Noble gas films on a decagonal AlNiCo quasicrystal

Thermodynamic properties of Ne, Ar, Kr, and Xe adsorbed on an Al-Ni-Co quasicrystalline surface (QC) are studied with Grand Canonical Monte Carlo by employing Lennard-Jones interactions with parameter values derived from experiments and traditional combining rules. In all the gas/QC systems, a layer-by-layer film growth is observed at low temperature. The monolayers have regular epitaxial fivefold arrangements which evolve toward sixfold close-packed structures as the pressure is increased. The final states can contain either considerable or negligible amounts of defects. In the latter case, there occurs a structural transition from five to sixfold symmetry which can be described by introducing an order parameter, whose evolution characterizes the transition to be continuous or discontinuous as in the case of Xe/QC (first-order transition with associated latent heat). By simulating fictitious noble gases, we find that the existence of the transition is correlated with the size mismatch between adsorbate and substrate's characteristic lengths. A simple rule is proposed to predict the phenomenon.Comment: 19 pages. 8 figures. (color figures can be seen at http://alpha.mems.duke.edu/wahyu/ or http://www.iop.org/EJ/abstract/0953-8984/19/1/016007/

Xe films on a decagonal Al-Ni-Co quasicrystal surface

The grand canonical Monte Carlo method is employed to study the adsorption of Xe on a quasicrystalline Al-Ni-Co surface. The calculation uses a semiempirical gas-surface interaction, based on conventional combining rules and the usual Lennard-Jones Xe-Xe interaction. The resulting adsorption isotherms and calculated structures are consistent with the results of LEED experimental data. In this paper we focus on five features not discussed earlier (Phys. Rev. Lett. 95, 136104 (2005)): the range of the average density of the adsorbate, the order of the transition, the orientational degeneracy of the ground state, the isosteric heat of adsorption of the system, and the effect of the vertical cell dimension.Comment: 6 pages, 5 pic

Stacked clusters of polycyclic aromatic hydrocarbon molecules

Clusters of polycyclic aromatic hydrocarbon (PAH) molecules are modelled using explicit all-atom potentials using a rigid body approximation. The PAH's considered range from pyrene (C10H8) to circumcoronene (C54H18), and clusters containing between 2 and 32 molecules are investigated. In addition to the usual repulsion-dispersion interactions, electrostatic point-charge interactions are incorporated, as obtained from density functional theory calculations. The general electrostatic distribution in neutral or singly charged PAH's is reproduced well using a fluctuating charges analysis, which provides an adequate description of the multipolar distribution. Global optimization is performed using a variety of methods, including basin-hopping and parallel tempering Monte Carlo. We find evidence that stacking the PAH molecules generally yields the most stable motif. A structural transition between one-dimensional stacks and three-dimensional shapes built from mutiple stacks is observed at larger sizes, and the threshold for this transition increases with the size of the monomer. Larger aggregates seem to evolve toward the packing observed for benzene in bulk.Difficulties met in optimizing these clusters are analysed in terms of the strong anisotropy of the molecules. We also discuss segregation in heterogeneous clusters and vibrational properties in the context of astrophysical observations.Comment: 12 pages, 7 figure

Engineering chiral density waves and topological band structures by multiple-QQ superpositions of collinear up-up-down-down orders

Magnetic orders characterized by multiple ordering vectors harbor noncollinear and noncoplanar spin textures and can be a source of unusual electronic properties through the spin Berry phase mechanism. We theoretically show that such multiple-$Q$ states are stabilized in itinerant magnets in the form of superpositions of collinear up-up-down-down (UUDD) spin states, which accompany the density waves of vector and scalar chirality. The result is drawn by examining the ground state of the Kondo lattice model with classical localized moments, especially when the Fermi surface is tuned to be partially nested by the symmetry-related commensurate vectors. We unveil the instability toward the multiple-$Q$ UUDD states with chirality density waves, using the perturbative theory, variational calculations, and large-scale Langevin dynamics simulations. We also show that the chirality density waves can induce rich nontrivial topology of electronic structures, such as the massless Dirac semimetal, Chern insulator with quantized topological Hall response, and peculiar edge states which depend on the phase of chirality density waves at the edges.Comment: 10 pages, 7 figure

Order parameter and detection for crystallized dipolar bosons in lattices

We explore the ground-state properties of bosons with dipole-dipole interactions in a one-dimensional optical lattice. Remarkably, a crystallization process happens for strong dipolar interactions. Herein, we provide a detailed characterization and a way to measure the resulting crystal phase. Using the eigenvalues of the reduced one-body density matrix we define an order parameter that yields a phase diagram in agreement with an analysis of the density and two-body density. We demonstrate that the phase diagram can be detected experimentally using the variance of single-shot measurements.Comment: 6 pages, 3 figures. Supplementary Information included. Software available at http://ultracold.org

Quantum gases in trimerized kagom\'e lattices

We study low temperature properties of atomic gases in trimerized optical kagom\'{e} lattices. The laser arrangements that can be used to create these lattices are briefly described. We also present explicit results for the coupling constants of the generalized Hubbard models that can be realized in such lattices. In the case of a single component Bose gas the existence of a Mott insulator phase with fractional numbers of particles per trimer is verified in a mean field approach. The main emphasis of the paper is on an atomic spinless interacting Fermi gas in the trimerized kagom\'{e} lattice with two fermions per site. This system is shown to be described by a quantum spin 1/2 model on the triangular lattice with couplings that depend on the bond directions. We investigate this model by means of exact diagonalization. Our key finding is that the system exhibits non-standard properties of a quantum spin-liquid crystal: it combines planar antiferromagnetic order in the ground state with an exceptionally large number of low energy excitations. The possibilities of experimental verification of our theoretical results are critically discussed.Comment: 19 pages/14 figures, version to appear in Phys. Rev. A., numerous minor corrections with respect to former lanl submissio

Fluctuation induced vortex pattern and its disordering in the fully frustrated XY model on a dice lattice

A highly degenerate family of states [proposed in PRB 63, 134503 (2001)] is proven to really minimize the Hamiltonian of the fully frustrated XY model on a dice lattice. The harmonic fluctuations are shown to be no consequence for the removal of the accidental degeneracy of these states, so a particular vortex pattern can be stabilized only by the anharmonic fluctuations. The structure of this pattern is found and the temperature of its disordering due to the proliferation of domain walls is estimated. The extreme smallness of the fluctuations induced free energy of domain walls leads to the anomalous prominence of the finite-size effects, which prevent the observation of vortex-pattern ordering in numerical simulations. In such a situation the loss of phase coherence may be related to the dissociation of fractional vortices with topological charges 1/8. In a physical situation the magnetic interaction of currents in a Josephson junction array will be a more important source for the stabilization of a particular vortex pattern than the anharmonic fluctuations.Comment: 20 pages, 7 figure

Enhanced Quantum Effects in an Ultra-Small Coulomb Blockaded Device Operating at Room-Temperature

An ultra-small Coulomb blockade device can be regarded as a mesoscopic artificial atom system and provides a rich experimental environment for studying quantum transport phenomena[1]. Previously, these quantum effects have been investigated using relatively large devices at ultra-low temperatures, where they give rise to a fine additional structure on the Coulomb oscillations [2-13]. Here, we report transport measurements carried out on a sub-2nm single-electron device; this size is sufficiently small that Coulomb blockade, and other quantum effects, persist up to room temperature (RT). These devices were made by scaling the size of a FinFET structure down to an ultimate limiting form, resulting in the reliable formation of a sub-2nm silicon Coulomb island. Four clear Coulomb diamonds can be observed at RT and the 2nd Coulomb diamond is unusually large, due to quantum confinement. The observed characteristics are successfully modeled on the basis of a very low electron number on the island, combined with Pauli spin exclusion. These effects offer additional functionality for future RT-operating single-electron device applicationsComment: 7 pages, 4 figure

Low-temperature properties of single-crystal CrB2_{2}

We report the low-temperature properties of $^{11}$B-enriched single-crystal CrB$_{2}$ as prepared from high-purity Cr and B powder by a solid-state reaction and optical float zoning. The electrical resistivity, $\rho_{\rm xx}$, Hall effect, $\rho_{\rm xy}$, and specific heat, $C$, are characteristic of an exchange-enhanced Fermi liquid ground state, which develops a slightly anisotropic spin gap $\Delta \approx 220\,{\rm K}$ below $T_{\rm N}=88\,{\rm K}$. This observation is corroborated by the absence of a Curie dependence in the magnetization for $T\to0$ reported in the literature. Comparison of $C$ with $d\rho_{\rm xx}/dT$, where we infer lattice contributions from measurements of VB$_2$, reveals strong antiferromagnetic spin fluctuations with a characteristic spin fluctuation temperature $T_{\rm sf}\approx 257\,{\rm K}$ in the paramagnetic state, followed by a pronounced second-order mean-field transition at $T_{\rm N}$, and unusual excitations around $\approx T_{\rm N}/2$. The pronounced anisotropy of $\rho_{\rm xx}$ above $T_{\rm N}$ is characteristic of an easy-plane anisotropy of the spin fluctuations consistent with the magnetization. The ratio of the Curie-Weiss to the N$\acute{\rm{e}}$el temperatures, $f=-\Theta_{\rm CW}/T_{\rm N}\approx 8.5$, inferred from the magnetization, implies strong geometric frustration. All physical properties are remarkably invariant under applied magnetic fields up to $14\,\,{\rm T}$, the highest field studied. In contrast to earlier suggestions of local-moment magnetism our study identifies CrB$_{2}$ as a weak itinerant antiferromagnet par excellence with strong geometric frustration.Comment: 15 pages, 9 figure

Global priority areas for ecosystem restoration

Extensive ecosystem restoration is increasingly seen as being central to conserving biodiversity1 and stabilizing the climate of the Earth2. Although ambitious national and global targets have been set, global priority areas that account for spatial variation in benefits and costs have yet to be identified. Here we develop and apply a multicriteria optimization approach that identifies priority areas for restoration across all terrestrial biomes, and estimates their benefits and costs. We find that restoring 15% of converted lands in priority areas could avoid 60% of expected extinctions while sequestering 299 gigatonnes of CO2—30% of the total CO2 increase in the atmosphere since the Industrial Revolution. The inclusion of several biomes is key to achieving multiple benefits. Cost effectiveness can increase up to 13-fold when spatial allocation is optimized using our multicriteria approach, which highlights the importance of spatial planning. Our results confirm the vast potential contributions of restoration to addressing global challenges, while underscoring the necessity of pursuing these goals synergistically.Fil: Strassburg, Bernardo B. N.. Pontifícia Universidade Católica do Rio de Janeiro; Brasil. Universidade Federal do Rio de Janeiro; BrasilFil: Iribarrem, Alvaro. Pontifícia Universidade Católica do Rio de Janeiro; BrasilFil: Beyer, Hawthorne L.. The University of Queensland; Australia. University of Queensland; AustraliaFil: Cordeiro, Carlos Leandro. Pontifícia Universidade Católica do Rio de Janeiro; BrasilFil: Crouzeilles, Renato. Universidade Federal do Rio de Janeiro; Brasil. Pontifícia Universidade Católica do Rio de Janeiro; BrasilFil: Jakovac, Catarina C.. Pontifícia Universidade Católica do Rio de Janeiro; BrasilFil: Braga Junqueira, André. Pontifícia Universidade Católica do Rio de Janeiro; BrasilFil: Lacerda, Eduardo. Pontifícia Universidade Católica do Rio de Janeiro; Brasil. Universidade Federal Fluminense; BrasilFil: Latawiec, Agnieszka E.. University of East Anglia; Reino Unido. Pontifícia Universidade Católica do Rio de Janeiro; BrasilFil: Balmford, Andrew. University of Cambridge; Estados UnidosFil: Brooks, Thomas M.. University Of The Philippines Los Banos; Filipinas. Institute For Marine And Antarctic Studies; Australia. International Union For Conservation Of Nature And Natural Resources; SuizaFil: Butchart, Stuart H. M.. University of Cambridge; Estados UnidosFil: Chazdon, Robin L.. University Of The Sunshine Coast; Australia. University of Connecticut; Estados UnidosFil: Erb, Karl-Heinz. Universitat Fur Bodenkultur Wien; AustriaFil: Brancalion, Pedro. Universidade de Sao Paulo; BrasilFil: Buchanan, Graeme. Royal Society For The Protection Of Birds; Reino UnidoFil: Cooper, David. Secretariat Of The Convention On Biological Diversity; CanadáFil: Díaz, Sandra Myrna. Universidad Nacional de Córdoba; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto Multidisciplinario de Biología Vegetal. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Instituto Multidisciplinario de Biología Vegetal; ArgentinaFil: Donald, Paul F.. University of Cambridge; Estados UnidosFil: Kapos, Valerie. United Nations Environment Programme World Conservation Monitoring Centre; Reino UnidoFil: Leclère, David. International Institute For Applied Systems Analysis, Laxenburg; AustriaFil: Miles, Lera. United Nations Environment Programme World Conservation Monitoring Centre; Reino UnidoFil: Obersteiner, Michael. Oxford Social Sciences Division; Reino Unido. International Institute For Applied Systems Analysis, Laxenburg; AustriaFil: Plutzar, Christoph. Universitat Fur Bodenkultur Wien; Austria. Universidad de Viena; AustriaFil: de M. Scaramuzza, Carlos Alberto. International Institute For Sustainability; BrasilFil: Scarano, Fabio R.. Universidade Federal do Rio de Janeiro; BrasilFil: Visconti, Piero. International Institute For Applied Systems Analysis, Laxenburg; Austri