Centrosymmetric Materials Research Articles

Tunable room temperature magnetic skyrmions in centrosymmetric kagome magnet Mn4Ga2Sn

The successful realization of skyrmion-based spintronic devices depends on the easy manipulation of underlying magnetic interactions in the skyrmion-hosting materials. Although the mechanism of skyrmion formation in non-centrosymmetric magnets is comprehensively established, the stabilization process of different skyrmion-like magnetic textures in centrosymmetric magnets needs further investigation. Here, we utilize Lorentz transmission electron microscopy study to report the finding of a tunable skyrmion lattice up to room temperature in a centrosymmetric kagome ferromagnet Mn4Ga2Sn. We demonstrate that a controlled switching between the topological skyrmions and non-topological type-II magnetic bubbles can be realized at the optimal magnetic anisotropy. We find that the topological skyrmions are the energetically most stable magnetic objects in the centrosymmetric hexagonal magnets, whereas application of in-plane magnetic field stabilizes type-II magnetic bubbles as an excited state. The present study is a significant step towards understanding of the skyrmion stabilization mechanism in centrosymmetric materials for their future applications.

Spin Swapping Effect of Band Structure Origin in Centrosymmetric Ferromagnets.

We theoretically demonstrate the spin swapping effect of band structure origin in centrosymmetric ferromagnets. It is mediated by an orbital degree of freedom but does not require inversion asymmetry or impurity spin-orbit scattering. Analytic and tight-binding models reveal that it originates mainly from k points where bands with different spins and different orbitals are nearly degenerate, and thus it has no counterpart in normal metals. First-principle calculations for centrosymmetric 3d transition-metal ferromagnets show that the spin swapping conductivity of band structure origin can be comparable in magnitude to the intrinsic spin Hall conductivity of Pt. Our theory generalizes transverse spin currents generated by ferromagnets and emphasizes the important role of the orbital degree of freedom in describing spin-orbit-coupled transport in centrosymmetric materials.

(Digital Presentation) Characterization of Passivation Dielectrics on Silicon Through Second Harmonic Generation: Effect of Fixed Charge

In applications such as image sensors or solar cells, the performance of the device can be improved thanks to the passivation of the silicon substrate using high-k dielectrics such as alumina, hafnium dioxide etc. To assess the quality of the dielectric/semiconductor interface, conventional electrical characterization techniques can be employed, but they require fabrication of dedicated test devices. Among the methods that can be directly used at wafer-level, COCOS [1] allows obtaining electrical information such as the interface states density, but the charging of the sample surface can be an issue. In this paper, we discuss an alternative sensitive and non-destructive optical technique to characterize the electrical properties of a passivation scheme: the second harmonic generation (SHG). In centrosymmetric materials, in the dipolar approximation, the bulk SHG response is zero [2], so the signal mainly contains interface-related information. In particular, when a static electric field (Edc) is present at the interface, the SHG is reinforced and a phenomenon called EFISH (electric field SHG) appears [3]. The electric field at the interface between a dielectric and a semiconductor is related to fixed charges (Qox) and/or interface states (Dit), so the SHG signal can provide these two parameters. Additionally the technique has a potential to probe small areas (within the micrometer size).In this work, the study is focused on the ability of the SHG to measure Qox. The samples used have a layer of Al2O3 of 13nm thickness on Si (100), deposited with different processes, followed or not by an annealing. The annealing step is known to activate negative charges in alumina layers, which promotes the field effect passivation and it can also modify the interface defects [4]. The samples were measured with the SHG tool from FemtoMetrix [5]. Complementary electrical characterizations were performed with the COCOS technique on the full wafer and with capacitance versus voltage (C-V) on dedicated MOS capacitors, in order to obtain Qox. The electric field at the interface can then be calculated through Gauss’ law.Figure 1a shows a typical experimental response of the SHG versus time, obtained on two different samples. The parameters Qtot and Dit were extracted through COCOS. The time-dependence of the SHG is due to the time-dependence of the electric field during the measurement because of charging/discharging effects by carriers generated under the incident laser of the SHG. The SHG signature is obviously stronger for a high Qtot, which indeed is associated with a higher electric field at the interface. Figure 1b shows (with symbols) the experimental results of SHG versus the incidence angle (AOI) for the two samples. The increase of SHG signal for the sample with higher Qtot is confirmed, regardless of the angle of incidence.A general calibration, only based on these types of measurements is not sufficient, since optical phenomena such as transmission & interferences depend on the thickness of the layers involved. In order to better anticipate the effect of these phenomena for a future calibration, we developed a home-made simulator, based on absorption in each layer, boundary conditions at each interface and which contains the “static” interface electric field between materials. Using the electric field calculated with the charge parameters extracted from the COCOS/C-V measurements, we traced the simulated SHG versus AOI (Figure 1b, lines). The tendency observed in experiments is reproduced, even though the simulations are not fully superimposed. This mismatch is probably due to the fact that some of the material parameters in the simulations (e.g. the non-linear susceptibility values) are not very well known.The full paper will show additional samples, corresponding C-V and SHG experimental and simulated data, with the aim of proposing a calibration method that would allow the direct extraction of Qox based only on the SHG measurement.AcknowledgementsThis work was supported by the French National Plan Nano2022, within the IPCEI Nanoelectronics for Europe program.[1] M. Wilson, J. Lagowski, L. Jastrzebski, A. Savtchouk, and V. Faifer, AIP Conference Proceedings, 2001, vol. 550, no. 1, pp. 220–225.[2] R. W. Boyd, Nonlinear optics. Academic press, 2020.[3] N. M. Terlinden, G. Dingemans, V. Vandalon, R. Bosch, and W. M. M. Kessels, J. Appl. Phys., vol. 115, no. 3, p. 033708, 2014.[4] D. K. Schroder. John Wiley & Sons, 2015.[5] http://femtometrix.com Figure 1

(Invited) Second Harmonic Generation: Non-Linear Optics for Characterization of Electrical Properties of Dielectric-on-Semiconductor Interfaces

Dielectric layers are deposited on semiconductor materials for many applications, either in the active stacks of the devices (e.g. the gate dielectric in a metal-oxide-semiconductor transistor) or as passivation solutions (e.g. in image sensors [1], solar cells [2] etc.). The progress at the material and technology levels must be accompanied by the development of new characterization tools. One of the challenges in terms of characterization is to obtain the information on the electrical quality of the dielectric-on-semiconductor interface during the fabrication flow, non-destructively, just after the corresponding deposition step (if possible) and without needing to fully fabricate a particular test structure. At wafer level, a typical electrical evaluation method is the Corona characterization of semiconductors [3] that provides the interface state density (Dit) and the “total” charge responding in the structure. Its main drawback is the charging of the surface during the measurement. Optical methods (like the photoconductance decay or the photoluminescence) are non-destructive, but they are directly related to material and interface quality through the carrier lifetime [4] and they don’t allow a simple separation between fixed charge in the oxide (Qox) and Dit.However optical-based methods are still a recommended strategy, since they ensure non-destructive measurements. In this context, the second harmonic generation (SHG) is a good option, because it can be sensitive to the “static” electric field induced between layers. In the SHG, the surface of the sample is irradiated with a femtosecond laser and a second harmonic wave is then generated and detected. In general, the SHG contains both bulk and surface contributions, but for centrosymmetric materials (such as silicon, silicon dioxide, alumina, etc.) in the dipolar approximation, the interface signal is dominant and it is related to the symmetry breaking due to both the interface itself and to the “static” electric field [5]. The SHG has already been used for dielectric characterization using various modalities: SHG versus power, time, wavelength, etc. [6], [7], [8].In this paper, we focus on the analysis of the interface electric field, which is related to Qox and Dit. If trapping/detrapping phenomena occur during the illumination of the sample, this field can actually depend on time. The value of the SHG (proportional to the square of the field) will then be related to Qox and its evolution in time should correlate to Dit. The technique is promising but it triggers two questions: (1) how to separate optical phenomena (absorption, interferences) specific to multi-layer structures and access electrical field only and (2) how to actually extract Qox and Dit. During the presentation we explain recent results in these two areas, using simulation results obtained with our “home-made” code and a wide panel of samples for the experiments. Alternative electrical characterization performed through capacitance versus voltage measurements gives hints for future calibration.AcknowledgementsThis work was supported by Region Rhône Alpes (ARC6 program), the French National Research Agency within the framework of the OXYGENE project (ANR-17-CE05-0034) and French National Plan Nano2022, within the IPCEI Nanoelectronics for Europe program.[1] J. L. Regolini, D. Benoit, and P. Morin, "Passivation issues in active pixel CMOS image sensors," Microelectronics Reliability, vol. 47, pp. 739-742, 2007.[2] A. G. Aberle, "Surface passivation of crystalline silicon solar cells: a review," Progress in Photovoltaics: Research and Applications, vol. 8, pp. 473-487, 2000.[3] M. Wilson, J. Lagowski, L. Jastrzebski, et al., "COCOS (corona oxide characterization of semiconductor) non-contact metrology for gate dielectrics," AIP Conference Proceedings, vol. 550, pp. 220-225, 2001.[4] D. K. Schroder, "Carrier lifetimes in silicon," Electron Devices, IEEE Transactions on, vol. 44, pp. 160-170, 1997.[5] J. E. Sipe, D. J. Moss, and H. M. van Driel, "Phenomenological theory of optical second- and third-harmonic generation from cubic centrosymmetric crystals," Physical Review B, vol. 35, pp. 1129-1141, 1987.[6] J. Price, M. Lei, P. S. Lysaght, et al., "Charge trapping defects in Si/SiO2/Hf(1−x)SixO2 film stacks characterized by spectroscopic second-harmonic generation," Journal of Vacuum Science & Technology B, vol. 29, p. 04D101, 2011.[7] N. M. Terlinden, G. Dingemans, V. Vandalon, et al., "Influence of the SiO2 interlayer thickness on the density and polarity of charges in Si/SiO2/Al2O3 stacks as studied by optical second-harmonic generation," Journal of Applied Physics, vol. 115, p. 033708, 2014.[8] H. Park, J. Qi, Y. Xu, et al., "Boron induced charge traps near the interface of Si/SiO2 probed by second harmonic generation," Physica Status Solidi (b), vol. 247, pp. 1997-2001, 2010.

Femtosecond laser-induced sub-wavelength plasma inside dielectrics. II. Second-harmonic generation

Second-harmonic emission at a frequency that is twice the laser frequency is an important diagnostic for nonlinear laser–plasma interaction. It is forbidden for centrosymmetric materials such as the bulk of sapphire. The symmetry, however, can be broken by dielectric discontinuities as a result of plasma generation inside a solid dielectric. In the present work, we explore the basic characteristics of experimentally observed second-harmonic emission during focusing a femtosecond Bessel beam inside sapphire. We employ three-dimensional particle-in-cell simulations and the Helmholtz wave equation for theoretical investigations. We analyze how the efficiency of second-harmonic generation and its polarization depend on the plasma parameters. We find that the second-harmonic is generated either due to the coalescence of two-surface electromagnetic waves or nonlinear interaction between the transverse electromagnetic wave and the longitudinal electron plasma wave driven by linear mode conversion. Experimental results agree with the theoretical predictions and confirm the existence of over-critical plasma inside the sapphire that is essential for the resonance of plasma waves or excitation of surface plasmons.

Symmetry-breaking induced piezocatalysis of Bi2S3 nanorods and boosted by alternating magnetic field

In this work, the centrosymmetry of Bi2S3 nanorods is broken due to the introduction of large quantities of Si defects in the crystal lattice, which then endows the Bi2S3 nanorods with the ability to efficiently reduce Cr(VI) by piezocatalysis. Most importantly, the piezocatalytic reduction could be significantly boosted by applying an alternating magnetic field (AMF). The AMF coupled piezocatalysis is achieved by H2O2 and·O2-. AMF is found to facilitate the separation of electrons and holes due to the magnetoresistance effect of Bi2S3 nanorods, thus remarkably enhancing the piezocatalysis by largely increasing the generation of H2O2. This work paves a new way for inducing the piezoelectricity of centrosymmetric materials by introducing defects and improving their catalytic activity through AMF, which has a great application potential in energy and environment fields.

Emergent ferroelectricity in subnanometer binary oxide films on silicon.

The critical size limit of voltage-switchable electric dipoles has extensive implications for energy-efficient electronics, underlying the importance of ferroelectric order stabilized at reduced dimensionality. We report on the thickness-dependent antiferroelectric-to-ferroelectric phase transition in zirconium dioxide (ZrO2) thin films on silicon. The emergent ferroelectricity and hysteretic polarization switching in ultrathin ZrO2, conventionally a paraelectric material, notably persists down to a film thickness of 5 angstroms, the fluorite-structure unit-cell size. This approach to exploit three-dimensional centrosymmetric materials deposited down to the two-dimensional thickness limit, particularly within this model fluorite-structure system that possesses unconventional ferroelectric size effects, offers substantial promise for electronics, demonstrated by proof-of-principle atomic-scale nonvolatile ferroelectric memory on silicon. Additionally, it is also indicative of hidden electronic phenomena that are achievable across a wide class of simple binary materials.

A unified model for the dynamical flexoelectric effect in isotropic dielectric materials

The present work proposes a unified model for studying the dynamical flexoelectricity by including effects of the micro-inertia, the dynamical polarization, and the dynamical flexocoupling between displacement and polarization upon the mechanical and electrical states of the dielectric material. The mathematical description is presented in the framework of strain gradient elasticity. Using the variational technique for internal energies and external forces, the governing equations are derived with their concomitant boundary conditions for homogeneous anisotropic flexoelectric materials. The focus is on the study of the effects of such dynamical size-dependent and dynamical flexocoupling tensor on the class of centrosymmetric isotropic dielectric materials, which referred to as the flexoelectric effect, by means of the integral transform technique. A one-dimensional application is presented for a semi-infinite flexoelectric material. The solution is obtained using the integral transform method, precisely, the Laplace transform. The strontium titanate (STO) is chosen for the numerical simulation. It is emphasized that the size-dependent factor diminishes the dynamical wave feature of the response variables. Otherwise, the flexocoupling parameter is found to play a key role in polarization production inside the dielectric medium.

Square and rhombic lattices of magnetic skyrmions in a centrosymmetric binary compound

Magnetic skyrmions are topologically stable swirling spin textures with particle-like character, and have been intensively studied as a candidate of high-density information bit. While magnetic skyrmions were originally discovered in noncentrosymmetric systems with Dzyaloshinskii-Moriya interaction, recently a nanometric skyrmion lattice has also been reported for centrosymmetric rare-earth compounds, such as Gd2PdSi3 and GdRu2Si2. For the latter systems, a distinct skyrmion formation mechanism mediated by itinerant electrons has been proposed, and the search of a simpler model system allowing for a better understanding of their intricate magnetic phase diagram is highly demanded. Here, we report the discovery of square and rhombic lattices of nanometric skyrmions in a centrosymmetric binary compound EuAl4, by performing small-angle neutron and resonant elastic X-ray scattering experiments. Unlike previously reported centrosymmetric skyrmion-hosting materials, EuAl4 shows multiple-step reorientation of the fundamental magnetic modulation vector as a function of magnetic field, probably reflecting a delicate balance of associated itinerant-electron-mediated interactions. The present results demonstrate that a variety of distinctive skyrmion orders can be derived even in a simple centrosymmetric binary compound, which highlights rare-earth intermetallic systems as a promising platform to realize/control the competition of multiple topological magnetic phases in a single material.

Symmetry-allowed nonlinear orbital response across the topological phase transition in centrosymmetric materials

Nonlinear optical responses are commonly used as a probe for studying the electronic properties of materials. For topological materials, studies thus far focused on photogalvanic electric currents, which are forbidden in centrosymmetric materials because they require broken inversion symmetry. In this Letter, we propose a class of symmetry-allowed responses for inversion-symmetric topological insulators with two doubly degenerate bands. We consider a specific example of such a response, the orbital current, and show that the sign of the response reflects the ${\mathbb{Z}}_{2}$ topological index, i.e., the orbital current changes sign at the transition between trivial and topological insulator phases. This is illustrated in two models of topological insulators: the Bernevig-Hughes-Zhang model and the $1{T}^{\ensuremath{'}}$ phase of transition metal dichalcogenides.

Efficient Second‐Harmonic Generation from Silicon Slotted Nanocubes with Bound States in the Continuum

AbstractOptical materials with centrosymmetry, such as silicon and germanium, are unfortunately absent of second‐order nonlinear optical responses, hindering their developments in efficient nonlinear optical devices. Here, a design with an array of slotted nanocubes is proposed to realize remarkable second harmonic generation (SHG) from the centrosymmetric silicon, which takes advantage of enlarged surface second‐order nonlinearity, strengthened electric field over the surface of the air‐slot, as well as the resonance enhancement by the bound states in the continuum. Compared with that from the array of silicon nanocubes without air‐slots, SHG from the slotted nanocube array is improved by more than two orders of magnitude. The experimentally measured SHG efficiency of the silicon slotted nanocube array is high as 1.8 × 10−4 W−1, which is expected to be further engineered by modifying the air‐slot geometries. The result can provide a new strategy to expand nonlinear optical effects and devices of centrosymmetric materials.

Induced giant piezoelectricity in centrosymmetric oxides.

Piezoelectrics are materials that linearly deform in response to an applied electric field. As a fundamental prerequisite, piezoelectric materials must have a noncentrosymmetric crystal structure. For more than a century, this has remained a major obstacle for finding piezoelectric materials. We circumvented this limitation by breaking the crystallographic symmetry and inducing large and sustainable piezoelectric effects in centrosymmetric materials by the electric field-induced rearrangement of oxygen vacancies. Our results show the generation of extraordinarily large piezoelectric responses [with piezoelectric strain coefficients (d33) of ~200,000 picometers per volt at millihertz frequencies] in cubic fluorite gadolinium-doped CeO2-x films, which are two orders of magnitude larger than the responses observed in the presently best-known lead-based piezoelectric relaxor-ferroelectric oxide at kilohertz frequencies. These findings provide opportunities to design piezoelectric materials from environmentally friendly centrosymmetric ones.

A non-classical theory of elastic dielectrics incorporating couple stress and quadrupole effects: part II - variational formulations and applications in plates

A new Mindlin plate model is developed incorporating the couple stress, quadrupole, and curvature-based flexoelectric effects using a non-classical elastic dielectric theory. The general governing equations and complete boundary conditions are derived simultaneously through a variational approach. The new general plate model includes the model for centrosymmetric materials as a special case. As direct applications of the new plate and to illustrate the flexoelectric effect, the static bending and free vibration problems of a simply supported rectangular plate are analytically solved by directly applying the new centrosymmetric model. The numerical results reveal that the flexoelectric effect is present in centrosymmetric materials and is important when the plate thickness is very thin. The dispersion relations for flexure and fundamental shear modes are obtained. We find electrostatic wave also propagates with the flexure or fundamental shear waves by flexoelectric coupling. The present work can be seen as an extension of mechanical analysis of plate structures with couple stress effects and may be useful to the devices design of micro- or nano-sized objects.

Contact electrification induced mechanoluminescence

Mechanoluminescence (ML) is a luminescence response to mechanical stimuli that shows a variety of significant applications. It is generally believed that ML is due to piezoluminescence in asymmetric piezoelectric materials, but this model encounters difficulties in explaining the ML for centrosymmetric nonpiezoelectric materials, especially the Lu3Al5O12: Ce3+/PDMS elastomer with ultrastrong nonpreirradiation ML. This theoretical problem has become the prominent “dark cloud” in the sky of the ML field. This work reveals for the first time that the ML for the centrosymmetric Lu3Al5O12: Ce3+/PDMS elastomer is essentially induced by contact electrification. According to contact electrification, an electron-cloud model of contact electrification is successfully established to understand the ML mechanism for the elastomer. This demonstrates that contact electrification arising from the interaction of two dissimilar surfaces creates a broadband excitation energy, which directly excites the luminescence levels to emit ML. Accordingly, the unique ML properties of the centrosymmetric Lu3Al5O12: Ce3+/PDMS elastomer that cannot be explained by piezoelectricity can be well understood by contact electrification. Particularly, these results also inspire us to have reasonable new insight into the trap-dependent piezoluminescence mechanism for typical SrAl2O4: Eu2+ and ZnS: Cu/Mn2+ materials.

Piezoelectricity in nominally centrosymmetric phases

Compound phases often display properties that are symmetry forbidden relative to their nominal, average crystallographic symmetry, even if extrinsic reasons (defects, strain, or imperfections) are not apparent. Specifically, breaking the macroscopic inversion symmetry of a centrosymmetric phase can dominate or significantly change its observed properties while the detailed mechanisms and magnitudes of the deviations of symmetry breaking are often obscure. Here, we choose piezoelectricity as a tool to investigate macroscopic inversion-symmetry breaking in nominally centrosymmetric materials as a prominent example and measure resonant piezoelectric spectroscopy (RPS) and Resonant Ultrasound Spectroscopy (RUS) in 15 compounds, 18 samples, and 21 different phases, including unpoled ferroelectrics, paraelectrics, relaxors, ferroelastics, incipient ferroelectrics, and isotropic materials with low defect concentrations, i.e., NaCl, fused silica, and ${\mathrm{CaF}}_{2}$. We exclude the flexoelectric effect as a source of the observed piezoelectricity yet observe piezoelectricity in all nominally cubic phases of these samples. By scaling the RPS intensities with those of RUS, we calibrate the effective piezoelectric coefficients using single-crystal quartz as standard. Using this scaling we determine the effective piezoelectric modulus in nominally nonpiezoelectric phases, finding that the ``symmetry-forbidden'' piezoelectric effect ranges from $\ensuremath{\sim}1$ to ${10}^{\ensuremath{-}5}$ pm/V ($\ensuremath{\sim}0.5%$ to $\ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}%$ of the piezoelectric coefficient of poled ferroelectric lead zirconate titanate). The values for the unpoled ferroelectric phase are only slightly higher than those in the paraelectric phase. The extremely low coefficients are well below the detection limit of conventional piezoelectric measurements and demonstrate RPS as a convenient and ultrahighly sensitive method to measure piezoelectricity. We suggest that symmetry-breaking piezoelectricity in nominally centrosymmetric materials and disordered, unpoled ferroelectrics is a common phenomenon.

Self-Reduction-Related Defects, Long Afterglow, and Mechanoluminescence in Centrosymmetric Li2ZnGeO4:Mn2.

Mechanoluminescent materials have shown great application potential in the fields of stress detection, anti-counterfeiting, and optical storage; however, its development is hindered by the unclear mechanism. Different from the mainstream exploration of new mechanoluminescent materials in non-centrosymmetric structures, a centrosymmetric mechanoluminescent material Li2ZnGeO4:Mn2+ is synthesized by a standard high-temperature solid-state reaction in an ambient atmosphere. Combined with the Rietveld refinement, photoluminescence, electron spin resonance, and X-ray photoelectron spectroscopy, it is proved that the increase in oxygen vacancies is accompanied by the self-reduction process from Mn4+ to Mn2+, and the mechanism of mechanoluminescence is clarified through the afterglow and thermoluminescence spectra. The carriers trapped by the shallow traps participate in the mechanoluminescence process through the tunneling effect, while the carriers trapped by the deep traps take part in the mechanoluminescence process via conduction band or tunneling. A signature anti-counterfeiting application is designed using the new mechanoluminescent material Li2ZnGeO4:0.004Mn2+. Utilizing the afterglow characteristics of Li2ZnGeO4:xMn2+ phosphors, we designed an intelligent long-persistent luminescence quick response code (QR-code) and visualized information encoding/decoding model, which provides a fast, simple, and effective method for information encryption, transformation, and dynamic anti-counterfeiting. This study not only analyzes the self-reduction and mechanoluminescence processes in detail but also breaks the limitation of crystal symmetry and provides a new strategy for the exploration of novel mechanoluminescent materials.

Coherence control of directional nonlinear photocurrent in spatially symmetric systems

The interplay between crystal symmetry and its optical responses is at the heart of tremendous recent advances in light-matter interactions and applications. Nonlinear optical processes that produce electric currents, for example the bulk photovoltaic (BPV) effect, require inversion-symmetry-broken materials, such as ferroelectrics. In this work, we demonstrate that such BPV current could be generated in centrosymmetric materials with excitation of out-of-equilibrium coherent phonons. This is very different from the generally studied static or thermally excited states. We show that depending on the coherent phonon phase factor, the photoconductivity contains a static zero-frequency component, as well as THz frequency components. This indicates that a unidirectional current may emerge under synergistic effects of atomic vibration and light irradiation. We also generalize the conventional injection charge current into angular momentum (spin and orbital) degrees of freedom, and demonstrate spin and orbital BPV photoconductivities under coherent phonons. Our findings open the pathway to exploring the exotic phonon-photon-electron coherent interactions in quantum materials.

Unraveling Strain Gradient Induced Electromechanical Coupling in Twisted Double Bilayer Graphene Moiré Superlattices.

Moiré superlattices of 2D materials with a small twist angle are thought to exhibit appreciable flexoelectric effect, though unambiguous confirmation of their flexoelectricity is challenging due to artifacts associated with commonly used piezoresponse force microscopy (PFM). For example, unexpectedly small phase contrast (≈8°) between opposite flexoelectric polarizations is reported in twisted bilayer graphene (tBG), though theoretically predicted value is 180°. Here a methodology is developed to extract intrinsic moiré flexoelectricity using twisted double bilayer graphene (tDBG) as a model system, probed by lateral PFM. For small twist angle samples, it is found that a vectorial decomposition is essential to recover the small intrinsic flexoelectric response at domain walls from a large background signal. The obtained threefold symmetry of commensurate domains with significant flexoelectric response at domain walls is fully consistent with the theoretical calculations. Incommensurate domains in tDBG with relatively large twist angles can also be observed by this technique. A general strategy is provided here for unraveling intrinsic flexoelectricity in van der Waals moiré superlattices while providing insights into engineered symmetry breaking in centrosymmetric materials.

Analytical theory for three-wave mixing processes in a slightly deformed nanowire

The second order optical response of centrosymmetric materials manifests itself mostly at their surface, being strongly suppressed in their bulk. However, the overall surface response is also suppressed in nanoparticles with a centrosymmetric geometry subjected to homogeneous fields. Nevertheless, nanoparticles with a noncentrosymmetric geometry do exhibit second order optical properties. We develop an analytical theory to investigate the second order optical response of a noncentrosymmetric thin cylinder with a slightly deformed cross-section made up of a centrosymmetric material subjected to two monochromatic fields. We calculate the linear and nonlinear near fields perturbatively using the extent of the deformation away from a circular cross-section as the perturbation parameter. We obtain expressions for the quadratic hyperpolarizabilities in terms of the linear response evaluated at the three frequencies involved. We analyze the spectral features of the nonlinear response functions and explore their resonant structure for a model dielectric cylinder. Furthermore, we evaluate the second order radiated fields, the radiation patterns and efficiency of the different quadratic processes. We obtain a strong competition between electric dipolar, magnetic dipolar and electric quadrupolar contributions even for very small deformations.

Flexoelectric nanodomains in rare-earth iron garnet thin films under strain gradient

Flexoelectricity is a universal property associated with dielectric materials, wherein they exhibit remanent polarization induced by strain gradient. Rare-earth iron garnets, R3Fe5O12, are ferrimagnetic insulators with useful magnetic properties. However, they are unlikely to show remanent dielectric polarization because of their centrosymmetric structure. Here, to induce flexoelectricity, we investigate various rare-earth iron-garnet thin films deposited on lattice-mismatched substrates. Atomic-resolution scanning transmission electron microscopy demonstrates the presence of 15 nm-thick strain gradients in Sm3Fe5O12 films between epitaxially strained tetragonal and relaxed cubic structures. Furthermore, negatively polarized nanodomains are imaged by scanning nonlinear dielectric microscopy. It suggests a generation of flexoelectricity, where the polarization points down toward the substrate in the out-of-plane direction. X-ray magnetic circular dichroism demonstrates hysteresis with a large coercive field originating from the strain-gradient layer. We believe that our study will pave the way for achieving dielectric polarization even in nonpolar centrosymmetric materials by strain-gradient engineering.