Stable Polaron Research Articles

Exact solution for polarons on the anharmonic lattice and charge transfer in biopolymers

The possible mechanism of charge transfer via polarons to long distances in biopolymers was considered. A set of accurately integrable equations was obtained for a lattice with a potential of interaction of neighboring particles with cubic nonlinearity and at a certain ratio of the parameters of the problem. This system has many-soliton solutions, while the polaron is a one-soliton solution. Numerical modeling proved high stability of the obtained solutions. A new class of stable polarons with several peaks were detected for arbitrary values of the parameters. The behavior of polarons on a lattice with defects was studied by numerical methods. The applicability of the results to rationalization of recent experiments on the effective charge transfer in biopolymers was analyzed.

Variational study of polarons in Bose-Einstein condensates

We use a class of variational wave functions to study the properties of an impurity in a Bose-Einstein condensate, i.e., the Bose polaron. The impurity interacts with the condensate through a contact interaction, which can be tuned by a Feshbach resonance. We find a stable attractive polaron branch that evolves continuously across the resonance to a tight-binding diatomic molecule deep in the positive scattering length side. A repulsive polaron branch with finite lifetime is also observed and it becomes unstable as the interaction strength increases. The effective mass of the attractive polaron also changes smoothly across the resonance connecting the two well-understood limits deep on both sides.

Solid State Spectroelectrochemistry of Redox Reactions in Polypyrrole/Oxide Molecular Heterojunctions

To understand the mechanism of bias-induced resistance switching observed in polypyrrole (PPy) based solid state junctions, in situ UV-vis absorption spectroscopy was employed to monitor oxidation states within PPy layers in solution and in PPy/metal oxide junctions. For PPy layers in acetonitrile, oxidation led primarily to cationic polaron formation, while oxidation in 0.1 M NaOH in H(2)O resulted in imine formation, caused by deprotonation of polarons. On the basis of these results in solution, spectroelectrochemistry was used to monitor bias-induced formation of polarons and imines in PPy layers incorporated into solid state carbon/PPy/Al(2)O(3)/Pt junctions. A positive bias on the carbon electrode caused PPy oxidation, with the formation of polaron and imine species strongly dependent on the surrounding environment. The spectral changes associated with polarons or imines were stable for at least several hours after the applied bias, while a negative bias reversed the absorbance changes back to the initial PPy spectrum. These results indicate that PPy can be oxidized in nominally solid state devices, and the formation of stable polarons is dependent on the tendency for deprotonation of the polaron to the imine. Since PPy conductivity depends strongly on the polaron concentration, monitoring its concentration is critical to determining resistance switching mechanisms. Furthermore, the importance of ion mobility and OH(-) generation through H(2)O reduction at the Pt contact are discussed.

Charge Localization and Transport in Lithiated Olivine Phosphate Materials

We report density functional theory (DFT) calculations for olivine-type LiTMPO4 and TMPO4 (TM=Mn, Fe, Co, Ni) structures, using GGA+U and the B3LYP hybrid density functional that includes nonlocal Fock exchange. TM is typically characterized in terms of the formal oxide states of 2+ or 3+, corresponding to TM with localized charge in LiTMPO4 and TMPO4 structures, respectively, in which electron transport would take place by thermally activated hopping of electrons strongly localized on the transition metal (small polarons). In this work, we assess the validity of the concept of formal TM oxidation states in these materials, and conclude that the valence depends in large part on the strength of d-p hybridization. Stable small polaron formation, i.e., mixed 2+ and 3+ valence states, appears to require that the ratio of differences in the metal and oxygen ionic charges (dQTM/dQO) of the two end member phases is larger than 2, corresponding to the mixed-valence TM system. If the ratio of dQTM/dQO is smaller than 2, excess electrons prefer delocalization and the system behaves more single-valence like with charge transport more akin to metallic conduction. The critical ratio emerging from our analysis may turn out to be relevant to other transition metal systems more » as well, as a criterion to discriminate single-valence or mixed-valence characteristics and hence the predominant conduction mechanism. « less

Electrical resistivity and thermopower measurements of the hole- and electron-doped cobaltitesLnCoO3

Two extreme members of the cobaltite series, LaCoO3 and DyCoO3, were investigated by the electrical resistivity and thermopower measurements up to 800-1000 K. Special attention was given to effects of extra holes or electrons, introduced by light doping of Co sites by Mg2+ or Ti4+ ions. The experiments on the La based compounds were complemented with magnetic measurements. The study shows that both kinds of charge carriers induce magnetic states on surrounding CoIII sites and form thus thermally stable polarons of large total spin. Their itinerancy is characterized by low temperature resistivity, which is of Arrhenius type r~exp(EA/kT) for the hole (CoIV) doped samples, while an unusual dependence r~1/Tn (n=8-10) is observed for the electron (CoII) doped samples. At higher temperatures, additional hole carriers are massively populated in the CoIII background, leading to a resistivity drop. This transition become evident at ~300 K and 450 K and culminates at TI-M=540 and 780 K for the La and Dy based samples, respectively. The electronic behaviours of the cobaltites are explained considering two excitation processes in parent compounds. The first one is related to a local excitation from the diamagnetic LS CoIII to close-lying paramagnetic HS CoIII state. Secondarily, a metallic phase of the IS CoIII character is formed through a charge transfer mechanism between LS/HS pairs. The magnetic polarons associated with doped carriers are interpreted as droplets of such IS phase.

Valence and spin states in perovskites LaCo 0.95M 0.05O 3 (M=Mg, Ga, Ti)

The samples LaCoO 3 with dilute substitutions on cobalt sites have been studied using the resistivity, thermopower and magnetic susceptibility measurements over the temperature range up to ∼900 K. The Co-site substitution does not affect the magnetic transition at ∼100 K and the onset of massive population of hole carriers at ∼500 K, characteristic for undoped LaCoO 3. On the other hand, the low-temperature transport and magnetism is markedly distinct for samples with extra charge on cobalt ions introduced by the heterovalent dopants (Mg 2+, Ti 4+) compared to samples with minor non-stoichiometry (LaCoO 3, Ga 3+-doped sample). Magnetic properties suggest that these extra charges create thermally stable magnetic polarons of total S∼2–3. Common features of Co-site doped and La-site doped samples (La 1− x Sr x CoO 3) are discussed.

Polaron physics in optical lattices

We investigate the effects of a nearly uniform Bose-Einstein condensate (BEC) on the properties of immersed trapped impurity atoms. Using a weak-coupling expansion in the BEC-impurity interaction strength, we derive a model describing polarons, i.e., impurities dressed by a coherent state of Bogoliubov phonons, and apply it to ultracold bosonic atoms in an optical lattice. We show that, with increasing BEC temperature, the transport properties of the impurities change from coherent to diffusive. Furthermore, stable polaron clusters are formed via a phonon-mediated off-site attraction.

Ultrafast carrier and phonon dynamics in GaAs and GaN quantum dots

We analyse the electronic and phononic dynamics in GaAs and GaN quantum dot structures due to their interaction with acoustic phonons. We compare results for two specific quantum dot heterostructures which have been proposed as hardware building blocks for a quantum computer in recent quantum computation/information schemes. In particular, we are interested in the loss of coherence after excitation with an ultrashort laser pulse and in the dynamics of phonons which are created as a consequence of the optical excitation process. Our results are non-perturbative with respect to both carrier–phonon and carrier–light interaction and therefore include multi-phonon processes of arbitrary order. We find that, due to different quantum dot sizes, involved electric fields and material parameters, the decoherence is stronger in the GaN dots. The interplay of these effects also strongly determines the details of the created phonon occupation, which splits into two parts: one remains in the vicinity of the dot and forms a stable polaron and another leaves the dot as a phononic wave packet travelling with the velocity of sound. Due to the dot geometry and the carrier–phonon coupling matrix elements this phonon emission is strongly anisotropic.

Electron-phonon dynamics in optically excited quantum dots: Exact solution for multiple ultrashort laser pulses

A model of semiconductor quantum dots coupled to phonons and laser light, which is relevant in the limit of strong electronic confinement, is investigated. For an arbitrary sequence of excitations by ultrafast pulses, analytical solutions are obtained for all pertinent density-matrix elements. The results are nonperturbative with respect to the coupling to both phonons and laser light. The derivation depends neither on a particular choice of electronic wave functions nor on a particular wave-vector dependence of the phonon interaction or phonon dispersion. All components of the density matrix exhibit oscillatory dependences on the pulse areas of the exciting pulses analogously to the Rabi oscillations known from simple two-level systems. Three typical applications of the general results are worked out: (a) the influence of phonons on four-wave-mixing spectra, (b) the coherent control of electronic dot occupations in the presence of phonons, and (c) the dynamics of phonon correlations including the formation of a stable polaron spatially localized in the dot and the emission of a pulsed phonon wave packet that leaves the dot.

Analytical approach to the Davydov-Scott theory with on-site potential

We propose an analytical approach to study the one-dimensional acoustic polaron model that includes an on-site external potential applied to each chain molecule. The key to the approach is an exact discrete solution for the chain-deformation field given in terms of a (quasi)particle wave function. For this purpose we introduce a set of polynomial series that resemble the Chebyshev polynomials. We call these series the hyperbolic Chebyshev polynomials. Using next a properly chosen discrete trial function for the wave function envelope, we obtain simple expressions for the variational energy of the system. Contrary to an isolated (without any external potential) molecular chain, the polaron state (Davydov soliton) is shown to exist only for appropriate system parameters while the delocalized (exciton) state can always exist. As a result, the following three regimes can be specified for the chain with an on-site potential: (i) the polaron is a ground state and the exciton is a metastable state, (ii) the polaron is a metastable state and the exciton is a (delocalized) ground state, and (iii) the polaron state does not exist and only the exciton exists, being a ground state. Two characteristic dimensionless parameters are found in terms of which a criterion of existence of (stable and metastable) polaron states and their nonexistence is formulated. Finally, the Davydov soliton experiences depinning in a particular case of system parameters, resulting in a transparent regime of uniform propagation of the soliton with very small size.

Low oxidation potential conducting polymers based on 1,4-bis[2-(3,4-ethylenedioxy)thienyl]-2,5-dialkoxybenzenes

Palladium-catalyzed coupling techniques have been used to prepare a series of 1,4-bis[2-(3,4-ethylenedioxy)thienyl]-2,5-dialkoxybenzenes which can be polymerized chemically or electrochemically. The polymers have been characterized by electrochemistry, spectroelectrochemistry, in situ EPR electrochemistry, and in situ conductivity. The highly electron rich nature of these polymers leads to extremely low oxidation potentials (E 1/2.p= −0.15 to −0.35V vs. Ag/Ag +). Mild polymerization conditions can be used, resulting in fewer side reactions and more stable polymers; e.g. only a 25% loss in electro activity occurs over 3000 redox cycles when switching polymer 2c. These polymers are stable over a broad potential range, allowing them to be used as modified electrode materials; for example, ferrocene can be repeatedly oxidized and reduced on the polymer surface. In situ EPR electrochemistry and in situ conductivity were used to better understand the polymer redox processes, which are solvent-dependent. Maximum conductivity is attained in the bipolaronic state, but stable polarons can be formed in certain solvents. The polymers are electrochromic, reversibly switching from red to blue upon oxidation with electronic bandgaps at ca. 1.9 eV.

Quantum theory of unusual heat-resistant superconductivity in polymers

Abstract A new theory of unusual superconductivity is developed for states that may exist in both amorphous polymers and biological systems based on flexible polar dielectric substanccs. Its key points are firstly a low concentration of stable ions and free polarons, sccondly a flexible medium which allows their structural self-organization resulting in the formation of quasi-one-dimensional (1D) electron strings and thirdly a stable fine structure of any string corresponding to two electron subcrystals moving through one another with the single permitted velocity v. Both the localization and the strong exchange interaction of all electrons having parallel spins lead to triplet pairing with an energy of electron binding 2 δE ≈ 1 eV. The resulting ground state of that system is quasi-ID ferromagnetic superconductivity with an expected Tc ≫ 103 K. The good agreement between the theory and known experimental data predicts new quantum effects.

Polaronic excitations in the doped polyacene

The polyacene as two coupled chains of transpolyacetylene has been studied based on an extended SSHHubbard model. The dimerized displacement u0 is found to be similar to the case of trans-polyacetylene, and equals to 0.04 A. The energy-band gap is 0.38 eV, in agreement with other authors. In particular, we have considered some cases where polyacene is doped with one and two donor electrons. In the case of doping with one electron, a polaron spreading over two chains has been found; in the case of doping with two electrons having different spins, a stable bipolaron has been obtained, which is different from the one of transpolyacetylene; in the case of doping with two electrons having the same spins, two stable polarons which spread over two chains have been found. The bound electronic states corresponding to these cases are obtained.

Polaron nature of critical size of ammonia cluster

A consistent polaron model is shown to result in a critical size of a cluster made of polar molecules. Over this size the existence of a bound state of an excess electron inside the cluster is possible. In the case of an ammonia cluster composed of n molecules the theory predicts an appearance of a stable polaron state of excess electron at n = 35. This threshold value and photodetachment energies are in a good agreement with existing experimental data. The theory predicts also metastable states of the ammonia cluster anions for 21 ≤ n < 35.

Experimental and theoretical studies of the electronic structure of Na-doped poly (para-phenylenevinylene)

The electronic structure of sodium-doped poly ( p-phenylenevinylene), or PPV, has been studied using photoelectron spectroscopy, UPS and XPS. Upon doping, two new states are created in the previously forbidden electronic bandgap. No finite density- of-states is observed at the Fermi energy. The UPS spectra are analysed with the help of VEH-level quantum chemical calculations. It is determined that the Na-doping of PPV results in the formation of bipolaron bands in the otherwise forbidden energy gap at saturation doping. These results are in contrast with the case of poly-hexyl-thiophene doped from NOPF 6, where the existence of a finite density-of-states at E F and a stable polaron lattice was observed at saturation doping at room temperature. This work represents the first direct measure of multiple, resolved gap states in a doped conjugated polymer.

Surface structural study of aniline and hydroxyaniline thin films

The techniques of scanning tunneling microscopy and tunneling spectroscopy were used to study the surface morphological and electronic structural features of electrochemically prepared aniline and hydroxyaniline thin films. The aniline films were found to exhibit a terracedlike structure, with individual large terraces being flat to within a few tens of angstroms. Local density of states features included many intragap maxima, however no evidence of a stable polaron band was observed. Hydroxyaniline films exhibited a nanometer-scale bundle structure, with electronic features similar to the aniline films. A 0.8 eV polaron band was observed for the hydroxyaniline films at approximately 25% of the surface locations examined.

First-principles and model calculations on carbon chains

First-principle calculations are performed on periodic, linear carbon chains. The results are parametrized within a model related to the Su-Schrieffer-Heeger model, but it is found that H σ contains anharmonic terms. The model predicts the existence of a variety of (meta-)stable polarons and that polarons have lower total energy than solitons. Also interchain interactions are studied. Finally, twistons are examined with a model derived from first-principles calculations on periodic zigzag chains and are found to be unstable.

Bipolarons in Qn(TCNQ) 2 and (NMP) x(Phen) 1− x(TCNQ) for [formula omitted

Quinolinium ditetracyanoquinodimethanide [Qn(TCNQ) 2] and N-methylphenazinium) x (phenazine) 1− x tetracyanoquinodimethanide[(NMP) x (Phen) 1− x (TCNQ)] for x ⋍ 0.5 are quarter-filled-band quasi-one-dimensional conductors with conduction on the TCNQ chains only and strong on-site electron-electron correlations (‘large U’). Optical absorption data have shown that they are semiconductors up to 300 K. We consider whether the gaps are predominantly Peierls or Coulomb in origin and conclude that the former is the case. To explain the observed slow decrease of the Peierls gaps with temperature, there must be a small additional contribution to the gap due to the cation potential and perhaps Coulomb effects. We discuss the effects of such a potential and two other effects that influence the gap strongly, particularly close to the transition temperature: thermal generation of soliton-like defects and a small percentage of excess electrons beyond the commensurate band filling. All three of these effects must be included in the gap equation to obtain agreement of experiment with theory. Another result of the interchain potential is that the stable soliton defects arising from electrons in excess of 0.5/TCNQ molecule are polarons rather than kinks. We find that the stable polarons are pairs of − e/2, − e/2 kinks or + e/2, + e/2 kinks, thus being bipolarons, and calculate their properties in terms of the interchain potential Δ e. To obtain an approximate value for Δ e, the Peierls portion of the gap, 2 Δ o, is calculated as a function of temperature T for different Δ e values and compared with the temperature variation of Δ o obtained from optical absorption. It is found that the theoretical Δ o( T) agrees best with the Qn(TCNQ) 2 data for a Δ e value of ∼25 K. The optical absorption edges predicted from the energy levels of the resulting bipolarons have not been seen. We suggest that this absorption is smeared out because the data have been taken on powder samples.

Bipolarons in quinolinium di-tetracyanoquinodimethanide

Optical-absorption data have shown that Qn(TCNQ${)}_{2}$ and (NMP${)}_{\mathrm{x}}$(Phen${)}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$(TCNQ) are semiconductors up to T=300 K, with gaps due mainly to Peierls distortion on the TCNQ chains. To explain the observed slow falloff of the Peierls gaps with temperature, there must be a small additional contribution to the gap due to the cation potential and perhaps Coulomb effects. As a result of the interchain potential, the stable soliton defects arising from electrons in excess of 0.5/TCNQ molecule are polarons rather than kinks. The stable polarons are found to consist of pairs of -(1/2)e,-(1/2)e kinks or +(1/2)e,+(1/2)e kinks, thus being bipolarons. The energy levels, creation energy, and length of the bipolarons are calculated in terms of the interchain potential ${\ensuremath{\Delta}}_{e}$. To obtain an approximate value for ${\ensuremath{\Delta}}_{e}$ the Peierls portion of the gap ${\ensuremath{\Delta}}_{0}$ is calculated as a function of temperature T for different ${\ensuremath{\Delta}}_{e}$ values and compared with the temperature variation of ${\ensuremath{\Delta}}_{0}$ obtained from optical absorption. To carry this out it is necessary to determine the numbers of positive and negative bipolarons as functions of T. This is done by calculating their chemical potentials and relating these to the Fermi energy. It is found that the theoretical ${\ensuremath{\Delta}}_{0}$(T) agrees best with experiment for a ${\ensuremath{\Delta}}_{e}$/${k}_{B}$ value of \ensuremath{\sim}25 K. The energy levels of the electrons bound in a bipolaron may then be calculated. Unambiguous evidence for the presence of bipolarons is not found in the optical absorption. Consideration shows that the bipolaron absorption may be quite broad, due to structural imperfections as well as to overlap, and therefore difficult to detect.

On a Self-Trapped Acoustic Polaron with the Site Diagonal and Site Off-Diagonal Electron-Phonon Interaction

By including the site off-diagonal electron-phonon interaction which is not taken into consideration in usual, self-trapped acoustic polaron is studied generally on the basis of the complete discrete model of the classical lattice. The self-consistent equations for the polaron are solved without any assumption except that the polaron in the ground state has the same symmetry as the original crystal lattice. The following facts are shown. The effect of the site off-diagonal interaction on one center trapped state makes the polaron radius large and the lattice deformation reduced and thus the polaron unstable in comparison with that of the site diagonal interaction. Moreover, the site off-diagonal interaction makes many center trapped state more favourable than one center trapped state. The stable polaron states for various cases of trapping and the stability condition for one center trapping are discussed. Brief comment for the magnetic polaron coupled with the lattice deformation is also given.