A class of chalcogenide alloy materials that shows significant changes in optical properties upon an amorphous-to-crystalline phase transition has lead to development of large data capacities in modern optical data storage. Among chalcogenide phase-change materials, Ge2Sb2Te5 (GST) is most widely used because of its reliability. We use a pair of femtosecond light pulses to demonstrate the ultrafast optical manipulation of atomic arrangements from tetrahedral (amorphous) to octahedral (crystalline) Ge-coordination in GST superlattices. Depending on the parameters of the second pump-pulse, ultrafast nonthermal phase-change occurred within only few-cycles (≈1 picosecond) of the coherent motion corresponding to a GeTe4 local vibration. Using the ultrafast switch in chalcogenide alloy memory could lead to a major paradigm shift in memory devices beyond the current generation of silicon-based flash-memory.

Interfacial phase change memory (iPCM), that has a structure of a superlattice made of alternating atomically thin GeTe and Sb2Te3 layers, has recently attracted attention not only due to its superior performance compared to the alloy of the same average composition in terms of energy consumption but also due to its strong response to an external magnetic field (giant magnetoresistance) that has been speculated to arise from switching between topological insulator (RESET) and normal insulator (SET) phases. Here we report magneto-optical Kerr rotation loops in the visible range, that have mirror symmetric resonances with respect to the magnetic field polarity at temperatures above 380 K when the material is in the SET phase that has Kramers-pairs in spin-split bands. We further found that this threshold temperature may be controlled if the sample was cooled in a magnetic field. The observed results open new possibilities for use of iPCM beyond phase-change memory applications.

No abstract

We have used a femtosecond pump-probe impulsive Raman technique to explore the ultrafast dynamics of micelle suspended single-walled carbon nanotubes ͑SWNTs͒ in various pH environments. The structures of coherent phonon spectra of the radial breathing modes exhibit significant pH dependence, to which we attribute the effect of the protonation at the surface of SWNTs, resulting in the modification of electronic properties of semiconductor SWNTs. Analysis of the time-domain data using a time-frequency transformation uncovers also a second transient longitudinal breathing mode, which vanishes after 1 ps of the photoexcitation. DOI: 10.1103/PhysRevB.80.245428 PACS number͑s͒: 78.47.JϪ, 78.67.Ch, 63.22.Gh, 63.20.kd Carbon nanotubes ͑CNTs͒ are one of the most intriguing nanomaterials, which are useful for molecular devices, such as a chemical sensor 1 and a nanomachine capable for medical science. 2 One potential way to realize the nanomachine by the use of CNTs, which can work in biological systems, e.g., a drug delivery system, is to use them in solution with proteins. 2 Therefore, CNT devices will be used in various pH environments in biological systems, e.g., the typical pH of human arterial blood is Ϸ7.4 that is weakly alkaline. Since the chemical reactivity of CNTs is dominated by the fundamental physical processes on their surface, investigation of dynamics at surfaces, such as charge transfer, 3 excitonplasmon coupling, 4 and electron-phonon energy transfer, 5 is crucial to prepare suitable environments for CNT devices.Single-walled carbon nanotubes ͑SWNTs͒ possess simplest structure of CNTs and it has metallic or semiconducting conductivity, depending on the arrangement of carbon atoms, referred as roll-up vectors. 6 Recently frequency-domain spectroscopy has revealed the diameter-selective Raman scattering from radial breathing modes ͑RBMs͒ in SWNT,7,8 where the frequency of the RBMs depended on the inverse of the diameter. 3 Moreover, Strano et al. 9 have investigated the change in the Raman spectra of RBM induced by the protonation and showed that the protons change electronic structure of SWNT, i.e., protonation is band-gap selective and consequently the condition of resonant Raman scattering was modified, depending on the pH values. Coherent phonon spectroscopy is another potential method to study phonon dynamics in time-domain. Up to date, there have been only several studies on ultrafast phenomena in CNTs, 10-14 and early time stage dynamics of photoexcitation of electronic system and subsequent phonon excitation and relaxation was not been well explored. Motivated by the observation of the early time stage dynamics of electron-phonon coupling, Lüer et al. 15 have recently investigated the electron-phonon coupling by means of coherent phonon spectroscopy with 10 fs time resolution, where they focused their attention to the high-frequency optical modes rather than the RBMs.In this paper, the coherent oscillation of the RBMs in SWNTs has been measured in various pH environments to show that the subpic...

Ge–Sb–Te (GST) phase change materials exhibit a metal-to-insulator transition and therefore are expected to be useful for a variety of terahertz wave applications in addition to their primary application in optical and electrical memory devices.