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Charge and dielectric response to terahertz pulses in the charge-ordered phase of
摘要： We investigated the dielectric properties of the charge-ordered phase of α-(BEDT-TTF)2I3 using exact numerical calculations of an extended Hubbard model. The electronic contribution to the electric polarization (electronic polarization) ˉP of the charge-ordered ground state is obtained by directly calculating the current when transfer integrals were changed adiabatically from symmetric integrals to integrals for the charge-ordered phase without inversion symmetry. The angle of ˉP from the positive b axis is 36?, which is consistent with experimental results and previous theoretical results based on density-functional theory. Furthermore, we numerically calculated the dynamics induced by terahertz- (THz-) pulse excitation. Both the THz-pulse-induced variation of the electronic polarization magnitude and that of the charge disproportionation that shows the charge-order amplitude are largest when the electric field of the THz pulse and ˉP have almost the same direction. This originates from the charge transfer through bond b2(cid:3) being dominant in both the adiabatic flow of the current and the THz-pulse excitation. These results reproduce important features of the experimental results of THz-pulse-induced dynamics.
关键词： charge-ordered phase,ferroelectricity,extended Hubbard model,THz-pulse excitation,electronic polarization
更新于2025-09-23 15:21:01
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Ferroelectricity of Hf <sub/>x</sub> Zr <sub/>1?x</sub> O <sub/>2</sub> Thin Films Fabricated Using TiN Stressor and ZrO <sub/>2</sub> Nucleation Techniques
摘要： We studied the ferroelectricity and the leakage current (J) properties of the TiN/ZrO2/HfxZr1?xO2/ZrO2/TiN capacitor (D-ZrO2) with the ZrO2 nucleation layers, comparing to the TiN/HfxZr1?xO2/TiN capacitor (D-TiN) with the TiN stressor layers. The larger ferroelectric phase ratio was achieved by the epitaxial like grain growth of the HfxZr1?xO2 film on the surface of the top- and bottom-ZrO2 nucleation layers. Thus, the larger remanent polarization (2Pr = 29 μC/cm2) of D-ZrO2 was obtained compared to that (24 μC/cm2) of D-TiN. Moreover, the J value of D-ZrO2 was approximately one order of magnitude lower than that of D-TiN. Therefore, these results suggest that the superior ferroelectricity of HZO film can be obtained using the ZrO2 nucleation technique instead of the TiN stressor technique.
关键词： HfxZr1?xO2,Thin Films,Ferroelectricity,ZrO2 Nucleation,TiN Stressor
更新于2025-09-23 15:21:01
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Opto-mechanics driven fast martensitic transition in two-dimensional materials
摘要： Diffusional phase-change materials, such as Ge-Sb-Te alloys, are used in rewritable non-volatile memory devices. But the continuous pursuit of readout/write speed and reduced energy consumption in miniaturized devices calls for optically driven, diffusionless phase change scheme in ultra-thin materials. Inspired by optical tweezers, in this work, we illustrate theoretically and computationally that a linearly polarized laser pulse with selected frequency can drive an ultrafast diffusionless martensitic phase transition of two-dimensional ferroelastic materials such as SnO and SnSe monolayers, where the unit-cell strain is tweezed as a generalized coordinate that affects the anisotropic dielectric function and electromagnetic energy density. At laser power of 2.0×1010 and 7.7×109 W/cm2, the transition potential energy barrier vanishes between two 90°-orientation variants of ferroelastic SnO and SnSe monolayer, respectively, so displacive domain switching can occur within picoseconds. The estimated adiabatic thermal limit of energy input in such optomechanical martensitic transition (OMT) is at least two orders of magnitude lower than that in Ge-Sb-Te alloy.
关键词： martensitic phase transition,ferroelasticity/ferroelectricity,two-dimensional materials,dielectric function,density functional theory,opto-mechanics
更新于2025-09-23 15:21:01
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First-principles theory of giant Rashba-like spin splitting in bulk GeTe
摘要： Large Rashba-like spin splitting has been recently found in the band structure of certain bulk ferroelectrics. In contrast to the relativistic Rashba effect, the chiral spin texture and large spin splitting of the electronic bands depend strongly on the character of the band and atomic spin-orbit coupling. We establish that this can be traced back to an interplay between the orbital angular momentum and ferroelectricity. The additional dependence on the orbital composition of the bands is crucial to the complete picture of the effect. Results from ?rst-principles calculations on ferroelectic GeTe verify the key predictions of the model.
关键词： Rashba-like spin splitting,orbital angular momentum,ferroelectricity,spin-orbit coupling,GeTe
更新于2025-09-23 15:21:01
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Stability, spontaneous and induced polarization in monolayer MoC, WC, WS, and WSe
摘要： Using density functional theory based calculations, we design buckled honeycomb binary monolayer MoC, WC, WS, and WSe. The feasibility of their experimental synthesis is evident from their low formation energies, stable lattice vibrations, and high elastic stiffnesses. The difference in electronegativity of the building atoms and lack of inversion symmetry results in high spontaneous out-of-plane polarization whereas all the in-plane components cancel out. The existence of the mirror plane along the armchair direction vanishes the in-plane piezoelectric stress effects for a strain applied along the zigzag direction. However, a strain along the armchair direction on MoC and WC results in a high piezoelectric coefficient up to 10×10?10 C/m, about three times larger than that reported for monolayer MoS2. The polarization switching barriers for the out-of-plane components range from 0.55 to 2.70 eV per formula unit.
关键词： ferroelectricity,monolayer,density functional theory,polarization,piezoelectricity
更新于2025-09-23 15:21:01
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In-situ observation of evolving microstructural damage and associated effective electro-mechanical properties of PZT during bipolar electrical fatigue
摘要： We investigate the fatigue behavior of bulk polycrystalline lead zirconate titanate (PZT) during bipolar electric field cycling. We characterize the frequency- and cycle-dependent degradation in both the effective electro-mechanical properties (specifically, the electrical hysteresis and the macroscopic viscoelastic stiffness and damping measured by Broadband Electromechanical Spectroscopy, BES) and the microstructural damage evolution (quantified via scanning electron microscopy). The BES setup enables the mechanical characterization while performing electrical cycling so as to measure the evolving viscoelasticity without remounting the sample; particularly measuring the viscoelastic damping allows us to gain insight into the ferroelectric domain wall activity across the full electric hysteresis and over the full range of cycles. A clear dependence on the electric cycling frequency is observed in the rates of degradation of all measured properties including an up to 10% increase in dynamic compliance and a 70% decrease in electric displacement magnitude. We quantify the evolving micro-crack density across wide ranges of numbers of cycles and compare with changes in the effective compliance. Interestingly, the observed strong degradation in the ferroelectric hysteresis is contrasted by relatively mild changes in the effective viscoelastic moduli, while samples clearly indicate increasing levels of micro-damage.
关键词： fatigue,polycrystal,electro-mechanical coupling,ferroelectricity
更新于2025-09-23 15:21:01
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Relaxor Behavior in Ordered Lead Magnesium Niobate (PbMg <sub/>1/3</sub> Nb <sub/>2/3</sub> O <sub/>3</sub> ) Thin Films
摘要： The local compositional heterogeneity associated with the short-range ordering of Mg and Nb in PbMg1/3Nb2/3O3 (PMN) is correlated with its characteristic relaxor ferroelectric behavior. Fully ordered PMN is not prepared as a bulk material. This work examines the relaxor behavior in PMN thin films grown at temperatures below 1073 K by artificially reducing the degree of disorder via synthesis of heterostructures with alternate layers of Pb(Mg2/3Nb1/3)O3 and PbNbO3, as suggested by the random-site model. 100 nm thick, phase-pure films are grown epitaxially on (111) SrTiO3 substrates using alternate target timed pulsed-laser deposition of Pb(Mg2/3Nb1/3)O3 and PbNbO3 targets with 20% excess Pb. Selected area electron diffraction confirms the emergence of (1/2, 1/2, 1/2) superlattice spots with randomly distributed ordered domains as large as ≈150 nm. These heterostructures exhibit a dielectric constant of 800, loss tangents of ≈0.03 and 2× remanent polarization of ≈11 μC cm?2 at room temperature. Polarization–electric field hysteresis loops, Rayleigh data, and optical second-harmonic generation measurements are consistent with the development of ferroelectric domains below 140 K. Temperature-dependent permittivity measurements demonstrate reduced frequency dispersion compared to short range ordered PMN films. This work suggests a continuum between normal and relaxor ferroelectric behavior in the engineered PMN thin films.
关键词： random site model,relaxor,ferroelectricity,short- and long-range order,temperature-dependent Rayleigh analysis
更新于2025-09-23 15:21:01
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Spectral dynamics of shift current in ferroelectric semiconductor SbSI
摘要： Photoexcitation in solids brings about transitions of electrons/holes between different electronic bands. If the solid lacks an inversion symmetry, these electronic transitions support spontaneous photocurrent due to the geometric phase of the constituting electronic bands: the Berry connection. This photocurrent, termed shift current, is expected to emerge on the timescale of primary photoexcitation process. We observe ultrafast evolution of the shift current in a prototypical ferroelectric semiconductor antimony sulfur iodide (SbSI) by detecting emitted terahertz electromagnetic waves. By sweeping the excitation photon energy across the bandgap, ultrafast electron dynamics as a source of terahertz emission abruptly changes its nature, reflecting a contribution of Berry connection on interband optical transition. The shift excitation carries a net charge flow and is followed by a swing over of the electron cloud on a subpicosecond timescale. Understanding these substantive characters of the shift current with the help of first-principles calculation will pave the way for its application to ultrafast sensors and solar cells.
关键词： photovoltaic effect,ferroelectricity,picosecond techniques,solar cells,bulk matter
更新于2025-09-19 17:15:36
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Electrical response of mixed phase (1-x)BiFeO3-xPbTiO3 solid solution: Role of tetragonal phase and tetragonality
摘要： We present the study of structural, morphological, dielectric, transport and ferroelectric properties of (1-x)BFO–xPTO solid solutions, with 0.3 ≤ x ≤ 0.6, prepared via non-conventional synthesis methods. These methods include Sol-gel and Single-step solid state method. Structural analysis revealed presence of mixed phases i.e. monoclinic (CC) and tetragonal (P4mm) phases, for all compositions showing a Morphotropic Phase Boundary. For the compositions with higher concentration of PTO, an increase in tetragonal phase fraction has been observed. Quantitative analysis showed, in general, higher value of c/a (i.e. tetragonality) for all samples as compared to the bulk PTO. The morphological analysis shows small grain size irrespective of synthesis method and composition. The low temperature frequency dependent tangent loss shows dielectric relaxation with small magnitude of dielectric constant indicating absence of extrinsic contributions. High temperature dielectric anomaly is observed around 400-500 K corresponding to magnetic phase transformation of BFO at Neel temperature which suggest the presence of magneto-electric coupling in specific compositions. Sol-gel prepared composite appeared to be more resistive than the Single-step synthesized composite and shows Arrhenius type dependence of high temperature ac conductivity. Ferroelectricity was observed in all ceramic samples which sustained high applied electric field up to 190 kV/cm. Finally, a correlation between polarization, tetragonal phase fraction and c/a ratio, has been drawn and discussed. It is concluded that c/a ratio (i.e. tetragonality) is more important parameter which can be tuned to achieve enhanced ferroelectric response as compared to the tetragonal phase fraction in (1-x)BFO–xPTO solid solutions.
关键词： Electric polarization,Non-conventional synthesis methods,Tetragonality,Morphotropic phase boundary,Ferroelectricity,Phase evolution,Dielectric anomalies
更新于2025-09-19 17:15:36
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Emerging Conductive Atomic Force Microscopy for Metal Halide Perovskite Materials and Solar Cells
摘要： Metal halide perovskite materials, benefiting from a combination of outstanding optoelectronic properties and low-cost solution-preparation processes, show tremendous potential for optoelectronics and photovoltaics. However, the nanoscale inhomogeneities of the electronic properties of perovskite materials cause a number of difficulties, such as recombination, stability, and hysteresis, all of which seriously restrict device performance. Scanning probe microscopy, as a high-resolution imaging technique, has been widely used to connect local properties and micro-area morphologies to overall device performance. Conductive atomic force microscopy (C-AFM) can realize a real-space visualization of topography coupled with optoelectronic properties on a microscopic scale and thereby is uniquely suited to probe the local effects of perovskite materials and devices. The fundamental principles, alternative operation modes, and development of C-AFM are comprehensively reviewed, and applications in perovskite solar cells (PSCs) for electronic transport behavior, ion migration and hysteresis, ferroelectric polarization, and facet orientation investigation are discussed. A comprehensive understanding and summary of up-to-date applications in PSCs is beneficial to further fully exploit the potential of such an emerging technique, so as to provide a novel and effective approach for perovskite materials analysis.
关键词： ferroelectricity,perovskite solar cells,conductive atomic force microscopy,ion migration,perovskite materials,electronic transport behavior
更新于2025-09-19 17:13:59