A conceptual design and operational characteristics for a Mars rover for a 1979 or 1981 Viking science mission

The feasibility of a small Mars rover for use on a 1979 or 1981 Viking mission was studied and a preliminary design concept was developed. Three variations of the concept were developed to provide comparisons in mobility and science capability of the rover. Final masses of the three rover designs were approximately 35 kg, 40 kg, and 69 kg. The smallest rover is umbilically connected to the lander for power and communications purposes whereas the larger two rovers have secondary battery power and a 2-way very high frequency communication link to the lander. The capability for carrying Viking rovers (including development system) to the surface of Mars was considered first. It was found to be feasible to carry rovers of over 100 kg. Virtually all rover systems were then studied briefly to determine a feasible system concept and a practical interface with the comparable system of a 1979 or 1981 lander vehicle

Wang-Landau sampling for quantum systems: algorithms to overcome tunneling problems and calculate the free energy

We present a generalization of the classical Wang-Landau algorithm [Phys. Rev. Lett. 86, 2050 (2001)] to quantum systems. The algorithm proceeds by stochastically evaluating the coefficients of a high temperature series expansion or a finite temperature perturbation expansion to arbitrary order. Similar to their classical counterpart, the algorithms are efficient at thermal and quantum phase transitions, greatly reducing the tunneling problem at first order phase transitions, and allow the direct calculation of the free energy and entropy.Comment: Added a plot showing the efficiency at first order phase transition

Field-Induced Magnetic Order in Quantum Spin Liquids

We study magnetic field-induced three-dimensional ordering transitions in low-dimensional quantum spin liquids, such as weakly coupled, antiferromagnetic spin-1/2 Heisenberg dimers and ladders. Using stochastic series expansion quantum Monte Carlo simulations, thermodynamic response functions are obtained down to ultra-low temperatures. We extract the critical scaling exponents which dictate the power-law dependence of the transition temperature on the applied magnetic field. These are compared with recent experiments on candidate materials and with predictions for the Bose-Einstein condensation of magnons obtained in mean-field theory.Comment: RevTex, 4 pages with 5 figure

NMR and dc-susceptibility studies of NaVGe2O6

We report the results of measurements of the dc magnetic susceptibility chi(T) and of the 23Na nuclear magnetic resonance (NMR) response of NaVGe2O6, a material in which the V ions form a network of interacting one-dimensional spin S=1 chains. The experiments were made at temperatures between 2.5 and 300 K. The chi(T) data suggest that the formation of the expected low-temperature Haldane phase is intercepted by an antiferromagnetic phase transition at 18 K. The transition is also reflected in the 23Na NMR spectra and the corresponding spin-lattice relaxation rate 1/T1(T). In the ordered phase, 1/T1(T) decreases by orders of magnitude with decreasing temperature, indicating the formation of a gap of the order of 12 K in the magnetic excitation spectrum.Comment: 10 pages, 15 figures; v2 with minor revisions of the tex

A nitrogen model for European catchments: INCA, new model structure and equations

International audienceA new version of the Integrated Nitrogen in Catchments model (INCA) was developed and tested using flow and streamwater nitrate concentration data collected from the River Kennet during 1998. INCA is a process-based model of the nitrogen cycle in the plant/soil and in-stream systems. The model simulates the nitrogen export from different land-use types within a river system, and the in-stream nitrate and ammonium concentrations at a daily time-step. The structure of the new version differs from the original, in that soil-water retention volumes have been added and the interface adapted to permit multiple crop and vegetation growth periods and fertiliser applications. The process equations are now written in terms of loads rather than concentrations allowing a more robust tracking of mass conservation when using numerical integration. The new version is able to reproduce the seasonal dynamics observed in the streamwater nitrogen concentration data, and the loads associated with plant/soil system nitrogen processes reported in the literature. As such, the model results suggest that the new structure is appropriate for the simulation of nitrogen in the River Kennet and an improvement on the original model. The utility of the INCA model is discussed in terms of improving scientific understanding and catchment management. Keywords: modelling, water quality, nitrogen, nitrate, River Kennet, River Thames</p

Synthesis of Alkaline Earth Diazenides MAEN2 (MAE = Ca, Sr, Ba) by Controlled Thermal Decomposition of Azides under High Pressure

The alkaline earth diazenides MAEN2 with MAE = Ca, Sr and Ba were synthesized by a novel synthetic approach, namely, a controlled decomposition of the corresponding azides in a multianvil press at highpressure/ high-temperature conditions. The crystal structure of hitherto unknown calcium diazenide (space group I4/mmm (no. 139), a = 3.5747(6) Å, c = 5.9844(9) Å, Z = 2, wRp = 0.078) was solved and refined on the basis of powder X-ray diffraction data as well as that of SrN2 and BaN2. Accordingly, CaN2 is isotypic with SrN2 (space group I4/mmm (no. 139), a = 3.8054(2) Å, c = 6.8961(4) Å, Z = 2, wRp = 0.057) and the corresponding alkaline earth acetylenides (MAEC2) crystallizing in a tetragonally distorted NaCl structure type. In accordance with literature data, BaN2 adopts a more distorted structure in space group C2/c (no. 15) with a = 7.1608(4) Å, b = 4.3776(3) Å, c = 7.2188(4) Å, β = 104.9679(33)°, Z = 4 and wRp = 0.049). The N−N bond lengths of 1.202(4) Å in CaN2 (SrN2 1.239(4) Å, BaN2 1.23(2) Å) correspond well with a double-bonded dinitrogen unit confirming a diazenide ion [N2]2−. Temperature-dependent in situ powder X-ray diffractometry of the three alkaline earth diazenides resulted in formation of the corresponding subnitrides MAE2N (MAE = Ca, Sr, Ba) at higher temperatures. FTIR spectroscopy revealed a band at about 1380 cm−1 assigned to the N−N stretching vibration of the diazenide unit. Electronic structure calculations support the metallic character of alkaline earth diazenides

Bosons in optical lattices - from the Mott transition to the Tonks-Girardeau gas

We present results from quantum Monte Carlo simulations of trapped bosons in optical lattices, focusing on the crossover from a gas of softcore bosons to a Tonks-Girardeau gas in a one-dimensional optical lattice. We find that depending on the quantity being measured, the behavior found in the Tonks-Girardeau regime is observed already at relatively small values of the interaction strength. A finite critical value for entering the Tonks-Girardeau regime does not exist. Furthermore, we discuss the computational efficiency of two quantum Monte Carlo methods to simulate large scale trapped bosonic systems: directed loops in stochastic series expansions and the worm algorithm.Comment: 7 pages with 9 figures;v2: improved discussion on Tonks-Girardeau ga

Novel Mechanism of Supersolid of Ultracold Polar Molecules in Optical Lattices

We study the checkerboard supersolid of the hard-core Bose-Hubbard model with the dipole-dipole interaction. This supersolid is different from all other supersolids found in lattice models in the sense that superflow paths through which interstitials or vacancies can hop freely are absent in the crystal. By focusing on repulsive interactions between interstitials, we reveal that the long-range tail of the dipole-dipole interaction have the role of increasing the energy cost of domain wall formations. This effect produces the supersolid by the second-order hopping process of defects. We also perform exact quantum Monte Carlo simulations and observe a novel double peak structure in the momentum distribution of bosons, which is a clear evidence for supersolid. This can be measured by the time-of-flight experiment in optical lattice systems

Emergence of magnetism in graphene materials and nanostructures

Magnetic materials and nanostructures based on carbon offer unique opportunities for future technological applications such as spintronics. This article reviews graphene-derived systems in which magnetic correlations emerge as a result of reduced dimensions, disorder and other possible scenarios. In particular, zero-dimensional graphene nanofragments, one-dimensional graphene nanoribbons, and defect-induced magnetism in graphene and graphite are covered. Possible physical mechanisms of the emergence of magnetism in these systems are illustrated with the help of computational examples based on simple model Hamiltonians. In addition, this review covers spin transport properties, proposed designs of graphene-based spintronic devices, magnetic ordering at finite temperatures as well as the most recent experimental achievements.Comment: tutorial-style review article -- 18 pages, 19 figure

Basal and LPS-stimulated inflammatory markers and the course of individual symptoms of depression

Multiple studies show an association between inflammatory markers and major depressive disorder (MDD). People with chronic low-grade inflammation may be at an increased risk of MDD, often in the form of sickness behaviors. We hypothesized that inflammation is predictive of the severity and the course of a subset of MDD symptoms, especially symptoms that overlap with sickness behavior, such as anhedonia, anorexia, low concentration, low energy, loss of libido, psychomotor slowness, irritability, and malaise. We tested the association between basal and lipopolysaccharide (LPS)-induced inflammatory markers with individual MDD symptoms (measured using the Inventory of Depressive Symptomatology Self-Report) over a period of up to 9 years using multivariate-adjusted mixed models in 1147–2872 Netherlands Study of Depression and Anxiety (NESDA) participants. At baseline, participants were on average 42.2 years old, 66.5% were women and 53.9% had a current mood or anxiety disorder. We found that basal and LPS-stimulated inflammatory markers were more strongly associated with sickness behavior symptoms at up to 9-year follow-up compared with non-sickness behavior symptoms of depression. However, we also found significant associations with some symptoms that are not typical of sickness behavior (e.g., sympathetic arousal among others). Inflammation was not related to depression as a unified syndrome but rather to the presence and the course of specific MDD symptoms, of which the majority were related to sickness behavior. Anti-inflammatory strategies should be tested in the subgroup of MDD patients who report depressive symptoms related to sickness behavior