Emergent quantum materials
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Introduction Throughout human history, major advances in technology have been usually accompanied by a revolution in materials. Examples include the invention of bronze tools after the stone age, the displacement of bronze by iron, the development of the metallurgy of steel and aluminum that has been stimulated by and has fueled the industrial revolution, and the optimization and surface passivation of silicon that enabled much of the multibillion-dollar technological sector in the present day. Today, similarly disruptive advances in quantum materials may germinate from the burgeoning field of quantum information science (QIS), which utilizes quantum degrees of freedom for information storage and processing.1 Technologies to manipulate and harness quantum states are poised to revolutionize current paradigms of computation, sensing, storage, and communications. The term “quantum materials” is fairly broad, encompassing all materials whose properties are largely determined by quantum mechanical principles and phenomena. A key distinction of quantum materials from other materials lies in the manifestation of quantum mechanical effects at macroscopic length scales. In fact, all materials are composed of basic quantum particles and quasiparticles (i.e., electrons, holes, spins, and phonons) on the foundation of quantum mechanical principles at microscopic length scales; for example, the
wave-particle duality of basic quantum particles or quasiparticles, and uncertainty in energy/momentum of basic particles or quasiparticles. However, the quantum mechanical effects manifested at the molecular and atomic scale of classical materials are overwhelmed by the classical statistical mechanics of a large particle ensemble in macroscopic length scales. Generally, quantum materials are characterized by at least one, and often several, of the following attributes. The first attribute is the quantum confinement of basic quantum particles and quasiparticles (i.e., electrons, holes, excitons, spins, and phonons) due to the reduced dimensionality of materials. Quantum confinement occurs when the physical dimension of the materials is comparable to or smaller than the characteristic length scale of quantum particles. The definition of characteristic length scale varies with quantum particles, for instance, as the physical extension of excitonic wave function (excitonic Bohr radius) for excitons and as mean scattering-free paths for electrons, holes, spins, and phonons. Quantum confinement leads to properties that are either dramatically modified from the bulk or entirely new properties not found in the parent materials, such as the size-dependent energy-level spacing of charges in semiconductor nanocrystals that form the basis of quantum dot displays,2 and the controlled entangled spin states in semiconductor quantum dots that are now a promising platform for quantum computation.3
Chun Ning Lau, Department of Physics, The Ohio State University, USA; [email protected] Fengnian Xia, Department of Electrical Engineering, Yale University, U
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