Ferroelectric LiNbO3 Research Articles

Nonlocal nonlinear phononics

Nonlinear phononics relies on the resonant optical excitation of infrared-active lattice vibrations to induce targeted structural deformations in solids. This form of dynamical crystal structure design has been applied to control the functional properties of many complex solids, including magnetic materials, superconductors and ferroelectrics. However, phononics has so far been restricted to protocols in which structural deformations occur within the optically excited volume, sometimes resulting in unwanted heating. Here, we extend nonlinear phononics to propagating polaritons, spatially separating the functional response from the optical drive. We use mid-infrared optical pulses to resonantly drive a phonon at the surface of ferroelectric LiNbO3. Time-resolved stimulated Raman scattering reveals that the ferroelectric polarization is reduced over the entire 50 µm depth of the sample, far beyond the micrometre depth of the evanescent phonon field. We attribute this effect to the anharmonic coupling between the driven mode and a polariton that propagates into the material. For high excitation amplitudes, we reach a regime in which the ferroelectric polarization is reversed, as revealed by a sign change in the Raman tensor coefficients of all the polar modes.

Dramatic n-Type Doping of Monolayer Graphene with Ferroelectric LiNbO3 Crystals and Bilayer Two-Dimensional Electron Gases

Dramatic n-Type Doping of Monolayer Graphene with Ferroelectric LiNbO₃ Crystals and Bilayer Two-Dimensional Electron Gases

Validation of hybrid WC1LYP functional for ferroelectric LiNbO3 and LiTaO3 using Compton spectroscopy and first-principles computations

Electronic structure and theoretical momentum densities (EMDs) of LiNbO3 and LiTaO3 using linear combination of atomic orbitals (LCAO) scheme are deduced. Present energy bands and density of states using local density approximation, generalised gradient approximation and hybrid (HF + DFT) PBE0 and WC1LYP schemes show a direct band gap (varying between 3.13- 5.13 eV and 3.11–5.18 eV for LiNbO3 and LiTaO3, respectively) at Γ point of Brillouin zones. The WC1LYP hybrid functional based EMDs depict reasonable agreement with present experimental Compton lines (using 100 mCi Am-241 γ-ray Compton spectrometer) than other investigated exchange-correlation potentials, thereby suggesting the applicability of hybrid model like WC1LYP in such ferroelectrics. Accuracy of energy bands is also ensured via electronic and optical properties using modified Becke–Johnson potential as facilitated in the full-potential linearized augmented plane wave method. Presently reported UV–vis measurements for optical band gaps are in consonance with the hybrid potential (WC1LYP) for the both ferroelectrics, which further confirms usefulness of hybrid scheme. Moreover, equal–valence–electron–density scaled experimental data reveal more covalent character of LiNbO3 than that in LiTaO3, which is in agreement with the present LCAO based Mulliken's population analysis.

Light and Thermally Induced Charge Transfer and Ejection of Micro‐/Nanoparticles from Ferroelectric Crystal Surfaces

AbstractDuring real‐time operation of photovoltaic optoelectronic tweezers, a novel intriguing phenomenon has been recently observed, namely, silver nanoparticles previously deposited on ferroelectric LiNbO3:Fe surfaces are ejected from them by illumination. Here, it is shown that this phenomenon results from the electrical charging of the micro‐/nanoparticles previously trapped on the LiNbO3:Fe surfaces and the subsequent Coulomb repulsion. Specific experiments are performed to determine the sign of the transferred charges, which is negative/positive for the +c/−c sample face, i.e., it coincides with that of the stored charges on each surface. The charging/ejection process is proved to occur regardless of the illuminated crystal surface, the dielectric or metallic nature of the particles, and in different surrounding media. Besides, the role of the excitation light intensity is also explored. Next, to assess the generality of the ejection phenomenon, similar experiments based on the pyroelectric effect are performed, i.e., generating the electric fields by changing the crystal temperature instead of illuminating it. Thereby, thermally driven particle ejection is demonstrated using different ferroelectric crystals. The similarities found for the two approaches throw light on the charge transfer and ejection mechanism and remark the universality of the phenomenon for ferroelectrics, which can find many technological applications.

Electrostatic doping determined by band alignment in graphene on ferroelectric LiNbO3(0001) polar surfaces

Herein, first-principles density functional theory calculations are performed to investigate the interface coupling and electrostatic doping effect in the graphene/LiNbO3(0001) [denoted as Gr/LNO(0001)] heterostructure. The results show that, in addition to the coupling mechanism of the large ionic–van der Waals interaction, an unusual electrostatic doping, which is in contrast with the polarization compensation mechanism, appears in the Gr/LNO(0001) system: for graphene adsorbed on the thermodynamically preferred ferroelectric LiNbO3(0001) [denoted as LNO(0001)] positive (Z+) and negative (Z−) surfaces, carriers are of p- and n-type, respectively. Further analysis indicates that the electrostatically doped effect of the graphene layer adsorbed on the LNO(0001) polar surfaces should be determined by the particular band alignment between the LNO(0001) polar surfaces and graphene. Our studies provide theoretical evidence for designing p–n homojunctions in a graphene sheet by engineering the domain structure of the LNO(0001) ferroelectric substrates. Furthermore, the significant change in carrier density caused by the reversal of the ferroelectric polarization direction suggests that the Gr/LNO(0001) system could be a promising platform to explore ferroelectric field-effect transistors.

Specific features of SH-waves propagation in structures with prestressed inhomogeneous coating made of piezoceramics based on LiNbO3

Within the framework of the linearized theory of electroelastic wave propagation, a model of a piezoelectric structure with a prestressed functionally graded coating made of piezoceramics of a trigonal system with a symmetry class of 3m is considered. The ferroelectric LiNbO3 is used as the main material of the structure. The initial deformed state of the coating material is homogeneous, induced by the action of initial mechanical stresses and an external electrostatic field, the properties of the coating continuously change in thickness. By the example of the problem of the propagation of SH-waves from a remote source for structures with an inhomogeneous prestressed coating in the case of an electrically free and short-circuited surface, the influence of the nature and localization of the inhomogeneity of the coating on the features of SAW propagation is studied. The separate and combined effects of initial actions on changes in the physical properties of the structure, the transformation of the surface wave field, and the change in the SAW velocities in a wide frequency range is studied. The results obtained in this work are useful for understanding the dynamic processes in prestressed piezoelectric structures, in the optimization and design of new structures and devices on SAW with high performance characteristics.

Size-Controlled Polarization Retention and Wall Current in Lithium Niobate Single-Crystal Memories.

Highly conductive domain walls in insulating ferroelectric LiNbO3 (LNO) single-crystal thin films with atomic smoothness are attractive for use in high-density integration of the ferroelectric domain wall random access memory (DWRAM) because of their excellent reliability and high read currents. However, downscaling of the memory size to the nanoscale could cause poor polarization retention. Understanding the size-dependent electrical performance of a memory cell is therefore crucial. In this work, highly insulating X-cut LNO thin films were bonded to SiO2/Si wafers and lateral mesa-like cells were fabricated on the film surfaces, where contact occurred with two-sided electrodes along the polar z-axis. Under application of an in-plane electric field above a coercive field (Ec), the domain within each memory cell was switched to be antiparallel to the unswitched referencing domain at the bottom; this resulted in the formation of a conducting domain wall, which enables the nondestructive readout strategy of the DWRAM. The cell, which has a lateral length (l) above a critical size (l0) of 105 nm, is found to be a mixture of two phases across the cell area. The inner area of the cell suffers from poor polarization retention because Ec = 150 kV/cm, as demonstrated by in-plane piezoresponse force microscopy imaging. In comparison, the outer periphery domains, which have lengths of 70 nm (∼l0/2), show good retention but require a much higher Ec of 785 kV/cm. The relevant physics is discussed as phase reconstruction occurs after release of the in-plane compressive strain near the outer regions; the results show good agreement with those of one-dimensional thermodynamic calculations and phase-field simulations. The measured current-voltage curves demonstrated a sudden enhancement of the wall current across the cell when l < l0, thus implying higher readout wall currents and better retention for the DWRAM at higher storage densities.

Strong modulation of electronic properties of monolayer MoTe2 using a ferroelectric LiNbO3(0001) substrate

Projected band (red dots) of the 2H-MoTe2 monolayer adsorbed on the LiNbO3(0001) substrates. The ferroelectric LiNbO3 substrates is able to modulate the electronic properties of MoTe2 very well.

Nonsymmetric spin pumping in a multiferroic heterostructure

We present spin pumping experiments in Co/Pt bilayers grown on Al2O3(0001) and on ferroelectric Y-cut LiNbO3 substrates. Spin pumping is triggered by resonant ferromagnetic resonance induced by surface acoustic waves. We observe that spin pumping efficiency varies when both the magnetization vector and the acoustic wave vector are reversed in Pt/Co/LiNbO3. This phenomenon is not observed in Pt/Co/Al2O3. We propose that the in-plane electric polarization of LiNbO3 is at the origin of the observed phenomenon. These observations open up the perspective of an electric field control of spin pumping efficiency.

Record-Low Subthreshold-Swing Negative-Capacitance 2D Field-Effect Transistors.

Power consumption is one of the most challenging bottlenecks for complementary metal-oxide-semiconductor integration. Negative-capacitance field-effect transistors (NC-FETs) offer a promising platform to break the thermionic limit defined by the Boltzmann tyranny and architect energy-efficient devices. However, it is a great challenge to achieving ultralow-subthreshold-swing (SS) (10mV dec-1 ) and small-hysteresis NC-FETs simultaneously at room temperature, which has only been reported using the hafnium zirconium oxide system. Here, based on a ferroelectric LiNbO3 thin film with great spontaneous polarization, an ultralow-SS NC-FET with small hysteresis is designed. The LiNbO3 NC-FET platform exhibits a record-low SS of 4.97mV dec-1 with great repeatability due to the superior capacitance matching characteristic as evidenced by the negative differential resistance phenomenon. By modulating the structure and operating parameters (such as channel length (Lch ), drain-sourse bias (Vds ), and gate bias (Vg )) of devices, an optimized SS from ≈40 to ≈10mV dec-1 and hysteresis from ≈900 to ≈60mV are achieved simultaneously. The results provide a new potential method for future highly integrated electronic and optical integrated energy-efficient devices.

Dielectric, electrooptic and viscoelastic properties in cybotactic nematic phase doped with ferroelectric nanoparticles

Hydrogen bonded liquid crystal, possessing a wide temperature range of cybotactic phase was used as a matrix to disperse ferroelectric LiNbO3 nanoparticles at the concentrations from 0.5 wt.% to 3.0 wt.%. The influence of nanoparticles on the physical properties has been studied with the help of polarizing optical microscopy (POM), impedance spectroscopy and electro-optic measurements. POM investigation revealed a nematic droplet dispersed in the isotropic phase due to the addition of ferroelectric nanoparticles. The suppression of the polarization has been observed and explained by the occurrence of antiparallel dipole configuration. The isotropic-nematic phase transition temperature was decreased due to the reduction in the parameter order. The decreasing in dielectric anisotropy (∆ε), response time (τ) and in threshold Freedericksz voltage (Vth) was associated to the decrease in splay constant elastic (K11) and in the rotational viscosity (γ). The curves of ∆ε, Vth, τ, K11 and γ when NPs concentration increases, showed a minimum at the same concentration (c = 1 wt.%) allowing the determination of the optimal NPs concentration.

Raman opalescence and central peak scattering near phase transition point in crystals

Comparison of isotemperature and isofrequency temperature dependences was used for revealing the soft mode behavior near phase transition points in ferroelectric and ferro-structural phase transitions in crystals. Sharp increase of the Raman scattering spectral intensity maximum near phase transition points was found and classified as soft mode Raman opalescence effect. Such phenomenon was observed in ferroelectric crystals LiNbO3, LiTaO3, and Pb5Ge3O11, and crystalline quartz. Presence of the high intensive and narrow central peak in low frequency area of light scattering spectra in vicinity of phase transition point was explained on the base of two coupled oscillator model.

Polarity Control of ZnO Films Grown on Ferroelectric (0001) LiNbO3 Substrates without Buffer Layers by Pulsed-Laser Deposition.

For this study, polarity-controlled ZnO films were grown on lithium niobate (LiNbO3) substrates without buffer layers using the pulsed-laser deposition technique. The interfacial structure between the ZnO films and the LiNbO3 was inspected using high-resolution transmission electron microscopy (HR-TEM) measurements, and X-ray diffraction (XRD) measurements were performed to support these HR-TEM results. The polarity determination of the ZnO films was investigated using piezoresponse force microscopy (PFM) and a chemical-etching analysis. It was verified from the PFM and chemical-etching analyses that the ZnO film grown on the (+z) LiNbO3 was Zn-polar ZnO, while the O-polar ZnO occurred on the (-z) LiNbO3. Further, a possible mechanism of the interfacial atomic configuration between the ZnO on the (+z) LiNbO3 and that on the (-z) LiNbO3 was suggested. It appears that the electrostatic stability at the substrate surface determines the initial nucleation of the ZnO films, leading to the different polarities in the ZnO systems.

Transparent ferroelectric crystals with ultrahigh piezoelectricity.

Transparent piezoelectrics are highly desirable for numerous hybrid ultrasound-optical devices ranging from photoacoustic imaging transducers to transparent actuators for haptic applications1-7. However, it is challenging to achieve high piezoelectricity and perfect transparency simultaneously because most high-performance piezoelectrics are ferroelectrics that contain high-density light-scattering domain walls. Here, through a combination of phase-field simulations and experiments, we demonstrate a relatively simple method of using an alternating-current electric field to engineer the domain structures of originally opaque rhombohedral Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) crystals to simultaneously generate near-perfect transparency, an ultrahigh piezoelectric coefficient d33 (greater than 2,100picocoulombs per newton), an excellent electromechanical coupling factor k33 (about 94 per cent) and a large electro-optical coefficient γ33 (approximately 220picometres per volt), which is far beyond the performance of the commonly used transparent ferroelectric crystal LiNbO3. We find that increasing the domain size leads to a higher d33 value for the [001]-oriented rhombohedral PMN-PT crystals, challenging the conventional wisdom that decreasing the domain size always results in higher piezoelectricity8-10. This work presents a paradigm for achieving high transparency and piezoelectricity by ferroelectricdomain engineering, and we expect the transparent ferroelectric crystals reported here to provide a route to a wide range of hybrid device applications, such as medical imaging, self-energy-harvesting touch screens and invisible robotic devices.

Magnetic Scattering in Ni Wires Fabricated on Ferroelectric LiNbO3 Substrate for Magnetic Sensor Application

Magnetic Scattering in Ni Wires Fabricated on Ferroelectric LiNbO3 Substrate for Magnetic Sensor Application

Electrical Tunability of Domain Wall Conductivity in LiNbO3 Thin Films.

Domain wall nanoelectronics is a rapidly evolving field, which explores the diverse electronic properties of the ferroelectric domain walls for application in low-dimensional electronic systems. One of the most prominent features of the ferroelectric domain walls is their electrical conductivity. Here, using a combination of scanning probe and scanning transmission electron microscopy, the mechanism of the tunable conducting behavior of the domain walls in the sub-micrometer thick films of the technologically important ferroelectric LiNbO3 is explored. It is found that the electric bias generates stable domains with strongly inclined domain boundaries with the inclination angle reaching 20° with respect to the polar axis. The head-to-head domain boundaries exhibit high conductance, which can be modulated by application of the sub-coercive voltage. Electron microscopy visualization of the electrically written domains and piezoresponse force microscopy imaging of the very same domains reveals that the gradual and reversible transition between the conducting and insulating states of the domain walls results from the electrically induced wall bending near the sample surface. The observed modulation of the wall conductance is corroborated by the phase-field modeling. The results open a possibility for exploiting the conducting domain walls as the electrically controllable functional elements in the multilevel logic nanoelectronics devices.

Hyperbolic plasmon\u2013phonon dispersion on group velocity reversal and tunable spontaneous emission in graphene\u2013ferroelectric substrate

This work presents a new substrate platform, which provides tunability of the group velocity and spontaneous emission of a dipolar scatterer graphene–ferroelectric slab hybrid system in the terahertz ranges. We use analytical models to determine the hybridization of graphene surface plasmon and ferroelectric LiNbO3 type I and type II reststrahlen hyperbolic phonon–polariton. The variation of the chemical potential of graphene and the thickness of the ferroelectric layer results in several distinct features. Flipping the group velocity, strongly coupled hybrid hyperbolic surface plasmon–polaritons, and surface plasmon–polariton mode exists for the same momentum at different frequencies. The group velocity sign reversal for both a single-graphene- and double-graphene-integrated system depends on the thickness of the hyperbolic layer and the chemical potential of graphene. Comparative analysis of Purcell radiation is presented for a quantum emitter positioned at different locations between ferroelectric and graphene-integrated ferroelectric layers, revealing that this system can support strong spontaneous emission that can be modulated with the graphene chemical potential. Changing the chemical potential through selective voltage biasing demonstrates a substantial increase or decrease in the decay rate for spontaneous emission. Further analysis of the emission phenomenon shows a dependence on factors, such as the relative radiating source position and the thickness of the ferroelectric film. These characteristics make graphene–ferroelectric materials promising candidates to modify the light–matter interaction at the low terahertz ranges.

Charging and domain switching in ferroelectrics LiNbO3 by electron beam

Comprehensive experimental results on the kinetic characteristics of LiNbO3 charging process are reported. Secondary electron emission (SEE) yield, surface potential, and the sum of leakage and displacement currents were experimentally obtained during electron beam irradiation in the energy range from 200 eV to 10 keV for both sides of the Z-cut. We found that at the initial stage of charging, the SEE current that were collected by a hemispherical electrode changes its sign. It was explained by the emergence of a large displacement current on the collector. The direction of this current is opposite to the directions of SEE and backscattered electron currents. If a large positive charge arises on the surface of ferroelectrics, the positive displacement current dominates the secondary emission current; this fact was confirmed experimentally. Also, a high value of the negative surface potential in the electron energy range 200–700 eV for −Z cut of lithium niobate was found, though in this energy region, positive charging is observed for nonpolar dielectrics. The influence of domain repolarization currents on the measured emitted electrons and displacement current signals is shown.

Rare earth ions feel the electric: A novel strategy to obtain efficient near-infrared photoluminescence

Ferroelectrics doped with active luminescent ions have gradually paved the path of optoelectronic industry due to their multifunctionality and the dynamic modulation of luminescent properties through electric method widely broadens their application prospect in future devices. In this work, a novel strategy without compositional change is employed on the Er3+ doped transparent hybrid enriched with nano-sized ferroelectric LiNbO3 crystals to investigate the tunable photoluminescence performance. The results demonstrate that the photoluminescence acquires a tremendous boost after polarization engineering, that is, a six times enlargement. This notably light amplification effect is propelled by the reduction of environmental symmetry around luminescent Er3+ ions that is originated from electric polarization. Additionally, the prepared samples are proved to possess optical and electric properties simultaneously. Therefore, this multifunctional material with tunable luminescent properties is beneficial for regulating the physical couplings of electric and optics, which highlights the prospect in future optoelectronic industry.

Anharmonic phonon-polariton dynamics in ferroelectric LiNbO3 studied with single-shot pump-probe imaging spectroscopy

We demonstrate that single-shot pump-probe imaging spectroscopy with an echelon mirror enables us to disclose the ferroelectric phonon-polariton dynamics across a wide temperature range from 10 K to 375 K while avoiding the photorefractive effects that appear prominently at low temperatures. The E-mode phonon-polaritons corresponding to the two transverse optical modes, TO1 and TO3, up to ∼7 THz were induced in LiNbO3 through an impulsive stimulated Raman scattering process. Subsequently, using single-shot pump-probe imaging spectroscopy over a minimal cumulative time, we successfully visualized the phonon-polariton dynamics in time-wavelength space even at low temperatures. We found that the phase-matching condition significantly affected the observed temperature-dependent phonon-polariton frequency shift. The anharmonicity of the TO1 and TO3 modes was then evaluated based on an anharmonic model involving higher-order interactions with acoustic phonons while eliminating the influence of the frequency shift due to the phase-matching condition. The observed wavenumber-dependent damping rate was analyzed by considering the bilinear coupling of the TO1 or TO3 modes with the thermally activated relaxation mode. We found that the phonon-polariton with a higher frequency and wavenumber had a higher damping rate at high temperatures because of its frequent interaction with the thermally activated relaxation mode and acoustic phonons. The TO3 mode displayed greater bilinear coupling than the TO1 mode, which may also have contributed to the observed high damping rate. Thus, using our unique single-shot spectroscopy technique, we could reveal the overall anharmonic characteristics of the E-mode phonon-polaritons arising from both the acoustic phonons and the relaxation mode.