Coexistence Regime Research Articles

The effect of environmental uncertainty and diapause investment on the occurrence of specialist and generalist species

The evolution of specialist and generalist strategies is a central topic in ecology with strong implications for the biodiversity and structure of communities. Environmental unpredictability has been suggested as a key factor affecting the relative advantages of generalist species. However, life cycle features, like diapause, can also play a major role in the competitive dynamics between generalists and specialists. Zooplanktonic communities of continental waters are suitable models to study this; they inhabit water bodies that vary temporally with different degrees of uncertainty and rely on the production of diapause stages to survive across the year. We developed a simple theoretical model with three competing species, i.e. two specialists and one generalist with lower competitive capability than the specialists. We explore the effect of environmental unpredictability and diapause modes on the coexistence regime and on the evolution of diapause investment patterns. Our aim is to get insight into factors affecting the diversity of zooplankton over an ecological time scale. Our results show that environmental uncertainty and diapause investment are important factors affecting the competitive dynamics between generalists and specialists strategies. Generalist species are able to persist through a hundred of growing seasons due to the “protection effect” afforded by density‐dependent diapause investment, and can even exclude the specialists (52% of replicates). Priority effects (i.e. which species has its period of habitat suitability earlier) and cross‐induction of diapause investment also have an impact on the structure of zooplanktonic communities. Diapause investment patterns were studied for a specialist species. Patterns to spread the risks of environmental variability by a means of a continuous and early production of diapausing eggs evolved within a hundred growing seasons.

Nanoscale structural evolution of electrically driven insulator to metal transition in vanadium dioxide

The structural evolution of tensile strained vanadium dioxide thin films was examined across the electrically driven insulator-to-metal transition by nanoscale hard X-ray diffraction. A metallic filament with rutile (R) structure was found to be the dominant conduction pathway for an electrically driven transition, while the majority of the channel area remained in the monoclinic M1 phase. The filament dimensions were estimated using simultaneous electrical probing and nanoscale X-ray diffraction. Analysis revealed that the width of the conducting channel can be tuned externally using resistive loads in series, enabling the M1/R phase ratio in the phase coexistence regime to be tuned.

Entanglement entropy and macroscopic quantum states with dipolar bosons in a triple-well potential

We study interacting dipolar atomic bosons in a triple-well potential within a ring geometry. This system is shown to be equivalent to a three-site Bose-Hubbard model. We analyze the ground state of dipolar bosons by varying the effective on-site interaction. This analysis is performed both numerically and analytically by using suitable coherent-state representations of the ground state. The latter exhibits a variety of forms ranging from the su(3) coherent state in the delocalization regime to a macroscopic cat-like state with fully localized populations, passing for a coexistence regime where the ground state displays a mixed character. We characterize the quantum correlations of the ground state from the bi-partition perspective. We calculate both numerically and analytically (within the previous coherent-state representation) the single-site entanglement entropy which, among various interesting properties, exhibits a maximum value in correspondence to the transition from the cat-like to the coexistence regime. In the latter case, we show that the ground-state mixed form corresponds, semiclassically, to an energy exhibiting two almost-degenerate minima.

Solvable model for template coexistence in protocells

Compartmentalization of self-replicating molecules (templates) in protocells is a necessary step towards the evolution of modern cells. However, coexistence between distinct template types inside a protocell can be achieved only if there is a selective pressure favoring protocells with a mixed template composition. Here we study analytically a group selection model for the coexistence between two template types using the diffusion approximation of population genetics. The model combines competition at the template and protocell levels as well as genetic drift inside protocells. At the steady state, we find a continuous phase transition separating the coexistence and segregation regimes, with the order parameter vanishing linearly with the distance to the critical point. In addition, we derive explicit analytical expressions for the critical steady-state probability density of protocell compositions.

Visualizing a dilute vortex liquid to solid phase transition in a Bi2Sr2CaCu2O8 single crystal

Using high-sensitivity magneto-optical imaging, we find evidence for a jump in local vortex density associated with a vortex liquid to vortex solid phase transition just above the lower critical field in a single crystal of Bi2Sr2CaCu2O8. We find that the regions of the sample where the jump in vortex density occurs are associated with low screening currents. In the field–temperature vortex phase diagram, we identify phase boundaries demarcating a dilute vortex liquid phase and the vortex solid phase. The phase diagram also identifies a coexistence regime of the dilute vortex liquid and solid phases and shows the effect of pinning on the vortex liquid to vortex solid phase transition line. We find that the phase boundary lines can be fitted to the theoretically predicted expression for the low-field portion of the phase boundary delineating a dilute vortex solid from a vortex liquid phase. We show that the same theoretical fit can be used to describe the pinning dependence of the low-field phase boundary lines provided that the dependence of the Lindemann number on pinning strength is considered.

Finite Size Scaling of the Dynamical Free-Energy in a Kinetically Constrained Model

We determine the finite size corrections to the large deviation function of the activity in a kinetically constrained model (the Fredrickson-Andersen model in one dimension), in the regime of dynamical phase coexistence. Numerical results agree with an effective model where the boundary between active and inactive regions is described by a Brownian interface.

Fe/MnAs bilayers: Magnetic anisotropy and the role of the interface

We have studied different aspects of the magnetic behavior of Fe(5nm)/MnAs(100nm) bilayers epitaxially grown on GaAs(100). Ferromagnetic resonance (FMR) measurements were performed in order to characterize the magnetic anisotropies of the films and the interlayer coupling between them. The chemical composition of the interface was investigated by X-ray photoemission spectroscopy (XPS).The iron layer FMR spectrum is highly temperature dependent between 280K and 320K as its parameters are strongly altered in the α/β phase coexistence regime of the MnAs layer, revealing a strong interlayer coupling in this T-region. Additionally, the XPS experiments demonstrate the existence of an Fe–MnAs intermixed interface. This result would explain the appearance of an additional uniaxial anisotropy in the plane of the Fe films, together with a strong reduction in its magnetocrystalline anisotropy and a negligible direct exchange coupling in the MnAs ferromagnetic state.

Evolution of the spin-density wave-superconductivity texture in the organic superconductor (TMTSF)2PF6 under pressure

We report an investigation at the endpoint region of the spin density wave state in (TMTSF)2PF6 where metal and superconductivity emerge. Thanks to resistivity measurements along the three main crystallographic directions, we are able to follow the texture in this phase coexistence regime. In this respect, superconductivity is used as a decoration technique of the metallic pattern. We show that metal (superconductivity) emerges first along the c⁎ direction in a counterintuitive manner. Then metal (superconductivity) domains evolves from filaments along the c⁎ axis towards slabs perpendicular to the a-axis which melt together in the homogeneous phase at high pressure. This evolution is compatible with the proposition of the formation of a soliton phase in the vicinity of the critical pressure of the (TMTSF)2PF6 phase diagram.

Collapse of a weak polyelectrolyte star in a poor solvent

We present a theory of intramolecular collapse transition in a weak (pH sensitive) polyelectrolyte (PE) star induced by a decrease in the solvent strength and/or variation in the ionic strength in the solution. Our system mimics conformational coil-to-globule transitions in individual star-shaped thermo- and pH-sensitive (e.g. poly(dimethylaminoethyl methacrylate)) macromolecules in dilute aqueous solutions. Systematic comparison with the behaviour of non-ionic and strong (quenched) polyelectrolyte stars in poor solvents enables us to unravel specific features of the collapse transition in weak polyelectrolyte stars. We demonstrate that, depending on temperature and the ionic strength of the solution, a vast diversity of different scenarios for the salt- or temperature-induced collapse transitions may take place. Both at high or low ionic strength a collapse transition induced by a decrease in the solvent strength occurs continuously resembling the collapse of a neutral polymer star. On the contrary, at intermediate salt concentrations the collapse of a weak polyelectrolyte star may feature a first order phase transition, which involves the co-existence of a collapsed, weakly ionized state with a swollen, strongly ionized state. At a fixed temperature the collapse transition can be triggered by an increase in salt concentration. In the latter case the transition may occur via a sequence of two co-existence regimes separated by continuous though non-monotonous variation in the star dimensions as a function of salt concentration.

Dynamically induced locking and unlocking transitions in driven layered systems with quenched disorder

Using numerical simulations, we examine a simple model of two or more coupled one-dimensional channels of driven particles with repulsive interactions in the presence of quenched disorder. We find that this model exhibits a remarkably rich variety of dynamical behavior as a function of the strength of the quenched disorder, coupling between channels, and external drive. For weaker disorder, the channels depin in a single step. For two channels we find dynamically induced decoupling transitions that result in coexisting pinned and moving phases as well as moving decoupled phases where particles in both channels move at different average velocities and slide past one another. Decoupling can also be induced by changing the relative strength of the disorder in neighboring channels. At higher drives, we observe a dynamical recoupling or locking transition into a state with no relative motion between the channels. This recoupling produces unusual velocity-force signatures, including negative differential conductivity. The depinning threshold shows distinct changes near the decoupling and coupling transitions and exhibits a peak effect phenomenon of the type that has been associated with transitions from elastic to plastic flow in other systems. We map several dynamic phase diagrams showing the coupling-decoupling transitions and the regions in which hysteresis occurs. We also examine the coexistence regime for channels with unequal amounts of quenched disorder. For multiple channels, multiple coupling and decoupling transitions can occur; however, many of the general features found for the two-channel system are still present. Our results should be relevant to depinning in layered geometries in systems such as vortices in layered or nanostructured superconductors and Wigner or colloidal particles confined in nanochannels; they are also relevant to the general understanding of plastic flow.

Magnetic resonance from the interplay of frustration and superconductivity

Motivated by iron-based superconductors, we develop a self-consistent electronic theory for the itinerant spin excitations in the regime of coexistence of the antiferromagnetic stripe order with wave vector ${\mathbf{Q}}_{1}=(\ensuremath{\pi},0)$ and ${s}^{+\ensuremath{-}}$ superconductivity. The onset of superconductivity leads to the appearance of a magnetic resonance near the wave vector ${\mathbf{Q}}_{2}=(0,\ensuremath{\pi})$, where magnetic order is absent. This resonance is isotropic in spin space, unlike the excitations near ${\mathbf{Q}}_{1}$, where the magnetic Goldstone mode resides. We discuss several features which can be observed experimentally.

Single-Nanowire Raman Microprobe Studies of Doping-, Temperature-, and Voltage-Induced Metal–Insulator Transitions of WxV1–xO2 Nanowires

Considerable recent research interest has focused on mapping the structural phase diagrams of anisotropic VO(2) nanobeams as model systems for elucidating single-domain behavior within strongly correlated electronic materials, to examine in particular the coupling of lattice and orbital degrees of freedom. Nevertheless, the role of substitutional doping in altering the phase stabilities of competing ground states of VO(2) remains underexplored. In this study, we use individual nanowire Raman microprobe mapping to examine the structural phase progressions underlying the metal-insulator transitions of solution-grown W(x)V(1-x)O(2) nanowires. The structural phase progressions have been monitored for three distinctive modes of inducing the electronic metal-insulator phase transition: as a function of (a) W doping at constant temperature, (b) varying temperature for specific W dopant concentrations, and (c) varying applied voltage for specific W dopant concentrations. Our results suggest the establishment of a coexistence regime within individual nanowires wherein M1 and R phases simultaneously exist before the percolation threshold is reached and the nanowire becomes entirely metallic. Such a coexistence regime has been found to exist during both temperature- and voltage-induced transitions. No evidence of an M2 phase is observed upon inducing the electronic phase transition by any of the three distinctive methods (temperature, doping, and applied voltage), suggesting that substitutional tungsten doping stabilizes the M1 phase over its M2 counterpart and further corroborating that the latter phase is not required to mediate M1→R transformations.

Melting of microgel colloidal crystals

We experimentally studied the melting of colloidal crystals composed of diameter-tunable microgel spheres by bright-field and confocal video microscopies at the single-particle level. Thick (> 4 layers) and thin (≤ 4 layers) films exhibited dramatically different melting kinetics. Thick films melted heterogeneously from grain boundaries in polycrystals and from surfaces in single crystals, while thin films melted homogeneously even in polycrystals. A novel heterogeneous melting at dislocation was observed in 5- to 12-layer films. The equilibrium phase behaviors in three thickness regimes were all different: thick films had a crystal-liquid coexistence regime which decreased with the film thickness and vanished at 4 layers, thin crystalline films melted into liquids in one step, while monolayers melted in two steps with an intermediate hexatic phase.

Competing epidemics on complex networks

Human diseases spread over networks of contacts between individuals and a substantial body of recent research has focused on the dynamics of the spreading process. Here we examine a model of two competing diseases spreading over the same network at the same time, where infection with either disease gives an individual subsequent immunity to both. Using a combination of analytic and numerical methods, we derive the phase diagram of the system and estimates of the expected final numbers of individuals infected with each disease. The system shows an unusual dynamical transition between dominance of one disease and dominance of the other as a function of their relative rates of growth. Close to this transition the final outcomes show strong dependence on stochastic fluctuations in the early stages of growth, dependence that decreases with increasing network size, but does so sufficiently slowly as still to be easily visible in systems with millions or billions of individuals. In most regions of the phase diagram we find that one disease eventually dominates while the other reaches only a vanishing fraction of the network, but the system also displays a significant coexistence regime in which both diseases reach epidemic proportions and infect an extensive fraction of the network.

Bimodal response in periodically driven diffusive systems

We study the response of one-dimensional diffusive systems, consisting of particles interacting via symmetric or asymmetric exclusion, to time-periodic driving from two reservoirs coupled to the ends. The dynamical response of the system can be characterized in terms of the structure factor. We find an interesting frequency-dependent response; the current-carrying majority excitons cyclically crosses over from a short wavelength mode to a long wavelength mode with an intermediate regime of coexistence. This effect being boundary driven decays inversely with system size. Analytic calculations show that this behavior is common to diffusive systems, both in the absence and presence of correlations.

Label-Free Determination of Compositional Fluctuations and Macroscopic Phase Separation in a Ternary Lipid Mixture

Reports on phase coexistence regimes and directions of tie lines of ternary lipid mixtures are often controversial. The origin of these controversies is typically the experimental window of the applied experimental techniques. Additional complications arrive due to putative influences of labels on the phase behavior. Therefore, we combined small- and wide-angle x-ray scattering, differential scanning calorimetry and attenuated total reflection-Fourier transform infrared spectroscopy to probe the stability and physical properties of coexisting domains under label-free conditions.

Quasiparticle interference in iron-based superconductors

We systematically calculate quasiparticle interference (QPI) signatures for the whole phase diagram of iron-based superconductors. Impurities inherent in the sample together with ordered phases lead to distinct features in the QPI images that are believed to be measured in spectroscopic imaging-scanning tunneling microscopy (SI-STM). In the spin-density wave phase the rotational symmetry of the electronic structure is broken, signatures of which are also seen in the coexistence regime with both superconducting and magnetic order. In the superconducting regime we show how the different scattering behavior for magnetic and non-magnetic impurities allows to verify the $s^{+-}$ symmetry of the order parameter. The effect of possible gap minima or nodes is discussed.

Thermally induced phase separation in supported bilayers of glycosphingolipid and phospholipid mixtures

The authors have studied microstructure evolution during thermally induced phase separation in a class of binary supported lipid bilayers using a quantitative application of imaging ellipsometry. The bilayers consist of binary mixtures consisting of a higher melting glycosphingolipid, galactosylceramide (GalCer), which resides primarily in the outer leaflet, and a lower melting, unsaturated phospholipid, 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC). Three different bilayer compositions of GalCer/DLPC mixtures at 35:65, 20:80, and 10:90 molar ratios were cooled at controlled rates from their high-temperature homogeneous phase to temperatures corresponding to their phase coexistence regime and imaged in real time using imaging ellipsometry. During the thermotropic course of GalCer gelation, we find that two distinct types of morphological features modulate. First, the formation and growth of chain and fractal-like defects ascribed to the net change in molecular areas during the phase transition. The formation of these defects is consistent with the expected contraction in the molecular area during the liquid crystalline to gel-phase transition. Second, the nucleation and growth of irregularly shaped gel-phase domains, which exhibit either line-tension dominated compact shape or dendritic domains with extended interfaces. Quantifying domain morphology within the fractal framework reveals a close correspondence, and the quantization of the transition width confirms previous estimates of reduced phase transition cooperativity in supported bilayers. A comparison of domain properties indicates that thermal history, bilayer composition, and cooling rate all influence microstructure details including shapes, sizes, and distributions of domains and defects: At lower cooling rates and lower GalCer fractions compact domains form and at higher GalCer fractions (or at higher cooling rates) dendritic domains are evident. This transition of domain morphology from compact shapes to dendritic shapes at higher cooling rates and higher relative fractions of GalCer suggests kinetic control of shape equilibration in these phospho- and glycolipid mixtures.

First-principles coexistence simulations of supercooled liquid silicon

We perform first-principles coexistence simulations of the low-density and the high-density phases of supercooled liquid silicon and find a negative slope for the coexisting line in the temperature–pressure plane. Electron density maps and electron-localization function plots of the two phases of silicon show marked differences. The calculated differences suggest more localized electrons in the low-density liquid compared to the high-density liquid, coming from an increased population of covalent bonds, which further explain the calculated negative slope in the two phase coexistence regime. This is consistent with the presence of a pseudo-gap in low-density liquid silicon, absent in the high-density liquid which shows a metallic behavior.

Melting of Colloidal Crystal Films

We study melting mechanisms in single and polycrystalline colloidal films composed of diameter-tunable microgel spheres with short-ranged repulsive interactions and confined between two glass walls. Thick films (>4 layers), thin-films (≤4 layers), and monolayers exhibit different melting behaviors. Thick films melt from grain boundaries in polycrystalline solid films and from film-wall interfaces in single-crystal films; a liquid-solid coexistence regime is observed in thick films but vanishes at a critical thickness of 4 layers. Thin solid films (2 to 4 layers) melt into the liquid phase in one step from both grain boundaries and from within crystalline domains. Monolayers melt in two steps with a middle hexatic phase.