Phase Coexistence Regime Research Articles

Fixed-Magnetization Ising Model with a Slowly Varying Magnetic Field

The motivation for this paper is the analysis of the fixed-density Ising lattice gas in the presence of a gravitational field. This is seen as a particular instance of an Ising model with a slowly varying magnetic field in the fixed magnetization ensemble. We first characterize the typical magnetization profiles in the regime in which the contribution of the magnetic field competes with the bulk energy term. We then discuss in more detail the particular case of a gravitational field and the arising interfacial phenomena. In particular, we identify the macroscopic profile and propose several conjectures concerning the interface appearing in the phase coexistence regime. The latter are supported by explicit computations in an effective model. Finally, we state some conjectures concerning equilibrium crystal shapes in the presence of a gravitational field, when the latter contributes to the energy only to surface order.

Spin density waves and ground state helices in EuGa2.4Al1.6

The Eu(Ga1−xAlx)4 series is composed of centrosymmetric structures which exhibit a wide range of rich topological phenomena, including some members hosting magnetic skyrmions. In this letter, we investigate the previously unreported intermediate compound EuGa2.4Al1.6, which hosts two distinct phase transitions under zero applied magnetic field. We have used resonant elastic x-ray scattering with full linear polarization analysis to unambiguously determine the zero-field magnetic structures, which consist of a transition between a basal plane transverse spin density wave at higher temperatures into a noncollinear helical ground state. Furthermore, we demonstrate a phase coexistence regime below the transition and reveal an elliptically modulated helical magnetic structure emerging from wavevector splitting. Published by the American Physical Society 2024

Metal–insulator transition in composition-tuned nickel oxide films

Thin films of the solid solution NdLaNiO3 are grown in order to study the expected 0 K phase transitions at a specific composition. We experimentally map out the structural, electronic and magnetic properties as a function of x and a discontinuous, possibly first order, insulator–metal transition is observed at low temperature when x = 0.2. Raman spectroscopy and scanning transmission electron microscopy show that this is not associated with a correspondingly discontinuous global structural change. On the other hand, results from density functional theory (DFT) and combined DFT and dynamical mean field theory calculations produce a 0 K first order transition at around this composition. We further estimate the temperature-dependence of the transition from thermodynamic considerations and find that a discontinuous insulator–metal transition can be reproduced theoretically and implies a narrow insulator–metal phase coexistence with x. Finally, muon spin rotation (µSR) measurements suggest that there are non-static magnetic moments in the system that may be understood in the context of the first order nature of the 0 K transition and its associated phase coexistence regime.

Universality and microstrain origin of the ramp reversal memory effect

The recently discovered ramp reversal memory (RRM) is a nonvolatile memory effect observed in correlated oxides with temperature-driven insulator--metal transitions (IMT). It appears as a resistance increase at predefined temperatures that are set or erased by simple heating--cooling (i.e., ramp reversal) protocols. Until now RRM was measured for two materials: ${\mathrm{VO}}_{2}$ and ${\mathrm{NdNiO}}_{3}$. A heuristic model suggests that the RRM is caused by a local transition temperature increase at boundaries of spatially separated metallic and insulating domains during ramp reversal. However, there is no experimental measure of the magnitude of the effect, which is crucial for the development of a theoretical account of the RRM. Here we show that ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ also shows RRM, including all related features, highlighting the generality of the effect. Moreover, an analysis of the RRM as an effective (average) increase of the critical temperature provides a quantitative measure of its magnitude as a function of temperature and ramp reversal protocols. We provide clear evidence that the RRM is the outcome of a local increase in transition temperature of the microscopic-scale phase boundaries that are created during temperature ramp reversal (from heating to cooling) within the insulator--metal phase coexistence regime.

Pressure-induced magnetic transformations in Cd3As2+MnAs hybrid composite

Considerable interest to magnetism of MnAs both in bulk or in the form of epitaxial films is stimulated by its applications as a magnetocaloric material and in spintronic devices. Since the MnAs films deposited on GaAs reproduce well a magnetic transformation related to α–β magnetostructural transition that occurs in bulk MnAs, this first-order phase transition occurs through a phase coexistence over a wide temperature range. Here, we considered the same magnetostructural transition in a bulk hybrid structure based on micrometer-scaled MnAs inclusions embedded into the Cd3As2 matrix. In particular, the effect of high pressure and magnetic fields on the ferromagnetic transition temperature, TC, in a composite Cd3As2 + 30 mol. % MnAs has been studied. We found that at ambient pressure, the transition from α-MnAs to β-MnAs is accompanied by the absence of thermal hysteresis of magnetization, implying a phase coexistence regime. The hysteresis width does not markedly increase even at pressures about P = 0.35 GPa, and displacement of TC occurs with a rate of dTC/dP ∼ −91.42 K/GPa. In the temperature region of the α–β phase coexistence, a local peak at T = 283 K and P = 1 GPa is observed, which is associated with an antiferromagnetic order of MnAs inclusions. Direct measurements of isothermal magnetization vs pressure indicate both the stabilization of the ferromagnetic hexagonal α phase at P < Pmax and the development of an orthorhombic antiferromagnetic long-range order, which propagate up to 5 GPa.

Asymmetric Character of the Ferroelectric Phase Transition and Charged Domain Walls in a Hybrid Improper Ferroelectric

AbstractIn improper ferroelectrics, the spontaneous ordering is typically driven by a structural distortion or a magnetic spin alignment. The induced electric polarization is only a secondary effect. This dependence is a rich source for unusual phenomena and ferroelectric domain configurations for proper, polarization‐driven ferroelectrics. This study focuses on the polar domain structure and the hysteretic behavior at the ferroelectric phase transition in Ca3Mn1.9Ti0.1O7 as a representative of the recently discovered hybrid improper ferroelectric class of multiferroics. Combining optical second harmonic generation and Raman spectroscopy gives access to the spontaneous structural distortion and the resulting improper electric polarization. This study shows that hybrid improper ferroelectrics contrast proper and improper ferroelectrics in several ways. Most intriguingly, adjacent ferroelectric domains favor head‐to‐head and tail‐to‐tail domain walls over charge‐neutral configurations. Furthermore, the phase transition occurs in an asymmetric fashion. The regime of phase coexistence of the nonpolar and polar phases shows a clear and abrupt upper temperature limit. In contrast, the coexistence toward low temperatures is best described as a fade‐out process, where 100‐nm‐sized islands of the nonpolar phase expand deep into the polar phase.

Phase coexistence and negative thermal expansion in the triple perovskite iridate Ba3CoIr2O9

The anomalous thermal expansion in a layered $3d\text{\ensuremath{-}}5d$ based triple perovskite iridate ${\mathrm{Ba}}_{3}{\mathrm{CoIr}}_{2}{\mathrm{O}}_{9}$ is investigated using high resolution synchrotron diffraction. Below the magnetostructural transition at 107 K, the onset of antiferromagnetic order is associated with a monoclinic distortion of the hexagonal structure. Deeper within the magnetically ordered state, a part of the monoclinic phase distorts even further, and both these structural phases co-exist down to the lowest measured temperatures. We observe negative thermal expansion in this phase co-existence regime, which appears to be intimately connected to the temperature driven relative fractions of these monoclinic phases. The significant NTE observed in this system could be driven by magnetic exchange striction and is of relevance to a number of systems with pronounced spin orbit interactions.

Modeling the Metal–Insulator Phase Transition in LixCoO2 for Energy and Information Storage

AbstractAn electro‐chemomechanical phase‐field model is developed to capture the metal–insulator phase transformation along with the structural and chemical changes that occur in LixCoO2 in the regular operating range of 0.5 < x < 1. Under equilibrium, in the regime of phase coexistence, it is found that transport limitations lead to kinetically arrested states that are not determined by strain‐energy minimization. Further, lithiation profiles are obtained for different discharging rates and the experimentally observed voltage plateau is observed. Finally, a simple model is developed to account for the conductivity changes for a polycrystalline LixCoO2 thin film as it transforms from the metallic phase to the insulating phase and a strategy is outlined for memristor design. The theory can therefore be used for modeling LixCoO2‐electrode batteries as well as low voltage nonvolatile redox transistors for neuromorphic computing architectures.

Studies on magnetic field and temperature driven magneto-structural phase transition in La0.5Sr0.5MnO3+δ

Abstract The influence of magnetic field on the crystal structure of colossal magnetoresistance (CMR) material La 0.5 Sr 0.5 MnO 3+δ has been investigated. Low temperature high-magnetic field powder X-ray diffraction (XRD) measurements were carried out on La 0.5 Sr 0.5 MnO 3+δ in temperature range from 4.2 to 300 K and magnetic field range from 0 to 8 T. A first order structural phase transition from tetragonal I 4/ mcm to orthorhombic Fmmm phase, coupled with reported electronic and magnetic phase transition from ferromagnetic metal to antiferromagnetic insulator state has been started at ∼200 K and completed at ∼100 K. On the application of 8 T magnetic field this magneto-structural phase transition shifted to lower temperature (∼162 K) with phase coexistence regime down to 4.2 K. Isothermal X-ray diffraction measurements at 150 K infer the evidence of magnetic field driven first order structural phase transition from Fmmm to I 4/ mcm . These results consistent with the resistivity and magnetoresistance results; present the microscopic evidence of strong spin-lattice coupling and reveal the magnetic field driven structural phase transition as the origin of observed colossal magnetoresistance in this material.

Structural change and phase coexistence upon magnetic ordering in the magnetodielectric spinelMn3O4

Cooperative Jahn-Teller ordering is well-known to drive the cubic $Fd\overline{3}m$ to tetragonal $I$4$_1$/$amd$ structural distortion in Mn$_3$O$_4$ at 1170 $^{\circ}$C. Further structural distortion occurs in Mn$_3$O$_4$ upon magnetic ordering at 42 K. Employing high-resolution variable-temperature synchrotron x-ray diffraction we show that tetragonal $I$4$_1$/$amd$ and orthorhombic $Fddd$ phases coexist, with nearly equal fractions, below the N\'eel temperature of Mn$_3$O$_4$. Significant variation of the orthorhombic $a$ and $b$ lattice constants from the tetragonal $a$ lattice constant is observed. Structural phase coexistence in Mn$_3$O$_4$ is attributed to large strains due to the lattice mismatch between the tetragonal $I$4$_1$/$amd$ and the orthorhombic $Fddd$ phases. Strain tensors determined from Rietveld refinement show a highly strained matrix of the $I4_1/amd$ phase that accommodates the nucleated orthorhombic $Fddd$ phase in the phase coexistence regime. A comparison of the deformation observed in Mn$_3$O$_4$ to structural deformations of other magnetic spinels shows that phase coexistence may be a common theme when structural distortions occur below 50 K.

Bilayer registry in a multicomponent asymmetric membrane: dependence on lipid composition and chain length.

A question of considerable interest to cell membrane biology is whether phase segregated domains across an asymmetric bilayer are strongly correlated with each other and whether phase segregation in one leaflet can induce segregation in the other. We answer both these questions in the affirmative, using an atomistic molecular dynamics simulation to study the equilibrium statistical properties of a 3-component asymmetric lipid bilayer comprising an unsaturated palmitoyl-oleoyl-phosphatidyl-choline, a saturated sphingomyelin, and cholesterol with different composition ratios. Our simulations are done by fixing the composition of the upper leaflet to be at the coexistence of the liquid ordered (l(o))-liquid disordered (l(d)) phases, while the composition of the lower leaflet is varied from the phase coexistence regime to the mixed l(d) phase, across a first-order phase boundary. In the regime of phase coexistence in each leaflet, we find strong transbilayer correlations of the l(o) domains across the two leaflets, resulting in bilayer registry. This transbilayer correlation depends sensitively upon the chain length of the participating lipids and possibly other features of lipid chemistry, such as degree of saturation. We find that the l(o) domains in the upper leaflet can induce phase segregation in the lower leaflet, when the latter is nominally in the mixed (l(d)) phase.

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.

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.

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.

The solid–fluid transition in a yield stress shear thinning physical gel

We present an experimental investigation of the solid–fluid transition in a yield stress shear thinning physical gel (Carbopol® 940) under shear. Upon a gradual increase of the external forcing, we observe three distinct deformation regimes: an elastic solid-like regime (characterized by a linear stress–strain dependence), a solid–fluid phase coexistence regime (characterized by a competition between destruction and reformation of the gel), and a purely viscous regime (characterized by a power law stress-rate of strain dependence). The competition between destruction and reformation of the gel is investigated via both systematic measurements of the dynamic elastic moduli (as a function of stress, the amplitude, and temperature) and unsteady flow ramps. The transition from solid behavior to fluid behavior displays a clear hysteresis upon increasing and decreasing values of the external forcing. We find that the deformation power corresponding to the hysteresis region scales linearly with the rate at which the material is being forced (the degree of flow unsteadiness). In the asymptotic limit of small forcing rates, our results agree well with previous steady state investigations of the yielding transition. Based on these experimental findings, we suggest an analogy between the solid–fluid transition and a first-order phase transition, e.g., the magnetization of a ferro-magnet where irreversibility and hysteresis emerge as a consequence of a phase coexistence regime. In order to get further insight into the solid–fluid transition, our experimental findings are complemented by a simple kinetic model that qualitatively describes the structural hysteresis observed in our rheological experiments. The model is fairly well validated against oscillatory flow data by a partial reconstruction of the Pipkin space of the material’s response and its nonlinear spectral behavior.

Magnetic coupling of ferromagnetic stripe arrays: Analytical model for theα−β-phase coexistence regime of MnAs/GaAs(001)

We investigate the temperature-dependent hysteresis of the stripe state of MnAs thin films on GaAs(001) in the phase coexistence regime. The underlying magnetic domain structure is described employing an analytic model for stripe arrays with perpendicular anisotropy. In the framework of this model the magnetic properties of the MnAs stripe array can be unraveled as a combined effect of magnetostatic coupling of neighboring ferromagnetic stripes and the tendency to form antiparallel magnetic domains within the individual ferromagnetic stripes. The detailed analysis reveals the balance of demagnetization energy and domain-wall energy for the domain structure. It is capable to quantitatively predict the temperature dependency of the coercive field of MnAs thin films on GaAs(001) in the phase coexistence regime. Further, the analytic model allows for an understanding of the unusual magnetic reversal properties as a consequence of the temperature-driven geometrical variations in the stripe array. Here, it is the energy difference of the single and the multidomain states associated with the geometrical variations, which is the driving factor, rather than the temperature dependence of the magnetic properties themselves. Although the stripe array of MnAs thin films can be in an interstripe as well as in an intrastripe coupling state, the magnetization reversal is entirely determined by interstripe coupling.

Precise texture determination of the spin density wave/superconductivity mixture in the phase separation regime of

Abstract At low temperature, in a narrow range of pressure, the organic conductor ( TMTSF ) 2 PF 6 exhibits a phase coexistence between two ground states, a spin density wave (SDW) state and a metallic state which presents a superconducting (SC) transition at low temperature. This phase coexistence regime extends from 6.7 kbar up to 9.5 kbar . From resistivity measurements performed along the three crystallographic a -, b ′ - and c * -axes, we are able to extract an image of the evolution of the texture of ( TMTSF ) 2 PF 6 in its phase coexistence regime at low temperatures: at low SC concentration, SC regions form bubbles mostly extended along the c * -axis. Upon increasing the pressure, these bubbles extend to filaments along the c * -axis which then stacks to form slabs perpendicular to the a -axis in agreement with theoretical work on the formation of solitons in an SDW system.