Elementary excitations in isotope-mixed crystals

Over the last five decades, the isotope effect has been one of the major research in solids. 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 has made it possible to date to grow crystals of C, LiH, ZnO, ZnSe, CuCl, GaN, GaAs, CdS, Cu 2 O , Si, Ge and α -Sn with a controllable isotopic composition. The use of such objects allows the investigation of not only the isotope effects in lattice dynamics (vibrational, elastic and thermal 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 Δ LT ). The thermal conductivity enhancement in the isotopically enriched materials amounts (C; Ge; Si) to almost 10% at room temperature and is close to a factor of six at the thermal conductivity maximum around 20 K (Si-case). The change in the lattice constant is Δ a / a ∼ 10 - 3 – 10 - 4 , while the change δ c ik in the elastic constants amounts to several percent. The nonlinear dependence of the free exciton luminescence (especially C x 13 12 C 1 - x , LiH x D 1 - x ) intensity on the excitation density allows 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 single atoms) within a bulk crystalline (or thin film) structure. Isotope information storage and isotope quantum computers are briefly discussed.