X-Ray Scattering Measurements of the Transient Structure of a Driven Charge-Density-Wave

We report time-resolved x-ray scattering measurements of the transient structural response of the sliding {\bf Q}$_{1}$ charge-density-wave (CDW) in NbSe$_{3}$ to a reversal of the driving electric field. The observed time scale characterizing this response at 70K varies from $\sim$ 15 msec for driving fields near threshold to $\sim$ 2 msec for fields well above threshold. The position and time-dependent strain of the CDW is analyzed in terms of a phenomenological equation of motion for the phase of the CDW order parameter. The value of the damping constant, $\gamma = (3.2 \pm 0.7) \times 10^{-19}$ eV $\cdot$ seconds $\cdot$ \AA$^{-3}$, is in excellent agreement with the value determined from transport measurements. As the driving field approaches threshold from above, the line shape becomes bimodal, suggesting that the CDW does not depin throughout the entire sample at one well-defined voltage.Comment: revtex 3.0, 7 figure

Evanescent wave transport and shot noise in graphene: ballistic regime and effect of disorder

We have investigated electrical transport and shot noise in graphene field effect devices. In large width over length ratio $W/L$ graphene strips, we have measured shot noise at low frequency ($f$ = 600--850 MHz) in the temperature range of 4.2--30 K. We observe a minimum conductivity of $\frac{4e^{2}}{\pi h}$ and a finite and gate dependent Fano factor reaching the universal value of 1/3 at the Dirac point, i.e. where the density of states vanishes. These findings are in good agreement with the theory describing that transport at the Dirac point should occur via evanescent waves in perfect graphene samples with large $W/L$. Moreover, we show and discuss how disorder and non-parallel leads affect both conductivity and shot noise.Comment: Extended version (19 pages, 10 figures) of Phys. Rev. Lett. 100, 196802 (2008). Additional data on the effect of disorder and non-parallel leads. Submitted for publication in Journal of Low Temperature Physics for the Proceedings of the International Symposium on Quantum Phenomena and Devices at Low Temperatures (ULTI 2008), Espoo, Finlan

CO2 Leakage Prevention by Introducing Engineered Nanoparticles to the In-situ Brine

AbstractIntroducing engineered nanoparticles into an aquifer or reservoir can potentially increase the storage efficiency and mitigate the risk of leakage of stored CO2. We have measured flow pattern and pressure drop during core floods in which high pressure liquid CO2 or a CO2 analogue fluid displaces an aqueous suspension of nanoparticles. The displacement front is more spatially uniform and travels more slowly compared to a control displacement with no in- situ nanoparticles. Pressure measurements are consistent with generation of a viscous phase such as an emulsion during the displacement. These observations suggest that a nanoparticle stabilized emulsion is formed during the displacement which acts to suppress the viscous instability

Structure and kinetics of the sliding Q1 CDW in NbSe3

Using crystallographically perfect single crystal whiskers of NbSe3 and the synchrotron x-ray scattering facilities at CHESS, we have performed detailed high resolution measurements of the structure of a sliding CDW. We observe the longitudinal strain of the CDW associated with the phase slip necessary to convert between normal and collective current at the electrical contacts. Using a stroboscopic x-ray scattering technique, we have extended these measurements into the time domain and studied the evolution of the structure of the sliding CDW as it responds to a reversal of the direction of an applied d.c. electric field. The time constant characterizing this structural evolution is on the order of milliseconds for fields well above threshold. Near threshold the CDW splits into two components ; one component is pinned and the other is sliding. As the electric field strength is increased, the fraction of the CDW which remains pinned decreases monotonicaily