Combining high pressure and coherent diffraction: a first feasibility test

We present a first experiment combining high pression and coherent X-ray diffraction. By using a dedicated diamond anvil cell, we show that the degree of coherence of the X-ray beam is preserved when the X-ray beam passes through the diamond cell. This observation opens the possibility of studying the dynamics of slow fluctuations under high pressure.Comment: 3 pages, 2 figures, GHPR 2009 conferenc

Friedel Oscillations and Charge-density Waves Pinning in Quasi-one-dimensional Conductors: An X-ray Access

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.Comment: 4 pages REVTEX, 5 figure

Comment on "Two Dimensional Ordering and Fluctuations in alpha'-NaV2O5"

We claim that the analysis presented by Gaulin et al. (Phys. Rev. Lett. 84, 3446 (2000)) does not prove the two-dimensional character of the transition in NaV2O5 nor the charge ordering mechanism of the transition.Comment: 1 pag

Heritage Language Maintenance and Use among 1.5 Generation Khmer College Students

Most studies of heritage language maintenance have reported a steep attrition in heritage language use among the 1.5 and 2nd generation children of immigrants, in particular among East Asian groups. However, not much is known about the role of heritage languages and the patterns of language maintenance within refugee communities. This study focuses on heritage language use and maintenance among 1.5 generation Khmer college students. The findings show that Khmer students report a high frequency of heritage language use within the home with their parents as well as outside of the home with their co-ethnic peers. The data reveal that oral proficiency in Khmer is significantly more developed than literacy skills and is a necessity for bridging communication with parents and participating in co-ethnic peer social networks. The results of the study also indicate that these students development of English oral and literacy skills were significantly higher than their heritage language skills suggesting a similar trajectory of language loss similar to other immigrant groups, but perhaps at a slower rate

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.

The SrTiO3_3 displacive transition revisited by Coherent X-ray Diffraction

We present a Coherent X-ray Diffraction study of the antiferrodistortive displacive transition of SrTiO$_3$, 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 $\mu$m-scale spatial variations of the latter. Moreover, both components exhibit a speckle pattern, which is static on a $\sim$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 \emph{central} peak seen in inelastic neutron scattering.Comment: 4 pages, 3 figures, accepted in PR

X-ray spectrum of a pinned charge density wave

We calculate the X-ray diffraction spectrum produced by a pinned charge density wave (CDW). The signature of the presence of a CDW consists of two satellite peaks, asymmetric as a consequence of disorder. The shape and the intensity of these peaks are determined in the case of a collective weak pinning using the variational method. We predict divergent asymmetric peaks, revealing the presence of a Bragg glass phase. We deal also with the long range Coulomb interactions, concluding that both peak divergence and anisotropy are enhanced. Finally we discuss how to detect experimentally the Bragg glass phase in the view of the role played by the finite resolution of measurements.Comment: 13 pages 10 figure