Transport properties of supercooled confined water

This article presents an overview of recent experiments performed on transport properties of water in the deeply supercooled region, a temperature region of fundamental importance in the science of water. We report data of nuclear magnetic resonance, quasi-elastic neutron scattering, Fourier-transform infrared spectroscopy, and Raman spectroscopy, studying water confined in nanometer-scale environments. When contained within small pores, water does not crystallise, and can be supercooled well below its homogeneous nucleation temperature T_h. On this basis it is possible to carry out a careful analysis of the well known thermodynamical anomalies of water. Studying the temperature and pressure dependencies of water dynamics, we show that the liquid-liquid phase transition (LLPT) hypothesis represents a reliable model for describing liquid water. In this model, water in the liquid state is a mixture of two different local structures, characterised by different densities, namely the low density liquid (LDL) and the high-density liquid (HDL). The LLPT line should terminate at a special transition point: a low-T liquid-liquid critical point. We discuss the following experimental findings on liquid water: (i) a crossover from non-Arrhenius behaviour at high T to Arrhenius behaviour at low T in transport parameters; (ii) a breakdown of the Stokes-Einstein relation; (iii) the existence of a Widom line, which is the locus of points corresponding to maximum correlation length in the p-T phase diagram and which ends in the liquid-liquid critical point; (iv) the direct observation of the LDL phase; (v) a minimum in the density at approximately 70 K below the temperature of the density maximum. In our opinion these results represent the experimental proofs of the validity of the LLPT hypothesis.     

Glass transition in biomolecules and the liquid-liquid critical point of water

Using molecular dynamics simulations, we investigate the relation between the dynamic transitions of biomolecules (lysozyme and DNA) and the dynamic and thermodynamic properties of hydration water. We find that the dynamic transition of the macromolecules, sometimes called a "protein glass transition," occurs at the temperature of dynamic crossover in the diffusivity of hydration water and also coincides with the maxima of the isobaric specific heat C_{P} and the temperature derivative of the orientational order parameter. We relate these findings to the hypothesis of a liquid-liquid critical point in water. Our simulations are consistent with the possibility that the protein glass transition results from crossing the Widom line, which is defined as the locus of correlation length maxima emanating from the hypothesized second critical point of water.     

Microscopic theory of a strongly correlated two-dimensional electron gas

The rearrangement of the Fermi surface in a diluted two-dimensional electron gas beyond the topological quantum critical point has been examined within an approach based on the Landau theory of Fermi liquid and a nonperturbative functional method. The possibility of a transition of the first order in the coupling constant at zero temperature between the states with a three-sheet Fermi surface and a transition of the first order in temperature between these states at a fixed coupling constant has been shown. It has also been shown that a topological crossover, which is associated with the joining of two sheets of the Fermi surface and is characterized by the maxima of the density of states N(T) and ratio C(T)/T of the specific heat to the temperature, occurs at a very low temperature T _⋄ determined by the structure of a state with the three-sheet Fermi surface. A momentum region where the distribution n(p, T) depends slightly on the temperature, which is manifested in the maximum of the specific heat C(T) near T _*, appears through a crossover at temperatures T ∼ T _* > T _⋄. It has been shown that the flattening of the single-particle spectrum of the strongly correlated two-dimensional electron gas results in the crossover from the Fermi liquid behavior to a non-Fermi liquid one with the density of states N(T) ∝ T ^−α with the exponent α }~ 2/3.     

Liquid-liquid critical point: an analytical approach

Theoretical simulations and experimental studies have showed that many systems (like liquid metals) can exhibit two phase transitions: gas-liquid and liquid-liquid. Consequently the fluid phase of these systems presents two critical points, namely the usual gas-liquid (G-L) critical point and the liquid-liquid critical point that results from a phase transition between two liquids of different densities: a low density liquid (LDL) and a high density liquid (HDL). The van der Waals theory for simple fluids [Phys. Rev. E 50, 2913 (1994)] is based on taking a system with purely repulsive forces as a reference, is able to describe two stable first-order phase transitions between fluids of different densities. The particles in our system interact via a total pair potential, which splits into a repulsive V_R and a density-dependent attractive V_A part.     

Fragile to strong crossover and Widom line in supercooled water: A comparative study

The aim of this paper is to discuss the relationship between the dynamics and thermodynamics of water in the supercooled region. Reviewed case studies comprehend bulk water simulated with the SPC/E, TIP4P and TIP4P/2005 potentials, water at protein interfaces, and water in solution with electrolytes. Upon supercooling, the fragile to strong crossover in the α-relaxation of water is found to occur when the Widom line emanating from the liquid-liquid critical point is crossed. This appears to be a general characteristic of supercooled water, not depending on the applied interaction potential and/or different local environments.     

Predictions of dynamic behavior under pressure for two scenarios to explain water anomalies

Using Monte Carlo simulations and mean field calculations for a cell model of water we find a dynamic crossover in the orientational correlation time tau from non-Arrhenius behavior at high temperatures to Arrhenius behavior at low temperatures. This dynamic crossover is independent of whether water at very low temperature is characterized by a "liquid-liquid critical point" or by the "singularity-free" scenario. We relate tau to fluctuations of hydrogen bond network and show that the crossover found for tau for both scenarios is a consequence of the sharp change in the average number of hydrogen bonds at the temperature of the specific heat maximum. We find that the effect of pressure on the dynamics is strikingly different in the two scenarios, offering means to distinguish between them.     

Thermodynamic behavior of a water model with a liquid-liquid critical point

In this paper we calculate the constant pressure specific heat response of a water model with a liquid-liquid critical point. We show how, due to the existence of the critical point, there is a secondary maximum in the specific heat at some temperature T_*>T_h for any pressure P>P_c, being T_h the first order transition temperature between the high and the low density liquid phases and P_c the pressure of the critical point. This secondary maximum does not correspond to any long range correlated phase transition and does not show up in the temperature dependence of the isothermal compressibility.     

Conventional and inverse magnetocaloric effects,and critical behaviors in Ni43Mn46Sn8In3 alloy

A systematic study of the conventional and inverse magnetocaloric effects, and critical behaviors in an alloy ingot of Ni₄₃Mn₄₆Sn₈In₃ has been performed. Our results reveal the sample exhibiting structural and magnetic phase transitions at temperatures T_C^M = 166 K (T_C of the martensitic phase), T_M–A = 260 K (the martensitic-to-austenitic phase transformation) and T_C^A = 296 K (T_C of the austenitic phase). The large values of refrigerant capacity (RC) around T_M–A and T_C^A are found to be RC_M–A = 172.6 and RC_A = 155.9 J kg⁻¹, respectively, under an applied field change of 30 kOe. Our critical analyses near the T_C^M and T_C^A reveal that a coexistence of the long- and short-range ferromagnetic order in the martensitic phase, while the long-range ferromagnetic order exists in the austenitic phase. Interestingly, at around T_C^A, the maximum magnetic entropy change (|ΔS_max|) versus magnetic field H obeys a power law, |ΔS_max| = a·Hⁿ, where the exponent n is found to be about 0.66.     

Positronium decay and formation in paraffin wax from polarized and unpolarized positrons

Summary Ps formation and decay in heterogeneousn-alkane samples (paraffin waxes) have been studied both in the solid and in the liquid phase; then, in the solid phase, the positron's residual degree of polarization was measured at the instant of Ps formation. Differently from what is already known in homogeneousn-alkane samples, Ps shows, many degrees below the melting point, a mean lifetime longer than that typical of the liquid phase; furthermore, the mean lifetime's values pertaining to the transition between solid and liquid do not show a sharp variation across the melting temperature but gradually decrease over a range of temperatures of several degrees. Positronium decay in static magnetic fields indicates that o-Ps magnetic quenching in liquid phase is regular, and corresponds to a contact density value α=|ψ(0)|²/|ψ(0)|_vac ²=0.79±0.07; instead, in the solid phase, o-Ps magnetic quenching shows anomalous behaviour for fields weaker than 7kG. Positrons' residual polarization measurements do not reveal the presence of depolarization effects during the whole slowing-down process until Ps is formed.     

The effects of sintering temperatures on dielectric,ferroelectric and electric field-induced strain of lead-free Bi0.5(Na0.78K0.22)0.5TiO3 piezoelectric ceramics synthesized by the sol–gel technique

Lead-free Bi_0.5(Na_0.78K_0.22)_0.5TiO₃ (BNKT) piezoelectric ceramics were synthesized by the sol–gel technique. The effects of sintering temperatures on the crystal structure, microstructure, densification, dielectric, ferroelectric and electric field-induced strain behaviors of the BNKT ceramics were investigated. X-ray diffraction patterns exhibited a pure perovskite structure from 1075 to 1150 °C. A scanning electron microscopy study revealed an increase in grain size with increasing sintering temperature. The density of the ceramics sintered at 1150 °C reaches a maximum value of 5.55 g/cm³, which is 96% of the theoretical density. BNKT ceramics sintered at an optimum temperature of 1150 °C exhibited a high remnant polarization of 18.5 μC/cm², a high electric field-induced strain of 0.20% and dynamic piezoelectric coefficient d₃₃¹ = (S_max/E_max) of 247 pm/V.     

Mössbauer spectroscopic study of the thermal spin crossover in [Fe(II)(isoxazole)6](ClO4)2

The ⁵⁷Fe Mössbauer spectroscopy of mononuclear [Fe(II)(isoxazole)₆](ClO₄)₂ has been studied to reveal the thermal spin crossover of Fe(II) between low-spin (S=0) and high-spin (S=2) states. Temperature-dependent spin transition curves have been constructed with the least-square fitted data obtained from the Mössbauer spectra measured at various temperatures between 84 and 270 K during a cooling and heating cycle. This compound exhibits an unusual temperature-dependent spin transition behaviour with T_C(↓)=223 and T_C(↑)=213 K occurring in the reverse order in comparison to those observed in SQUID observation and many other spin transition compounds. The compound has three high-spin Fe(II) sites at the highest temperature of study of which two undergo spin transitions. The compound seems to undergo a structural phase transition around the spin transition temperature, which plays a significant role in the spin crossover behaviour as well as the magnetic properties of the compound at temperatures below T_C. The present study reveals an increase in high-spin fraction upon heating in the temperature range below T_C, and an explanation is provided.     

Universal crossover of liquid dynamics in supercritical region

We demonstrate that all liquids in supercritical region may exist in two qualitatively different states: solid-like and gas-like. Solid-like to gas-like crossover corresponds to the condition τ ≈ τ₀, where τ is liquid relaxation time and τ₀ is the minimum period of transverse waves. This condition corresponds to the loss of shear stiffness of a liquid at all frequencies and defines a new narrow crossover zone on the phase diagram. We show that the intersection of this zone corresponds to the disappearance of high-frequency sound, qualitative changes of diffusion and viscous flow, increase in particle thermal speed to half of the speed of sound and reduction of the specific heat at constant volume to 2k _B per particle. The new crossover is universal: it separates two liquid states at arbitrarily high pressure and temperature, and even exists in systems where liquid-gas transition and the critical point are absent overall.     

Dynamics of vortices in type-II superconductors with periodic square columnar pins

The dynamics of a two-dimensional vortex system with strong periodic square columnar pins is investigated. For the case vortex number matching pinning number, we find that the vortex liquid is frozen into square lattice via a continuous transition, and the freezing (melting) temperature T_m is the same as the thermal depinning temperature of vortices, which are different from the first-order phase transition at weak pinning. The zero-temperature critical depinning force F_c0 is exactly the same as the maximum pinning force, and the depinning property at T = 0 can be expressed by scaling v ～ (F ? F_c0)^β with the exponent β close to 0.5. The v–F curves at temperatures below T_m show that vortices are pinned at small driving force.     

Diffusion anomaly in a three-dimensional lattice gas

We investigate the relationship between thermodynamic and dynamic properties of an associating lattice-gas (ALG) model. The ALG combines a three-dimensional lattice gas with particles interacting through a soft core potential and orientational degrees of freedom. The competition between the directional attractive forces and the soft core potential results in two liquid phases, double criticality and density anomaly. We study the mobility of the molecules in this model by calculating the diffusion constant at a constant temperature, D. We show that D has a maximum at a density ρ_max and a minimum at a density ρ_min<ρ_max. Between these densities the diffusivity differs from the one expected for normal liquids. We also show that in the pressure-temperature phase-diagram the line of extrema in diffusivity is close to the liquid-liquid critical point and it is partially inside the temperature of maximum density (TMD) line.     

The measurements of coexistence curves and critical behavior of a binary mixture with a high molecular weight polymer

The critical behavior of solutions of poly (diallydimethylammonium chloride) (PDDAC, M_w = 3.2 × 10⁵) in the critical binary mixture of isobutyric acid (I) + water (W) was studied by the refractive index measurements. The measurements were performed at three different PDDAC concentrations near and far away from the critical point. We observed that the critical temperatures increase linearly with increasing the concentration of PDDAC. As the distance from the critical point increases, the system exhibits a crossover from the renormalized Ising critical behavior to the mean-field one. For the solutions with the highest PDDAC concentration, experiments suggest a crossover to tricritical behavior.     

Critical currents in polycrystalline thin films of high-T c superconductors

Summary It is shown that the behaviour of the temperature dependence of the critical current in polycrystalline thin films of high-T _c superconductors depends crucially on the assumption made concerning the nature of the intergranular material. The usual assumption of a superconductor-insulator-superconductor (=SIS) ?sandwich? between each grain leads to a crossover fromI _c∼(1−T/T _c) toI _c∼(1−T/T _c)^3/2, for temperatures nearT _c (whereI _c is the critical current,T the absolute temperature, andT _c the superconducting transition temperature). Instead, for a superconductor-normal metal-superconductor (=SNS) sandwich the dependenceI _c∼(1−T/T _c)² is found for all temperatures. Consideration is given to the effect of self-magnetic field on the analysis. The comparison between expressions for continuous and granular systems is extended. Due to the relevance of its scientific content, this paper has been given priority by the Journal Direction.     

Dynamic magnetic hysteresis behavior and dynamic phase transition in the spin-1 Blume-Capel model

The nature (time variation) of response magnetization m(wt) of the spin-1 Blume-Capel model in the presence of a periodically varying external magnetic field h(wt) is studied by employing the effective-field theory (EFT) with correlations as well as the Glauber-type stochastic dynamics. We determine the time variations of m(wt) and h(wt) for various temperatures, and investigate the dynamic magnetic hysteresis behavior. We also investigate the temperature dependence of the dynamic magnetization, hysteresis loop area and correlation near the transition point in order to characterize the nature (first- or second-order) of the dynamic transitions as well as obtain the dynamic phase transition temperatures. The hysteresis loops are obtained for different reduced temperatures and we find that the areas of the loops are decreasing with the increasing of the reduced temperatures. We also present the dynamic phase diagrams and compare the results of the EFT with the results of the dynamic mean-field approximation. The phase diagrams exhibit many dynamic critical points, such as tricritical (•), zero-temperature critical (Z), triple (TP) and multicritical (A) points. According to values of Hamiltonian parameters, besides the paramagnetic (P), ferromagnetic (F) fundamental phases, one coexistence or mixed phase region, (F+P) and the reentrant behavior exist in the system. The results are in good agreement with some experimental and theoretical results.     

Magnetocaloric effect in Y-doped La0.6Ca0.4MnO3 enhanced by Griffiths phase and re-entrance of first-order phase transition

It has been known that bulk La_0.6Ca_0.4MnO₃ is an intermediate material of the first- and second-order characters with the tricritical-point exponents, and the doping of a metal ion in it usually causes a continuous second-order transition. The present work reports the re-entrance of a discontinuous first-order transition in orthorhombic La_0.6-xY_xCa_0.4MnO₃ (x = 0.03–0.09) compounds. This enhances the magnetocaloric effect. For the field H = 30 kOe, the maximum magnetic-entropy change (|ΔS_max|) and relative cooling power (RCP) have been evaluated being about 5.45–6.3 J/kg·K and 130–185 J/kg, respectively. If combining these compounds as refrigerant blocks in a rotary ring model, a magnetic cooling device can operate at temperatures T = 85–280 K, with |ΔS_max| ≈ 5.5 J/kg⋅K and RCP ≈ 1073 J/kg. Aside from the re-entranced first-order phase transition, the magnetization and structural analyses have proved the enhanced magnetocaloric effect in La_0.6-xY_xCa_0.4MnO₃ related to a Griffiths singularity, and local Jahn-Teller distortions of the perovskite structure (since the Mn³⁺/Mn⁴⁺ ratio and orthorhombic structural phase are unchanged vs. x).     

CW one-micron (4F3/2-4I11/2) laser oscillation of Nd3+ ions in the melilite-type crystal Ca2MgSi2O7:Nd3+(Na+) at its incommensurate–commensurate phase transition

The spectroscopic and stimulated-emission (⁴F_3/2→⁴I_11/2) properties of the novel melilite-type laser crystal Ca₂MgSi₂O₇:Nd³⁺(Na⁺) were studied in a temperature range that covers its incommensurate–commensurate (IC↔N) phase transition. The phase transition temperatures of both undoped Ca₂MgSi₂O₇ and the doped crystal were ascertained at 346.6 K (undoped) and 341.3 K (doped) by means of differential scanning calorimetry (DSC). The temperature-dependent spectroscopic and laser experiments showed a significant decrease in CW output power and a strong distortion of the generated lasing beam in the region of the phase transition. The observed crystal field disorder of Nd³⁺ lasants in Ca₂MgSi₂O₇:Nd³⁺(Na⁺) is dominantly due to occupation of the position of Ca²⁺ by cations of different valency, while the influence of incommensurability is of minor importance.