Backflow in a Fermi Liquid

We calculate the backflow current around a fixed impurity in a Fermi liquid. The leading contribution at long distances is radial and proportional to 1/r^2. It is caused by the current induced density modulation first discussed by Landauer. The familiar 1/r^3 dipolar backflow obtained in linear response by Pines and Nozieres is only the next to leading term, whose strength is calculated here to all orders in the scattering. In the charged case the condition of perfect screening gives rise to a novel sum rule for the phase shifts. Similar to the behavior in a classical viscous liquid, the friction force is due only to the leading contribution in the backflow while the dipolar term does not contribute.Comment: 4 pages, 1 postscript figure, uses ReVTeX and epsfig macro, submitted to Physical Review Letter

If it does take a village to raise a child, how should the village do it? Insights from the kids in places initiative

Cross-sector collaborations are some of the strategies used to promote early childhood development and wellbeing. Without these collaborations, key services for families with young children may be missed or even duplicated. By drawing from experiences in Canada and Italy, we share findings from a study that aimed to understand the factors that make cross-sector collaborations (CSC) succeed or fail. Specifically, the study focused on understanding how CSC promoting early child development are created, maintained, and consolidated; and on identifying the social psychological, organizational, and economic aspects of CSC that help or hinder their functioning. Based on qualitative analysis of data gathered from four focus groups and thirteen interviews conducted across seven Canadian and Italian communities, we conclude that the success of CSC depend of a series of factors that transcend context, language and culture

LENS Deposition of Complex Geometries

The Laser Engineered Net Shaping (LENS®) system at Sandia National Laboratories, a laser-based direct metal deposition process, was recently used for the fabrication of a complex prototype. The LENS process involved the challenges of adjacent areas of thick and thin cross section, overhanging unsupported features, high aspect ratios, and a hemispherical substrate. These challenges were overcome through the use of closed-loop weld pool control, height monitoring, a strong understanding of build parameters, and unique process planning strategies. The near-net shape LENS part was completed with annealing and conventional machining to produce the complex components in a reduced timeframe.Mechanical Engineerin

Looking at electronic wave functions on metal surfaces

The project described here is not only a beautiful example of the visual side tophysics, it is also a beautiful example of international cooperation. The first use of the idea—to apply a Fourier transform to STM pictures to see electron waves instead of just the surface atoms—came out of a collaboration between Plummer, Sprunger and the Aarhus group headed by Besenbacher. Hofman, who had beenworking at Tennessee, took Be(1010) samples to Berlin where the images shown in this pictorial were taken. All of the participants are now preparing a paper on the use of a Fourier transform to map the Fermi contour at metal surfaces

Implication of the overlap representation for modelling generalized parton distributions

Based on a field theoretically inspired model of light-cone wave functions, we derive valence-like generalized parton distributions and their double distributions from the wave function overlap in the parton number conserved s-channel. The parton number changing contributions in the t-channel are restored from duality. In our construction constraints of positivity and polynomiality are simultaneously satisfied and it also implies a model dependent relation between generalized parton distributions and transverse momentum dependent parton distribution functions. The model predicts that the t-behavior of resulting hadronic amplitudes depends on the Bjorken variable x_Bj. We also propose an improved ansatz for double distributions that embeds this property.Comment: 15 pages, 8 eps figure

Local densities, distribution functions, and wave function correlations for spatially resolved shot noise at nanocontacts

We consider a current-carrying, phase-coherent multi-probe conductor to which a small tunneling contact is attached. We treat the conductor and the tunneling contact as a phase-coherent entity and use a Green's function formulation of the scattering approach. We show that the average current and the current fluctuations at the tunneling contact are determined by an effective local non-equilibrium distribution function. This function characterizes the distribution of charge-carriers (or quasi-particles) inside the conductor. It is an exact quantum-mechanical expression and contains the phase-coherence of the particles via local partial densities of states, called injectivities. The distribution function is analyzed for different systems in the zero-temperature limit as well as at finite temperature. Furthermore, we investigate in detail the correlations of the currents measured at two different contacts of a four-probe sample, where two of the probes are only weakly coupled contacts. In particular, we show that the correlations of the currents are at zero-temperature given by spatially non-diagonal injectivities and emissivities. These non-diagonal densities are sensitive to correlations of wave functions and the phase of the wave functions. We consider ballistic conductors and metallic diffusive conductors. We also analyze the Aharonov-Bohm oscillations in the shot noise correlations of a conductor which in the absence of the nano-contacts exhibits no flux-sensitivity in the conductance.Comment: 17 pages, 8 figure

Nanoscopic Tunneling Contacts on Mesoscopic Multiprobe Conductors

We derive Bardeen-like expressions for the transmission probabilities between two multi-probe mesoscopic conductors coupled by a weak tunneling contact. We emphasize especially the dual role of a weak coupling contact as a current source and sink and analyze the magnetic field symmetry. In the limit of a point-like tunneling contact the transmission probability becomes a product of local, partial density of states of the two mesoscopic conductors. We present expressions for the partial density of states in terms of functional derivatives of the scattering matrix with respect to the local potential and in terms of wave functions. We discuss voltage measurements and resistance measurements in the transport state of conductors. We illustrate the theory for the simple case of a scatterer in an otherwise perfect wire. In particular, we investigate the development of the Hall-resistance as measured with weak coupling probes.Comment: 10 pages, 5 figures, revte

Combinatorial CRISPR-Cas9 screens for de novo mapping of genetic interactions.

We developed a systematic approach to map human genetic networks by combinatorial CRISPR-Cas9 perturbations coupled to robust analysis of growth kinetics. We targeted all pairs of 73 cancer genes with dual guide RNAs in three cell lines, comprising 141,912 tests of interaction. Numerous therapeutically relevant interactions were identified, and these patterns replicated with combinatorial drugs at 75% precision. From these results, we anticipate that cellular context will be critical to synthetic-lethal therapies

The impact of model resolution on the simulated Holocene retreat of the southwestern Greenland ice sheet using the Ice Sheet System Model (ISSM)

Geologic archives constraining the variability of the Greenland ice sheet (GrIS) during the Holocene provide targets for ice sheet models to test sensitivities to variations in past climate and model formulation. Even as data–model comparisons are becoming more common, many models simulating the behavior of the GrIS during the past rely on meshes with coarse horizontal resolutions (≥10&thinsp;km). In this study, we explore the impact of model resolution on the simulated nature of retreat across southwestern Greenland during the Holocene. Four simulations are performed using the Ice Sheet System Model (ISSM): three that use a uniform mesh and horizontal mesh resolutions of 20, 10, and 5&thinsp;km, and one that uses a nonuniform mesh with a resolution ranging from 2 to 15&thinsp;km. We find that the simulated retreat can vary significantly between models with different horizontal resolutions based on how well the bed topography is resolved. In areas of low topographic relief, the horizontal resolution plays a negligible role in simulated differences in retreat, with each model instead responding similarly to retreat driven by surface mass balance (SMB). Conversely, in areas where the bed topography is complex and high in relief, such as fjords, the lower-resolution models (10 and 20&thinsp;km) simulate unrealistic retreat that occurs as ice surface lowering intersects bumps in the bed topography that would otherwise be resolved as troughs using the higher-resolution grids. Our results highlight the important role that high-resolution grids play in simulating retreat in areas of complex bed topography, but also suggest that models using nonuniform grids can save computational resources through coarsening the mesh in areas of noncomplex bed topography where the SMB predominantly drives retreat. Additionally, these results emphasize that care must be taken with ice sheet models when tuning model parameters to match reconstructed margins, particularly for lower-resolution models in regions where complex bed topography is poorly resolved.</p