Polaron Motion Research Articles

Effect of temperature on polaron dynamics in poly-DNA

We investigate the effect of temperature on polaron dynamics in the framework of a tight-binding model. The dissociation of a polaron will become fast with the increase of temperature. There exists a crossover of the charge localization time from strong to weak temperature-dependence. Instead of the uniform motion at zero temperature, a polaron moves un-uniformly under a driven field at a finite temperature, which indicates a discrete hopping between base pairs. It is also found that the polaron motion is thermally activated. A high temperature will result in a fast movement of a polaron under a deriving field.

Dynamical Simulations of Polaron Transport in Conjugated Polymers with the Inclusion of Electron−Electron Interactions

Dynamical simulations of polaron transport in conjugated polymers in the presence of an external time-dependent electric field have been performed within a combined extended Hubbard model (EHM) and Su-Schrieffer-Heeger (SSH) model. Nearly all relevant electron-phonon and electron-electron interactions are fully taken into account by solving the time-dependent Schrödinger equation for the pi electrons and the Newton's equation of motion for the backbone monomer displacements by virtue of the combination of the adaptive time-dependent density matrix renormalization group (TDDMRG) and classical molecular dynamics (MD). We find that after a smooth turn-on of the external electric field, the polaron is accelerated at first and then moves with a nearly constant velocity as one entity consisting of both the charge and the lattice deformation. An ohmic region (3 < or = E0 < or = 9 mV/A) where the stationary velocity increases linearly with the electric field strength is observed for the case of U = 2.0 eV and V = 1.0 eV. The maximal velocity is well above the speed of sound. Below 3 mV/A, the polaron velocity increases nonlinearly, and in high electric fields with strengths of E0 > or = 10.0 mV/A, the polaron will become unstable and dissociate. The relationship between electron-electron interaction strengths and polaron transport is also studied in detail. We find that the on-site Coulomb interactions U will suppress the polaron transport, and small nearest-neighbor interactions V values are also not beneficial to the polaronic motion while large V values favor the polaron transport.

Electron spin resonance study of polycrystallineLa0.75(CaxSr1−x)0.25MnO3 (x = 0,0.45, 1)

Electron spin resonance (ESR) spectra of polycrystallineLa0.75(CaxSr1−x)0.25MnO3 (x = 0, 0.45, 1) were studied within the temperature range110 K≤T≤470 K. The temperature dependence of the ESR intensity for the samples is described by athermally activated model in the paramagnetic regime. It is found that the activationenergy in the orthorhombic phase is higher than that in the rhombohedral phase forLa0.75(Ca0.45Sr0.55)0.25MnO3. It is suggested that a higher energy is required to destroy the correlated polarons due tothe fact that correlated polarons only exist in the orthorhombic phase. This proposition isconfirmed by the analysis of the ESR linewidth data, which can be well fitted by the modelof adiabatic hopping motion of small polarons. In addition, it is found that, ata fixed temperature, the linewidth decreases with increasing Sr doping, whichreveals that the structural tolerance factor has a significant effect on the linewidth.

Boundary between coherent and noncoherent small polaron motion: Influence of the phonon hardening

The transition from band to hopping conductivity of small polaron is examined within Holstein's molecular crystal model. The conditions under which each of these mechanisms prevail are formulated in terms of the values of the coupling constant S and adiabatic parameter B. Particular attention was paid to the possible influence of the polaron induced modification of phonon spectrum.

Small polaron transport in Li 7MnN 4 containing isolated MnN 4 tetrahedra

The electrical transport properties related to the Mn 3d-band in Li 7MnN 4, which has been considered as an electrode material of Li ion secondary battery, are studied by combining with the optical absorption and electron paramagnetic resonance (EPR). The optical absorption studies show that the empty Mn 3d-band attributed to isolated MnN 4 tetrahedra in Li 7MnN 4 lies at about 1.15 eV above the top of the valence band. An EPR signal with g -value = 1.9848 is observed at 77 K, suggesting the presence of Mn 5+ in the isolated MnN 4 tetrahedra like that in Li 3MnO 4. The temperature dependence of electrical resistivity shows the hopping conduction with the activation energy of 112 meV at higher temperature above 180 K and the tunneling conduction at lower temperature below 60 K, suggesting the small polaron motion in the Mn-3d band as predicted by Holstein.

Effect of helix angles on polaron motion in poly-DNA

We investigate the effect of helix angles on polaron motion in poly-DNA in the framework of a tight binding model. A DNA molecule is supposed to have a uniform or a random distribution of the helix angles. It is found that a polaron moves faster along a DNA molecule with an overwinding helix than that with an unwinding one. In DNA molecule with randomly distributed helix angles, charge transport is suggested to be field-facilitated tunneling between spatial disordered potential wells.

Intrachain polaron motion and geminate combination in donor-acceptor copolymers: Effects of level offset and interfacial coupling

A model study of polaron motion and geminate combination in a molecular chain of donor-acceptor copolymers is presented. The simulations are performed within the framework of an extended version of the onedimensional Su-Schrieffer-Heeger tight-binding model. Two effects are mainly concerned: One is level offset, and the other is interfacial coupling. A general rule associated with the ratio of level offset to polaron or exciton binding energy is obtained to contribute to optimizing donor-acceptor copolymers for optoelectronic applications. According to the rule, we identify two cases for polaron motion and four cases for geminate combination of oppositely charged polarons. It is found that an interface with weak coupling serves as an energy barrier and that with strong coupling as a well, both of which can significantly affect the intrachain process in copolymers. The effect of electron-electron interaction is briefly discussed.

Dynamics of polarons in conjugated polymers: An adaptive time-dependent density-matrix renormalization-group study

The motion of polarons, which serve as charge carriers in conjugated polymers, is of fundamental importance for understanding transport properties of organic optoelectronic devices. We investigate the dynamics of a charged polaron in the presence of both electron-phonon and electron-electron interactions under the influence of an external electric field, which is modeled by the one-dimensional tight-binding Su-Schrieffer-Heeger (SSH) model supplemented with a Hubbard on-site repulsion term. For this many-body dynamical evolution problem, we develop an adaptive time-dependent density matrix renormalization group ($t$-DMRG) method in combination with a Newtonian equation of motion for atomic displacements. Our results show that the velocity of the polaron is suppressed by the on-site Coulomb interaction $U$. The polaron moves with a supersonic velocity, about four times the sound velocity at the small $U$ limit, and approaches the sound velocity at the large $U$ limit. Furthermore, the dependence of the polaron velocity and the polaron effective mass on the lattice structures are discussed.

Triplet energy transfer in conjugated polymers. II. A polaron theory description addressing the influence of disorder

Motivated by experiments monitoring motion of triplet excitations in a conjugated polymer containing Pt-atoms in the main chain (see Paper I), a theoretical formalism for electronic transport has been developed. It considers the interplay between polaronic distortion of the excited chain elements and disorder treated in terms of effective-medium theory. The essential parameters are the electronic coupling $J$, the polaronic binding energy $\ensuremath{\lambda}$ that determines the activation energy of polaron motion ${E}_{a}$, and the variance $\ensuremath{\sigma}$ of the density of states distribution controlling the incoherent hopping motion. It turns out that for the weak electronic coupling associated with triplet motion ($J$ a few meV), the transfer is nonadiabatic. For a critical ratio of $\ensuremath{\sigma}/{E}_{a}l0.3$, Marcus-type multiphonon transport prevails above a certain transition temperature. At lower temperatures, transport is disorder controlled consistent with the Miller-Abrahams formalism. Theoretical results are consistent with triplet transport in the Pt-polymer. Implications for charge and triplet motion in random organic semiconductors in general are discussed.

Dynamics of charge carriers photoinduced in poly(3-dodecylthiophene)/fullerene bulk heterojunction

Magnetic and dynamics properties of mobile polarons and fullerene anion-radicals photoinduced by laser beam in the poly(3-dodecylthiophene)/fullerene bulk heterojunction were studied. The number of these charge carriers was found to decrease with the growth in the photon energy. Monotonic temperature dependence for main electron relaxation parameters was obtained. The 1D polaron diffusion along the polymer chain and the fullerene rotation was shown to follow activation Elliot hopping model and to govern by the photon energy. The deviation in activation energies for polaron and anion-radical motion and the difference in their dependence on the laser irradiation photon energy prove a non-interacting character of these charge carriers photoinduced in the polymer/fullerene bulk heterojunction.

Dynamics of charge carriers photoinduced in poly(3-dodecylthiophene)/fullerene composite

Radical pairs, polarons and fullerene anion-radicals, photoinduced by laser with photon energies of 1.88, 2.22 and 2.75 eV in the poly(3-dodecylthiophene)/[6,6]-phenyl-C 61-butanoic acid methyl ester (P3DDT/PCBM) bulk heterojunction, were studied over a wide temperature range. The number of these centers was found to decrease with the increase in laser photon energy. Both the spin–lattice and spin–spin relaxation times of fullerene anion-radicals and the spin–spin relaxation time of polarons change monotonically with temperature, whereas the interaction of polarons with the lattice is characterized by extreme temperature dependence. The one-dimensional polaron diffusion along the polymer chain and the rotation of fullerene near it own main axis was shown to follow the activation Elliot hopping model and to be governed by the photon energy. The deviation in activation energies for polaron and anion-radical motion and the difference in their dependence on the laser photon energy prove the non-interacting character of these charge carriers photoinduced in the P3DDT/PCBM bulk heterojunction.

Evolution of conductivity, structure and thermochemical stability of lanthanum manganese iron nickelate perovskites

A series of solid solutions in the La0.8Sr0.2MnO3±δ (LSM)–xLa0.95Ni0.6Fe0.4O3−δ (LNF) system were synthesised by solid-state reaction to investigate the effect of Ni/Fe substitution for Mn on their DC electrical conductivity, structure and thermochemical stability. A non-linear change in structure, conductivity, magnetic properties and thermochemical stability of solid solutions was observed with the increase in the Ni/Fe content. A new phase (100 − x)LSM·xLNF with orthorhombic symmetry was found in the concentration range 35 ≤ x < 70 mol%, and exhibits the lowest value of the conductivity and the highest activation energy in the single-phase concentration range 43 ≤ x ≤ 53. This is, most probably, caused by a low concentration of Mn3+–O–Mn4+ units available for small polaron motion through the B sublattice. The compositions containing more than 50 mol% Ni in the B sublattice shows metallic-like conductivity behaviour. The temperature of the semiconducting metallic transition decreases with increase in the Ni concentration. All compositions had good thermochemical stability in air, in contrast to that under reducing conditions. The higher the Ni/Fe content in the sample, the faster the reduction of the solid solutions occurred in non-humidified Ar–H2 (95 : 5) atmosphere. The overall oxygen stoichiometry of the solid solutions is discussed.

Valence changes of manganese and praseodymium in Pr 1−xSr xMn 1−yIn yO 3−δ perovskites upon cation substitution as determined with XANES and ELNES

Systematic valence changes in Pr 1− x Sr x Mn 1− y In y O 3− δ upon cation substitution with Sr 2+ and In 3+ have been found using Mn K-edge and Pr L-edge X-ray absorption, and Mn L II,III and Pr M IV,V electron energy-loss spectroscopy. The average valence of the praseodymium ions is close to +3.0 and virtually constant over the sample set when the samples also contained manganese ions. Pr 0.5Sr 0.5InO 3− δ showed a distinct increase in the praseodymium valence state. In contrast, the average valence of the manganese ions changed from the trivalent state to intermediate values between +3.0 and +4.0 and approached the tetravalent state depending on the level of substitution. The knowledge of the valence is required to understand the conduction mechanisms in the material due to the small polaron hopping (electronic conductivity) and motion of oxygen ions along the vacancies (ionic conductivity). Addition of strontium and indium led to the formation of oxygen vacancies. A previously assumed intermediate valence of praseodymium as causal factor for the higher oxygen catalytic activity cannot be confirmed with room temperature measurements.

Unusual metallic in-plane and non-metallic c-axis charge transports in underdoped cuprates

We develop the theory of the metallic in-plane and non-metallic c-axis conductivities in the underdoped cuprates within the large (bi) polaron model and the precursor pairing model by considering a two-component charge carrier picture and the carrier-acoustic phonon scattering. The metallic transports of large polarons above the pairing pseudogap temperature T ∗ and polaron Cooper pairs below T ∗ in the CuO 2 layers of the cuprates are studied by using the appropriate Boltzmann equations in the relaxation time approximation and the extended BCS-like pairing model. We show that the in-plane resistivity in underdoped cuprates follows a T-linear law above T ∗ and deviates from the T-linear behavior below T ∗due to the precursor BCS-like pairing of large polarons. We argue that the non-metallic c-axis transport in these systems is caused by the dissociation of localized large bipolarons into two polarons and the motion of nondegenerate polarons scattered at acoustic phonons. The resistivity anisotropy is determined and compared with experimental results.

Nonlocal interactions in doped cuprates: Correlated motion of Zhang-Rice polarons

Intercarrier correlations in hole doped cuprates are investigated by ab initio multiconfiguration calculations. The dressed carriers display features that are reminiscent of both Zhang-Rice (ZR) $\mathrm{Cu}{\mathrm{O}}_{4}$ states and Jahn-Teller polarons. The interaction between these quasiparticles is repulsive. At dopings that are high enough, the interplay between long-range unscreened Coulomb interactions and long-range phase coherence among the O-ion half-breathing vibrations on the ZR plaquettes may lead to a strong reduction of the effective adiabatic energy barrier associated to each polaronic state. Tunneling effects cannot be neglected for a relatively flat, multiwell energy landscape. We suggest that the coherent, superconducting quantum state is the result of such coherent quantum lattice fluctuations involving in-plane O ions. Our findings appear to support phenomenological models where the superconductivity is related to a lowering of the in-plane kinetic energy.

Holstein light quantum polarons on the one-dimensional lattice

The polaron formation is investigated in the intermediate regime of the Holstein model by using an exact diagonalization technique for the one-dimensional infinite lattice. The numerical results for the electron and phonon propagators are compared to the nonadiabatic weak- and strong-coupling perturbation theories, as well as with the harmonic adiabatic approximation. A qualitative explanation of the crossover regime between the self-trapped and free-particle-like behaviors, not well understood previously, is proposed. It is shown that a fine balance of nonadiabatic and adiabatic contributions determines the motion of small polarons, making them light. A comprehensive analysis of spatially and temporally resolved low-frequency lattice correlations that characterize the translationally invariant polaron states is derived. Various behaviors of the polaronic deformation field, ranging from classical adiabatic for strong couplings to quantum nonadiabatic for weak couplings, are discussed.

Trapping and hopping of polaron in DNA periodic stack

The dynamic properties of a polaron in DNA stacks are calculated using the time-depended Su–Schrieffer–Heeger model containing the electric field term. The calculation results show that the on-site energy is an important factor influencing the polaron motion. When base pairs with different on-site energy arrange alternatively, the polaron would hop far quickly along this kind of DNA stack. But the polaron would be trapped when the two base pairs with the same on-site energy arrange together. The reason of the above phenomena is revealed according to the calculations.

Influence of A-site substitution on ESR spectra of lanthanum manganite perovskites

We studied electron spin resonance (ESR) spectra of La 0.7A′ 0.3MnO 3 (A′=Sr, Ba, Pb, and Cd) perovskites in a large range of temperature, from their Curie temperature to ∼500 K. Asymmetrical EPR signals at temperatures T C < T < T min , due to FM correlations, became Lorentzian at T ⩾ T min , where T min is the temperature corresponding to the narrowest ESR linewidth. The ESR linewidth vs. temperature, Δ H( T), in the region T > T min well fits to the model of adiabatic hopping motion of small polarons Δ H ( T ) = Δ H 0 + ( a / T ) exp ( - E a / k B T ) , where E a and k B are the activation energy and the Boltzmann factor, respectively; Δ H 0 and a are constants. It is worth noticing that, in this temperature range, the Lande factor g for the samples is about 2.00, indicating spin–spin interaction among electrons of Mn 3+ and Mn 4+ ions playing an important role.

Electrical and magnetic behaviors in Nd 0.7Sr 0.3MnO 3 with different annealing periods

We studied electrical and magnetic behaviors of polycrystalline samples of Nd 0.7Sr 0.3MnO 3 prepared by the solid-state reaction method at 1300 °C for 10, 20, 30, 40, and 80 h. Magnetization M( T), resistivity ρ( T) and X-band electron paramagnetic resonance (EPR) measurements were carried out. Experimental results showed that the Curie temperature T C decreased considerably with increasing the annealing time t a from 10 to 30 h, but became unchanged with t a>30 h. The insulating-metallic transition was observed at T MI. Spin-dynamic processes in the ferromagnetic and paramagnetic regimes were recorded by EPR. Electrical conductivity and the EPR linewidth (Δ H) versus temperature were analyzed by means of the adiabatic hoping motion of small polarons.

Correlated polaron transport in a quasi-one-dimensional liquid crystal

Charge-carrier transport in the columnar phase of the discotic liquid crystal, hexapentyloxytriphenylene, has been investigated by the time-of-flight technique over a range of temperatures and electric fields. The hole mobility was found to be temperature and electric-field-dependent with a maximum value of $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}{\mathrm{cm}}^{2}∕\mathrm{V}\phantom{\rule{0.2em}{0ex}}\mathrm{s}$. Its temperature dependence is consistent with a ${T}^{\ensuremath{-}n}$ power law, with the factor $n$ being dependent on the electric field. The drift velocity is a linear function of the electric field below ${10}^{\ensuremath{-}5}\phantom{\rule{0.3em}{0ex}}\mathrm{V}∕\mathrm{cm}$ and tends to saturate at higher fields. These results were interpreted in the framework of correlated polaron motion as described by the nonadiabatic low-temperature limit of Holstein's polaron theory developed for a one-dimensional diatomic chain.