Explanation of 100-fold reduction of spectral shifts for hydrogen on helium films

We show that helium film-mediated hydrogen-hydrogen interactions account for a two orders of magnitude discrepancy between previous theory and recent experiments on cold collision shifts in spin-polarized hydrogen adsorbed on a helium film. These attractive interactions also explain the anomalous dependence of the cold collision frequency shifts on the $^3$He covering of the film. Our findings suggest that the gas will become mechanically unstable before reaching the Kosterlitz-Thouless transition unless the experiment is performed in a drastically different regime, for example with a much different helium film geometry.Comment: 4+ pages, 1 figure (3 subfigures), revtex

Local versus global equilibration near the bosonic Mott-superfluid transition

We study the response of trapped two dimensional cold bosons to time dependent lattices. We find that in lattice ramps from 11 (superfluid, $\hbar/U_{\text{i}} = 3$ms, $\hbar/J_{\text{i}} = 45$ms) to 16 recoils (Mott, $\hbar/U_{\text{f}} = 2$ms, $\hbar/J_{\text{f}} = 130$ms) the local number fluctuations remains at their equilibrium values if ramps are slower than 3 ms. Global transport, however, is much slower (1s), especially in the presence of Mott shells. This separation of timescales has practical implications for cold atom experiments and cooling protocols.Comment: 4 pages, 4 figs. 6 subfigure

Development of Simplified Models of Regional Groundwater and Surface Water Flow Processes based on Computational Experiments with Comprehensive Models

The development of complex decision support model systems for the analysis of regional water policies for regions with intense socio-economic development affecting and being affected by the water resources system is of increasing importance. One of the most illustrative examples are regions with open-pit lignite mining. Such model systems have to be based on appropriate submodels, e.g. for water quantity processes. The paper describes submodels for groundwater and surface water flow with special regard to open-pit lignite mining regions. Starting with a problem definition in Section 2 the methodological background is given. The state-of-the-art of comprehensive models of regional water flow processes based on groundwater flow models and of stochastic long-term management modeling are described in details. Section 3 gives the methodological approach for model reduction. The application of this approach is illustrated in Section 4 for the modeling of mine drainage and groundwater tables, for the modeling of remaining pit management and of groundwater-surface water interactions. In the appendix computer programs of some submodels are given being suitable for a more general application

Water Policies: Regions with Open-Pit Lignite Mining (Introduction to the IIASA Study)

There is an apparent need for the analysis of long-term regional water policies to reconcile conflicting interests in regions with open-pit lignite mining. The most important. interest groups in such regions are mining, municipal and industrial water supply, agriculture as well as the "environment". A scientifically sound and practically simple policy-oriented system of methods and computerized procedures has to be developed. To develop such a system is part of the research work in the Regional Water Policies project carried out at the International Institute for Applied Systems Analysis (IIASA) in collaboration with research institutes in the German Democratic Republic, Poland, and in other countries as well. A test area that includes typical water-related elements of mining regions and significant conflicts and interest groups has been chosen. The first stage in the analysis is oriented towards developing a scenario generating system as a tool to choose "good" policies from the regional point of view. Therefore a policy-oriented interactive decision support model system is under development, considering the dynamic, nonlinear and uncertain systems behaviour. It combines a model for multi-criteria analysis in planning periods with a simulation model for monthly systems behaviour. The paper outlines the methodological approach. describes the test region in the GDR, and the submodels for the test region

Frustrated H-Induced Instability of Mo(110)

Using helium atom scattering Hulpke and L"udecke recently observed a giant phonon anomaly for the hydrogen covered W(110) and Mo(110) surfaces. An explanation which is able to account for this and other experiments is still lacking. Below we present density-functional theory calculations of the atomic and electronic structure of the clean and hydrogen-covered Mo(110) surfaces. For the full adsorbate monolayer the calculations provide evidence for a strong Fermi surface nesting instability. This explains the observed anomalies and resolves the apparent inconsistencies of different experiments.Comment: 4 pages, 2 figures, submitted to PR

Second-scale rotational coherence and dipolar interactions in a gas of ultracold polar molecules

Ultracold polar molecules combine a rich structure of long-lived internal states with access to controllable long-range anisotropic dipole–dipole interactions. In particular, the rotational states of polar molecules confined in optical tweezers or optical lattices may be used to encode interacting qubits for quantum computation or pseudo-spins for simulating quantum magnetism. As with all quantum platforms, the engineering of robust coherent superpositions of states is vital. However, for optically trapped molecules, the coherence time between rotational states is typically limited by inhomogeneous differential light shifts. Here we demonstrate a rotationally magic optical trap for 87Rb133Cs molecules that supports a Ramsey coherence time of 0.78(4) s in the absence of dipole–dipole interactions. This is estimated to extend to >1.4 s at the 95% confidence level using a single spin-echo pulse. In our trap, dipolar interactions become the dominant mechanism by which Ramsey contrast is lost for superpositions that generate oscillating dipoles. By changing the states forming the superposition, we tune the effective dipole moment and show that the coherence time is inversely proportional to the strength of the dipolar interaction. Our work unlocks the full potential of the rotational degree of freedom in molecules for quantum computation and quantum simulation