Showing papers on "Phase (matter) published in 2021"

Chiral flux phase in the Kagome superconductor AV3Sb5

Abstract: We argue that the topological charge density wave phase in the quasi-2D Kagome superconductor AV3Sb5 is a chiral flux phase. Considering the symmetry of the Kagome lattice, we show that the chiral flux phase has the lowest energy among those states which exhibit 2 × 2 charge orders observed experimentally. This state breaks the time-reversal symmetry and displays anomalous Hall effect. The explicit pattern of the density of state in real space is calculated. These results are supported by recent experiments and suggest that these materials are new platforms to investigate the interplay between topology, superconductivity and electron–electron correlations.

Quantum Criticality in Twisted Transition Metal Dichalcogenides

Abstract: In moir\'e heterostructures, gate-tunable insulating phases driven by electronic correlations have been recently discovered. Here, we use transport measurements to characterize the gate-driven metal-insulator transitions and the metallic phase in twisted WSe$_2$ near half filling of the first moir\'e subband. We find that the metal-insulator transition as a function of both density and displacement field is continuous. At the metal-insulator boundary, the resistivity displays strange metal behaviour at low temperature with dissipation comparable to the Planckian limit. Further into the metallic phase, Fermi-liquid behaviour is recovered at low temperature which evolves into a quantum critical fan at intermediate temperatures before eventually reaching an anomalous saturated regime near room temperature. An analysis of the residual resistivity indicates the presence of strong quantum fluctuations in the insulating phase. These results establish twisted WSe$_2$ as a new platform to study doping and bandwidth controlled metal-insulator quantum phase transitions on the triangular lattice.

Isospin Pomeranchuk effect in twisted bilayer graphene

Abstract: In condensed-matter systems, higher temperatures typically disfavour ordered phases, leading to an upper critical temperature for magnetism, superconductivity and other phenomena. An exception is the Pomeranchuk effect in 3He, in which the liquid ground state freezes upon increasing the temperature1, owing to the large entropy of the paramagnetic solid phase. Here we show that a similar mechanism describes the finite-temperature dynamics of spin and valley isospins in magic-angle twisted bilayer graphene2. Notably, a resistivity peak appears at high temperatures near a superlattice filling factor of −1, despite no signs of a commensurate correlated phase appearing in the low-temperature limit. Tilted-field magnetotransport and thermodynamic measurements of the in-plane magnetic moment show that the resistivity peak is connected to a finite-field magnetic phase transition3 at which the system develops finite isospin polarization. These data are suggestive of a Pomeranchuk-type mechanism, in which the entropy of disordered isospin moments in the ferromagnetic phase stabilizes the phase relative to an isospin-unpolarized Fermi liquid phase at higher temperatures. We find the entropy, in units of Boltzmann’s constant, to be of the order of unity per unit cell area, with a measurable fraction that is suppressed by an in-plane magnetic field consistent with a contribution from disordered spins. In contrast to 3He, however, no discontinuities are observed in the thermodynamic quantities across this transition. Our findings imply a small isospin stiffness4,5, with implications for the nature of finite-temperature electron transport6–8, as well as for the mechanisms underlying isospin ordering and superconductivity9,10 in twisted bilayer graphene and related systems. An electronic analogue of the Pomeranchuk effect is present in twisted bilayer graphene, shown by the stability of entropy in a ferromagnetic phase compared to an unpolarized Fermi liquid phase at certain high temperatures.

Simulation of FUS Protein Condensates with an Adapted Coarse-Grained Model

Abstract: Disordered proteins and nucleic acids can condense into droplets that resemble the membraneless organelles observed in living cells. MD simulations offer a unique tool to characterize the molecular interactions governing the formation of these biomolecular condensates, their physicochemical properties, and the factors controlling their composition and size. However, biopolymer condensation depends sensitively on the balance between different energetic and entropic contributions. Here, we develop a general strategy to fine-tune the potential energy function for molecular dynamics simulations of biopolymer phase separation. We rebalance protein-protein interactions against solvation and entropic contributions to match the excess free energy of transferring proteins between dilute solution and condensate. We illustrate this formalism by simulating liquid droplet formation of the FUS low-complexity domain (LCD) with a rebalanced MARTINI model. By scaling the strength of the nonbonded interactions in the coarse-grained MARTINI potential energy function, we map out a phase diagram in the plane of protein concentration and interaction strength. Above a critical scaling factor of αc ≈ 0.6, FUS-LCD condensation is observed, where α = 1 and 0 correspond to full and repulsive interactions in the MARTINI model. For a scaling factor α = 0.65, we recover experimental densities of the dilute and dense phases, and thus the excess protein transfer free energy into the droplet and the saturation concentration where FUS-LCD condenses. In the region of phase separation, we simulate FUS-LCD droplets of four different sizes in stable equilibrium with the dilute phase and slabs of condensed FUS-LCD for tens of microseconds, and over one millisecond in aggregate. We determine surface tensions in the range of 0.01-0.4 mN/m from the fluctuations of the droplet shape and from the capillary-wave-like broadening of the interface between the two phases. From the dynamics of the protein end-to-end distance, we estimate shear viscosities from 0.001 to 0.02 Pa s for the FUS-LCD droplets with scaling factors α in the range of 0.625-0.75, where we observe liquid droplets. Significant hydration of the interior of the droplets keeps the proteins mobile and the droplets fluid.

Generating Dual-Polarized Vortex Beam by Detour Phase: From Phase Gradient Metasurfaces to Metagratings

Abstract: Traditional methods of generating vortex beams based on metasurfaces consist mainly in modulating propagation phase or geometric phase. Here, by introducing detour phase, we propose the construction of dual-polarized vortex beam generators in the form of metasurface and metagrating (MG). The phase is modulated through moving the position of meta-atoms instead of varying the geometrical parameters or rotating the unit cells. To use detour phase, two kinds of unit cells are designed to achieve specific diffraction order. Each unit can arbitrarily and independently adjust the operation frequency and diffraction angle of transverse electric (TE) and transverse magnetic (TM) polarizations. Two vortex beam generators are designed and fabricated with different topological charges carried by orthogonal polarizations. To demonstrate the ability to independently manipulate, two polarizations of the generator based on MG are designed in different frequency bands. Both the simulation and experimental results validate the proposed method, showing great potential for polarization division multiplexing in orbital angular momentum (OAM) communication systems.

Unique surface patterns emerging during solidification of liquid metal alloys

Abstract: It is well-understood that during the liquid-to-solid phase transition of alloys, elements segregate in the bulk phase with the formation of microstructures. In contrast, we show here that in a Bi–Ga alloy system, highly ordered nanopatterns emerge preferentially at the alloy surfaces during solidification. We observed a variety of transition, hybrid and crystal-defect-like patterns, in addition to lamellar and rod-like structures. Combining experiments and molecular dynamics simulations, we investigated the influence of the superficial Bi and Ga2O3 layers during surface solidification and elucidated the pattern-formation mechanisms, which involve surface-catalysed heterogeneous nucleation. We further demonstrated the dynamic nature and robustness of the phenomenon under different solidification conditions and for various alloy systems. The surface patterns we observed enable high-spatial-resolution nanoscale-infrared and surface-enhanced Raman mapping, which reveal promising potential for surface- and nanoscale-based applications. During a liquid-to-solid phase transition, a Bi–Ga alloy forms ordered nanostructured patterns on its surface.

Observation of fluctuation-mediated picosecond nucleation of a topological phase

Abstract: Topological states of matter exhibit fascinating physics combined with an intrinsic stability. A key challenge is the fast creation of topological phases, which requires massive reorientation of charge or spin degrees of freedom. Here we report the picosecond emergence of an extended topological phase that comprises many magnetic skyrmions. The nucleation of this phase, followed in real time via single-shot soft X-ray scattering after infrared laser excitation, is mediated by a transient topological fluctuation state. This state is enabled by the presence of a time-reversal symmetry-breaking perpendicular magnetic field and exists for less than 300 ps. Atomistic simulations indicate that the fluctuation state largely reduces the topological energy barrier and thereby enables the observed rapid and homogeneous nucleation of the skyrmion phase. These observations provide fundamental insights into the nature of topological phase transitions, and suggest a path towards ultrafast topological switching in a wide variety of materials through intermediate fluctuating states. Time-resolved X-ray scattering is utilized to demonstrate an ultrafast 300 ps topological phase transition to a skyrmionic phase. This transition is enabled by the formation of a transient topological fluctuation state.

Preserving a robust CsPbI3 perovskite phase via pressure-directed octahedral tilt.

Abstract: Functional CsPbI3 perovskite phases are not stable at ambient conditions and spontaneously convert to a non-perovskite δ phase, limiting their applications as solar cell materials. We demonstrate the preservation of a black CsPbI3 perovskite structure to room temperature by subjecting the δ phase to pressures of 0.1 – 0.6 GPa followed by heating and rapid cooling. Synchrotron X-ray diffraction and Raman spectroscopy indicate that this perovskite phase is consistent with orthorhombic γ-CsPbI3. Once formed, γ-CsPbI3 could be then retained after releasing pressure to ambient conditions and shows substantial stability at 35% relative humidity. First-principles density functional theory calculations indicate that compression directs the out-of-phase and in-phase tilt between the [PbI6]4− octahedra which in turn tune the energy difference between δ- and γ-CsPbI3, leading to the preservation of γ-CsPbI3. Here, we present a high-pressure strategy for manipulating the (meta)stability of halide perovskites for the synthesis of desirable phases with enhanced materials functionality. Inorganic lead halide perovskites are structurally unstable, which prevents their application in solar cells. Here the authors synthesize, using high pressure and temperature, a perovskite CsPbI3 phase that is metastably preserved to ambient conditions through a structural deformation induced at high pressure.

Phase-Separated Mo–Ni Alloy for Hydrogen Oxidation and Evolution Reactions with High Activity and Enhanced Stability

Abstract: DOI: 10.1002/aenm.202003511 advantage of alkaline polymer electrolyte fuel cells (APEFCs) and alkaline water electrolysis (AWE) is the use of nonprecious metal catalysts, which is expected to significantly reduce the cost and promote the practical application of hydrogen energy.[6–8] At present, major obstacles to the application of APEFC and AWE are the slow kinetics of hydrogen electrode reactions and oxygen electrode reactions.[9,10] Hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER) belong to hydrogen electrode reactions, and Mo–Ni alloy is one of the most promising electrocatalysts for hydrogen electrode reactions.[11–13] Intermetallic compound Mo–Ni alloy (IC-MoNi), such as MoNi4, MoNi3, and MoNi, will produce a synergistic effect between Mo and Ni for enhanced hydrogen electrode reactions.[14–17] However, since the Mo element in IC-MoNi is unstable during the electrode reaction process, the breakdown potential of IC-MoNi is low, resulting in poor electrochemical stability.[14–17] Moreover, Ni also suffers from low stability at potentials about 0.1 V versus reversible hydrogen electrode (RHE) due to its relatively strong binding affinity toward oxygen species.[18] How to improve the stability of IC-MoNi has become an important challenge. The phase-separated alloy is different from the intermetallic compound alloy. For example, the phase-separated Mo–Ni alloy (PS-MoNi) is composed of phase-separated Mo metal phase and phase-separated Ni metal phase. Because Mo metal phase is stable and has a high breakdown potential,[17] the PS-MoNi may have better structural and electrochemical stability than IC-MoNi. For PS-MoNi, the electron density of Mo metal can also be adjusted by Ni metal, which is similar to IC-MoNi.[14–22] In addition, amorphous materials are more resistant to corrosion due to the absence of grain boundaries. Therefore, amorphous PS-MoNi may be a good material for HOR and HER with high activity and enhanced stability. Herein, we synthesized the PS-MoNi composed of phaseseparated Mo and Ni metal phases. The X-ray absorption spectroscopy (XAS) and the energy dispersive X-ray spectroscopy (EDS) mappings illustrate that we have synthesized PS-MoNi composed of Mo metal and embedded Ni metal nanoparticles. Excitingly, PS-MoNi shows excellent hydrogen electrode activity with the high exchange current density (−4.883 mA cm−2), which is comparable to the reported highest value for non-noble The development of alkaline polymer electrolyte fuel cells and alkaline water electrolysis requires nonprecious metal catalysts for the hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER). Herein, it is reported a phase-separated Mo–Ni alloy (PS-MoNi) that is composed of Mo metal and embedded Ni metal nanoparticles. The PS-MoNi shows excellent hydrogen electrode activity with a high exchange current density (−4.883 mA cm−2), which is comparable to the reported highest value for non-noble catalysts. Moreover, the amorphous phase-separated Mo–Ni alloy has better structural and electrochemical stability than the intermetallic compound Mo–Ni alloy (IC-MoNi). The breakdown potential of PS-MoNi is as high as 0.32 V, which is much higher than that of reported IC-MoNi. The X-ray absorption near edge structure (XANES) and density functional theory (DFT) calculations indicate the electrons transfer from Mo to Ni for PS-MoNi, leading to suitable adsorption free energies of H* (ΔGH*) on the surface of Mo. This means that the electron density modulation of Mo metal by embedded Ni metal nanoparticles can produce excellent HOR and HER performance.

Inhibiting in situ phase transition in Ruddlesden-Popper perovskite via tailoring bond hybridization and its application in oxygen permeation

Abstract: Summary Ruddlesden-Popper perovskite oxide (An+1BnO3n+1) mixed ionic-electronic conducting membranes are proposed as a new method for oxygen separation from air. Element doping was used to improve the ionic conductivity and to stabilize the crystal structure. The doping of orthorhombic Pr2NiO4 with Mo resulted in the ex situ collapse of the crystal together with the generation of impurities by the rearrangement of Pr atoms. Mo doping also inhibited the in situ phase transition from low-order Pr2NiO4 to high-order Pr4Ni3O10 by weakening the covalent interaction between Pr and O. Membranes made from Pr2Ni0.95Mo0.05O4+δ showed an oxygen flux of 3.35 mL min−1 cm−2 at 1,000°C, high permeation stability in air and helium, and high CO2 tolerance with no decline of oxygen flux during 500 h at 900°C. This work advances a comprehensive understanding of phase transitions on Pr2Ni1−xMoxO4 and provides an effective way to improve the oxygen permeability via in situ stabilization of the phase structure.

Active phase separation by turning towards regions of higher density

Abstract: Studies of active matter, from molecular assemblies to animal groups, have revealed two broad classes of behaviour: a tendency to align yields orientational order and collective motion, whereas particle repulsion leads to self-trapping and motility-induced phase separation. Here we report a third class of behaviour: orientational interactions that produce active phase separation. Combining theory and experiments on self-propelled Janus colloids, we show that stronger repulsion on the rear than on the front of these particles produces non-reciprocal torques that reorient particle motion towards high-density regions. Particles thus self-propel towards crowded areas, which leads to phase separation. Clusters remain fluid and exhibit fast particle turnover, in contrast to the jammed clusters that typically arise from self-trapping, and interfaces are sufficiently wide that they span entire clusters. Overall, our work identifies a torque-based mechanism for phase separation in active fluids, and our theory predicts that these orientational interactions yield coexisting phases that lack internal orientational order. Self-propelled particles are shown to orient themselves towards areas of high density, phase separating into fluid-like clusters. This behaviour is unique to active systems, forming a distinct class of motility-induced phase separation.

Study of the formation effect of the cubic phase of LiTiO 2 on the structural, optical, and mechanical properties of Li 2±x Ti 1±x O 3 ceramics with different contents of the X component

Abstract: The paper presents the results of the study of structural, morphological, optical, and mechanical properties of lithium-containing ceramics obtained using the method of mechanochemical solid-phase synthesis. The purpose of this work is to assess the possibility of obtaining lithium-containing ceramics of the two-phase type, as well as the formation effect of the cubic phase of LiTiO2 on changes in the properties of ceramics. The relevance of this study is to obtain new data on the properties of lithium-containing ceramics, which have great prospects in their use as blanket materials for tritium reproduction. During the study, it was found that the formation of the LiTiO2 cubic phase leads to a change in the morphological features of ceramics, with the formation of sphere-like agglomerates of a nanoscale scale. An increase in the contribution of the LiTiO2 phase leads to a shift of the fundamental absorption edge, as well as the appearance of additional absorption bands. During mechanical tests for the determination of resistance to destruction by single compression, it was found that an increase in ceramic density, which is due to an increase in the contribution of the cubic phase, leads to an increase in resistance by 70–85%.

Nitrogen-Doped Cobalt Diselenide with Cubic Phase Maintained for Enhanced Alkaline Hydrogen Evolution.

Abstract: The introduction of heteroatoms is one of the most important ways to modulate the intrinsic electronic structure of electrocatalysts to improve their catalytic activity. However, for transition metal chalcogenides with highly symmetric crystal structure (HS-TMC), the introduction of heteroatoms, especially those with large atomic radius, often induces large lattice distortion and vacancy defects, which may lead to structural phase transition of doped materials or structural phase reconstruction during the catalytic reaction. Such unpredictable situations will make it difficult to explore the connection between the intrinsic electronic structure of doped catalysts and catalytic activity. Herein, taking thermodynamically stable cubic CoSe2 phase as an example, we demonstrate that nitrogen incorporation can effectively regulate the intrinsic electronic structure of HS-TMC with structural phase stability and thus promote its electrocatalytic activity for the hydrogen evolution activity (HER). In contrast, the introduction of phosphorus can lead to structural phase transition from cubic CoSe2 to orthorhombic phase, and the structural phase of phosphorus-doped CoSe2 is unstable for HER.

Phase- and Surface Composition-Dependent Electrochemical Stability of Ir-Ru Nanoparticles during Oxygen Evolution Reaction

Abstract: The increasing scarcity of iridium (Ir) and its rutile-type oxide (IrO2), the current state-of-the-art oxygen evolution reaction (OER) catalysts, is driving the transition toward the use of mixed Ir oxides with a highly active yet inexpensive metal (IrxM1−xO2). Ruthenium (Ru) has been commonly employed due to its high OER activity although its electrochemical stability in IrRu mixed oxide nanoparticles (IrxRu1−xO2 NPs), especially at high relative contents, is rarely evaluated for long-term application as water electrolyzers. In this work, we bridge the knowledge gap by performing a thorough study on the compositionand phase-dependent stability of welldefined IrxRu1−xO2 NPs prepared by flame spray pyrolysis under dynamic operating conditions. As-prepared NPs (IrxRu1−xOy) present an amorphous coral-like structure with a hydrous Ir-Ru oxide phase, which upon post-synthetic thermal treatment fully converts to a rutile-type structure followed by a selective Ir enrichment at the NP topmost surface. It was demonstrated that Ir incorporation into a RuO2 matrix drastically reduced Ru dissolution by ca. 10-fold at the expense of worsening Ir inherent stability, regardless of the oxide phase present. Hydrous IrxRu1−xOy NPs, however, were shown to be 1000-fold less stable than rutile-type IrxRu1−xO2, where the severe Ru leaching yielded a fast convergence toward the activity of monometallic hydrous IrOy. For rutiletype IrxRu1−xO2, the sequential start-up/shut-down OER protocol employed revealed a steady-state dissolution for both Ir and Ru, as well as the key role of surface Ru species in OER activity: minimal Ru surface losses (<1 at. %) yielded OER activities for tested Ir0.2Ru0.8O2 equivalent to those of untested Ir0.8Ru0.2O2. Ir enrichment at the NP topmost surface, which mitigates selective subsurface Ru dissolution, is identified as the origin of the NP stabilization. These results suggest Ru-rich IrxRu1−xO2 NPs to be viable electrocatalysts for long-term water electrolysis, with significant repercussions in cost reduction.