S. Ravy

High-pressure study of x-ray diffuse scattering in ferroelectric perovskites.

We present a high-pressure x-ray diffuse scattering study of the ABO3 ferroelectric perovskites BaTiO3 and KNbO3. The well-known diffuse lines are observed in all the phases studied. In KNbO3, we show that the lines are present up to 21.8 GPa, with constant width and a slightly decreasing intensity. At variance, the intensity of the diffuse lines observed in the cubic phase of BaTiO3 linearly decreases to zero at approximately 11 GPa. These results are discussed with respect to x-ray absorption measurements, which leads to the conclusion that the diffuse lines are only observed when the B atom is off the center of the oxygen tetrahedron. The role of such disorder on the ferroelectric instability of perovskites is discussed.

X-ray study of the density wave instability of α-(BEDT-TTF)2MHg(SCN)4 with M=K and Rb

We present an X-ray diffraction study of the quasi-2D conductors α-(BEDTTTF) 2 MHg(SCN), with M=K and Rb. They exhibit a phase transition of the density wave type at T DW =8-10K and 12-13K respectively, evidenced by magnetoresistivity, specific heat, NMR and Hall constant measurements. The structural study shows the presence of satellite reflections already at ambient temperature. The related modulation is incommensurate with multiple harmonics. For some of the compounds studied, the intensity of the satellite reflections strongly increases below T DW . According to Fermi surface (FS) calculations, the wave vector of the structural modulation achieves a quite good nesting of the global FS. This suggests a coupling of the modulation with the electronic degrees of freedom leading to a charge density wave ground state.

SrTiO3 displacive transition revisited via coherent X-ray diffraction.

We present a coherent x-ray diffraction study of the antiferrodistortive displacive transition of SrTiO3, a prototypical example of a phase transition for which the critical fluctuations exhibit two length scales and two time scales. From the microbeam x-ray coherent diffraction patterns, we show that the broad (short-length scale) and the narrow (long-length scale) components can be spatially disentangled, due to 100-microm-scale spatial variations of the latter. Moreover, both components exhibit a speckle pattern, which is static on a approximately 10 mn time scale. This gives evidence that the narrow component corresponds to static ordered domains. We interpret the speckles in the broad component as due to a very slow dynamical process, corresponding to the well-known central peak seen in inelastic neutron scattering.

Structural fluctuations and spin-peierls transitions revisited

Abstract Pretransitional structural fluctuations have been measured in the organic compounds (BCPTTF) 2 X (X = PF 6 , AsF 6 ) and MEM(TCNQ) 2 , which undergo spin-Peierls phase transitions at T SP =36 K, 32.5 K and 18 K respectively. In the (BCPTTF) 2 X materials, the structural fluctuations have been measured up to T F =100 K and 120 K respectively, temperatures at which the in-chain correlation length almost amounts to the inter-spin distance. In the AsF 6 compound, accurate measurements show that above 60 K these fluctuations are quasi one-dimensional. Consistently, magnetic susceptibility measurements show deviaations from the Bonner-Fisher law below T F , that we interpret as caused by a local pairing of the spins due to the lattice critical fluctuations. In contrast MEM(TCNQ) 2 , where the magnetic susceptibility is known to perfectly follow the Bonner-Fisher law down to T SP , exhibits tridimensional critical effects only 20 K above T SP . These results show that the (BCPTTF) 2 X compounds exhibit a true spin-Peierls transition triggered by one-dimensional fluctuations while an unconventional spin-Peierls transition occurs in MEM(TCNQ) 2 .

Structural phase transitions in (TMTSF)2X and related compounds

Abstract Structural studies of the anion ordering phase transitions of some (TMTTF) 2 X and (TMTSF) 2 X salts are presented, with special attention on the Ising order parameter and on its kinetics. (TMTSF) 2 X salts containing dipolar anions (SCN − , PF 2 O 2 − ) exhibit unexpected behaviours, such as incommensurability or the occurrence of two successive phase transitions. Finally, the structural origin at the 77 K phase transition of the first sulphur-based organic superconductor (BEDT-TTF) 2 ReO 4 is established.

Ultrafast Formation of a Charge Density Wave State in 1T-TaS_{2}: Observation at Nanometer Scales Using Time-Resolved X-Ray Diffraction.

C. Laulhé, 2, ∗ T. Huber, G. Lantz, 3 A. Ferrer, S.O. Mariager, S. Grübel, J. Rittmann, J.A. Johnson, V. Esposito, A. Lübcke, † L. Huber, M. Kubli, M. Savoini, V.L.R. Jacques, L. Cario, B. Corraze, E. Janod, G. Ingold, P. Beaud, S.L. Johnson, and S. Ravy Synchrotron SOLEIL, L’Orme des Merisiers, Saint Aubin BP 48, F-91192 Gif-sur-Yvette, France Université Paris-Saclay (Univ. Paris-Sud), F-91405 Orsay Cedex, France Institute for Quantum Electronics, Physics Department, ETH Zurich, CH-8093 Zurich, Switzerland Laboratoire de Physique des Solides, Université Paris-Sud, CNRS, UMR 8502, F-91405 Orsay, France Swiss Light Source, Paul Scherrer Institute, CH-5232, Villigen, Switzerland Institut des Matériaux Jean Rouxel UMR 6502, Université de Nantes, 2 rue de la Houssinière, F-44322 Nantes, France (Dated: September 29, 2018)

Friedel oscillations and charge-density wave pinning in quasi-one-dimensional conductors: An x-ray diffraction study

We present an x-ray diffraction study of the Vanadium-doped blue bronze K0.3(Mo0.972V0.028)O3. At low temperature, we have observed both an intensity asymmetry of the +-2kF satellite reflections relative to the pure compound, and a profile asymmetry of each satellite reflections. We show that the profile asymmetry is due to Friedel oscillation around the V substituant and that the intensity asymmetry is related to the charge density wave (CDW) pinning. These two effects, intensity and profile asymmetries, gives for the first time access to the local properties of CDW in disordered systems, including the pinning and even the phase shift of FOs.

Laser-Induced Charge-Density-Wave Transient Depinning in Chromium

We report on time-resolved x-ray diffraction measurements following femtosecond laser excitation in pure bulk chromium. Comparing the evolution of incommensurate charge-density-wave (CDW) and atomic lattice reflections, we show that, a few nanoseconds after laser excitation, the CDW undergoes different structural changes than the atomic lattice. We give evidence for a transient CDW shear strain that breaks the lattice point symmetry. This strain is characteristic of sliding CDWs, as observed in other incommensurate CDW systems, suggesting the laser-induced CDW sliding capability in 3D systems. This first evidence opens perspectives for unconventional laser-assisted transport of correlated charges.

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

AbstractWe 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.

Ultrafast evolution and transient phases of a prototype out-of-equilibrium Mott-Hubbard material

The study of photoexcited strongly correlated materials is attracting growing interest since their rich phase diagram often translates into an equally rich out-of-equilibrium behaviour. With femtosecond optical pulses, electronic and lattice degrees of freedom can be transiently decoupled, giving the opportunity of stabilizing new states inaccessible by quasi-adiabatic pathways. Here we show that the prototype Mott–Hubbard material V2O3 presents a transient non-thermal phase developing immediately after ultrafast photoexcitation and lasting few picoseconds. For both the insulating and the metallic phase, the formation of the transient configuration is triggered by the excitation of electrons into the bonding a1g orbital, and is then stabilized by a lattice distortion characterized by a hardening of the A1g coherent phonon, in stark contrast with the softening observed upon heating. Our results show the importance of selective electron–lattice interplay for the ultrafast control of material parameters, and are relevant for the optical manipulation of strongly correlated systems.