Optical data carriers such as DVDs, Blu-rays and CD-RWs store data in layers of so-called "phase-change materials." In the future, these materials will enable the development of fast, nonvolatile and energy-saving main memories.

A prerequisite for the advancement of these materials is low thermal conductivity, as described "Phase-Change Materials: Vibrational Softening upon Crystallization and Its Impact on Thermal Properties," which appears inAdvanced Functional Materials. Part of an international research team, the authors said their findings will facilitate a targeted search for materials with the desired properties.

Phase-change materials, which display a surprisingly low thermal conductivity even in the crystalline state, are among the favorite candidates for developing a universal memory that is as fast as DRAM (dynamic RAM), has high storage density, is always ready for use, and does not lose data even when inadvertently turned off. The data is stored in tiny areas of different electrical resistance, which are written to by heating with the aid of electric pulses. In doing so, the atomic ordering of the material and its electrical resistance is changed.

When heated, phase-change materials switch from the unordered (amorphous) to the ordered (crystalline) state, which leads to a change in their physical properties. This feature has been exploited by industry for many years in optical data carriers such as DVDs, Blu-rays and CD-RWs. By means of a laser, the atomic structure and thus the optical properties are changed in minute areas of the discs, which allows bits to be written to the disc and be read out again by a laser.

Normally, the propagation of sound waves in material correlates with the thermal conductivity. However, this is not the case with phase-change materials, because in the crystalline state, atoms experience resonance bonding - in other words, the bonding electrons are shared between several atomic pairs. In contrast, in amorphous material, the atoms are covalently - more strongly and more locally bonded. The crystalline material is therefore softer and the atoms vibrate more gently. In addition, there is more disorder in the local range. Both of these aspects impair the conductivity for heat carriers, which are partially of short wavelength, but not for the long-wavelength sound waves.