Localized exciton magnetic polarons inCd 1 − x Mn
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Using the method of selective excitation of the exciton luminescence in ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Te epilayers we have measured energies of localized magnetic polarons (LMP's) for alloys with manganese mole fractions x\ensuremath{\le}0.34. The suppression of the LMP energy has been studied in external magnetic fields and with temperature increase. Polaron formation times and exciton lifetimes have been measured by time-resolved photoluminescence. We have found that in alloys with x0.17 the polaron formation process is interrupted by exciton recombination and, as a result, the LMP does not reach its equilibrium energy. This dynamical effect on the polaron energy together with the strong sensitivity of the LMP formation to the conditions of primary exciton localization causes the absence of the LMP formation in layers with x0.1. Antiferromagnetic clustering of Mn ions, which leads to the spin-glass phase formation at low temperatures, affects the polaron energy and results in the increasing stability of LMP's against suppression by temperature increase and magnetic fields. In ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Te with x>0.20 a considerable part of the polaron energy is controlled by the input of clusters of antiferromagnetically coupled Mn spins located in the nonuniform molecular field of localized excitons. The comparison of the exciton Zeeman splitting and the LMP magnetic-field suppression provides insight into the internal structure of LMP's.The velocity of polaron migration in the long poly-DNA chain (~40 base pairs) in an applied electric field has been studied within a polaron model. We found that the polaron velocity strongly depends on the polaron size. A small polaron shows a slow propagation and strong tolerance to the electric field, while a large polaron is much faster and less stable with increasing electric field. Moreover, the conductance of the DNA molecule within the polaron model is found to be sensitive to structural disorders in the DNA geometry, but that dependence diminishes with increasing temperature.
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We study the charge transport by polarons in conjugated polymers in the presence of impurities. The effects on the polaron motion due to the symmetry of the chain is considered. The polaron dynamics in doped conjugated polymers is numerically studied using the Su-Schrieffer-Heeger (SSH) model combined with the Pariser-Parr-Pople (PPP) model modified to include an external electric field and the Brazovskii-Kirova (BK) symmetry breaking interaction. The time dependent Hartree-Fock approximation is used. The electric field is used to accelerate the polaron in a polymer chain. Polaron trapped by the impurity and polaron-impurity collisions are considered. When the polaron-structure is around the impurity site a charge oscillation can be observed. Nevertheless, when there are collisions between the polaron and the impurity, the polaron-structure can pass, be trapped or be reflected. These effects are determined by the strength of the radical parameter and by the electric field intensity. The effects on the polaron are analyzed and an effective potential is determined. Therefore, the effective potential determines the polaron behavior for each case, as the polaron pass, or it is trapped or reflected by an impurity.
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Bi2WO6 (BWO) is considered as a promising material for photocatalytic water splitting. Its unique layered structure leads the charge separation, and transport is different from other materials. However, the charge transport mechanisms in BWO are not well understood. In this work, we investigated polaron formation and transport in BWO using the DFT+U and hybrid PBE0 functional approaches. We found that the electron will form 2-dimensional (2D)-shaped polarons among W sites in the ab plane of BWO with approximately 55% polaron density state on the central W site. This type of polaron is similar to the electron polarons in WO3. For other W-based materials, the electrons may also form a 2D-shaped polaron. We found that the W 6s orbital plays an important role in these 2D-shaped electron polarons. The calculated mobility of electron polarons in BWO was consistent with experimental findings. For the hole state, it could form a small hole polaron on the O site with O 2p in character. However, it will not form a polaron on the Bi site, which is quite different from BiVO4. This study provides insight for understanding polaron formation and transport in materials with W and Bi ions. It also provides understanding regarding charge separation and transport for materials with layered structures.
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An analytical theory based on the effective medium approach is formulated to describe nondispersive hopping charge transport in a disordered organic material where polaron effects are important. The treatment of polaron transport in solids with superimposed disorder and polaron effects is based on the Marcus jump rate equation, while the conventional Miller-Abrahams formalism is used to describe charge mobility in polaron-free systems. It is shown that the Poole-Frenkel-type field dependence of mobility $\mathrm{ln}\ensuremath{\mu}\ensuremath{\propto}\sqrt{E}$ occurs for both the bare charge carrier and the polaron transport provided that energetic correlation effects have been taken into account. We show that our polaron model can quantitatively explain the observed magnitudes of temperature- and electric-field-dependent polaron mobilities assuming physically reasonable values of polaron binding energies and transfer integrals; it gives a background for the development of the method for estimation of polaron binding energy and the energetic disorder parameter from these dependences. The results of the calculations are found to be in good agreement with both experimental results obtained for some \ensuremath{\sigma}-conjugated polysilylenes where polaron formation was straightforwardly demonstrated and recent computer simulations of polaron transport.
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Abstract Defect formation in insulators induced by electronic excitation is reviewed, emphasizing possible role on formation of tracks of heavy ions. Relaxation of excitons that produces selftrapped excitons and defects is discussed first. Evidence for defect formation by interaction of a selftrapped excitons with another excitation event is described. It is pointed out that track registration has been observed so far only in materials in which excitons are selftrapped. It is suggested that track registration occurs though interaction of selftrapped excitons among each other or with other excitation events.
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Explicit two-polaron solution is found in the continuum model of trans -polyacetylene. The solution is explored to examine stability of two polarons. It is found that two free polarons attract each other with a long range force until they form a single polaron with two carriers therein, which then dissociates into charged kink-antikink pair owing to a short range repulsion. Thus the reaction mechanism of two polarons into two charged kinks is clarified.
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