arXiv subjects

A. M. Bratkovsky

Publications and source records attributed to A. M. Bratkovsky.

At least 19 recordsLinked to original sources

arXiv2026

Fast and Slow Sound Excitations in Nematic Aerogel in superfluid 3He

Nematic aerogel (nAG) supports so-called polar phase in liquid 3He. The experiment [Dmitriev et al, JETP Lett. 112, 780 (2020)] showed that the onset of polar phase inside the nAG is accompanied by emergence of a sound wave with frequency quickly growing with cooling down from transition temperature and reaching a plateau. To describe this behavior, we start by calculating the elastic properties of the dry nematic AG that appear to depend only on Young's modulus of the parent material (e.g. mullite), the volume fraction of the solid phase and the aspect ratio of the representative volume of nAG. The elastic constants are then used to solve elasto-hydrodynamic equations for various sound vibrations of nAG filled with 3He. The (isotropic) first sound and anisotropic second sound in the polar phase are strongly hybridized with fourth sound and standard elastic modes in nAG. The hybrid second and the transverse fourth sound start with zero velocity at the transition, similar to pure 3He, and quickly grow with lowering temperature until they hit the sample finite size cutoff.

cond-mat.supr-con↗

arXiv2023

Simple Analytical Model for Optimizing Integrating Sphere Port Sizes

The integrating sphere (IS) is an indispensable tool for measuring transmission and scattering of materials and their colorimetry, as well as other photometric tasks. The accuracy of its data depends critically on port sizes used for measurement and control, usually defined by trial and error or brute-force optical simulations. To find the optimal port sizes of this powerful tool, a sample visibility function is defined and optimized using the energy conservation principle. This yields an analytical expression that should be useful in a variety of applications, especially those where signal is rather small (low-haze materials).

physics.optics↗

arXiv2020

Photoluminescence in PbS nanocrystal thin films: Nanocrystal density, film morphology and energy transfer

We show that photoluminescence properties of PbS nanocrystal thin films are directly related to film morphology and nanocrystal density. In densely packed PbS nanocrystal films, low-temperature donor-to-acceptor energy transfer is mainly responsible for the photoluminescence spectra narrowing and shift toward longer wavelengths. At elevated temperatures, back energy transfer is proposed to be responsible for an unusual photoluminescence intensity temperature dependence. In thin films with a low PbS nanocrystal density, the energy transfer is suppressed, and the effect is dramatically reduced.

cond-mat.mes-hall↗

arXiv2016

Plasmon-enhanced second harmonic generation in the laser-irradiated cubic metal nanoparticles

The plasmon-enhanced second harmonic generation in the subwavelength neutral metal cubic nanoparticles is calculated for the first time in the hydrodynamic and cold plasma approximations. The theory is developed that takes into account all singularities of the electromagnetic field at the cubic nanoparticle surface. The results are compared to the linear case for the nanocube and to those for a spherical nanoparticle. In the latter case, they demonstrate very strong enhancement of the local field strength and the second harmonic signal.

cond-mat.mes-hall↗

arXiv2011

Gap opening in graphene by simple periodic inhomogeneous strain

Using ab-initio methods, we show that the uniform deformation either leaves graphene (semi)metallic or opens up a small gap yet only beyond the mechanical breaking point of the graphene, contrary to claims in the literature based on tight-binding (TB) calculations. It is possible, however, to open up a global gap by a sine-like one-dimensional inhomogeneous deformation applied along any direction but the armchair one, with the largest gap for the corrugation along the zigzag direction (~0.5 eV) without any electrostatic gating. The gap opening has a threshold character with very sharp rise when the ratio of the amplitude A and the period of the sine wave deformation lambda exceeds (A/lambda)_c ~0.1 and the inversion symmetry is preserved, while it is threshold-less when the symmetry is broken, in contrast with TB-derived pseudo-magnetic field models.

cond-mat.mes-hall↗

arXiv2011

Semiconducting graphene nanomeshes

Symmetry arguments are used to describe all possible two-dimensional periodic corrugations of graphene ("nanomeshes") capable of inducing tangible semiconducting gap. Such nanomeshes or superlattices break the initial graphene translational symmetry in a way that produces mixing and subsequent splitting of the Dirac K and K' states. All of them have hexagonal Bravais lattice and are described by space groups that are subgroups of the graphene group. The first-principles calculations show that the gaps of about 0.5 eV can be induced at strains safely smaller than the graphene failure strain.

cond-mat.mes-hall↗

arXiv2011

Current-Controlled Negative Differential Resistance due to Joule Heating in TiO2

We show that Joule heating causes current-controlled negative differential resistance (CC-NDR) in TiO2 by constructing an analytical model of the voltage-current V(I) characteristic based on polaronic transport for Ohm's Law and Newton's Law of Cooling, and fitting this model to experimental data. This threshold switching is the 'soft breakdown' observed during electroforming of TiO2 and other transition-metal-oxide based memristors, as well as a precursor to 'ON' or 'SET' switching of unipolar memristors from their high to their low resistance states. The shape of the V(I) curve is a sensitive indicator of the nature of the polaronic conduction.

cond-mat.mes-hall↗

arXiv2011

Molecular dynamics simulations of oxide memristors: crystal field effects

We present molecular-dynamic simulations of memory resistors (memristors) including the crystal field effects on mobile ionic species such as oxygen vacancies appearing during operation of the device. Vacancy distributions show different patterns depending on the ratio of a spatial period of the crystal field to a characteristic radius of the vacancy-vacancy interaction. There are signatures of the orientational order and of spatial voids in the vacancy distributions for some crystal field potentials. The crystal field stabilizes the patterns after they are formed, resulting in a non-volatile switching of the simulated devices.

cond-mat.mtrl-sci↗

arXiv2011

Effects of anisotropic elasticity in the problem of domain formation and stability of monodomain state in ferroelectric films

We study cubic ferroelectrics films that become uniaxial with a polar axis perpendicular to the film because of a misfit strain due to a substrate. The main present result is the analytical account for the elastic anisotropy as well as the anisotropy of the electrostriction. They define, in particular, an orientation of the domain boundaries and stabilizing or destabilizing effect of inhomogeneous elastic strains on the single domain state. We apply the general results to perovskite systems like BaTiO3/SrRuO3/SrTiO3 films and find that at least not far from the ferroelectric phase transition the equilibrium domain structure consists of the stripes along the cubic axes or at 45 degrees to them. We have also showed that in this system the inhomogeneous strains increase stability with regards to the small fluctuations of the metastable single domain state, which may exist not very close to the ferroelectric transition. The latter analytical result is in qualitative agreement with the numerical result by Pertsev and Kohlstedt [Phys. Rev. Lett. 98, 257603 (2007)], but we show that the effect is much smaller than those authors claim. We have found also that under certain conditions on the material constants, which are not satisfied in the perovskites but are not forbidden either, a checkerboard domain structure can be realized instead of the stripe-like one and that the polarization-strain coupling decreases stability of a single domain state instead of increasing it. The single domain state is metastable at certain large thicknesses and becomes suitable for memory applications at even larger thicknesses when the lifetime of the metastable state becomes sufficiently large.

cond-mat.mes-hall↗

arXiv2010

Molecular dynamics simulations of oxide memristors: thermal effects

We have extended our recent molecular-dynamic simulations of memristors to include the effect of thermal inhomogeneities on mobile ionic species appearing during operation of the device. Simulations show a competition between an attractive short-ranged interaction between oxygen vacancies and an enhanced local temperature in creating/destroying the conducting oxygen channels. Such a competition would strongly affect the performance of the memristive devices.

cond-mat.mes-hall↗

arXiv2010

Molecular dynamics simulations of oxide memory resistors (memristors)

Reversible bipolar nano-switches that can be set and read electronically in a solid-state two-terminal device are very promising for applications. We have performed molecular-dynamics simulations that mimic systems with oxygen vacancies interacting via realistic potentials and driven by an external bias voltage. The competing short- and long-range interactions among charged mobile vacancies lead to density fluctuations and short-range ordering, while illustrating some aspects of observed experimental behavior, such as memristor polarity inversion.

cond-mat.mes-hall↗

arXiv2010

Key pairing interaction in layered doped ionic insulators

A controversial issue on whether the electron-phonon interaction (EPI) is crucial for high-temperature superconductivity or it is weak and inessential has remained one of the most challenging problems of contemporary condensed matter physics. We employ a continuum RPA approximation for the dielectric response function allowing for a selfconsistent semi-analytical evaluation of the EPI strength, electron-electron attractions, and the carrier mass renormalisation in layered high-temperature superconductors. We show that the Fröhlich EPI with high-frequency optical phonons in doped ionic lattices is the key pairing interaction, which is beyond the BCS-Migdal-Eliashberg approximation in underdoped superconductors, and it remains a significant player in overdoped compounds.

cond-mat.supr-con↗

arXiv2010

Negative c-axis magnetoresistance in graphite

We have studied the c-axis interlayer magnetoresistance (ILMR), R_c(B) in graphite. The measurements have been performed on strongly anisotropic highly oriented pyrolytic graphite (HOPG) samples in magnetic field up to B = 9 T applied both parallel and perpendicular to the sample c-axis in the temperature interval 2 K < T < 300 K. We have observed negative magnetoresistance, dR_c/dB < 0, for B || c-axis above a certain field B_m(T) that reaches its minimum value B_m = 5.4 T at T = 150 K. The results can be consistently understood assuming that ILMR is related to a tunneling between zero-energy Landau levels of quasi-two-dimensional Dirac fermions, in a close analogy with the behavior reported for alpha-(BEDT-TTF)2I3 [N. Tajima et al., Phys. Rev. Lett. 102, 176403 (2009)], another multilayer Dirac electron system.

cond-mat.mtrl-sci↗

arXiv2009

Polarons in 2+epsilon dimensions and giant figure of merit of semiconducting nanolayers

Polarons - electrons coupled with lattice vibrations - play a key role in the transport and optical properties of many systems of reduced dimension and dimensionality. Their confinement affects drastically the phonon, polaron, bipolaron and multi-polaron properties of quantum wells. Here we calculate the energy spectrum and thermopower of polarons confined to a potential well as a function of the well thickness. We show that the polaron mass enhancement in 2+epsilon dimensions explains a giant thermolectric power recently observed in doped semiconducting nanolayers (multiple quantum wells (MQWs)), and propose a route for enhancing the performance of thermoelectric energy nanoconverters increasing their figure of merit by more than one order of magnitude.

cond-mat.mes-hall↗

arXiv2009

Polaronic memristor strongly coupled to electrodes

Attractive electron correlations due to an electron-vibron interaction (EVI) can overcome the direct Coulomb repulsion of polarons in strongly deformable molecular quantum dots (MQDs). If it realizes, a switching appears in the I-V characteristics of the degenerate nonadiabatic molecular bridges weakly coupled to electrodes providing a route to ultrafast `memristors' (memory-resistors) as the basis for future oscillators, amplifers, and other important circuit elements. Here, we extend our theory of polaronic memristors to adiabatic MQDs strongly coupled to the leads and show that the degeneracy of MQD (or other multilevel energy structure) along with the polaron-polaron attraction is a necessary ingredient of its switching behavior in the strong-coupling regime as well.

cond-mat.mes-hall↗

arXiv2008

Auger-mediated radiative recombination in three-dimensional silicon/silicon-germanium nanostructures

In a semiconductor heterostructure with type II energy band alignment, the spatial separation between electrons and holes slows down their radiative recombination. With increasing excitation intensity, Auger recombination quickly becomes the dominate recombination channel, and it produces carrier ejection from the quantum well. Here, we show that in Si/SiGe three-dimensional nanostructures, this efficient process facilitates the formation of an electron-hole plasma (EHP) and/or electron-hole droplets (EHDs) in thin Si barriers separating SiGe clusters. In contrast to conventional, strongly temperature dependent and slow radiative carrier recombination in bulk Si, this EHD/EHP luminescence in nanometer-thick Si layers is found to be nearly temperature independent with radiative lifetime approaching 10^-8 s, which is only slightly slower than that found in direct band gap III-V semiconductors.

cond-mat.mtrl-sci↗

arXiv2008

Path to finding the critical thickness for memory in thin ferroelectric films

The finite screening length by real metallic electrodes, albeit very small (<1A), results in finite depolarizing field that tends to split the film into domains. In very thin ferroelectric films the domain structure reduces to sinusoidal distribution of polarization considered first in the 1980s. We discuss the phase transition between this structure and a single domain state and show that it is first order, if it exists at all. The alternative possibility is that the single domain state at zero bias voltage would be metastable for all temperatures in most systems. This scenario defines a path towards solution to a problem of finding parameters of a system that can sustain the ferroelectric memory over a desired period of time.

cond-mat.mtrl-sci↗

arXiv2008

Continuous theory of ferroelectric states in ultrathin films with real electrodes

According to a continuous medium theory, in very thin ferroelectric films with real metallic electrodes (or dead layers near the electrodes) the domain structure reduces to sinusoidal distribution of ferroelectric polarization. Such a sinusoidal structure was considered in 1980s for para-ferroelectric phase transition in a capacitor with dead layers near electrode. We give a review of this theory and its further development for the case of real metallic electrodes. The goal of the general theory is to consistently interpret the experimental data in very thin films with real metallic electrodes. This is illustrated on a recent experimental data for 5-30 nm BaTiO3 films with SrRuO3/SrTiO3 electrodes. The screening length by real metallic electrodes is very small small (<1A), but it has a profound effect on ferroelectric properties and its phase behavior. This general theory also allows to formulate the important open problems and show paths towards their solution. In particular, this is a problem of finding parameters of the system, which can sustain the ferroelectric memory over a desired lifetime.

cond-mat.mtrl-sci↗