Collective and quasiparticle properties of nanocrystals and their arrays

The fundamental competition between order and disorder lies at the heart of materials science and technology. Interactions between atoms or electrons with sub nanometer spacings can lead to collective organization into states with long-range order. Order in electronic states gives rise to physical consequences such as magnetism, ferroelectricity and superconductivity. In bulk materials, the collective electronic properties associated with ferromagnetic and superconducting order yield effects that are exceptionally robust and useful. Collective organization of quasiparticles gives rise to a number of characteristic length scales. Examples include the coherence length and the minimum grain size in superconductors, and in ferromagnets the domain wall width, the superparamagnetic limit, and the maximum single-domain radius. These length scales generally exceed or are comparable to geometrical parameters of nanostructured materials and are, on the other hand, not far removed from the microscopic length scales such as inter-electron spacing. For these reasons nanostructured materials represent an important new frontier for the study of collective electronic behavior. This interdisciplinary research team will study the physical consequences of nanometer-size dimensions on collective and independent electron properties in individual nanocrystals and in controlled nanocrystal arrays. This program brings together expertise in the epitaxial growth of self-assembled semiconductor quantum dots, expertise in colloidal synthesis of both metal and semiconductor nanoparticles, expertise in nanoscale optical spectroscopy, expertise in scanning tunneling and magnetic force microscopy, SQUID magnetometry and cantilever micro-magnetometry, and expertise in mesoscopic and many-body condensed matter theory. We intend to work closely together, with materials synthesis efforts informed by theoretical ideas on interesting effects that might be realized in novel nanoparticle systems, and the modeling and theory effort informed by materials ideas on nanoparticle synthesis. Both efforts will ultimately be guided by optical, magnetic, tunneling, and transport measurements of nanoparticle physical properties. The materials systems we will focus on will be the controlled two-and three-dimensional arrays of metal and semiconductor nanocrystals realized by colloidal synthesis, and molecular beam epitaxy. We intend to study ferromagnetic, superconducting, and normal metal nanocrystals, and ferromagnetic, semimagnetic, and normal semiconductor quantum dots. Coulomb blockade and independent electron properties of both metallic and semiconductor nanoparticles have already been extensively studied. In ferromagnetic and superconducting systems the emphasis to date has been on the collective properties that underlie, for example, the use of ferromagnets for information storage and the potential use of small superconductors as quantum-bits. As these particles become smaller, the physics of the magnetic anisotropy …