Mykhaylo Evstigneev

Dielectric properties of BaTiO3/SrTiO3 multilayered thin films prepared by pulsed laser deposition

BaTiO3/SrTiO3 multilayered thin films were deposited on Si and Pt/Si substrates. X-ray diffraction clearly shows the formation of the superstructures. Phase transition properties were studied via dielectric measurements. Glassy behavior, characterized by a strong frequency dispersion of dielectric properties, was found in samples with a total thickness of 400 nm, while in samples with a total thickness of 800 nm, normal ferroelectric phase transitions with two dielectric peaks were observed. A preliminary interpretation assumes that size effects which frustrate long range ferroelectric ordering may lead to the relaxational behavior in BaTiO3/SrTiO3 superstructures.

Enhanced stability of rocksalt-type AIN phase in AIN/TiN superlattices synthesized by room-temperature pulsed laser deposition

AlN/TiN superlattice coatings with bilayer thickness, Λ, of 1–20 nm were synthesized by pulsed laser deposition (PLD) at room temperature. Below a critical value of bilayer thickness, Λc≈3 nm, the coatings formed a single-phase NaCl-type structure due to pseudomorphic stabilization of the nonequilibrium rocksalt-type (B1) AlN phase. Exceeding Λc resulted in the loss of continuous coherent growth of alternating AlN and TiN layers that is usually attributed to a rocksalt→wurtzite AlN phase transformation occurring in AlN layers when their thickness exceeds a certain critical value. However, in contrast to previous reports, a substantial amount of crystalline B1–AlN has been detected within the volume of AlN layers in AlN/TiN superlattices with Λ>Λc. The observed enhanced pseudomorphic stability of B1–AlN in superlattices fabricated by room-temperature PLD is explained by kinetically limited conditions inherent to this deposition technique. The presence of the B1–AlN phase in AlN/TiN multilayers is assumed t...

Description of atomic friction as forced Brownian motion

A theoretical description of friction force microscopy experiments in terms of a forced Brownian motion model is derived on the basis of microscopic considerations. Particular emphasis is put on the discussion of the relevant state variables/collective coordinates and on a realistic description of dissipation and inertia effects by means of comparison with experimental findings. The main new prediction of the model is a non-monotonic dependence of the friction force upon the pulling velocity of the AFM-tip relative to an atomically flat surface. The region around the force maximum can be approximately described by a universal scaling law and should be observable under experimentally realistic conditions.

Dimer motion on a periodic substrate: Spontaneous symmetry breaking and absolute negative mobility

We consider two coupled particles moving along a periodic substrate potential with negligible inertia effects (overdamped limit). Even when the particles are identical and the substrate spatially symmetric, a sinusoidal external driving of appropriate amplitude and frequency may lead to spontaneous symmetry breaking in the form of a permanent directed motion of the dimer. Thermal noise restores ergodicity and thus zero net velocity, but entails arbitrarily fast diffusion of the dimer for sufficiently weak noise. Moreover, upon application of a static bias force, the dimer exhibits a motion opposite to that force (absolute negative mobility). The key requirement for all these effects is a nonconvex interaction potential of the two particles.

Single-molecule force spectroscopy: Practical limitations beyond Bell's model

Single-molecule force spectroscopy experiments, and a number of other physical systems, are governed by thermally activated transitions out of a metastable state under the action of a steadily increasing external force. The main observable in such experiments is the distribution of the forces, at which the escape events occur. The challenge in interpreting the experimental data is to relate them to the microscopic system properties. We work out a maximum likelihood approach and show that it is the optimal method to tackle this problem. When fitting actual experimental data it is unavoidable to assume some functional form for the force-dependent escape rate. Focusing on escape processes over a single activation barrier, we consider a quite general and common such functional form and demonstrate by means of data from a realistic computer experiment that the maximum number of fit parameters that can be determined reliably is three. They are related to the force-free escape rate and the position and height of the activation barrier. Furthermore, the results for the first two of these fit parameters show little dependence on the assumption about the manner in which the barrier decreases with the applied force, while the last one, the barrier height in the absence of force, depends strongly on this assumption.

Temperature dependence of photoconversion efficiency in silicon heterojunction solar cells: Theory vs experiment

Silicon heterojunction solar cells (HJSC) with the efficiency of about 20% are manufactured. Their short-circuit current, open-circuit voltage, photoconversion efficiency, and fill factor of the current–voltage curve are measured in a broad temperature range from 80 to 420 K. It is established that the open-circuit voltage, the fill factor, and the photoconversion efficiency are non-monotonic functions of temperature, having a maximum in the vicinity of 200 K. A new approach to modeling of HJSCs is proposed, which allows one to obtain quantitative agreement with the experimental results at temperatures above 200 K, as well as to describe the results published in the literature on the solar cells under AM1.5 conditions. The temperature coefficient of photoconversion efficiency in HJSCs is discussed, and its low value is shown to be related to the low surface and volume recombination rates. Finally, a theoretical expression for the SCs temperature under natural working conditions is derived.

Role of substrate in the pseudomorphic stabilization of rocksalt-type AlN phase in AlN/TiN superlattices

The nonequilibrium B1 (rocksalt-type) AlN phase was successfully stabilized in stress-free AlN/TiN superlattices prepared by room-temperature pulsed laser deposition on thin film amorphous carbon substrates. The coatings displayed the same properties as reported for multilayers grown on thick crystalline substrates; namely, the existence of a critical thickness of AlN (about 1 nm for the present study) below which AlN crystallizes in the B1–AlN phase. Exceeding this value lead to AlN crystallization in its equilibrium hexagonal wurtzite-type structure, accompanied by considerable degradation of coating crystallinity due to the loss of coherent epitaxial growth of alternating TiN and AlN layers in the coating. It is concluded that neither an orientational effect of the substrate nor its stress-sustaining ability is required to provide a pseudomorphic stabilization of the nonequilibrium B1–AlN phase in AlN/TiN multilayered coatings.

Application of the method of moments for calculating the dynamic response of periodically driven nonlinear stochastic systems

It is shown that the method of moments allows one to calculate the first- and higher-harmonic susceptibilities of nonlinear stochastic systems with high accuracy. The dependence of the spectral amplification at the first three harmonics on the noise intensity is studied. It is shown that stochastic resonance at the third harmonic occurs at two separate values of the noise intensity for not too large a bias. Also, it is demonstrated that even when the bias is so large that the bistable system turns into a monostable one, the stochastic resonant enhancement of the systems sensitivity to the external driving field is still observed at some optimal non-zero value of the noise intensity.

Production and structural characterization of BN/TiN multilayers

A pulsed excimer laser was used to evaporate targets of boron nitride and titanium nitride in an attempt to produce hard thin films on crystalline silicon substrates. The films were either pure TiN or BN layers, as well as alternating multilayers and mixed layers. Deposition could be assisted by ion bombardment. The films were characterized by Auger electron spectroscopy, Fourier transform infrared spectroscopy (FTIR), and x-ray diffraction. A selection of films was also studied by profilometry in order to determine deposition rate and the type of stress present. The level of stress in TiN films was also a function of the deposition temperature and could be varied with the use of ion bombardment. Amorphous, cubic, and hexagonal BN films were produced and the effect of the stress of the substrate on these layers was investigated. Multilayers were stressed, having alternating layers of nanocrystalline TiN and amorphous BN. Mixtures consisted of nanometer-sized regions of crystalline TiN and sp2 coordinated ...

Pulsating potential ratchet

We consider Brownian motion in a periodic array of potential wells. The potential barriers between adjacent wells are spatially symmetric and pulsating periodically in time. While the modulation amplitude is the same for all barriers, the spatial symmetry is broken by sequentially alternating the respective pulsation frequencies among three different incommensurate values. The result is a net motion (ratchet effect), whose direction depends in an intriguing way on the detailed choice of parameters.