Mixed acoustic phonons and phase modes in an aperiodic composite crystal.

Aperiodic crystals which are long range ordered materials present original dynamics features due to the lack of translational symmetry formally implying the nonvalidity of the Brillouin zone concept. This Letter reports the observation by neutron scattering of an overdamped acousticlike mode at a Bragg peak position in a n-alkane-urea inclusion crystal. This result implies the existence of a gap in the dispersion branch.

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.

Sliding-induced decoupling and charge transfer between the coexisting Q1 and Q2 charge density waves in NbSe3.

Using high-resolution x-ray scattering in the presence of an applied current, we report evidence for a dynamical decoupling between the two NbSe3 charge-density waves (CDWs), Q1 (T(C1)=145 K) and Q2 (T(C2)=59 K), coexisting below T(C2). Simultaneous and oppositely directed shifts of the relevant CDW superlattice spots develop above a threshold current which we identify as the depinning threshold I(C1) for the more strongly pinned upper CDW Q1 (I(C1) approximately 10I(C2)). In contrast with shifts induced by current conversion processes, the present effect is not current polarized and is not limited to the current-contact regions.

Motional ordering of a charge-density wave in the sliding state.

We have used high-resolution x-ray scattering, in the presence of an applied direct current, for studying the correlation lengths in the sliding charge-density wave (CDW) state. Transport properties were simultaneously measured in situ during the experiment. We find that, while the transverse correlation is reduced when the CDW moves, the CDW becomes more ordered in the direction of motion.