arXiv subjects

A. V. Dvurechenskii

Publications and source records attributed to A. V. Dvurechenskii.

14 recordsLinked to original sources

arXiv2022

Comment on "Interplay of Structural and Optoelectronic Properties in Formamidinium Mixed Tin-Lead Triiodide Perovskites"

Studying optoelectronic properties in FAPb$_{1-x}$Sn$_x$I$_3$ perovskites as a function of the lead:tin content, Parrott et al. observed the broadest luminescence linewidth and the largest luminescence Stokes shift in mixed compositions with Sn $ < 25$% and with $> 0.85$%. Since the largest effects of alloy disorder were expected for the 50:50 composition, it was concluded that the revealed disorder effects might arise from extrinsic factors that can be eliminated upon further crystal growth optimization. This comment shows that the largest effects of alloy disorder for perfectly random fluctuations in FAPb$_{1-x}$Sn$_x$I$_3$ perovskite are, in fact, expected for $x < 0.25$ and for $x > 0.85$. Therefore, further crystal growth optimization is futile.

cond-mat.mtrl-sci↗

arXiv2019

Percolation description of charge transport in the random barrier model applied to amorphous oxide semiconductors

Charge transport in amorphous oxide semiconductors is often described as the band transport affected by disorder in the form of random potential barriers (RB). Theoretical studies in the framework of this approach neglected so far the percolation nature of the phenomenon. In this article, a recipe for theoretical description of charge transport in the RB model is formulated using percolation arguments. Comparison with the results published so far evidences the superiority of the percolation approach.

cond-mat.dis-nn↗

arXiv2019

Percolation description of charge transport in amorphous oxide semiconductors

The charge transport mechanism in amorphous oxide semiconductors (AOS) is a matter of controversial debates. Most theoretical studies so far neglected the percolation nature of the phenomenon. In this article, a recipe for theoretical description of charge transport in AOSs is formulated using the percolation arguments. Comparison with the previous theoretical studies shows a superiority of the percolation approach. The results of the percolation theory are compared to experimental data obtained in various InGaZnO materials revealing parameters of the disorder potential in such AOS.

cond-mat.dis-nn↗

arXiv2017

Fundamental characteristic length scale for the field dependence of hopping charge transport in disordered organic semiconductors

Using analytical arguments and computer simulations we show that the dependence of the hopping carrier mobility on the electric field $μ(F)/μ(0)$ in a system of random sites is determined by the localization length $a$, and not by the concentration of sites $N$. This result is in drastic contrast to what is usually assumed in the literature for a theoretical description of experimental data and for device modeling, where $N^{-1/3}$ is considered as the decisive length scale for $μ(F)$. We show that although the limiting value $μ(F \rightarrow 0)$ is determined by the ratio $N^{-1/3}/a$, the dependence $μ(F)/μ(0)$ is sensitive to the magnitude of $a$, and not to $N^{-1/3}$. Furthermore, our numerical and analytical results prove that the effective temperature responsible for the combined effect of the electric field $F$ and the real temperature $T$ on the hopping transport via spatially random sites can contain the electric field only in the combination $eFa$.

cond-mat.mes-hall↗

arXiv2017

Elastic strain field due to an inclusion of a polyhedral shape with a non-uniform lattice misfit

An analytical solution in a closed form is obtained for the three-dimensional elastic strain distribution in an unlimited medium containing an inclusion with a coordinate-dependent lattice mismatch (an eigenstrain). Quantum dots consisting of a solid solution with a spatially varying composition are examples of such inclusions. It is assumed that both the inclusion and the surrounding medium (the matrix) are elastically isotropic and have the same Young modulus and Poisson ratio. The inclusion shape is supposed to be an arbitrary polyhedron, and the coordinate dependence of the lattice misfit, with respect to the matrix, is assumed to be a polynomial of any degree. It is shown that, both inside and outside the inclusion, the strain tensor is expressed as a sum of contributions of all faces, edges and vertices of the inclusion. Each of these contributions, as a function of the observation point's coordinates, is a product of some polynomial and a simple analytical function, which is the solid angle subtended by the face from the observation point (for a contribution of a face), or the potential of the uniformly charged edge (for a contribution of an edge), or the distance from the vertex to the observation point (for a contribution of a vertex). The method of constructing the relevant polynomial functions is suggested. We also found out that similar expressions describe an electrostatic or gravitational potential, as well as its first and second derivatives, of a polyhedral body with a charge/mass density that depends on coordinates polynomially.

cond-mat.mes-hall↗

arXiv2016

Structures with Vertically Stacked Ge/Si Quantum Dots for Logical Operations

Ge/Si structures with vertically stacked quantum dots are simulated to implement the basic elements of a quantum computer for operation with electron spin states. Elastic-strain fields are simulated using the conjugate gradient method and an atomistic model based on the Keating potential. Calculations are performed in the cluster approximation using clusters containing about three million atoms belonging to 150 coordination spheres. The spatial distributions of the strain energy density and electron potential energy are calculated for different valleys forming the bottom of the silicon conduction band. It is shown that the development of multilayer structures with vertically stacked quantum dots makes it possible to fabricate deep potential wells for electrons with vertical tunnel coupling.

cond-mat.mes-hall↗

arXiv2015

Quantum logic gates from time-dependent global magnetic field in a system with constant exchange

We propose a method for implementation of an universal set of one- and two-quantum-bit gates for quantum computation in the system of two coupled electrons with constant non-diagonal exchange interaction. Suppression of the exchange interaction is offered to implement by all-the-time repetition of single spin rotations. Small g-factor difference of electrons allows to address qubits and to avoid strong magnetic field pulses. It is shown by means of numerical experiments that for implementation of one- and two-qubit operations it is sufficient to change the amplitude of the magnetic field within a few Gauss, introducing in a resonance one and then the other electron. To find the evolution of the two-qubit system, we use the algorithms of the optimal control theory.

cond-mat.mes-hall↗

arXiv2013

Spin relaxation in inhomogeneous quantum dot arrays studied by electron spin resonance

Electron states in a inhomogeneous Ge/Si quantum dot array with groups of closely spaced quantum dots were studied by conventional continuous wave ($cw$) ESR and spin-echo methods. We find that the existence of quantum dot groups allows to increase the spin relaxation time in the system. Created structures allow us to change an effective localization radius of electrons by external magnetic field. With the localization radius close to the size of a quantum dot group, we obtain fourfold increasing spin relaxation time $T_1$, as compared to conventional homogeneous quantum dot arrays. This effect is attributed to averaging of local magnetic fields related to nuclear spins $^{29}$Si and stabilization of $S_z$-polarization during electron back-and-forth motion within a quantum dot group.

cond-mat.mes-hall↗

arXiv2013

Universal behavior of magnetoresistance in quantum dot arrays with different degree of disorder

Magnetoresistance in two-dimensional array of Ge/Si quantum dots was studied in a wide range of zero-magnetic field conductances, where the transport regime changes from hopping to diffusive one. The behavior of magnetoresistance is found to be similar for all samples - it is negative in weak fields and becomes positive with increase of magnetic field. The result apparently contradicts to existing theories. To explain experimental data we suggest that clusters of overlapping quantum dots are formed. These clusters are assumed to have metal-like conductance, the charge transfer taking place via hopping between the clusters. Relatively strong magnetic field shrinks electron wave functions decreasing inter-cluster hopping and, therefore, leading to a positive magnetoresistance. Weak magnetic field acts on "metallic" clusters destroying interference of electron wave function corresponding to different paths (weak localization) inside clusters. The interference may be restricted either by inelastic processes, or by the cluster size. Taking into account WL inside clusters and hopping between them within the effective medium approximation we extract effective parameters characterizing charge (magneto) transport.

cond-mat.mes-hall↗

arXiv2010

Role of Diffusion in Two-dimensional Bimolecular Recombination

Experiments on carrier recombination in two-dimensional organic structures are often interpreted in the frame of the Langevin model with taking into account only the drift of the charge carriers in their mutual electric field. While this approach is well justified for three-dimensional systems, it is in general not valid for two-dimensional structures, where the contribution of diffusion can play a dominant role. We study the two-dimensional Langevin recombination theoretically and find the critical concentration below which diffusion cannot be neglected. For typical experimental conditions, neglecting the diffusion leads to an underestimation of the recombination rate by several times.

cond-mat.dis-nn↗

arXiv2009

Effect of Electric Field on Diffusion in Disordered Materials I. One-dimensional Hopping Transport

An exact analytical theory is developed for calculating the diffusion coefficient of charge carriers in strongly anisotropic disordered solids with one-dimensional hopping transport mode for any dependence of the hopping rates on space and energy. So far such a theory existed only for calculating the carrier mobility. The dependence of the diffusion coefficient on the electric field evidences a linear, non-analytic behavior at low fields for all considered models of disorder. The mobility, on the contrary, demonstrates a parabolic, analytic field dependence for a random-barrier model, being linear, non-analytic for a random energy model. For both models the Einstein relation between the diffusion coefficient and mobility is proven to be violated at any finite electric field. The question on whether these non-analytic field dependences of the transport coefficients and the concomitant violation of the Einstein's formula are due to the dimensionality of space or due to the considered models of disorder is resolved in the following paper [Nenashev et al., arXiv:0912.3169], where analytical calculations and computer simulations are carried out for two- and three-dimensional systems.

cond-mat.dis-nn↗

arXiv2009

Effect of Electric Field on Diffusion in Disordered Materials II. Two- and Three-dimensional Hopping Transport

In the previous paper [Nenashev et al., arXiv:0912.3161] an analytical theory confirmed by numerical simulations has been developed for the field-dependent hopping diffusion coefficient D(F) in one-dimensional systems with Gaussian disorder. The main result of that paper is the linear, non-analytic field dependence of the diffusion coefficient at low electric fields. In the current paper, an analytical theory is developed for the field-dependent diffusion coefficient in three- and two-dimensional Gaussian disordered systems in the hopping transport regime. The theory predicts a smooth parabolic field dependence for the diffusion coefficient at low fields. The result is supported by Monte Carlo computer simulations. In spite of the smooth field dependences for the mobility and for the longitudinal diffusivity, the traditional Einstein form of the relation between these transport coefficients is shown to be violated even at very low electric fields.

cond-mat.dis-nn↗

arXiv2009

Strain distribution in quantum dot of arbitrary polyhedral shape: Analytical solution in closed form

An analytical expression of the strain distribution due to lattice mismatch is obtained in an infinite isotropic elastic medium (a matrix) with a three-dimensional polyhedron-shaped inclusion (a quantum dot). The expression was obtained utilizing the analogy between electrostatic and elastic theory problems. The main idea lies in similarity of behavior of point charge electric field and the strain field induced by point inclusion in the matrix. This opens a way to simplify the structure of the expression for the strain tensor. In the solution, the strain distribution consists of contributions related to faces and edges of the inclusion. A contribution of each face is proportional to the solid angle at which the face is seen from the point where the strain is calculated. A contribution of an edge is proportional to the electrostatic potential which would be induced by this edge if it is charged with a constant linear charge density. The solution is valid for the case of inclusion having the same elastic constants as the matrix. Our method can be applied also to the case of semi-infinite matrix with a free surface. Three particular cases of the general solution are considered--for inclusions of pyramidal, truncated pyramidal, and "hut-cluster" shape. In these cases considerable simplification was achieved in comparison with previously published solutions. A generalization of the obtained solution to the case of anisotropic media is discussed.

cond-mat.other↗

arXiv2008

Hopping conduction in strong electric fields: Negative differential conductivity

Effects of strong electric fields on hopping conductivity are studied theoretically. Monte-Carlo computer simulations show that the analytical theory of Nguyen and Shklovskii [Solid State Commun. 38, 99 (1981)] provides an accurate description of hopping transport in the limit of very high electric fields and low concentrations of charge carriers as compared to the concentration of localization sites and also at the relative concentration of carriers equal to 0.5. At intermediate concentrations of carriers between 0.1 and 0.5 computer simulations evidence essential deviations from the results of the existing analytical theories. The theory of Nguyen and Shklovskii also predicts a negative differential hopping conductivity at high electric fields. Our numerical calculations confirm this prediction qualitatively. However the field dependence of the drift velocity of charge carriers obtained numerically differs essentially from the one predicted so far. Analytical theory is further developed so that its agreement with numerical results is essentially improved.

cond-mat.dis-nn↗