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X-CITED! Report Summary

Project ID: 259709
Funded under: FP7-IDEAS-ERC
Country: Hungary

Final Report Summary - X-CITED! (Electronic transitions and bistability: states, switches, transitions and dynamics studied with high-resolution X-ray spectroscopy)

Information technology requires advanced novel materials with special transport and magnetic properties and excitation characteristics. Beside nano-scaled devices, transition-metal-based molecular systems with switchable states have great potential as very high density devices. In order to exploit them, first we need to explore the characteristics of their different states and the possible switching mechanisms, which should allow us to tailor their properties to applications with chemical engineering. Another type of system, compounds of transition metals with strongly correlated electrons already play an important role in innovative technologies, which exploit their special magnetic and transport properties. Understanding the microscopic origin of the various uncommon behaviour of these materials requires a thorough exploration of their different structural, electronic, magnetic, lattice and spin degrees of freedoms under different conditions. Most of these can be explored with powerful high-resolution X-ray spectroscopy techniques -- in this project, we have employed such static and time-resolved X-ray tools to reveal the fine details of their characteristics in 21 synchrotron and 8 X-ray free electron laser (XFEL) experiments.

Unveiling the elementary steps of light-induced molecular switching requires ultrafast pump-probe experiments. We have been making efforts to introduce element-sensitive high-resolution X-ray spectroscopy as probes in ultrafast experiments. Already in the first year of the project, we made a relevant advance from our first proof-of-principle X-ray emission experiment to the actual collection of picosecond-resolved data employing a MHz laser system with count rates similar to those in static experiments. Moreover, in collaboration with groups from Copenhagen, Hamburg and Lund, we have realized a setup to combine scattering and spectroscopy, in order to obtain information on the electronic and structural dynamics at the same time. This was used, for instance, to study the photoinduced spin-state transition of an iron complex in aqueous solution, where the combined techniques permitted us to closely follow the structural and electronic changes, the excitation fraction, as well as the temperature and density changes of the solvent (including the variation of the solvent cage) on the subnanosecond time scale of the life­time of the transient excited state.
We have also utilized the same approach to extend these dynamics studies to the femtosecond region at X-ray free electron lasers in several experiments, which brought us close to making the first femtosecond-resolved molecular movies.
Theoretical modelling is essential to understand the mechanism of the photoinduced switching of molecular states. We have successfully modelled the studied molecules with quantum chemistry, and shown that description at the DFT/TD-DFT level describes Fe(II) systems with sufficient accuracy, allowing us to use these tools even for meaningful predictions.
Exploring alternative modes for molecular switching can provide us with insights into the interplay of the strongly coupled degrees of freedoms that shape the electronic structure and magnetic properties of such materials. Following our recent finding that hard X-rays trigger spin-state switching in Fe(II) complexes, we have now observed that they can also induce a thermal hysteresis at the switching of molecular magnets; the observation can provide us with a new tool to monitor cooperativity among the switching units. Moreover, we found the X-ray equivalent for a light-induced switching anomaly, the so called strong-field light-induced excited-spin-state trapping, and even exploited it as an alternative excitation method. Finally, we have explored a mechanically induced switching phenomenon in inorganic one dimensional polymers, which can have applications in identifying shock or impact history.
A part of our activity is focused on solids with special transport properties, from which we mention one example. In complex oxides that exhibit extraordinary magnetic and transport properties, (for example, those utilized in the heads of hard disk drives) a nanometre-scale separation of magnetic and electronic properties occurs. Curiously, this phenomena, which is believed to be behind the "colossal" variation in the magnetic resistance, takes place in a chemically homogeneous phase. It is difficult to grasp this phenomenon, the coexistence of the tiny magnetic phases experimentally. Also, the origin of the phenomenon is far from being understood despite relevant research efforts.
Utilizing local investigation techniques, we were able to give a direct experimental evidence for the coexistence of the separated nanoscale phases in Sr-doped LaCoO3 perovskites. Mössbauer spectroscopy and synchrotron hard X-ray spectroscopy provided a consistent local picture of the composition, electronic and spin structure, and relative amount of these nanoscale phases. A simple model describing how the doping affects the local spin momentum and the magnetism is in excellent agreement with the data, which sheds light on the microscopic origin of the nanoscale magneto-electronic phase separation.

During these research efforts, we have developed a flexible, cost-effective, uncomplicated X-ray spectrometer that can also be used in a laboratory to characterise the local atomic and electronic structure of selected elements in concentrated samples. Offering access to X-ray absorption spectra in hours, this instrument fills a technology gap, as it allows investigations that were thought to be possible only at synchrotrons. This is expected to make X-ray spectroscopy investigations available routinely at many institutes, and largely contribute to the more efficient training of future X-ray scientists.

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