Fundamentals and applications of isotope effect in solids

Over the last five decades, the isotope effect in solids has been one of the major researches in solid state science. Most of the physical properties of a solid depend to a greater or lesser degree on its isotopic composition. Scientific interest, technological promise and increased availability of highly enriched isotopes have led to a sharp rise in the number of experimental and theoretical studies with isotopically controlled semiconductor and insulator crystals. A great number of stable isotopes and well-developed methods of their separation have made it possible to date to grow crystals of C, LiH, ZnO, ZnSe, CuCl, GaN, GaAs, US, Cu2O, Si, Ge and alpha-Sn with a controllable isotopic composition. The use of such objects allows the investigation of not only the isotope effects in lattice dynamics (elastic, thermal and vibrational properties) but also the influence of such effects on the electronic states via electron-phonon coupling (the renormalization of the band-to-band transition energy E-g, the exciton binding energy E-b and the size of the longitudinal-transverse splitting Delta(LT)). Capture of thermal neutrons by isotope nuclei followed by nuclear decay produces new elements, resulting in a large number of possibilities for isotope selective doping of solids used in opto-, quantum electronics, fiber optics, etc. The nonlinear dependence of the free exciton luminescence (especially (CxC1-x)-C-12-C-13, LiHxD1-x) intensity on the excitation density allows us to consider these crystals as potential solid state lasers in the UV part of the spectrum. Isotopic information storage may consist in assigning the information zero or one to mono-isotopic microislands (or even to a single atom) within a bulk crystalline (or thin film) structure. Recent theoretical results confirm that quantum theory provides the possibility of new ways of performing efficient calculations. It shows how the use of quantum physics could revolutionize the way of communication and process information. Although modern computers rely on quantum mechanics to operate, the information itself is still encoded classically. A new approach is to treat information as a quantum concept and to ask what new insights can be gained by encoding this information in an individual quantum system. Isotope information storage and isotope quantum computers are briefly discussed. The review concludes with an outline of the main features of isotope physics in solids, and avenues for future research and applications. (c) 2005 Elsevier Ltd. All rights reserved.