We compare femtosecond pump-probe experiments in Ni and micromagnetic modelling based on the Landau-Lifshitz-Bloch equation coupled to a two-temperature model, revealing a predominant thermal ultrafast demagnetization mechanism. We show that both spin (femtosecond demagnetization) and electron-phonon (magnetization recovery) rates in Ni increase as a function of the laser pump fluence. The slowing down for high fluences arises from the increased longitudinal relaxation time. PACS numbers: 75.40Gb,78.47.+p, The implementation of novel magnetic recording and spintronic devices requires well-funded knowledge of the limits of spin manipulation. Pump-probe experiments with powerful femtosecond lasers [1,2,3,4,5,6,7,8] have pushed these limits down to the femtosecond timescale in the past decade. These experiments have attracted many researchers with the aim to understand both fundamental mechanisms of the magnetization dynamics in a strongly out-of-equilibrium regime and to control the magnetic properties of materials on the femtosecond timescale. Very recently the involvement of the spin in thermoelectric processes, spin Peltier or Seebeck effects, has become of strong interest [9]. Therefore a vital understanding of energy transport between the spin, electron and phonon system in ferromagnets is needed. Even for the most simple itinerant ferromagnets, such as Ni, the processes connecting the elementary spin scattering process and the terahertz (THz) spin-wave generation have not been identified yet. However, they are the key to understand the macroscopic demagnetization on the femtosecond time scale.Different non-thermal mechanisms of how light could couple to the spin system have been put forward: the excitation of non-magnetic states mediated by the enhanced spin-orbit interaction (SOC) [10], the inverse Faraday [4] or the Barnet effect [11]. At the moment, no clear proof of these effects has been presented for the ultrafast magnetization dynamics in Ni: (i) The change of the light polarization has no influence on the femtosecond demagnetization and estimations suggest that the amount of direct angular momentum transfer from photons to spins is negligible [5], (ii) the time-dependent density functional theory based on the SOC mechanism at its present state of the art srongly underestimates the experimentally observed timescales in itinerant ferromagnets [10].Differenty from the latter, it has been shown that more applicable is a "thermal" ansatz for the description of the femtosecond magnetization dynamics in the itinerant ferromagnets [1,3,12,13,14]. Within this description, it is assumed that the excited state is a statistical ensemble of many electronic excitations, based on the undisputed fact that the photons of a femtosecond laser, focused on a metal, pass the energy to the subsystems of electrons, phonons and spins. Photons are absorbed by electrons close to the Fermi level leading to a non-equilibrium distribution that thermalizes within several femtoseconds. In the so-called two-temperature (2T ) model [1...

Dynamical information on spin degrees of freedom of proteins or solids can be obtained by NMR and electron spin resonance. A technique with similar versatility for charge degrees of freedom and their ultrafast correlations could move the understanding of systems like unconventional superconductors forward. By perturbing the superconducting state in a high-T c cuprate, using a femtosecond laser pulse, we generate coherent oscillations of the Cooper pair condensate that can be described by an NMR/electron spin resonance formalism. The oscillations are detected by transient broad-band reflectivity and are found to resonate at the typical scale of Mott physics (2.6 eV), suggesting the existence of a nonretarded contribution to the pairing interaction, as in unconventional (non-Migdal-Eliashberg) theories.A ccording to Bardeen-Cooper-Schrieffer (BCS) theory (1), superconductivity requires that electrons bind in Cooper pairs and condense collectively in a macroscopic quantum state. In conventional superconductors, the observation of a shift in the superconductivity transition temperature upon isotope substitution (2, 3) was an experimental breakthrough leading to the conclusion that lattice vibrations (phonons) act as a glue among electrons, promoting the required pairing. Since the discovery of high-temperature superconductivity in cuprates in 1986 (4), the observation of an analogous fingerprint of the glue involved in the pairing mechanism, if any, has been lacking.A fertile route to obtain information on excitations in solids and their coupling to electrons is pump-probe spectroscopy (5). Typically, the sample is illuminated by an ultrashort laser pulse lasting a few tens of femtoseconds and carrying 1.5 eV photons. This "pump" pulse creates an out-of-equilibrium distribution of particle-hole excitations that decays to states within a few hundreds of millielectronvolts of the chemical potential (6) in the pulse duration timescale. There, phase space restrictions slow down the dynamics (7) and the subsequent evolution can be studied in real time by a probe pulse. The dynamical response of the system can be observed with a temporal resolution comparable to the timescale of relevant processes in the material, like the pairs breaking, their recombination, or the electron-phonon coupling time.For example, the photoinduced quenching of the superconducting order parameter and its subsequent recovery were followed by recording the temporal evolution of the gap amplitude in the optical spectrum of different cuprates (8-11). Remarkably, it was found that the energy needed to suppress the superconducting state in these materials is several times larger than the condensation energy (9, 10), in contrast to what happens in conventional superconductors where it is of the same order (9, 11). Optical studies also provided insights on the relaxation dynamics of the excited quasiparticles (11-14) and on the optical spectral weight transfers associated with the carriers' kinetic energy changes across the photoinduced phase transition...

Synchronization scenarios of coupled mechanical metronomes are studied by means of numerical simulations showing the onset of synchronization for two, three, and 100 globally coupled metronomes in terms of Arnol'd tongues in parameter space and a Kuramoto transition as a function of coupling strength. Furthermore, we study the dynamics of metronomes where overturning is possible. In this case hyperchaotic dynamics associated with some diffusion process in configuration space is observed, indicating the potential complexity of metronome dynamics.

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