question or criticise the regime. Even making a joke at work may send you to the gulag. Over the last decade, I have noticed a cultural shift in universities where there seems to be a culture of fear in many different aspects. A few examples are below. I should be clear that I am not suggesting that universities today are anything like Syria, China, or the former Soviet Union. Nevertheless, it is worth reflecting on whether there is a culture of fear and what its impli

The Ising model is a paradigm in both statistical mechanics and condensed matter physics. Today for most theorists it is so familiar that some of its historical and conceptual significance is lost. Previously, I posted about what students can l earn from computer simulations of the Ising model. If you had to talk about the Ising model to an experimental chemist what would you say? [Last week I had to do this]. The Ising model is the simplest effective model Hamiltonian that can describe a thermodynamic system that undergoes a first-order phase transition and has a phase diagram containing a critical point. On each site i of a lattice one defines a spin sigma_i= +1 or -1, representing spin up or spin down. The Hamiltonian H is J_ij describes the interaction between spins on sites i and j. In the simplest version the interactions are only between nearest neighbours, and have the same value J. h is the external magnetic field. If J is positive, the ground state at h=0

My mental health this year has been up and down. It is not particularly clear why I have struggled at times, given the sources of stress were not particularly bad. Thankfully, now I am the best I have been all year. This may be because I have been quite proactive in taking action. First, there are the basics: adequate sleep, downtime, exercise, and diet. At one point I also cut out all caffeine and alcohol. I also went to the psychologist several times, did more mindfulness exercises, and increased my medication, in consultation with my doctor. This experience underscores some of the complexities and associated poor understanding of both mental illness and healing. There are biomedical, psychological, social, and spiritual dimensions. There is a high causal density , just like in public policy. Why did I get worse? Why did I get better? As a patient, I don't want to do a series of clinical trials on myself and just change one variable, one after the other. It is better to attack

Expert readers: please note this post is written for the general audience of a Very Short Introduction . General comments welcome. Condensed matter physics is not just defined by the objects it studies: condensed states of matter. Rather, the field is also defined by a particular approach. The focus is on finding unifying concepts and organizing principles to address fundamental questions concerning a wide range of phenomena in materials that are chemically and structurally diverse. This approach means looking at the different scales (length, time, and energy) associated with phenomena. In particular, CMP often looks at scales intermediate between the macroscopic and atomic scales. I argued before, that in this sense Kamerlingh Onnes was the first condensed matter experimentalist. In a similar sense, Lev Landau (1908 - 1968) is arguably the first condensed matter theorist, with three papers that he published in 1937, marking the beginning of theoretical CMP. Landau lived in the So

Finishing a thesis (honours, masters, or PhD) can be exhausting: physically, emotionally, and intellectually. When you finally submit it, the last thing you want to do is look at it again or reflect on the experience. Unfortunately, many students do not have a break and soon they are caught up in a job search or starting a new job. Furthermore, it is easy for students to default to the academic track: Masters, PhD, postdoc1, postdoc2, .... I have posted before about how the privileged few who get tenure may not make the most of  transitions within an academic career . Here the focus is on students. After a well-earned break, it is worth reflecting on the following questions, particularly before deciding what you might do next and how to make that a positive experience. What are some things you enjoyed? did not enjoy? What do you think you did well? not well? What did you learn about yourself, particularly your strengths and weaknesses? What did you learn about those you w

An important question concerning spin-crossover compounds concerns the origin and the magnitude of the interactions between the individual molecular units. There is a nice paper Evolution of cooperativity in the spin transition of an iron(II) complex on a graphite surface Lalminthang Kipgen, Matthias Bernien, Sascha Ossinger, Fabian Nickel, Andrew J. Britton, Lucas M. Arruda, Holger Naggert, Chen Luo, Christian Lotze, Hanjo Ryll, Florin Radu, Enrico Schierle, Eugen Weschke, Felix Tuczek, and Wolfgang Kuch An impressive achievement is the control of the number of monolayers (ML) of SCO molecules deposited on a highly oriented surface pyrolytic graphite. The coverage varies between 0.35 and 10 ML. The shape of the spin-crossover curve changes significantly as the number of monolayers varies, as shown in the upper panel below. The natural interpretation is that as the number of monolayers increases the interaction between molecules (co-operativity) increases. This can be quantifie

Oral exams (vivas) are quite common for most postgraduate degrees involving research. The basic goal is to provide an efficient mechanism for the examiners to determine a student's level of understanding of what they have done. Most committees comprise both experts and non-experts. Most are actually quite friendly. If the non-experts learn something new they will be happy. Sometimes an examiner may ``grill'' a student simply because they want to understand what is going on. I think the main reason thinks occasionally get tense is when there is a member of the committee who has a poor relationship with the student's advisor or doesn't think much of their research. To prepare take any opportunity to attend another student's oral exam or ask them about what questions  they were asked and tips. Some common mistakes that students make are to assume: Everyone on the committee has read the thesis in detail. The committee is going to ask highly technical and nua

I was asked for an update on this. The challenges are formidable, but not insurmountable. Here are slides from a talk on the subject. As always, it is important not to reinvent the wheel. There are already  some excellent resources and organisations.  A relevant organisation is  AuthorAID  which is related to  inasp , and has  online course s on writing. People I know who have taken these courses, or acted as mentors, speak highly of them. Authors should also make use of software to correct English such as  Grammarly . Publishing Scientific Papers in the Developing World  is a helpful book, stemming from a 2010 conference. Erik Thulstrup  has a nice chapter "How should a Young Researcher Write and Publish a Good Research Paper?"

I have now finished my first draft of chapter 3 , of Condensed Matter Physics: A Very Short Introduction.  I welcome comments and suggestions. However, bear in mind my target audience is not the typical reader of this blog, but rather your non-physicist friends and family.  I think it still needs a lot of work. The goal is for it to be interesting, accessible, and bring out the excitement and importance of condensed matter physics. This is quite hard work, particularly to try and explain things in an accessible manner. I am also learning a lot. I have a couple of basic questions. How is the symmetry of the rectangular lattice and the centred lattice different? When was the crystal structure of ice determined by X-ray diffraction? [Pauling proposed the structure in 1935.]

There are several independent lines of argument that are used to support the idea of a multiverse: the many-worlds interpretation of quantum mechanics, the ``landscape problem'' in string theory, and the fine-tuning of fundamental physical constants . Previously, I wrote about four distinct responses to the fine-tuning of the cosmological constant. I was recently trying to explain the above to a group of non-physicists. One of them [Joanna] had the following objection that I had not heard before. Schrodinger's cat can only exist in one universe within the multiverse. The multiverse involves zillions of universes. However, because of fine-tuning carbon-based life is so improbable that it can only exist in one (or maybe a handful?) of the universes, within the multiverse. Thus, when one observes whether the cat is dead or alive, and the universe ``branches" into two distinct universes, one with a dead cat and the other with a living cat, there is a problem. It is possi

It is that time of year again. I have not made predictions for a few years. For physics this year I predict Experiments for testing Bell inequalities and elucidating the role of entanglement in quantum physics Alan Aspect, John Clauser, and Anton Zeilinger They received the Wolf Prize in 2010, a common precursor to the Nobel. My personal preference for the next Nobel for CMP would be centred around Kondo physics, since that is such a paradigm for many-body physics, maybe even comparable to BCS. Kondo effect and heavy fermions Jun Kondo , Frank Steglich , David Goldhaber-Gordon Arguably the latter two might be replaced with others who worked on heavy fermions and/or Kondo in quantum dots. Steglich discovered heavy fermion superconductivity. Goldhaber-Gordon realised tuneable Kondo and Anderson models in quantum dots (single-electron transistors). Unlike many, I still remain to be convinced that topological insulators is worthy of a Nobel. For chemistry, my knowledge is

I previously discussed how one of the simplest model effective Hamiltonians that can describe many physical properties of spin-crossover compounds is an Ising model in an external "field". The s_i=+/-1 is a pseudo-spin denoting the low-spin (LS) and high-spin (HS) states of a transition metal molecular complex at site i. The ``external field" is one half of the Gibbs free energy difference between the LS and HS states. The physical origin of the J interaction is ``believed to be'' elastic, not magnetic interactions. A short and helpful review of the literature is by Pavlik and Boca. Important questions are: 1. What is a realistic model that can explain how J arises due to elastic interactions? 2. How does one calculate J from quantum chemistry calculations? 3. How does one estimate J for a specific material from experimental data? 4. What are typical values of J? I will focus on the last two questions. One can do a mean-field treatment of the Ising m

As years go by the PhD thesis in science and engineering is less and less of a ``thesis'' and more just a box to tick. There was a time when the thesis was largely the work of the student and tackled one serious problem. Decades ago at the University of Chicago, students were meant to write a single author paper that was based on their thesis. At some universities, including my own, students can now staple several papers together, write an introductory chapter, and submit that as a thesis. One obvious problem with that system is the question of how large was the contribution of the student multi-author papers, both in terms of the writing and doing the experiments or calculations. Previously I have argued that A PhD is more than a thesis , a PhD should involve scholarship , and a thesis should suggest future directions and be self-critical.  In some sense these posts were negative, focusing on what may be missing. Here I just want to highlight several positive things I recen

In Phil Anderson's  review of  Lucifer's Legacy: The Meaning of Asymmetry  by Frank Close, Anderson makes the following profound and cryptic comment. In a book focusing, as this does, on symmetry, it seems misleading not to explain the fundamental principle that all interaction follows from symmetry: the gauge principle of London and Weyl, modelled on and foreshadowed by Einstein's derivation of gravity from general relativity (Einstein seems to be at the root of everything). The beautiful idea that every continuous symmetry implies a conservation law, and an accompanying interaction between the conserved charges, determines the structure of all of the interactions of physics. It is not appropriate to try to approach advanced topics such as electroweak unification and supersymmetry without this foundation block. To see how this plays out in electrodynamics see here.

In terms of institutional structures, Condensed Matter Physics did not really exist until the 1970s. A landmark being when the Division of Solid State Physics of the American Physical Society changed its name. On the other hand, long before that people were clearly doing CMP! If we think of CMP as a unified approach to studying different states of matter that enterprise began in earnest during the twentieth century. Kamerlingh Onnes (1853-1924) was a pioneer in low-temperature physics but is best known for the discovery of superconductivity in 1911. In many ways, Onnes embodied the beginning of an integrated and multi-faceted approach to CMP: development of experimental techniques, the interaction of theory and experiment, and addressing fundamental questions. 1. Onnes played the long game, spending years developing and improving experimental methods and techniques, whether glass blowing, sample purification, or building vacuum pumps. He realized that this approach required a large

In his beautiful book, Lucifer's Legacy: The Meaning of Asymmetry, Frank Close gives several nice examples of symmetry breaking that make the concept more accessible to a popular audience. One is shown in the video below. Consider a spherical drop of liquid that hits the flat surface of a liquid. Prior to impact, the system has continuous rotational symmetry about an axis normal to the plane of the liquid and through the centre of the drop. However, after impact, a structure emerges which does not have this continuous rotational symmetry, but rather a discrete rotational symmetry. Another example that Close gives is illustrated below. Which napkin should a diner take? One on their left or right? Before anyone makes a choice there is no chirality in the system. However, if one diner chooses left others will follow, symmetry is broken and a spontaneous order emerges.

In my last post, I asked a number of questions about Condensed Matter Physics (CMP) that my son asked me. On reflection, my title ``basic questions" was a misnomer, because these are actually rather profound questions. Also, it should be acknowledged that the answers are quite personal and subjective. Here are my current answers. 1. What do you think is the coolest or most exciting thing that CMP has discovered?  Superconductivity. explained? BCS theory of superconductivity. Renormalisation group (RG) theory of critical exponents. 2. Scientific knowledge changes with time. Sometimes long-accepted ``facts''  and ``theories'' become overturned.   What ideas and results are you presenting that you are almost absolutely certain of?  Phase diagrams of pure substances. Crystallography. Landau theory and symmetry breaking as a means to understand almost all phase transitions. RG theory. Bloch's theorem and band theory as a framework to understand the

I am trying out draft chapters of Condensed matter physics: A very short introduction, on a few people who I see as representative of my target audience. My son is an economist but has not studied science beyond high school. He enjoys reading widely. He kindly agreed to give me feedback on each draft chapter. Last week he read the first two chapters and his feedback was extremely helpful. He asked me several excellent questions that he thought I should answer. 1. What do you think is the coolest or most exciting thing that CMP has discovered? explained? 2. Scientific knowledge changes with time. Sometimes long-accepted ``facts''  and ``theories'' become overturned? What ideas and results are you presenting that you are almost absolutely certain of? What might be overturned? 3. What are the most interesting historical anecdotes? What are the most significant historical events? Who were the major players? 4. What are the sexy questions that CMP might answer in th

A recent issue of The Economist has an interesting article about the massive expansion in higher education, both private and public, in Africa. The thing I found most surprising and interesting is the graphic below. It compares the percentage of the population within 5 years of secondary school graduation are enrolled in higher education, in 2000 and 2017. In almost all parts of the world the percentage enrollment has doubled in just 17 years! I knew there was rapid expansion in China and Africa, but did not realise it is such a global phenomenon. Is this expansion good, bad, or neutral? It is helpful to consider the iron triangle of access, cost, and quality . You cannot change one without changing at least one of the others. I think that this expansion is based on parents, students, governments, and philanthropies holding the following implicit beliefs uncritically. Based on the history of universities until about the 1970s. Prior to that universities were fewer, smaller,

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

This is a basic question that I have been puzzling about. I welcome solutions. Consider a diatomic molecule containing atoms with mass m1 and m2. It has a stretch vibration that can be described by a harmonic oscillator with a reduced mass mu given by . Now consider a polyatomic molecule containing N atoms. It will have 3N-6 normal modes of vibration. [The 6 is due to the fact that there are 6 zero-frequency modes: 3 rigid translations and 3 rotations of the whole molecule]. In the harmonic limit, the normal mode problem is solved below. [I follow the classic text Wilson et al., Molecular Vibrations ]. The problem is also solved in matrix form in Chapter 6 of Goldstein, Classical Mechanics ]. One now has a collection of non-interacting harmonic oscillators. All have mass = 1. This is because the normal mode co-ordinates have units of length * sqrt(mass). The quantum chemistry package Gaussian does more. It calculates a reduced mass mu_i for each normal mode i u

Since I am working on a Very Short Introduction (VSI) to condensed matter physics I am looking at a lot of writing about science for popular audiences. I have noticed several distinct approaches that different authors take. They all have strengths and weaknesses. Historical The story of discoveries and the associated scientists is told. A beautiful example is A Short History of Nearly Everything by Bill Bryson. When done well this approach has many positives. Stories can be fun and easy to read, particularly when they involve quirky personalities, serendipity, and fascinating anecdotes. Furthermore, this shows how hard and messy real science is, and that science is a verb, not just a noun. On the other hand, it can be a bit challenging for readers as they have to understand not just the successes but also why certain theories, experiments, and interpretations were wrong along the way.  Many writers also seem eager to burden readers will all sorts of historical background details a

On a recent flight, I watched the HBO documentary The Inventor: Out for Blood in Silicon Valley . It chronicles the dramatic rise and fall of Elizabeth Holmes , founder of a start-up, Theranos , that claimed to have revolutionised blood testing. There is a good article in the New Republic What the Theranos Documentary Misses Instead of examining Elizabeth Holmes’s personality, look at the people and systems that aided the company’s rise. In spite of the weaknesses described in that article, the documentary made me think about a range of issues at the interface of science, technology, philosophy, and social justice. The story underscores Kauzmann's maxim , `` people will often believe what they want to believe rather than what the evidence before them suggests they should believe.'' Truth matters. Eventually, we all bounce up against reality: scientific, technological, economic, legal, ...  It does not matter how much hype and BS one can get away, eventually, it

In my view, the central question that Condensed Matter Physics (CMP) seeks to answer is: How do the properties of a distinct phase in a material emerge from the interactions between the atoms of which the material is composed?  CMP aims to find a connection between the microscopic properties and macroscopic properties of a material. This requires determining three things: what the microscopic properties are, what the macroscopic properties are, and how the two are related. None of the three is particularly straightforward. Historically, the order of discovery is usually: macroscopic, microscopic, connection. Making the connection between microscopic and macroscopic can take decades, as exemplified in the BCS theory of superconductivity. Arguably, the central concept to describe the macroscopic properties is broken symmetry , which can be quantified in terms of an order parameter . Connecting this microscopics is not obvious. For example, with superconductivity, the sequence of di