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- ItemStructural Transitions in Ge2Sb2Te5 Phase Change Memory Thin Films Induced by Nanosecond UV Optical Pulses(Basel : MDPI, 2020) Behrens, Mario; Lotnyk, Andriy; Bryja, Hagen; Gerlach, Jürgen W.; Rauschenbach, BerndGe-Sb-Te-based phase change memory alloys have recently attracted a lot of attention due to their promising applications in the fields of photonics, non-volatile data storage, and neuromorphic computing. Of particular interest is the understanding of the structural changes and underlying mechanisms induced by short optical pulses. This work reports on structural changes induced by single nanosecond UV laser pulses in amorphous and epitaxial Ge2Sb2Te5 (GST) thin films. The phase changes within the thin films are studied by a combined approach using X-ray diffraction and transmission electron microscopy. The results reveal different phase transitions such as crystalline-to-amorphous phase changes, interface assisted crystallization of the cubic GST phase and structural transformations within crystalline phases. In particular, it is found that crystalline interfaces serve as crystallization templates for epitaxial formation of metastable cubic GST phase upon phase transitions. By varying the laser fluence, GST thin films consisting of multiple phases and different amorphous to crystalline volume ratios can be achieved in this approach, offering a possibility of multilevel data storage and realization of memory devices with very low resistance drift. In addition, this work demonstrates amorphization and crystallization of GST thin films by using only one UV laser with one single pulse duration and one wavelength. Overall, the presented results offer new perspectives on switching pathways in Ge-Sb-Te-based materials and show the potential of epitaxial Ge-Sb-Te thin films for applications in advanced phase change memory concepts.
- ItemDynamical Phase Transitions for Flows on Finite Graphs(New York, NY [u.a.] : Springer Science + Business Media B.V., 2020) Gabrielli, Davide; Renger, D.R. MichielWe study the time-averaged flow in a model of particles that randomly hop on a finite directed graph. In the limit as the number of particles and the time window go to infinity but the graph remains finite, the large-deviation rate functional of the average flow is given by a variational formulation involving paths of the density and flow. We give sufficient conditions under which the large deviations of a given time averaged flow is determined by paths that are constant in time. We then consider a class of models on a discrete ring for which it is possible to show that a better strategy is obtained producing a time-dependent path. This phenomenon, called a dynamical phase transition, is known to occur for some particle systems in the hydrodynamic scaling limit, which is thus extended to the setting of a finite graph.
- ItemModelling and simulation of flame cutting for steel plates with solid phases and melting(Berlin ; Heidelberg : Springer, 2020) Arenas, Manuel J.; Hömberg, Dietmar; Lasarzik, Robert; Mikkonen, Pertti; Petzold, ThomasThe goal of this work is to describe in detail a quasi-stationary state model which can be used to deeply understand the distribution of the heat in a steel plate and the changes in the solid phases of the steel and into liquid phase during the flame cutting process. We use a 3D-model similar to previous works from Thiébaud (J. Mater. Process. Technol. 214(2):304–310, 2014) and expand it to consider phases changes, in particular, austenite formation and melting of material. Experimental data is used to validate the model and study its capabilities. Parameters defining the shape of the volumetric heat source and the power density are calibrated to achieve good agreement with temperature measurements. Similarities and differences with other models from literature are discussed. © 2020, The Author(s).
- ItemSuitability of binary oxides for molecular-beam epitaxy source materials: A comprehensive thermodynamic analysis(Melville, NY : AIP Publ., 2020) Adkison, Kate M.; Shang, Shun-Li; Bocklund, Brandon J.; Klimm, Detlef; Schlom, Darrell G.; Liu, Zi-KuiWe have conducted a comprehensive thermodynamic analysis of the volatility of 128 binary oxides to evaluate their suitability as source materials for oxide molecular-beam epitaxy (MBE). 16 solid or liquid oxides are identified that evaporate nearly congruently from stable oxide sources to gas species: As2O3, B2O3, BaO, MoO3, OsO4, P2O5, PbO, PuO2, Rb2O, Re2O7, Sb2O3, SeO2, SnO, ThO2, Tl2O, and WO3. An additional 24 oxides could provide molecular beams with dominant gas species of CeO, Cs2O, DyO, ErO, Ga2O, GdO, GeO, HfO, HoO, In2O, LaO, LuO, NdO, PmO, PrO, PuO, ScO, SiO, SmO, TbO, Te2O2, U2O6, VO2, and YO2. The present findings are in close accord with available experimental results in the literature. For example, As2O3, B2O3, BaO, MoO3, PbO, Sb2O3, and WO3 are the only oxides in the ideal category that have been used in MBE. The remaining oxides deemed ideal for MBE awaiting experimental verification. We also consider two-phase mixtures as a route to achieve the desired congruent evaporation characteristic of an ideal MBE source. These include (Ga2O3 + Ga) to produce a molecular beam of Ga2O(g), (GeO2 + Ge) to produce GeO(g), (SiO2 + Si) to produce SiO(g), (SnO2 + Sn) to produce SnO(g), etc.; these suboxide sources enable suboxide MBE. Our analysis provides the vapor pressures of the gas species over the condensed phases of 128 binary oxides, which may be either solid or liquid depending on the melting temperature. © 2020 Author(s).
- ItemSelf-stabilization of the equilibrium state in ferroelectric thin films(Amsterdam : Elsevier, 2022) Gaal, Peter; Schmidt, Daniel; Khosla, Mallika; Richter, Carsten; Boesecke, Peter; Novikov, Dmitri; Schmidbauer, Martin; Schwarzkopf, Jutta(K,Na)NbO3 is a lead-free and sustainable ferroelectric material with electromechanical parameters comparable to Pb(Zr,Ti)O3 (PZT) and other lead-based solid solutions. It is therefore a promising candidate for caloric cooling and energy harvesting applications. Specifically, the structural transition from the low-temperature Mc- to the high-temperature c-phase displays a rich hierarchical order of domains and superdomains, that forms at specific strain conditions. The relevant length scales are few tens of nanometers for the domain and few micrometers for the superdomain size, respectively. Phase-field calculations show that this hierarchical order adds to the total free energy of the solid. Thus, domains and their formation has a strong impact on the functional properties relevant for electrocaloric cooling or energy harvesting applications. However, monitoring the formation of domains and superdomains is difficult and requires both, high spatial and high temporal resolution of the experiment. Synchrotron-based time-resolved X-ray diffraction methods in combination with scanning imaging X-ray microscopy is applied to resolve the local dynamics of the domain morphology with sub-micrometer spatial and nanosecond temporal resolution. In this regime, the material displays a novel self-stabilization mechanism of the domain morphology, which may be a general property of first-order phase transitions.
- ItemTracking ultrafast solid-state dynamics using high harmonic spectroscopy(College Park, MD : APS, 2021) Bionta, Mina R.; Haddad, Elissa; Leblanc, Adrien; Gruson, Vincent; Lassonde, Philippe; Ibrahim, Heide; Chaillou, Jérémie; Émond, Nicolas; Otto, Martin R.; Siwick, Bradley J.; Chaker, Mohamed; Légaré, FrançoisWWe establish time-resolved high harmonic generation (tr-HHG) as a powerful spectroscopy method for tracking photoinduced dynamics in strongly correlated materials through a detailed investigation of the insulator-to-metal phase transitions in vanadium dioxide. We benchmark the technique by comparing our measurements to established momentum-resolved ultrafast electron diffraction, and theoretical density functional calculations. Tr-HHG allows distinguishing of individual dynamic channels, including a transition to a thermodynamically hidden phase. In addition, the HHG yield is shown to be modulated at a frequency characteristic of a coherent phonon of the equilibrium monoclinic phase over a wide range of excitation fluences. These results demonstrate that tr-HHG is capable of tracking complex dynamics in solids through its sensitivity to the band structure.
- ItemQuantitative hyperspectral coherent diffractive imaging spectroscopy of a solid-state phase transition in vanadium dioxide(Washington, DC [u.a.] : Assoc., 2021) Johnson, Allan S.; Conesa, Jordi Valls; Vidas, Luciana; Perez-Salinas, Daniel; Günther, Christian M.; Pfau, Bastian; Hallman, Kent A.; Haglund, Richard F.; Eisebitt, Stefan; Wall, SimonSolid-state systems can host a variety of thermodynamic phases that can be controlled with magnetic fields, strain, or laser excitation. Many phases that are believed to exhibit exotic properties only exist on the nanoscale, coexisting with other phases that make them challenging to study, as measurements require both nanometer spatial resolution and spectroscopic information, which are not easily accessible with traditional x-ray spectromicroscopy techniques. Here, we use coherent diffractive imaging spectroscopy (CDIS) to acquire quantitative hyperspectral images of the prototypical quantum material vanadium oxide across the vanadium L2,3 and oxygen K x-ray absorption edges with nanometer-scale resolution. We extract the full complex refractive indices of the monoclinic insulating and rutile conducting phases of VO2 from a single sample and find no evidence for correlation-driven phase transitions. CDIS will enable quantitative full-field x-ray spectromicroscopy for studying phase separation in time-resolved experiments and other extreme sample environments where other methods cannot operate.