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- ItemKMHK 1762: Another star cluster in the Large Magellanic Cloud age gap(Les Ulis : EDP Sciences, 2022) Gatto, M.; Ripepi, V.; Bellazzini, M.; Tosi, M.; Tortora, C.; Cignoni, M.; Dall’Ora, M.; Cioni, M.-R.L.; Cusano, F.; Longo, G.; Marconi, M.; Musella, I.; Schipani, P.; Spavone, M.Context. The star cluster (SC) age distribution of the Large Magellanic Cloud (LMC) exhibits a gap from ∼4 to 10 Gyr ago, with an almost total absence of SCs. Within this age gap, only two confirmed SCs have been identified hitherto. Nonetheless, the star field counterpart does not show the same characteristics, making the LMC a peculiar galaxy where the star formation history and cluster formation history appear to differ significantly. Aims. We re-analysed the colour-magnitude diagram (CMD) of the KMHK 1762 SC by using the deep optical photometry provided by the ‘Yes, Magellanic Clouds Again’ survey, so as to robustly assess its age. Methods. First, we partially removed foreground and/or field stars by means of parallaxes and proper motions obtained from the Gaia Early Data Release 3. Then, we applied the Automated Stellar Cluster Analysis package to the cleaned photometric catalogue to identify the isochrone that best matches the CMD of KMHK 1762. Results. The estimated age of KMHK 1762 is log(t) = 9.74 ± 0.15 dex (∼5.5 Gyr), which is more than 2 Gyr older than the previous estimation which was obtained with shallower photometry. This value makes KMHK 1762 the third confirmed age-gap SC of the LMC. Conclusions. The physical existence of a quiescent period of the LMC SC formation is questioned. We suggest it can be the result of an observational bias, originating from the combination of shallow photometry and limited investigation of the LMC periphery.
- ItemDevelopment of structural correlations and synchronization from adaptive rewiring in networks of Kuramoto oscillators(Woodbury, NY : American Institute of Physics, 2017) Papadopoulos, Lia; Kim, Jason Z.; Kurths, Jürgen; Bassett, Danielle S.Synchronization of non-identical oscillators coupled through complex networks is an important example of collective behavior, and it is interesting to ask how the structural organization of network interactions influences this process. Several studies have explored and uncovered optimal topologies for synchronization by making purposeful alterations to a network. On the other hand, the connectivity patterns of many natural systems are often not static, but are rather modulated over time according to their dynamics. However, this co-evolution and the extent to which the dynamics of the individual units can shape the organization of the network itself are less well understood. Here, we study initially randomly connected but locally adaptive networks of Kuramoto oscillators. In particular, the system employs a co-evolutionary rewiring strategy that depends only on the instantaneous, pairwise phase differences of neighboring oscillators, and that conserves the total number of edges, allowing the effects of local reorganization to be isolated. We find that a simple rule-which preserves connections between more outof- phase oscillators while rewiring connections between more in-phase oscillators-can cause initially disordered networks to organize into more structured topologies that support enhanced synchronization dynamics. We examine how this process unfolds over time, finding a dependence on the intrinsic frequencies of the oscillators, the global coupling, and the network density, in terms of how the adaptive mechanism reorganizes the network and influences the dynamics. Importantly, for large enough coupling and after sufficient adaptation, the resulting networks exhibit interesting characteristics, including degree-frequency and frequency-neighbor frequency correlations. These properties have previously been associated with optimal synchronization or explosive transitions in which the networks were constructed using global information. On the contrary, by considering a time-dependent interplay between structure and dynamics, this work offers a mechanism through which emergent phenomena and organization can arise in complex systems utilizing local rules.
- ItemDisparate ultrafast dynamics of itinerant and localized magnetic moments in gadolinium metal([London] : Nature Publishing Group UK, 2015) Frietsch, B.; Bowlan, J.; Carley, R.; Teichmann, M.; Wienholdt, S.; Hinzke, D.; Nowak, U.; Carva, K.; Oppeneer, P. M.; Weinelt, M.The Heisenberg–Dirac intra-atomic exchange coupling is responsible for the formation of the atomic spin moment and thus the strongest interaction in magnetism. Therefore, it is generally assumed that intra-atomic exchange leads to a quasi-instantaneous aligning process in the magnetic moment dynamics of spins in separate, on-site atomic orbitals. Following ultrashort optical excitation of gadolinium metal, we concurrently record in photoemission the 4f magnetic linear dichroism and 5d exchange splitting. Their dynamics differ by one order of magnitude, with decay constants of 14 versus 0.8 ps, respectively. Spin dynamics simulations based on an orbital-resolved Heisenberg Hamiltonian combined with first-principles calculations explain the particular dynamics of 5d and 4f spin moments well, and corroborate that the 5d exchange splitting traces closely the 5d spin-moment dynamics. Thus gadolinium shows disparate dynamics of the localized 4f and the itinerant 5d spin moments, demonstrating a breakdown of their intra-atomic exchange alignment on a picosecond timescale.
- ItemOptical inter-site spin transfer probed by energy and spin-resolved transient absorption spectroscopy([London] : Nature Publishing Group UK, 2020) Willems, Felix; von Korff Schmising, Clemens; Strüber, Christian; Schick, Daniel; Engel, Dieter W.; Dewhurst, J. K.; Elliott, Peter; Sharma, Sangeeta; Eisebitt, StefanOptically driven spin transport is the fastest and most efficient process to manipulate macroscopic magnetization as it does not rely on secondary mechanisms to dissipate angular momentum. In the present work, we show that such an optical inter-site spin transfer (OISTR) from Pt to Co emerges as a dominant mechanism governing the ultrafast magnetization dynamics of a CoPt alloy. To demonstrate this, we perform a joint theoretical and experimental investigation to determine the transient changes of the helicity dependent absorption in the extreme ultraviolet spectral range. We show that the helicity dependent absorption is directly related to changes of the transient spin-split density of states, allowing us to link the origin of OISTR to the available minority states above the Fermi level. This makes OISTR a general phenomenon in optical manipulation of multi-component magnetic systems.
- ItemIdentifying causal gateways and mediators in complex spatio-temporal systems(London : Nature Publishing Group, 2015) Runge, J.; Petoukhov, V.; Donges, J.F.; Hlinka, J.; Jajcay, N.; Vejmelka, M.; Hartman, D.; Marwan, N.; Paluš, M.; Kurths, J.
- ItemUnraveling the nature of spin excitations disentangled from charge contributions in a doped cuprate superconductor([London] : Nature Publishing Group, 2022) Zhang, Wenliang; Agrapidis, Cliò Efthimia; Tseng, Yi; Asmara, Teguh Citra; Paris, Eugenio; Strocov, Vladimir N.; Giannini, Enrico; Nishimoto, Satoshi; Wohlfeld, Krzysztof; Schmitt, ThorstenThe nature of the spin excitations in superconducting cuprates is a key question toward a unified understanding of the cuprate physics from long-range antiferromagnetism to superconductivity. The intense spin excitations up to the over-doped regime revealed by resonant inelastic X-ray scattering bring new insights as well as questions like how to understand their persistence or their relation to the collective excitations in ordered magnets (magnons). Here, we study the evolution of the spin excitations upon hole-doping the superconducting cuprate Bi2Sr2CaCu2O8+δ by disentangling the spin from the charge excitations in the experimental cross section. We compare our experimental results against density matrix renormalization group calculations for a t-J-like model on a square lattice. Our results unambiguously confirm the persistence of the spin excitations, which are closely connected to the persistence of short-range magnetic correlations up to high doping. This suggests that the spin excitations in hole-doped cuprates are related to magnons—albeit short-ranged.
- ItemDiffraction imaging of light induced dynamics in xenon-doped helium nanodroplets([London] : IOP, 2022-11-30) Langbehn, B.; Ovcharenko, Y.; Clark, A.; Coreno, M.; Cucini, R.; Demidovich, A.; Drabbels, M.; Finetti, P.; Di Fraia, M.; Giannessi, L.; Grazioli, C.; Iablonskyi, D.; LaForge, A.C.; Nishiyama, T.; Oliver Álvarez de Lara, V.; Peltz, C.; Piseri, P.; Plekan, O.; Sander, K.; Ueda, K.; Fennel, T.; Prince, K.C.; Stienkemeier, F.; Callegari, C.; Möller, T.; Rupp, D.We explore the light induced dynamics in superfluid helium nanodroplets with wide-angle scattering in a pump–probe measurement scheme. The droplets are doped with xenon atoms to facilitate the ignition of a nanoplasma through irradiation with near-infrared laser pulses. After a variable time delay of up to 800 ps, we image the subsequent dynamics using intense extreme ultraviolet pulses from the FERMI free-electron laser. The recorded scattering images exhibit complex intensity fluctuations that are categorized based on their characteristic features. Systematic simulations of wide-angle diffraction patterns are performed, which can qualitatively explain the observed features by employing model shapes with both randomly distributed as well as structured, symmetric distortions. This points to a connection between the dynamics and the positions of the dopants in the droplets. In particular, the structured fluctuations might be governed by an underlying array of quantized vortices in the superfluid droplet as has been observed in previous small-angle diffraction experiments. Our results provide a basis for further investigations of dopant–droplet interactions and associated heating mechanisms.
- ItemTopological transitions in ac/dc-driven superconductor nanotubes([London] : Macmillan Publishers Limited, part of Springer Nature, 2022) Fomin, Vladimir M.; Rezaev, Roman O.; Dobrovolskiy, Oleksandr V.Extending of nanostructures into the third dimension has become a major research avenue in condensed-matter physics, because of geometry- and topology-induced phenomena. In this regard, superconductor 3D nanoarchitectures feature magnetic field inhomogeneity, non-trivial topology of Meissner currents and complex dynamics of topological defects. Here, we investigate theoretically topological transitions in the dynamics of vortices and slips of the phase of the order parameter in open superconductor nanotubes under a modulated transport current. Relying upon the time-dependent Ginzburg–Landau equation, we reveal two distinct voltage regimes when (i) a dominant part of the tube is in either the normal or superconducting state and (ii) a complex interplay between vortices, phase-slip regions and screening currents determines a rich FFT voltage spectrum. Our findings unveil novel dynamical states in superconductor open nanotubes, such as paraxial and azimuthal phase-slip regions, their branching and coexistence with vortices, and allow for control of these states by superimposed dc and ac current stimuli.
- ItemNumerical study of coherence of optical feedback in semiconductor laser dynamics(Berlin : Weierstraß-Institut für Angewandte Analysis und Stochastik, 2019) Radziunas, Mindaugas; Little, Douglas J.; Kane, Deborah M.The nonlinear dynamics of semiconductor laser with coherent, as compared to incoherent, delayed optical feedback systems have been discussed and contrasted in prior research literature. Here, we report simulations of how the dynamics change as the coherence of the optical feedback is systematically varied from being coherent to incoherent. An increasing rate of phase disturbance is used to vary the coherence. An edge emitting, 830nm, Fabry Perot semiconductor laser with a long external cavity is simulated. Following this study, consideration of prior and future experimental studies should include evaluation of where on the continuum of partial coherence the delayed optical feedback sits. Partial coherence is a parameter that will affect the dynamics.
- ItemCohesive zone-type delamination in visco-elasticity : to the occasion of the 60th anniversary of Tomaš Roubícek(Berlin : Weierstraß-Institut für Angewandte Analysis und Stochastik, 2016) Thomas, Marita; Zanini, ChiaraWe study a model for the rate-independent evolution of cohesive zone delamination in a viscoelastic solid, also exposed to dynamics effects. The main feature of this model, inspired by [OP99], is that the surface energy related to the crack opening depends on the history of the crack separation between the two sides of the crack path, and allows for different responses upon loading and unloading. Due to the presence of multivalued and unbounded operators featuring non-penetration and the memory-constraint in the strong formulation of the problem, we prove existence of a weaker notion of solution, known as semistable energetic solution, pioneered in [Rou09] and refined in [RT15a].