Localization of charge carriers in crystalline solids has been the subject of numerous investigations over more than half a century. Materials that show a metal-insulator transition without a structural change are therefore of interest. Mechanisms leading to metal-insulator transition include electron correlation (Mott transition) or disorder (Anderson localization), but a clear distinction is difficult. Here we report on a metal-insulator transition on increasing annealing temperature for a group of crystalline phase-change materials, where the metal-insulator transition is due to strong disorder usually associated only with amorphous solids. With pronounced disorder but weak electron correlation, these phase-change materials form an unparalleled quantum state of matter. Their universal electronic behaviour seems to be at the origin of the remarkable reproducibility of the resistance switching that is crucial to their applications in non-volatile-memory devices. Controlling the degree of disorder in crystalline phase-change materials might enable multilevel resistance states in upcoming storage devices.

The electrical switching behavior of GeTe-based phase change memory devices is characterized by time resolved experiments. SET pulses with a duration of less than 16 ns are shown to crystallize the material. Depending on the resistance of the RESET state, the minimum SET pulse duration can even be reduced down to 1 ns. This finding is attributed to the increasing impact of crystal growth upon decreasing switchable volume. Using GeTe or materials with similar crystal growth velocities, hence promises nonvolatile phase change memories with dynamic random access memorylike switching speeds.

Rubrene single crystals have been grown by a vapor-phase process. Two additional compounds that contaminate rubrene have been identified and their structures determined. Single crystals of rubrene show excellent crystallinity and very small rocking curve width. Field effect transistors based on pure rubrene single crystals with colloidal graphite electrodes and Parylene as a dielectric demonstrate a maximal mobility of 13 cm 2 /Vs with strong anisotropy. The mobility increases very slightly with cooling, but decreases significantly at low temperatures.

Controlling thermal transport in solids is of paramount importance for many applications. Often thermal management is crucial for a device's performance, as it affects both reliability and power consumption. A number of intricate concepts have been developed to address this challenge, such as diamond-like coatings to enhance the thermal conductivity or low symmetry complex super-structures to reduce it. Here, a different approach is pursued, where we explore the potential of solids with a high yet controllable degree of disorder. Recently, it has been demonstrated that an unconventionally high degree of structural disorder characterizes a number of crystalline phase-change materials (PCMs). This disorder strongly impacts electronic transport and even leads to disorder induced localization (Anderson localization). This raises the question how thermal transport is affected by such conditions. Here thermal transport in highly disordered crystalline Ge-Sb-Te (GST) based PCMs is investigated. Glass-like thermal properties are observed for several crystalline PCMs, which are attributed to strong scattering by disordered point defects. A systematic study of different compounds along the pseudo-binary line between GeTe and Sb2Te3 reveals that disordered vacancies act as point defects responsible for pronounced phonon scattering. Annealing causes a gradual ordering of the vacancies and leads to a more 'crystal-like' thermal conductivity. While both vibrational and electronic degrees of freedom are affected by disorder, the consequences differ for different stoichiometries. This opens up a pathway to tune electrical and thermal transport by controlling the degree of disorder. Materials with tailored transport properties may not only help to improve power efficiency and scaling in upcoming phase-change memories but are also of fundamental interest in the field of thermoelectric materials.

We simulate the optical fields and optical transmission through nanoarrays of silica rings embedded in thin gold films using the finite-difference-time-domain method. By examining the optical transmission spectra for varying ring geometries we uncover large enhancements in the transmission at wavelengths much longer than the usual cutoffs for cylindrical apertures or where the usual planar surface plasmons or other periodic effects from the array could play a role. We attribute these enhancements to closely coupled cylindrical surface plasmons on the inner and outer surfaces of the rings, and this coupling is more efficient as the inner and outer ring radii approach each other. We confirm this hypothesis by comparing the transmission peaks of the simulation with cylindrical surface plasmon ͑CSP͒ dispersion curves calculated for the geometries of interest. One important result is that a transmission peak appears in the simulations close to the frequency where the longitudinal wave number k z in the ring satisfies k z = m / L, where m is an integer and L the length of the aperture, for a normal CSP TE 1 or TM 1 mode. The behavior of the CSP dispersion is such that propagating modes can be sent through the rings for ever longer wavelengths as the ring radii approach, whereas the transmission decreases only in proportion to the ring area.