Atomic-Scale Measurement of Flexoelectric Polarization at 
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 Dislocations

Gao, Peng; Yang, Shu; Ishikawa, Ryo; Li, Ning; Feng, Bin; Kumamoto, Akihito; Shibata, Naoya; Yu, Pu; Ikuhara, Yuichi

doi:10.1103/physrevlett.120.267601

Cited by 97 publications

(55 citation statements)

References 44 publications

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“…SrTiO 3 has been reported to undergo a strain induced phase transition into an SHG active ferroelectric phase . The absence of first‐order peaks other than the TO 3 however leads us to the conclusion that the strain induced in our experiment is too small to generate the substrate‐induced phase transition invoked by Pertsev et al, thus suggesting that the strain gradient causing loss of centrosymmetry is due to the presence of dislocations near the nanoindentations, consistently with previously reported transmission electron microscopy investigations . The strain gradient induces a shift of the Ti ion charge, thereby creating a polarization in the material .…”

Section: Resultssupporting

confidence: 89%

Second Harmonic Generation Investigation of Symmetry Breaking and Flexoelectricity Induced by Nanoindentations in SrTiO₃

Kolhatkar¹,

Nicklaus²,

Youssef³

et al. 2019

Adv Funct Materials

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Strain gradients induced by nanoindentations in bulk undoped strontium titanate (SrTiO 3 ) are investigated. After indenting a 850 nm deep pattern on the SrTiO 3 surface with a 8 GPa hardness and an 83 GPa indentation modulus by a Berkovich tip, the resulting deformation patterns are imaged by Raman spectroscopy and by second harmonic generation microscopy. Cross-shaped compressively stressed regions along the cubic SrTiO 3 primary slip planes are observed. These zones relax toward the unstrained bulk material through localized strain gradients on a length scale of several microns. Second harmonic tomography reveals the reduction of the crystal's centrosymmetry in the vicinity of the crosslike features due to a strain gradient. These results indicate the appearance of flexoelectricity following the nanoindentation process due to the creation of an inhomogeneous strain.

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Section: Resultssupporting

confidence: 89%

Second Harmonic Generation Investigation of Symmetry Breaking and Flexoelectricity Induced by Nanoindentations in SrTiO₃

Kolhatkar¹,

Nicklaus²,

Youssef³

et al. 2019

Adv Funct Materials

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“…While flexoelectric effects are typically negligible in bulk materials, they appear to be a generic feature of nanometer structures in FEs-including domain walls [ 43,44], grain boundaries [45], and cracks [46]-where the strain gradient ∂ z η is often enormous. Flexoelectricity substantially affects the performance of nanocapacitors [47], and numerous proposals for flexo-mechanical devices have been made [34].…”

mentioning

confidence: 99%

Possible flexoelectric origin of the Lifshitz transition in LaAlO3/SrTiO3 interfaces

Raslan

Atkinson

2018

Phys. Rev. B

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Multiple experiments have observed a sharp transition in the band structure of LaAlO3/SrTiO3 (001) interfaces as a function of applied gate voltage. This Lifshitz transition, between a single occupied band at low electron density and multiple occupied bands at high density, is remarkable for its abruptness. In this work, we propose a mechanism by which such a transition might happen. We show via numerical modeling that the simultaneous coupling of the dielectric polarization to the interfacial strain ("electrostrictive coupling") and strain gradient ("flexoelectric coupling") generates a thin polarized layer whose direction reverses at a critical density. The Lifshitz transition occurs concomitantly with the polarization reversal and is first-order at T = 0. A secondary Lifshitz transition, in which electrons spread out into semiclassical tails, occurs at a higher density.LaAlO 3 (LAO) and SrTiO 3 (STO) are band insulators; however, when four or more monolayers of LAO are grown on top of a STO substrate, a mobile twodimensional electron gas (2DEG) forms on the STO side of the interface [1, 2]. One compelling feature of these interfaces is that the character of the 2DEG changes dramatically with the application of a gate voltage. Indeed, for (001) interfaces there is a narrow doping range over which the superconducting transition temperature [3][4][5][6][7], the spin-orbit coupling [7-10], and the metamagnetic response [11] change by an order of magnitude. Furthermore, the superconducting gap and resistive transition appear at different temperatures at low electron densities, n 2D , but track each other closely at high n 2D [12]. This qualitative distinction between low and high doping has also been seen in quantum dot transport experiments, which reveal a crossover from attractive to repulsive pairing interactions with increasing n 2D [13]. While there is general agreement that the sensitivity to doping is connected to an observed Lifshitz transition [5, 14-17] between a single occupied band at low density and multiple occupied bands at high density [6,[17][18][19][20][21][22][23][24], the mechanism by which this transition happens is not established.Density functional theory (DFT), while instrumental in establishing fundamental interface properties [25][26][27][28], finds electron densities that are an order of magnitude larger than the Lifshitz transition density n L ∼ 0.02-0.05 electrons per 2D unit cell, and cannot easily be tuned through the transition. Schrödinger-Poisson calculations, for which n 2D can be continuously tuned, persistently find multiple occupied bands even for n 2D n L [5,15,[29][30][31]. Indeed, we showed previously that, because of STO is close to a ferroelectric (FE) quantum critical point [32], electrons become deconfined from an ideal interface as n 2D → 0, and form a dilute quasi-threedimensional (quasi-3D) gas extending far into the STO substrate [33]. This result is incompatible with experiments and raises the question, why is only a single band occupied at low n 2D ? Furthermore, t...

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“…This eigenstrain influences the total strain distribution in the nanoparticles. As shown in recent work on strontium titanate, atomic-scale measurements of local displacements due to the flexoelectric effect have been reported 31 . However, for the NBT-25STnanoparticle system, the measurement of atomic-displacements for the whole nanoparticle is nontrivial.…”

Section: Resultsmentioning

confidence: 82%

Enabling nanoscale flexoelectricity at extreme temperature by tuning cation diffusion

et al. 2018

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Any dielectric material under a strain gradient presents flexoelectricity. Here, we synthesized 0.75 sodium bismuth titanate −0.25 strontium titanate (NBT-25ST) core–shell nanoparticles via a solid-state chemical reaction directly inside a transmission electron microscope (TEM) and observed domain-like nanoregions (DLNRs) up to an extreme temperature of 800 °C. We attribute this abnormal phenomenon to a chemically induced lattice strain gradient present in the core–shell nanoparticle. The strain gradient was generated by controlling the diffusion of strontium cations. By combining electrical biasing and temperature-dependent in situ TEM with phase field simulations, we analyzed the resulting strain gradient and local polarization distribution within a single nanoparticle. The analysis confirms that a local symmetry breaking, occurring due to a strain gradient (i.e. flexoelectricity), accounts for switchable polarization beyond the conventional temperature range of existing polar materials. We demonstrate that polar nanomaterials can be obtained through flexoelectricity at extreme temperature by tuning the cation diffusion.

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Atomic-Scale Measurement of Flexoelectric Polarization at SrTiO3 Dislocations

Cited by 97 publications

References 44 publications

Second Harmonic Generation Investigation of Symmetry Breaking and Flexoelectricity Induced by Nanoindentations in SrTiO₃

Second Harmonic Generation Investigation of Symmetry Breaking and Flexoelectricity Induced by Nanoindentations in SrTiO₃

Possible flexoelectric origin of the Lifshitz transition in LaAlO3/SrTiO3 interfaces

Enabling nanoscale flexoelectricity at extreme temperature by tuning cation diffusion

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Atomic-Scale Measurement of Flexoelectric Polarization at SrTiO3 Dislocations

Cited by 97 publications

References 44 publications

Second Harmonic Generation Investigation of Symmetry Breaking and Flexoelectricity Induced by Nanoindentations in SrTiO3

Second Harmonic Generation Investigation of Symmetry Breaking and Flexoelectricity Induced by Nanoindentations in SrTiO3

Possible flexoelectric origin of the Lifshitz transition in LaAlO3/SrTiO3 interfaces

Enabling nanoscale flexoelectricity at extreme temperature by tuning cation diffusion

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Product

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Second Harmonic Generation Investigation of Symmetry Breaking and Flexoelectricity Induced by Nanoindentations in SrTiO₃

Second Harmonic Generation Investigation of Symmetry Breaking and Flexoelectricity Induced by Nanoindentations in SrTiO₃