Emergent quanta of momentum and charge, called quasiparticles, govern many of the properties of materials. The development of a quasiparticle collider promises to reveal fundamental insights into these peculiar entities. See Letter p.225
Electrons and atomic nuclei in solids are bound together by their electric charges. If an electron is moved to a new position inside a metal, then the other electrons and nuclei respond by shifting their own positions. An electron that is accompanied by this response of the surrounding electrons and nuclei is an example of a quasiparticle (Fig. 1). It would be fascinating to prepare and manipulate the trajectories of quasiparticles to make them collide and then to study the effect of the collision, similar to experiments in a particle accelerator. On page 225 of this issue, Langer et al.1 report the realization of such an experiment.
The authors studied an electrical insulator, tungsten diselenide (WSe2). They generated pairs of quasiparticles in the material, one negatively charged and the other positively charged, using an ultrashort light pulse (10–100 femtoseconds in duration; 1 fs is 10−15 seconds). The light pulse's energy, intensity and duration were precisely adjusted so that the initial distance of the quasiparticles from each other, and their relative speeds, were well defined.
Langer and colleagues then launched the quasiparticles along a linear track. This track was created with the help of the electric field from a second light pulse; the field strength, duration and oscillation period of the light pulse were adjusted to direct the quasiparticles into a head-on collision. The collision caused mutual annihilation of the quasiparticles and the emission of a photon, which the authors detected. The experiment is therefore similar to studies of electron–positron annihilation in high-energy particle accelerators (positrons are the antiparticles of electrons, which means that they have opposite charge and equal mass to an electron).
The researchers can tune the conditions of their system in many ways by adjusting the aforementioned experimental parameters and the time interval between the generation of the pulses and their detection. They particularly examined the effect of the electrical (Coulomb) interaction between the two oppositely charged quasiparticles. Under stable conditions, this interaction would bind the quasiparticles into a neutral composite particle called an exciton. Excitons are another example of a collective state that can exist in solids, somewhat like positronium atoms (which form from one electron and one positron). The authors obtained material-specific information such as the exciton binding energy, and observed an enhancement of the collisional cross-section (a quantity that governs the rate at which the quasiparticles collide) as a result of the Coulomb force between the oppositely charged quasiparticles.
The beauty of Langer and colleagues' experimental toolkit is that it might finally allow quasiparticles and their mutual interactions to be studied in the materials in which they arise. The negative and positive quasiparticles in the authors' experiment are similar to electrons and positrons in a vacuum, but a rich variety of unconventional quasiparticles could also be studied, for which no equivalent elementary particles are known. For example, when an electron is introduced into an insulating transition-metal oxide such as strontium titanate (SrTiO3), the electron slightly attracts the positive ions in the material (Ti4+ and Sr3+), but slightly repels the negative oxide ions (O2−). When the electron moves around the compound's lattice, the ionic displacements move with the electron. The resulting object — the electron plus the co-moving lattice distortion — is called a polaron2,3. Its properties and behaviour are different from those of an electron; for example, its mass is typically two or three times higher.
Quasiparticles that are even more bizarre emerge in two-dimensional gases of interacting electrons in a strong magnetic field. The charge on these quasiparticles is a fraction of that for an electron: it can be one-third (the same as for elementary particles called quarks), one-fifth, one-seventh, or smaller4,5.
When a magnetic field is applied to certain superconductors, peculiar topological states known as vortices appear, equivalent to tubes of magnetic flux. Vortices and antivortices form spontaneously6,7 in 2D superconductors, but it might also be possible to generate them using light pulses. This would open the way to studies of their interactions using Langer and colleagues' approach, including the annihilation of vortex–antivortex pairs.
Quasiparticles are not only of academic interest — they also determine many of the properties and functionalities of materials, such as electrical resistivity, heat capacity and magnetism. There are thus many reasons to study quasiparticles in the materials in which they are manifested. Langer and co-workers have provided a fresh strategy with which condensed-matter physicists can tackle such studies. This promises fundamental insights, but also offers ways to control and handle the quasiparticles characteristic of the various states of matter that can be realized in solids.
That said, only a few experimental facilities will have the combination of technologies required to study quasiparticles that have fractional charges, or vortex–antivortex annihilation in two dimensions. But for those that do, Langer and colleagues' approach can be readily applied to investigate the properties of polarons in strontium titanate or other transition-metal oxides, or the 'heavy electrons' that occur in several materials owing to the coupling of mobile electrons to fluctuations of magnetic polarization8,9,10. According to some schools of thought, the quasiparticle concept does not apply in certain materials or under special conditions11. Collision experiments might therefore help to identify the boundaries of the quasiparticle concept.Footnote 1
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