I just finished presenting my March Meeting talk, *Infinite boundary conditions as a current source for impurity conductance in a quantum wire. *Slides here.

I just finished presenting my March Meeting talk, *Infinite boundary conditions as a current source for impurity conductance in a quantum wire. *Slides here.

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I’m about to set off to Boston for the APS March Meeting 2019 (March 4-8). I’ll be presenting my newest work on using infinite boundary conditions are current reservoirs for measuring steady-state currents in quantum wires using tensor network methods. My talk is at Wednesday 6 March at 8:48am in room 156C. If you want to chat with me at the March Meeting drop me a line.

After the March Meeting I’ll be visiting the Sandvik group at Boston University 11-20 March. I’m really looking forward to seeing all my old friends and colleagues at BU.

Last week I was invited to give a seminar “Quantum criticality under the influence of magnetic fields” as a part of the Center for Quantum Science and Engineering (CQSE) series at NTU. Thanks to Prof. Guin-Dar Lin for the invitation and for Zih-Ying Huang for taking this photo of me presenting.

A few months ago I signed up for Skype a Scientist, a service that connects scientists with classrooms around the world so students have a chance to meet a real scientist. Today I had my first session with a 7th grade IB class in Bangkok, Thailand. It was a lot of fun! I introduced myself and my field and talked a little bit about what it’s like being a scientist, then I answered questions from the students for the remaining time. There were all sorts of questions from “What challenges did you overcome to become a computational physicist?” to “Is the Earth’s magnetic field changing?”

If you’re a fellow scientist or a teacher who wants to skype a scientist you can sign up on their website: https://www.skypeascientist.com/ The commitment is small (you can sign up to do just one session) and there’s no need to prepare a lecture. I had a blast and I’m looking forward to more skype sessions!

I’m thrilled to announce that my dissertation “Magnetic field effects in low-dimensional quantum magnets” has been selected for a Springer Thesis Award and will be published by Springer. The manuscript is still in production (currently scheduled for publication November 27), but the listing is live on Springer’s website now.* Thanks again to my PhD advisor, Anders Sandvik and my committee, Rob Carey, Shyam Erramilli, Claudio Chamon and David Campbell as well as my department chair Andrei Ruckenstein. A special thanks to David for nominating my dissertation for this award.

*Let me know if you want to read it.

Below is a lightly-edited excerpt from Ch. 1 of my dissertation in which I describe my field in the broadest possible terms. My dissertation is currently in production for publication in the “Springer Theses” series.

This dissertation is in the field of condensed matter physics, which in the most informal sense possible, could be described as ‘the study of stuff that is not especially hot nor moving especially fast’ [1]. A more formal (but no less vague) definition is ‘the study of the behavior of large collections of interacting particles’ [2]. The haziness of this definition is appropriate since condensed matter is a very broad field encompassing the study of almost all everyday matter including liquids, solids and gels as well as exotic matter like superconductors. Condensed matter physics is a tool for answering questions like: Why are some materials liquids? Why are others magnetic? What sorts of materials make good conductors of electricity? Why are ceramics brittle? Our understanding of condensed matter physics underlies much of modern technology; some prominent examples include ultra-precise atomic clocks, transistors [3], lasers, and both the superconducting magnets and the superconducting magnetometers used for magnetic resonance imaging (MRI). Condensed matter physics overlaps with the fields of magnetism, optics, materials science and solid state physics.

Condensed matter physics is concerned with the behavior of large collections of particles. These particles are easy to define: they will sometimes be atoms or molecules and occasionally electrons and nuclei; condensed matter is almost never concerned with any behavior at higher energy scales (i.e. no need to worry about quarks). The key word in the definition is large. Atoms are very small, so any macroscopic amount of matter has a huge number of them, somewhere around Avogadro’s number: 10^{23}. Large ensembles of particles display emergent phenomena that are not obvious consequences of underlying laws that govern the behavior of their microscopic components. In the words of P.W. Anderson:

*The ability to reduce everything to simple fundamental laws does not imply the ability to start from those laws and reconstruct the universe. …hierarchy does not imply that science X is “just applied Y.” At each stage entirely new laws, concepts, and generalizations are necessary, requiring inspiration and creativity to just as great a degree as in the previous one. Psychology is not applied biology, nor is biology applied chemistry. **[4]*

Emergent phenomena are not merely difficult to predict from the underlying microscopic laws, but they are effectively unrelated. At the most extreme scale, no one would argue that consciousness is somehow a property of standard model particles, or that democracy is a state that could ever be described in terms of quantum field theory. Here I will focus on two such emergent phenomena: *phase transitions*, where symmetries of the underlying laws are spontaneously violated and behavior is independent of microscopic details, and *quasiparticles*, an almost infinite variety of excitations of many-body states of matter that bear no resemblance to the ‘real’ particles that make up the matter itself [5].

The most interesting problems tend to involve systems with *interactions*. To highlight the importance of interactions, let us first consider the case of noninteracting particles. The canonical example here is the ideal gas, which is composed of classical point-like particles that do not interact with each other. Because they do not interact, the motion of the particles is independent; if we want to know the energy of any particle, it is easy to calculate from its speed (E = mv^{2}∕2). The behavior of the whole system can be described by an ensemble of independent single particles. When the particles are interacting things are very different. Instead of an ideal gas, let us consider a gas of classical electrons interacting via the Coulomb force 1∕r. For two electrons the equations of motion can be solved analytically, but in a solid there are 10^{23} electrons (for all practical purposes, we can round 10^{23} up to infinity). To write down the energy of of one of them, we must account for the position of every single other electron. Thus the energy of just one electron is a function of 3N variables. Even with just three particles, analytic (pen and paper) solutions are impossible in most cases. An analytic solution for the motion of 10^{23} electrons is impossible, and “it’s not clear that such a solution, if it existed, would be useful” [6]. This is many-body physics. Instead of following individual particles, we describe their collective motion and the resulting emergent phenomena such as *quasiparticles* and *phase transitions*. Consider waves crashing on the beach. It would be foolish to try to understand this phenomenon by following the motion of all the individual water molecules. Instead, we can treat the water as a continuous substance with some emergent properties like density and viscosity. We can then study the waves as excitations the ground state of the water (the state without waves).

[1] This definition distinguishes condensed matter from particle physics (the other broad subdiscipline of physics), which is the ‘study of really hot and really fast-moving objects.’

[2] In practice, condensed matter tends to be the term used to describe physics that does not fit into one of the smaller, more well-defined subdisciplines like high-energy physics or cosmology.

[3] Both transistors and atomic clocks are essential to cellular telephones and satellite navigation systems like GPS.

[4]This quote is taken from “More is different” Science **177**, 393 (1972) by P.W. Anderson , an excellent refutation of reductionism and discussion of emergent phenomena written in a manner that should be accessible to non-physicists.

[5] I hope to post non-technical descriptions of phase transitions and quasiparticles at some point in the future.

[6] Chaikin and Lubensky, 1998, p. 1

I had a great time at the uni10 hackathon at National Tsing-Hua University last weekend. Members of the Ying-Jer Kao (NTU) and Pochung Chen (NTHU) groups presented the updates on the development of the uni10 tensor network library. Also in attendance were developers from industry (and SOLVCON) and the local python user community.