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- ItemThe influence of Mg doping on the nucleation of self-induced GaN nanowires(New York : American Institute of Physics, 2012) Limbach, F.; Caterino, R.; Gotschke, T.; Stoica, T.; Calarco, R.; Geelhaar, L.; Riechert, H.GaN nanowires were grown without any catalyst by plasma-assisted molecular beam epitaxy. Under supply of Mg, nanowire nucleation is faster, the areal density of wires increases to a higher value, and nanowire coalescence is more pronounced than without Mg. During nanowire nucleation the Ga desorption was monitored insitu by line-of-sight quadrupolemass spectrometry for various substrate temperatures. Nucleation energies of 4.0±0.3 eV and 3.2±0.3 eV without and with Mg supply were deduced, respectively. This effect has to be taken into account for the fabrication of nanowire devices and could be employed to tune the NW areal density.
- 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.
- ItemLarge superplastic strain in non-modulated epitaxial Ni-Mn-Ga films(Amsterdam : Elsevier, 2010) Yeduru, S.R.; Backen, A.; Fahler, S.; Schultz, L.; Kohl, M.The phase transformation and superplastic characteristics of free-standing epitaxial Ni-Mn-Ga stripes are reported. The stripes are prepared by micromachining a 1 μm thick Ni-Mn-Ga film sputter-deposited on a single crystalline MgO (100) substrate using optical lithography and a Chromium-based sacrificial layer technology. The stripes are oriented at angles of 0 and 45 degrees with respect to the Ni-Mn-Ga unit cell. Electrical resistance versus temperature characteristics reveal a reversible thermally induced phase transformation between 169°C and 191°C. Stress-strain measurements are performed with the stress applied along the [100]Ni-Mn-Ga as well as [110]Ni-Mn-Ga direction. Depending on the orientation, the twinning stress ranges between 25 and 30 MPa, respectively. For the [100] Ni-Mn-Ga and [110]Ni-Mn-Ga directions, superplastic behaviour with a strain plateau of 12 % and 4% are observed, respectively, indicating stress-induced reorientation of non-modulated martensite variants.
- ItemNonequilibrium phase transitions in finite arrays of globally coupled Stratonovich models: Strong coupling limit(College Park, MD : Institute of Physics Publishing, 2009) Senf, F.; Altrock, P.M.; Behn, U.A finite array of N globally coupled Stratonovich models exhibits a continuous nonequilibrium phase transition. In the limit of strong coupling, there is a clear separation of timescales of centre of mass and relative coordinates. The latter relax very fast to zero and the array behaves as a single entity described by the centre of mass coordinate. We compute analytically the stationary probability distribution and the moments of the centre of mass coordinate. The scaling behaviour of the moments near the critical value of the control parameter ac(N) is determined. We identify a crossover from linear to square root scaling with increasing distance from ac. The crossover point approaches ac in the limit N →∞ which reproduces previous results for infinite arrays. Our results are obtained in both the Fokker-Planck and the Langevin approach and are corroborated by numerical simulations. For a general class of models we show that the transition manifold in the parameter space depends on N and is determined by the scaling behaviour near a fixed point of the stochastic flow. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
- ItemFerroelectric switching in epitaxial GeTe films(New York : American Institute of Physics, 2014) Kolobov, A.V.; Kim, D.J.; Giussani, A.; Fons, P.; Tominaga, J.; Calarco, R.; Gruverman, A.In this paper, using a resonance-enhanced piezoresponse force microscopy approach supported by density functional theory computer simulations, we have demonstrated the ferroelectric switching in epitaxial GeTe films. It has been shown that in films with thickness on the order of several nanometers reversible reorientation of polarization occurs due to swapping of the shorter and longer Ge-Te bonds in the interior of the material. It is also hinted that for ultra thin films consisting of just several atomic layers weakly bonded to the substrate, ferroelectric switching may proceed through exchange of Ge and Te planes within individual GeTe layers.
- 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.
- ItemAbsorption and photoemission spectroscopy of rare-earth oxypnictides(Milton Park : Taylor & Francis, 2009) Kroll, T.; Roth, F.; Koitzsch, A.; Kraus, R.; Batchelor, D.R.; Werner, J.; Behr, G.; Büchner, B.; Knupfer, M.The electronic structure of various rare-earth oxypnictides has been investigated by performing Fe L2, 3 x-ray absorption spectroscopy, and Fe 2p and valence band x-ray photoemission spectroscopy. As representative samples the non-superconducting parent compounds LnFeAsO (Ln=La, Ce, Sm and Gd) have been chosen and measured at 25 and 300 K, i.e. below and above the structural and magnetic phase transition at ~150 K. We find no significant change of the electronic structure of the FeAs layers when switching between the different rare-earth ions or when varying the temperature below and above the transition temperatures. Using a simple two-configuration model, we find qualitative agreement with the Fe 2p3/2 core-level spectrum, which allows for a qualitative explanation of the experimental spectral shapes.
- 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.