Probing the role of single defects on the thermodynamics of electric-field induced phase transitions

The kinetics and thermodynamics of first order transitions are universally controlled by defects that act as nucleation sites and pinning centers. Here we demonstrate that defect-domain interactions during polarization reversal processes in ferroelectric materials result in a pronounced fine structure in electromechanical hysteresis loops. Spatially resolved imaging of a single defect center in multiferroic BiFeO3 thin film is achieved, and the defect size and built-in field are determined self-consistently from the single-point spectroscopic measurements and spatially resolved images. This methodology is universal and can be applied to other reversible bias-induced transitions including electrochemical reactions.     

Dimer structures formed in smectic films by inclusions with parallel and antiparallel topological dipole moments

Self-organization of cholesteric and nematic droplets in smectic free-standing films is investigated. Strong tangential anchoring at the droplet boundary leads to nucleation of a topological defect and formation of a topological dipole from the droplet and the defect. The interaction between droplets results in assembly of the droplets in dimers and line chains. Topological dipoles of the droplet-defect pairs can be oriented in dimers with polar (ferroelectric) and nonpolar (antiferroelectric) ordering. In this paper we found novel dimer structures formed by droplets with different handedness of the c-director field near the droplets. Depending on the relative magnitude of the droplet dipoles the resulting structure may be dipolar or quadrupolar. Formation of antiferroelectric dimers is discussed on the basis of electromagnetic analogy.     

Defect-induced instability of the surface layer involving static coupled Lamb and Rayleigh waves as a universal mechanism of the formation of an ensemble of nanodot nucleation centers

It is demonstrated that a stressed flat surface nanolayer saturated with mobile point defects exhibits a threshold (with respect to the defect concentration or mechanical stress) transition to a periodic spatially bent state with a simultaneous formation of the spatially periodic defect pile-ups at the extrema of the spontaneously emerging surface relief. In this case, the layer deformation corresponds to the displacements in a static bending Lamb wave and the deformation of the underlying elastic continuum corresponds to the displacements in the static Rayleigh wave. For the first time, we demonstrate that the analysis simultaneously involving the nonlocal character of the defect interaction with the lattice atoms and both (normal and lateral) defect-induced forces that cause the bending of the surface layer yields two maxima on the curve of the instability growth rate versus the period of the generated relief. This corresponds to the experimentally observed two scales of the surface relief modulation upon the laser and ion irradiation of semiconductors. Based on the results obtained, we propose a cooperative defect-deformational (DD) mechanism for the formation of an ensemble of the nanoparticle nucleation centers above the critical levels of the stress or the defect concentration. An approach to the calculation of a bimodal distribution function of the nanoparticle nucleation centers with respect to their size is adequately developed to the DD mechanism of nucleation which represents the distribution function in terms of the growth rate. The calculated results are compared with the experimental data for the molecular beam epitaxy of the nanodot ensemble and the pulsed laser nanostructuring of a solid surface.     

Ultrashort microwave pulses generated due to three magnon interactions

Density-functional theory is applied to the interaction site model to study gas-liquid nucleation in a partially immiscible binary system consisting of spherical Lennard-Jones atoms (monomers) and dumbbell molecules of two Lennard-Jones atoms (dimers). Increased interaction anisotropy between the dimer sites and monomers is shown to result in mutual enhancement of nucleation. Critical nuclei with a lamellar structure are observed at high dimer activities.     

钨酸铅晶体的本征色心和辐照诱导色心

根据钨酸铅晶体(PbWO4,简称PWO)的缺陷化学和晶体结构特点,用光吸收谱、广延X射线吸收精细结构(EXAFS)和精密X射线衍射(XRD)方法对高温退火后PWO晶体进行微结构研究,获得了其退火前后缺陷变化的情况,据此提出生成态(asgrown)晶体中350nm本征色心吸收带起源于V-F空穴心,并指出PWO中紫外区色心吸收带的强度取决于晶体中铅空位和氧空位浓度之差:[VPb]-[VO];然后,结合晶体在紫外光(UV)辐照过程中色心的转化规律和偏振吸收谱的实验结果,提高420nm辐照诱导色心吸收带起源于V0F双空穴心.并对所提出的PWO晶体色心模型的合理性进行了讨论. 关键词： 钨酸铅 本征色心 辐照诱导色心 色心模型     

CO adsorption on Rh, Pd and Ag atoms deposited on the MgO surface: a comparative ab initio study

The adsorption properties of CO molecules adsorbed on Rh, Pd, and Ag atoms supported on various sites of the MgO surface have been studied by means of a density functional cluster model approach. The metal atoms are stabilized with different binding energies on the regular and morphological defect sites of the surface. Among others we considered oxide anions, neutral and charged anion vacancies (F centers) located at terraces, steps, edges, and corners. CO is used as a probe molecule to characterize where the metal atoms are located. This is done by analyzing how the metal-CO binding energy and the C-O stretching frequency change as function of the substrate site where the metal atom is bound.     

Theoretical Studies of g Factors and Defect Structures for Cubic, Tetragonal, and Orthorhombic Fe+ Centers in Alkali Halides MX (M=Li,Na;X=F,Cl)

The EPR g factors for cubic, tetragonal and orthorhombic Fe centers in alkali halides MX (M = Li, Na;X = F, Cl) are calculated from second-order perturbation formulas of g factors based on cluster approach for 3d7 ions in three symmetries. From calculations, the g factors of these Fe centers in MX crystals are reasonably explained and the defect structural data for the tetragonal and orthorhombic Fe centers are estimated. The results are discussed.     

Geometric characterization of a singly charged oxygen vacancy on a single-crystalline MgO(001) film by electron paramagnetic resonance spectroscopy

Electron paramagnetic resonance spectra of singly charged surface oxygen vacancies (F or color centers) formed by electron bombardment on a single-crystalline MgO film under UHV conditions are reported. The embedding of the defect in a well-defined geometrical environment allows not only for the determination of the magnetic quantities but also, in conjunction with STM studies, for the geometrical assignment of the observed signal to color centers located on the edges of the MgO film.     

The effect of surface cracks and irregularities on reverse domain nucleation in Nd2Fe14B

Numerical modelling of a single spheroidal grain of Nd₂Fe₁₄B has shown that local demagnetization fields at defect sites are enhanced by up to 6kOe over those of a perfect spheroidal grain. The defect sites appeared to act as nucleation centres of reverse magnetic domains.     

XPS study on the evaporation of gold submonolayers on carbon surfaces

We investigated the early nucleation stages of evaporated gold submonolayers on different carbon surfaces (pristine HOPG, argon-ion irradiated HOPG and amorphous carbon). Gold core-level and valence band spectra were measured by monochromatised X-ray photoelectron spectroscopy (MXPS). The Au 4f spectra for the lowest coverages (0.1 Å equivalent thickness) on irradiated HOPG and amorphous carbon surprisingly exhibited two well-separated doublets. We attribute this phenomenon to a bimodal particle size distribution caused by gold atom pinning at carbon defect sites. Deposition at elevated temperatures (on irradiated HOPG) opens a possibility to grow particles preferentially on defect sites. The influence of carbon surface defects on the cluster morphology was checked by SEM imaging. These results are interesting for future applications as they help to improve control over metal nanodots growth.     

Theoretical Studies of g Factors and Defect Structures for Cubic, Tetragonal, and Orthorhombic Fe^＋ Centers in Alkali Halides MX （M=Li, Na; X = F, C1）

The EPR 9 factors for cubic, tetragonal and orthorhombic Fe^＋ centers in alkali halides MX （M= Li, Na; X = F, CI） are calculated from second-order perturbation formulas of g factors based on cluster approach for 3d^7 ions in three symmetries. From calculations, the g factors of these Fe^＋ centers in MX crystals are reasonably explained and the defect structural data for the tetragonal and orthorhombic Fe^＋ centers are estimated. The results are discussed.     

Theoretical explorations on the armchair BN nanotube with defects

A systematic study of armchair boron nitride nanotubes (BNNTs) with defects has been carried out within density functional theory. The effect brought by the defects is localized. The defect sites have major contribution to the frontier molecular orbital and change the conductivity of the BNNTs. The defect sites are reactive centers. The substitution of boron with carbon enhances the field emission of the tubes. Doping or vacancy defect creates active center on nanotubes, thus broadening the applications of nanotubes in chemistry and material sciences through functionalization.     

Interaction of Magnetization Centers of Different Signs as the Cause of the Nonmonotonic Field Dependence of the Domain Wall Velocity in Synthetic Pt/Co/Ir/Co/Pt Ferrimagnets

The purpose of the work is to find the dependence of growth rate of magnetization centers of various types on their surrounding by other nucleation centers in a synthetic Pt/Co/Ir/Co/Pt ferrimagnet with perpendicular magnetic anisotropy. The following four types of nucleation centers exist in a sample with two ferromagnetic layers of different thicknesses: P+ centers correspond to the regions where the magnetizations of the thick and thin Co layers are directed along an applied field (↑↑); P– centers are the regions where the magnetizations of the layers are opposite to an applied field (↓↓); and AP+ and AP– centers correspond to the regions where the magnetizations of the thick and thin Co layers are opposite to each other and the total magnetic moment is along (↑↓) or opposite to (↓↑) an applied field, respectively. P– nucleation centers are found to be surrounded by AP+ regions in any field and exhibit a monotonic field dependence of their boundary. The field dependence of the growth rate of AP– nucleation centers is nonmonotonic since, as the field increases, they are surrounded by AP+ nucleation centers, AP+ and P– regions, and only P– nucleation centers in strong fields.     

A theoretical study of structural and electronic properties of a missing dimer defect on Si- and C-terminated SiC(0 0 1)

We present a theoretical study of the geometrical and electronic properties of the C- and Si-terminated -SiC(0 0 1) surfaces in the vicinity of the missing dimer defect. The experimental results suggest that the atomic structures of these two surfaces may be considerably modified by external stress. In our present study we have considered the possible influence of this factor on the surface geometry of both systems. We have shown that the structural differences between the C- and Si-terminated surfaces lead to their different behaviour in the presence of a missing dimer and applied stress. In the case of the C-terminated c(2×2) surface, the missing dimer defect causes the buckling of the adjacent carbon dimers lying in the line of the defect (dimer atoms adjacent to the defect have vertical positions lower by 0.18 Å). This effect becomes more pronounced in the presence of compressive stress — the stress of 8% leads to the buckling of these two dimers of around 0.5 Å. The vertical positions of silicon atoms located directly below the defect were increased by 0.2 Å. We have also found that the missing dimer influences the structure of the carbon dimers on the neighbouring lines of dimers. Contrary to the C-terminated surface, the missing dimer defect on the Si-terminated SiC(0 0 1)-p(2×1) surface remains neutral for silicon dimers located in the line of defect, i.e. the dimers do not change their geometrical properties in unstrained structure nor in the presence of a tensile stress. On the other hand, this defect modifies considerably the geometry of the dimers from the two neighbouring lines of dimers by reducing their bond lengths and vertical positions. Changes in the geometrical properties of the second neighbour dimers (with respect to the defect) in these two lines are also noticeable. Moreover, we have found that the presence of a missing dimer modifies significantly the positions of the adjacent subsurface carbon atoms.     

Transformations of C-type defects on Si(100)-2 × 1 surface at room temperature – STM/STS study

We present a scanning tunneling microscopy study of the C-type defects on the Si(100)-2 × 1 surface and their transformations into other defect forms at room temperature. A model of the C defect as a dissociated water molecule was adopted for interpretation of the observed transformations. We explained the transformations by hopping the H or OH between bonding sites on Si dimers. Newly, the most stable defect form, corresponding to the H and hydroxyl group adsorbed on the same dimer, is reported. Real time observations provided an explanation for the defect C2-C2 described earlier. A reversible transition of this defect into another form, not revealed yet, is presented. Electronic structure of the observed defects is studied by means of scanning tunneling spectroscopy. Measured spectra show semiconducting character of the C defect. Spectra of the other defect forms are discussed.     

Surface Nucleation Kinetics on Active Centers from Supersaturated Vapor

The formation of nuclei from a supersaturated vapor on a surface with active centers is treated within standard self-consistent classical model with exhaustion of active centers. Basic characteristics of nucleation process (total number of nuclei, nucleation rate, time lag and size distribution of nuclei) are determined by numerical solution of kinetic equations. It is shown that standard approach to nucleation on active centers based on Avrami model coincides with our approach in the values of time lag of nucleation process, but it differs in the total number of nuclei.     

Effects of an anomalous strain accommodating behavior of Si ad-dimers on the nucleation of regular and defect islands

Computer simulations, based on a theory relating the binding energy of a Si adatom on the Si(001)-2 × 1 surface to the excess strain and local configuration energy resulting from the presence of the adatom, have been performed to study the nucleation of ID dimer strings. We found an anomalous strain accommodating behavior of ad-dimers that explains the greater stability of adatoms adsorbed at ad-dimer sites. This behavior also accounts for the occurrence of non-lattice binding sites, which are closely connected to defect formation at low temperatures, in the neighborhood of ad-dimers.     

重离子辐照SiO2通过能量损失产生发光色心的研究

通过红外光谱和荧光发射光谱分别对600 keV、4 MeV和5 MeV Kr离子辐照的SiO2进行发光特性的研究。在低能量辐照体系中,简单色心(F2色心)的形成在损伤过程中占据主导地位,其主要诱发蓝光发射带;在高能离子辐照条件下,离子径迹上的能量密度较大,因此缺陷浓度的增大产生了一些缺陷团簇和离子径迹,形成了复杂的色心(F2+和F3+色心等)并诱发了强烈的绿光发射带和红光发射带。该实验结果与能量损失过程中统一热峰理论模型(一个综合的基于电子能损与核能损的非弹性碰撞模型和弹性碰撞模型)的模拟结果能够很好地吻合,表明在keV～MeV能区上存在电子能损过程与核能损过程的协同效应。     

Polaron jump conductivity in films of vitreous oxide semiconductors in an intense DC magnetic field

Experimental data on the field dependence of static conductivity of vitreous oxide semiconductors (VOS) based on V₂O₅ are analyzed. To describe the effects of an intense electric field a model is proposed which assumes localization of small radius polarons (SRP) by the Coulomb field of positively charged defect centers (the bound SRP model). Within the framework of this model an expression is obtained which defines the flux density of charge carriers (J) in an electric field of arbitrary intensity (F), which in the limit of an intense electric field leads to a linear dependence of ln J on F^1/2. With the parameters found for the model the calculated function J(F) agrees well with experimental dependences of current upon F for VOS films in the vanadium-borate system. The most probable nature of defect centers in the given materials is anion (oxygen) vacancies. The results obtained agree with a previous analysis of field dependence of low-frequency dielectric permittivity also based on the bound SRP model.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 95–100, December, 1988.     

Role of defects in the kink mechanism of state switching

State switching occurring via the kink mechanism in linear systems has been described considering the effect of defects forming centers of enhanced kink pair nucleation and obstacles for kink propagation. An equation describing the switching kinetics has been derived considering the stochastic nature of kink pair formation in time and the random distribution of defects in space. Using this equation, the average switching time has been calculated as a function of the defect density and delay times caused by defects, and the ranges of parameters with domination of this or that defect type have been determined. The theory has been applicable to magnetic nanowires, dislocations, biological macromolecules, and many other systems.