Giant Electrocaloric Strength in Single‐Crystal BaTiO3

Electrocaloric effect driven by electric fields displays great potential in realizing highly efficient solid-state refrigeration. Nevertheless, most known electrocaloric materials exhibit relatively poor cooling performance near room temperature, which hinders their further applications. The emerging family of hybrid perovskite ferroelectrics, which exhibits superior structural diversity, large heat exchange and broad property tenability, offers an ideal platform. Herein, we report an exceptionally large electrocaloric effect near room temperature in a designed hybrid perovskite ferroelectric [(CH3)2CHCH2NH3]2PbCl4, which exhibits a sharp first-order phase transition at 302 K, superior spontaneous polarization (>4.8 μC/cm2) and relatively small coercive field (<15 kV/cm). Strikingly, a large isothermal entropy change ΔS of 25.64 J/kg/K and adiabatic temperature change ΔT of 11.06 K under a small electric field ΔE of 29.7 kV/cm at room temperature are achieved, with giant electrocaloric strengths of isothermal ΔS/ΔE of 0.86 J·cm/kg/K/kV and adiabatic ΔT/ΔE of 370 mK·cm/kV, which is larger than those of traditional ferroelectrics. This work presents a general approach to the design of hybrid perovskite ferroelectrics, as well as provides a family of candidate materials with potentially prominent electrocaloric performance for room temperature solid-state refrigeration.

An applied stress field ${\ensuremath{\sigma}}_{3}$ can reversibly change the temperature of an elastocaloric material under adiabatic conditions, and the temperature change $\ensuremath{\Delta}{T}_{{\ensuremath{\sigma}}_{3}}$ is usually maximized near phase transitions. Using a thermodynamic approach, we demonstrate that an elastocaloric strength $\ensuremath{\alpha}=|\ensuremath{\Delta}{T}_{{\ensuremath{\sigma}}_{3}}|/|{\ensuremath{\sigma}}_{3}|$ of 0.016 K/MPa can be achieved benefiting from the full first-order phase transition in ${\mathrm{BaTiO}}_{3}$ single crystals, which is comparable with typical elastocaloric materials reported in the literature. The elastocaloric temperature change is found to be giant (3.2 K) under a stress of 200 MPa with a temperature span of over 50 K, which can be significantly larger than its electrocaloric counterpart ($\ensuremath{\sim}1$ K). Moreover, it is found that the elastocaloric strength can be remarkably enhanced (2.32 K/MPa) as long as the phase transition is triggered even by a modest stress near the sharp first-order phase transition, which is two orders of magnitude larger than those accomplished by full transition. Therefore, even a low stress ($&lt;30$ MPa) can induce a modest elastocaloric effect (1.3 K) comparable with the electrocaloric counterpart, which is accompanied by a reduction of the working temperature span. In addition, it is found that the electrocaloric peak under tensile stresses moves towards higher temperatures with its magnitude slightly enhanced. Hopefully, our study will stimulate further investigations on elastocaloric and stress-mediated electrocaloric effects in ferroelectrics.

Anti-ferroelectric thin films are renowned for their signature double hysteresis loops and sheds light on the distinguished energy storage capabilities of dielectric capacitors in modern electronic devices. However, anti-ferroelectric capacitors are still facing the dual challenges of low energy density and efficiency to achieve state-of-the-art performance. Their large hysteresis and sharp first-order phase transition usually results in a low energy storage efficiency and easy breakdown, severely obscuring its future application. In this study, we demonstrate that anti-ferroelectric (Pb0.97La0.02)(Zr1−xSnx)O3 epitaxial thin films exhibit enhanced energy storage performance through local structural heterogeneity to moderate the first-order phase transition by calculating the corresponding polarization as a function of switching time for the first time. The films exhibit remarkable enhanced breakdown strength (∼3.47 MV/cm, ∼5 times the value for PbZrO3) and energy storage performance. Our endeavors have culminated in the ingenious formulation of a novel strategy, namely, the postponement of polarization processes, thereby elevating the breakdown strength and total energy storage performance. This landmark achievement has unveiled a fresh vista of investigative opportunities for advancing the energy storage prowess of electric dielectrics.

The underpotential deposition (UPD) of metal submonolayers and monolayers on metal substrates for the systems Ag/Au(100), Au/Ag(100), Ag/Pt(100), Pt/Ag(100), Au/Pt(100), Pt/Au(100), Au/Pd(100), and Pd/Au(100) is studied by means of lattice Monte Carlo simulations. Interaction energies among different metal atoms are evaluated by using the embedded-atom method. A wide variety of physical situations are found and discussed, including systems exhibiting the sequential adsorption of atoms on kink and step sites, prior to the completion of the monolayer. On the other hand, for other systems, we observe the formation of 2D alloys between substrate and adsorbate atoms, and our predictions are compared with available experimental data. The adsorption isotherms determined for most of the systems studied exhibit sharp transitions in the coverage when the chemical potential is finely tuned. In particular, on the basis of the fact that the UPD of Ag atoms on the Au(100) surface exhibits a sharp first-order phase transition, at a well-defined value of the (coexistence) chemical potential, we also performed extensive simulations aimed at investigating the hysteretic dynamic behavior of the system close to coexistence upon the application of a periodic potential signal.

First-order transition to a novel metallic state in [Na y+(CH) − y] x: In situ electron spin resonance during chemical and electrochemical doping

We study a dimerized spin-1/2 chain, such as ${\mathrm{CuGeO}}_{3}$, hosting triplon excitations coupled to optical phonons under weak terahertz laser driving. Both phonons and triplons weakly lose energy into the surrounding baths, forming a nonequilibrium steady state. In the strong phonon-triplon coupling regime, phonons near the two-triplon continuum hybridize strongly with triplons. Using mean-field Lindblad dynamics, we show that strong hybridization induces sharp first-order phase transitions---either single or simultaneous double---in the emission spectrum, mainly due to nonlinearities in the chain and the instantaneous breaking of spatial inversion symmetry, as captured by the phonon displacement. Using mean-field Floquet analysis of harmonic modes in both sectors, we analytically confirm the existence of these phase transitions. Furthermore, we map the complete steady-state phase diagram by varying key control parameters and provide experimentally relevant parameters for observing these transitions in laser-driven ${\mathrm{CuGeO}}_{3}$.

Heat capacity and phase transition of tetrahydrofuran clathrate hydrate

Sharp quantum phase transitions typically require a large system with many particles. Here we show that, for a frustrated fully connected Ising spin network represented by trapped atomic ions, the competition between different spin orders leads to rich phase transitions whose sharpness scales exponentially with the number of spins. This unusual finite-size scaling behavior opens up the possibility of observing sharp quantum phase transitions in a system of just a few trapped ion spins.

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One of the most actively debated issues in the study of the glass transition is whether a mean-field description is a reasonable starting point for understanding experimental glass formers. Although the mean-field theory of the glass transition--like that of other statistical systems--is exact when the spatial dimension d → ∞, the evolution of systems properties with d may not be smooth. Finite-dimensional effects could dramatically change what happens in physical dimensions,d = 2, 3. For standard phase transitions finite-dimensional effects are typically captured by renormalization group methods, but for glasses the corrections are much more subtle and only partially understood. Here, we investigate hopping between localized cages formed by neighboring particles in a model that allows to cleanly isolate that effect. By bringing together results from replica theory, cavity reconstruction, void percolation, and molecular dynamics, we obtain insights into how hopping induces a breakdown of the Stokes-Einstein relation and modifies the mean-field scenario in experimental systems. Although hopping is found to supersede the dynamical glass transition, it nonetheless leaves a sizable part of the critical regime untouched. By providing a constructive framework for identifying and quantifying the role of hopping, we thus take an important step toward describing dynamic facilitation in the framework of the mean-field theory of glasses.

Using the parity doublet model (PDM) for hadronic matter and a modified Nambu-Jona-Lasinio (NJL) model for quark matter, we investigate the potential existence of two- and three-flavor quark matter in neutron star cores. Both models respect chiral symmetry, and a sharp first-order phase transition is implemented via Maxwell construction. We find stable neutron stars with quark cores within a specific parameter space that satisfies current astronomical observations. Typical neutron stars with masses around $1.4 \ M_\odot$ may possess deconfined quark matter in their centers. The hybrid star scenario with a two-flavor quark core offers enough parameter space to allow the neutron stars with large quark cores exceeding $\sim 1\ M_\odot$, and allow the early deconfinement position before $2\ ρ_0$, where $ρ_0$ is the nuclear saturation density. The observations of gravitational wave event GW170817 suggest a relatively large chiral invariant mass $m_0=600\ \rm MeV$ in the PDM for scenarios involving three-flavor quark matter cores. The maximum mass of the hybrid star with a quark core is found to be approximately $2.2\ M_\odot$ for both two- or three-flavor quark matter in their centers.

A microscopic Hamiltonian of the hydrogen-bond network in two-dimensional lattice water is proposed, which describes the formation and disruption of the H bonds, their bending, and which satisfies the Bernal-Fowler rules [J. D. Bernal and R. H. Fowler, J. Chem. Phys. 1, 515 (1933)]. The thermodynamic properties of the H-bond network are studied using the method of many-particle irreducible distribution functions, which is a generalization of the Kikuchi cluster approach [R. Kikuchi, Phys. Rev. 81, 988 (1951)] and the Bethe-Peierls quasiactivities method [H. A. Bethe, Prog. R. Soc. A 150, 552 (1935)]. The temperature dependencies of the average number of H bonds per molecules, the contribution of the H bonds into the heat capacity of the system, and the parameters describing the correlations between the states of molecules on the neighboring sites are investigated. It is shown that depending on the magnitude of the interaction between the H bonds in the H-bond subsystem either smooth or sharp first-order phase transition can occur. The role of different factors in the formation of the properties of the H-bond network is discussed.

By using Monte Carlo simulation we investigate the structure of monolayer filmsformed on the (100) plane of a face centered cubic crystal, being a latticeof square symmetry. It is demonstrated that besides the commensuratec(2 × 2) phase, different incommensurate structures develop. In particular, we concentrate on theformation of rather peculiar structure which exhibits Archimedean tiling (AT) of type(32.4.3.4). The mechanism leading to the formation of the AT phase is identified and it is shown that itdevelops via small displacements of adatoms from registry positions. It is shown that thestability of the AT phase depends on the misfit between the adsorbate and the surfacelattice as well as on the properties of the surface corrugation potential. The AT phase maybe stable in the ground state or develop at finite temperatures from the commensuratec(2 × 2) structure, via a sharp first-order phase transition. The AT phase remains stable over acertain temperature range, and then it undergoes a transition to either the partiallyordered phase of square symmetry or the incommensurate floating phase of triangularsymmetry. In both cases a further increase of temperature leads to the formation of aliquid-like phase.

An investigation of the thermodynamic and structural properties of a system of LJ particles around the solid-liquid transition has been performed using isobaric-isothermal (NPT) moleclar dynamics computer simulation techniques. A sharp first-order phase transition is observed. However, the solid is shown to superheat considerably giving an apparent melting temperature ∼ 14 per cent too high at zero pressure. Extensive tests of the simulation run, etc., were made to separate simulation artifacts from genuine physical phenomena. We conclude that the observed superheating is a real effect resulting in a metastable state. A comparison of these results with previous computer simulation studies were made. An investigation of apparent diffusion coefficient, the isothermal compressibility, and the constant pressure specific heat was also made.

Small (down to 1 mV/mm) electric fields will polarize neurons by only a small amount; for this reason small electric fields have previously been considered to have no physiologically relevant effects. However, here we propose a novel mechanism by which the non-linear properties of single neurons 'amplify' very small electric fields. Specifically, an amplified change in timing of action potential firing (DeltaT) is inversely proportional to the slope of depolarizing ramp stimulation and proportional to the amount of polarization (DeltaV) caused by the electric fields: DeltaT=DeltaV/(ramp slope). Thus, when responding to slow depolarizing synaptic input, small electric fields can have significant effects on spike timing. Hippocampal CA1 pyramidal neurons were depolarized with injections of depolarizing current ramps approximating synaptic input. Simultaneously, neurons were polarized by either DC holding currents or extracellular uniform DC electrical fields and the resulting changes in spike timing quantified. Consistent with our hypothesis, the polarization induced by each method was found to affect firing time linearly with the amount of polarization, scaled (amplified) with the inverse of the injected ramp slope consistent with our hypothesis.