Ionic polarizability of conductive metal oxides and critical thickness for ferroelectricity in BaTiO3

We report a first-principles investigation of ultrathin BaTiO(3) films with SrRuO(3) electrodes. We find that the ionic relaxations in the metal-oxide electrode play a crucial role in stabilizing the ferroelectric phase. Comparison with frozen-phonon calculations shows that the degree of softness of the SrRuO(3) lattice has an essential impact on the screening of ferroelectric polarization in BaTiO(3). The critical thickness for ferroelectricity in BaTiO(3) is found to be 1.2 nm. The results of our calculations provide a possible explanation for the beneficial impact of oxide electrodes on the switching and dielectric properties of ferroelectric capacitors.     

Effect of ferroelectricity on electron transport in Pt/BaTiO3/Pt tunnel junctions

Based on first-principles calculations, we demonstrate the impact of the electric polarization on electron transport in ferroelectric tunnel junctions (FTJs). Using a Pt/BaTiO3/Pt FTJ as a model system, we show that the polarization of the BaTiO3 barrier leads to a substantial drop in the tunneling conductance due to changes in the electronic structure driven by ferroelectric displacements. We find a sizable change in the transmission probability across the Pt/BaTiO3 interface with polarization reversal, a signature of the electroresistance effect. These results reveal exciting prospects that FTJs offer as resistive switches in nanoscale electronic devices.     

Anisotropy contrast in phonon-enhanced apertureless near-field microscopy using a free-electron laser

We demonstrate the imaging of ferroelectric domains in BaTiO3, using an infrared-emitting free-electron laser as a tunable optical source for scattering scanning near-field optical microscopy and spectroscopy. When the laser is tuned into the spectral vicinity of a phonon resonance, ferroelectric domains can be resolved due to the anisotropy of the dielectric properties of the material. Slight detuning of the wavelength gives rise to a contrast reversal clearly evidencing the resonant character of the excitation. The near-field domain contrast shows that the orientation of the dielectric tensor with respect to the sample surface has a clear influence on the near-field signal.     

High-pressure study of x-ray diffuse scattering in ferroelectric perovskites

We present a high-pressure x-ray diffuse scattering study of the ABO3 ferroelectric perovskites BaTiO3 and KNbO3. The well-known diffuse lines are observed in all the phases studied. In KNbO3, we show that the lines are present up to 21.8 GPa, with constant width and a slightly decreasing intensity. At variance, the intensity of the diffuse lines observed in the cubic phase of BaTiO3 linearly decreases to zero at approximately 11 GPa. These results are discussed with respect to x-ray absorption measurements, which leads to the conclusion that the diffuse lines are only observed when the B atom is off the center of the oxygen tetrahedron. The role of such disorder on the ferroelectric instability of perovskites is discussed.     

Ferroelectric control of the magnetocrystalline anisotropy of the Fe/BaTiO(3)(001) interface

Density-functional calculations are employed to investigate the effect of ferroelectric polarization of BaTiO(3) on the magnetocrystalline anisotropy of the Fe /BaTiO(3)(001) interface. It is found that the interface magnetocrystalline anisotropy energy changes from 1.33 to 1.02 erg cm (-2) when the ferroelectric polarization is reversed. This strong magnetoelectric coupling is explained in terms of the changing population of the Fe 3d orbitals at the Fe/BaTiO(3) interface driven by polarization reversal. Our results indicate that the electronically assisted magnetoelectric effects at the ferromagnetic/ferroelectric interfaces may be a viable alternative to the strain mediated coupling in related heterostructures and the electric field-induced effects on the interface magnetic anisotropy in ferromagnet/dielectric structures.     

四方相BaTiO_3:Ce及Mn离子注入的BaTiO_3:Ce的拉曼散射的研究

四方相ＢａＴｉＯ３：Ｃｅ及Ｍｎ离子注入的ＢａＴｉＯ３：Ｃｅ的拉曼散射的研究刘玉龙朱恪张昊朱镛（中科院物理研究所北京１０００８０）赵金涛（张掖师范高等专科学校物理系甘肃张掖７３４０００）萧季驹（香港城市大学物理及材料科学系香港九龙）ＲａｍａｎＳｐｅｃ...     

Off-center displacements and hydrostatic pressure induced phase transition in perovskites

Pressure has a profound effect on the paraelectric and ferroelectric properties of perovskite crystals. In this paper we theoretically investigate the effect of pressure on the cubic-to-tetragonal phase transition and on the soft mode dynamics of some classical perovskite crystals: BaTiO(3), PbTiO(3), and KNbO(3). We use a model consisting of three subsystems: electrons, phonons, and off-center displacements treated as spins. Experiments show that pressure has a large effect on the tunneling and hopping of the off-center displacements, that in turn strongly affect the pressure dependence of the transition temperature and the soft mode frequency. This model, with a very small number of adjustable parameters, accounts quantitatively for the experimentally measured nonlinear pressure dependence of the cubic-to-tetragonal phase transition temperature, up to the critical pressure where the transition temperature is zero. It also accounts quantitatively for the pressure dependence of the soft mode frequency, which is finite at the phase transition in spite of the fact that the phase transition at elevated pressures is second order, and for the pressure dependence of the electronic gap energy.     

Ferroelectric phase transitions in ultrathin films of BaTiO3

We present molecular dynamics simulations of a realistic model of an ultrathin film of BaTiO3 sandwiched between short-circuited electrodes to determine and understand effects of film thickness, epitaxial strain, and the nature of electrodes on its ferroelectric phase transitions as a function of temperature. We determine a full epitaxial strain-temperature phase diagram in the presence of perfect electrodes. Even with the vanishing depolarization field, we find that ferroelectric phase transitions to states with in-plane and out-of-plane components of polarization exhibit dependence on thickness; it arises from the interactions of local dipoles with their electrostatic images in the presence of electrodes. Secondly, in the presence of relatively bad metal electrodes which only partly compensate the surface charges and depolarization field, a qualitatively different phase with stripelike domains is stabilized at low temperature.     

Desorption of carbon dioxide from small potassium niobate particles induced by the particles’ ferroelectric transition

The aim of this work is to understand surface properties of ferroelectric crystals related to gas adsorption. Various ferroelectric crystals involved in these studies readily adsorb carbon dioxide, thus our studies were centered on adsorption studies of this molecule. It has been claimed that a dipole moment is induced on carbon dioxide molecules that are near an oxide surface. Our experiments explored the possibility of a dipole-dipole interaction between the gas molecule and the ferroelectric oxide surface in order to explain its adsorption. We characterized the samples with scanning electron microscopy, X-ray diffraction and Raman spectroscopy. We determined the ferroelectric nature of the particles and studied the temperature-dependent phase transitions in small particles of KNbO₃ using Raman spectroscopy. We were able to correlate desorption of CO₂ from one surface state of KNbO₃ with the occurrence of the orthorhombic to tetragonal transition in KNbO₃ in particles of 1 μm size. This CO₂ surface site was not observed in KTaO₃, which does not show ferroelectricity at room temperature.     

Potential relief on the surface of polarized ferroelectric electrets from an analysis of the anomalous electron emission spectra

A technique is proposed for determining the specific features in the potential distribution over a free surface of polarized ferroelectric electrets from analyzing the anomalous electron emission spectra. This technique is applied to examine the potential distribution over the surface of a lead magnoniobate single crystal and ferroelectric ceramics. Reasoning from the results obtained for the lead magnoniobate crystal, the inference is drawn that the degree of perfection of surface layers of ferroelectric single crystals can be controlled using the anomalous electron emission spectra.     

Displacement-type ferroelectricity with off-center magnetic ions in perovskite Sr(1-x)Ba(x)MnO3

We report a ferroelectric transition driven by the off-centering of magnetic Mn(4+) ions in antiferromagnetic Mott insulators Sr(1-x)Ba(x)MnO(3) with a perovskite structure. As x increases, the perovskite lattice shows the typical soft-mode dynamics, as revealed by the momentum-resolved inelastic x-ray scattering and far-infrared spectroscopy, and the ferroelectricity shows up for x ≥ 0.45. The observed polarization is comparable to that for a prototypical ferroelectric BaTiO(3). We further demonstrate that the magnetic order suppresses the ferroelectric lattice dilation by ～70% and increases the soft-phonon energy by ～50%, indicating the largest magnetoelectric effects yet attained.     

Analysis of ferroelectric 180 degrees -domain structures in BaTiO(3) by use of second-harmonic scattering

We present a method of determining the distribution of sizes of ferroelectric 180 degrees domains in BaTiO(3), using second-harmonic scattering. We find that the domains are rods with approximately square cross sections that extend from the -c to the +c face of the crystal, with widths ranging from 2 to 150 mum.     

Persistence of surface domain structures for a bulk ferroelectric above TC

Photoemission electron microscopy performed on a well-prepared surface of BaTiO3 reveals the persistence of surface domains at temperatures well above the bulk Curie temperature. Their patterns follow the ferroelectric domain structure observed at 300 K. The contrast between formerly outward polarized domains and in-plane polarized domains is preserved across the transition, while the contrast of inward polarized domains changes sign. The work functions of different possible structures are compared by first-principles calculations. The domain contrast in photoemission above the bulk Curie temperature is associated with a remaining tetragonal distortion of the topmost unit cells which is stabilized by an ionic surface relaxation.     

First-principles determination of electromechanical responses of solids under finite electric fields

We describe a first-principles, easy-to-implement, and efficient approach for determining the structural geometry of insulating solids under finite electric fields. This method consists of simultaneously minimizing the field-induced total ionic forces and the electric free energy. Moreover, we present a theory to analyze its predictions that provides a microscopic understanding of electro-mechanical responses in materials. We illustrate this approach by computing piezoelectric and dielectric responses of two rather different compounds, namely, ferroelectric PbTiO3 and semiconductor GaN.     

Polarization dependent chemistry of ferroelectric BaTiO3(001) domains

Recent works suggest that the surface chemistry, in particular the presence of oxygen vacancies, can affect the polarization in a ferroelectric material. This should, in turn, influence the domain ordering driven by the need to screen the depolarizing field. Here we show using density-functional theory that the presence of oxygen vacancies at the surface of BaTiO(3)(001) preferentially stabilizes an inward pointing, P-, polarization. Mirror electron microscopy measurements of the domain ordering confirm the theoretical results.     

Observation of disorder-induced 2D mott-hubbard states of the alkali-earth metal (Mg,Ba)-adsorbed Si(111) surface

We report evidence of a disorder-driven Mott-Hubbard-type localization on the alkali-earth metal (AEM) (Mg,Ba)-adsorbed Si(111)-(7x7) surface. The clean metallic Si(111) surface is found to undergo a two-dimensional (2D) metal-insulator transition as randomly distributed AEM adsorbates cause disorder on the surface. A well-defined electron-energy-loss peak unique to the insulating phase is attributed to an interband excitation between the split Hubbard bands originated from a metallic surface band at Fermi energy. A quantitative analysis of the loss peak reveals that the AEM-induced insulating surfaces are of a Mott-Hubbard type driven essentially by disorder.     

Lattice dynamics and the ferroelectric and antiferrodistorsive instabilities in a bulk crystal and thin films of SrZrO3

The lattice dynamics and energies of phases related to antiferrodistorsive and ferroelectric distortions of bulk crystals and thin films of the SrZrO₃ crystal have been calculated within the framework of the ab initio model of an ionic crystal. In the case of a bulk crystal, it has been found that the most energetically favorable phases are related to antiferrodistorsive lattice distortions. Ferroelectricity in the SrZrO₃ crystal is suppressed by structural lattice distortions. In the case of thin films, it has been found that the ferroelectric instability is retained after the ??rotation?? of the oxygen octahedron and the film remains polar both in the case of a free surface and with the inclusion of the SrTiO₃ substrate in the calculation. The spontaneous polarization of thin films of different thicknesses in the ferroelectric phase has been calculated.     

Strain dependence of polarization and piezoelectric response in epitaxial BiFeO3 thin films

Epitaxial strain has recently emerged as a powerful means to engineer the properties of ferroelectric thin films, for instance to enhance the ferroelectric Curie temperature (T(C)) in BaTiO(3). However, in multiferroic BiFeO(3) thin films an unanticipated strain-driven decrease of T(C) was reported and ascribed to the peculiar competition between polar and antiferrodistortive instabilities. Here, we report a systematic characterization of the room-temperature ferroelectric and piezoelectric properties for strain levels ranging between -2.5% and +1%. We find that polarization and the piezoelectric coefficient increase by about 20% and 250%, respectively, in this strain range. These trends are well reproduced by first-principles-based techniques.     

Polarization relaxation induced by a depolarization field in ultrathin ferroelectric capacitors

Time-dependent polarization relaxation behavior induced by a depolarization field E(d) was investigated on high-quality ultrathin SrRuO3/BaTiO3/SrRuO3 capacitors. The E(d) values were determined experimentally from an applied external field to stop the net polarization relaxation. These values agree with those from the electrostatic calculations, demonstrating that a large E(d) inside the ultrathin ferroelectric layer could cause severe polarization relaxation. For numerous ferroelectric devices of capacitor configuration, this effect will set a stricter size limit than the critical thickness issue.     

Stress-induced phase transition in ferroelectric domain walls of BaTiO3

The seminal paper by Zhirnov (1958 Zh. Eksp. Teor. Fiz. 35 1175-80) explained why the structure of domain walls in ferroelectrics and ferromagnets is drastically different. Here we show that the antiparallel ferroelectric walls in rhombohedral ferroelectric BaTiO(3) can be switched between the Ising-like state (typical for ferroelectrics) and a Bloch-like state (unusual for ferroelectric walls but typical for magnetic ones). Phase-field simulations using a Ginzburg-Landau-Devonshire model suggest that this symmetry-breaking transition can be induced by a compressive epitaxial stress. The strain-tunable chiral properties of these domain walls promise a range of novel phenomena in epitaxial ferroelectric thin films.