Condensed concepts

I have often wondered about my personal experience and the anecdotal evidence that when you get stressed you seem more likely to get a cold or the flu. I finally found some research literature on the subject. A helpful review is Modern Approaches to Conceptualizing and Measuring Human Life Stress Scott M. Monroe A seminal paper from 50 years ago The social readjustment rating scale  TH Holmes, RH Rahe The authors developed a quiz to estimate how your recent life circumstances and changes may be producing different levels of stress. They then correlated the stressful life circumstances to recent illness of the subjects. It is worth occasionally doing the quiz. Here is one version . Aside: One interesting aspect is that positive changes can create stress (e.g. starting a new job, getting married, having a baby, ...). However, as discussed in detail by Monroe in his review all of this is more complicated than we might like. Measures of stress are subjective, subtle, personal

One of the simplest examples of a first-order phase transition is occurs in a ferromagnet at a temperature below the critical temperature and in an external magnetic field. The transition occurs when the field is varied so that it changes sign. This can be described in terms of the following Landau free energy where H is the external field and r is negative. One observes hysteresis as for non-zero H there is a metastable state. The order parameter phi versus H is shown below The boundaries of the region of metastability are defined by the field Hc given by The above description is taken from a review article by Kurt Binder . I have never seen this in a textbook. Have you? Any clear detailed presentations of this topic would be appreciated.

In a time when misunderstandings of science anti-science views are rising around the world, it is important that scientists do a better job of communicating to the broader public what science actually is, what it can do, and what it cannot do. An interesting and important question is what it is that people should know and understand. There is a multitude of views on this (which is not necessarily a completely bad thing). I only learned last week that in 1994, Phil Anderson had tackled this issue in a short article he wrote for The Daily Telegraph, a London-based newspaper. An interesting paper about Anderson's article just appeared. It nicely places the article in a broader context and gives a more recent perspective on the issues he raised. Four Facts Everyone Ought to Know about Science: The Two-Culture Concerns of Philip W. Anderson Andrew Zhang and Andrew Zangwill The four ``facts'' that Anderson chose were (as paraphrased by Zhang and Zangwill): 1. Science

I am a vocal critic of the use of metrics to evaluate individuals, single scientific papers, journals, sub-fields, institutions, .... However, my problem is really one of abuse. I don't think metrics are totally meaningless or useless. Rather, it is the mindless use of metrics, with a disregard for their limitations, that is a problem. This post is not about metrics, jobs, and funding. I have probably already written too many posts on that. Rather, I want to give two examples where I have found some multi-dimensional metrics helpful, when considering issues relating to public policy and development, particularly in the Majority World. The case is that of the HDI ( Human Development Index ). Prior to its introduction people tended to use GDP (Gross Domestic Product) as a measure of how a country was performing and where it ranked in the world. In contrast, the HDI is a composite metric, factoring in income per capita, life expectancy, and education. The map below gives a sense

I am working with a graduate student beginning research and she has asked this important question. I don't think there is a simple universal answer. Background material includes review articles of a field, details of an experimental technique or computer code, details of derivations, seminal articles on the topic, .... At the UQ condensed matter theory group meeting, we had a brief discussion about the question. Answers from students, both beginning and advanced, were helpful. It also underscored how important the question is because students really do struggle with this issue. One shared how he developed some mental health problems because at the beginning of his Ph.D. he was too obsessive about understanding all the details. The question and discussion underscored to me how we need to have more discussions of this nature. Beginning research is a difficult transition for most graduate students. When they were undergraduates they often could understand all the details and wor

On Friday there was a nice UQ Maths and Physics Colloquium, Beyond the Gaussian Universality Class , given by Ivan Corwin , The talk was a very nice example of synergy between fundamental physics and maths research. There are interesting connections with simple one-dimensional models for surface growth, the Kardan-Parisi-Zhang equation , the KPZ universality class, traffic models, random matrix theory, directed polymers in random media, .... Much of the content of Corwin's talk is in a helpful article , from 2016 in the American Mathematical Society Notices. Although this field is motivated by very concrete physical problems, actually doing experiments to test the theory is difficult. One of the few examples is in a  paper , by Takeuchi and Sano, who studied ``topological-defect turbulence in the electroconvection of nematic liquid crystals.'' A good summary of the field from the physics side is A KPZ Cocktail-Shaken, not Stirred... Toasting 30 Years of Kinetic

In the next month, I have been asked to give a talk and to write an article about universities in two different forums. What are universities for? How do they promote human flourishing? Before I get too carried away I thought I would float a few ideas/claims/hypotheses that will be central to my argument that there needs to be a greater debate about fundamental issues and about the history of universities. Some of the claims are interconnected. In future posts, I may expand on some of these claims. Universities are currently having a crisis of identity, mission, and purpose. This crisis arises because there is a multitude of competing and conflicting visions from a range of "stakeholders". This crisis and the degree of conflict is far greater and deeper than those faced by other institutions : government, schools, hospitals, business, charities, ... Over time universities have been one of the most successful human institutions for promoting human flourishing (br

There is a nice paper Bad metallic transport in a cold atom Fermi-Hubbard system  Peter T. Brown, Debayan Mitra, Elmer Guardado-Sanchez, Reza Nourafkan, Alexis Reymbaut, Simon Bergeron, A.-M. S. Tremblay, Jure Kokalj, David A. Huse, Peter Schaus, and Waseem S. Bakr The paper represents a significant experimental advance in using ultracold atoms to investigate questions directly relevant to strongly correlated electron systems. In this case, the system Hamiltonian can be tuned to be a Hubbard model on a square lattice, such that the model parameters, U and t, and the doping, n are known. One limitation is that current experiments can only be performed down to the lowest temperature of T/t =0.3. [For comparison, for cuprates this is of the order of 1000 K!]. Using imaging techniques the authors are able to directly extract the density (charge) diffusion constant D and the density susceptibility, chi, shown below. The experimental data are red dots. The blue curve is the result of c

Over the past few decades, I have taught a wide range of courses in diverse contexts. Perhaps I have been slow to learn how to be a better teacher. Since I began teaching things have changed dramatically. Our goals and the content of most curricula have changed little, and should not. However, advances in technology provide new opportunities but also challenges and potential distractions. The social context has changed significantly in terms of the expectations of both students and institutions. Here are a few of the ideas that I think are important to keep in mind.  Some seem obvious, particularly in hindsight. On the other hand, practical implementations are a challenge. I think keeping the ideas in mind is also important for maintaining your sanity and motivation. The ideas are listed in no particular order and many are interconnected. The amount of learning that happens is correlated with the level of student engagement. Engagement happens at many levels and in many ways: thr

A friend asked me for any advice I have before he launches a blog. What mistakes have I made? How do I manage comments? What is the best platform? So here are my rough thoughts. Just do it! This applies to both starting, persevering, and what you write. Blogging is not for perfectionists and procrastinators. A major strength (and weakness) of the medium is that one can float tentative and controversial ideas and not worry about endless editing and polishing. It can be an incredibly enriching experience, for both yourself and others. The biggest impact of your blog may be on you not on your audience. This is really true in my case. Blogging has clarified my thinking on a wide range of issues, from science to politics to religion. Blogging saves time rather than taking time. Don't be driven by metrics. It is easy to keep track of page views and an abundance of other data. It is not clear how accurate or helpful it is. Furthermore, this can easily lead to feelin

I have had some interesting discussions with an editor at Oxford University Press about the Very Short Introductions series.  The upshot is that I have been asked to write a VSI Condensed Matter Physics. I find it amazing and concerning that after 500 titles there wasn't one about CMP. There are excellent ones on Magnetism, Superconductivity, Complexity, and Crystallography. I am very happy about this and will post more about it later. At first, we discussed a VSI on Solid State Physics. Here is my outline for that. 1. Introduction     Solid state physics    - is central to technology (d iodes, transistors, LEDs, photovoltaic cells, and computer memories)    - provides important lessons in scientific model building    - is one of the largest fields of physics    - is a rich source of ideas and concepts that have cross-fertilised with other fields of science 2. Solids are quantum matter Solids are made of atoms (nuclei and electrons). Electrons are waves. Electrons are f

There is no doubt that the world is changing very rapidly. This is true in many spheres: technology, politics, economics, and social. These changes present significant challenges to individuals, families, communities, businesses, institutions, and countries. On this blog there have been many posts and comments about how science and universities are changing. I think there are three common mistakes in how people respond to these changes. 1. Denial. Claim that the changes are not really that significant (either qualitatively or quantitatively) and we should just keep on operating in the same way. This response will mostly come from those who are not directly affected in the short term. 2. On the other hand, some claim everything has changed and that everything is up for grabs , and they begin to lose sight of basic truths and goals, whether it is human aspirations or the content of physics curricula. 3. Seduction by the "change merchants." These are the opportunists: wh

Tomorrow I am giving a talk about academic publishing for a group of faculty and Ph.D students from African universities. The challenges they face are formidable. Here are my slides. As always, it is important not to reinvent the wheel. There are already some excellent resources and organisations.  A particularly relevant organisation is  AuthorAID  which is related to  inasp , and has an online course starting right now. 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?"

Scientists often like to talk about how much they know and understand. On one level this is fine and appropriate because it is truly amazing how much we do now know and understand about the material world. Yet there are many things we don't really understand, and in some cases it may be argued we may never (at least in our lifetimes) understand certain things. Furthermore, the preponderance of hype in science today tends to obscure and confuse what we don't understand. Humility can be a good thing for at least two reasons. First, it makes us more open to seeing our mistakes and misunderstandings. Maybe some things we think we do know and understand we may actually be wrong about. Second, in contrast to hype, humility helps us more clearly see and acknowledge the limitations of our current knowledge, so that we can explore ways forward. Economics is an interesting case. My son, pointed out this quote from Hayek, a Nobel Laureate. “The curious task of economics is to demonst

Computer simulations can provide significant insight into different physical phenomena. Two decades ago the best one could do in a class or seminar was show screen shots of simulations and try and explain what was going on. Now one can show a simulation live and even vary parameters in real time to provide insight. I have done this quite a bit with Solid State Simulations. One simulation I like but have never used effectively is that of the Ising model. See for example, Daniel Schroeder's  simulation  or James Sethna  or Matt Bierbaum. What does it help me understand? The main ideas are the concept of symmetry breaking, the correlation length, and the divergence of the correlation length at the critical point. 1. Watching the different configurations changing with time illustrates the notion of an ensemble. 2. At high temperatures one sees the paramagnetic phase where the spins are independent of each other and so there are no domains. 3. As the temperature approaches t

The BCS theory of superconductivity is one of the towering intellectual achievements of the twentieth century. There are many ingredients to the theory and many significant results. One key step is to consider an effective interaction that is responsible for the Cooper pairing . A key result is that many properties are universal in that one can rescale temperatures and energies by the energy gap (at zero temperature), Delta(0) or the transition temperature Tc. In the limit of weak-coupling there is a universal ratio 2 Delta(0)/kTc = 3.5 Most elemental superconductors are consistent with this value. Some such as Hg and Pb have larger values, but these can actually be calculated when strong coupling effects are taken into account, via the Eliashberg equations. Unconventional superconductors (cuprate, organic, heavy fermion, iron based) have resisted a simple unifying theory and universal trends, comparable to the stellar success of BCS theory. For example, the gap/Tc ratio is all ove

It is easy to take for granted many scientific ideas and results that seem commonplace and reasonable to us today. However, when some ideas were first proposed or discoveries made they were greeted with scepticism, and even ridicule. In 1913 the Braggs determined the crystal structure of sodium chloride. They were awarded the Nobel Prize in Physics in 1915. However, as late as 1927, the following letter appeared in Nature. [Double click if you want to read a larger version]. I recently became aware of this in Crystallography: A Very Short Introduction by A.M. Glazer.

For the past few months I have been travelling in North America and the UK, for a mixture of work and holiday. In a range of professional and social settings I have had conversations with different people associated with universities: faculty, students, parents, and NGOs. It is amazing (and disturbing) how many times one subject keeps coming come up: student mental health problems. It was never me who brought up the subject and I don't think any of the people who did bring it up knew that I am interested in the issue, partly because of my own struggles. Two important questions people have asked are: Has the incidence of student mental health problems increased or is it just that reporting of problems has increased? Is the situation likely to improve in the near future? Unfortunately, I think the problems have substantially increased and that they unlikely to decrease in the near future. I hope I am wrong. But, I think that there are a multitude of inter-related social, econom

Suppose a motivated and intelligent high school student or first year undergraduate comes to you and says, ``Condensed matter physics sounds really cool! What should I read or look at to learn more about it?" Obviously, suggesting the student look at classic graduate texts such as Ashcroft and Mermin or Chaikin and Lubensky is not helpful. They need something that will inspire them to want to learn more as well as introduce them to some of the basic ideas and topics. I would suggest the following. David Pines, Unit 8 in Physics for the 21st Century , an on-line course Emergent Behavior in Quantum Matter Robert Laughlin, A Different Universe: Reinventing Physics from the Bottom Down Stephen Blundell, Superconductivity: A Very Short Introduction Rodney Cotterell, The Material World But when then have read some of these it would be nice if the student could look at something more technical. To second year undergrads I give a series of lectures on Thermodynamics and Con

I was recently asked to give a talk to an NGO about how universities are changing. There is no doubt that there are rapid changes, many for the worse, happening. For me the biggest change has been the rising influence of neoliberalism (free market ideology) in the values, goals, and decision making within universities. But that is another story... In the past few weeks some relevant articles "came across my desk" [through my web browser...]. I would be particularly interested to hear readers comments on the first one. My wife sent me this New York Times piece to read and I really despaired The iGen Shift: Colleges Are Changing to Reach the Next Generation  The newest students are transforming the way schools serve and educate them, including sending presidents and deans to Instagram and Twitter. Why do I despair? I believe a university education should largely be about two things. The first goal is explicit and the second is implicit. The first and primary goal of

The two terms are often used interchangeably, but that is not appropriate. Condensed matter physics does not just involve solids but also phenomena in liquids, liquid crystals, superfluids, and polymer melts.  Solid state physics is a subset of condensed matter physics. The latter term was arguably coined by Phil Anderson, when he and Mott renamed their research group at Cambridge in the 1970s. One can view research fields or course titles as a list of topics or as a way of thinking about certain parts of reality. Solids exhibit rich phenomena including magnetism and superconductivity. However, it is best to actually view the solids as (an almost irrelevant) substrate for the phenomena . Like many things, this perspective arguably started with Landau. His theory of phase transitions in the 1930s did not consider atomic structure or chemical composition. Even structural phase transitions were viewed in terms of symmetry change, not in terms of explicit microscopic details. In 1950

Unfortunately, hype in science reflects hype in broader society, including in business. The complete DNA sequencing of the human genome was an amazing scientific achievement. Unfortunately, it was also associated with a lot of hype about what this would mean for medicine and for the pharmaceutical industry. This issue is made painfully and succinctly in a recent column in the business section of  The Guardian by Nils Pratley. It has been almost two decades since the first bosses of the newly merged GlaxoSmithKline talked up the medical wonders that would flow from the unravelling of the human genome. GSK would become the “Microsoft of the pharmaceutical industry”, they said.    To put it mildly, the corporate vision hasn’t been realised. GSK’s share price stood at £20 at the time of the turn-of-the-century merger and is £15.42 today. Lack of productivity in the labs has been a constant complaint. The genetics revolution is happening, but not at the pace originally promised, at lea

An important and fascinating issue in many-body physics is the emergence of new energy scales, particularly scales that are orders of magnitude smaller than the energy scales in the underlying Hamiltonian. One example is the coherence temperature associated with the crossover from a Fermi liquid (with coherent quasi-particles) to a bad metal. Recently, I posted about the crossover from a Hund's metal to a bad metal, seen in the c ollapse of the Drude peak in the optical conductivity , and the issue of capturing this slave-particle theories . One commenter mentioned the relevance of the paper below and another asked about the claim that the Kondo effect is associated with the collapse. I agree that Kondo physics is associated with the crossover. Although, far from obvious this is also the case in the single-band Hubbard model. The Kondo effect was first studied with isolated magnetic impurities in metals and can be described by a single-impurity Anderson model (SIAM). Alt

Condensed matter physics is a source of a multitude of beautiful examples of emergence.  On the other hand, f or more than a century philosophers have thought seriously about emergence , partly motivated by profound and difficult questions concerning human consciousness and free will. Prior to the past decade, there appear to have been no substantial interactions between physicists and philosophers about the subject. A few years ago I posted about some recent work by philosophers of science on quasi-particles. One of the big issues that philosophers wrestle with is the relative merits of weak emergence and strong emergence , which are sometimes distinguished as epistemological and ontological emergence. I am very happy that in the past year or so that philosophy journals have published more than half a dozen papers about emergence in condensed matter. One of the papers, by Stephen Blundell, I blogged about earlier. Here I will mention two others and discuss one. All the papers a

I like "collecting" interesting phase diagrams, partly because they are fun to show students when teaching introductory thermodynamics. I recently discovered the one below that I feel I really should have known about. It shows the morphology of different snow crystals as a function of temperature and water supersaturation (relative to ice). It should be pointed out that this is a non-equilibrium phase diagram as it involves supercooled liquid water. The figure below is taken from the beautiful review The physics of snow crystals  Kenneth G Libbrecht This diagram was originally constructed by Ukichiro Nakaya in the 1930's. The physics behind it is still poorly understood. I came across the diagram while browsing through the Forces of Nature book by Brian Cox and Andrew Cohen. While on the subject here is a nice video.

A definitive experimental signature of the crossover from a Fermi liquid metal to a bad metal is the disappearance of a Drude peak in the optical conductivity. In single band systems this occurs in proximity to a Mott insulator and is particularly clearly seen in organic charge transfer salts and is nicely captured by Dynamical Mean-Field Theory (DMFT). An important question concerning multi-band systems with Hund's rule coupling, such as iron-based superconductors, is whether there is a similar collapse of the Drude peak. This is clearly seen in one material in a recent paper Observation of an emergent coherent state in the iron-based superconductor KFe2As2  Run Yang, Zhiping Yin, Yilin Wang, Yaomin Dai, Hu Miao, Bing Xu, Xianggang Qiu, and Christopher C. Homes Note how as the temperature increases from 15 K to 200 K that the Drude peak collapses.  The authors give a detailed analysis of the shifts in spectral weight with varying temperature by fitting the optical c

A snarky mathematician [Stanislaw Ulam] once challenged the great Paul Samuelson to name an economic proposition that is true but not obvious. Samuelson’s choice was comparative advantage , which shows, among other things, that there are mutual gains from trade even if one nation is better than another at producing everything.    Here’s a homespun illustration. Suppose a surgeon is also a whiz at house painting—better than most professional painters. Should she therefore take time off from her medical practice to paint her own house? Certainly not. For while she may have a slight edge over most painters when it comes to painting walls, she has an enormous edge when it comes to performing surgery. Surgery is her comparative advantage, so she should specialize in it and let some others, who don’t know their way around an operating room, specialize in painting—their comparative advantage. That way, the whole economy becomes more efficient.    The same principle applies to nations. E

One of most interesting new ideas about quantum matter from the last decade is that of a Hund's metal. This is a strongly correlated metal that can occurs in a multi-orbital material (model) as a result of the Hund's rule (exchange interaction) J that favours parallel spins in different orbitals. Above some relatively low temperature (i.e. compared to the bare energy scales such as non-interacting band-widths, J, and Hubbard U) the metal becomes a bad metal , associated with incoherent excitations. An important question concerns the extent to which slave mean-field theories can capture the stability of the Hund's metal, and its properties including the emergence of a bad metal above some coherence temperature, T*. In a single-band Hubbard model, the strongly correlated metallic phase that occurs in proximity to a Mott insulator is associated with a small quasi-particle weight and suppression of double occupancy, reflecting suppressed charge fluctuations. This is captur

When is a system "complex"? Even though we have intuition (e.g. complexity is associated with many interacting degrees of freedom) coming up with definitive criteria for complexity is not easy. I just finished reading, Complexity: A Very Short Introduction , by John Holland. His perspective is that a system is "complicated" if it has many interacting degrees of freedom, but is "complex" if in addition it exhibits emergent properties. The criteria for emergence is the existence of new hierarchies , containing new entities or agents (defined by the formation of  boundaries ) that are coupled by new interactions , and described by new " laws ". Holland distinguishes complex physical systems (CPS) from complex adaptive systems (CAS). The latter involve elements (agents) that can change (learn or adapt) in response to interactions with other agents. Cellular automata and pattern formation in biology are CPS, whereas genetic algorithms, econom

We all want to increase our productivity. But too often we are distracted, procrastinate, stressed, or waste time going down dead ends. I think there are two distinct kinds of productivity. The first is creative , where we can clearly conceive a project, solve a problem, or draft a useful outline. The second is the actual completion of a task , whether writing a paper or report, or making corrections, ... This is less creative and more mundane, but can consume large amounts of time, particularly if one stops and starts on the task many times. How might you increase your productivity? I think this is quite personal and maybe even somewhat random. It might be very different for different people. It can be different at different times. Factors to consider include the following. Physical space and environment.  Some people need a regular quiet work space that is free from distractions. Others will function well in a noisy cafe or an open plan office, maybe with headphones with l