2018 Lightning Round – Physics Subfields Experimental Condensed Matter Physics – Lucas Peeters More is Different isn’t just the title of Phil Anderson’s classic essay on the limitations of reductionism in science - if condensed matter physics was ever in need of an official tagline, it would also be a prime candidate. Indeed, why do we even need a field dedicated to the behavior of the trillions of electrons in materials all around us? Haven’t particle physicists figured out all the fundamental properties of those particles a while ago? But more is indeed different. The zoo of emergent phenomena arising when we put myriads of such particles together - magnetism, superconductivity and multiferroicity, to name a few - requires its own language, its own concepts and its own tools. Our understanding of and control over such effects are not only a rewarding scientific enterprise in their own right, but have also spurred a host of technological breakthroughs which continue to transform our everyday lives. For example, having been at the root of our current, silicon-based computing architecture, condensed matter physics is now also one of the main contenders to deliver the first industrially useful quantum computing devices. Experimental condensed matter physicists have several approaches at their disposal to advance our understanding. Materials growth is one of these. This field, with its often-exceptional sensitivity to a slew of control parameters can appear like a dark art to many outsiders. Those who master it, though, can discover materials which combine effects in ways never seen before in nature, and their controlled realization of such systems can provide invaluable insight. Not all effects are visible at all length scales though, so others specialize in the fabrication of micro- or even nanoscale devices, combining different materials and techniques to create minute devices which bridge the gap between the single-atom scale and macroscopic scales. Another line of research focuses on the development of new measurement techniques; the boundary of spatial and temporal resolution is always being pushed, and increased access to the charge and spin properties of electrons can shed new light on long-standing questions. Since its debut about a century ago, condensed matter physics has dramatically advanced our understanding of electronic behavior in a wide range of contexts. If the past is any guide, the future will give us even more, and it’ll be even more different. Computational Condensed Matter Physics – Yanbing Zhu (some sections from Harrison Ruiz) What is it? Theoretical/computational condensed matter physics involves studying condensed phases of matter. This usually refers to solid states of matter (anything you can touch) plus liquids. A big goal is characterizing certain materials and their resulting phenomena - i.e. finding materials with certain properties, designing a new material from scratch, understanding a (quantum) phase transition, or studying how a material behaves under external influences. Examples of properties are studied are stability, conductivity, specific heat, and magnetic susceptibility. Often these properties studied are in exotic phases of matter such as superconductors, topological insulators, and spin liquids, where some unusual and often purely quantum mechanical phenomena are observed in solids. These phases can be studied both in out of equilibrium. There’s a big interplay between physics, materials science, chemistry, and of course, computational tools. The labels in research between the different subfields becomes blurred - i.e. statistical mechanics, scattering behavior, analytical models, and theories of strongly correlated phenomena are used from physics, physical considerations like the behavior of a material under strain or stress, dislocations, and grain boundaries are used from materials science, and elemental properties, reaction properties, and bonding rules are used from chemistry. Why do we do it? Research is driven by both understanding fundamental science problems and also more practical purposes. Oftentimes both goals can be part of the same project. For instance, topological materials are interesting its exotic behavior was only fairly recently understood, but they are also of interest for quantum computing purposes since there is no decoherence. Batteries and solar materials like new lithium based electrolytes or hybrid inorganic- perovskites are studied for their practical purposes (computational/theory work is often used to guide experiments or explain behavior seen in the lab). The work also enables us to better understand topics like conductivity or to figure out which models and approximations work in certain limits. What does a typical research do? The daily work consists of making calculations using various models and possibly collaborating with experimentalists working on the same problem. The way the calculations are done can vary depending on the type of research. On one end, you have computational condensed matter physicists who focus on numerical calculations. This can involve using methods such as density functional theory, quantum Monte Carlo simulations, molecular dynamics, or exact diagonalization. Running these types of calculations can either involve using libraries designed for ab initio simulations, such as density functional theory (DFT), or writing code for them from scratch. Common programming languages used for this are C++ and Python. In many cases parallel computing techniques are used as the problem is computationally intensive and even some of the simplest models can only be solved exactly for around 20 particles. Recently, machine learning tools also become more popular. Other theorists may use analytical techniques to work on these problems. Some of the approaches they may use are mean-field theory, renormalization group, and quantum field theory. There is not always a clear division between the computational physicist and the traditional theorist so a combination of numerical and analytical methods. Some experimentalists also often use computational tools to compare with their experimental findings. Experimental Particle Physics - Kelly Stifter Particle physics is the study of the smallest pieces of matter and their interactions. The biggest success of the field is the Standard Model, which describes all known particles and the interactions between them (except gravity). Despite the success of the Standard Model, there are still many mysteries to be solved that are considered “beyond” the Standard Model, as well as some open questions within the Standard Model itself. On the experimental side, scientists build detectors in order to learn about these particles. When conducting these experiments, we often try to do one of two things. First, we search for new physics that is not included in the Standard Model. Second, we precisely measure, or look for deviations from, “known” physics. We can break it down even further and specify three main ways that we conduct these searches or measurements. They are often referred to as “frontiers”, since in each category, we push the limits of our current technology and understanding. First is the energy frontier, which uses the world’s largest particle accelerator, the LHC, to recreate the universe as it was a billionth of a second after the big bang and make huge discoveries about the smallest pieces of the universe. The two main experiments on the LHC are CMS and ATLAS. Together, these two detectors discovered the Higgs Boson in 2012. Now, in addition to precisely measuring the masses and couplings of all Standard Model particles, they are looking for dark matter and more exotic new physics such as supersymmetry and extra dimensions. Next is the intensity frontier, which investigates some of the rarest processes in nature, including unusual interactions of fundamental particles and subtle effects that require large data sets to observe and measure. Of particular note is the effort to measure neutrino properties, including their mass and parameters associated with their oscillations. Others open questions include matter-antimatter asymmetry, proton decay, the anomalous magnetic dipole moment of the muon, and many more. Last but not least is the cosmic frontier, which strives to understand the structure, evolution, and contents of the universe. It turns out that only 5% of the energy in the universe is regular matter, and then rest is dark matter and dark energy. There are many experiments trying to understand the physical nature of dark matter either by directly detecting dark matter particles that pass through a detector, or by indirectly detecting specific signatures in cosmic rays coming from the universe. Cosmic rays can also be used to look for other new particles beyond the Standard Model. The very beginning of the universe was marked by a period of intense inflation, and there are experiments that study the cosmic microwave background radiation in order to understand this phenomenon. A similar expansion is happening today and is driven by dark energy, which is being studied by large ground-based telescopes. Everything I have mentioned so far is considered “high energy” particle physics, since it deals with particles and interactions up at the scale of millions of electron volts (eV). Scientists also work on medium and low energy particle physics, which are concerned with structure and stability of hadrons and atomic nuclei. The interactions in these subfields have relatively lower energy - only