Strong Metastability Research Articles

Wrinkled Shrinkage on Surface Monophasic Structure of Floating Refractory Alloy Droplets Solidified in Space Environment

AbstractThe surface‐wrinkled shrinkage structure of spheres is very common and also quite intriguing to explore and reveal the analogous droplet solidification kinetics in the space environment. Here, a liquid‐to‐solid phase transition experiment of floating refractory alloy droplets is designed to study the surface wrinkled shrinkage structures aboard China Space Station with a 10−5 gravity level. The experimental Ti‐Ni‐V alloy droplets undergo an extremely high overheating temperature beyond 1800 K and then achieve a supercooling of 307 K, displaying a strong thermodynamic metastability. The spherical surface deformation structures of this alloy under microgravity coupled with extreme metastable solidification are explored. Moreover, through the active control of metastable rapid solidification, a surface monophasic structure and a B2 structure capable of stress‐induced martensitic transformation are facilitated by wrinkled shrinkage dynamics under microgravity. Their findings shed further light on the surface deformation structures during the microgravity solidification process.

Stabilizing metastable ferroelectric tungsten trioxide phase at room temperature via solvothermal synthesis and millisecond pulsed laser irradiation

Epsilon tungsten trioxide (ε-WO3) has drawn much attention for its unique gas sensing and ferroelectric properties. However, the strong metastability of ε-phase makes it extremely difficult to stabilize the phase at room temperature. For a long time, it was believed that the ε-phase can only be stabilized by rapid solidification processing. In this study, ε-WO3 was stabilized by employing solvothermal synthesis and subsequent annealing, with the help of Cr- and Ti-dopants. Structural characterization using XRD, Raman and TEM analysis revealed that the as-annealed powders are a mixture of ε-WO3 and γ-WO3 and the fraction of ε-WO3 is directly correlated with the Cr- and Ti-doping levels. Rietveld refinement suggests that the maximum obtainable fractions of ε-phase in as-annealed WO3 are ∼68 % for Cr-doping and ∼87 % for Ti-doping. The growth mechanism is that solvothermal synthesis produced Cr- and Ti-doped W18O49 and subsequent annealing resulted in phase transition from W18O49 to ε-WO3. The XPS analysis reveals the phase stabilization mechanism to involve the interstitial sites of WO3 being occupied by Cr3+ and Ti4+ dopants, resulting in structure distortions. The fraction of ε-WO3 can be further improved to almost 100 % by introducing kinetic constraints using pulsed laser irradiation to rapidly melt and solidify the WO3 (in several milliseconds). A viable solvothermal synthesis route for ε-WO3 and the feasibility to produce ε-phase via additive manufacturing are illustrated in this work.

Spiral eutectic growth dynamics facilitated by space Marangoni convection and liquid surface wave

Eutectic alloys display excellent application performances since the essential function of coupled microstructures is quite different from that of single-phase and peritectic alloys. However, due to the strong natural convection within liquid alloys under normal gravity, the eutectic growth process on earth usually produces traditional rod-like or lamellar composite microstructures, which hinders the exploration of distinctive coupled growth patterns. Here, we carried out the rapid solidification of hypoeutectic Zr64V36 alloy to explore novel coupled growth dynamics aboard the China Space Station under a long-term stable microgravity condition. An extreme liquid undercooling of 253 K was achieved for this refractory alloy, displaying a strong metastability in outer space. We find that a radial coupled pattern grew out of the nucleation site, accompanying a ripple-like surface microstructure. This resulted from the rapid eutectic growth within a highly undercooled alloy in combination with a liquid surface wave excited by the electrostatic field under microgravity. Especially, a spiral coupled growth mode occurred during radial eutectic growth and surface wave spreading, which were controlled by the Marangoni convection effect on the fluid flow pattern and eutectic growth dynamics. Our findings contribute to the coupled growth investigation by modulating gravity levels to develop multi-pattern microstructures.

Distinct vibrational signatures and complex phase behavior in metallic oxygen

Evidence for metallization in dense oxygen has been reported for over 30 years [Desgreniers et al., J. Phys. Chem. 94, 1117 (1990)] at a now routinely accessible 95 GPa [Shimizu et al., Nature 393, 767 (1998)]. However, despite the longevity of this result and the technological advances since, the nature of the metallic phase remains poorly constrained [Akahama et al., Phys. Rev. Lett. 74, 4690 (1995); Goncharov et al., Phys. Rev. B 68, 224108 (2003); Ma, Phys. Rev. B 76, 064101 (2007); and Weck et al., Phys. Rev. Lett. 102, 255503 (2009)]. In this work, through Raman spectroscopy, we report the distinct vibrational characteristics of metallic ζ-O2 from 85 to 225 GPa. In comparison with numerical simulations, we find reasonable agreement with the C2/m candidate structure up to about 150 GPa. At higher pressures, the C2/m structure is found to be unstable and incompatible with experimental observations. Alternative candidate structures, C2/c and Ci, with only two molecules in the primitive unit cell, are found to be stable and more compatible with measurements above 175 GPa, indicative of the dissociation of (O2)4 units. Further, we report and discuss a strong hysteresis and metastability with the precursory phase ϵ-O2. These findings will reinvigorate experimental and theoretical work into the dense oxygen system, which will have importance for oxygen-bearing chemistry, prevalent in the deep Earth, as well as fundamental physics.

Vendi sampling for molecular simulations: Diversity as a force for faster convergence and better exploration.

Molecular dynamics (MD) is the method of choice for understanding the structure, function, and interactions of molecules. However, MD simulations are limited by the strong metastability of many molecules, which traps them in a single conformation basin for an extended amount of time. Enhanced sampling techniques, such as metadynamics and replica exchange, have been developed to overcome this limitation and accelerate the exploration of complex free energy landscapes. In this paper, we propose Vendi Sampling, a replica-based algorithm for increasing the efficiency and efficacy of the exploration of molecular conformation spaces. In Vendi sampling, replicas are simulated in parallel and coupled via a global statistical measure, the Vendi Score, to enhance diversity. Vendi sampling allows for the recovery of unbiased sampling statistics and dramatically improves sampling efficiency. We demonstrate the effectiveness of Vendi sampling in improving molecular dynamics simulations by showing significant improvements in coverage and mixing between metastable states and convergence of free energy estimates for four common benchmarks, including Alanine Dipeptide and Chignolin.

Nonlinear and thermal effects in the absorption of microwaves by random magnets

We study the temperature dependence of the absorption of microwaves by random-anisotropy magnets. It is governed by strong metastability due to the broad distribution of energy barriers separating different spin configurations. At a low microwave power, when the heating is negligible, the spin dynamics is close to linear. It corresponds to the precession of ferromagnetically ordered regions that are in resonance with the microwave field. Previously, we showed [Phys. Rev. B 103, 214414 (2021)] that in this regime a dielectric substance packed with random magnets would be a strong microwave absorber in a broad frequency range. Here we demonstrate that on increasing the power, heating and over-barrier spin transitions come into play, resulting in the nonlinear behavior. At elevated temperatures the absorption of microwave power decreases dramatically, making the dielectric substance with random magnets transparent for the microwaves.

An Endogenous miRNA-Initiated Hybridization Chain Reaction and Subsequent DNAzyme Activation for Cellular Theranostics

An Endogenous miRNA-Initiated Hybridization Chain Reaction and Subsequent DNAzyme Activation for Cellular Theranostics

Can One Predict a Drop Contact Angle?

AbstractThe study of wetting phenomena is of great interest due to the multifaceted technological applications of hydrophobic and hydrophilic surfaces. The theoretical approaches proposed by Wenzel and later by Cassie and Baxter to describe the behavior of a droplet of water on a rough solid are extensively used and continuously updated to characterize the apparent contact angle of a droplet. However, the equilibrium hypothesis implied in these models means that they are not always predictive of experimental contact angles due to the strong metastabilities typically occurring in the wetting of heterogeneous surfaces. A predictive scheme for contact angles is thus urgently needed both to characterize a surface by contact angle measurements and to design super‐hydrophobic and ‐oleophobic surfaces with the desired properties, for example, contact angle hysteresis. In this work, a combination of Monte Carlo simulation and the string method is employed to calculate the free energy profile of a liquid droplet deposited on a pillared surface. For the analyzed surfaces, it is shown that there is only one minimum of the free energy that corresponds to the superhydrophobic wetting state while the wet state can present multiple minima. Furthermore, when the surface roughness decreases the amount of local minima observed in the free energy profile increases. The presented approach clarifies the origin of contact angle hysteresis providing quantitative tools for understanding and controlling wetting at structured surfaces.

Thermal Casimir interactions for higher derivative field Lagrangians: generalized Brazovskii models

We examine the Casimir effect for free statistical field theories which have Hamiltonians with second order derivative terms. Examples of such Hamiltonians arise from models of non-local electrostatics, membranes with non-zero bending rigidities and field theories of the Brazovskii type that arise for polymer systems. The presence of a second derivative term means that new types of boundary conditions can be imposed, leading to a richer phenomenology of interaction phenomena. In addition zero modes can be generated that are not present in standard first derivative models, and it is these zero modes which give rise to long range Casimir forces. Two physically distinct cases are considered: (i) unconfined fields, usually considered for finite size embedded inclusions in an infinite fluctuating medium, here in a two plate geometry the fluctuating field exists both inside and outside the plates, (ii) confined fields, where the field is absent outside the slab confined between the two plates. We show how these two physically distinct cases are mathematically related and discuss a wide range of commonly applied boundary conditions. We concentrate our analysis to the critical region where the underlying bulk Hamiltonian has zero modes and show that very exotic Casimir forces can arise, characterised by very long range effects and oscillatory behavior that can lead to strong metastability in the system.

Observation of Volkov-Pankratov states in topological HgTe heterojunctions using high-frequency compressibility

It is well established that topological insulators sustain Dirac fermion surface states as a consequence of band inversion in the bulk. These states have a helical spin polarization and a linear dispersion with large Fermi velocity. In this article we report on a set of experimental observations indicating the existence of massive surface states. These states are confined at the interface and dominate equilibrium and transport properties at high energy and/or high electric field. By monitoring the AC admittance of HgTe topological insulator field-effect capacitors, we access the compressibility and conductivity of surface states in a broad range of energy and electric fields. The Dirac surface states are characterized by a compressibility minimum, a linear energy dependence and a high mobility persisting up to energies much larger than the transport bandgap of the bulk. New features are revealed at high energies with signatures such as conductance peaks, compressibility bumps, a strong charge metastability and a Hall resistance anomaly. These features point to the existence of excited massive surface states, responsible for a strong intersubband scattering with the Dirac states and the nucleation of metastable bulk carriers. The spectrum of excited states agrees with predictions of a phenomenological model of the topological-trivial semiconductor interface. The model accounts for the finite interface depth and the effect of electric fields. The existence of excited topological states is essential for the understanding of topological phases and opens a route for engineering and exploiting topological resources in quantum technology.

Pressure-induced Transformations of Dense Carbonyl Sulfide to Singly Bonded Amorphous Metallic Solid.

The application of pressure, internal or external, transforms molecular solids into non-molecular extended network solids with diverse crystal structures and electronic properties. These transformations can be understood in terms of pressure-induced electron delocalization; however, the governing mechanisms are complex because of strong lattice strains, phase metastability and path dependent phase behaviors. Here, we present the pressure-induced transformations of linear OCS (R3m, Phase I) to bent OCS (Cm, Phase II) at 9 GPa; an amorphous, one-dimensional (1D) polymer at 20 GPa (Phase III); and an extended 3D network above ~35 GPa (Phase IV) that metallizes at ~105 GPa. These results underscore the significance of long-range dipole interactions in dense OCS, leading to an extended molecular alloy that can be considered a chemical intermediate of its two end members, CO2 and CS2.

Elastic Frustration Triggering Photoinduced Hidden Hysteresis and Multistability in a Two-Dimensional Photoswitchable Hofmann-Like Spin-Crossover Metal-Organic Framework.

We report a two-dimensional Hofmann-like spin-crossover (SCO) material, [Fe(trz-py)2{Pt(CN)4}]·3H2O, built from [FePt(CN)4] layers separated by interdigitated 4-(2-pyridyl)-1,2,4,4H-triazole (trz-py) ligands with two symmetrically inequivalent FeII sites. This compound exhibits an incomplete first-order spin transition at 153 K between fully high-spin (HS-HS) and intermediate high-spin low-spin (HS-LS) ordered states. At low temperature, it undergoes a bidirectional photoswitching to HS-HS and fully low-spin (LS-LS) states with green and near-IR light irradiation, respectively, with associated T(LIESST = Light-Induced Excited Spin-State Trapping) and T(reverse-LIESST) values of 52 and 85 K, respectively. Photomagnetic investigations show that the reverse-LIESST process, performed from either HS-HS or HS-LS states, enables access to a hidden stable LS-LS state, revealing the existence of a hidden thermal hysteresis. Crystallographic investigations allowed to identify that the strong metastability of the HS-LS state originates from the existence of a strong elastic frustration causing antiferroelastic interactions within the [FePt(CN)4] layers, through the rigid NC-Pt-CN bridges connecting the inequivalent FeII sites. The existence of the stable LS-LS state paves the way for a multidirectional photoswitching and allows potential applications for electronic devices based on ternary digits.

An efficient algorithm for numerical computations of continuous densities of states

In Wang–Landau type algorithms, Monte-Carlo updates are performed with respect to the density of states, which is iteratively refined during simulations. The partition function and thermodynamic observables are then obtained by standard integration. In this work, our recently introduced method in this class (the LLR approach) is analysed and further developed. Our approach is a histogram free method particularly suited for systems with continuous degrees of freedom giving rise to a continuum density of states, as it is commonly found in lattice gauge theories and in some statistical mechanics systems. We show that the method possesses an exponential error suppression that allows us to estimate the density of states over several orders of magnitude with nearly constant relative precision. We explain how ergodicity issues can be avoided and how expectation values of arbitrary observables can be obtained within this framework. We then demonstrate the method using compact U(1) lattice gauge theory as a show case. A thorough study of the algorithm parameter dependence of the results is performed and compared with the analytically expected behaviour. We obtain high precision values for the critical coupling for the phase transition and for the peak value of the specific heat for lattice sizes ranging from \(8^4\) to \(20^4\). Our results perfectly agree with the reference values reported in the literature, which covers lattice sizes up to \(18^4\). Robust results for the \(20^4\) volume are obtained for the first time. This latter investigation, which, due to strong metastabilities developed at the pseudo-critical coupling of the system, so far has been out of reach even on supercomputers with importance sampling approaches, has been performed to high accuracy with modest computational resources. This shows the potential of the method for studies of first order phase transitions. Other situations where the method is expected to be superior to importance sampling techniques are pointed out.

Dynamic transition in current-driven disordered flux-line lattice in single-crystal of Bi-2212

We have measured the current–voltage characteristics for both as-grown and irradiated Bi2Sr2CaCu2O8+δ single crystals at T=5K in a magnetic field applied parallel to c axis. The results show a variety of dynamical behavior above the depinning threshold, depending on the vortex–vortex interaction (λab/a0) strength and the nature of the quenched disorder (point-like or columnar defects). When the flux lattice is soft, our experimental measurements in both samples have been attributed to plastic flow, including strong metastability and history dependence of the depinning process. The vortex motion in this regime is thought of relatively weakly pinned vortices past more strongly pinned neighbors. A power-law scaling, fit between voltage and applied current can be obtained for the onset of motion in both samples, with different apparent critical exponent depending on defect nature and the strength of interactions. In the plastic regime, the usual scaling ansatz associated with dynamic critical phenomena V scales as (I−Ic)β, where β∼2.2±0.1 and 1.22±0.021 for as-grown and β≈1.49±0.07 for irradiated samples, respectively. Finally, in both cases of defects, with increasing the strength of vortex–vortex interaction a dynamical transition is observed as confirmed by the discontinuity in the vortex–vortex interactions dependence of the critical exponent β. More, our results confirm the important role of the system dimensionnality on vortex dynamics.

Nature of the glassy transition in simulations of the ferromagnetic plaquette Ising model

The homogeneous plaquette Ising model in two and three dimensions is investigated by means of Monte Carlo simulations. By introducing a suitable order parameter for the two-dimensional lattice, and the finite-size scaling of the corresponding fourth-order cumulant, it is found that, consistent with the previous theoretical indications, the model in two dimensions is disordered at finite temperature and exhibits a zero-temperature phase transition characteristic of the one-dimensional Ising model with an essential (exponential) singularity of the order-parameter susceptibility as opposed to a Curie-law (power-law) divergence. In three dimensions, however, the model is believed to have a first-order phase transition at Tc approximately 3.6 screened by strong metastability leading to a so-called "glassy transition" at T approximately 3.4 when subjected to slow cooling. By computing the configurational entropy Sc identical withS(liquid)-S(crystal) in the supercooled temperature range via thermodynamic integration of the internal energy results, the Kauzmann temperature defined as that temperature where the extrapolated configurational entropy Sc(T) vanishes, is estimated to be TK approximately 3.18 . By finding ways to estimate the equilibration time of the supercooled liquid and the nucleation time of the stable crystal droplets, it is shown that T approximately 3.4 is indeed the limit of stability or the effective spinodal temperature Tsp, at which the two time-scales associated with the quasiequilibration of the supercooled liquid, taueq, and the nucleation of the stable crystal droplets, taunuc, cross one another, with the former rising above the latter such that the supercooled liquid state becomes physically irrelevant below Tsp approximately 3.4 and the impending entropy crisis at TK approximately 3.18 (<Tsp) is thus avoided. Hence, what is sometimes called "glassy temperature," is really a kinetic spinodal temperature that may be regarded as the remnant of the mean-field spinodal.

Theoretical zero‐temperature isotherm and structural phase stability of thorium dioxide

AbstractThe static‐lattice equation of state (EOS) and structural phase stability of Thorium Dioxide (ThO2) have been investigated for pressures up to 500 kbar using the relativistic linear combinations of Gaussian type orbitals‐fitting function (LCGTO‐FF) technique, within the generalized gradient approximation (GGA) to density functional theory (DFT). Two observed room temperature crystal structures for ThO2 have been considered here, the ambient fluorite structure and the high‐pressure cotunnite structure. The EOS parameters calculated for the fluorite structure and the external cell parameters found for the cotunnite structure are consistent with experiment to within the known limitations of the GGA model. The internal lattice parameters obtained for the cotunnite phase differ greatly from the experimental values reported in the literature. There is strong theoretical evidence that the measured values of the internal parameters are in error. The predicted transition pressure (275 kbar) is in reasonable agreement with the measured value of 330 kbar, given the likelihood of strong metastability in this oxide. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

Interplay between coarsening and nucleation in an Ising model with dipolar interactions

We study the dynamical behavior of a square lattice Ising model with exchange and dipolar interactions by means of Monte Carlo simulations. After a sudden quench to low temperatures, we find that the system may undergo a coarsening process where stripe phases with different orientations compete, or alternatively it can relax initially to a metastable nematic phase and then decay to the equilibrium stripe phase through nucleation. We measure the distribution of equilibration times for both processes and compute their relative probability of occurrence as a function of temperature and system size. This peculiar relaxation mechanism is due to the strong metastability of the nematic phase, which goes deep into the low-temperature stripe phase. We also measure quasiequilibrium autocorrelations in a wide range of temperatures. They show a distinct decay to a plateau that we identify as due to a finite fraction of frozen spins in the nematic phase. We find indications that the plateau is a finite-size effect. Relaxation times as a function of temperature in the metastable region show super-Arrhenius behavior, suggesting a possible glassy behavior of the system at low temperatures.

Ordered, Disordered, and Coexistent Stable Vortex Lattices inNbSe2Single Crystals

The peak effect (PE) in the critical current density of type II superconductors has been related to an order-disorder transition in the vortex lattice (VL), but its underlying physics remains a controversial issue. Intrinsic to the PE are strong metastabilities that frequently mask the stationary VL configurations. We follow shaking and thermal protocols in NbSe2 single crystals to access these configurations and examine them by linear ac susceptibility measurements that avoid VL reorganization. We identify three different regions. For T<T1(H), stable VL configurations are maximally ordered. For T>T2(H), configurations are fully disordered and no metastability is observed. In the T1<T<T2 region, we find temperature-dependent stable configurations with an intermediate degree of disorder, possibly associated with the coexistence of ordered and disordered lattices throughout the PE.

Intermittency, metastability and coarse graining for coupled deterministic–stochastic lattice systems

We study the role of strong particle/particle interactions and stochastic fluctuations emanating from the micro-/sub-grid scale, in the context of a simple prototype hybrid system consisting of a scalar linear ordinary differential equation (ODE), coupled to a microscopic spin flip Ising lattice system. Due to the presence of strong interactions in the lattice model, the mean-field approximation of this system is a Fitzhugh–Nagumo-type system of ODE. However, microscopic noise and local interactions will significantly alter the deterministic and spatially homogeneous mean-field Fitzhugh–Nagumo behaviours (excitable, bistable and oscillatory) and will yield corresponding regimes with phenomena driven by the interaction of nonlinearity and noise across scales, such as strong intermittency, metastability and random oscillations. Motivated by these observations we consider a class of stochastic numerical approximations based on systematic coarse-grainings of stochastic lattice dynamics. The resulting stochastic closures give rise to computationally inexpensive reduced hybrid models that capture correctly the transient and long-time behaviour of the full system; this is demonstrated by detailed time series analysis that includes comparisons of power spectra and auto- and cross-correlations in time and space, especially in examples dominated by strong interactions between scales and fluctuations, such as nucleation, intermittent and random oscillation regimes.

Bulk first-order phase transition in three-flavor lattice QCD withO(a)-improved Wilson fermion action at zero temperature

Three-flavor QCD simulation with the $O(a)$-improved Wilson fermion action is made employing an exact fermion algorithm developed for odd number of quark flavors. For the plaquette gauge action, an unexpected first-order phase transition is found in the strong coupling regime ($\beta\lesssim$ 5.0) at relatively heavy quark masses ($m_{\mathrm{PS}}/m_{\mathrm{V}}\sim$ 0.74--0.87). Strong metastability persists on a large lattice of size $12^3\times 32$, which indicates that the transition has a bulk nature. The phase gap becomes smaller toward weaker couplings and vanishes at $\beta\simeq 5.0$, which corresponds to a lattice spacing $a\simeq$ 0.1 fm. The phase transition is not found if the improved gauge actions are employed. Our results imply that realistic simulations of QCD with three flavors of dynamical Wilson-type fermions at lattice spacings in the range $a=$ 0.1--0.2 fm require use of improved gauge actions. Possible origins of the phase transition is discussed.