Order Phase Transition Research Articles

Evolution of P-E loop in phase transition region of first order phase transition ferroelectrics

Polarization-electric field (E) hysteresis loop is crucial to ferroelectrics and their applications. First order phase transition ferroelectric has a transition region, where two phases coexist from ferroelectric phase at T0 to paraelectric phase at T1, and stretching to an E-enhanced phase transition temperature T2. Shape of the loops in the transition region changes with temperature confirmed by experiments. Evolution of the shape is derived by three-dimension Landau-Devonshire theory, on the basis of variations of E induced valleys of Gibbs free energy. The principles used are rotations of dipoles by the E to cause critical effect, and coupling between rotated dipoles to produce polarization hysteresis. The theoretic results exhibit that double loops converge to the origin and then separate when temperature rises through T1. The reasons for the double loops are ascribed to combination of the critical effects for the abrupt changes in polarization and the coupling effect for the hysteresis between two critical points. The theory can explain the experimental phenomena.

Dielectric and piezoelectric properties of 0.8Na0.5Bi0.5TiO3-0.2BaTiO3 modified with sodium niobate

In this paper, we present the dielectric and piezoelectric properties of tetragonal 0.8Na0.5Bi0.5TiO3-0.2BaTiO3 modified with NaNbO3 ((1-x)[0.8Na0.5Bi0.5TiO3-0.2BaTiO3]-xNaNbO3). Our experimental study has revealed that the ferroelectric phase in these compositions is suppressed with the increase of sodium niobate concentration. A broad anomaly, resembling relaxor ferroelectrics, appears in the 325–450 K temperature interval. The investigation of the electric field dependence of polarization has indicated that the double hysteresis loop behaviour is characteristic of the modified compositions, which is associated with the 1st order phase transition under the applied electric field. The experiments below room temperature have revealed that the range of stability of the ferroelectric phase is shifted to lower temperatures upon the increase of sodium niobate concentration. The electromechanical displacement in the modified compositions shows a similar maximum displacement in the whole concentration range. The electromechanical response in 0.8Na0.5Bi0.5TiO3-0.2BaTiO3 solid solutions is due to the piezoelectric effect, while, in the mixed compositions, it is related to the jump-like change of the lattice constants in the vicinity of electric fieldinduced 1st order phase transition.

QCD Equilibrium and Dynamical Properties from Holographic Black Holes

By using gravity/gauge correspondence, we employ an Einstein-Maxwell-Dilaton model to compute the equilibrium and out-of-equilibrium properties of a hot and baryon rich strongly coupled quark-gluon plasma. The family of 5-dimensional holographic black holes, which are constrained to mimic the lattice QCD equation of state at zero density, is used to investigate the temperature and baryon chemical potential dependence of the equation of state. We also obtained the baryon charge conductivity, and the bulk and shear viscosities with a particular focus on the behavior of these observables on top of the critical end point and the line of first order phase transition predicted by the model.

Transport in deformed centrosymmetric networks.

Centrosymmetry often mediates perfect state transfer (PST) in various complex systems ranging from quantum wires to photosynthetic networks. We introduce the deformed centrosymmetric ensemble (DCE) of random matrices H(λ)≡H_{+}+λH_{-}, where H_{+} is centrosymmetric while H_{-} is skew-centrosymmetric. The relative strength of the H_{±} prompts the system size scaling of the control parameter as λ=N^{-γ/2}. We propose two quantities, P and C, quantifying centro and skewcentrosymmetry, respectively, exhibiting second-order phase transitions at γ_{P}≡1 and γ_{C}≡-1. In addition, DCE posses an ergodic transition at γ_{E}≡0. Thus equipped with a precise control of the extent of centrosymmetry in DCE, we study the manifestation of γ on the transport properties of complex networks. We propose that such random networks can be constructed using the eigenvectors of H(λ) and establish that the maximum transfer fidelity F_{T} is equivalent to the degree of centrosymmetry P.

Unravelling the nature of spin reorientation transition in quasi-2D vdW magnetic material, Fe[formula omitted]GeTe[formula omitted

Fe4GeTe2, a recently discovered van der Waals magnet, hosts rather complex magnetic phases with two distinct transitions: paramagnetic to ferromagnetic at around TC∼ 266 K and another spin reorientation transition (SRT) at around TSRT∼ 100 K, unlike the other family member (Fe3GeTe2). Understanding the magnetic phases, nature of phase transitions and magnetic anisotropy have both fundamental and technological importance. Here, we report a detailed investigation of magnetization (M (T, H)), magneto-caloric effect (MCE), and heat capacity (Cp) in Fe4GeTe2 single crystal prepared via chemical vapor transport (CVT) method to shed light on the nature the magnetic transitions and the magnetic anisotropy, in particular, near the spin reorientation transition. M (T) measurements exhibit a prominent thermal hysteresis in proximity to TSRT, implying the first-order nature of SRT. A change in magnetic easy axis around SRT observed from M (T) and M (H) concludes a magnetic anisotropy is present at around TSRT. SRT is further observed in Cp (T) measurement, in terms of a prominent anomaly around SRT, suggesting the transition as real thermo-magnetic. Not only that we observe a higher MCE value near TSRT in comparison to near TC (near TC, -ΔSMmax = 1.95 and 1.99 J Kg−1K−1 and near TSRT, -ΔSMmax= 3.9 and 2.4 J Kg−1K−1 along H∥ab and H∥c, respectively, at 50 kOe magnetic field). The scaling analysis of MCE at TC, shows that the rescaled ΔSM(T,H) follows a universal curve confirming the second-order character of the ferromagnetic transition, while the same analysis breaks down at TSRT, suggesting that SRT is indeed not a second order phase transition. Thus, our experimental findings not only confirm the first-order nature of SRT at round 100 K in Fe4GeTe2, but provide additional insight into the complex spin texture with the possibility of canted or ferrimagnetic ordering.

Exact renormalization group equation for lattice Ginzburg–Landau models adapted to the solution in the local potential approximation

The Wilson Green’s function approach and, alternatively, Feynman’s diffusion equation and the Hori representation have been used to derive an exact functional RG equation (EFRGE) that in the course of the RG flow interpolates between the interaction part of the lattice Ginzburg–Landau Hamiltonian and the logarithm of the generating functional of the S-matrix. Because the S-matrix vertices are the amputated correlation functions of the fluctuating field, it has been suggested that in the critical region the amputation of the long-range tails makes the S-matrix functional more localized and thus more amenable to the local potential approximation (LPA) than the renormalized free energy functional used in Wilson’s EFRGE. By means of a functional Legendre transform the S-matrix EFRGE has been converted into an EFRGE for the effective action (EA). It has been found that the field-dependent part of EA predicted by the equation is the same as calculated within the known EA EFRGE approaches but in addition it is accurately accounts for the field-independent terms. These are indispensable in calculation of such important quantities as the specific heat, the latent heat, etc. With the use of the derived EFRGE a closed expression for the renormalization counterterm has been obtained which when subtracted from the divergent solution of the Wetterich equation would lead to a finite exact expression for the EA thus making two approaches formally equivalent. The S-matrix equation has been found to be simply connected with a generalized functional Burgers’ equation which establishes a direct correspondence between the first order phase transitions and the shock wave solutions of the RG equation. The transparent semi-group structure of the S-matrix RG equation makes possible the use of different RG techniques at different stages of the RG flow in order to improve the LPA solution.

Evidence of fluctuation-induced first-order phase transition in active matter

We investigate the effects of density fluctuations on the near-ordering phase of a flock by studying the Malthusian Toner–Tu theory. Because of the birth/death process, characteristic of this Malthusian model, density fluctuations are partially suppressed. We show that unlike its incompressible counterpart, where the absence of the density fluctuations renders the ordering phase transition similar to a second-order phase transition, in the Malthusian theory density fluctuations may turn the phase from continuous to first-order. We study the model using a perturbative renormalization group approach. At one loop, we find that the renormalization group flow drives the system in an unstable region, suggesting a fluctuation-induced first-order phase transition.

Particle production in a hybrid approach for a beam energy scan of Au+Au/Pb+Pb collisions between sqrt{s_textrm{NN}} = 4.3 GeV and sqrt{s_textrm{NN}} = 200.0 GeV

Heavy-ion collisions at varying collision energies provide access to different regions of the QCD phase diagram. In particular collisions at intermediate energies are promising candidates to experimentally identify the postulated first order phase transition and critical end point. While heavy-ion collisions at low and high collision energies are theoretically well described by transport approaches and hydrodynamics+transport hybrid approaches, respectively, intermediate energy collisions remain a challenge. In this work, a modular hybrid approach, the SMASH-vHLLE-hybrid coupling 3+1D viscous hydrodynamics (vHLLE) to hadronic transport (SMASH), is introduced. It is validated and subsequently applied in Au+Au/Pb+Pb collisions between sqrt{s_textrm{NN}} = 4.3 GeV and sqrt{s_textrm{NN}} = 200.0 GeV to study the rapidity and transverse mass distributions of identified particles as well as excitation functions for textrm{dN}/textrm{d}y|_{y = 0} and langle p_textrm{T} rangle . A good agreement with experimental measurements is obtained, including the baryon stopping dynamics. The transition from a Gaussian rapidity spectrum of protons at lower energies to the double-hump structure at high energies is reproduced. The centrality and energy dependence of charged particle v_2 is also described reasonably well. This work serves as a basis for further studies, e.g. systematic investigations of different equations of state or transport coefficients.

Efficient and scalable path integral Monte Carlo simulations with worm-type updates for Bose-Hubbard and XXZ models

We present a novel and open-source implementation of the worm algorithm, which is an algorithm to simulate Bose-Hubbard and sign-positive spin models using a path-integral representation of the partition function. The code can deal with arbitrary lattice structures and assumes spin-exchange terms, or bosonic hopping amplitudes, between nearest-neighbor sites, and local or nearest-neighbor interactions of the density-density type. We explicitly demonstrate the near-linear scaling of the algorithm with respect to the system volume and the inverse temperature and analyze the autocorrelation times in the vicinity of a U(1)U(1) second order phase transition. The code is written in such a way that extensions to other lattice models as well as closely-related sign-positive models can be done straightforwardly on top of the provided framework.

Codebase release 1.0 Worm

We present a novel and open-source implementation of the worm algorithm, which is an algorithm to simulate Bose-Hubbard and sign-positive spin models using a path-integral representation of the partition function. The code can deal with arbitrary lattice structures and assumes spin-exchange terms, or bosonic hopping amplitudes, between nearest-neighbor sites, and local or nearest-neighbor interactions of the density-density type. We explicitly demonstrate the near-linear scaling of the algorithm with respect to the system volume and the inverse temperature and analyze the autocorrelation times in the vicinity of a U(1)U(1) second order phase transition. The code is written in such a way that extensions to other lattice models as well as closely-related sign-positive models can be done straightforwardly on top of the provided framework.

Interpolating geometries and the stretched dS2 horizon

We investigate dilaton-gravity models whose solutions contain a large portion of the static patch of dS2. The thermodynamic properties of these theories are considered both in the presence of a finite Dirichlet wall, as well as for asymptotically near-AdS2 boundaries. We show that under certain circumstances such geometries, including those endowed with an asymptotically near-AdS2 boundary, can be locally and even globally thermodynamically stable within particular temperature regimes. First order phase transitions reminiscent of the Hawking-Page transition are discussed. For judiciously chosen models, the near-AdS2 boundary can be viewed as a completion of the stretched cosmological dS2 horizon. We speculate on candidate microphysical models.

Leptogenesis and dark matter through relativistic bubble walls with observable gravitational waves

We study a scenario where both dark matter and heavy right handed neutrino (RHN) responsible for leptogenesis acquire masses by crossing the relativistic bubble walls formed as a result of a TeV scale supercooled first order phase transition (FOPT). While this leads to a large out-of-equilibrium abundance of right handed neutrino inside the bubble sufficient to produce the required lepton asymmetry, the dark matter being lighter can still remain in equilibrium with its relic being set by subsequent thermal freeze-out. A classical conformal symmetry ensures the origin of mass via FOPT induced by a singlet scalar while also ensuring supercooling leading to enhanced gravitational wave amplitude within the sensitivity of the LISA experiment. A minimal scenario with three RHN, one inert scalar doublet and one singlet scalar as additional fields beyond the standard model is sufficient to realize this possibility which also favours inert RHN dark matter over inert scalar doublet.

Freezing and Thawing Processes of Highways in Kazakhstan

This paper presents the results of an experimental study of freezing and thawing patterns of highways in Kazakhstan. Special sensors measure temperature and moisture change every hour in automatic mode. The purpose of this work is to develop a methodology for determining the depth of freezing of subgrade soils of roads of Kazakhstan, and the task is to establish the pattern of cold temperature change (temperature “0 °C”) through certain points (sensors) at any time. In the upper part of the pavement (up to 30–40 cm), the temperature changes in annual and daily cycles. As the depth increases, the daily temperature fluctuations disappear, leaving only the annual fluctuation. At a depth of 180 cm and below, temperature fluctuations occur only in the annual cycle. The freezing rate varied from 14 cm/day to 0.33 cm/day. The maximum freezing depth was 227 cm. The descending branch of thawing occurs almost uniformly, with an average rate of 6.25 cm/day to a depth of 220 cm; the average rate of the ascending branch of thawing is 0.9 cm/day. Asphalt–concrete layers of the pavement and the upper part of the subgrade were in a frozen state for 151 and 166 days, respectively. In the subgrade at the beginning and end of the cold period, there are abrupt changes in moisture, which are explained by phase transitions of the second order: the transition from the liquid state to the solid (ice) at the beginning of the cold period and the transition of moisture from the solid state to liquid at the end of the cold period.

Have pulsar timing arrays detected the hot big bang: Gravitational waves from strong first order phase transitions in the early Universe

The origins of matter and radiation in the universe lie in a hot big bang. We present a number of well-motivated cosmologies in which the big bang occurs through a strong first-order phase transition---either at the end of inflation, after a period of kination (``kination-induced big bang''), or after a second period of vacuum domination in the early Universe (``supercooled big bang''); we also propose a ``dark big bang'' where only the dark matter in the Universe is created in a first-order phase transition much after inflation. In all of these scenarios, the resulting gravitational radiation can explain the tentative signals reported by the NANOGrav, Parkes, and European Pulsar Timing Array experiments if the reheating temperature of the hot big bang, and correspondingly the energy scale of the false vacuum, falls in the range ${T}_{*}\ensuremath{\sim}{\ensuremath{\rho}}_{\mathrm{vac}}^{1/4}=\mathrm{MeV}--100\text{ }\mathrm{GeV}$. All of the same models at higher reheating temperatures will be of interest to upcoming ground- and space-based interferometer searches for gravitational waves at larger frequency.

Transient ordering in the Gross-Pitaevskii lattice after an energy quench within a nonordered phase

We numerically investigate heating-and-cooling quenches taking place entirely in the non-ordered phase of the discrete Gross-Pitaevskii equation on a three-dimensional cubic lattice. In equilibrium, this system exhibits a U(1)-ordering phase transition at an energy density which is significantly lower than the minimum one during the quench. Yet, we observe that the post-quench relaxation is accompanied by a transient U(1) ordering, namely, the correlation length of U(1) fluctuations significantly exceeds its equilibrium pre-quench value. The longer and the stronger the heating stage of the quench, the stronger the U(1) transient ordering. We identify the origin of this ordering with the emergence of a small group of slowly relaxing lattice sites accumulating a large fraction of the total energy of the system. Our findings suggest that the transient ordering may be a robust feature of a broad class of physical systems. This premise is consistent with the growing experimental evidence of the transient U(1) order in rather dissimilar settings.

A general relation between the largest nucleus and all nuclei distributions for free energy calculations.

Prediction of nucleation rates in first order phase transitions requires the knowledge of the barrier associated with the free energy profile W. Molecular simulations offer a direct route through W = -kT ln pa, where k is Boltzmann's constant, T is temperature, and pa is the probability distribution of the size of any nucleus. However, in practice, the extremely scarce spontaneous occurrence of large nuclei impedes the full determination of pa, and a numerical bias must be introduced, which is generally done on the size of the largest nucleus in the system, leading to the probability size distribution of the largest nucleus pl. Although pl is known to be system size dependent, unlike pa, it has extensively been used as an approximation for pa. This communication demonstrates an improved relation between pa and pl, which cures this approximation and allows an accurate calculation of free energy barriers from biased simulations.

Mind the miscibility gap: Cation mixing and current density driven non-equilibrium phase transformations in spinel cathode materials

Cathode materials that exhibit phase transitions with large structural rearrangements during electrochemical cycling are generally seen as disadvantageous. Large volume changes and lattice mismatches between intermediate phases tend to lead to significant kinetic barriers, as well as strain and particle cracking. In this regard, solid solution reactions are more desirable as they provide lower energy barriers and no miscibility gap between co-existing phases. The high-voltage cathode material LiNi0.5Mn1.5O4 is an interesting candidate for high power and rate capability applications, however little is known on how its phase transitions occur on the particle level. In the presented work operando X-ray diffraction was utilized together with detailed peak profile analysis to elucidate the phase transition mechanism dependency on transition metal cation order and current density. When fully disordered, the material was found to undergo a bulk single-phase solid solution reaction between the intermediate phases LiNi0.44Mn1.56O4 and Li0.5Ni0.44Mn1.56O4 followed by a first order phase transition with a coherent interphase between the intermediates Li0.5Ni0.44Mn1.56O4 and Ni0.44Mn1.6O4. When fully ordered and slightly less ordered, two separate first order phase transitions with a coherent interphase between the same intermediate phases were observed. On discharge, the fast kinetics of the transition between Li0.5Ni0.44Mn1.56O4 and LiNi0.44Mn1.56O4 resulted in less strain on the former phase. For all samples the miscibility gap between the intermediate phases narrowed with increased current density, suggesting that the solid solution domain formed at the coherent interphase can be extended when the rate of (de)lithiation exceeds the movement speed of the interphase at the phase transition. This effect was found to be larger with increasing cation disorder. The influence of transition metal ordering on the ability to form solid solutions is in good agreement with computational phase diagrams of LiNi0.5Mn1.5O4, showing that disorder is important for promoting and stabilizing solid solutions. These results indicate that the degree of transition metal ordering within the material is of importance for obtaining a material with small lattice mismatches between the involved intermediate phases and for rational design of full solid solution materials.

Statistical mechanics of phase transitions in elastic media with vanishing thermal expansion.

We consider a minimal spin model for Ising transitions in an isotropic elastic medium in the zero thermal expansion (ZTE) limit. We set up the elastic theory for this system. We use this theory to identify and study the nature of the fluctuations in the system near the second order phase transitions at T_{c} in the ZTE limit given by dT_{c}/dV=0, where V is the system volume, and explore anomalous elasticity. Allowing for the local strain to couple asymmetrically or selectively with the states of the order parameter, we uncover the dramatic effects of these couplings on the fluctuations of the local displacements near T_{c}, and also on the nature of the phase transition itself. Near second-order phase transitions and with weak asymmetry in the order parameter-strain couplings, the variance of the displacement fluctuations in two dimensions scale with the system size L in a universal fashion as [ln(L/a_{0})]^{2/3}; a_{0} is a small-scale cutoff. Likewise, the correlation functions of the difference of the local displacements at two different points separated by r scale as [ln(r/a_{0})]^{2/3} for large r. For stronger selectivity above a finite threshold, this variance diverge as L exceeds beyond a (nonuniversal) size, determined by the model parameters, signaling a transition to a phase with only short-range order or the loss of the positional order of the elastic medium. At dimensions higher than two, for sufficiently weak selectivity, the variance of the displacement fluctuations is L-independent corresponding to long-range order. However, if the selectivity parameters rise beyond a dimension-dependent threshold value, then again the positional order is lost with a concomitant transition to a phase with short-range order. Large values of the order parameter-strain couplings can turn the phase transition into a first order as well. Our theory establishes a one-to-one correspondence between the order of phase transitions and anomalous elasticity near the transitions. Our theory should be a useful guide to possible synthesis of appropriate ZTE materials.

Magnetic properties and universal behaviour of magnetocaloric effect for polycrystalline [formula omitted] compound

In the present study, we have systematically investigated the magnetic and magnetocaloric properties as well as the universal behaviour of magnetocaloric effect (MCE) for polycrystalline (Nd0.5Sm0.5)0.5Sr0.5MnO3 compound. The universal behaviour of magnetic entropy change is proposed here for conventional magnetocaloric effect (CMCE) materials with second order phase transition. Thus, the construction of a universal master curve for MCE is possible in our system by rescaling the temperature axis. For such CMCE compounds, ΔS follows the power law dependence of H: ΔS ∝ Hn where ‘n’ is dependent on both H and T. In this paper, we have studied the universal behaviour of MCE at different conditions of initial and final field values. The result suggests that the universality behaviour of MCE is satisfied if and only if the initial base field remains zero or at a constant value.

Landau theory for the Mpemba effect through phase transitions

The Mpemba effect describes the situation in which a hot system cools faster than an identical copy that is initiated at a colder temperature. In many of the experimental observations of the effect, e.g. in water and clathrate hydrates, it is defined by the phase transition timing. However, none of the theoretical investigations so far considered the timing of the phase transition, and most of the abstract models used to explore the Mpemba effect do not have a phase transition. We use the phenomenological Landau theory for phase transitions to identify the second order phase transition time, and demonstrate with a concrete example that a Mpemba effect can exist in such models.