As the oldest known magnetic material, magnetite (Fe3O4) has fascinated mankind for millennia. As the first oxide in which a relationship between electrical conductivity and fluctuating/localized electronic order was shown1, magnetite represents a model system for understanding correlated oxides in general. Nevertheless, the exact mechanism of the insulator–metal, or Verwey, transition has long remained inaccessible2, 3, 4, 5, 6, 7, 8. Recently, three-Fe-site lattice distortions called trimerons were identified as the characteristic building blocks of the low-temperature insulating electronically ordered phase9. Here we investigate the Verwey transition with pump–probe X-ray diffraction and optical reflectivity techniques, and show how trimerons become mobile across the insulator–metal transition. We find this to be a two-step process. After an initial 300 fs destruction of individual trimerons, phase separation occurs on a 1.5±0.2 ps timescale to yield residual insulating and metallic regions. This work establishes the speed limit for switching in future oxide electronics10

Controlling the blend morphology is one of the ways to achieve high power conversion efficiency in organic bulk heterojunction (BHJ) photovoltaic devices. One simple yet effective method is ''solvent additive'' approach, which involves the addition of a small fraction of high boiling point solvent into the blend of donor/acceptor dissolved in another host solvent. Even though this method has been successfully applied in a number of polymer/fullerene BHJ devices, the selection rule of the choice of additive and the host solvent has yet to be fully established. In this work, we performed a systematic study of the effect of alkyl lengths of alkanedithiol additives on the nanoscale phase separation of P3HT:PC 61 BM blends and consequently, the power conversion efficiency (PCE) of the devices. The extent of the additive-induced phase separation is related to the additive boiling point and the degree of interaction between the additive and fullerene, as evident from grazing incidence X-ray diffractometry (GIXRD) and scanning transmission X-ray microscopy (STXM) data. We found that both the boiling point and the degree of interaction are correlated and should be considered simultaneously in the selection of the appropriate solvent additives. Lastly, PCE as high as 3.1% can be achieved in an optimally phase-separated blend due to an improvement in the charge dissociation and a decrease in bimolecular recombination.

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Thin films of organic molecular semiconductors from the class of phthalocyanines are investigated by means of magneto-optical Kerr effect ͑MOKE͒ spectroscopy at room temperature. A numerical analysis of the energy dispersion of the real and imaginary part of the complex magneto-optical Kerr rotation angle in the visible to near ultraviolet spectral range allows the first determination of the magneto-optical material constant, the so-called Voigt constant, for a molecular material and the results are compared to density-functional theory calculations. The Voigt constant is only 1-2 orders of magnitude smaller compared with soft ferromagnetic materials such as Ni. The amplitude of the Voigt constant is exploited to determine the molecular orientation in thin films with high accuracy.