Stability of a ferroelectric phase with electrical domains in multilayers

Multilayer BaTiO3-SrTiO3 and PbTiO3-SrTiO3 structures with different electrical domain states are studied using a Landau-Ginzburg-Devonshire free energy. Polarizations in the layers are computed for multi-domain and single-domain states where the paraelectric-to-ferroelectric volumetric layer ratio is varied. It is shown that the ferroelectric layers with electrical domains are thermodynamically more stable than the single-domain ferroelectric state. High domain wall energies result in the stabilization of the paraelectric state in the ferroelectric layers for large depolarizing fields. It is concluded that the stability of single-domain state ferroelectric layers correspond to a very small paraelectric-to-ferroelectric ratio after which multi-domain ferroelectric state is favored

Phase transitions in ferroelectric-paraelectric superlattices

Within the phenomenological Landau–Ginzburg–Devonshire theory, we discuss the paraelectric-ferrolectric transition in superstructures consisting of ferroelectric and paraelectric layers of equal thickness. The polar axis of the ferroelectric is perpendicular to the layer plane as expected in fully strained BaTiO3/SrTiO3 superstructures on SrTiO3 substrates with pseudomorphic electrodes. We concentrate on the electrostatic effects and do not take into account the boundary conditions other than the electrostatic ones. We find that when the ferroelectric phase transition in the superstructures is into a multidomain state, both its temperature and its character, i. e., the profile of the polarization appearing at the phase transition is strongly influenced by the nature of the near-electrode region. This is also the case for the layer thickness separating the single-and multidomain regimes of the transition. Such a finding makes us question the idea that these superstructures can be thought of as infinite systems, i.e., periodic superstructures similar to a crystal. The irrelevance of this idea in certain conditions is demonstrated by comparing the phase transitions in two different superstructures consisting of ferroelectric and paraelectric layers of the same thickness. In one of them, the ferroelectric layer is in immediate contact with an ideal metallic electrode, whereas at the other boundary, it is the paraelectric layer that is in contact with the electrode. In another superstructure, one paraelectric layer is split in two equal parts which are placed as the first and last layer between the electrodes and the ferroelectric layers which are closest to the electrodes. We show (with some formal reservations) that the phase transition temperature in the first superstructure can be over 100 °C more than in the second one if the material parameters of BaTiO3/SrTiO3 are used for the estimations. Moreover, the profile of the polarization arising at the phase transition is inhomogeneous along the superstructure and has the maximum amplitude in the ferroelectric layer contacting the electrode. We argue that this situation is general and results in smearing of the phase transition anomalies for the layer thicknesses corresponding to multidomain transitions. The work is mainly analyical but numerical methods have been used to support some statements that have been put forward as hypotheses

Recurrent Solitary Clavicular Parosteal Osteoma 25 Years after Surgical Resection

Osteoma is a slowly growing, asymptomatic, benign osteogenic tumor. An extremely rare case of clavicular parosteal osteoma (PO) is reported. A 46-year-old female patient was treated with marginal resection after an open biopsy for a large, firm symptomatic mass originating from the middle part of the left clavicle, which recurred 25 years after surgery. In the fifth year postoperatively, the patient was followed up with a full range of motion of the left shoulder without any problem in her daily life. In this case report, the clinical course, imaging findings, diagnosis, and long-term results of a rare case of parosteal osteoma recurrence of the clavicle are described for the first time in the literature

Low-temperature monoclinic phase in epitaxial (001) barium titanate on (001) cubic substrates

The possibility of the existence of a low temperature monoclinic phase in epitaxial (001) BaTiO3 films on a (001) compressive substrate is analyzed theoretically and compared to recent experimental data from literature. There is good agreement between the theoretical findings and the experimentally observed behavior. The formation of the monoclinic phase arises from the point group reduction due to the rotation of the polarization vector commensurate with the variations in the in-plane strain state

Phase transformation characteristics of barium strontium titanate films on anisotropic substrates with (001)//(001) epitaxy

The role of anisotropic misfit strains on the spontaneous polarization of (100) oriented Ba0.6Sr0.4TiO3 thin films on (100) orthorhombic substrates is theoretically analyzed. A modified thermodynamic model is utilized to evaluate the equilibrium polarization values as a function of the anisotropic misfit strains. Results show that ferroelectric phases that cannot be observed in single-crystal Ba0.6Sr0.4TiO3 can be stabilized due to the reduction in the symmetry induced by the anisotropic strain state

Very large dielectric response from ferroelectric nanocapacitor films due to collective surface and strain relaxation effects

Dependence of the dielectric response of ferroelectrics on defect types, particularly those with long range strain fields in confined geometries have been often mentioned, especially in interpreting experimental results in films. However, detailed discussions on the mechanisms with which defects alter properties, particularly in the presence of interfaces imposing certain boundary conditions, are seldom made. Here, we studied the thickness dependence of transition temperatures and dielectric response of Metal/BaTiO3/Metal ferroelectric nanocapacitor structures grown on SrTiO3 using a phenomenological approach accounting for the equations of electrostatics and semiconductors. Relaxation of the misfit strain via misfit dislocations amplify the surface effects in films below a critical thickness and favor electrical domains leading to very large dielectric responses in this regime. Thin film structures with relaxed misfit strain in this work are fully-depleted in the presence of moderate densities of impurities (~1025 m-3). This is due to the reduction of polarization amplitude parallel to the film normal and its mplications for near-micron thick films are discussed. Consequently, the misfit dislocation sites have nearly no free carrier localization, making the role of these sites on leakage currents highly questionable. Dielectric response of intrinsic thicker films (>40 nm) are mostly under the influence of strain relaxation only with minimal interface impact in the limit of ideal electrodes. Our results point out that control of the dislocation density can lead to non-conventional functionalities in ferroelectric thin film capacitors via electromechanical coupling of properties to structure and domain stabilization

Invariant Subspace Theorems For Families Of Operators On Banach Spaces And Banach Lattices

Tez (Doktora) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 2006Thesis (PhD) -- İstanbul Technical University, Institute of Science and Technology, 2006Bu çalışmada, Banach uzayları üzerinde tanımlı doğrusal sınırlı operatörlerin oluşturduğu bazı aileler ile Banach örgüleri üzerinde tanımlı pozitif operatörlerin oluşturduğu bazı aileler için değişmez altuzay problemi incelenmiştir. İlk olarak, ortomorfizmaları ayırma özelliğine sahip bir Banach örgüsü üzerinde tanımlı yerel yarınilpotent ve kompakt-yakın olan her sıfırdan farklı operatörün, aşikar olmayan kapalı değişmez bir ideale sahip olduğu gösterilmiştir. Ayrıca bu sonuç, kompakt-yakınlık kavramından faydalanalarak, “ortomorfizmaları ayırma özelliğine sahip bir Banach örgüsü üzerinde tanımlı pozitif operatörlerden oluşan her yerel sonlu yarınilpotent aile, bu ailenin komutantı bir kompakt pozitif operatör tarafından bastırılan bir operatöre göre baskın olan bir pozitif operatör içeriyorsa, aşikar olmayan ortak kapalı değişmez bir ideale sahiptir”, şeklinde genelleştirilmiştir. İkinci olarak, Schauder tabanına sahip bir Banach uzayı üzerinde tanımlı sürekli pozitif operatörlerden oluşan yerel sonlu yarınilpotent çarpımsal her yarıgrubun, aşikar olmayan kapalı değişmez bir altuzaya sahip olduğu gösterilmiştir. Daha sonra bu sonuç, zayıf yarınilpotentlik kavramı kullanılarak, Markushevich tabanına sahip topolojik vektör uzaylarına genişletilmiştir. Son olarak, Banach uzayları üzerinde tanımlı doğrusal sınırlı operatörlerden oluşan birlikte kompakt kümeler, değişmez altuzay problemi ile bağlantılı olarak ele alınmıştır. Birlikte kompakt kümeler için, ortak spektral yarıçap ve bunun yerel versiyonuna göre, bazı değişmez altuzay teoremleri verilmiştir. Ayrıca, birlikte kompakt kümelerin, özel bir durumda, Berger-Wang formülünü gerçeklediği gösterilmiştir.In this work, the invariant subspace problem is studied for certain families of linear bounded operators on Banach spaces. We also consider families of positive operators on Banach lattices. First, we prove that every non-zero locally quasinilpotent compact-friendly operator on a Banach lattice with separating orthomorphisms has a non-trivial closed invariant ideal. We then generalize it by using the concept of compact-friendliness as follows: Every locally finitely quasinilpotent family of positive operators on a Banach lattice with separating orthomorphisms, whose commutant contains a positive operator which dominates an operator which is dominated by a compact positive operator, has a common non-trivial closed invariant ideal. Secondly, we prove that a locally finitely quasinilpotent multiplicative semigroup of positive continuous operators on a Banach space with a Schauder basis has a non-trivial closed invariant subspace, and then, we generalize our result to topological vector spaces with Markushevich basis by using the notion of weakly quasinilpotence. Finally, collectively compact sets of linear bounded operators on infinite dimensional Banach spaces are studied in connection with the invariant subspace problem. We give some invariant subspace results for these sets with respect to the joint spectral radius and its local version. It is also shown, in a special case, that any collectively compact set of operators satisfies the Berger-Wang formula.DoktoraPh