arXiv subjects

Eugene A. Eliseev

Publications and source records attributed to Eugene A. Eliseev.

At least 91 records · Page 5Linked to original sources

arXiv2020

Investigating phase transitions from local crystallographic analysis based on machine learning of atomic environments

Traditionally, phase transitions are explored using a combination of macroscopic functional characterization and scattering techniques, providing insight into average properties and symmetries of the lattice but local atomic level mechanisms during phase transitions generally remain unknown. Here we explore the mechanisms of a phase transition between the trigonal prismatic and distorted octahedral phases of layered chalogenides in the MoS2 ReS2 system from the observations of local degrees of freedom, namely atomic positions by Scanning Transmission Electron Microscopy (STEM). We employ local crystallographic analysis based on machine learning of atomic environments to build a picture of the transition from the atomic level up and determine local and global variables controlling the local symmetry breaking. In particular, we argue that the dependence of the average symmetry breaking distortion amplitude on global and local concentration can be used to separate local chemical and global electronic effects on transition. This approach allows exploring atomic mechanisms beyond the traditional macroscopic descriptions, utilizing the imaging of compositional fluctuations in solids to explore phase transitions over a range of realized and observed local stoichiometries and atomic configurations.

cond-mat.mtrl-sci↗

arXiv2020

Flexo-induced ferroelectricity in low dimensional transition metal dichalcogenides

We developed a Landau type theory for the description of polar phenomena in low-dimensional transition metal dichalcogenides (TMDs), specifically exploring flexoelectric origin of the polarization induced by a spontaneous bending and by inversion symmetry breaking due to the interactions with substrate. We consider the appearance of the spontaneous out-of-plane polarization due to the flexoelectric coupling with the strain gradient of the spontaneous surface rippling and surface-induced piezoelectricity. Performed calculations proved that the out-of-plane spontaneous polarization, originated from flexoelectric effect in a rippled TMD, is bistable and reversible by a non-uniform electric field. In contrast, the spontaneous polarization induced by a misfit strain and symmetry-sensitive surface-induced piezoelectric coupling, cannot be reversed by an external electric field. The special attention is paid to the spectral analysis of the linear dielectric susceptibility and gain factor, which enhancement is critically important for the observation of the polar phenomena in low-dimensional TMDs by the surface-enhanced vibrational spectroscopy.

cond-mat.mtrl-sci↗

arXiv2020

Strain engineering of ferromagnetic-graphene-ferroelectric nanostructures

We calculated a spin-polarized conductance in the almost unexplored nanostructure "high temperature ferromagnetic insulator/ graphene/ ferroelectric film" with a special attention to the impact of electric polarization rotation in a strained multiaxial ferroelectric film. The rotation and value of polarization vector are controlled by a misfit strain. We proposed a phenomenological model, which takes into account the shift of the Dirac point due to the proximity of ferromagnetic insulator and uses the Landauer formula for the conductivity of the graphene channel. We derived analytical expressions, which show that the strain-dependent ferroelectric polarization governs the concentration of two-dimensional charge carriers and Fermi level in graphene in a self-consistent way. We demonstrate the realistic opportunity to control the spin-polarized conductance of graphene by a misfit strain ("strain engineering") at room and higher temperatures in the nanostructures CoFeO4/graphene/PZT and Y3Fe5O12/graphene/PZT. Obtained results open the possibilities for the applications of ferromagnetic/graphene/ferroelectric nanostructures as non-volatile spin filters and spin valves.

cond-mat.mtrl-sci↗

arXiv2020

Electric Field Control of Three-Dimensional Vortex States in Core-Shell Ferroelectric Nanoparticles

In the framework of the Landau-Ginzburg-Devonshire approach coupled with electrostatic equations, we performed finite element modeling of the polarization behavior in a ferroelectric barium titanate core covered with a tunable paraelectric strontium titanate shell placed in a polymer or liquid medium. A stable two-dimensional vortex is formed in the core after a zero-field relaxation of an initial random or poly-domain distribution of the polarization, where the vortex axis is directed along one of the core crystallographic axes. Subsequently, sinusoidal pulses of a homogeneous electric field with variable period, strength, and direction are applied. The field-induced changes of the vortex structure consist in the appearance of an axial kernel in the form of a prolate nanodomain, the kernel growth, an increasing orientation of the polarization along the field, and the onset of a single-domain state. We introduced the term "kernel" to name the prolate nanodomain developed near the vortex axis and polarized perpendicular to the vortex plane. In ferromagnetism, this region is generally known as the vortex core. The vortex states with a kernel possess a manifold degeneracy, appearing from three equiprobable directions of vortex axis, clockwise and counterclockwise directions of polarization rotation along the vortex axis, and two polarization directions in the kernel. This multitude of the vortex states in a single core are promising for applications of core-shell nanoparticles and their ensembles as multi-bit memory and related logic units. The rotation of a vortex kernel over a sphere, possible for the core-shell nanoparticles in a soft matter medium with controllable viscosity, may be used to imitate qubit features.

cond-mat.mes-hall↗

arXiv2020

Dynamic manipulation in piezoresponse force microscopy: creating non-equilibrium phases with large electromechanical response

Domains walls and topological defects in ferroelectric materials have emerged as a powerful new paradigm for functional electronic devices including memory and logic. Similarly, wall interactions and dynamics underpin a broad range of mesoscale phenomena ranging from giant electromechanical responses to memory effects. Exploring the functionalities of individual domain walls, their interactions, and controlled modifications of the domain structures is crucial for applications and fundamental physical studies. However, the dynamic nature of these features severely limits studies of their local physics since application of local biases or pressures in piezoresponse force microscopy induce wall displacement as a primary response. Here, we introduce a fundamentally new approach for the control and modification of domain structures based on automated experimentation whereby real space image-based feedback is used to control the tip bias during ferroelectric switching, allowing for modification routes conditioned on domain states under the tip. This automated experiment approach is demonstrated for the exploration of domain wall dynamics and creation of metastable phases with large electromechanical response.

cond-mat.mes-hall↗

arXiv2019

Phase Diagrams of Single Layer Two-Dimensional Transition Metal Dichalcogenides: Landau Theory

Single layer (SL) two-dimensional transition metal dichalcogenides (TMDs), such as MoS2, ReS2, WSe2, and MoTe2 have now become the focus of intensive fundamental and applied researches due to their intriguing and tunable physical properties. These materials exhibit a broad range of structural phases that can be induced via elastic strain, chemical doping, and electrostatic field effect. These transitions in turn can open and close the band gap of SL-TMDs, leading to metal-insulator transitions, and lead to emergence of more complex quantum phenomena. These considerations necessitate detailed understanding of the mesoscopic mechanisms of these structural phase transitions. Here we develop the Landau-type thermodynamic description of SL-TMDs on example of SL-(MoS2)1-x-(ReS2)x system and analyze the free energy surfaces, phase diagrams, and order parameter behavior. Our results predict the existence of multiple structural phases with 2-, 6- and 12-fold degenerated energy minima for in-plane and out of plane order parameters. This analysis suggests that out-of-plane ferroelectricity can exist in many of these phases, with the switchable polarization being proportional to the out-of-plane order parameter. We further predict that the domain walls in SL-(MoS2)1-x-(ReS2)x should become conductive above a certain strain threshold.

cond-mat.mtrl-sci↗

arXiv2019

Melting of Spatially Modulated Phases in La-doped BiFeO3 at Surfaces and Surface-Domain Wall Junctions

The interplay between the surface and domain wall phenomena in multiferroic LaxBi1-xFeO3 in the vicinity of morphotropic phase transition is explored on the atomic level. Scanning Transmission Electron Microscopy (STEM) has enabled mapping of atomic structures of the material with picometer-level precision, providing direct insight into the spatial distribution of the order parameters in this material and their behavior at surfaces and interfaces. Here, we use the thermodynamic Landau-Ginzburg-Devonshire (LGD) approach to explain the emergence of spatially modulated phases (SMP) in La0.22Bi0.78FeO3 films, and establish that the change of polarization gradient coefficients caused by La-doping is the primary driving mechanisms. The suppression, or "melting", of the SMP in the vicinity of the domain wall surface junction is observed experimentally and simulated in the framework of LGD theory. The melting originated from the system tendency to minimize electrostatic energy caused by long-range stray electric fields outside the film and related depolarization effects inside it. The observed behavior provides insight to the origin of surface and interface behaviors in multiferroics.

cond-mat.mtrl-sci↗

arXiv2019

Controlling the domain structure of ferroelectric nanoparticles using tunable shells

The possibility of controlling the domain structure in spherical nanoparticles of uniaxial and multiaxial ferroelectrics using a shell with tunable dielectric properties is studied in the framework of Landau-Ginzburg-Devonshire theory. Finite element modeling and analytical calculations are performed for Sn2P2S6 and BaTiO3 nanoparticles covered with high-k polymer, temperature dependent isotropic paraelectric strontium titanate, or anisotropic liquid crystal shells with a strongly temperature dependent dielectric permittivity tensor. It appeared that the tunable paraelectric shell with a temperature dependent high dielectric permittivity (~300 - 3000) provides much more efficient screening of the nanoparticle polarization than the polymer shell with a much smaller (~10) temperature-independent permittivity. The tunable dielectric anisotropy of the liquid crystal shell (~ 1 - 100) adds a new level of functionality for the control of ferroelectric domains morphology (including a single-domain state, domain stripes and cylinders, meandering and labyrinthine domains, and polarization flux-closure domains and vortexes) in comparison with isotropic paraelectric and polymer shells. The obtained results indicate the opportunities to control the domain structure morphology of ferroelectric nanoparticles covered with tunable shells, which can lead to the generation of new ferroelectric memory and advanced cryptographic materials.

cond-mat.mtrl-sci↗

arXiv2019

Analytical description of the size effect on pyroelectric and electrocaloric properties of ferroelectric nanoparticles

Using Landau-Ginzburg-Devonshire theory and effective medium approximation, we analytically calculate typical dependences of the pyroelectric and electrocaloric coefficients on external electric field, temperature and radius for spherical single-domain ferroelectric nanoparticles. The considered physical model corresponds to the nanocomposite with small fraction of ferroelectric nanoparticles. Within the framework of the analytical model we establish how the size changes determine the temperature and field behavior pyroelectric and electrocaloric coefficients on example of BaTiO3 nanoparticles covered by a semiconducting shell and placed in a dielectric polymer. We show that by changing the particle size one can induce maxima of the pyroelectric coefficient and electrocaloric temperature variation, control their width, height and sign. Obtained analytical expressions allow selecting the interval of particle sizes, voltage, and/or temperature for which pyroelectric energy conversion and electrocaloric coefficient are optimal for applications. The observed size effect opens the possibility to control pyroelectric and electrocaloric properties of ferroelectric nanocomposites that can be important for their advanced applications in energy convertors and cooling systems.

cond-mat.mtrl-sci↗

arXiv2019

Building Free Energy Functional from Atomically-Resolved Imaging: Atomic Scale Phenomena in La-doped BiFeO3

Scanning Transmission Electron Microscopy (STEM) has enabled mapping of atomic structures of solids with sub-pm precision, providing insight to the physics of ferroic phenomena and chemical expansion. However, only a subset of information is available, due to projective nature of imaging in the beam direction. Correspondingly, the analysis often relies on the postulated known form of macroscopic Landau-Ginzburg energy, and some predefined relationship between experimentally determined atomic coordinates and the order parameter field. Here, we propose an approach for exploring the structure of ferroics using reduced order parameter models constructed based on experimental data only. We develop a four sublattices model (FSM) for the analytical description of A-cation displacement in (anti)ferroelectric-antiferrodistortive perovskites of ABO3-type. The model describes the displacements of cation A in four neighboring unit cells and determines the conditions of different structural phases appearance and stability in ABO3. We show that FSM explains the coexistence of rhombohedral (R), orthorhombic (O) and spatially modulated (SM) phases, observed by atomic-resolution STEM in La-doped BiFeO3. Using this approach, we atomically resolve and theoretically model the sublattice asymmetry inherent to the case of the A-site La/Bi cation sublattice in LaxBi1-xFeO3 polymorphs. This approach allows exploring the ferroics behaviors from experimental data only, without additional assumptions on the nature of the order parameter.

cond-mat.mtrl-sci↗

arXiv2019

Symmetry of flexoelectric response in ferroics

Using direct matrix method we establish the structure, including the number of nonzero independent elements, of the static flexoelectric coupling tensor f_ijkl for all 32 point groups. We use the point symmetry of elementary cell, previously known evident index-permutation symmetry (f_ijkl= f_jikl) and recently established "hidden" index-permutation symmetry (f_ijkl= f_ilkj). We compare these results and demonstrated that the hidden symmetry of f_ijkl significantly reduces the number of its nonzero independent elements. Using group theory, we find out the explicit form of the tensor f_ijkl and the flexoelectric coupling energy in the form of Lifshitz invariant for several point symmetries most important for applications. For these symmetries we visualize the effective flexoelectric response of the bended plate allowing for both evident and hidden index-permutation symmetries, analyze its anisotropy, and angular dependences. We discuss how the hidden symmetry can significantly simplify the problem of the flexoelectric constants determination from inelastic neutron and Raman scattering experiments due to the minimization of f_ijkl independent components, opening the way for its unambiguous determination from the scattering experiments.

cond-mat.mtrl-sci↗

arXiv2019

Giant negative electrostriction and dielectric tunability in a van der Waals layered ferroelectric

The interest in ferroelectric van der Waals crystals arises from the potential to realize ultrathin ferroic systems owing to the reduced surface energy of these materials and the layered structure that allows for exfoliation. Here, we quantitatively unravel giant negative electrostriction of van der Waals layered copper indium thiophosphate (CIPS), which exhibits an electrostrictive coefficient Q33 as high as -3.2 m4/C2 and a resulting bulk piezoelectric coefficient d33 up to -85 pm/V. As a result, the electromechanical response of CIPS is comparable in magnitude to established perovskite ferroelectrics despite possessing a much smaller spontaneous polarization of only a few uC/cm2. In the paraelectric state, readily accessible owing to low transition temperatures, CIPS exhibits large dielectric tunability, similar to widely-used barium strontium titanate, and as a result both giant and continuously tunable electromechanical response. The persistence of electrostrictive and tunable responses in the paraelectric state indicates that even few layer films or nanoparticles will sustain significant electromechanical functionality, offsetting the inevitable suppression of ferroelectric properties in the nanoscale limit. These findings can likely be extended to other ferroelectric transition metal thiophosphates and (quasi-) two-dimensional materials and might facilitate the quest towards novel ultrathin functional devices incorporating electromechanical response.

cond-mat.mtrl-sci↗

arXiv2018

Intrinsic structural instabilities of domain walls driven by gradient couplings: meandering anferrodistortive-ferroelectric domain walls in BiFeO3

Using Landau-Ginzburg-Devonshire approach, we predict the intrinsic instability of the ferroelectric-ferroelastic domain walls in the multiferroic BiFeO3 emerging from the interplay between the gradient terms of the antiferrodistortive and ferroelectric order parameters at the walls. These instabilities are the interface analogue of the structural instabilities in the vicinity of phase coexistence in the bulk; and so they do not steam from incomplete polarization screening in thin films or its spatial confinement, electrostrictive or flexoelectric coupling. The effect of BiFeO3 material parameters on the 71 degree, 109 degree, and 180 degree walls is explored, and it is shown that the meandering instability appears at 109 degree, and 180 degree walls for small gradient energies, and the walls become straight and broaden for higher gradients. In contrast to the 180 degree and 109 degree domain walls, uncharged 71 degree walls are always straight, and their width increases with increasing the tilt gradient coefficient. The wall instability and associated intrinsic meandering provide a new insight into the behavior of morphotropic and relaxor materials, wall pinning, and mechanisms of interactions between order parameter fields and local microstructure.

cond-mat.mtrl-sci↗

arXiv2018

Size effect of soft phonon dispersion law in nanosized ferroics

Using Landau-Ginsburg-Devonshire theory, we derive and analyze analytical expressions for the frequency dispersion of soft phonon modes in nanosized ferroics and perform numerical calculations for a thin SrTiO3 film. We revealed the pronounced "true" size effect in the dependence of soft phonon spatial dispersion on the film thickness and predict that it can lead to the "apparent" or "false" size effect of dynamic flexoelectric coupling constants

cond-mat.mtrl-sci↗

arXiv2018

Oxygen vacancies induced ferroelectricity in relaxors with ABO3 structure

The consideration of oxygen vacancies influence on the relaxors with perovskite structure was considered in the framework of Landau-Ginzburg-Devonshire phenomenological theory. The theory applicability for relaxors is based on the existence of some hidden soft phonon polar mode in them, and its frequency could be zero at some negative temperature TC*. Main attention was paid to PZN-PLZT relaxor described by formula 0.3Pb(Zn1/3Nb2/3)O3-0.7(Pb0.96La0.04(ZrxTi1-x)0.99O3) with x = 0.52, where earlier experimental investigation of oxygen vacancies influence on the polar properties was performed and the evidence of oxygen vacancies induced ferroelectricity was obtained. Since the oxygen vacancies are known to be elastic dipoles, they influence upon elastic and electric fields due to Vegard and flexoelectric coupling. We include the vacancies elastic and electrostrictive contribution into free energy functional. The calculations of the vacancies impact on polar properties were performed using their concentration distribution function. It was shown that the negative Curie temperature of a relaxor TC* is renormalized by the elastic dipoles due to the electrostrictive coupling and can become positive at some large enough concentration of the vacancies. We calculated the local polarization and electric field induced by the flexo-chemical coupling in dependence on the concentration of oxygen vacancies. The coexistence of FE phase and relaxor state can take place because of inhomogeneity of vacancies concentration distribution.

cond-mat.mtrl-sci↗

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

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↗