Goodness-of-fit measures of evenness: a new tool for exploring changes in community structure

Growing concern about the fate of biodiversity, highlighted by the Convention on Biological Diversity's 2010 and 2020 targets for stemming biodiversity loss, has intensified interest in methods of assessing change in ecological communities through time. Biodiversity is a multivariate concept, which cannot be well‐represented by a single measure. However, diversity profiles summarize the multivariate nature of multi‐species datasets, and allow a more nuanced interpretation of biodiversity trends than unitary metrics. Here we introduce a new approach to diversity profiling. Our method is based on the knowledge that an ecological community is never completely even and uses this departure from perfect evenness as a novel and insightful way of measuring diversity. We plot our measure of departure as a function of a free parameter, to generate “evenness profiles”. These profiles allow us to separate changes due to dominant species from those due to rare species, and relate these patterns to shifts in overall diversity. This separation of the influence of dominance and rarity on overall diversity enables the user to uncover changes in diversity that would be masked in other methods. We discuss profiling techniques based on this parametric family, and explore its connections with existing diversity indices. Next, we evaluate our approach in terms of predicted community structure (following Tokeshi's niche models) and present an example assessing temporal trends in diversity of British farmland birds. We conclude that this method is an informative and tractable parametric approach for quantifying evenness. It provides novel insights into community structure, revealing the contributions of both rare and common species to biodiversity trends

Detecting neutral hydrogen in emission at redshift z ~ 1

We use a large N-body simulation to examine the detectability of HI in emission at redshift z ~ 1, and the constraints imposed by current observations on the neutral hydrogen mass function of galaxies at this epoch. We consider three different models for populating dark matter halos with HI, designed to encompass uncertainties at this redshift. These models are consistent with recent observations of the detection of HI in emission at z ~ 0.8. Whilst detection of 21 cm emission from individual halos requires extremely long integrations with existing radio interferometers, such as the Giant Meter Radio Telescope (GMRT), we show that the stacked 21 cm signal from a large number of halos can be easily detected. However, the stacking procedure requires accurate redshifts of galaxies. We show that radio observations of the field of the DEEP2 spectroscopic galaxy redshift survey should allow detection of the HI mass function at the 5-12 sigma level in the mass range 10^(11.4) M_sun/h < M_halo < 10^(12.5)M_sun/h, with a moderate amount of observation time. Assuming a larger noise level that corresponds to an upper bound for the expected noise for the GMRT, the detection significance for the HI mass function is still at the 1.7-3 sigma level. We find that optically undetected satellite galaxies enhance the HI emission profile of the parent halo, leading to broader wings as well as a higher peak signal in the stacked profile of a large number of halos. We show that it is in principle possible to discern the contribution of undetected satellites to the total HI signal, even though cosmic variance limitation make this challenging for some of our models.Comment: 14 pages, 9 figures, Submitted To MNRA

The Information Geometry of the Ising Model on Planar Random Graphs

It has been suggested that an information geometric view of statistical mechanics in which a metric is introduced onto the space of parameters provides an interesting alternative characterisation of the phase structure, particularly in the case where there are two such parameters -- such as the Ising model with inverse temperature $\beta$ and external field $h$. In various two parameter calculable models the scalar curvature ${\cal R}$ of the information metric has been found to diverge at the phase transition point $\beta_c$ and a plausible scaling relation postulated: ${\cal R} \sim |\beta- \beta_c|^{\alpha - 2}$. For spin models the necessity of calculating in non-zero field has limited analytic consideration to 1D, mean-field and Bethe lattice Ising models. In this letter we use the solution in field of the Ising model on an ensemble of planar random graphs (where $\alpha=-1, \beta=1/2, \gamma=2$) to evaluate the scaling behaviour of the scalar curvature, and find ${\cal R} \sim | \beta- \beta_c |^{-2}$. The apparent discrepancy is traced back to the effect of a negative $\alpha$.Comment: Version accepted for publication in PRE, revtex

Tight-binding g-Factor Calculations of CdSe Nanostructures

The Lande g-factors for CdSe quantum dots and rods are investigated within the framework of the semiempirical tight-binding method. We describe methods for treating both the n-doped and neutral nanostructures, and then apply these to a selection of nanocrystals of variable size and shape, focusing on approximately spherical dots and rods of differing aspect ratio. For the negatively charged n-doped systems, we observe that the g-factors for near-spherical CdSe dots are approximately independent of size, but show strong shape dependence as one axis of the quantum dot is extended to form rod-like structures. In particular, there is a discontinuity in the magnitude of g-factor and a transition from anisotropic to isotropic g-factor tensor at aspect ratio ~1.3. For the neutral systems, we analyze the electron g-factor of both the conduction and valence band electrons. We find that the behavior of the electron g-factor in the neutral nanocrystals is generally similar to that in the n-doped case, showing the same strong shape dependence and discontinuity in magnitude and anisotropy. In smaller systems the g-factor value is dependent on the details of the surface model. Comparison with recent measurements of g-factors for CdSe nanocrystals suggests that the shape dependent transition may be responsible for the observations of anomalous numbers of g-factors at certain nanocrystal sizes.Comment: 15 pages, 6 figures. Fixed typos to match published versio

Quantum fluctuations of charge order induce phonon softening in a superconducting cuprate

Quantum phase transitions play an important role in shaping the phase diagram of high-temperature cuprate superconductors. These cuprates possess intertwined orders which interact strongly with superconductivity. However, the evidence for the quantum critical point associated with the charge order in the superconducting phase remains elusive. Here we show the short-range charge orders and the spectral signature of the quantum fluctuations in La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) near the optimal doping using high-resolution resonant inelastic X-ray scattering. On performing calculations through a diagrammatic framework, we discovered that the charge correlations significantly soften several branches of phonons. These results elucidate the role of charge order in the LSCO compound, providing evidence for quantum critical scaling and discommensurations associated with charge order

Quantum fluctuations of charge order induce phonon softening in a superconducting cuprate

Quantum phase transitions play an important role in shaping the phase diagram of high-temperature cuprate superconductors. These cuprates possess intertwined orders which interact strongly with superconductivity. However, the evidence for the quantum critical point associated with the charge order in the superconducting phase remains elusive. Here we show the short-range charge orders and the spectral signature of the quantum fluctuations in La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) near the optimal doping using high-resolution resonant inelastic X-ray scattering. On performing calculations through a diagrammatic framework, we discovered that the charge correlations significantly soften several branches of phonons. These results elucidate the role of charge order in the LSCO compound, providing evidence for quantum critical scaling and discommensurations associated with charge order

The role of beach state and the timing of pre-storm surveys in determining the accuracy of storm impact assessments

Dune erosion principally occurs when water level exceeds the elevation of the beach and predicting erosion is progressively becoming more important for management as coastal populations increase, sea level rises, and storms become more powerful. This study assesses storm impacts using a simple model from Stockdon et al. (2007) configured with oceanographic information from the ADCIRC + SWAN model and frequently collected beach profiles. We applied that model to barrier islands in North Carolina including: Core Banks with a more dissipative beach morphology and Shackleford Banks and Onslow Beach with intermediate beach morphologies. The study periods captured 10 events where wave collision with the dunes and/or overwash were either predicted or observed, including large multiple-day events caused by hurricanes and smaller events caused by onshore winds and high tide. Comparing model output with a time series of beach photographs shows the predictive power and sensitivity of the model was consistently high at the Core Banks Site with its wide and low-gradient beach, high-elevation dunes (2.58 m), and high resistance to overwash. Model predictive power and sensitivity was lowest at the Shackleford Banks Site because frequent and large changes to beach slope and intermediate dune elevation (0.54–1.25 m) caused small variations of modeled total water level to either overpredict or underpredict storm impacts. In addition, storm impacts were always overpredicted during hurricanes at the Shackleford Banks Site, which was likely due to storm waves decreasing the beach slope from what was measured prior to the event and used as model input. Like Shackleford Banks, the beach slope of the Onslow Beach Site was steep and variable, but the low-elevation dunes (0.24–0.28 m) made resistance to overwash low and the predictive power and sensitivity of the model higher than at the Shackleford Banks Site. Results suggest that storm impacts and the associated potential for dune erosion is predicted more accurately at beaches where the threshold for overwash is high or low because total water level during most events will commonly fall short of or exceed the overwash threshold, respectively. The accuracy of predicting the storm impact regime is sensitive to beach slope. The slope of intermediate beaches is more variable than dissipative beaches and requires frequent measurement if it is to be represented accurately in the model, but this can be impractical and costly even using the latest drone-surveying methods. To maximize the accuracy of predicting storm impacts, intermediate beach morphology should be constrained by surveying at seasonal or yearly time scales and used as input to numerical models that estimate beach slope over short time scales (hours during an event or daily), configured with the latest wave and water-level forecasts

Sr2V3O9 and Ba2V3O9: quasi one-dimensional spin-systems with an anomalous low temperature susceptibility

The magnetic behaviour of the low-dimensional Vanadium-oxides Sr2V3O9 and Ba2V3O9 was investigated by means of magnetic susceptibility and specific heat measurements. In both compounds, the results can be very well described by an S=1/2 Heisenberg antiferromagnetic chain with an intrachain exchange of J = 82 K and J = 94 K in Sr2V3O9 and Ba2V3O9, respectively. In Sr2V3O9, antiferromagnetic ordering at T_N = 5.3 K indicate a weak interchain exchange of the order of J_perp ~ 2 K. In contrast, no evidence for magnetic order was found in Ba2V3O9 down to 0.5 K, pointing to an even smaller interchain coupling. In both compounds, we observe a pronounced Curie-like increase of the susceptibility below 30 K, which we tentatively attribute to a staggered field effect induced by the applied magnetic field. Results of LDA calculations support the quasi one-dimensional character and indicate that in Sr2V3O9, the magnetic chain is perpendicular to the structural one with the magnetic exchange being transferred through VO4 tetrahedra.Comment: Submitted to Phy. Rev.

From nodal liquid to nodal Mottness in a frustrated Hubbard model

We investigate the physics of frustrated 3-leg Hubbard ladders in the band limit, when hopping across the ladder's rungs (t$_{\perp}$) is of the same order as hopping along them (t) much greater than the onsite Coulomb repulsion (U). We show that this model exhibits a striking electron-hole asymmetry close to half-filling: the hole-doped system at low temperatures develops a Resonating Valence Bond (RVB)-like d-wave gap (pseudogap close to ($\pi$,0)) coinciding with gapless nodal excitations (nodal liquid); in contrast, the electron-doped system is seen to develop a Mott gap at the nodes, whilst retaining a metallic character of its majority Fermi surface. At lower temperatures in the electron-doped case, d-wave superconducting correlations -- here, coexisting with gapped nodal excitations -- are already seen to arise. Upon further doping the hole-doped case, the RVB-like state yields to d-wave superconductivity. Such physics is reminiscent of that exhibited by the high temperature cuprate superconductors--notably electron-hole asymmetry as noted by Angle Resolved PhotoEmission Spectroscopy (ARPES) and the resistivity exponents observed. This toy model also reinforces the importance of a more thorough experimental investigation of the known 3-leg ladder cuprate systems, and may have some bearing on low dimensional organic superconductors.Comment: 26 pages, 16 figure

The Information Geometry of the Spherical Model

Motivated by previous observations that geometrizing statistical mechanics offers an interesting alternative to more standard approaches,we have recently calculated the curvature (the fundamental object in this approach) of the information geometry metric for the Ising model on an ensemble of planar random graphs. The standard critical exponents for this model are alpha=-1, beta=1/2, gamma=2 and we found that the scalar curvature, R, behaves as epsilon^(-2),where epsilon = beta_c - beta is the distance from criticality. This contrasts with the naively expected R ~ epsilon^(-3) and the apparent discrepancy was traced back to the effect of a negative alpha on the scaling of R. Oddly,the set of standard critical exponents is shared with the 3D spherical model. In this paper we calculate the scaling behaviour of R for the 3D spherical model, again finding that R ~ epsilon^(-2), coinciding with the scaling behaviour of the Ising model on planar random graphs. We also discuss briefly the scaling of R in higher dimensions, where mean-field behaviour sets in.Comment: 7 pages, no figure