arXiv subjects

A. N. Semenov

Publications and source records attributed to A. N. Semenov.

14 recordsLinked to original sources

arXiv2026

Fractional-Monolayer 2D-GaN/AlN Structures: Growth Kinetics and UVC-emitter Applications

The paper reports on fundamental properties of the GaN/AlN quantum wells (QWs) with nominal subcritical thicknesses of 0.75-2 monolayers (MLs). They are grown by plasma-activated molecular beam epitaxy, varying either the nominal thickness or the gallium-to-nitrogen flux ratio. In situ monitoring reveals difference in 2D nucleation and step-flow growth modes of the QWs. The emission charactestics of QWs with integer thicknesses of 1 and 2 MLs depend weakly on the growth mechanism. In contrast, the intensity and spectral position luminescence of QWs with fractional-ML thicknesses are determined by the growth mechanism. Using ab initio calculations, a phenomenological model is proposed that describes fractional-ML QWs either as arrays of 2D quantum disks or as arrays of 2D quantum ribbons, in cases where 2D nucleation or step-flow growth mechanisms predominate, respectively. This model is generally consistent with experimental data on photo- and cathodoluminescence of heterostructures with multiple (250) GaN/AlN QWs. These heterostructures, when pumped by electrom beam at an energy 12.5 keV with a maximum pulse current of 2 A, exhibit linear current dependences of optical peak powers up to 1 and 37 W for wavelengths of 228 and 256 nm, respectively, making them promising for use as powerfull ultraviolet-C emitters.

cond-mat.mtrl-sci↗

arXiv2023

Strain correlation functions in isotropic elastic bodies: Large wavelength limit for two-dimensional systems

Strain correlation functions in two-dimensional isotropic elastic bodies are shown both theoretically (using the general structure of isotropic tensor fields) and numerically (using a glass-forming model system) to depend on the coordinates of the field variable (position vector r in real space or wavevector q in reciprocal space) and thus on the direction of the field vector and the orientation of the coordinate system. Since the fluctuations of the longitudinal and transverse components of the strain field in reciprocal space are known in the long-wavelength limit from the equipartition theorem, all components of the correlation function tensor field are imposed and no additional physical assumptions are needed. An observed dependence on the field vector direction thus cannot be used as an indication for anisotropy or for a plastic rearrangement. This dependence is different for the associated strain response field containing also information on the localized stress perturbation

cond-mat.stat-mech↗

arXiv2023

Correlations of tensor field components in isotropic systems with an application to stress correlations in elastic bodies

Correlation functions of components of second-order tensor fields in isotropic systems can be reduced to an isotropic forth-order tensor field characterized by a few invariant correlation functions (ICFs). It is emphasized that components of this field depend in general on the coordinates of the field vector variable and thus on the orientation of the coordinate system. These angular dependencies are distinct from those of ordinary anisotropic systems. As a simple example of the procedure to obtain the ICFs we discuss correlations of time-averaged stresses in isotropic glasses where only one ICF in reciprocal space becomes a finite constant e for large sampling times and small wavevectors. It is shown that e is set by the typical size of the frozen-in stress components normal to the wavevectors, i.e. it is caused by the symmetry breaking of the stress for each independent configuration. Using the presented general mathematical formalism for isotropic tensor fields this finding explains in turn the observed long-range stress correlations in real space. Under additional but rather general assumptions e is shown to be given by a thermodynamic quantity, the equilibrium Young modulus E. We thus relate for certain isotropic amorphous bodies the existence of finite Young or shear moduli to the symmetry breaking of a stress component in reciprocal space.

cond-mat.stat-mech↗

arXiv2022

Spatial correlation functions for non-ergodic stochastic processes of macroscopic system

Focusing on non-ergodic macroscopic systems we reconsider the variances of time averages time-series. The total variance (direct average over all time-series) is known to be the sum of an internal variance (fluctuations within the meta-basins) and an external variance (fluctuations between meta-basins). It is shown that whenever the time-averaged observable can be expressed as a volume average of a local field the three variances can be written as volume averages of correlation functions with the total correlation function being the sum of an internal and an external correlation function. The dependences of the the different variancescan thus be traced back to the internal and the external correlation function. Various relations are illustrated using lattice spring models with spatially correlated spring constants.

cond-mat.stat-mech↗

arXiv2021

Simple models for strictly non-ergodic stochastic processes of macroscopic systems

We investigate simple models for strictly non-ergodic stochastic processes $x_t$ ($t$ being the discrete time step) focusing on the expectation value $v$ and the standard deviation $δv$ of the empirical variance $v[x]$ of finite time series $x$. $x_t$ is averaged over a fluctuating field $σ_{r}$ ($r$ being the microcell position) characterized by a quenched spatially correlated Gaussian field. Due to the quenched field $δv(Δt)$ becomes a finite constant, $Δ_{ne} > 0$, for large sampling times $Δt$. The volume dependence of the non-ergodicity parameter $Δ_{ne}$ is investigated for different spatial correlations. Models with marginally long-ranged $\fr$-correlations are successfully mapped on shear-stress data from simulated amorphous glasses of polydisperse beads.

cond-mat.dis-nn↗

arXiv2021

Fluctuations of non-ergodic stochastic processes

We investigate the standard deviation $δv(\tsamp)$ of the variance $v[\xbf]$ of time series $\xbf$ measured over a finite sampling time $\tsamp$ focusing on non-ergodic systems where independent "configurations" $c$ get trapped in meta-basins of a generalized phase space. It is thus relevant in which order averages over the configurations $c$ and over time series $k$ of a configuration $c$ are performed. Three variances of $v[\xbf_{ck}]$ must be distinguished: the total variance $\dvtot = \dvint + \dvext$ and its contributions $\dvint$, the typical internal variance within the meta-basins, and $\dvext$, characterizing the dispersion between the different basins. We discuss simplifications for physical systems where the stochastic variable $x(t)$ is due to a density field averaged over a large system volume $V$. The relations are illustrated for the shear-stress fluctuations in quenched elastic networks and low-temperature glasses formed by polydisperse particles and free-standing polymer films. The different statistics of $\svint$ and $\svext$ are manifested by their different system-size dependence

cond-mat.stat-mech↗

arXiv2020

Ensemble fluctuations matter for variances of macroscopic variables

Extending recent work on stress fluctuations in complex fluids and amorphous solids we describe in general terms the ensemble average $v(Δt)$ and the standard deviation $δv(Δt)$ of the variance $v[\mathbf{x}]$ of time series $\mathbf{x}$ of a stochastic process $x(t)$ measured over a finite sampling time $Δt$. Assuming a stationary, Gaussian and ergodic process, $δv$ is given by a functional $δv_G[h]$ of the autocorrelation function $h(t)$. $δv(Δt)$ is shown to become large and similar to $v(Δt)$ if $Δt$ corresponds to a fast relaxation process. Albeit $δv = δv_G[h]$ does not hold in general for non-ergodic systems, the deviations for common systems with many microstates are merely finite-size corrections. Various issues are illustrated for shear-stress fluctuations in simple coarse-grained model systems.

cond-mat.stat-mech↗

arXiv2013

Electrical anisotropy of heteroepitaxial InSb/GaAs layers

We report on study of electrical and structural properties of InSb/GaAs(001) heteroepitaxial layers in [110] and [1-10] crystallographic directions. Strong anisotropy of electron transport parameters measured at a low magnetic field has been found, whereas the electrical transport through the InSb bulk layer studied by Shubnikov-de Haas oscillations is shown to be independent of the crystallographic directions. The low-field electrical anisotropy of the InSb films appears to be governed by two competitive anisotropic effects: influence of spontaneously formed In nanoclusters inhomogeneously distributed within the InSb layers and conductivity through the near-interface layer with high anisotropic density of extended defects.

cond-mat.mtrl-sci↗

arXiv2012

Photoluminescence studies of Zeeman effect in type-II InSb/InAs nanostructures

Electron spin polarization up to 100% has been observed in type-II narrow-gap heterostructures with ultrathin InSb insertions in an InAs matrix via investigation of circularly polarized photoluminescence in an external magnetic field applied in Faraday geometry. The polarization degree decreases drastically, changes its sign, and saturates finally at the value of 10% in the limit of either high temperature or strong excitation. The observed effect is explained in terms of strong Zeeman splitting of the electron conduction band in the InAs matrix and a heavy-hole state confined in the InSb insertion, due to a large intrinsic g-factor of both types of carriers. The hole ground state in a monolayer scale InSb/InAs quantum well, calculated using a tight-binding approach, fits well the observed emission wavelength. Temperature dependence of the emission polarization degree is in good agreement with its theoretical estimation performed in the framework of a proposed phenomenological model.

cond-mat.mes-hall↗

arXiv2012

Spin-polarized electric currents in diluted magnetic semiconductor heterostructures induced by terahertz and microwave radiation

We report on the study of spin-polarized electric currents in diluted magnetic semiconductor (DMS) quantum wells subjected to an in-plane external magnetic field and illuminated by microwave or terahertz radiation. The effect is studied in (Cd,Mn)Te/(Cd,Mg)Te quantum wells (QWs) and (In,Ga)As/InAlAs:Mn QWs belonging to the well known II-VI and III-V DMS material systems, as well as, in heterovalent AlSb/InAs/(Zn,Mn)Te QWs which represent a promising combination of II-VI and III-V semiconductors. Experimental data and developed theory demonstrate that the photocurrent originates from a spin-dependent scattering of free carriers by static defects or phonons in the Drude absorption of radiation and subsequent relaxation of carriers. We show that in DMS structures the efficiency of the current generation is drastically enhanced compared to non-magnetic semiconductors. The enhancement is caused by the exchange interaction of carrier spins with localized spins of magnetic ions resulting, on the one hand, in the giant Zeeman spin-splitting, and, on the other hand, in the spin-dependent carrier scattering by localized Mn2+ ions polarized by an external magnetic field.

cond-mat.mes-hall↗

arXiv2011

Diluted magnetic semiconductor heterostructure AlSb/InAs/ZnMnTe with giant Zeeman effect for two dimensional electrons in InAs

A new approach to the growth of diluted magnetic semiconductors with two dimensional electron gas in InAs quantum well has been developed. The method is based on molecular-beam epitaxy of coherent "hybrid" AlSb/InAs/(Zn,Mn)Te heterostructures with a III-V/II-VI interface inside. The giant Zeeman splitting of the InAs conduction band caused by exchange interaction with Mn2+ ions has been proved by measuring the microwave radiation induced spin polarized electric currents.

cond-mat.mes-hall↗

arXiv2007

Intramolecular Form Factor in Dense Polymer Systems: Systematic Deviations from the Debye formula

We discuss theoretically and numerically the intramolecular form factor $F(q)$ in dense polymer systems. Following Flory's ideality hypothesis, chains in the melt adopt Gaussian configurations and their form factor is supposed to be given by Debye's formula. At striking variance to this, we obtain noticeable (up to 20%) non-monotonic deviations which can be traced back to the incompressibility of dense polymer solutions beyond a local scale. The Kratky plot ($q^2F(q)$ {\it vs.} wavevector $q$) does not exhibit the plateau expected for Gaussian chains in the intermediate $q$-range. One rather finds a significant decrease according to the correction $δ(F^{-1}(q)) = q^3/32ρ$ that only depends on the concentration $ρ$ of the solution, but neither on the persistence length or the interaction strength.

cond-mat.soft↗

arXiv2007

Why polymer chains in a melt are not random walks

A cornerstone of modern polymer physics is the `Flory ideality hypothesis' which states that a chain in a polymer melt adopts `ideal' random-walk-like conformations. Here we revisit theoretically and numerically this pivotal assumption and demonstrate that there are noticeable deviations from ideality. The deviations come from the interplay of chain connectivity and the incompressibility of the melt, leading to an effective repulsion between chain segments of all sizes $s$. The amplitude of this repulsion increases with decreasing $s$ where chain segments become more and more swollen. We illustrate this swelling by an analysis of the form factor $F(q)$, i.e. the scattered intensity at wavevector $q$ resulting from intramolecular interferences of a chain. A `Kratky plot' of $q^2F(q)$ {\em vs.} $q$ does not exhibit the plateau for intermediate wavevectors characteristic of ideal chains. One rather finds a conspicuous depression of the plateau, $δ(F^{-1}(q)) = |q|^3/32ρ$, which increases with $q$ and only depends on the monomer density $ρ$.

cond-mat.soft↗

arXiv2004

Long Range Bond-Bond Correlations in Dense Polymer Solutions

The scaling of the bond-bond correlation function $C(s)$ along linear polymer chains is investigated with respect to the curvilinear distance, $s$, along the flexible chain and the monomer density, $ρ$, via Monte Carlo and molecular dynamics simulations. % Surprisingly, the correlations in dense three dimensional solutions are found to decay with a power law $C(s) \sim s^{-ω}$ with $ω=3/2$ and the exponential behavior commonly assumed is clearly ruled out for long chains. % In semidilute solutions, the density dependent scaling of $C(s) \approx g^{-ω_0} (s/g)^{-ω}$ with $ω_0=2-2ν=0.824$ ($ν=0.588$ being Flory's exponent) is set by the number of monomers $g(ρ)$ contained in an excluded volume blob of size $ξ$. % Our computational findings compare well with simple scaling arguments and perturbation calculation. The power-law behavior is due to self-interactions of chains on distances $s \gg g$ caused by the connectivity of chains and the incompressibility of the melt. %

cond-mat.soft↗