Experimental Heat Transfer Supporting Simulated Water Well Performance on Mars

Favorable indications of massive quantities of water on Mars have initiated studies of potential changes to human Mars missions. Using a technique known as a Rodriguez Well to melt the ice, store the resulting water in a subsurface ice cavity until needed, and then pump water to the surface for use is one potential means to effect these changes. A computer simulation of the Rodriguez Well in a terrestrial environment is one of the engineering tools being used to characterize the performance of this type of well on Mars. An experiment at the NASA Johnson Space Center is gathering data for convective heat transfer and evaporation rates at Mars surface conditions so that this computer simulation can be properly modified to predict performance on Mars. While quantitative results await processing, tests have indicated that a pool of water can be maintained at 1C to 2 C while at Mars surface temperatures and pressures

Approximate Newton Methods for Policy Search in Markov Decision Processes

Approximate Newton methods are standard optimization tools which aim to maintain the benefits of Newton's method, such as a fast rate of convergence, while alleviating its drawbacks, such as computationally expensive calculation or estimation of the inverse Hessian. In this work we investigate approximate Newton methods for policy optimization in Markov decision processes (MDPs). We first analyse the structure of the Hessian of the total expected reward, which is a standard objective function for MDPs. We show that, like the gradient, the Hessian exhibits useful structure in the context of MDPs and we use this analysis to motivate two Gauss-Newton methods for MDPs. Like the Gauss- Newton method for non-linear least squares, these methods drop certain terms in the Hessian. The approximate Hessians possess desirable properties, such as negative definiteness, and we demonstrate several important performance guarantees including guaranteed ascent directions, invariance to affine transformation of the parameter space and convergence guarantees. We finally provide a unifying perspective of key policy search algorithms, demonstrating that our second Gauss- Newton algorithm is closely related to both the EM-algorithm and natural gradient ascent applied to MDPs, but performs significantly better in practice on a range of challenging domains

Si/SiGe bound-to-continuum quantum cascade emitters

Si/SiGe bound-to-continuum quantum cascade emitters designed by self-consistent 6-band k.p modeling and grown by low energy plasma enhanced chemical vapour deposition are presented demonstrating electroluminescence between 1.5 and 3 THz. The electroluminescence is Stark shifted by an electric field and demonstrates polarized emission consistent with the design. Transmission electron microscopy and x-ray diffraction are also presented to characterize the thick heterolayer structure

Darwin's small and medium ground finches might have taste preferences, but not for human foods

Urbanization is rapidly changing ecological niches. On the inhabited Galapagos Islands, Darwin's finches consume human-introduced foods preferentially; however, it remains unclear why. Here, we presented pastry with flavour profiles typical of human foods (oily, salty and sweet) to small ground finches (Geospiza fuliginosa) and medium ground finches (Geospiza fortis) to test if latent taste preferences might drive the selection of human foods. If human food flavours were consumed more than a neutral or bitter control only at sites with human foods, then we predicted tastes were acquired after urbanization; however, if no site differences were found then this would indicate latent taste preferences. Contrary to both predictions, we found little evidence that human food flavours were preferred compared with control flavours at any site. Instead, finches showed a weak aversion to oily foods, but only at remote (no human foods present) sites. This was further supported by behavioural responses, with beak-wiping occurring more often at remote sites after finches tasted flavours associated with human foods. Our results suggest, therefore, that while Darwin's finches regularly exposed to human foods might have acquired a tolerance to human food flavours, latent taste preferences are unlikely to have played a major role in their dietary response to increased urbanization.Peer reviewe

Electrostatic considerations affecting the calculated HOMO-LUMO gap in protein molecules.

A detailed study of energy differences between the highest occupied and lowest unoccupied molecular orbitals (HOMO-LUMO gaps) in protein systems and water clusters is presented. Recent work questioning the applicability of Kohn-Sham density-functional theory to proteins and large water clusters (E. Rudberg, J. Phys.: Condens. Mat. 2012, 24, 072202) has demonstrated vanishing HOMO-LUMO gaps for these systems, which is generally attributed to the treatment of exchange in the functional used. The present work shows that the vanishing gap is, in fact, an electrostatic artefact of the method used to prepare the system. Practical solutions for ensuring the gap is maintained when the system size is increased are demonstrated. This work has important implications for the use of large-scale density-functional theory in biomolecular systems, particularly in the simulation of photoemission, optical absorption and electronic transport, all of which depend critically on differences between energies of molecular orbitals.Comment: 13 pages, 4 figure

Theory and design of quantum cascade lasers in (111) n-type Si/SiGe

Although most work towards the realization of group IV quantum cascade lasers (QCLs) has focused on valence band transitions, there are many desirable properties associated with the conduction band. We show that the commonly cited shortcomings of n-type Si/SiGe heterostructures can be overcome by moving to the (111) growth direction. Specifically, a large band offset and low effective mass are achievable and subband degeneracy is preserved. We predict net gain up to lattice temperatures of 90 K in a bound-to-continuum QCL with a double-metal waveguide, and show that a Ge interdiffusion length of at least 8 Å across interfaces is tolerable

A scattering rate approach to the understanding of absorption line broadening in near-infrared AlGaN/GaN quantum wells

There has been much interest in the advancement of III-Nitride growth technology to fabricate AlGaN/GaN heterostructures for intersubband transitions (ISBTs). The large conduction band offset in these structures (up to 2 eV) allows transition energies in the near- to the far-infrared region, which have applications from telecommunications, such as in all-optical switches, to infra-red detectors for sensing and imaging. To date, ISBT electroluminescence has been elusive and absorption measurements remain an important method to verify band structure calculations. The growth quality can be inferred from the absorption spectrum, which will have line broadening with contributions that are both inhomogeneous (large-scale interface roughness, and non-parabolicity) and homogeneous (electron scattering related lifetime broadening). In the present work we calculated the contributions of various homogeneous broadening mechanisms (electron interaction with longitudinal-optical (LO) phonons, acoustic phonons, impurities and alloy disorder) to the full linewidth, and also the contribution of band non-parabolicity, which contributes to the inhomogeneous broadening. Calculations are then compared to the measured absorption spectra of several samples