Energy harvesting from sunlight is essential in order to save fossil fuels, which are found in limited amount in the earth's crust. Photovoltaic devices converting light into electrical energy are presently made of semiconducting materials, but ferroelectrics are also natural candidates because of their internal built‐in electric field. Although they are clearly uncompetitive for mainstream applications, the possibility to output high photovoltages is making these materials reconsidered for targeted applications. However, their intrinsic properties regarding electronic transport and the origin of their internal field are poorly known. Here, it is demonstrated that under intense illumination and electric field, oxygen vacancies can be controllably generated in BiFeO3 to dramatically increase the conductance of BiFeO3 single crystals to a controllable value spanning 6 orders of magnitude while at the same time triggering light sensitivity in the form of photoconductivity, diode, and photovoltaic effects. Properties of the bulk and the Schottky interfaces with gold contacts are disentangled and it is shown that bulk effects are time dependent. The photocurrent has a direction that can be set by an applied field without changing the ferroelectric polarization direction. The self‐doping procedure is found to be essential in both the generation of electron hole pairs and the establishment of the internal field that separates them.
The recent development of spin dynamics opens perspectives for various applications
based on spin waves, including logic devices. The first important step in the
realization of spin-wave-based logics is the manipulation of spin-wave interference.
Here, we present the experimental realization of a micrometre-scale spin-wave
interferometer consisting of two parallel spin-wave waveguides. The spin waves
propagate through the waveguides and the superposition or interference of the
electrical signals corresponding to the spin waves is measured. A direct current
flowing through a metal wire underneath one of the spin-wave waveguides affects the
propagation properties of the corresponding spin wave. The signal of constructive or
destructive interference depends on the magnitude and direction of the applied
direct current. Thus, the present work demonstrates a unique manipulation of
spin-wave interference.
Optical measurements have been carried out on high-quality BiFeO3 single crystals in order to show the presence of electronic defect states and calculate the band gap. The photoluminescence spectra show an intense electronic transition peak at a wavelength of 410 nm. This peak is large and asymmetric, indicating the existence of defects inside the gap, which we attribute to oxygen vacancies. These defects are likely to originate from the slow heating rate and long sintering times necessary to synthesize BiFeO3 single crystals. The optical band gap is measured to be 3 eV, a larger value than those previously reported.
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