Phase Transitions at the Nanoscale in Functional Materials
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the Nanoscale in Functional Materials Avadh Saxena and Gabriel Aeppli, Guest Editors
Abstract Many properties of functional materials are quite different at the nanoscale, because at this length scale, the surface/interface energy becomes comparable to the bulk energy. Thus, the nature of various phase transitions at the nanoscale (e.g., structural, electronic, magnetic, metal-insulator) is altered. In addition, in functional materials with many coupled order parameters, quantum effects can dominate the response. We use the term nanoscale with three different context-specific connotations: it could refer to a cluster of atoms or molecules, a confined geometry as in a nanoscale grain or a superlattice, and a nanoscale region in the bulk. This field is still in its infancy, and much needs to be learned in terms of nucleation and thermodynamics at this scale. Materials of interest that we consider in this issue include, but are not limited to, ferroics (ferroelectrics, ferromagnets, ferroelastics), multiferroics (magnetoelectrics, ferrotoroidics), and complex functional materials such as those that exhibit colossal magnetoresistance and high-temperature superconductivity, including the recently discovered iron pnictide superconductors. Superconductors provide a fertile ground for quantum phase transitions.
Introduction Functional materials possess a specific functionality (e.g., piezoelectricity or magnetoresistance) and exhibit a sensitive dependence of their physical and chemical properties on the environment, such as temperature, pressure, electric and magnetic fields, chemical doping, and optical wavelength. The functionality usually emerges below a certain critical temperature or pressure as a result of a phase transition. These aspects make them very desirable for applications involving control, sensing, actuation, and information storage. In this article, we describe key materials issues in the context of phase transitions at the nanoscale. In certain cases, the phase transition may be arrested below a critical size because the surface energy overwhelms the bulk energy. We comment on the physics of nanoscale phase transitions compared with those of bulk transitions. In addition, the meaning of various tensor quantities, 1 such as elastic constants, electrostrictive coefficients, and magnetostric804
tive coefficients, becomes either different or not well-defined because the definition of these quantities assumes underlying crystal translation symmetry, which is missing at the nanoscale due to relaxation near the surface of a cluster or nanocrystal. We discuss both the fundamental materials issues and potential applications of nanoscale transitions as well as propose open questions. Nanoscale microstructures (hereafter abbreviated as nanostructures) have been widely observed in a broad range of important materials, including ferroelastics (e.g., martensites and shape memory alloys2), ferroelectrics, ferromagnets, and strongly correlated systems (such as hightemperature superconductors and colossal magnetoresis
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