Quantum confinement in oxide quantum wells
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Introduction Quantum confinement involves the use of spatial modulations of chemical composition and electric fields to localize electrons to regions that are sufficiently small (at least in one direction) that their quantum mechanical properties are affected. Quantum wells, sketched in Figure 1, are basic to semiconductor science and technology.1 In its simplest form, a quantum well involves a thin layer of one material, such as InGaN, sandwiched between thick layers of another (often a wider bandgap) material, such as GaN. This creates a potential well in which carriers can move in two directions but are confined in the third. The basic ideas of quantum confinement—using a gradient of the electrochemical potential, such as band offsets, band bending, and applied fields, to localize the electrons in particular spatial regions—apply to transition metal oxides, but the physics is much richer. First and foremost, electrons in the narrow d-bands of transition metal oxides are subject to strong electron-electron interactions (thus the electrons are referred to as being “correlated,” and materials containing such electrons are referred to as “correlated electron materials”). The electron-electron interactions (“correlations”) lead to a rich variety of physical phenomena that can be accessed, modified, and controlled in quantum wells. Examples include magnetism with high Curie temperature, “Mott” (electron correlation-driven) metal-to-insulator transitions,
high transition temperature (Tc) superconductivity, and unique charge and magnetic order. The electron densities required to obtain the Mott insulator and many of the related correlation-driven effects are typically very high, on the order of one electron per 1022 cm–3, or a sheet carrier density of several 1014 cm–2 in a single atomic plane.2 Such sheet carrier densities are an order of magnitude higher than the highest density two-dimensional electron gases (2DEGs) achievable in conventional semiconductors (the III-nitrides). One of the unique aspects of oxide quantum wells is that these densities can be achieved.3–5 The article by Hilgenkamp in this issue discusses one example of such an interface, namely that between the band insulators LaAlO3 and SrTiO3. In this article, we discuss the issues arising in the theoretical description of new high density oxide quantum wells, describe some aspects of the materials, fabrication techniques and diagnostics used, and discuss a few prototypical examples at the forefront of current research interests.
Theoretical challenges Theoretical and practical challenges arise for high electron density quantum wells. The high carrier densities imply that the physics can be much more local than in conventional semiconductors, because the interelectron distances become comparable to the distances between atoms in the crystal lattice. Materials must be controlled on subnanometer length
Susanne Stemmer, Materials Department, University of California, Santa Barbara, CA; [email protected] Andrew J. Millis, Department of Physics, Columbia Unive
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