Excitation spectra of a 3He impurity on 4He clusters

The diffusion Monte Carlo technique is used to calculate and analyze the excitation spectrum of a single 3He atom bound to a cluster with N 4He atoms, with the aim of establishing the most adequate filling ordering of single-fermion orbits to the mixed clusters with a large number of 3He atoms. The resulting ordering looks like the rotational spectrum of a diatomic molecule, being classified only by the angular momentum of the level, although vibrational-like excitations appear at higher energies for sufficiently large N

A zero dimensional model of lithium-sulfur batteries during charge and discharge

Lithium-sulfur cells present an attractive alternative to Li-ion batteries due to their large energy density, safety, and possible low cost. Their successful commercialisation is dependent on improving their performance, but also on acquiring sufficient understanding of the underlying mechanisms to allow for the development of predictive models for operational cells. To address the latter, we present a zero dimensional model that predicts many observed features in the behaviour of a lithium-sulfur cell during charge and discharge. The model accounts for two electrochemical reactions via the Nernst formulation, power limitations through Butler-Volmer kinetics, and precipitation/dissolution of one species, including nucleation. It is shown that the precipitation/dissolution causes the flat shape of the low voltage plateau, typical of the lithium-sulfur cell discharge. During charge, it is predicted that the dissolution can act as a bottleneck, as for large enough currents smaller amounts dissolve. This results in reduced charge capacity and an earlier onset of the high plateau reaction, such that the two plateaus merge. By including these effects, the model improves on the existing zero dimensional models, while requiring considerably fewer input parameters and computational resources. The model also predicts that, due to precipitation, the customary way of experimentally measuring the open circuit voltage from a low rate discharge might not be suitable for lithium-sulfur. This model can provide the basis for mechanistic studies, identification of dominant effects in a real cell, predictions of operational behaviour under realistic loads, and control algorithms for applications

Performance pay and adverse selection

We study equilibrium wage contracts in a labour market with adverse selection and moral hazard. Firms offer incentive contracts to their employees to motivate them to exert effort. Providing incentives comes, however, at a cost, as it leads to misallocation of effort across tasks. With ex ante identical workers, the optimal wage contract is linear, and the equilibrium resource allocation optimal. With ex ante heterogenous workers, firms may increase the incentive power of the wage contract to attract the better workers. The resulting equilibrium is separating, in the sense that workers self-select on contracts. Furthermore, the contracts offered to the good workers are too high powered compared to the contracts that maximise welfare.-

Cosmology with Twisted Tori

We consider the cosmological role of the scalar fields generated by the compactification of 11-dimensional Einstein gravity on a 7D elliptic twisted torus, which has the attractive features of giving rise to a positive semi-definite potential, and partially fixing the moduli. This compactification is therefore relevant for low energy M-theory, 11D supergravity. We find that slow-roll inflation with the moduli is not possible, but that there is a novel scaling solution in Friedmann cosmologies in which the massive moduli oscillate but maintain a constant energy density relative to the background barotropic fluid

Cluster-Exact Approximation of Spin Glass Groundstates

We present an algorithm which calculates groundstates of Ising spin glasses approximately. It works by randomly selecting clusters of spins which exhibit no frustrations. The spins which were not selected, contribute to the local fields of the selected spins. For the spin--cluster a groundstate is exactly calaculated by using graphtheoretical methods. The other spins remain unchanged. This procedure is repeated many times resulting in a state with low energy. The total time complexity of this scheme is approximately cubic. We estimate that the groundstate energy density of the infinite system for the +/- J model is -1.400 +/- 0.005 (2d) and -1.766 +/- 0.002 (3d). The distribution of overlaps for selected systems is calculated in order to characterize the algorithm.Comment: 13 pages, LaTeX (including figures in LaTeX-format

Inverse seesaw and dark matter in models with exotic lepton triplets

We show that models with exotic leptons transforming as E ~ (1,3,-1) under the standard model gauge symmetry are well suited for generating neutrino mass via a radiative inverse seesaw. This approach realizes natural neutrino masses and allows multiple new states to appear at the TeV scale. The exotic leptons are therefore good candidates for new physics that can be probed at the LHC. Furthermore, remnant low-energy symmetries ensure a stable dark matter candidate, providing a link between dark matter and the origins of neutrino mass.Comment: 6 pages, 3 figures (revtex4.1, two-columns

Bound states of neutral particles in external electric fields

Neutral fermions of spin $\frac 12$ with magnetic moment can interact with electromagnetic fields through nonminimal coupling. The Dirac--Pauli equation for such a fermion coupled to a spherically symmetric or central electric field can be reduced to two simultaneous ordinary differential equations by separation of variables in spherical coordinates. For a wide variety of central electric fields, bound-state solutions of critical energy values can be found analytically. The degeneracy of these energy levels turns out to be numerably infinite. This reveals the possibility of condensing infinitely many fermions into a single energy level. For radially constant and radially linear electric fields, the system of ordinary differential equations can be completely solved, and all bound-state solutions are obtained in closed forms. The radially constant field supports scattering solutions as well. For radially linear fields, more energy levels (in addition to the critical one) are infinitely degenerate. The simultaneous presence of central magnetic and electric fields is discussed.Comment: REVTeX, 14 pages, no figur