Fantastic Beasts

editorial Fantastic beasts

Elementary particles are the building blocks of matter, but there is also a zoo of quasiparticles that are crucial for understanding how this matter behaves.

The idea that the world can be broken down at their core. Take electrons, Bosons Fermions into elementary particles has a long history, for example. When Bogoliubov Topological Relativistic Phonons Anyons dating back to ancient India and Greece. And electrons move inside quasiparticles defects fermions while physicists may have discovered most a material they interact of these building blocks in the seemingly with the surrounding

Collective Amplitude Plasmons Magnons infallible standard model of particle physics, atoms and electrons. modes the language of particles does not stop once Because of all these they merge together to create matter. On interactions, the electrons page 1085 of this issue, we discuss the history often behave as though Excitons Polaritons Polarons and role of a selection of quasiparticles — by they are heavier than Dressed no means an exhaustive list — to highlight they should be. But these how this concept is not only crucial for are elementary particles understanding and predicting a range of whose mass is not really Schematic depiction of a selection of quasiparticles. phenomena inside materials, but could negotiable. The electrons also provide a platform for probing physics inside materials are beyond the standard model. not described as elementary electrons anyons play a major role in the fractional John Dalton and Robert Brown laid the anymore, but should be thought of as quantum Hall effect. And although scientific framework for elementary particles electron quasiparticles. non-Abelian anyon quasiparticles are yet in the early nineteenth century, but decades A distinction is sometimes made to be seen experimentally, they appear passed before an elementary particle was between quasiparticles that arise from to be within tantalizing reach, and actually found experimentally. That situation collective excitations and those that could be used as a basis for topological changed very quickly in the early to mid- have real particles at their core, usually quantum computation. twentieth century, with particles discovered referred to as dressed particles — a real Anyons are not the only type of exotic at such a rate that Enrico Fermi quipped, particle that has been dressed by some quasiparticle that does not behave like “If I could remember the names of these form of excitation. Polarons, for example, any of the elementary particles. A flurry particles, I would have been a botanist”1. are electrons that are dressed with lattice of quasiparticles akin to Weyl fermions The particle zoo has been successfully deformation (a cloud of virtual phonons). were reported last year, which led to popularized to general audiences — the But whether or not it’s important to make excitement about the discovery of all three excitement around the Higgs announcement this distinction is somewhat debatable. of the fundamental fermions: Dirac, Weyl shows how well particle physics captures A schematic depiction highlighting and Majorana4. But due to the different the imagination of the public. But a second, the different groups (pictured) may symmetry rules between high-energy lesser-known particle zoo emerged almost have its use but what’s important is that physics and solid-state systems — high- in parallel. And despite the lack of fame, these quasiparticles provide a powerful energy has constraints imposed by Poincaré it also played an important role in the framework for explaining complex symmetry — it is now thought that there development of physics in the last century. many-body phenomena. are several different types of fermionic One reason why these particles generate The analogy with elementary particles quasiparticle with no high-energy analogue. less interest may be because they are not can go quite far. And just as accelerators The list of fermions just got longer5. viewed as real particles — which they can be built to probe the dynamics The elementary particles that form the are and they aren’t. Starting from the and structure of particles, colliders particle zoo have understandably stolen 1930s, physicists realized that new entities can be built to probe the dynamics of the spotlight — they’re even available as could emerge from the interaction and quasiparticles3. Such colliders could not soft toys http://particlezoo.net/( ). But the dynamics of a collection of particles. These only provide insights into the formation quasiparticles that are key to understanding could also be described as particles, even and characteristics of quasiparticles, but and describing the behaviour of materials without having a real particle at their also help to identify the boundaries of the sadly receive much less attention. As they core. A popular example of such emergent quasiparticle concept itself. extend into ever-more exotic realms, behaviour is a ‘Mexican wave’ at sporting Quasiparticles already open the door beyond the standard model, we hope that events, which arises from the collective to realms out of reach of conventional the public will become more aware of these behaviour of a large number of interacting particle physics. In the standard model, fantastic beasts. spectators. From phonons and plasmons particles come in two classes: bosons and References to excitons and skyrmions, a whole host of fermions. In two-dimensional systems, 1. Baggott, J. Higgs: The Invention and Discovery of the ‘God Particle’ quasiparticles emerge from the complex quasiparticles can belong to a third, distinct (Oxford Univ. Press, 2012). interactions inside materials2. class, known as anyons. Rather interestingly, 2. Pines, D. Rep. Prog. Phys. 79, 092501 (2016). 3. Langer, F. et al. Nature 533, 225–229 (2016). But not all quasiparticles are collective anyons themselves come in two different 4. Nat. Phys. 11, 697 (2015). excitations and some do have real particles classes: Abelian or non-Abelian. Abelian 5. Bradlyn, B. et al. Science http://doi.org/bs56 (2016).