Elastic Softness of Hybrid Lead Halide Perovskites.

Much recent attention has been devoted towards unraveling the microscopic optoelectronic properties of hybrid organic-inorganic perovskites. Here we investigate by coherent inelastic neutron scattering spectroscopy and Brillouin light scattering, low frequency acoustic phonons in four different hybrid perovskite single crystals: MAPbBr_{3}, FAPbBr_{3}, MAPbI_{3}, and α-FAPbI_{3} (MA: methylammonium, FA: formamidinium). We report a complete set of elastic constants characterized by a very soft shear modulus C_{44}. Further, a tendency towards an incipient ferroelastic transition is observed in FAPbBr_{3}. We observe a systematic lower sound group velocity in the technologically important iodide-based compounds compared to the bromide-based ones. The findings suggest that low thermal conductivity and hot phonon bottleneck phenomena are expected to be enhanced by low elastic stiffness, particularly in the case of the ultrasoft α-FAPbI_{3}.

Nitrogen-related intermediate band in P-rich GaNxPyAs1-x-y alloys.

The electronic band structure of phosphorus-rich GaNxPyAs1-x-y alloys (x ~ 0.025 and y ≥ 0.6) is studied experimentally using optical absorption, photomodulated transmission, contactless electroreflectance, and photoluminescence. It is shown that incorporation of a few percent of N atoms has a drastic effect on the electronic structure of the alloys. The change of the electronic band structure is very well described by the band anticrossing (BAC) model in which localized nitrogen states interact with the extended states of the conduction band of GaAsP host. The BAC interaction results in the formation of a narrow intermediate band (E- band in BAC model) with the minimum at the Γ point of the Brillouin zone resulting in a change of the nature of the fundamental band gap from indirect to direct. The splitting of the conduction band by the BAC interaction is further confirmed by a direct observation of the optical transitions to the E+ band using contactless electroreflectance spectroscopy.

Antiphase domain tailoring for combination of modal and 4¯ -quasi-phase matching in gallium phosphide microdisks.

We propose a novel phase-matching scheme in GaP whispering-gallery-mode microdisks grown on Si substrate combining modal and 4¯ -quasi-phase-matching for second-harmonic-generation. The technique consists in unlocking parity-forbidden processes by tailoring the antiphase domain distribution in the GaP layer. Our proposal can be used to overcome the limitations of form birefringence phase-matching and 4¯ -quasi-phase-matching using high order whispering-gallery-modes. The high frequency conversion efficiency of this new scheme demonstrates the competitiveness of nonlinear photonic devices monolithically integrated on silicon.

Self-organized growth of nanoparticles on a surface patterned by a buried dislocation network.

The self-organized growth of Co nanoparticles is achieved at room temperature on an inhomogenously strained Ag(001) surface arising from an underlying square misfit dislocation network of 10 nm periodicity buried at the interface between a 5 nm-thick Ag film and a MgO(001) substrate. This is revealed by in situ grazing-incidence small-angle x-ray scattering. Simulations of the data performed in the distorted wave Born approximation framework demonstrate that the Co clusters grow above the dislocation crossing lines. This is confirmed by molecular dynamic simulations indicating preferential Co adsorption on tensile sites.

Strain, size, and composition of InAs quantum sticks embedded in InP determined via grazing incidence x-ray anomalous diffraction.

We have used x-ray anomalous diffraction to recover the model-independent Fourier transform (x-ray structure factor) of InAs quantum sticklike islands embedded in InP. The average height of the quantum sticks, as deduced from the width of the structure factor profile, is 2.54 nm. The InAs out-of-plane deformation, relative to InP, is 6.1%. Diffraction anomalous fine structure provides evidence of pure InAs quantum sticks. Finite difference method calculations reproduce well the diffraction data, and give the strain along the growth direction. The chemical mixing at interfaces is also analyzed.

Dynamics of phason fluctuations in the i-AlPdMn quasicrystal.

We report on the study of the dynamics of long wavelength phason fluctuations in the i-AlPdMn icosahedral phase using coherent x-ray scattering. When measured with a coherent x-ray beam, the diffuse intensity due to phasons presents strong fluctuations or speckles pattern. From room temperature to 500 degrees C the speckle pattern is time independent. At 650 degrees C the time correlation of the speckle pattern exhibits an exponential time decay, from which a characteristic time tau is extracted. We find that tau is proportional to the square of the phason wavelength, which demonstrates that phasons are collective diffusive modes in quasicrystals, in agreement with theoretical predictions.

Diffraction anomalous fine-structure spectroscopy at beamline BM2 at the European Synchrotron Radiation Facility.

Diffraction anomalous fine-structure (DAFS) spectroscopy uses resonant elastic X-rays scattering as an atomic, shell and site-selective probe that provides information on the electronic structure and the local atomic environment as well as on the long-range-ordered crystallographic structure. A DAFS experiment consists of measuring the Bragg peak intensities as a function of the energy of the incoming X-ray beam. The French CRG (Collaborative Research Group) beamline BM2-D2AM (Diffraction Diffusion Anomale Multi-longueurs d'Onde) at the ESRF (European Synchrotron Radiation Facility) has developed a state-of-the-art energy scan diffraction set-up. In this article the requirements for obtaining reliable DAFS data are presented and recent technical achievements are reported.

Coherent small-angle scattering on a bending-magnet beamline at the ESRF.

A coherent X-ray beam is obtained from the D2AM bending-magnet beamline (BM2) of the European Synchrotron Radiation Facility. As this line has permanent convergent optics, monochromated by an Si(111) double monochromator, coherence conditions are satisfied by selecting a part of the beam close to the focal point. Low intensity (10(6) X-rays s(-1)) is partially compensated by a high degree of coherence and by the use of a high-resolution direct-illumination CCD area detector. The stability of the small-angle set-up makes quantitative analysis possible. The calculated and measured degrees of coherence are compared. The distributions of speckle intensities are explained by a beam composed of two-thirds coherent and one-third incoherent parts. This incoherent component is caused by Kapton windows, which will be removed in future experiments.