Shift Of Critical Point Research Articles

Adsorption effects on CO2-oil minimum miscibility pressure in tight reservoirs

CO2 miscible injection holds immense potential for enhancing tight oil recovery, where achieving the minimum miscibility pressure (MMP) is pivotal. Adsorption of CO2 and oil in pores affects the CO2-oil MMP in tight reservoirs, necessitating precise nanoscale MMP calculations and understanding influencing factors. Here, we applied a modified Peng-Robinson equation of state (PR-EOS) for nanoscale MMP calculations, incorporating adsorption layers and effective molar volume to describe adsorption effects. Validation against molecular simulations and nanofluidic experiments shows a maximum deviation of 4.6 %. We found that in nanopores, achieving miscibility demands less CO2 than in bulk. The CO2-oil MMP reduces as pore size decreasing, influenced by adsorption, critical point shift and capillarity. At 5 nm, the MMP is 11.12 MPa, 27.8 % lower than the bulk value (15.4 MPa). Adsorption intensifies this reduction by curtailing free molecules and effective pore radius, and becomes more pronounced for lighter hydrocarbon mixtures. However, the nanoscale CO2-oil MMP is equal to the bulk value when rp ≥ 350 nm. Furthermore, a maximum MMP and the corresponding transition temperature exist for each pore size, and increase as pore size increasing. This method provides a valuable tool for optimizing CO2 miscible injection and carbon storage in challenging nanoscale-pore reservoirs.

Supercooled solutions of sodium perchlorate in TIP4P/2005 water: The effect of martian solutes on thermodynamics and structure.

We study the thermodynamic behavior of sodium perchlorate solutions in supercooled water through molecular dynamics numerical simulations. These solutions are of special interest because of the recent experimental results that led to hypothesize the presence of liquid water in perchlorate solutions beneath the Martian soil. We model water using the TIP4P/2005 potential. The results we obtain for solutions with concentrations 1.63 and 15.4 wt% are in agreement with those of a system undergoing a liquid-liquid phase transition where the liquid-liquid critical point shifts to slightly higher temperatures and lower pressures. The structure of the system is also analyzed, and we come to the conclusion that, even at the highest concentration considered, water retains its anomalous behavior.

Effects of site dilution on Compensation in Ising Spin-1/2 trilayered triangular Ferrimagnets with non-equivalent planes

Using Monte Carlo simulations with the Metropolis algorithm, the magnetic and thermodynamic behaviours of a spin-1/2, trilayered ferrimagnetic system on triangular monolayers with quenched nonmagnetic impurities are studied. Two different theoretical atoms, A and B, make up the ABA and AAB types of distinct configurations. Like atoms (A-A and B-B) interact ferromagnetically, while unlike atoms (A-B) interact antiferromagnetically. Only the A-layers are randomly site-diluted with dilution percentages ranging from 5% to 45%. Such diluted magnetic thin systems exhibit magnetic compensation which depends sensitively on the concentration of impurities. The phase diagram in the Hamiltonian parameter space related to the occurrence of magnetic compensation phenomenon and the effect of site dilution is discussed in detail. Special attention is given to the mathematical dependencies of compensation temperature on the concentration of nonmagnetic impurities. Depending upon the concentration of nonmagnetic impurities, the compensation and critical points shift with the equilibrium magnetic behaviours changing between distinct ferrimagnetic behaviours. For each combination of the coupling strengths, with values of the impurity concentration above a threshold, compensation appears where previously was absent. Suggested mathematical formulae show how threshold impurity concentration relies on Hamiltonian parameters.

Finite-temperature ferromagnetic transition in coherently coupled Bose gases

A paramagnetic-ferromagnetic quantum phase transition is known to occur at zero temperature in a two-dimensional coherently coupled Bose mixture of dilute ultracold atomic gases provided the interspecies interaction strength is large enough. Here we study the fate of such a transition at finite temperature by performing numerical simulations with the stochastic (projected) Gross-Pitaevskii formalism, which includes both thermal and beyond mean-field effects. By extracting the average magnetization, the magnetic fluctuations and characteristic relaxation frequency (or critical slowing down), we identify a finite-temperature critical line for the transition. We find that the critical point shifts linearly with temperature and, in addition, the three quantities used to probe the transition exhibit a temperature power-law scaling. The scaling of the critical slowing down is found to be consistent with thermal critical exponents and is very well approximated by the square of the spin excitation gap at zero temperature.

Quantized topological Anderson-Thouless pump

Thouless pump with quantized transports is topologically robust against small perturbations and disorders, while breaks down under sufficiently strong disorders. Here we propose counter-intuitive topological pumps induced by disorders in noninteracting and interacting systems. We first show an extrinsic topological pump driven by the on-site quasiperiodic potential for a two-loop sequence, where the disorder inequivalently suppresses the topology of two pump loops. Moreover, we reveal an intrinsic topological pump induced by the hopping quasiperiodic disorder from a trivial single-loop pump in the clean limit, dubbed the topological Anderson-Thouless pump (TATP) as a dynamical analogue of topological Anderson insulators. We demonstrate that the mechanism of the TATP is the disorder-induced shift of gapless critical points and the TATP can even exhibit in the dynamic disorder and interacting cases. Finally, we extend the TATP to higher-order topological systems with disorder-induced quantized corner transports. Our proposed TATPs present new members of the topological pump family and could be realized with ultracold atoms or photonic waveguides.

Perils of embedding for quantum sampling

Given quantum hardware that enables sampling from a family of natively implemented Hamiltonians, how well can one use that hardware to sample from a Hamiltonian outside that family? A common approach is to minor embed the desired Hamiltonian in a native Hamiltonian. In Phys. Rev. Research 2, 023020 (2020) it was shown that minor embedding can be detrimental for classical thermal sampling. Here, we generalize these results by considering quantum thermal sampling in the transverse-field Ising model, i.e. sampling a Hamiltonian with non-zero off diagonal terms. To study these systems numerically we introduce a modification to standard cluster update quantum Monte-Carlo (QMC) techniques, which allows us to much more efficiently obtain thermal samples of an embedded Hamiltonian, enabling us to simulate systems of much larger sizes and larger transverse-field strengths than would otherwise be possible. Our numerics focus on models that can be implemented on current quantum devices using planar two-dimensional lattices, which exhibit finite-temperature quantum phase transitions. Our results include: i) An estimate on the probability to sample the logical subspace directly as a function of transverse-field, temperature, and total system size, which agrees with QMC simulations. ii) We show that typically measured observables (diagonal energy and magnetization) are biased by the embedding process, in the regime of intermediate transverse field strength, meaning that the extracted values are not the same as in the native model. iii) By considering individual embedding realizations akin to 'realizations of disorder', we provide numerical evidence suggesting that as the embedding size is increased, the critical point shifts to increasingly large values of the transverse-field.

Effect of a uniform random external magnetic field with spatiotemporal variation on compensation in Ising spin-1/2 trilayered square ferrimagnets.

Trilayered spin-1/2 Ising ferrimagnets are interesting thin systems for compensation phenomenon. In this work, a Metropolis Monte Carlo study is performed on the magnetic and thermodynamic response of such a system on square Bravais lattice, driven by uniform random external magnetic field with spatiotemporal variations. In two distinct configurations, the surface layers are made up of A and the middle layer is made up of B atoms in an ABA-type stacking while in AAB-type stacking, the top layer and the middle layer is made up of A atoms while the bottom layer is made up of B atoms. The magnetic coupling between the like atoms (A-A and B-B) is ferromagnetic while between the unlike atoms (A-B), it is antiferromagnetic. For the time-dependent external uniform random field, the mean is always set to zero and the standard deviation is varied until spin-field energy is comparable to the dominant cooperative energy of the system. The findings show that the observed compensation and critical points shift and steady-state magnetic behaviors shift among N, L, P, and Q, etc., type of ferrimagnetic behaviors, depending on the strength of external uniform random field. The compensation phenomenon even vanishes after crossing a finite threshold of standard deviation of the magnetic field for particular choices of the other controlling parameters. Thus islands of ferrimagnetic phase without compensation appear within the phase area with compensation of field-free case, in the two-dimensional Hamiltonian parameter space. For both the configurations, the areas of such islands even grow with increasing standard deviation of the external field, σ, obeying a scaling relation of the form: f(σ,A(σ))=σ^{-b}A(σ) with b_{ABA}=1.958±0.122 and b_{AAB}=1.783±0.118.

Thermodynamic behaviour of magnetocaloric quantities in spin-1/2 Ising square trilayer

A spin-1/2, Ising trilayered ferrimagnetic system on square Bravais lattice is studied, employing Monte-Carlo simulation with the single spin-flip Metropolis algorithm. The bulk of such a system is formed by three layers, each of which is composed entirely either by A or B type of atoms, resulting in two distinct compositions: ABA and AAB and two different types of magnetic interactions: ferromagnetic between like atoms and antiferromagnetic between unlike atoms. For such systems, Inverse Absolute of Reduced Residual Magnetisation is the absolute value of the ratio of the extremum of the magnetisation in between compensation and critical points and the saturation magnetisation. Variation of relative interaction strengths in the Hamiltonian, for a range of values, leads to the shift of compensation point and critical point and changes in the magnitude of Inverse absolute of Reduced Residual magnetisation. Probable mathematical forms of dependences of the Inverse absolute of Reduced Residual magnetisation and temperature interval between the compensation and critical points on controlling parameters were proposed in the absence of applied magnetic field and have obtained phase diagrams for both types of configurations from these relations. This alternative description of the simulated systems may help technologists design magnetocaloric materials according to desired characteristics.

Theoretical Charge density Proof of N–N weak bonds of RDX Energetic Molecule

The bond topological analysis of Cyclotrimethylene-trinitramine (RDX) energetic molecule has been carried out for the wave function obtained from the ab initio and DFT methods of quantum chemical calculations. The geometrical parameters of all bonds are compared with that of experimental reports. The inclusion of diffuse function in HF basis set levels makes the significant shift of bond critical point towards carbon atoms of C–N bonds. The heteroatomic bond density character is well understood from unequal C-cp and cp-N distances in all C–N bonds. For all the level of calculations, the maximum bond density was found for all N=O bonds, attributes the maximum potential energy V(r). The N–N bond properties are strongly depends upon the equilibrium bond length which clears from charge concentration in shorter N1–N4 bond and charge depletion found in longer N2–N5 and N3–N6 bonding regions. The bond topological analysis of all bonds in RDX molecule resulted that the N–N bond is the weakest among all the other bonds. The weakness of N2–N5 and N3–N6 bonds than N1–N4 bond of RDX has also been analyzed from energy density calculation from various level of theories as an alternate for Laplacian of electron density. From the analysis of CHELPG charges at the MP2 level, the N–N bonds of RDX appears to have a significant ionic nature which attributes strong hyperconjugation effect. The hyperconjugation effect of RDX, due to polarization of N–N bonds, is the additional proof of weak N–N bonds in RDX explosive. The isosurface electrostatic potential shows the electro positive and negative region in the molecule. A large negative potential found at the vicinity of oxygen atoms.

Control strategies for Hopf bifurcation in a chaotic associative memory

Abstract This paper presents a set of control approaches that can deal with unwanted dynamics such as the Hopf Bifurcation in a Chaotic Associative Memory (CAM) for two distinct goals: the bifurcation critical point shift to anticipate or to postpone the start of a Hopf bifurcation; or to extinguish it. A Washout Filter Control, a Polynomial Control, and an adaptive control for data-driven called the Model-Free Adaptive Control were used in this study. We present the results of numerical simulations in a CAM with three neurons (a system with six dimensions), which were trained classically to retrieve a set of memories. Our analysis includes the dynamics of the bifurcated system, the ability of the network to recover the memories learned and how it is affected by the control strategies. The generality of the proposed control approaches was tested to control a different system and for a different behavior (chaotic dynamics).

The Joule-Thomson Effect for Refrigerants with Dopants of the Fullerenes and Carbon Nanotubes

The importance of thermodynamic and phase behavior of working fluids embedded with nanostructured materials is fundamental to new nanotechnology applications. The fullerenes (C60) and carbon nanotubes (CNT) adding to refrigerants change their thermodynamic properties the Joule – Thomson effect such as dislocation of critical point, gas – liquid equilibria shift at alias. Algorithm of refrigerant thermodynamic property calculations based on the NIST (National Institute of Standards and Technologies) equation of state at different carbon nanotube concentrations is proposed. Thermodynamic properties of carbon dioxide in the C60 and CNT presence are given. Considering the extremely large number of different both nanoparticle types and reference fluids, it is obvious that there is need for developing theoretically sound methods of the prompt estimation thermodynamic properties and phase equilibria for emerging working media. The effect of nanoparticles on the critical point shift for classical fluids doped by nanoparticles is examined. The regular and singular parts of thermodynamic surface of reference fluid and nanofluid (volume nanoparticle concentration < 5%) are suggested to coincide in the reduced form. The shift of critical point for nanoliquids of industrial interest is theoretically predicted. Results of calculations of phase equilibria for some nanofluids are described.

Thermodynamic structure of supercritical LOX–GH2 diffusion flames

In this study, we evaluate the thermodynamic structure of laminar hydrogen/oxygen flames at supercritical pressures using 1D flame calculations and large-eddy simulation (LES) results. We find that the real fluid mixing behavior differs between inert (cold flow) and reactive (hot flow) conditions. Specifically, we show that combustion under transcritical conditions is not dominated by large-scale homogeneous real-fluid mixing: similar to subcritical atomization, the supercritical pure oxygen stream undergoes a distinct transition from liquid-like to gas-like conditions; significant mixing and combustion occurs primarily after this transition under ideal gas conditions. The joint study of 1D flame computations and LES demonstrates that real-fluid behavior is chiefly confined to the bulk LOX stream; real fluid mixing occurs but in a thin layer surrounding the LOX core, characterized by water mass fractions limited to 3%. A parameter study of 1D flame solutions shows that this structure holds for a wide range of relevant injection temperatures and chamber pressures. To analyze the mixing-induced shift of the local fluid critical point, we introduce a state-space representation of the flame trajectories in the reduced temperature and reduced pressure plane which allows for a direct assessment of the local thermodynamic state. In the flame, water increases the local mixture critical pressures, so that subcritical conditions are reached. This view of limited mixing under supercritical conditions may yield more efficient models and an improved understanding of the disintegration modes of supercritical flows.

Enhanced quantum spin fluctuations in a binary Bose-Einstein condensate

For quantum fluids, the role of quantum fluctuations may be significant in several regimes such as when the dimensionality is low, the density is high, the interactions are strong, or for low particle numbers. In this paper we propose a fundamentally different regime for enhanced quantum fluctuations without being restricted by any of the above conditions. Instead, our scheme relies on the engineering of an effective attractive interaction in a dilute, two-component Bose-Einstein condensate (BEC) consisting of thousands of atoms. In such a regime, the quantum spin fluctuations are significantly enhanced (atom bunching with respect to the noninteracting limit) since they act to reduce the interaction energy - a remarkable property given that spin fluctuations are normally suppressed (anti-bunching) at zero temperature. In contrast to the case of true attractive interactions, our approach is not vulnerable to BEC collapse. We numerically demonstrate that these quantum fluctuations are experimentally accessible by either spin or single-component Bragg spectroscopy, offering a useful platform on which to test beyond-mean-field theories. We also develop a variational model and use it to analytically predict the shift of the immiscibility critical point, finding good agreement with our numerics.

Annealing effect on ferromagnetic properties, hole concentration and electronic band structure of GaMnAs epitaxial layers

In this paper, we analyze the correlation of the magnetism and the carrier concentration with the shift of the spectroscopic critical points for low compensated GaMnAs samples with a high Curie temperature of around 150 K. The GaMnAs layers were grown by low-temperature molecular beam epitaxy. The low-temperature annealing leads to a reduction of Mn interstitials from 0.8 to 0.4% and an enhancement in the hole concentration. The saturation magnetization is 51 emu/cm $$^3$$ and the Curie temperature is 150 K after post-growth annealing, while those of as-grown layers are 37 emu/cm $$^3$$ and 80 K. The resistivity dropped significantly after the post-growth annealing, due to the fact that the number of Mn $$_I$$ , which act as double donors and compensate holes, was significantly reduced by the low-temperature and long-time annealing. The electronic band structure is investigated by spectroscopic ellipsometry. The transition energies of critical points show redshift after post-growth annealing due to the annealing-induced enhancement of the hole concentration. Our results support the valence band picture (the Zener model) in ferromagnetic GaMnAs.

Phase Behavior of Magnetic Nanocolloids of Different Sizes Suspended in an Apolar Solvent.

We employ classical density functional theory (DFT) to investigate the phase behavior and composition of binary mixtures; each compound consists of hard spheres of different sizes with superimposed dispersion attraction. In addition to the dispersion attraction, molecules of one component carry an additional three-dimensional magnetic "spin" where the orientation-dependent spin-spin interaction is accounted for by the Heisenberg model. We are treating the excess free energy using a modified mean-field approximation (second virial coefficient) for the orientation-dependent pair correlation function. Depending on the concentration of the magnetic particles, the strength of the spin-spin coupling, and the size ratio of the particles, the model predicts the formation of ordered (polar) phases in addition to the more conventional gas and isotropic liquid phases. Key features of our model are a particle-size dependent shift of the gas-liquid critical point (critical temperature and density) and a change in the width of the phase diagram. In the near-critical region, the latter can be analyzed quantitativly in terms of an effective critical exponent βeff that may differ from the classical critical exponent [Formula: see text]; the classical value is attained in the immediate vicinity of the critical point as it must. The deviation between βeff and β can be linked to nontrivial composition effects along the phase boundaries.

The Widom line for supercritical fluids

We review the different approaches to establish the boundary between the liquid and the quasi-gas fluid above the critical point. Some experimental data by inelastic X-ray scattering demonstrates different dynamical regimes: gas-like and liquid-like reminiscent of the subcritical domains. Theoretical models based on the Lennard-Jones and square well interparticle potentials together with phenomenological Van der Waals model confirm similar picture for the Widom line in supercritical domain. To minimize uncertainty experimental data we calculated extrema of response functions via fundamental equations of state. The shift of critical points under influences of impurities and nanoparticle adding is analyzed.

Vapour-liquid phase diagram for an ionic fluid in a random porous medium

We study the vapour-liquid phase behaviour of an ionic fluid confined in a random porous matrix formed by uncharged hard sphere particles. The ionic fluid is modelled as an equimolar binary mixture of oppositely charged equisized hard spheres, the so-called restricted primitive model (RPM). Considering the matrix-fluid system as a partly-quenched model, we develop a theoretical approach which combines the method of collective variables with the extension of the scaled-particle theory (SPT) for a hard-sphere fluid confined in a disordered hard-sphere matrix. The approach allows us to formulate the perturbation theory using the SPT for the description of the thermodynamics of the reference system. The phase diagrams of the RPM in matrices of different porosities and for different size ratios of matrix and fluid particles are calculated in the random-phase approximation and also when the effects of higher-order correlations between ions are taken into account. Both approximations correctly reproduce the basic effects of porous media on the vapour-liquid phase diagram, i.e. with a decrease of porosity the critical point shifts towards lower fluid densities and lower temperatures and the coexistence region gets narrower. For the fixed matrix porosity, both the critical temperature and the critical density increase with an increase of size of matrix particles and tend to the critical values of the bulk RPM.

Zeeman Splitting of Ferromagnetic GaMnAs on InP Observed by Magnetic Circular Dichroism in Reflection Mode

Systematic investigations of Ga1−xMnxAs grown on InP with different Mn concentrations have been conducted using magnetic circular dichroism (MCD) in reflection mode. The MCD spectrum of Ga0.97Mn0.03As/InP was decomposed into two dispersion curves originating from E1 and E1 + Δ1 optical transitions using the energy derivative of a Gaussian function. The Zeeman splitting energy E1 at the L critical point (0.6 meV) of ferromagnetic Ga0.97Mn0.03As/InP was estimated using a rigid band shift model. Based on the relationship between E1 and E0 (Γ critical point) observed in Cd1−xMnxTe dilute magnetic semiconductor (DMS), the Zeeman splitting energy E1 (9.6 meV) of ferromagnetic Ga1−xMnxAs/InP was calculated. In addition, it was established that the peaks in the MCD spectra at L critical points shift toward the lower energy side as the Mn concentration is increased, and the observed shift saturates for Mn content of x = 0.001. Furthermore, the measured absorption spectra for Ga1−xMnxAs/InP did not show noticeable peak shifts with increasing Mn content. This suggests that the s, p–d exchange interaction induced in Ga1−xMnxAs/InP has localized nature due to the presence of a Mn rigid sphere of influence.

Miscibility phase diagram of ring-polymer blends: A topological effect.

The miscibility of polymer blends, a classical problem in polymer science, may be altered, if one or both of the component do not have chain ends. Based on the idea of topological volume, we propose a mean-field theory to clarify how the topological constraints in ring polymers affect the phase behavior of the blends. While the large enhancement of the miscibility is expected for ring-linear polymer blends, the opposite trend toward demixing, albeit comparatively weak, is predicted for ring-ring polymer blends. Scaling formulas for the shift of critical point for both cases are derived. We discuss the valid range of the present theory, and the crossover to the linear polymer blends behaviors, which is expected for short chains. These analyses put forward a view that the topological constraints could be represented as an effective excluded-volume effects, in which the topological length plays a role of the screening factor.

Hysteresis behavior during facing target magnetron sputtering

Abstract The target-to-target distance affects the hysteresis behavior during reactive facing target sputtering. A decreasing target-to-target distance results in a shift of the first critical point towards lower reactive gas flows while only a minor effect on the position of the second critical point is noticed. Based on a time-dependent and spatially-resolved simulation model for reactive sputtering for multiple metallic targets, it is possible to elucidate the main mechanisms for these shifts. The slight difference in position of the second critical point can be understood from the perspective of target deposition. The shift in the first critical point under decreasing target separation is dominated by the decrease in the effective gettering area.