Charge and Spin Density Waves observed through their spatial fluctuations by coherent and simultaneous X-ray diffraction

Spatial uctuations of spin density wave (SDW) and charge density wave (CDW) in chromium have been compared by combining coherent and simultaneous X-ray diffraction experiments. Despite their close relationship, spatial fluctuations of the spin and of the charge density waves display a very different behavior: the satellite reflection associated to the charge density displays speckles while the spin one displays an impressive long-range order. This observation is hardly compatible with the commonly accepted magneto-elastic origin of CDW in chromium and is more consistent with a purely electronic scenario where CDW is the second harmonic of SDW. A BCS model taking into account a second order nesting predicts correctly the existence of a CDW and explains why the CDW is more sensitive to punctual defects.Comment: 4 pages, 4 figures; Accepted in Phys. Rev.

Application de la diffraction cohérente des rayons X à l étude de défauts topologiques dans les structures atomiques et électroniques

Le travail présenté dans ce manuscrit à pour fil conducteur l utilisation de la diffraction cohérente des rayons X pour mettre en lumière différents phénomènes physiques. La détection de défauts topologiques, auxquels les faisceaux cohérents sont très sensibles, permet de tirer des conclusions sur l ordre ou le désordre trouvés dans les systèmes étudiés. La notion de cohérence des rayons X est tout d abord exposée, puis la caractérisation de profils de diffraction cohérente en présence de défauts topologiques est présentée à travers une étude des boucles de dislocation dans le silicium. Cette technique a ensuite été appliquée à l étude de composés présentant des ordres électroniques incommensurables, tels que les Ondes de Densité de Charge (ODC) et les Ondes de Densité de Spin (ODS). Plusieurs systèmes ont été étudiés : le chrome pur monocristallin, le bronze bleu K0.3 MoO3 et NbSe3. Dans le cas du chrome, les résultats obtenus posent la question du lien réel existant entre ODC et ODS. Par ailleurs, une dislocation magnétique isolée en volume a été détectée et a permis de soulever la question des constantes de force de l ODS du spin. Le bronze bleu et NbSe3 ont été étudiés sous champ électrique. Un ordre à très grande distance a été observé dans les ODC du bronze bleu et de NbSe3, dans le régime de glissement. Nous proposons une description en termes de phase, à l aide d un modèle de type réseau de solitons.In the present work, coherent x-ray diffraction has been used to highlight various physical phenomena. In most cases, the detection of topological defects allows drawing conclusions about the order or disorder in the periodic arrangements, and coherent x-ray diffraction is well suited for their detection. First of all, the notion of coherence will be exposed, and the main characteistics of the profiles obtained by coherent x-ray diffraction when a topological defect is present in the illuminated volume will be exposed through measurements carried out in a silicon sample containing well-known dislocation loops. This technique was then used for the study of electronic crystals, such as incommensurate Charge Density Wave (CDW) and Spin Density Wave (SDW) compounds. Several systems were studied : pure monocrystalline chromium, blue bronze K0.3M0O3, and NbSe 3. In the case of chromium, our results show that the SDW and the CDW behave differently, and are not organized the same way, which raises the question of the link between these two modulations. Besides, an isolated bulk magnetic dislocation was detected, and allowed to extract the SDW force constant of chromium. Blue bronze and NbSe3 were studied under electric field, in the sliding CDW state. A very long range order was observed in this regime. A description in terms of a soliton-like lattice is proposed.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

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

International audienceHigh-resolution coherent X-ray diffraction experiment has been performed on the Charge-Density Wave (CDW) system K0.3MoO3K0.3MoO3. The 2kF2kF satellite reflection associated with the CDW has been measured with respect to external dc currents. In the sliding regime, the 2kF2kF 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

Structural studies of mesoporous alumina membranes by small angle X-ray scattering

Small-angle X-ray scattering (SAXS) has been used to study the structures of mesoporous alumina membranes. These membranes, produced by anodic deposition, have an arrangement of parallel cylindrical pores centred on a disordered hexagonal lattice. The SAXS intensity profile varies over five orders of magnitude, and is a convolution of a structure factor for the 2D distribution of pore axes with a cylinder form factor for the pores. Rotation of the plane of the membrane changes the pattern from a ring structure for channels parallel to the X-ray beam to a set of vertical spots for channels perpendicular to the beam. The oscillatory pattern changes systematically with the anodic deposition voltage, confirming a linear relationship between voltage and pore separation. Initial results are also reported for alumina membranes containing cobalt nanowires in the pore volume. The SAXS technique complements and extends direct-imaging methods such as electron microscopy, which view only the surface structure for a limited area of the sample. The results indicate that the membranes show greater disorder than normally deduced from other techniques

La courbure d'une onde de densité de charge observée par une source Xfel utilisée comme une lentille électronique pour les rayons X

Ultrafast X-ray diffraction by the LCLS free-electron laser has been used to probe Charge Density Wave (CDW) systems under applied external currents. At sufficiently low currents, CDW wavefronts bend in the direction transverse to the 2kF wave vector. We show that this shear effect has the ability to focus or defocus hard X-ray beams, depending of the current direction, making it an electronic lens of a new kind, tunable at will from the Fraunhofer to the Fresnel regime. The effect is interpreted using the fractional Fourier transform showing how the macroscopic curvature of a nanometric modulation can be beneficially used to modify the propagation of X-ray beams

Observation of large multiple scattering effects in ultrafast electron diffraction on monocrystalline silicon

International audienceWe report on ultrafast electron diffraction on high quality single crystal silicon. The ultrafast dynamics of the Bragg peaks exhibits a giant photoinduced response which can only be explained in the framework of dynamical diffraction theory, taking into account multiple scattering of the probing electrons in the sample. In particular, we show that lattice heating following photoexcitation can cause an unexpected increase of the Bragg peak intensities, in contradiction with the well-known Debye-Waller effect. We anticipate that multiple scattering should be systematically considered in ultrafast electron diffraction on high quality crystals as it dominates the Bragg peak dynamics. In addition, taking into account multiple scattering effects opens the way to quantitative studies of nonequilibrium dynamics of defects in quasiperfect crystals

Superconductivity, pseudo-gap, and stripe correlations in high-T c cuprates

International audienceUnder-doped La-214 cuprates show a charge-and spin-modulation known as " stripes " [1]. These stripe modulations are (quasi)-static close to 1/8 hole doping where superconductivity is suppressed. The pseudo-gap phase of other cuprate compounds recently also revealed charge modulation, but interpreted rather as a charge density wave (CDW) [2, 3, 4], that possibly competes with superconductivity. In this context, to better understand the interplay between the stripe phase and the superconductivity, we use angle-resolved photoemission spectroscopy to study the electronic band structure and gap in La-214 cuprates near 1/8 doping (La 2−x−y Nd y Sr x CuO 4 (x = 0.12; y = 0.0 & 0.4)) and compare with the previous results in the same system [5] and La 1.86 Ba 0.14 CuO 4 [6]. Our data shows a loss of spectral intensity towards the end of the Fermi arcs, that is possibly due to a strong renormalisation, as already pointed out elsewhere * [email protected] Néel CNRS-25, av des Martyrs-38042 Grenoble cedex 9; tel: (+33)(0)4 76 88 12 84 [5], with a noisy but still measurable gap. On the nodal direction no gap is observed within our statistics, but a sizeable decrease in intensity with temperature. Moreover, we do not see any shadow band, but our Fermi surface can be well modelled with a single electron band calculation in the tight binding approximation, even very close to the 1/8 doping La 2−x−y Nd y Sr x CuO 4 with and without Nd substitution