arXiv subjects

A. I. Shushin

Publications and source records attributed to A. I. Shushin.

14 recordsLinked to original sources

arXiv2018

Specific features of the kinetics of singlet fission in organic semiconductors. The effect of kinetic curves crossing

Experimental investigations of magnetic field dependent kinetics of singlet fission (SF)processes in some organic semiconductors [i.e. splitting of excited singlet (S_1) state into a triplet exciton pair] have revealed the important specific feature of obtained kinetic curves, associated with decaying intensities I(t) of fluorescence from S_1-state. Kinetic curves, measured in different magnetic fields, are found to cross each other. We show that this kinetic curves crossing (KCC) effect is a general feature of geminate condensed phase reactions, resulting from simple characteristic properties of kinetic schemes of processes. Specific features of the KCC-effect are analyzed in detail with some models of SF-processes.

cond-mat.mtrl-sci↗

arXiv2017

Manifestation of T-Exciton Migration in the Kinetics of Singlet Fission in Organic Semiconductors

Kinetics of singlet fission in organic semiconductors, in which the excited singlet state (S_1) spontaneously splits into a pair of triplet (T) excitons, is known to be strongly influenced by back geminate annihilation of TT-pairs. We show that this influence can be properly described only by taking into account the diffusive exciton migration. The migration effect is treated in the model of two kinetically coupled states: the intermediate state of interacting TT-pairs and the state of migrating excitons. Within this model the singlet fission (including magnetic field effects) is studied as applied to the fluorescence decay kinetics (FDK) I_{S_1}(t) for S_1-state. The analysis shows that migration strongly affects the FDK resulting, in particular, in the universal long-time dependence I_{S_1}(t) \sim t^{-3/2}. The model accurately describes the FDK, recently observed for a number of systems. Possible applications of the considered model to the analysis of mechanisms of migration, using experimentally measured FDK, are briefly discussed.

cond-mat.mtrl-sci↗

arXiv2016

Photoinduced polarons in polymers. Time-resolved ESR analysis of polaron pairs in polymer:fullerene blends

The work concerns the analysis of experimental time-resolved ESR spectra in photoexcited polymer:fullerene blend, consisting of poly(3-hexilthiophene) and fullerene [6,6]-phenyl C_{61} -butyric acid methyl ester (at low temperature T = 100 K). The spectra are assumed to be determined by spin-coherent pairs of charged polarons P^{+} and P^{-} generated in the singlet state. The analysis is made within simple model of a set of first order processes, in which P^{+}P$^{-}-pair spin evolution is described by the stochastic Liouville equation, allowing for fairly accurate description of experimental results. Simple analytical interpretation of obtained numerical results demonstrates that trESR spectra can be represented as a superposition of antiphase and CIDEP contributions together with the conventional thermal one. These contributions are shown to change their signs with the increase of time in agreement with experimental observations.

cond-mat.mtrl-sci↗

arXiv2012

Generation of electron spin polarization in disordered organic semiconductors

The generation mechanisms of electron spin polarization (ESP) of charge carriers (electrons and holes, called "doublets") in doublet-doublet recombination and triplet-doublet quenching in disordered organic semiconductors are analyzed in detail. The ESP is assumed to result from quantum transitions between the states of the spin Hamiltonian of the pair of interacting particles. The value of the ESP is essentially determined by the mechanism of relative motion of particles. In our work we have considered the cage and free diffusion models. The effect of possible attractive spin-independent interactions between particles is also analyzed. Estimation with obtained formulas shows that the proposed mechanisms can lead to a fairly strong ESP much larger than the thermal one (at room temperatures)

cond-mat.mtrl-sci↗

arXiv2011

Magnetic field effects on electron-hole recombination in disordered organic semiconductors

Characteristic properties of magnetic field effects on spin selective geminate and bulk electron-hole polaron pair (PP) recombination are analyzed in detail within the approach based on the stochastic Liouville equation. Simple expressions for the magnetic field (B) dependence of recombination yield and rate are derived within two models of relative PP motion: free diffusion and diffusion in the presence of well (cage). The spin evolution of PPs is described taking in account the relaxation induced by hyperfine interaction, anisotropic part of the Zeeman interaction induced, as well as $Δg$-mechanism. A large variety of the $B$-dependences of the recombination yield $Y(B)$ and rate $K(B)$ is obtained depending on the relative weights of above-mentioned mechanisms. The proposed general method and derived particular formulas are shown to be quite useful for the analysis of recent experimental results.

cond-mat.mtrl-sci↗

arXiv2010

The effect of measurements, randomly distributed in time, on quantum systems. Stochastic quantum Zeno effect

The manifestation of measurements, randomly distributed in time, on the evolution of quantum systems are analyzed in detail. The set of randomly distributed measurements (RDM) is modeled within the renewal theory, in which the distribution is characterized by the probability density function (PDF) W(t) of times t between successive events (measurements). The evolution of the quantum system affected by the RDM is shown to be described by the density matrix satisfying the stochastic Liouville equation. This equation is applied to the analysis of the RDM effect on the evolution of a two level systems for different types of RDM statistics, corresponding to different PDFs W(t). Obtained general results are illustrated as applied to the cases of the Poissonian [W(t) ~ e^{-w_r t}] and anomalous [W(t) ~ 1/t^{1+α}, (αis smaller or equal to 1)], RDM statistics. In particular, specific features of the quantum and inverse Zeno effects, resulting from the RDM, are thoroughly discussed.

quant-ph↗

arXiv2010

Relaxation in quantum systems. Manifestation of the state-selective reactive decay

The effect of state-selective reactive decay on the relaxation kinetics of quantum multistate systems is studied in detail in the Bloch-Redfield approach (BRA). The results are applied to the analysis of this effect in radical pair recombination kinetics. The BRA is shown to be able to describe quantitatively most important specific features of the recombination kinetics including those predicted by phenomenological treatment and by recently proposed approaches based on quantum measurement theories.

quant-ph↗

arXiv2009

The kinetics of escaping of Brownian particles from a potential well for different space dimensionality. The effect of external force

The kinetics of two (2D) and three (3D) dimensional diffusion-assisted escaping of Brownian particles from a potential well in the presence of an external force is analyzed in detail. The kinetics is studied within the two-state model (TSM) proposed for processes in the absence of external force. The generalized variant of this model, taking into account the force effect, is proposed which is shown to be quite accurate for some shapes of the well both for 2D and 3D processes. Within the generalized TSM simple expressions for the well depopulation kinetics and, in particular, for the escape rate are obtained. The effect of the force ($F$) is shown to manifest itself in the escape rate dependence on the only parameter $φ= Fa/(2k_b T)$, where $a$ is the Onsager radius of the attractive part of the well $U(r)$, defined by the relation $|U(a)| \approx k_b T$. The limiting behavior of this dependence in the cases of weak and strong force is studied in detail both in 2D and 3D processes. Some applications of obtained results to the analysis of experiments are briefly discussed.

cond-mat.stat-mech↗

arXiv2009

External force affected escape of Brownian particles from a potential well

The effect of an external force on the kinetics of diffusion-assisted escaping of Brownian particles from a potential well is analyzed in detail. The analysis is made within the two-state model of the process which is known to be valid in the deep well limit in the absence of external force. The generalized variant of this model, taking into account the effect of the force, is shown to be quite accurate as well for some shapes of the well. Within the generalized two-state model simple expressions for the well depopulation kinetics and, in particular, the for the escape rate are obtained. These expressions show that the effect of the force ($F$) manifests itself in the escape rate dependence on the only parameter $φ= Fa/(2k_b T)$, where $a$ is the Onzager radius of the attractive part of the well $U(r)$, defined by the relation $|U(a)| = k_b T$. The limiting behavior of this dependence in the cases of weak and strong force is analyzed in detail. Possible applications as well as the relation of the results of the analysis to those obtained earlier are briefly discussed.

cond-mat.soft↗

arXiv2008

Specific features of the effect of time dependent field on subdiffusing particles. The stochastic Liouville equation approach

We analyze the effect of time dependent external field on non-Markovian migration described by the continuous time random walk (CTRW) approach. The rigorous method of treating the problem is proposed which is based on the Markovian representations of the CTRW approach and field modulation. The method is applied to the case of subdiffusive migration in which the exact formulas for the first and second moments of spatial distribution are derived. For oscillating external field they predict unusual dependence of the first moment on oscillation phase and anomalous field dependent contribution to

cond-mat.stat-mech↗

arXiv2008

Kinetics of geminate recombination of subdiffusing particles in the presence of interparticle interaction

The kinetics of geminate subdiffusion-assisted reactions (SDARs) of interacting particles is analyzed in detail with the use of the non-Markovian fractional Smoluchowki equation (FSE). It is suggested that the interparticle interaction potential is of the shape of potential well and reactivity is located within the well. The reaction kinetics is studied in the limit of deep well, in which the FSE can be solved analytically. This solution enables one to obtain the kinetics in a simple analytical form. The analytical expression shows that the SDAR kinetics fairly substantially depends on the mechanism of reactivity within the well. Specific features of the kinetics are thoroughly analyzed in two models of reactivity: the subdiffusion assisted activated rate model and the first order reaction model. The theory developed is applied to the interpretation of experimental kinetics of photoluminescence decay in amorphous $a$-Si:H semiconductors governed by geminate recombination of electrons and holes which are recently found to undergo subdiffusive relative motion. Analysis of results demonstrates that the subdiffusion assisted activated rate mechanism of reaction is closer to reality as applied to amorphous $a$-Si:H semiconductors. Comparison of experimental and theoretical kinetics allowed for obtaining some kinetic parameters of the systems under study: the rate of escaping from the well and the parameter characterizing the deviation of the subdiffusive motion from the conventional one.

cond-mat.stat-mech↗

arXiv2007

Non-Markovian Stochastic Liouville equation and its Markovian representation. Extensions of the continuous time random walk approach

Some specific features and extensions of the continuous time random walk (CTRW) approach are analyzed in detail within the Markovian representation (MR) and CTRW-based non-Markovian stochastic Liouville equation (SLE). In the MR CTRW processes are represented by multidimensional Markovian ones. In this representation the probability distribution function (PDF) W(t) of fluctuation renewals is associated with that of reoccurrences in a certain jump state of some Markovian controlling process. Within the MR the non-Markovian SLE, which describes the effect of CTRW-like noise on relaxation of dynamic and stochastic systems, is generalized to take into account the influence of relaxing systems on statistical properties of noise. The generalized non-Markovian SLE is applied to study two modifications of the CTRW approach. One of them considers the cascaded CTRWs in which the controlling process is actually CTRW-like one controlled by another CTRW process, controlled in turn by the third one, etc. Within the MR simple expression for the PDF W(t) of total controlling process is obtained in terms of Markovian variants of controlling PDFs in the cascade. The expression is shown to be especially simple and instructive in the case of anomalous processes determined by long time tailed W(t). The cascaded CTRWs can model the effect of complexity of a system on relaxation kinetics (in glasses, fractals, branching media, ultrametric structures, etc.). Another CTRW-modification describes the kinetics of processes governed by fluctuating W(t). Within the MR the problem is analyzed in a general form without restrictive assumptions on correlations of PDFs of consecutive renewals. The analysis shows that W(t) can strongly affect the kinetics of the process. Possible manifestations of this effect are discussed.

cond-mat.stat-mech↗

arXiv2004

Anomalous relaxation in quantum systems and the non-Markovian stochastic Liouville equation

The kinetics of relaxation in quantum systems induced by anomalous fluctuating noise is studied in detail. In this study two processes are considered, as examples: (1) relaxation in two-level system caused by external noise with slowly decaying correlation function P (t) ~ (wt)^{-alpha}, where 0 < alpha < 1, and (2) anomalous-diffusion controlled radical pair recombination in which relaxation results from the reaction with slowly fluctuating reaction rate whose fluctuations are governed by subdiffusive relative motion of radicals in a potential well. Analysis of these two processes is made within continuous time random walk approach (CTRWA). Rigorous CTRWA-treatment of the processes under study results in the non-Markovian stochastic Liouville equation (SLE) for the density matrix of the systems. This SLE predicts important specific features of relaxation kinetics of quantum systems in the presence of the above mentioned anomalous noise.

cond-mat.stat-mech↗

arXiv2004

Non-Markovian stochastic Liouville equation and anomalous relaxation kinetics

The kinetics of phase and population relaxation in quantum systems induced by noise with anomalously slowly decaying correlation function P (t) ~ (wt)^{- alpha}, where 0 < alpha < 1 is analyzed within continuous time random walk approach. The relaxation kinetics is shown to be anomalously slow. Moreover for alpha < 1 in the limit of short characteristic time of fluctuations w^{-1} the kinetics is independent of w. As alpha \to 1 the relaxation regime changes from the static limit to fluctuation narrowing. Simple analytical expressions are obtained describing the specific features of the kinetics.

cond-mat.stat-mech↗