The study of metal-insulator transitions in crystalline solids is a subject of paramount importance, both from the fundamental point of view and for its relevance to the transport properties of materials. Recently, a metal-insulator transition governed by disorder was observed in crystalline phase-change materials. Here we report on calculations employing Density Functional Theory, which identify the microscopic mechanism that localizes the wave functions and is driving this transition. We show that, in the insulating phase, the electronic states responsible for charge transport are localized inside regions having large vacancy concentrations. The transition to the metallic state is driven by the dissolution of these vacancy clusters and the formation of ordered vacancy layers. These results provide important insights on controlling the wave function localization, which should help to develop conceptually new devices based on multiple resistance states.

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Recently, phase-change materials (PCMs) have gained a lot of interest in the field of active metamaterials and plasmonics due to their switchable optical properties. In the infrared spectral range the huge contrast in the refractive index between an amorphous and a crystalline phase can be employed for nonvolatile tuning of nanoantenna or metasurface resonances. To make use of such concepts in devices, the reversible switching of the active material has to be realized. Here we demonstrate such reversible cycling by applying femtosecond pulses from a Ti:sapphire laser. These optical pulses trigger the phase transitions of the PCM thin film, which is covering infrared nanoantennas. Ge3Sb2Te6 is chosen as the PCM, since it offers very low losses in the infrared spectral range. The layer geometry presented is exceptionally thin (∼1/50 of the operating wavelength) and the design intentionally avoids lossy capping layers. Infrared reflectivity measurements verify the laser-induced resonance shifts of the plasmonic nanoantenna arrays. This switching mechanism opens the possibility to optically perform active, reversible, and nonvolatile tuning of metasurfaces.

Two-dimensional materials are subject to intrinsic and dynamic rippling that modulates their optoelectronic and electromechanical properties. Here, we directly visualize the dynamics of these processes within monolayer transition metal dichalcogenide MoS2 using femtosecond electron scattering techniques as a real-time probe with atomic-scale resolution. We show that optical excitation induces large-amplitude in-plane displacements and ultrafast wrinkling of the monolayer on nanometer length-scales, developing on picosecond time-scales. These deformations are associated with several percent peak strains that are fully reversible over tens of millions of cycles. Direct measurements of electron-phonon coupling times and the subsequent interfacial thermal heat flow between the monolayer and substrate are also obtained. These measurements, coupled with first-principles modeling, provide a new understanding of the dynamic structural processes that underlie the functionality of two-dimensional materials and open up new opportunities for ultrafast strain engineering using all-optical methods.

In phase-change memory devices, a material is cycled between glassy and crystalline states. The highly temperature-dependent kinetics of its crystallization process enables application in memory technology, but the transition has not been resolved on an atomic scale. Using femtosecond x-ray diffraction and ab initio computer simulations, we determined the time-dependent pair-correlation function of phase-change materials throughout the melt-quenching and crystallization process. We found a liquid–liquid phase transition in the phase-change materials Ag4In3Sb67Te26 and Ge15Sb85 at 660 and 610 kelvin, respectively. The transition is predominantly caused by the onset of Peierls distortions, the amplitude of which correlates with an increase of the apparent activation energy of diffusivity. This reveals a relationship between atomic structure and kinetics, enabling a systematic optimization of the memory-switching kinetics.