X-ray Coherent diffraction interpreted through the fractional Fourier transform

Diffraction of coherent x-ray beams is treated through the Fractionnal Fourier transform. The transformation allow us to deal with coherent diffraction experiments from the Fresnel to the Fraunhofer regime. The analogy with the Huygens-Fresnel theory is first discussed and a generalized uncertainty principle is introduced.Comment: 7 pages, 8 figure

Observation of correlations up to the micrometer scale in sliding charge-density waves

High-resolution coherent x-ray diffraction experiment has been performed on the charge density wave (CDW) system K$_{0.3}$MoO$_3$. The $2k_F$ satellite reflection associated with the CDW has been measured with respect to external dc currents. In the sliding regime, the $2k_F$ satellite reflection displays secondary satellites along the chain axis which corresponds to correlations up to the micrometer scale. This super long range order is 1500 times larger than the CDW period itself. This new type of electronic correlation seems inherent to the collective dynamics of electrons in charge density wave systems. Several scenarios are discussed.Comment: 4 pages, 3 figures Typos added, references remove

Spin density wave dislocation in chromium probed by coherent x-ray diffraction

We report on the study of a magnetic dislocation in pure chromium. Coherent x-ray diffraction profiles obtained on the incommensurate Spin Density Wave (SDW) reflection are consistent with the presence of a dislocation of the magnetic order, embedded at a few micrometers from the surface of the sample. Beyond the specific case of magnetic dislocations in chromium, this work may open up a new method for the study of magnetic defects embedded in the bulk.Comment: 8 pages, 7 figure

New long-range sub-structure found in the tetragonal phase of CH3NH3PbI3 single crystals

International audienceHybrid organic-inorganic perovskites have become one of the most promising low-cost alternatives to traditional semiconductors in the field of photovoltaics and light emitting devices. It combines both attractive features of organic and inorganic materials within a single composite, for instance with excellent electronic properties. We used x-ray diffraction to reveal a sub-structure within CH 3 NH 3 PbI 3 single crystals. We could observe the presence of additional peaks with a square symmetry in several monocrystalline samples. We discuss these results in terms of two different models: a superstructure modulated in two in-plane orthogonal directions, and a model with tilted domains with a shallow angle of ~0.6°. In both cases, the modulated or tilted domains appear in regions with small lattice expansion. We show that this last model appears to be the most likely to explain our observations

Charge density waves tuned by biaxial tensile stress

International audienceAbstract The precise arrangement and nature of atoms drive electronic phase transitions in condensed matter. To explore this tenuous link, we developed a true biaxial mechanical deformation device working at cryogenic temperatures, compatible with X-ray diffraction and transport measurements, well adapted to layered samples. Here we show that a slight deformation of TbTe$_3$ can have a dramatic influence on its Charge Density Wave (CDW), with an orientational transition from c to a driven by the a/c parameter, a tiny coexistence region near a = c , and without space group change. The CDW transition temperature T$_c$ displays a linear dependence with $\left\vert a/c-1\right\vert$ while the gap saturates out of the coexistence region. This behaviour is well accounted for within a tight-binding model. Our results question the relationship between gap and T$_c$ in RTe$_3$ systems. This method opens a new route towards the study of coexisting or competing electronic orders in condensed matter