We apply a low-frequency (mHz) ac pressure gradient to a sample of solid helium in order to search for a superfluidlike response. Our results are consistent with zero supersolid flow. Through a statistical analysis of our data, we set a bound on the rate of mass flow between two chambers, and hence the mass current density j. At the 68% confidence level, we bound v≡j/Ï≤9.6× 10-4 nm/s for the mass transport velocity. In terms of a simple model for the supersolid, we find an upper bound of 8.4× 10-6 for the supersolid fraction at 25 mK, at this same confidence level.

We study interaction-induced localization of electrons in an inhomogeneous quasi-one-dimensional system-a wire with two regions, one at low density and the other high. Quantum Monte Carlo techniques are used to treat the strong Coulomb interactions in the low-density region, where localization of electrons occurs. The nature of the transition from high to low density depends on the density gradient-if it is steep, a barrier develops between the two regions, causing Coulomb blockade effects. Ferromagnetic spin polarization does not appear for any parameters studied.

The intriguing idea that strongly interacting electrons can generate spatially inhomogeneous electronic liquid-crystalline phases is over a decade old 1-5 , but these systems still represent an unexplored frontier of condensed-matter physics. One reason is that visualization of the many-body quantum states generated by the strong interactions, and of the resulting electronic phases, has not been achieved. Soft condensed-matter physics was transformed by microscopies that enabled imaging of real-space structures and patterns.

While slowly turning the ends of a single molecule of DNA at constant applied force, a discontinuity was recently observed at the supercoiling transition when a small plectoneme is suddenly formed. This can be understood as an abrupt transition into a state in which stretched and plectonemic DNA coexist. We argue that there should be discontinuities in both the extension and the torque at the transition and provide experimental evidence for both.

Quantum information processing requires accurate coherent control of quantum-mechanical two-level systems, but is hampered in practice by their coupling to an uncontrolled environment. For electron spins in III-V quantum dots, the random environment is mostly given by the nuclear spins in the quantum-dot host material; they collectively act on the electron spin through the hyperfine interaction, much like a random magnetic field. Here we show that the same hyperfine interaction can be harnessed such that partial control of the normally uncontrolled environment becomes possible.

An information theoretic criterion for the feasibility of reconstructing diffraction signals from noisy tomographs, when the positions of the tomographs within the signal are unknown, is derived. For shot-noise limited data, the number of detected photons per tomograph for successful reconstruction is much smaller than previously believed necessary, growing only logarithmically with the number of contrast elements of the diffracting object.

Cryoprotectant-free thaumatin crystals have been cooled from 300 to 100 K at a rate of 0.1 K s-1 - 103-104 times slower than in conventional flash cooling - while continuously collecting X-ray diffraction data, so as to follow the evolution of protein lattice and solvent properties during cooling. Diffraction patterns show no evidence of crystalline ice at any temperature.

The proposed experiments to image single molecules with x-ray free-electron lasers present an unprecedented challenge in data processing. We describe for non-experts the computational tasks and some recent progress in solving them. © 2009 OSA.

The ground and lowest three adiabatic excited states of methylene are computed using the variational Monte Carlo and diffusion Monte Carlo (DMC) methods using progressively larger Jastrow-Slater multideterminant complete active space (CAS) wave functions. The highest of these states has the same symmetry, A1 1, as the first excited state.

We demonstrate piezoresistive transduction of mechanical motion from out-of-plane flexural micromechanical resonators made from stacked thin films. The resonators are fabricated from two highly doped polycrystalline silicon layers separated by an interlayer dielectric. We examine two interlayer materials: thermal silicon dioxide and stoichiometric silicon nitride. We show that via one-time dielectric breakdown, the film stack functions as a vertical piezoresistor effectively transducing the motion of the resonators.