Dynamic hysteresis loop in a ferroelectric heterostructure

A Landau–Devonshire theory in combination with Landau–Khalatnikov dynamic equation has been firstly used to study the dynamic hysteresis loop of a ferroelectric heterostructure consisting of two different films. The surface transition layer within each component film and an antiferroelectric coupling at the interface between two films are considered. A parameter β is introduced to describe the differences of physical properties between two constituent films. The influence of parameterβ, surface transition layer, antiferroelectric coupling and electric field frequency on the dynamic hysteresis loop of the ferroelectric heterostructure is discussed in detail. The results show that the system can exhibit antiferroelectriclike behavior (i.e., multi-loop hysteresis) through tuning some critical factors.     

Effect of dielectric layer on ferroelectric responses of P(VDF-TrFE) thin films

The dielectric layer in the sandwich structural device plays a very important role in determining the electrical properties of the ferroelectric film. In this paper, we investigate the effect of the dielectric layers with different thicknesses on switching performance of ferroelectric P(VDF-TrFE) thin films. The hysteresis loops become slanting with increasing thickness of the dielectric layer. A negative slope of the ‘real’ hysteresis loop is apparently observed which demonstrates negative capacitance effect caused by the dielectric layer. This behavior is simulated qualitatively by the Weiss mean field model considering an interfacial dielectric layer in series with a ferroelectric layer. The agreement between experiments and simulations supports that negative capacitance results from the positive feedback among electric dipoles. Furthermore, the switching time of the ferroelectric film increases with the increase of dielectric layer thickness. This study shows that the ferroelectric sandwich structure provides great potential towards low power negative capacitance devices.     

The polarization and dielectric susceptibility of a ferroelectric bilayer film

A ferroelectric bilayer film consisting of two different ferroelectric constituent films with a transition layer within each constituent film and interfacial coupling between two materials is investigated based on the Ginzburg-Landau-Devonshire phenomenological theory. A parameter α, which describing the differences between physical properties of two constituent films is first introduced in our paper, and reflects a more realistic situation compared to the previous treatments assuming the same two constituent films. We study the polarization and dielectric susceptibility properties of the ferroelectric bilayer film with two different constituent films. The results present some interesting phenomena due to the introduction of parameter α.     

Integrated electro-optic platform using ferroelectric crystals with the point symmetry of orthorhombic mm2

We theoretically and numerically investigate the modulation efficiencies of integrated electro-optic (EO) modulators based on ferroelectric crystals with the point symmetry of orthorhombic mm2. An analytical expression describing the index-ellipsoid under the EO effect is derived, where all 5 non-zero linear EO coefficients of mm2-crystals are considered. As practical candidates for integrated EO platform, three mm2-crystals (KNbO₃, KTiOPO₄, and Ba₂NaNb₅O₁₅) that are well-established and commercially available are chosen for numerical simulations. The refractive-index (RI) changes caused by an external electric field applied to each crystal with an arbitrary direction are analyzed in detail for the given optical polarization states of input light wave. For each crystal, the modulation efficiency is evaluated in terms of the half-wave voltage required for a π-phase shift of input wave, which is compared with the established counterpart based on LiNbO₃. The results indicate that higher modulation efficiencies are achievable in the two schemes using KNbO₃ and Ba₂NaNb₅O₁₅ than those for other cases, meaning that either lower power-consuming or larger beam-window integrated EO devices are feasible.     

Switching dynamics and modeling of multi-domain Zr-Doped HfO2 ferroelectric thin films

HfO₂-based ferroelectrics have attracted attention as promising materials for advanced memory applications owing to their negative capacitance effect, high scalability, and full-CMOS compatibility. Accordingly, the switching dynamics of HfO₂ thin films have been actively discussed and simulated using the Landau-Khalatnikov equation (LK model). Although the simulated results agree with experimental results in many studies, there is a slight dissimilarity near the coercive field in the polarization-electric field curve. For accurate and general modeling, a new model that combines the conventional LK model and Euler's equation was proposed in this work. The model was examined under single-domain and multi-domain conditions. The simulated curves using the Landau-Euler method better fit with measured curves than those using the LK model.     

铁电双层膜的反转动力学行为

基于Landau-Khalatnikov运动方程,本文研究了含有表面过渡层和铁电界面耦合的反转动力学行为(包括平均极化、反转时间、反转电流和矫顽场).研究结果表明:在铁电双层膜系统中存在一个竞争的机理,即表面过渡层与界面耦合的竞争作用.我们发现在双层膜反转过程中出现了反常行为,这些反常行为归因于表面过渡层与界面耦合之间的竞争.表面过渡层与界面耦合的共同行为对铁电双层膜的动力学特性起到了决定性的作用.     

Oxygen vacancy effect on dielectric and hysteretic properties of zigzag ferroelectric iron dioxide nanoribbon

Zigzag FeO₂ nanoribbon defected by the removal of oxygen atoms is simulated using Monte Carlo simulations. All possible arrangements of positions and number of oxygen vacancy are investigated. Temperature dependence of polarization, dielectric susceptibility, internal energy, specific heat and dielectric hysteresis loops are all studied. Results show the presence of second order phase transition and $Q-$ type behavior. Dielectric properties dependence on ribbon's edge, positions and number of oxygen vacancy are discussed in detail. Moreover, single and square hysteresis loops are observed whatever the number of oxygen vacancy in the system.     

First-principles study of two-dimensional van der Waals heterostructure based on ZnO and Mg(OH)2: A potential photocatalyst for water splitting

In this study, the structural, electronic and optical properties of the two-dimensional heterostructure based on ZnO and Mg(OH)₂ are investigated by first-principle calculations. The ZnO/Mg(OH)₂ heterostructure, formed by van der Waals (vdW) interaction, possesses a type-II band structure, which can separate the photogenerated electron–holes constantly. The heterostructure has decent band edge positions for the redox reaction to decompose the water at pH 0 and 7. As for the interfacial properties of the heterostructure, the trend of band bending of the ZnO and Mg(OH)₂ layers in the heterostructure is addressed, which will result a built-in electric field. Besides, the charge-density difference and potential drop across the interface of the ZnO/Mg(OH)₂ vdW heterostructure are also calculated. Finally, the heterostructure is demonstrated that it not only has excellent ability to capture the light near the visible spectrum region, but also can improve the optical performance for the monolayered ZnO and Mg(OH)₂.     

Effect of sintering temperature on the structural, dielectric and ferroelectric properties of tungsten substituted SBT ceramics

Structural, dielectric and ferroelectric properties of tungsten (W) substituted SrBi₂(Ta_1−xW_x)₂O₉ (SBTW) [x=0.0, 0.025, 0.05, 0.075, 0.1 and 0.2] have been studied as a function of sintering temperature (1100-1250 °C). X-ray diffraction patterns confirm the single-phase layered perovskite structure formation up to x=0.05 at all sintering temperatures. The present study reveals an optimum sintering temperature of 1200 °C for the best properties of SBTW samples. Maximum T_c of ∼390 °C is observed for x=0.20 sample sintered at 1200 °C. Peak-dielectric constant (ε_r) increases from ∼270 to ∼700 on increasing x from 0.0 to 0.20 at 1200 °C sintering temperature. DC conductivity of the SBTW samples is nearly two to three orders lower than that of the pristine sample. Remnant polarization (P_r) increases with the W content up to x≤0.075. A maximum 2P_r (∼25 μC/cm²) is obtained with x=0.075 sample sintered at 1200 °C. The observed behavior is explained in terms of improved microstructural features, contribution from the oxygen and cationic vacancies in SBTW. Such tungsten substituted samples sintered at 1200 °C exhibiting enhanced dielectric and ferroelectric properties should be useful for memory applications.     

Finite-size scaling analysis of the critical behavior of a general epidemic process in 2D

We investigate the critical behavior of a stochastic lattice model describing a General Epidemic Process. By means of a Monte Carlo procedure, we simulate the model on a regular square lattice and follow the spreading of an epidemic process with immunization. A finite size scaling analysis is employed to determine the critical point as well as some critical exponents. We show that the usual scaling analysis of the order parameter moment ratio does not provide an accurate estimate of the critical point. Precise estimates of the critical quantities are obtained from data of the order parameter variation rate and its fluctuations. Our numerical results corroborate that this model belongs to the dynamic isotropic percolation universality class. We also check the validity of the hyperscaling relation and present data collapse curves which reinforce the accuracy of the estimated critical parameters.     

Asymptotic behavior of two-electron expectation values in two-electron excited states

Singly-excited states of the two-electron atom cease being bound when $Z\to 1$ (from above), the outer orbital becoming infinitely diffuse. The asymptotic relations$\underset{Z\to 1}{\lim }?{(Z?1)}^k〈{(1sns)}^{1,3}S\left|r_{12}^k\right|{(1sns)}^{1,3}S〉=〈(n?1)s^{(0)}\left|r^k\right|(n?1)s^{(0)}〉,$ where $k=?1,1,2,3,?$, are demonstrated to hold. Here, $(n?1)s^{(0)}$ is a hydrogenic s orbital with principal quantum number $(n?1)$. New, more nuanced light is shed on the already challenged dogma that the Pauli principle keeps the electrons further apart in the triplet than in the corresponding singlet.     

System size effect on the critical behavior in nuclear multifragmentation

Attempt has been made to examine the role of system size on the traditional signatures of critical behavior from a comparative study of Mg-Em at 4.5 A GeV and Kr-Em at 0.95 A GeV interactions. A number of relevant observables such as fluctuation in the sizes of the largest cluster, reduced variance and the mean value of second moment of charge distribution were estimated with the experimental data. From a comparison of our results with that of EOS collaboration for Au, La and Kr on carbon at 1 A GeV, a definite systematic variation in the heights and positions of the peaks could be observed with the change of fragmenting nuclei thereby confirming the effect of system size on MF mechanism.     

Optical investigation of the metal-insulator transition in the manganite films with the thickness dependence

We investigated the optical response of La_0.67Sr_0.33MnO₃ (LSMO) films which show the metal-insulator transition (MIT) with the film thickness dependence. By using spectroscopic ellipsometric technique we found that the optical spectra below the charge transfer gap exhibited the coherent-incoherent crossover behavior through MIT. The formation of the incoherent mode near 1.5 eV was reminiscent of the polaron absorption which had been widely observed in various manganites. We suggest that the electron-phonon coupling could be enhanced due to low dimensionality in the ultrathin LSMO film in consideration of orbital polarization.     

Micromagnetic modeling studies on the effects of stress on magnetization reversal and dynamic hysteresis

The complex behaviors of magnetic materials subjected to magneto-electro-mechanical coupled fields call for a better understanding of the mechanism of multi-fields coupling. In this paper, micromagnetic modeling is carried out to study the effect of stress on hysteresis loops and dynamic magnetization reversal. The time-dependent Landau–Lifshitz–Gilbert equation which governs the evolution of magnetization is solved using the fast Fourier transform technique in reciprocal space. The simulation results show that the stress changes the distribution of easy directions and, therefore, leads to the change of magnetic properties. Moreover, the positive product of the stress and saturation magnetostriction coefficients increases the coercivity, hence, increases the area enclosed by the hysteresis loop. A stress-induced magnetization reversal is investigated.     

Effect of calcium deficiency on the critical behavior near the paramagnetic to ferromagnetic phase transition temperature in La0.8Ca0.2MnO3 oxides

The critical behavior associated with the magnetic phase transition has been investigated by magnetization isotherms in La_0.8Ca_0.2MnO₃ and La_0.8Ca_0.1□_0.1MnO₃ (□ is the calcium deficiency). The critical exponents are estimated by various techniques such as the Modified Arrott plot, Kouvel–Fisher plot and critical isotherm technique. The critical exponents values for La_0.8Ca_0.2MnO₃ are very close to those found out by the 3D-Ising model (β=0.328, γ=1.180, and δ=4.826 at an average T_C=181.676 K). Moreover, the estimated critical exponents of La_0.8Ca_0.1□_0.1MnO₃ are consistent with the prediction of the 3D-Heisenberg model (β=0.357, γ=1.167, and δ=4.802 at an average T_C=178.182 K). We noted that the critical exponents γ are almost similar to the value of the mean-field theory which can be explained by the existence of a long-range dipole–dipole interaction. Following the Harris criterion, we deduced that the disorder in our case is relevant, which can be the cause of the change in the universality class.     

Impact of vacancy and Na substitutions on the critical magnetic behavior in polycrystalline La0.8Pb0.2MnO3

We present the effect of vacancy and Na doping on the critical behavior in La_0.8Pb_0.2MnO₃ via dc magnetization measurements. Critical exponents values for La_0.8Pb_0.2MnO₃ match well with those predicted for the three-dimensional Heisenberg model. The vacancy in La_0.8Pb_0.1□_0.1MnO₃ does not alter the Heisenberg universality class. However, the critical isotherm exponent is slightly larger than predicted consistent with the presence of a Griffiths phase. For La_0.8Pb_0.1Na_0.1MnO₃, we found that the critical exponents values are in a close agreement with those predicted for the mean-field theory. The reason for this behavior may be long-range interactions between spins in this system.     

Controlled trapping of single particle states on a periodic substrate by deterministic stubbing

A periodic array of atomic sites, described within a tight binding formalism is shown to be capable of trapping electronic states as it grows in size and gets stubbed by an ‘atom’ or an ‘atomic’ clusters from a side in a deterministic way. We prescribe a method based on a real space renormalization group method, that unravels a subtle correlation between the positions of the side coupled atoms and the energy eigenvalues for which the incoming particle finally gets trapped. We discuss how, in such conditions, the periodic backbone gets transformed into an array of infinite quantum wells in the thermodynamic limit. We present a case here, where the wells have a hierarchically distribution of widths, hosting standing wave solutions in the thermodynamic limit.     

低气压条件下的铁电体离子发射特性研究

以低气压条件下的铁电体离子发射特性为研究对象,通过结合高速相机、光谱的光学诊断和二维网格质点法耦合蒙特卡洛碰撞模型(PIC-MCC)的仿真模拟获得了锆钛酸铅(PZT)铁电体离子发射的主要成分、表面放电的发光及等离子体演化过程、离子的产生及离子电流形成机制,为进一步研究铁电体离子源奠定了基础。     

Note on the quantum displacement of the critical point

The quantum corrections to the law of corresponding states are studied by calculating the critical pressure, temperature, and density to first order in Planck's constanth on an exactly soluble model. The ratio of the critical parameters to the corresponding classical values are found to be (p _c/p _c ⁰)^1/2=_c/_c ⁰ = T_c/T_c ⁰ = 1–0.67, with=h _c ^1/3(mkT _c)^–1/2. The critical ratio is independent ofh to first order. The results are compared with critical data for noble gases and hydrogen isotopes.     

基于声发射传感器的高温超导线圈临界电流的研究

用经验失超判据Ec=1μV/cm可以对高温超导线圈在直流情况下的临界电流进行检测,但对于高温超导线圈交流时的临界电流还没有一个标准.利用声发射传感器对高温超导线圈的交流临界电流进行了初步研究,并把结果与直流临界电流进行了比较,结果表明在低于直流临界电流时,声发射传感器获得了明显改变的声信号,即高温超导线圈的交流临界电流...