Dynamic process of self-trapped polaron in photoexcited SrTiO3

Relaxation process of self-trapped polaron is investigated by a nonadiabatic molecular dynamic method. We show localized disorder due to lattice fluctuations can give rise to a tightly-bound electronic state in ultraviolet illuminated SrTiO₃ crystal. This bound state is actually a self-trapped polaron in accordance with the experimentally observed large Stokes-shift. The formation of the self-trapped polaron is shown to be an ultrafast process.     

Lattice vibrational analysis of polyacene

Within an extended Su-Schrieffer-Heeger model, we made a lattice vibrational analysis of polyacene. In a singly-charged polyacene, the ground state contains an interchain-coupled polaron of quasi-D2h symmetry, around which we found thirteen localized modes in total. Among these localized modes, five (three B2u and two B3u) are infrared active, six (four Ag and two B1g) modes are Raman active, and the other two localized modes are asymmetric, which are both infrared active and Raman active. For the case a charged polaron is coupled with a neutral soliton in a finite polyacene chain, the vibrational modes are also calculated to display the coupling effect between self-trapping excitations on phonons. It is found that the localized phonons are determined mainly by the charged polaron, but the number and frequencies of the localized modes are influenced by the existence of the neutral soliton.     

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.     

On the possibility of electric transport mediated by long living intrinsic localized solectron modes

We consider a polaron Hamiltonian in which not only the lattice and the electron-lattice interactions, but also the electron hopping term is affected by anharmonicity. We find that the one-electron ground states of this system are localized in a wide range of the parameter space. Furthermore, low energy excited states, generated either by additional momenta in the lattice sites or by appropriate initial electron conditions, lead to states constituted by a localized electron density and an associated lattice distortion, which move together through the system, at subsonic or supersonic velocities. Thus we investigate here the localized states above the ground state which correspond to moving electrons. We show that besides the stationary localized electron states (proper polaron states) there exist moving localized solectron states which can be easily excited. The evolution of these localized states suggests their potential as new carriers for fast electric charge transport.     

Polarization catastrophe in the polaronic Wigner crystal

We consider a three dimensional Wigner crystal of electrons lying in a host ionic dielectric. Owing to their interaction with the lattice polarization, each localized electron forms a polaron. We study the collective excitations of such a polaronic Wigner crystal at zero temperature, taking into account the quantum fluctuations of the polarization within the Feynman harmonic approximation. We show that, contrary to the ordinary electron crystal, the system undergoes a polarization catastrophe when the density is increased. An optical signature of this instability is derived, whose trend agrees with the experiments carried out in Nd-based cuprates. Received 4 July 2002 Published online 17 September 2002     

Self-consistent treatment of the polaron damping in the Holstein model

Using the Holstein model, the damping of the localized polaron state and the small polaron conductivity are represented by the series of contributions from the multiphonon processes in which the polaron damping is taken into account. The self-consistent treatment of the damping shows the enhancement of the small-polaron relaxation rate by the quantum fluctuations of the lattice corresponding to the energy non-conserving phonon processes. The conditions for the polaron localization as well as the dependence of the d.c. conductivity on the electron transfer integral are discussed.     

Polarons in quantum chains with XY exchange and Dzyaloshinsky-Moriya interaction

Excitations of the polaron types are investigated in the spin-1/2 quantum chain with XY exchange and Dzyaloshinsky-Moriya interaction, both coupled to acoustic vibrations of the substrate lattice. The study is carried out via Jordan-Wigner transformation with the help of which the spin chain is mapped onto a chain of spinless fermions. From the resulting effective fermion-lattice Hamiltonian, the discrete equations of motion are derived. These equations are solved in the continuum limit for self-trapped states near the bottom of the fermion spectrum interacting with long-wavelength acoustic lattice modes. The associate polaron solution, which has a pulse shape, is shown to propagate bound to the induced lattice kink distortion by translation along the chain at a constant velocity v. The pair can also experience an additional acceleration ϑ₀ when the free fermion charge is excited above its groundstate. The polaron binding energy is strongly reduced, depending quadratically on the ratio D/J of the Dzyaloshinsky-Moriya interaction strength D to the isotropic XY exchange interaction J. It is also found that polaron parameters depend only on the XY spin-lattice coupling but not on the Dzyaloshinsky-Moriya contribution.     

Polaron stability in molecular crystals

A semi-empirical Peierls–Holstein model is applied to studies of the stability of polarons in two-dimensional molecular crystal systems. Calculations for a broad range of intra- and inter-molecular parameters within this model were performed in order to obtain detailed knowledge concerning the stability of the polaron solution with respect to a rigid lattice band solution. For realistic values of the parameters the polaron solution is stable with a polaron energy in the range 50–100 meV. A metastable polaron solution is also identified. The polarons that result from our model are highly localized and it is questionable if adiabatic polaron transport can occur in the system.     

Low temperature specific heat and thermal conductivity of bulk metallic glass (Cu50Zr50)94Al6

The low temperature specific heat and thermal conductivity of (Cu₅₀Zr₅₀)₉₄Al₆ bulk metallic glass have been studied experimentally. A low temperature anomaly in the specific heat is observed in this alloy. It is also found that in addition to Debye oscillators, the localized vibration modes whose vibration density of state has a Gaussian distribution should be considered to explain the low temperature phonon specific heat anomaly. The phonon thermal conductivity dependence on temperature for the sample does not show apparent plateau characteristics as other glass materials do; however, the influence of the resonant scattering from the localized modes on the lattice thermal conductivity is prominent in the bulk metallic glass at low temperatures.     

Dispersion Theory of Polaron in a Parabolic Quantum Dot at Finite Temperatures

By using the dispersion theory instead of the Fröhlich Hamiltonian, the polaron energy in a quantum dot with a parabolic confinement potential is investigated at finite temperatures. It is found that the self-trapping energy of the polaron decreases with the increasing temperature, and the temperature effect is more obvious in quantum dots with weaker confinement.     

Universal crossover from band to hopping conduction in molecular organic semiconductors

The charge transport in a variety of herringbone-stacked organic molecular semiconductors is investigated in the temperature range from 10 to 550 K. A crossover from coherent bandlike charge transport with mobilities up to several thousand cm (2)/V s at low temperature to an incoherent hopping motion at high temperatures is observed. This is attributed to the localization of the charge carrier due to increased electron-phonon interaction and, finally, the formation of a lattice polaron.     

极化子有效质量与温度的关系

本文计及离子晶格的原子结构,用紧束缚法导出了极化子的自陷能和有效质量,给出了有效质量与温度的关系。 关键词：     

Electronic transport mechanisms in tetraphenyleprophyrin thin films

Thermally-evaporated thin films of tetraphenylporphyrin, TPP, with thickness range from (175 to 735) nm had been prepared. Annealing temperatures ranging from (273–473) K do not influence the amorphous structure of these films. The influence of environmental conditions: film thickness, temperature and frequency on the electrical properties of TPP thin films had been reported. It was found that dc conductivity increases with increasing temperature and film thickness. The extrinsic conduction mechanism is operating in temperature range of (293–380) K with activation energy of 0.13 eV. The intrinsic one is in temperatures >380 K via phonon assisted hopping of small polaron with activation energy of 0.855 eV. The ac electrical conductivity and dielectric relaxation in the temperature range (293–473) K and in frequency range (0.1–100) kHz had been also studied. It had been shown that theoretical curves generated from correlated barrier hopping (CBH) model gives the best fitting with experimental results. Analysis of these results proved that conduction occurs at low temperatures (300–370) K by phonon assisted hopping between localized states and it is performed by single polaron hopping process at higher temperatures. The temperature and frequency dependence of both the real and imaginary parts of dielectric constant had been reported.     

Angle-resolved photoemission spectroscopy of the insulating NaxWO3: Anderson localization, polaron formation, and remnant Fermi surface

The electronic structure of the insulating sodium tungsten bronze, Na(0.025)WO(3), is investigated by high-resolution angle-resolved photoemission spectroscopy. We find that near-E(F) states are localized due to the strong disorder arising from random distribution of Na+ ions in the WO(3) lattice, which makes the system insulating. The temperature dependence of photoemission spectra provides direct evidence for polaron formation. The remnant Fermi surface of the insulator is found to be the replica of the real Fermi surface in the metallic system.     

Orbital polaron lattice formation in lightly doped

By resonant x-ray scattering at the Mn K edge on La(7/8)Sr(1/8)MnO3, we show that an orbital polaron lattice (OPL) develops at the metal-insulator transition of this compound. This orbital reordering explains consistently the unexpected coexistence of ferromagnetic and insulating properties at low temperatures, the quadrupling of the lattice structure parallel to the MnO2 planes, and the observed polarization and azimuthal dependencies. The OPL is a clear manifestation of strong orbital-hole interactions, which play a crucial role for the colossal magnetoresistance effect and the doped manganites in general.     

The Electronic and Lattice Structures of the Groundand the Polaron States of Polymer Poly (Phenylene Vinylene) (PPV)

The electronic and lattice structures of poly (phenylene vinylene) (PPV) are studied theoretically. Both the electron-electron and electron-phonon interactions are taken into account in the Pariser-Parr-Pople model. The electronic band and the lattice structure of the ground state and the polaronic state are calculated by means of the unrestricted Hartree-Fock method. In the ground state, there exist eight bands in PPV including four valence bands and four conduction bands, and the benzenes can be considered to be rigid. The polaron induces the split of energy bands. There are four localized electronic states within the energy gap. The defect of the polaron appears to extend over about 5 units. The benzenes are strongly affected by the electron-phonon interaction. Our calculation for the energy band structure of the ground and polaron states are consistent with experimental absorption spectra. The results of our calculation show that the electron-phonon and inter-site electron-electron interactions play an important role in determining the electronic and lattice structures.     

The Electronic and Lattice Structures of the Ground and the Polaron States of Polymer Poly (Phenylene Vinylene) (PPV)

The electronic and lattice structures of poly (phenylene vinylene) (PPV) are studied theoretically. Both the electron-electron and electron-phonon interactions are taken into account in the Pariser-Parr-Pople model. The electronic band and the lattice structure of the ground state and the polaronic state are calculated by means of the unrestricted Hartree-Fock method. In the ground state, there exist eight bands in PPV including four valence bands and four conduction bands, and the benzenes can be considered to be rigid. The polaron induces the split of energy bands. There are four localized electronic states within the energy gap. The defect of the polaron appears to extend over about 5 units. The benzenes are strongly affected by the electron-phonon interaction. Our calculation for the energy band structure of the ground and polaron states are consistent with experimental absorption spectra. The results of our calculation show that the electron-phonon and inter-site electron-electron interactions play an important role in determining the electronic and lattice structures.     

The inertial mass tensor of a polaron in an isotropic medium

Owing to a funadmentally erroneous approach to calculations of the effective polaron mass (calculations that use a model without spatial dispersion of the lattice polarizability), the polaron inertial mass has never before been distinguished from the mass as a measure of kinetic energy. In this paper we derive an expression for the tensor of the inertial mass of a large polaron. The tensor is found to be fully determined by two components: the longitudinal component, corresponding to the case where the force acting on the polaron is parallel to the polaron velocity, and the transverse component, corresponding to the case where the acceleration is perpendicular to the polaron velocity. The components of the polaron inertial mass tensor depend quasirelativistically on the polaron velocity due to the quasirelativistic compression of the polarization field in the direction of motion, which constitutes the effect of spatial dispersion of the lattice polarizability. We derive a formula that approximates the dependence of the components of the polaron mass tensor on all the parameters: the frequency and dispersion of the phonons, the polaron velocity, and the effective dielectric constant. Zh. éksp. Teor. Fiz. 115, 180–186 (January 1999)     

Holstein极化子能带和有效质量的温度依赖性

使用正则变换方法,考察了一维Holstein极化子能带和有效质量的温度依赖性。结果表明,对于一定的电子声子耦合强度,Holstein极化子能带宽度随温度升高而变窄,有效质量随温度升高而增大。特别是当电子声子耦合强度足够大时,极化子能带宽度在很小的温度范围内会迅速地变为零,我们认为这种情况实际上是极化子从能带状态向自陷局域态的迅速转变,这与通常的相变现象有点相类似。当电子声子耦合常数越大时,极化子有效质量随温度的升高而增加得越快。很显然,研究电子声子相互作用,对理解固体的光学和输运等性质将有重要的意义。