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FeSe x Te 1 − x compounds display a rich phase diagram, ranging from the nematicity of FeSe to the ( π , π ) magnetism of FeTe. We focus on FeSe0.4Te0.6, and exploit tr-ARPES to study its ultrafast electron dynamics following photoexcitation by near-infrared pump pulses. By exploiting probe-polarization-dependent matrix element effects, we reveal a photoinduced long-lived state, lasting for a few tens of picoseconds, showing features compatible with a nematic state. The possibility to induce a long-lived state in this compound by using ultra-short pulses might shed a new light on the driving force behind the nematic symmetry breaking in iron-based superconductors. With the aid of a phenomenological model, we illustrate how our results possibly question the common belief that a low-energy coupling with fluctuations is a necessary condition to stabilize the nematic order. On the contrary, the tendency towards orbital differentiation due to strong electronic correlations induced by the Hund’s coupling could be at the origin of the nematic order in iron-based superconductors.

The properties of organic films are strongly influenced by their growth and structure. In this work, we link the results from three different techniques to elucidate the interplay between the morphology and the electronic structure of α-sexithiophene films grown on Au(111) surfaces. Photoelectron emission microscopy and thermal desorption spectroscopy reveal the growth of a two-layer thick wetting layer at room temperature. On top of the wetting layer, the molecules start to form μm-sized crystallites, which are statistically distributed over the surface. This transition in the growth mode is reflected in the electronic structure of the system, studied by two-photon photoemission spectroscopy. While in the wetting layer, exciton formation is suppressed by the interaction with the metal substrate, Frenkel excitons with picosecond lifetimes are observed in the three-dimensional crystallites.

Azobenzenealkanethiols in self-assembled monolayers (SAMs) on Au(111) exhibit reversible trans-cis photoisomerization when diluted with alkanethiol spacers. Using these mixed SAMs, we show switching of the linear optical and second-harmonic response. The effective switching of these surface optical properties relies on a reasonably large cross section and a high photoisomerization yield as well as a long lifetime of the metastable cis isomer. We quantified the switching process by X-ray absorption spectroscopy. The cross sections for the trans-cis and cis-trans photoisomerization with 365 and 455 nm light, respectively, are 1 order of magnitude smaller than in solution. In vacuum, the 365 nm photostationary state comprises 50-74% of the molecules in the cis form, limited by their rapid thermal isomerization back to the trans state. In contrast, the 455 nm photostationary state contains nearly 100% trans-azobenzene. We determined time constants for the thermal cis-trans isomerization of only a few minutes in vacuum and in a dry nitrogen atmosphere but of more than 1 day in ambient air. Our results suggest that adventitious water adsorbed on the surface of the SAM stabilizes the polar cis configuration of azobenzene under ambient conditions. The back reaction rate constants differing by 2 orders of magnitude underline the huge influence of the environment and, accordingly, its importance when comparing various experiments.

Self-assembled monolayers of azobenzene-functionalized alkanethiolates form molecular ensembles with preferential orientation and significant excitonic coupling among the azobenzene chromophores. We have studied their optical switching with differential reflectance and two-photon-photoemission spectroscopy tuning the excitation wavelength through the excitonically broadened S2 absorption band. While the effective isomerization cross-section increases towards shorter wavelengths, the fraction of cis molecules in the photostationary state decreases. We attribute this observation to the absorption of the cis isomer in the SAM. The photoisomerization in the SAM thereby follows the behavior of non-interacting chromophores in solution, despite the formation of H-aggregates. Our study thus reveals that photoswitching occurs via localized excitations while strongly excitonically coupled, delocalized states do not contribute significantly.