arXiv subjects

A. P. Levanyuk

Publications and source records attributed to A. P. Levanyuk.

36 records · Page 2Linked to original sources

arXiv2004

Smearing of phase transition due to surface effect in ferroelectric nanostructures

The boundary conditions, customarily used in the Landau-type approach to ferroelectric thin films and nanostructures, have to be modified to take into account that a surface of a ferroelectric (FE) is a defect of a ``field'' type. The surface (interface) field is coupled to a normal component of polarization and, as a result, the second order phase transitions are generally suppressed and anomalies in response are washed out, as observed experimentally.

cond-mat.mtrl-sci↗

arXiv2004

An explanation for the glass-like anomaly in the low-temperature specific heat of incommensurate phases

An explanation for the glass-like anomaly observed in the low-temperature specific heat of incommensurate phases is proposed. The key point of this explanation is the proper account for the phason damping when computing the thermodynamic magnitudes. The low-temperature specific heat of the incommensurate phases is discussed within three possible scenarios for the phason dynamics: no phason gap, static phason gap and a phason gap of dynamical origin. Existing NMR and inelastic scattering data indicate that these scenarios are possible in biphenyl, blue bronze $\rm K_{0.30}MoO_3$ and BCPS respectively. Estimates of the corresponding low-temperature specific heat are in reasonable agreement with the experiments.

cond-mat.stat-mech↗

arXiv2004

Vortex rectification effects in plain superconducting films

Superconducting films in perpendicular magnetic field are found to rectify alternating currents. The effect has been observed both in plain and nanostructured superconducting films (niobium and lead). The rectified voltage appears both along and transverse to alternating current and strongly depends on the magnetic field. Rectification phenomena is based on the property of superconductors to have permanent circulating currents in magnetic field (vortices) and to develop electric fields at high enough currents. In contrast to previous findings, at higher current frequencies no asymmetric pinning sites are needed to produce rectification and related guided flux motion.

cond-mat.supr-con↗

arXiv2004

Low-temperature structural phase transitions: Phonon-like and relaxation order-parameter dynamics

A theoretical study on low-temperature structural phase transitions is presented, in which both phonon-like and relaxation order-parameter dynamics are contemplated. While the first limiting case has been considered previously, the second one is studied here for the first time. Attention is put on the low-temperature asymptotics of the temperature dependence of the generalized susceptibility. In the relaxation case, it is found $\sim (T^2 - T_c^2)^{-1}$ for temperature-independent relaxation times in a broad region of the temperature-pressure phase diagram. In contrast to the obtained in the phonon-like case, this asymptotics is not modified by long-range interactions (dipolar forces, piezoelectric effect, etc.).

cond-mat.stat-mech↗

arXiv2004

Smearing of phase transition due to a surface effect or a bulk inhomogeneity in ferroelectric nanostructures

The boundary conditions, customarily used in the Landau-type approach to ferroelectric thin films and nanostructures, have to be modified to take into account that a surface of a ferroelectric (FE) is a defect of the ``field'' type. The surface (interface) field is coupled to a normal component of polarization and, as a result, the second order phase transitions are generally suppressed and anomalies in response are washed out. In FE films with a compositional (grading) or some other type of inhomogeneity, the transition into a monodomain state is suppressed, but a transition with formation of a domain structure may occur.

cond-mat.mtrl-sci↗

arXiv2003

On phase transitions in Pb/Ge(111) and Sn/Ge(111)

The present communication is a critical examination of two points relevant to the surface phase transitions of Pb and Sn overlayers on Ge(111). One is connected with the reading of the reported structural data, which lead to some consequences of paramount importance but overlooked up to now. The other point is the seeming contradiction in what concerns to the transition temperatures obtained from different experimental methods. The importance of this contradiction, in contrast to the previous point, has been overestimated.

cond-mat.mtrl-sci↗

arXiv2003

Elasticity-driven interaction between vortices in type-II superconductors

The contribution to the vortex lattice energy which is due to the vortex-induced strains is calculated covering all the magnetic field range which defines the vortex state. This contribution is compared with previously reported ones what shows that, in the most part of the vortex state, it has been notably underestimated until now. The reason of such underestimation is the assumption that only the vortex cores induce strains. In contrast to what is generally assumed, both core and non-core regions are important sources of strains in high-$κ$ superconductors.

cond-mat.supr-con↗

arXiv2002

Elasticity-driven attraction between Abrikosov vortices in high-$κ$ superconductors: Leading role of a non-core contribution

We show that the strain-induced attraction between Abrikosov vortices has a non-core contribution overlooked up until now. This contribution is an example of the universal mechanism of soliton attraction in solids revealed in [Phys. Rev. B {\bf 66}, 14111 (2002)]. The resulting interaction energy is larger than that due to the vortex cores at least by a factor $\ln ^2 κ$. Consequently, the non-core contribution must be included, for instance, in discussions about the orientation of vortex lattices with respect to the crystal axes. It is shown to be also important when interpreting the thermal anomalies of the transition between superconducting and mixed states.

cond-mat.supr-con↗

arXiv2002

Formation and rapid evolution of domain structure at phase transitions in slightly inhomogeneous ferroelectrics and ferroelastics

We present the analytical study of stability loss and evolution of domain structure in inhomogeneous ferroelectric and ferroelastic samples for exactly solvable models. The model assumes a short-circuited ferroelectric capacitor (free ferroelastic) with two regions with slightly different critical temperatures Tc1 Tc2, where Tc1-Tc2 << Tc1, Tc2. We show that even a tiny inhomogeneity like 10-5 K results in splitting the system into domains below the phase transition temperature. At T < Tc2 the domain width a is proportional to (Tc1-T)/(Tc1-Tc2) and quickly increases with lowering temperature. The minute inhomogeneities in Tc may result from structural (growth) inhomogeneities, which are always present in real samples, and a similar role can be played by inevitable temperature gradients.

cond-mat.stat-mech↗

arXiv2002

Formation and rapid evolution of domain structure at phase transitions in slightly inhomogeneous ferroelectrics

We present the first analytical study of stability loss and evolution of domain structure in inhomogeneous ferroelectric samples for exactly solvable model. The model assumes a short-circuited capacitor with two regions with slightly different critical temperatures Tc1 > Tc2, where Tc1-Tc2 << Tc1, Tc2. Even a tiny inhomogeneity like 10-5 K may result in splitting the system into domains below the phase transition temperature. At T < Tc2 the domain width $a$ is proportional to (Tc1 - T)/(Tc1 - Tc2) and quickly increases with lowering temperature. The minute inhomogeneities in Tc may result from structural (growth) inhomogeneities which are always present in real samples and a similar role can be played by inevitable temperature gradients.

cond-mat.stat-mech↗

arXiv2001

Rigidity of the elastic domain structure near the boundary of its existence in thin epitaxial films

We consider an interesting and practically important case of elastic domain structure, which is the analogue of c/a domain pattern with 90$^\circ$ walls in perovskites, and is solvable analytically for arbitrary misfit strain. There is no critical thickness, below which the domain structure cannot exist, when the "extrinsic" misfit is zero and the domains are of equal width. At the boundary of polydomain-monodomain transition the period of the pattern diverges, as does the dynamic stiffness of the domain structure. It is unlikely, therefore, that one can achieve a softness of the dielectric response of the c/a elastic domains in ferroelectric-ferroelastic thin films.

cond-mat.stat-mech↗

arXiv2001

Proper ferroelastic phase transitions in thin epitaxial films with symmetry-conserving and symmetry-breaking misfit strains

We study how the ferroelastic domain structure sets in in an epitaxial film of a material with second order proper ferroelastic transition. The domain structures considered are similar to either $a_{1}/a_{2}/a_{1}/a_{2}$ or $c/a/c/a$ structures in perovskite ferroelectrics. If the "extrinsic" misfit strain, not associated with the transition, does not break the symmetry of the high-temperature phase, the phase transition in the film occurs at somewhat lower temperature compared to the bulk. The loss of stability then occurs with respect to a sinusoidal strain wave, which evolves into the domain structure with practically the same geometry and approximately the same period. In the presence of the symmetry-breaking component of the misfit strain ("extrinsic" misfit) the character of the phase transition is qualitatively different. In this case it is a {\em topological} transition between single-domain and multi-domain states, which starts from a low density of the domain walls.

cond-mat.stat-mech↗

arXiv2001

Very large dielectric response of thin ferroelectric films with the dead layers

We study the dielectric response of ferroelectric (FE) thin films with "dead" dielectric layer at the interface with electrodes. The domain structure inevitably forms in the FE film in presence of the dead layer. As a result, the effective dielectric constant of the capacitor $ε_{eff}$ increases abruptly when the dead layer is thin and, consequently, the pattern of 180-degree domains becomes "soft". We compare the exact results for this problem with the description in terms of a popular "capacitor" model, which is shown to give qualitatively incorrect results. We relate the present results to fatigue observed in thin ferroelectric films.

cond-mat.stat-mech↗

arXiv2000

Easy collective polarization switching in ferroelectrics

The actual mechanism of polarization switching in ferroelectrics remains a puzzle for many decades, since the usually estimated barrier for nucleation and growth is insurmountable ("paradox of the coercive field"). To analyze the mechanisms of the nucleation we consider the exactly solvable case of a ferroelectric film with a "dead" layer at the interface with electrodes. The classical nucleation is easier in this case but still impossible, since the calculated barrier is huge. We have found that the {\em interaction} between the nuclei is, however, long range, hence one has to study an {\em ensemble} of the nuclei. We show that there are the ensembles of small (embryonic) nuclei that grow {\em without the barrier}. We submit that the interaction between nuclei is the key point for solving the paradox.

cond-mat.stat-mech↗

arXiv2000

Phase transitions, stability, and dielectric response of the domain structure in ferroelectric-ferroelastic thin films

We present the first analytical study of phase transitions in ferroelastic-ferroelectric epitaxial thin films on exactly solvable model. The emerging domain structure with domains of equal width (which may be exponentially large on a "soft" substrate) always remains stable irrespective of the film thickness. The dielectric response of an epitaxial film is {\em smaller} than that of a free film, in striking contrast with assertions in the literature.

cond-mat.stat-mech↗

arXiv2000

Defects as a reason of continuity of normal-incommensurate phase transitions

Almost all normal-incommensurate phase transitions observed experimentally are continuous. We show that there is not any theoretical reason for this general behaviour in perfect crystals. A normal-incommensurate phase transition that is not too far from the mean-field tricritical point should be discontinuous and it is highly improbable that so far reported normal-incommensurate phase transitions lie very far from this point. To understand this behaviour we study influence of defects on a hypothetical first-order normal-incommensurate phase transition in a pure material. We have found that this influence is strikingly different from that on other kinds of first-order phase transitions. The change of the discontinuity of the order parameter at the transition is negative and formally diverges within our approximate theory. At the same time the diminishing of the phase transition temperature remains finite. We interpret these results as an indication that at least some of the observed seemingly second-order normal-incommensurate transitions would be first-order transitions in defectless crystals.

cond-mat.stat-mech↗

arXiv2000

Ferroelectric Phase Transitions in Films with Depletion Charge

We consider ferroelectric phase transitions in both short-circuited and biased ferroelectric-semiconductor films with a space (depletion) charge which leads to some unusual behavior. It is shown that in the presence of the charge the polarization separates into `switchable' and `non-switchable' parts. The electric field, appearing due to the space charge, does not wash out the phase transition, which remains second order but takes place at somewhat reduced temperature. At the same time, it leads to a suppression of the ferroelectricity in a near-electrode layer. This conclusion is valid for materials with both second and first order phase transitions in pure bulk samples. Influence of the depletion charge on thermodynamic coercive field reduces mainly to the lowering of the phase transition temperature, and its effect is negligible. The depletion charge can, however, facilitate an appearance of the domain structure which would be detrimental for device performance (fatigue). We discuss some issues of conceptual character, which are generally known but were overlooked in previous works. The present results have general implications for small systems with depletion charge.

cond-mat.stat-mech↗

arXiv2000

Abrupt appearance of the domain pattern and fatigue of thin ferroelectric films

We study the domain structure in ferroelectric thin films with a `passive' layer (material with damaged ferroelectric properties) at the interface between the film and electrodes within a continuous medium approximation. An abrupt transition from a monodomain to a polydomain state has been found with the increase of the `passive' layer thickness $d$. The domain width changes very quickly at the transition (exponentially with $d^{-2}$). We have estimated the dielectric response $dP/dE$ (the slope of the hysteresis loop) in the `fatigued' multidomain state and found that it is in agreement with experiment, assuming realistic parameters of the layer. We derive a simple universal relation for the dielectric response, which scales as $1/d$, involving only the properties of the passive layer. This relation qualitatively reproduces the evolution of the hysteresis loop in fatigued samples and it could be tested with controlled experiments. It is expected that the coercive field should increase with decreasing lateral size of the film. We believe that specific properties of the domain structure under bias voltage in ferroelectrics with a passive layer can resolve the long-standing `paradox of the coercive field'.

cond-mat.stat-mech↗