Asymmetric Phase Diagram Research Articles

Observation of dichotomic field-tunable electronic structure in twisted monolayer-bilayer graphene

Twisted bilayer graphene (tBLG) provides a fascinating platform for engineering flat bands and inducing correlated phenomena. By designing the stacking architecture of graphene layers, twisted multilayer graphene can exhibit different symmetries with rich tunability. For example, in twisted monolayer-bilayer graphene (tMBG) which breaks the C2z symmetry, transport measurements reveal an asymmetric phase diagram under an out-of-plane electric field, exhibiting correlated insulating state and ferromagnetic state respectively when reversing the field direction. Revealing how the electronic structure evolves with electric field is critical for providing a better understanding of such asymmetric field-tunable properties. Here we report the experimental observation of field-tunable dichotomic electronic structure of tMBG by nanospot angle-resolved photoemission spectroscopy (NanoARPES) with operando gating. Interestingly, selective enhancement of the relative spectral weight contributions from monolayer and bilayer graphene is observed when switching the polarity of the bias voltage. Combining experimental results with theoretical calculations, the origin of such field-tunable electronic structure, resembling either tBLG or twisted double-bilayer graphene (tDBG), is attributed to the selectively enhanced contribution from different stacking graphene layers with a strong electron-hole asymmetry. Our work provides electronic structure insights for understanding the rich field-tunable physics of tMBG.

Method to Rapidly Identify Potential Solvent Systems for Crystallization of Cocrystals

This study aims at easing the design of crystallization and the cake washing step of cocrystals. It is assessed if solvent systems can be classified based on the dissolution behavior of the cocrystal by leveraging on the solubility of its pure components. Solubility data of pure components are typically readily accessible data in the pharmaceutical industry at an early stage of development. For the three model systems considered in this study, the composition of eutectic liquid phases was determined at 20 °C in numerous solvent systems and compared with the solubility of pure components at the same conditions. In the solvent systems in which the cocrystal dissolves congruently, the solubilities of the pure components are similar. Incongruent dissolution behavior is in turn consistently observed if the solubilities of the pure components differed by at least one order of magnitude. For these solvent systems, the eutectic liquid phases are consistently enriched in the more soluble component. Comparison of the solubilities of the pure components thus enables determination of the component that needs to be charged in excess to reach thermodynamic stability of the cocrystal, which is particularly relevant for the design of screening and solubility experiments. Finally, highly asymmetric phase diagrams, which are not suitable for crystallization and cake washing purposes, are easily identifiable by highly dissimilar solubilities of pure components.

Asymmetric phase diagram of DPPC/DOPC to investigate inter-leaflet coupling.

Cell membranes are asymmetric, meaning that the bilayer is composed of leaflets with different lipid composition. Simplified models for the exoplasmic leaflet typically form coexisting Ld (liquid disordered) + Lo (liquid ordered) phases in symmetric bilayers. In contrast, the cytoplasmic leaflet lacks high-melting lipids, and models for this leaflet always form a single fluid phase in symmetric bilayers. Thus, among the major open questions is how the phase behavior of these two leaflets is coupled in an asymmetric bilayer. Here, we report the first experimentally determined asymmetric phase diagram for a binary lipid mixture, DPPC/DOPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine/1,2-dioleoyl-sn-glycero-3-phosphocholine) at room temperature. Different from cholesterol-containing model membranes that separate into coexisting liquid phases, this simpler mixture exhibits gel + fluid coexistence over a range of composition and temperature. We used calcium-induced hemifusion to engineer asymmetric GUVs (aGUVs), and we measured the extent of outer leaflet exchange through quantitative analysis of fluorescent probe intensities. Our experiments can be directly compared to theoretical phase diagrams calculated from mean field theory, where phase boundaries and tieline orientations depend on the relative strength of in-plane and out-of-plane lipid interaction energies. The experimental DPPC/DOPC asymmetric phase diagram is consistent with an intermediate coupling strength whereby anti-registered domains coexist with registered domains at lower levels of asymmetry, but phase separation is completely suppressed at higher levels of asymmetry. Our results allow us to estimate a mismatch free energy of 0.2-0.3 kT per square nanometer for a gel domain directly opposite a fluid domain.

Planar Josephson junctions in germanium: Effect of cubic spin-orbit interaction

Planar Josephson junctions comprising semiconductors with strong spin-orbit interaction (SOI) are promising platforms to host Majorana bound states (MBSs). Previous works on MBSs in planar Josephson junctions have focused on electron gases, where SOI is linear in momentum. In contrast, a two-dimensional hole gas in planar germanium (Ge) exhibits SOI that is cubic in momentum. Nevertheless, we show here that due to the particularly large SOI, Ge is a favorable material. Using a discretized model, we numerically simulate a Ge planar Josephson junction and demonstrate that also cubic SOI can lead to the emergence of MBSs. Interestingly, we find that the cubic SOI yields an asymmetric phase diagram as a function of the superconducting phase difference across the junction. We also find that trivial Andreev bound states can imitate the signatures of MBSs in a Ge planar Josephson junction, therefore making the experimental detection of MBSs difficult. We use experimentally realistic parameters to assess if the topological phase is accessible within experimental limitations. Our analysis shows that two-dimensional Ge is an auspicious candidate for topological phases.

Asymmetric phase diagram of a binary lipid mixture

The plasma membranes of eukaryotic cells are asymmetric with respect to their lipid composition. Model membrane studies consistently find that simplified outer leaflet mixtures tend to separate into coexisting liquid phases in symmetric bilayers, while inner leaflet mixtures do not. Among the major unanswered questions is how the phase behavior of the two leaflets is coupled in an asymmetric bilayer. Mean field theory has been used to calculate asymmetric phase diagrams, where phase boundaries and tieline orientations depend on the relative strength of in-plane and out-of-plane lipid interaction energies.

Transfer Matrix Model of pH Effects in Polymeric Complex Coacervation.

Oppositely charged polyelectrolytes can undergo an associative phase separation, in a process known as polymeric complex coacervation. This phenomenon is driven by the electrostatic attraction between polyanion and polycation species, leading to the formation of a polymer-dense coacervate phase and a coexisting polymer-dilute supernatant phase. There has been significant recent interest in the physical origin and features of coacervation; yet notably, experiments often use weak polyelectrolytes the charge state of which depends on solution pH, while theoretical or computational efforts typically assume strong polyelectrolytes that remain fully charged. There have been only a few efforts to address this limitation, and thus there has been little exploration of how pH can affect complex coacervation. In this paper, we modify a transfer matrix theory of coacervation to account for acid-base equilibria, taking advantage of its ability to directly account for some local ion correlations that will affect monomer charging. We show that coacervation can stabilize the charged state of a weak polyelectrolyte via the proximity of oppositely charged monomers, and can lead to asymmetric phase diagrams where the positively and negatively charged polyelectrolytes exhibit different behaviors near the pKa of either chain. Specifically, there is a partitioning of one of the salt species to a coacervate to maintain electroneutrality when one of the polyelectrolytes is only partially charged. This results in the depletion of the same salt species in the supernatant, and overall can suppress phase separation. We also demonstrate that, when one of the species is only partially charged, mixtures that are off-stoichiometric in volume fraction but stoichiometric in charge exhibit the greatest propensity to form coacervate phases.

Influence of Charge Fraction on the Phase Behavior of Symmetric Single-Ion Conducting Diblock Copolymers.

A series of symmetric poly[(oligo(ethylene glycol) methyl ether methacrylate-co-oligo(ethylene glycol) propyl sodium sulfonate methacrylate)]-block-polystyrene (PsOEGMA-PS) diblock copolymers were synthesized as a model system to probe the effect of charge fraction on the phase behavior of charged-neutral single-ion conducting diblock copolymers. Small-angle X-ray scattering (SAXS) experiments showed that increasing the charge fraction does not alter the ordered phase morphology (lamellar) but increases the order-disorder transition temperature (TODT) significantly. Additionally, the effective Flory-Huggins interaction parameter (χeff) was found to increase linearly with the charge fraction, similar to the case of conventional salt-doped diblock copolymers. This indicates that the effect of counterion solvation, attributed to the significant mismatch between the dielectric constant of each block, provides the dominant effect in tuning the phase behavior of this charged diblock copolymer. We therefore infer that electrostatic cohesion (local charge ordering induced by Coulombic interactions), which is predicted to suppress microphase separation and lead to asymmetric phase diagrams, only plays a minor role in this model system.

Asymmetric Phase Diagram of a Three Component Model Membrane Mixture Containing Sphingomyelin: Coupling of Lateral and Transverse Lipid Organization

Lipid asymmetry is a fundamental aspect of plasma membrane structure. Despite the importance of asymmetry, how the molecular features of individual lipids influence coupling between the leaflets is largely unexplored. We investigated the composition dependent phase behavior of ternary lipid system composed of egg sphingomyelin, DOPC, and cholesterol. Symmetric giant unilamellar vesicles (GUVs) were prepared by electroformation. Asymmetric GUVs were prepared by calcium induced hemifusion of the symmetric GUVs with a supported lipid bilayer composed of DOPC and cholesterol.

Field-Free Switching of a Spin-Orbit-Torque Device Through Interlayer-Coupling-Induced Domain Walls

The spin-orbit-torque (SOT) device is a promising candidate for the next-generation magnetic random-access memory; however, the static in-plane field needed to induce deterministic switching is a main obstacle for its application in circuits. In this work, we employ the exchange coupling between the $\mathrm{Co}$/$\mathrm{Ni}$/$\mathrm{Co}$ trilayer and $\mathrm{Tb}$/$\mathrm{Co}$ multilayers in a device to form the domain wall (DW), by whose current-driven propagation the field-free magnetization switching is achieved. The SOT efficiency of the device is highly dependent on the chirality of the N\'eel-type DW. Meanwhile, the coexistence of two switching modes results in an asymmetric switching phase diagram. We find the competition between the uniform external field and the Dzyaloshinskii--Moriya interaction induces negative feedback to the pinning effect, resulting in a sharp switching process and straight DW profile. Finally, a synthetic antiferromagnetic device is investigated by experiment and micromagnetic simulation to verify the feasibility of using this proposal as the free layer of a magnetic tunneling junction.

Structurally driven asymmetric miscibility in the phase diagram of W-Ti

Phase diagrams for multi-component systems represent crucial information for understanding and designing materials but are very time consuming to assess experimentally. Computational modeling plays an increasingly important role in this regard but has been largely focused on systems with matching lattice structures and/or stable boundary phases. Here, using a combination of density functional theory calculations, alloy cluster expansions, free energy integration, and Monte Carlo simulations, we obtain the phase diagram of W-Ti, a system that features metastable boundary phases on both sides of the phase diagram. We find that the mixing energy on the body-centered cubic (BCC) lattice is asymmetric and negative with a minimum of about -120 meV/atom, whereas for the hexagonal closed packed (HCP) lattice the mixing energy is positive. By combining these data with a model for the vibrational free energy, we propose a revision of the W-rich end of the phase diagram with a much larger solubility of Ti in BCC-W than previous assessments. Finally, by comparison with the W-V and W-Re system we demonstrate how strongly asymmetric phase diagrams can arise from a subtle energy balance of stable and metastable lattice structures.

Influence of two independent sources with long-range correlated disorder in the localization properties of mixed transmission lines

We study the localization properties of correlated disordered mixed transmission lines. These systems are formed by the repetition of a set of p successive direct cells followed by q successive dual cells. The spectrum shows a set of d=p+q sub-bands for any kind of disorder where we can find extended and localized states and even gaps. The correlated disorder is introduced only in the capacitances Cj,yα,b and inductances Lj,yβ,b of the dual cells. Meanwhile, the capacitances Cj,x and inductances Lj,x of the direct cells are kept constant. b measures the amplitude of the disorder, and the correlation exponents α and β quantify the degree of long-range correlation imposed on the system, which is generated by the Fourier filtering method. For fixed p, q and b values it is possible to find extended states as a function of α and β in any of the d sub-bands. For each frequency corresponding to an extended state we can find an asymmetric phase diagram, so that for all α ≥ αc and β ≥ βc with αc ≥ βc, all states are extended. These critical values αc and βc were obtained studying the scaling behavior of the Cω average overlap amplitude. In addition, for fixed α and β, we also find a critical bc value, so for b ≥ bc all states are localized states because the large amplitude b of the fluctuation of the long-range correlated disorder destroys the extended states for all values of α and β.

Extremely asymmetric phase diagram of homopolymer-monotethered nanoparticles: Competition between chain conformational entropy and particle steric interaction.

The phase behaviors of homopolymer-monotethered nanoparticles (HMNs) in melt are investigated via a theoretical method combining self-consistent field theory for polymers and density functional theory for hard spheres. An extremely asymmetric phase diagram is observed: (i) microphases are only possible for the volume fraction of the tethered polymer fA > 0.35; (ii) in addition to lamellar phase, the system can only self-assemble into various morphologies with a polymer-rich matrix, including gyroid phase, cylindrical phase, and spherical phase. In the frame of this theory, the critical point for HMNs' microphase separation is significantly lower than that of linear diblock copolymers. Furthermore, the characteristic length of microphase-separated structures of HMNs is much smaller than that of linear diblock copolymers with the same molecular weight. Our calculation results on morphologies and characteristic length agree well with recent simulations and experimental observations.

Hypothetical biomolecular probe based on a genetic switch with tunable symmetry and stability.

BackgroundGenetic switches are ubiquitous in nature, frequently associated with the control of cellular functions and developmental programs. In the realm of synthetic biology, it is of great interest to engineer genetic circuits that can change their mode of operation from monostable to bistable, or even to multistable, based on the experimental fine-tuning of readily accessible parameters. In order to successfully design robust, bistable synthetic circuits to be used as biomolecular probes, or understand modes of operation of such naturally occurring circuits, we must identify parameters that are key in determining their characteristics.ResultsHere, we analyze the bistability properties of a general, asymmetric genetic toggle switch based on a chemical-reaction kinetic description. By making appropriate approximations, we are able to reduce the system to two coupled differential equations. Their deterministic stability analysis and stochastic numerical simulations are in excellent agreement. Drawing upon this general framework, we develop a model of an experimentally realized asymmetric bistable genetic switch based on the LacI and TetR repressors. By varying the concentrations of two synthetic inducers, doxycycline and isopropyl β-D-1-thiogalactopyranoside, we predict that it will be possible to repeatedly fine-tune the mode of operation of this genetic switch from monostable to bistable, as well as the switching rates over many orders of magnitude, in an experimental setting. Furthermore, we find that the shape and size of the bistability region is closely connected with plasmid copy number.ConclusionsBased on our numerical calculations of the LacI-TetR asymmetric bistable switch phase diagram, we propose a generic work-flow for developing and applying biomolecular probes: Their initial state of operation should be specified by controlling inducer concentrations, and dilution due to cellular division would turn the probes into memory devices in which information could be preserved over multiple generations. Additionally, insights from our analysis of the LacI-TetR system suggest that this particular system is readily available to be employed in this kind of probe.Electronic supplementary materialThe online version of this article (doi:10.1186/s12918-016-0279-y) contains supplementary material, which is available to authorized users.

Pattern-Formation During Self-organization in Three-Phase Eutectic Solidification

Eutectic growth is an interesting example for exploring the topic of pattern-formation in multi-phase systems, where the growth of the phases is coupled with the diffusive transport of one or more components in the melt. While in the case of binary alloys, the number of possibilities are limited (lamellae, rods, labyrinth etc.), their number rapidly increases with the number of components and phases. In this paper, we will investigate pattern formation during three-phase eutectic solidification using a state-of-the art phase-field method based on the grand-canonical density formulation. The major aim of the study is to highlight the role of two properties, which are the volume fraction of the solid phases and the solid–liquid interfacial energies, in the self-organization of the solid phases during directional growth. Thereafter, we will show representative phase-field simulations of a micro-structure in a real alloy (Ag–Al–Cu) using an asymmetric phase diagram as well as interfacial properties.

Odd q-state clock spin-glass models in three dimensions, asymmetric phase diagrams, and multiple algebraically ordered phases.

Distinctive orderings and phase diagram structures are found, from renormalization-group theory, for odd q-state clock spin-glass models in d=3 dimensions. These models exhibit asymmetric phase diagrams, as is also the case for quantum Heisenberg spin-glass models. No finite-temperature spin-glass phase occurs. For all odd q≥5, algebraically ordered antiferromagnetic phases occur. One such phase is dominant and occurs for all q≥5. Other such phases occupy small low-temperature portions of the phase diagrams and occur for 5≤q≤15. All algebraically ordered phases have the same structure, determined by an attractive finite-temperature sink fixed point where a dominant and a subdominant pair states have the only nonzero Boltzmann weights. The phase transition critical exponents quickly saturate to the high q value.

Enormously Wide Range Cylinder Phase of Liquid Crystalline PEO-b-PMA(Az) Block Copolymer

A series of amphiphilic liquid crystalline diblock copolymers, PEOm-b-PMA(Az)n, consisting of hydrophilic poly(ethylene oxide) and hydrophobic poly(methacrylate) moieties with side chains containing liquid crystalline (LC) azobenzene moieties, produced highly ordered microphase-separated films with PEO cylinders aligned perpendicular to the smectic LC layer of azobenzene in the PMA(Az) matrix. In this paper, morphological phase diagrams of PEOm-b-PMA(Az)n diblock copolymers above and below the isotropic transition temperature of LC azobenzene (Tiso) are presented. The diagrams are based on small-angle X-ray scattering (SAXS) measurements of approximately 70 kinds of polymers with varying degrees of polymerization in each block. An asymmetric phase diagram described against the volume fraction of PEO (fPEO) was obtained at temperatures above and below Tiso. The lamellar phase appears in the fPEO window 0.52 ≤ fPEO ≤ 0.78 above and below Tiso. Besides, the wide window, 0.087 ≤ fPEO < 0.52, allows the PEO cylinder phase to form below Tiso. In particular, the PEO sphere phase, observed above Tiso, was completely eliminated through an order–order transition (OOT) to the PEO cylinder phase in the window 0.087 ≤ fPEO ≤ 0.23. Such a large expansion in the PEO-cylinder-phase window could be attributed to the main chain of LC PMA(Az) being shorter than that of the flexible PEO chain, and LC azobenzene forming a smectic layer in the microphase separated system.

Coil fraction-dependent phase behaviour of a model globular protein–polymer diblock copolymer

The self-assembly of the model globular protein-polymer block copolymer mCherry-b-poly(N-isopropyl acrylamide) is explored across a range of polymer coil fractions from 0.21 to 0.82 to produce a phase diagram for these materials as a function of molecular composition. Overall, four types of morphologies were observed: hexagonally packed cylinders, perforated lamellae, lamellae, and disordered nanostructures. Across all coil fractions and morphologies, a lyotropic re-entrant order-disorder transition in water was observed, with disordered structures below 30 wt% and above 70 wt% and well-ordered morphologies at intermediate concentrations. Solid state samples prepared by solvent evaporation show moderately ordered structures similar to those observed in 60 wt% solutions, suggesting that bulk structures result from kinetic trapping of morphologies which appear at lower concentrations. While highly ordered cylindrical nanostructures are observed around a bioconjugate polymer volume fraction of 0.3 and well-ordered lamellae are seen near a volume fraction of 0.6, materials at lower or higher coil fractions become increasingly disordered. Notable differences between the phase behaviour of globular protein-polymer block copolymers and coil-coil diblock copolymers include the lack of spherical nanostructures at either high or low polymer coil fractions as well as shifted phase boundaries between morphologies which result in an asymmetric phase diagram.

Orientational interactions in block copolymer melts: Self-consistent field theory

We study the phase behavior of diblock copolymer melts with one block possessing orientation-dependent segmental interactions using self-consistent field theory. A generalized coarse-grained description is introduced based on the local (polar) orientational order parameter and K, an effective Frank elastic constant for orientational gradients. To explore the role played by orientational interactions in assembly thermodynamics, we apply the theory to two-dimensional melt morphologies for a range of K. As microphase segregation necessarily introduces splay deformations of the segment orientation, we find that increasing the stiffness K raises the critical χN at the onset of microphase separation. Furthermore, we find that strong orientational interactions in one block give rise to highly asymmetric phase diagrams due to the large penalty for high-splay morphologies, such as the cylindrical phase. Finally, we analyze the costs of inter-segmental splay as well as the size dependence of domain spacing on K based on a strong-segregation picture of morphologies.

Correlation of rheology and morphology and estimation of interfacial tension of immiscible COC/EVA blends

Rheology and morphology of cyclic olefin copolymer (COC) / ethylene vinyl acetate copolymer (EVA) immiscible blends with droplet and co-continuous morphologies were experimentally examined and theoretically analyzed using emulsion and micromechanical models. The blends showed an asymmetric phase diagram in which the EVA-rich blends had smaller dispersed size domains as compared to the COC-rich blends. This could be explained based on the higher melt elasticity and viscosity of COC as compared to EVA determined by the rheological investigations. The rheological tools were used to investigate the miscibility of the blends. From the melt viscosity data it is found that the COC/EVA blends show a positive deviation behavior at all compositions which is a hint for strong interaction between the COC and EVA. Analysis of Cole-Cole and Han diagrams revealed that COC/EVA blends, at high EVA contents, were more compatible than COC-rich blends. For the droplet morphology, Palierne model was more successful but, by increasing the dispersed phase content some deviation was observed. In the co-continuous region, the Coran model was in good correspondence with the experimental data as compared to the Veenstra’s model. The storage and loss modulus of EVA-rich blends had a better correspondence with the Palierne model than the COC-rich blends which further confirmed the morphological findings. Interfacial tension calculated for the COC/EVA blends using the Palierne model, were about 1.2 and 15 mN/m2 for EVA-rich (10/90) and COC-rich blends (90/10), respectively. In both EVA-rich and COC-rich systems the interfacial tension increased with increasing the dispersed phase content.

Phase-field simulation of lamellar growth for a binary eutectic alloy

Using general multi-phase-field model, detailed microstructures corresponding to different initial lamellar sets were simulated in a binary eutectic alloy with an asymmetric phase diagram. The simulation results show that regular or unstable oscillating lamellar structures depend on the initial lamellar widths of two solid phases. A lamellar morphology map associating with the initial widths has been derived, which is capable of showing the condition of forming various lamella structures. For instance, a regular lamella was formed with fast solidification while large lamella resulted from disorder growth with low interfacial velocity. The investigated interface velocities indicate that with fast solidification to form regular lamella, a disorder growth manner or a large lamellar spacing causes a low interface velocity. These results are in good agreement with those proposed by Jackson-Hunt model.