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We apply ultrafast x-ray diffraction with femtosecond temporal resolution to monitor the lattice dynamics in a thin film of multiferroic BiFeO 3 after above-band-gap photoexcitation. The sound-velocity limited evolution of the observed lattice strains indicates a quasi-instantaneous photoinduced stress which decays on a nanosecond time scale. This stress exhibits an inhomogeneous spatial profile evidenced by the broadening of the Bragg peak. These new data require substantial modification of existing models of photogenerated stresses in BiFeO 3 : the relevant excited charge carriers must remain localized to be consistent with the data. Multiferroics have a great potential for application due to their possible coupling of ferroelectricity and magnetism [1][2][3]. BiFeO 3 (BFO) is one of the few room temperature multiferroics today [4][5][6][7][8], and of these, the only one that is a stable phase. Its relatively small band gap of approximately 2.7 eV [9] renders BFO an ideal candidate for applications in spintronics and memory devices [5] with a perspective for ultrafast optical switching similar to purely ferroelectric [10] or magnetic materials [11]. The photovoltaic effect in this complex material and the underlying ultrafast carrier dynamics after above-band-gap femtosecond (fs) optical excitation have been studied thoroughly [12][13][14]. The photoinduced currents in BFO lead to THz emission [15,16] and to a photostrictive response [17]. Alloptical experiments showed that the rapid photoinduced mechanical stress excites coherent phonons [18,19]. The dynamics of photoinduced strains were directly and quantitatively measured in a recent synchrotron-based ultrafast x-ray diffraction (UXRD) study with a temporal resolution of 100 ps [20]. Combined optical measurements revealed a linear dependence of the transient strain and the number of excited carriers over several nanoseconds (ns). This led to the conclusion that depolarization field screening (DFS) including macroscopic transport of the carriers to the surface and interface could be the dominant stress generating process, although the effect of excited antibonding orbitals was not ruled out [20].In this Letter, we report complementing UXRD experiments at a laser-driven plasma x-ray source (PXS) in order to monitor the coherent and incoherent lattice dynamics in a BFO thin film sample with subpicosecond (ps) temporal resolution after above-band-gap excitation. We observe a sound-velocity limited evolution of the structural response within 10 ps indicating a quasi-instantaneous stress. The substantial Bragg peak broadening is a direct evidence of an inhomogeneous spatial stress profile. It appears quasiinstantaneously and decays on nanosecond time scales as reconfirmed by new synchrotron-based UXRD data recorded at the Advanced Photon Source (APS). We obtain quantitative agreement of the transient peak shift and broadening measured with both setups and can firmly conclude that the photogenerated stress driving the film expansion has a strongly inhomogeneous sp...

The nanosecond response of a PbTiO(3)/SrTiO(3) ferroelectric/dielectric superlattice to applied electric fields is closely linked to the dynamics of striped domains of the remnant polarization. The intensity of domain satellite reflections observed with time-resolved x-ray microdiffraction decays in 5-100 ns depending on the magnitude of the electric field. The piezoelectric response of the superlattice within stripe domains is strongly suppressed due to electromechanical clamping between adjacent regions of opposite polarization. Regions of the superlattice that have been switched into a uniform polarization state by the applied electric field, however, exhibit piezoelectricity during the course of the switching process. We propose a switching model different from previous models of the switching of superlattices, based instead on a spatially heterogeneous transformation between striped and uniform polarization states.

The heavy metal selenophosphate, Pb2P2Se6, is a promising new material for cost‐effective X‐ray/γ‐ray detection. Crystal boules of Pb2P2Se6 up to 25 mm in length and 15 mm in diameter are grown by a vertical Bridgman method. They are cut and processed into size‐appropriate wafers for physical, photo‐transport property studies, as well as γ‐ray detector testing. The material is a semiconductor with an indirect bandgap of 1.88 eV and has electrical resistivity in the range of 1 × 1010 Ω cm. Pb2P2Se6 single crystal samples display a significant photoconductivity response to optical, X‐ray, and γ‐ray radiation. When tested with a 57Co γ‐ray source, Pb2P2Se6 crystals show spectroscopic response and several generated pulse height spectra resolving the 122.1 and 136.5 keV 57Co radiation. The mobility–lifetime product of Pb2P2Se6 is estimated to be ≈3.5 × 10−5 cm2 V−1 for electron carriers. The Pb2P2Se6 compound melts congruently at 812 °C and has robust chemical/physical properties that promise low cost bulk production and detector development.

The remnant polarization of weakly coupled ferroelectric-dielectric superlattices is distributed unequally between the component layers, and as a result the components respond differently to applied electric fields. The difference is apparent in both the nanometer-scale structure of striped polarization domains and in the development of piezoelectric strain and field-induced polarization. Both effects are probed with in situ time-resolved synchrotron x-ray diffraction in a PbTiO(3)/SrTiO(3) superlattice in fields up to 2.38 MV/cm. Domains are initially distorted to increase the polarization in the SrTiO(3) layer while retaining the striped motif. The subsequent transformation to a uniform polarization state at a later time leads to piezoelectric expansion dominated by the field-induced polarization of the SrTiO(3) layers. The results are consistent with theoretical predictions of the field dependence of the domain structure and electrical polarization.

Through mapping of the spatiotemporal strain profile in ferroelectric BiFeO3 epitaxial thin films, we report an optically initiated dynamic enhancement of the strain gradient of 105–106 m−1 that lasts up to a few ns depending on the film thickness. Correlating with transient optical absorption measurements, the enhancement of the strain gradient is attributed to a piezoelectric effect driven by a transient screening field mediated by excitons. These findings not only demonstrate a new possible way of controlling the flexoelectric effect, but also reveal the important role of exciton dynamics in photostriction and photovoltaic effects in ferroelectrics.

The nanodomain pattern in ferroelectric/dielectric superlattices transforms to a uniform polarization state under above-bandgap optical excitation. X-ray scattering reveals a disappearance of domain diffuse scattering and an expansion of the lattice. The reappearance of the domain pattern occurs over a period of seconds at room temperature, suggesting a transformation mechanism in which charge carriers in long-lived trap states screen the depolarization field. A Landau-Ginzburg-Devonshire model predicts changes in lattice parameter and a critical carrier concentration for the transformation.

Domains of remnant polarization can be written into ferroelectrics with nanoscale precision using scanning probe nanolithography techniques such as piezoresponse force microscopy (PFM). Understanding the structural effects accompanying this process has been challenging due to the lack of appropriate structural characterization tools. Synchrotron X-ray nanodiffraction provides images of the domain structure written by PFM into an epitaxial Pb(Zr,Ti)O(3) thin film and simultaneously reveals structural effects arising from the writing process. A coherent scattering simulation including the superposition of the beams simultaneously diffracted by multiple mosaic blocks provides an excellent fit to the observed diffraction patterns. Domains in which the polarization is reversed from the as-grown state have a strain of up to 0.1% representing the piezoelectric response to unscreened surface charges. An additional X-ray microdiffraction study of the photon-energy dependence of the difference in diffracted intensity between opposite polarization states shows that this contrast has a crystallographic origin. The sign and magnitude of the intensity contrast between domains of opposite polarization are consistent with the polarization expected from PFM images and with the writing of domains through the entire thickness of the ferroelectric layer. The strain induced by writing provides a significant additional contribution to the increased free energy of the written domain state with respect to a uniformly polarized state.