In 1954 Dicke predicted the accelerated initial decay of multiple atomic excitations 1 , laying the foundation for the concept of superradiance. Further studies 2-4 suggested that emission of the total energy was similarly accelerated, provided that the system reaches the inversion threshold. Superradiant emission of the total energy has been confirmed by numerous studies [4][5][6][7][8][9][10][11][12] , yet the acceleration of the initial decay has not been experimentally demonstrated. Here we use resonant diffraction of X-rays from the Mössbauer transition 13 of 57 Fe nuclei to investigate superradiant decay, photon by photon, along the entire chain of the de-excitation cascade of up to 68 simultaneous coherent nuclear excitations created by a pulse of an X-ray free-electron laser. We find agreement with Dicke's theory 1 for the accelerated initial decay as the number of excitations is increased. We also find that our results are in agreement with a simple statistical model, providing a necessary baseline for discussing further properties of superradiance, within and beyond the low-excitation regime. Dicke's model introduces superradiance as an accelerated initial decay of multiple atomic excitations, and provides exact predictions for the ensemble behaviour as a function of the number of atoms and number of excitations in the system

For a future hydrogen economy, non-precious metal catalysts for the water splitting reactions are needed that can be implemented on a global scale. Metal-nitrogencarbon (MNC) catalysts with active sites constituting a metal center with fourfold coordination of nitrogen (MN 4 ) show promising performance, but an optimization rooted in structure-property relationships has been hampered by their low structural definition. Porphyrin model complexes are studied to transfer insights from well-defined molecules to MNC systems. This work combines experiment and theory to evaluate the influence of porphyrin substituents on the electronic and electrocatalytic properties of MN 4 centers with respect to the hydrogen evolution reaction (HER) in aqueous electrolyte. We found that the choice of substituent affects their utilization on the carbon support and their electrocatalytic performance. We propose an HER mechanism for supported iron porphyrin complexes involving a [Fe II (P * )] À radical anion intermediate, in which a porphinic nitrogen atom acts as an internal base. While this work focuses on the HER, the limited influence of a simultaneous interaction with the support and an aqueous electrolyte will likely be transferrable to other catalytic applications.

Cu2SnS3 (CTS) is a medium-temperature, ecofriendly, p-type thermoelectric material known for phonon-glass-electron-crystal characteristic. In the present work, ordered and disordered CTS samples were prepared from elemental powders, and their electronic and vibrational properties were systematically investigated by experimental methods and ab initio calculations. The disordered CTS polymorph presents a higher power factor, PF ∼ 1.5 μW/K2 cm, than the ordered and stable phase, PF ∼ 0.5 μW/K2 cm, above 700 K, as an effect of a smaller band gap and higher carrier concentration. Most importantly, the disordered CTS shows an ultralow thermal conductivity, k ∼ 0.4–0.2 W/m K, as compared to ordered, k ∼ 1.0–0.4W/m K, in the temperature range of 323–723 K. The combined effect of a higher PF and lower k results in a higher figure of merit, zT ∼ 0.5 at 723 K, obtained for disordered CTS without resorting to chemical alloying. It turns out that structural disorder contributes to the suppression of thermal conductivity. While group velocity of acoustic phonons, as shown both by experiments and ab initio calculations, is similar in the two polymorphs, a strong anharmonicity characterizes the disordered CTS, resulting in the presence of low-lying optical modes acting as traps for heat transmission. Density functional theory/density functional perturbation theory simulations and nuclear inelastic scattering combined with high-resolution diffraction studies of the lattice parameters reveal details of phonon–phonon interactions in CTS with unprecedented effectiveness.

The parameters governing the thermoelectric efficiency of a material, Seebeck coefficient, electrical, and thermal conductivities, are correlated and their reciprocal interdependence typically prevents a simultaneous optimization. Here, we present the case of disordered cubic kesterite Cu 2 ZnSnS 4 , a phase stabilized by structural disorder at low temperature. With respect to the ordered form, the introduction of disorder improves the three thermoelectric parameters at the same time. The origin of this peculiar behavior lies in the localization of some Sn lone pair electrons, leading to "rattling" Sn ions. On one hand, these rattlers remarkably suppress thermal conductivity, dissipating lattice energy via optical phonons located below 1.5 THz; on the other, they form electron-deficient Sn-S bonds leading to a p-type dopinglike effect and highly localized acceptor levels, simultaneously enhancing electrical conductivity and the Seebeck coefficient. This phenomenon leads to a 3 times reduced thermal conductivity and doubling of both electrical conductivity and the Seebeck coefficient, resulting in a more than 20 times increase in figure of merit, although still moderate in absolute terms.

We report a systematic lattice dynamics study of the technologically important Fe 3 Si/GaAs heterostructure for Fe 3 Si layer thicknesses of 3, 6, 8, and 36 monolayers. The Fe-partial phonon density of states obtained by nuclear inelastic scattering exhibits up to a twofold enhancement of the low-energy phonon states compared to the bulk material for layer thicknesses of 8 monolayers and below. First-principles calculations explain the observed effect by interface-specific phonon states originating from the significantly reduced atomic force constants and allow for achieving a comprehensive understanding of the lattice dynamics of epitaxial strain-free interfaces.