Strong interference effects in the resonant Auger decay of atoms induced by intense X-Ray fields

The theory of resonant Auger decay of atoms in a high intensity coherent X-ray pulse is presented. The theory includes the coupling between the ground state and the resonance due to an intense X-ray pulse, taking into account the decay of the resonance and the direct photoionization of the ground state, both populating the final ionic states coherently. The theory also considers the impact of the direct photoionization of the resonance state itself which typically populates highly-excited ionic states. The combined action of the resonant decay and of the direct ionization of the ground state in the field induces a non-hermitian time-dependent coupling between the ground and the 'dressed' resonance stats. The impact of these competing processes on the total electron yield and on the 2s$^2$2p$^{4}(^1\mathrm{D})$3p $^2$P spectator and 2s$^1$2p$^{6}$ $^2$S participator Auger decay spectra of the Ne 1s$\to$3p resonance is investigated. The role of the direct photoionization of the ground state and of the resonance increases dramatically with the field intensity. This results in strong interference effects with distinct patterns in the electron spectra, different for the participator and spectator final states.Comment: 31 pages, 6 figure

The role of noise and initial conditions in the asymptotic solution of a bounded confidence, continuous-opinion model

We study a model for continuous-opinion dynamics under bounded confidence. In particular, we analyze the importance of the initial distribution of opinions in determining the asymptotic configuration. Thus, we sketch the structure of attractors of the dynamical system, by means of the numerical computation of the time evolution of the agents density. We show that, for a given bound of confidence, a consensus can be encouraged or prevented by certain initial conditions. Furthermore, a noisy perturbation is added to the system with the purpose of modeling the free will of the agents. As a consequence, the importance of the initial condition is partially replaced by that of the statistical distribution of the noise. Nevertheless, we still find evidence of the influence of the initial state upon the final configuration for a short range of the bound of confidence parameter

Breaking the resilience of a two-dimensional Bose-Einstein condensate to fragmentation

A two-dimensional Bose-Einstein condensate (BEC) split by a radial potential barrier is investigated. We determine on an accurate many-body level the system's ground-state phase diagram as well as a time-dependent phase diagram of the splitting process. Whereas the ground state is condensed for a wide range of parameters, the time-dependent splitting process leads to substantial fragmentation. We demonstrate for the first time the dynamical fragmentation of a BEC despite its ground state being condensed. The results are analyzed by a mean-field model and suggest that a large manifold of low-lying fragmented excited states can significantly impact the dynamics of trapped two-dimensional BECs.Comment: 5+eps pages, 4 figure

Measurements of thermodynamic and transport properties of EuC2_2: a low-temperature analogue of EuO

EuC$_2$ is a ferromagnet with a Curie-temperature of $T_C \simeq 15\,$K. It is semiconducting with the particularity that the resistivity drops by about 5 orders of magnitude on cooling through $T_C$, which is therefore called a metal-insulator transition. In this paper we study the magnetization, specific heat, thermal expansion, and the resistivity around this ferromagnetic transition on high-quality EuC$_2$ samples. At $T_C$ we observe well defined anomalies in the specific heat $c_p(T)$ and thermal expansion $\alpha(T)$ data. The magnetic contributions of $c_p(T)$ and $\alpha(T)$ can satisfactorily be described within a mean-field theory, taking into account the magnetization data. In zero magnetic field the magnetic contributions of the specific heat and thermal expansion fulfill a Gr\"uneisen-scaling, which is not preserved in finite fields. From an estimation of the pressure dependence of $T_C$ via Ehrenfest's relation, we expect a considerable increase of $T_C$ under applied pressure due to a strong spin-lattice coupling. Furthermore the influence of weak off stoichiometries $\delta$ in EuC$_{2 \pm \delta}$ was studied. It is found that $\delta$ strongly affects the resistivity, but hardly changes the transition temperature. In all these aspects, the behavior of EuC$_2$ strongly resembles that of EuO.Comment: 7 pages, 6 figure

Impact of ultrafast electronic damage in single particle x-ray imaging experiments

In single particle coherent x-ray diffraction imaging experiments, performed at x-ray free-electron lasers (XFELs), samples are exposed to intense x-ray pulses to obtain single-shot diffraction patterns. The high intensity induces electronic dynamics on the femtosecond time scale in the system, which can reduce the contrast of the obtained diffraction patterns and adds an isotropic background. We quantify the degradation of the diffraction pattern from ultrafast electronic damage by performing simulations on a biological sample exposed to x-ray pulses with different parameters. We find that the contrast is substantially reduced and the background is considerably strong only if almost all electrons are removed from their parent atoms. This happens at fluences of at least one order of magnitude larger than provided at currently available XFEL sources.Comment: 15 pages, 3 figures submitted to PR

Resonant Auger decay of the core-excited C∗^\astO molecule in intense X-ray laser fields

The dynamics of the resonant Auger (RA) process of the core-excited C$^\ast$O(1s$^{-1}\pi^\ast,v_r=0$) molecule in an intense X-ray laser field is studied theoretically. The theoretical approach includes the analogue of the conical intersections of the complex potential energy surfaces of the ground and `dressed' resonant states due to intense X-ray pulses, taking into account the decay of the resonance and the direct photoionization of the ground state, both populating the same final ionic states coherently, as well as the direct photoionization of the resonance state itself. The light-induced non-adiabatic effect of the analogue of the conical intersections of the resulting complex potential energy surfaces gives rise to strong coupling between the electronic, vibrational and rotational degrees of freedom of the diatomic CO molecule. The interplay of the direct photoionization of the ground state and of the decay of the resonance increases dramatically with the field intensity. The coherent population of a final ionic state via both the direct photoionization and the resonant Auger decay channels induces strong interference effects with distinct patterns in the RA electron spectra. The individual impact of these physical processes on the total electron yield and on the CO$^+(A^2\Pi)$ electron spectrum are demonstrated.Comment: 13 figs, 1 tabe

Wave chaos as signature for depletion of a Bose-Einstein condensate

We study the expansion of repulsively interacting Bose-Einstein condensates (BECs) in shallow one-dimensional potentials. We show for these systems that the onset of wave chaos in the Gross-Pitaevskii equation (GPE), i.e. the onset of exponential separation in Hilbert space of two nearby condensate wave functions, can be used as indication for the onset of depletion of the BEC and the occupation of excited modes within a many-body description. Comparison between the multiconfigurational time-dependent Hartree for bosons (MCTDHB) method and the GPE reveals a close correspondence between the many-body effect of depletion and the mean-field effect of wave chaos for a wide range of single-particle external potentials. In the regime of wave chaos the GPE fails to account for the fine-scale quantum fluctuations because many-body effects beyond the validity of the GPE are non-negligible. Surprisingly, despite the failure of the GPE to account for the depletion, coarse grained expectation values of the single-particle density such as the overall width of the atomic cloud agree very well with the many-body simulations. The time dependent depletion of the condensate could be investigated experimentally, e.g., via decay of coherence of the expanding atom cloud.Comment: 12 pages, 10 figure