arXiv subjects

Anna N. Morozovska

Publications and source records attributed to Anna N. Morozovska.

At least 127 records · Page 7Linked to original sources

arXiv2018

Hidden Symmetry of Flexoelectric Coupling

Considering the importance of the flexoelectric coupling for the physical understanding of the gradient-driven couplings in mesoscale and nanoscale solids, one has to determine its full symmetry and numerical values. The totality of available experimental and theoretical information about the flexocoupling tensor symmetry (specifically the amount of measurable independent components) and numerical values is contradictory indicating on a limited understanding of the coupling properties. However the discrepancy between the theory and experiment can be eliminated by consideration all possible inner symmetries of the flexocoupling tensor and physical limits on it components values. Specifically this study reveals the inner "hidden" symmetry of the static flexoelectric tensor that allows minimizing the number of its independent components to the numbers observed experimentally. Revealed hidden symmetry leads to nontrivial physical sequences, namely it affects on the upper limits of the static flexocoupling constants. Also we analyze the dynamic flexoelectric coupling symmetry and established the upper limits for the numerical values of its components. These results can help to understand and quantify the fundamentals of the gradient-type couplings in different solids.

cond-mat.mtrl-sci↗

arXiv2018

Anomalies Of Phase Diagrams And Physical Properties Of Antiferrodistortive Perovskide Oxides

The influence of rotomagnetic (RM), rotoelectric (RE) and magnetoelectric (ME) coupling on phase diagram and properties of antiferrodistortive (AFD) perovskite oxides was reviewed. The main examples we consider in the review are typical AFD perovkites, such as incipient ferroelectrics EuTiO3, SrTiO3, EuxSr1-xTiO3, multiferroic BiFeO3 and Bi1-xRxFeO3 (x = La, Nd). The strong influence of RM, RE and ME couplings on the physical properties and phase diagrams including antiferromagnetic (AFM), ferroelectric (FE) and structural AFD phases has been revealed in the framework of Landau-Ginzburg-Devonshire (LGD) theory, as well as the prediction of novel (double and triple) multiferroic phases has been demonstrated. In the review we are especially focused on (a) the possibility to induce FM (FE) phase in EuTiO3 (as well as in other paraelectric AFM oxides) by the application of an electric (magnetic) field due to the ME coupling; (b) the analysis of the size effects and novel phases in EuxSr1-xTiO3 nanosystems, where the LGD predicts the presence of the triple AFD-FE-FM(AFM) phase at low temperatures; (c) the appearance of improper spontaneous polarization and pyroelectricity in the vicinity of antiphase domain boundaries, structural twin walls, surfaces and interphases in the AFD phase of non-ferroelectric SrTiO3 induced by the flexoelectricity and rotostriction; (d) the occurrence of low symmetry monoclinic phase with in-plane FE polarization in thin strained EuxSr1-xTiO3 films and its stabilization over wide temperature range by AFD oxygen octahedron tilts due to flexoelectric and rotostriction coupling; (e) discussion of a surprisingly strong size-induced increase of AFM transition temperature caused by the joint action of RM coupling with elastic stress accumulated in the intergrain spaces of BiFeO3 dense ceramics.

cond-mat.mtrl-sci↗

arXiv2018

Analytical description of domain morphology and phase diagrams of ferroelectric nanoparticles

Analytical description of domain structure morphology and phase diagrams of ferroelectric nanoparticles is developed in the framework of Landau Ginzburg Devonshire approach. To model realistic conditions of incomplete screening of spontaneous polarization at the particle surface, it was considered covered by an ultra-thin layer of screening charge with finite screening length. The phase diagrams, calculated for spherical Sn2P2S6 nanoparticles in coordinates "temperature vs surface screening length" by finite element modeling, demonstrate the emergence of polydomain region at the tricritical point and its broadening with increasing the screening length for the particle radius over a critical value. Metastable and stable labyrinthine domain structures exist in Sn2P2S6 nanoparticles with radius 8 and 10 nm and more, similarly to the case of CuInP2S6 nanoparticles considered previously. We derived simple analytical expressions for the boundaries between paraelectric, single-domain and poly-domain ferroelectric phases, tricritical point and the necessary condition for the appearance of labyrinthine domains, and demonstrated their high accuracy in comparison with finite element modeling results. Analytical description shows that phase diagrams and domain morphologies, which are qualitatively similar to the ones calculated in this work, can be expected in other ferroelectric nanoparticles covered by the screening charges, being rather different for the ferroelectrics with the first and second order ferroelectric-paraelectric transitions respectively.

cond-mat.mtrl-sci↗

arXiv2018

Labyrinthine domains in ferroelectric nanoparticles: a manifestation of gradient-driven morphological transition

In the framework of the Landau-Ginzburg-Devonshire (LGD) approach we studied finite size effects of the phase diagram and domain structure evolution in spherical nanoparticles of uniaxial ferroelectric. The particle surface is covered by a layer of screening charge characterized by finite screening length. The phase diagram, calculated in coordinates "particle radius vs. screening length" has a wide region of versatile poly-domain structures separating single-domain ferroelectric and nonpolar paraelectric phases. Unexpectedly, we revealed a region of stable irregular labyrinthine domains in the nanoparticles of uniaxial ferroelectric CuInP2S6 with the first order paraelectric-ferroelectric phase transition. We established that the origin of labyrinthine domains is the mutual balance of LGD, polarization gradient and electrostatic energies. The branching of the domain walls appears and increases rapidly when the polarization gradient energy decreases below the critical value. Allowing for the generality of LGD approach, we expect that the gradient-induced morphological transition can be the source of labyrinthine domains appearance in many spatially-confined ferroics with long-range order parameter, including relaxors, ferromagnetics, antiferrodistortive materials and materials with incommensurate ferroic phases.

cond-mat.mtrl-sci↗

arXiv2018

Control of polarization hysteresis temperature behavior by interfacial screening in thin ferroelectric films

Ferroelectric interfaces are unique model objects for fundamental studies of polar surface properties such as versatile screening mechanisms of spontaneous polarization by free carriers and possible ion exchange with ambient media. The effect of ionic adsorption by electrically-open (i.e. non-electroded) ferroelectric surface on polarization reversal in the ferroelectric had been investigated experimentally and theoretically, however the effect influence on the temperature behaviour of polarization hysteresis remains unexplored. Also the comparative theoretical analysis of the ferroelectric hysteresis for linear and nonlinear electronic screenings, and strongly nonlinear ionic screening of the spontaneous polarization was absent. In this work we study the free energy relief of a thin ferroelectric film covered by a screening charge layer of different nature and ultra-thin gap separating the film surface from the top electrode and calculate hysteresis loops of polarization and screening charge at different temperatures in the considered system. The dependence of the screening charge density on electric potential was considered for three basic models, namely for the linear Bardeen-type model of electronic surface states (BS), nonlinear Fermi-Dirac (FD) density of states describing two-dimensional electron gas at the film-gap interface, and strongly nonlinear Stephenson-Highland (SH) model describing the surface charge density of absorbed ions by Langmuir adsorption isotherms. Appeared that BS, FD and SH screening charges, which properties are principally different, determine the free energy relief (including polarization values corresponding to the free energy minima at different applied voltages) and control the shape of polarization hysteresis loops at different temperatures.

cond-mat.mes-hall↗

arXiv2018

Nanoferroics: state of art, gradient driven couplings and advanced applications (Authors' review)

Ferroics and multiferroics are unique objects for fundamental physical research of complex nonlinear processes and phenomena, which occur in them in micro and nanoscale. Due to the possibility of their physical properties control by size effects, nanostructured and nanosized ferroics are among the most promising for advanced applications in nanoelectronics, nanoelectromechanics, optoelectronics, nonlinear optics and information technologies. The review discuss and analyze that the thickness of the strained films, the size and shape of the ferroic and multiferroic nanoparticles are unique tools for controlling their phase diagrams, long range order parameters, magnitude of susceptibility, magnetoelectric coupling and domain structure characteristics at fixed temperature. Significant influence of the flexochemical effect on the phase transition temperature, polar and dielectric properties is revealed for thin films and nanoparticles. Obtained results are important for understanding of the nonlinear physical processes in nanoferroics as well as for the advanced applications in nanoelectronics.

cond-mat.mtrl-sci↗

arXiv2018

Temperature behavior of graphene conductance induced by piezoelectric effect in ferroelectric substrate

Graphene on ferroelectric structures can be promising candidates for advanced field effect transistors, modulators and electrical transducers, providing that research of their electrotransport and electromechanical performances can be lifted up from mostly empirical to prognostic theoretical level.Recently we have shown that alternating piezoelectric displacement of the ferroelectric domain surfaces can lead to the alternate stretching and separation of graphene areas at the steps between elongated and contracted domains, and the conductance of graphene channel can be increased essentially at room temperature, because electrons in the stretched section scatter on acoustic phonons.The piezoelectric mechanism of graphene conductance control requires systematic studies of the ambient condition impact on its manifestations. This theoretical work studies in details the temperature behavior of the graphene conductance changes induced by piezoelectric effect in a ferroelectric substrate with domain stripes.We revealed the possibility to control graphene conductance (that can change up to 100 times for PZT ferroelectric substrate) by tuning the ambient temperature from low values to the critical one for given gate voltage and channel length.Also we demonstrate the possibility to control graphene conductance changes up to one hundred of times by tuning the gate voltage from 0 to the critical value at a given temperature and channel length. Obtained results can be open the way towards graphene on ferroelectric applications in piezoresistive memories operating in a wide temperature range.

physics.app-ph↗

arXiv2018

Rotomagnetic coupling in fine grained multiferroic BiFeO3: Theory and experiment

Using Landau-Ginzburg-Devonshire (LGD) theory for BiFeO3 dense fine grained ceramics with quasi spherical grains and nanosized inter grain spaces enriched by elastic defects, we calculated a surprisingly strong size-induced increase of the AFM temperature caused by the joint action of rotomagnetic and magnetostrictive coupling. Notably that all parameters included in the LGD functional have been extracted from experiments, not assumed. Complementary we performed experiments for dense BiFeO3 ceramics, which revealed that the shift of antiferromagnetic transition to 690 K instead of 645 K for a single crystal. To explain theoretically the result, we consider the possibility to control antiferromagnetic state of multiferroic BiFeO3 via biquadratic antiferrodistortive rotomagnetic, rotoelectric, magnetostrictive and magnetoelectric couplings. According to our calculations the highest is the rotostriction contribution, the magnetostrictive and electrostriction contributions appeared smaller.

cond-mat.mtrl-sci↗

arXiv2018

Impact of the domain structure in ferroelectric substrate on graphene conductance (authors' review)

Review is devoted to the recent theoretical studies of the impact of domain structure of ferroelectric substrate on graphene conductance. An analytical description of the hysteresis memory effect in a field effect transistor based on graphene-on-ferroelectric, taking into account absorbed dipole layers on the free surface of graphene and localized states on its interfaces is considered. The aspects of the recently developed theory of p-n junctions conductivity in a graphene channel on a ferroelectric substrate, which are created by a 180-degree ferroelectric domain structure, are analyzed, and cases of different current regimes from ballistic to diffusion one are considered. The influence of size effects in such systems and the possibility of using the results for improving the characteristics of field effect transistors with a graphene channel, non-volatile ferroelectric memory cells with random access, sensors, as well as for miniaturization of various devices of functional nanoelectronics are discussed.

cond-mat.mtrl-sci↗

arXiv2018

Fixed Volume Effect on Polar Properties and Phase Diagrams of Ferroelectric Semi-ellipsoidal Nanoparticles

For advanced applications in modern industry it is very important to reduce the volume of ferroelectric nanoparticles without serious deterioration of their polar properties. In many practically important cases fixed volume (rather than fixed size) corresponds to realistic technological conditions of nanoparticles fabrication. The letter is focused on the theoretical study of the behavior of ferroelectric polarization, paramagnetoelectric coefficient and phase diagrams of semi-ellipsoidal nanoparticles with fixed volume V. Our approach combines the Landau-Ginzburg-Devonshire phenomenology, classical electrostatics and elasticity theory. Our results show that the size effects of the phase diagrams and polarization of semi-ellipsoidal BiFeO3 nanoparticles nontrivially depends on V. These findings provide a path to optimize the polar properties of nanoparticles by controlling their phase diagrams at a fixed volume.

cond-mat.mtrl-sci↗

arXiv2018

Integer Quantum Hall Effect in Graphene Channel with p-n Junction at Domain Wall in Ferroelectric Substrate

We revealed that 180 degree ferroelectrics domain walls (FDWs) in a ferroelectric substrate, which induce p-n junctions in a graphene channel, lead to the nontrivial temperature and gate voltage dependences of the perpendicular and parallel modes of the integer quantum Hall effect. In particular the number of perpendicular modes v, corresponding to the p-n junction across the graphene channel varies with gate voltage increase from small integers to higher fractional numbers, e.g.v=1, 1.5, 2, ... 5.1,..9.1, 23 in the vicinity of the transition from ferroelectric to paraelectric phase.These numbers and their irregular sequence principally differ from the sequence of fractional numbers v=1.5,2.5,... reported earlier. The origin of the unusual v-numbers is significantly different numbers of the edge modes, v1 and v2, corresponding to significantly different concentration of carriers in the left (n1) and right (n1) domains of p-n junction boundary. The concentrations n1 and n2 are determined by the gate voltage and polarization contributions, and so their difference originates from the different direction of spontaneous polarization in different domains of ferroelectric substrate. The difference between n1 and n2 disappears with the vanishing of spontaneous polarization in a paraelectric phase. The phase transition from the ferroelectric to paraelectric phase can take place either with the temperature increase (temperature-induced phase transition) or with the decrease of ferroelectric substrate thickness (thickness-induced phase transition).

cond-mat.mes-hall↗

arXiv2017

Nontrivial temperature behavior of the carrier concentration in the nanostructure "graphene channel on ferroelectric substrate with domain walls"

This work explores a nontrivial temperature behavior of the carriers concentration, which governs graphene channel conductance in the nano-structure "graphene channel on ferroelectric substrate" that is a basic element for FETs in non-volatile memory units of new generation. We revealed the transition from a single to double antiferroelectric-like hysteresis loop of the concentration voltage dependence that happens with the temperature increase and then exist in a wide temperature range (from 350 to 500 K). Unexpectedly we revealed the double loops of polarization and concentration can have irregular shape that remains irregular as long as the computation takes place, and the voltage position of the different features (jumps, secondary maxima, etc.) changes from one period to another, leading to the impression of quasi-chaotic behavior. It appeared that these effects originate from the nonlinear screening of ferroelectric polarization by graphene carriers, as well as it is conditioned by the temperature evolution of the domain structure kinetics in ferroelectric substrate. The nonlinearity rules the voltage behavior of polarization screening by graphene 2D-layer and at the same time induces the motion of separated domain walls accompanied by the motion of p-n junction along the graphene channel (2D-analog of Hann effect). Since the domain walls structure, period and kinetics can be controlled by varying the temperature, we concluded that the considered nano-structures based on graphene-on-ferroelectric are promising for the fabrication of new generation of modulators based on the graphene p-n junctions.

cond-mat.mtrl-sci↗

arXiv2017

Effect of surface ionic screening on polarization reversal scenario in ferroelectric thin films: crossover from ferroionic to antiferroionic states

Nonlinear electrostatic interaction between the surface ions of electrochemical nature and ferroelectric dipoles gives rise to the coupled ferroionic states in nanoscale ferroelectrics. Here, we investigated the role of the surface ions formation energy value on the polarization states and polarization reversal mechanisms, domain structure and corresponding phase diagrams of ferroelectric thin films. Using 3D finite elements modeling we analyze the distribution and hysteresis loops of ferroelectric polarization and ionic charge, and dynamics of the domain states. These calculations performed over large parameter space delineate the regions of single- and poly- domain ferroelectric, ferroionic, antiferroionic and non-ferroelectric states as a function of surface ions formation energy, film thickness, applied voltage and temperature. We further map the analytical theory for 1D system onto effective Landau-Ginzburg free energy and establish the correspondence between the 3D numerical and 1D analytical results. This approach allows performing the overview of the phase diagrams of ferroionic systems and exploring the specific of switching and domain evolution phenomena.

cond-mat.mtrl-sci↗

arXiv2017

Flexoelectricity induced spatially modulated phases in ferroics and liquid crystals (Author review)

In the review we briefly analyze the state-of-art in the theory of flexoelectric phenomena and analyze how significantly the flexoelectric coupling can change the polar order parameter distribution in different ferroics and liquid crystals. The special attention in paid to the appearance of the spatially modulated phases induced by the flexocoupling in condensed and soft matter. Results of theoretical modeling performed in the framework of the Landau-Ginzburg-Devonshire formalism revealed that the general feature, inherent to both ferroics and liquid crystals, is the appearance of the spatially-modulated phases is taking place with increasing of the flexocoupling strength. We'd like to underline that theoretical and experimental study of flexoelectricity and related phenomena in nanosized and bulk ferroics, liquid crystals and related materials are very important for their advanced applications in nanoelectronics, memory devices and LC displays.

cond-mat.mtrl-sci↗

arXiv2017

Graphene separation and stretching induced by piezoelectric effect of ferroelectric domains: impact on the conductance of graphene channel

P-N junctions in graphene on ferroelectric have been actively studied, but the impact of piezoelectric effect in ferroelectric substrate with ferroelectric domain walls (FDWs) on graphene characteristics was not considered. Due to the piezo-effect ferroelectric domain stripes with opposite spontaneous polarizations elongate or contract depending on the polarity of voltage applied to the substrate. We show that the alternating piezoelectric displacement of the ferroelectric domain surfaces can lead to the alternate stretching and separation of graphene areas at the steps between elongated and contracted domains. Graphene separation at FDWs induced by piezo-effect can cause unusual effects. In particular, the conductance of graphene channel in a field effect transistor increases essentially, because electrons in the stretched section scatter on acoustic phonons. At the same time the graphene conductance is determined by ferroelectric spontaneous polarization and varies greatly in the presence of FDWs. The revealed piezo-mechanism of graphene conductance control is promising for next generations of graphene-based field effect transistors, modulators, electrical transducers and piezo-resistive elements. Also our results propose the method of suspended graphene fabrication based on piezo-effect in a ferroelectric substrate that does not require any additional technological procedures.

cond-mat.mtrl-sci↗

arXiv2017

Defect driven flexo-chemical coupling in thin ferroelectric films

Using Landau-Ginzburg-Devonshire theory, we considered the impact of the flexoelectro-chemical coupling on the size effects inpolar properties and phase transitions of thin ferroelectric films with a layer of elastic defects. We investigated a typical case, when defects fill a thin layer below the top film surface with a constant concentration creating an additional gradient of elastic fields. The defective surface of the film is not covered with an electrode, but instead with an ultra-thin layer of ambient screening charges, characterized by a surface screening length. This geometry is typical for the scanning probe piezoelectric force microscopy. Obtained results revealed an unexpectedly strong effect of the joint action of Vegard stresses and flexoelectric effect (shortly flexo-chemical coupling) on the ferroelectric transition temperature, distribution of the spontaneous polarization and elastic fields, domain wall structure and period in thin PbTiO3 films containing a layer of elastic defects. A nontrivial result is the ferroelectricity persisting at film thicknesses below 4 nm, temperatures lower than 350 K and relatively high surface screening length (~0.1 nm). The origin of this phenomenon is the re-building of the domain structure in the film (namely the cross-over from c-domain stripes to a-type closure domains) when its thickness decreases below 4 nm, conditioned by the flexoelectric coupling and facilitated by negative Vegard effect. For positive Vegard effect, thicker films exhibit the appearance of pronounced maxima on the thickness dependence of the transition temperature, whose position and height can be controlled by the defect type and concentration. The revealed features may have important implications for miniaturization of ferroelectric-based devices.

cond-mat.mtrl-sci↗

arXiv2017

Thermal Contrast in Nanoscale Infrared Spectroscopy (AFM-IR): Low Frequency Limit

The contrast formation mechanism in Nanoscale Infrared Spectroscopy (Nano-IR or AFM-IR) is analyzed for the boundary between two layers with different light absorption, thermal and elastic parameters. Analytical results derived in the decoupling approximation for low frequency limit show that the response amplitude is linearly proportional to the intensity of the illuminating light and thermal expansion coefficient. The spatial resolution between two dissimilar materials is linearly proportional to the sum of inverse light adsorption coefficients and to the effective thermal transfer length. The difference of displacements height across the T-shape boundary ("thermo-elastic step") is proportional to the difference of the adsorption coefficients and inversely proportional to the heat transfer coefficient. The step height becomes thickness-independent for thick films and proportional to h^2 in a very thin film

cond-mat.mtrl-sci↗

arXiv2017

Flexocoupling induced soft acoustic mode and the spatially modulated phases in ferroelectrics

Using the Landau Ginzburg Devonshire theory and scalar approximation, we derived analytical expressions for the singular points (zeros, complex ranges) of the acoustic phonon mode (A mode) frequency in dependence on the wave vector k and examined the conditions of the soft A modes appearance in a ferroelectric depending on the magnitude of the flexoelectric coefficient f and temperature T. We predict that if the magnitude of the flexocoefficient f is equal to the temperature-dependent critical value fcr(T) at the temperature T=T_IC, the A mode frequency tends to zero at k=kr_0 and the spontaneous polarization becomes spatially modulated in a temperature range T<T_IC.The comparison of calculated physical properties with measured ones are performed for some ferroelectrics with SMP phases.In particular, temperature dependence of the calculated direct and inverse static dielectric susceptibility is in an agreement with experimental data in Sn2P2(SexS1-x)6 that gives us additional background to predict flexo-coupling induced soft acoustic amplitudon-type mode in the SMP phase.The available experimental data on neutron scattering in organic incommensurate ferroelectric (CH3)3NCH2COO*CaCl2*2H2O are in a semi-quantitative agreement with our theoretical results. To quantify the theory, it is necessary to measure the frequency dependence of the A mode in a uniaxial ferroelectric with a spatially modulated phase in the temperature interval near its occurrence.

cond-mat.mtrl-sci↗