Quantum Materials: Where Many Paths Meet
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Quantum materials: Where many paths meet By Philip Ball
A
ll matter, in the end, must be explained by quantum mechanics, which describes how atoms bind and electrons interact at a fundamental level. Typically, the quantum behavior can be approximated by a classical description, in which atoms become balls that stick together in welldefined arrangements via simple forces and vibrate much like balls on springs. Sometimes, however, that is not so. In some materials, the quantum aspects assert themselves tenaciously, and the only way to fully understand how the material behaves is to keep the quantum in view. Such substances are now grouped together under the banner of quantum materials. The US Department of Energy (DOE) describes quantum materials as “solids with exotic physical properties, arising from the quantum mechanical properties of their constituent electrons, that have great scientific and/or technological potential.” It’s a diverse class of materials—so much so that some question whether the designation is meaningful. It includes well-advertised materials such as superconductors and graphene, along with ones with less familiar names: topological insulators, Weyl semimetals, quantum spin liquids, and spin ices. A collection of oddballs they may be, but quantum materials are both a treasure trove of interesting physics and a potential source of useful substances. And while the compositions and behaviors of quantum materials cover a wide spectrum, some common themes recur. Many of them derive their properties from reduced dimensionality, in particular from confinement of electrons to two-dimensional (2D) sheets. In many, magnetism and electronic structure interact in curious ways. In particular, they tend to be materials in which electrons cannot be considered as independent particles but act as collective states dubbed quasiparticles.
Aside from the matter of their intrinsic properties, however, what tends to unite quantum materials is who is interested in them. The community of researchers that 20 years ago was grappling with high-temperature superconductivity is likely today to be pondering topological insulators and Weyl semimetals. A somewhat distinct community that used to focus around the 1980s on the exotic 2D phenomenon called the quantum Hall effect is now finding common cause. And quantum materials draw inspiration from a third direction too, apparently unlikely at first flush: particle physics, within which some unusual types of fundamental particles proposed around the mid-to-late 20th century are now finding analogues in the quasiparticles of condensed matter. This is not a matter of leaping onboard the latest fancy-titled bandwagon. Rather, it is a reflection of a common experience in physics, whereby concepts developed to explore one phenomenon turn out to be a subset of more general principles. Quantum materials reveal that properties once thought to be quirks confined to exotic conditions are in fact a significant feature of the materials universe. To turn those ideas into real materials, me
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