The influence of magnetic sublattice dilution on magnetic order in CeNiGe3 and UNiSi2

Polycrystalline samples of the Y-diluted antiferromagnet CeNiGe3 (T_N = 5.5 K) and Th-diluted ferromagnet UNiSi2 (T_C = 95 K) were studied by means of x-ray powder diffraction, magnetization and specific heat measurements performed in a wide temperature range. The lattice parameters of the Ce1-xYxNiGe3 alloys decrease linearly with increasing the Y content, while the unit cell volume of U1-xThxNiSi2 increases linearly with rising the Th content. The ordering temperatures of the systems decrease monotonically with increasing x down to about 1.2 K in Ce0.4Y0.6NiGe3 and 26 K in U0.3Th0.7NiSi2, forming a dome of a long-range magnetic order on their magnetic phase diagrams. The suppression of the magnetic order is associated with distinct broadening of the anomalies at T_N,C due to crystallographic disorder being a consequence of the alloying. Below the magnetic percolation threshold xc of about 0.68 and 0.75 in the Ce- and U-based alloys, respectively, the long-range magnetic order smoothly evolves into a short-range one, forming a tail on the magnetic phase diagrams. The observed behaviour Ce1-xYxNiGe3 and U1-xThxNiSi2 is characteristic of diluted magnetic alloys. (c) 2012 IOP Publishing Ltd.Comment: This is an author-created, un-copyedited version of an article accepted for publication in Journal of Physics: Condensed Matter. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The definitive publisher-authenticated version is available online at doi:10.1088/0953-8984/24/27/27600

Atmospheric Circulation of Brown Dwarfs: Jets, Vortices, and Time Variability

A variety of observational evidence demonstrates that brown dwarfs exhibit active atmospheric circulations. In this study we use a shallow-water model to investigate the global atmospheric dynamics in the stratified layer overlying the convective zone on these rapidly rotating objects. We show that the existence and properties of the atmospheric circulation crucially depend on key parameters including the energy injection rate and radiative timescale. Under conditions of strong internal heat flux and weak radiative dissipation, a banded flow pattern comprising east-west jet streams spontaneously emerges from the interaction of atmospheric turbulence with the planetary rotation. In contrast, when the internal heat flux is weak and/or radiative dissipation is strong, turbulence injected into the atmosphere damps before it can self-organize into jets, leading to a flow dominated by transient eddies and isotropic turbulence instead. The simulation results are not very sensitive to the form of the forcing. Based on the location of the transition between jet-dominated and eddy-dominated regimes, we suggest that many brown dwarfs may exhibit atmospheric circulations dominated by eddies and turbulence (rather than jets) due to the strong radiative damping on these worlds, but a jet structure is also possible under some realistic conditions. Our simulated light curves capture important features from observed infrared lightcurves of brown dwarfs, including amplitude variations of a few percent and shapes that fluctuate between single-peak and multi-peak structures. More broadly, our work shows that the shallow-water system provides a useful tool to illuminate fundamental aspects of the dynamics on these worlds

Global-mean Vertical Tracer Mixing in Planetary Atmospheres II: Tidally Locked Planets

In Zhang $\&$ Showman (2018, hereafter Paper I), we developed an analytical theory of 1D eddy diffusivity $K_{zz}$ for global-mean vertical tracer transport in a 3D atmosphere. We also presented 2D numerical simulations on fast-rotating planets to validate our theory. On a slowly rotating planet such as Venus or a tidally locked planet (not necessarily a slow-rotator) such as a hot Jupiter, the tracer distribution could exhibit significant longitudinal inhomogeneity and tracer transport is intrinsically 3D. Here we study the global-mean vertical tracer transport on tidally locked planets using 3D tracer-transport simulations. We find that our analytical $K_{zz}$ theory in Paper I is validated on tidally locked planets over a wide parameter space. $K_{zz}$ strongly depends on the large-scale circulation strength, horizontal mixing due to eddies and waves and local tracer sources and sinks due to chemistry and microphysics. As our analytical theory predicted, $K_{zz}$ on tidally locked planets also exhibit three regimes In Regime I where the chemical and microphysical processes are uniformly distributed across the globe, different chemical species should be transported via different eddy diffusivity. In Regime II where the chemical and microphysical processes are non-uniform---for example, photochemistry or cloud formation that exhibits strong day-night contrast---the global-mean vertical tracer mixing does not always behave diffusively. In the third regime where the tracer is long-lived, non-diffusive effects are significant. Using species-dependent eddy diffusivity, we provide a new analytical theory of the dynamical quench points for disequilibrium tracers on tidally locked planets from first principles.Comment: Accepted at ApJ, 16 pages, 12 figures. This is the part II. Part I is "Global-mean Vertical Tracer Mixing in Planetary Atmospheres I: Theory and Fast-rotating Planets

Three-body Hydrogen Bond Defects Contribute Significantly to the Dielectric Properties of the Liquid Water-Vapor Interface

In this Letter, we present a simple model of aqueous interfacial molecular structure and we use this model to isolate the effects of hydrogen bonding on the dielectric properties of the liquid water-vapor interface. By comparing this model to the results of atomistic simulation we show that the anisotropic distribution of molecular orientations at the interface can be understood by considering the behavior of a single water molecule interacting with the average interfacial density field via an empirical hydrogen bonding potential. We illustrate that the depth dependence of this orientational anisotropy is determined by the geometric constraints of hydrogen bonding and we show that the primary features of simulated orientational distributions can be reproduced by assuming an idealized, perfectly tetrahedral hydrogen bonding geometry. We also demonstrate that non-ideal hydrogen bond geometries are required to produce interfacial variations in the average orientational polarization and polarizability. We find that these interfacial properties contain significant contributions from a specific type of geometrically distorted three-body hydrogen bond defect that is preferentially stabilized at the interface. Our findings thus reveal that the dielectric properties of the liquid water-vapor interface are determined by collective molecular interactions that are unique to the interfacial environment.Comment: 5 pages, 4 figure, S

Energy spectrum of the low-lying gluon excitations in the Coulomb gauge

We compute the energy spectrum of low-lying gluonic excitations in the presence of static quark-antiquark sources using Coulomb gauge and the quasi-particle representation. Within the valence sector of the Fock space we reproduce both, the overall normalization and the ordering of the spin-parity multiplets. We discus how the interactions induced by the non-abelian Coulomb kernel are central in to fine structure of the spectrum.Comment: 8 pages, 8 figure