Showing papers on "Bipolaron published in 2007"

Bipolaron Mechanism for Organic Magnetoresistance

Abstract: We present a mechanism for the recently discovered magnetoresistance in disordered $\ensuremath{\pi}$-conjugated materials, based on hopping of polarons and bipolaron formation, in the presence of the random hyperfine fields of the hydrogen nuclei and an external magnetic field. Within a simple model we describe the magnetic field dependence of the bipolaron density. Monte Carlo simulations including on-site and longer-range Coulomb repulsion show how this leads to positive and negative magnetoresistance. Depending on the branching ratio between bipolaron formation or dissociation and hopping rates, two different line shapes in excellent agreement with experiment are obtained.

Electronic structure of hybrid interfaces for polymer-based electronics

Abstract: The fundamentals of the energy level alignment at anode and cathode electrodes in organic electronics are described. We focus on two different models that treat weakly interacting organic/metal (and organic/organic) interfaces: the induced density of interfacial states model and the so-called integer charge transfer model. The two models are compared and evaluated, mainly using photoelectron spectroscopy data of the energy level alignment of conjugated polymers and molecules at various organic/metal and organic/organic interfaces. We show that two different alignment regimes are generally observed: (i) vacuum level alignment, which corresponds to the lack of vacuum level offsets (Schottky-Mott limit) and hence the lack of charge transfer across the interface, and (ii) Fermi level pinning where the resulting work function of an organic/metal and organic/organic bilayer is independent of the substrate work function and an interface dipole is formed due to charge transfer across the interface. We argue that the experimental results are best described by the integer charge transfer model which predicts the vacuum level alignment when the substrate work function is above the positive charge transfer level and below the negative charge transfer level of the conjugated material. The model further predicts Fermi level pinning to the positive (negative) charge transfer level when the substrate work function is below (above) the positive (negative) charge transfer level. The nature of the integer charge transfer levels depend on the materials system: for conjugated large molecules and polymers, the integer charge transfer states are polarons or bipolarons; for small molecules' highest occupied and lowest unoccupied molecular orbitals and for crystalline systems, the relevant levels are the valence and conduction band edges. Finally, limits and further improvements to the integer charge transfer model are discussed as well as the impact on device design.

Superlight small bipolarons in the presence of a strong Coulomb repulsion.

Abstract: We study a lattice bipolaron on a staggered triangular ladder and triangular and hexagonal lattices with both long-range electron-phonon interaction and strong Coulomb repulsion using a novel continuous-time quantum Monte Carlo algorithm to solve the two-particle Coulomb-Frohlich model. The algorithm is preceded by an exact integration over phonon degrees of freedom, and as such is extremely efficient. The bipolaron effective mass and radius are computed. Bipolarons on lattices constructed from triangular plaquettes have a novel crablike motion, and are small but very light over a wide range of parameters. We discuss the conditions under which such particles may form a Bose-Einstein condensate with high transition temperature, proposing a route to room temperature superconductivity.

Localized trions in conjugated polymers

Abstract: Experimental and theoretical evidence for charged trions in films of poly[2-(N-carbazolyl)-5-(${2}^{\ensuremath{'}}$-ethyl)-hexoxy-1,4-phenylenevinylene] consisting of two on-chain polarons of the same sign and a trapped polaron of the opposite sign, which can be metastable with respect to both dissociation and recombination, is described. Such trions can be generated by fusion of a free polaron with a neutral polaron pair; this process eliminates the high Coulomb barrier for fusion of two like-charge carriers into a bipolaron (BP). We also argue that trions can be created by photoexcitation of a conjugated polymer, which may create a high density of geminate polaron pairs. Since the metastable trion is anchored by a deeply trapped charge, it is immobile and can be considered as a Coulombically trapped BP, even if the BP may not exist as a free particle. Such localized trions can account for the reversible photoinduced fatigue of the thermally stimulated luminescence (TSL) and the negative [photoluminescence (PL) quenching] spin $1∕2$ PL-detected magnetic resonance (PLDMR) of these films. We also show that the model of reversible UV photoinduced cyclization, recently suggested to explain metastable charged states in substituted poly(phenylene vinylene) derivatives, is inconsistent with the obtained TSL and PLDMR results.

Are there bipolarons in icosahedral boron-rich solids?

Abstract: The charge transport of boron carbide, often incorrectly denoted as B4C, has been controversially discussed. It is shown that the bipolaron hypothesis is not compatible with numerous experimental results. In particular, the determined real microstructure of boron carbide and its related electronic properties disprove several assumptions, which are fundamental to the bipolaron hypothesis. In contrast, the actual energy band scheme derived mainly from optical investigations is confirmed by careful evaluation of the high-temperature electrical conductivity, and allows a consistent description at most of the experimental results.

The Bipolaron in the Strong Coupling Limit

Abstract: The bipolaron are two electrons coupled to the elastic deformations of an ionic crystal. We study this system in the Frohlich approximation. If the Coulomb repulsion dominates, the lowest energy states are two well separated polarons. Otherwise the electrons form a bound pair. We prove the validity of the Pekar–Tomasevich energy functional in the strong coupling limit, yielding estimates on the coupling parameters for which the binding energy is strictly positive. Under the condition of a strictly positive binding energy we prove the existence of a ground state at fixed total momentum P, provided P is not too large.

Exact Green’s functions for the two-site Hubbard-Holstein Hamiltonian

Abstract: We derive exactly all Green's functions for the two-site Hubbard-Holstein model. We then study the bipolaron phase diagram, with emphasis on its unbinding into two polarons and on the crossover from a two-site to a one-site bipolaron. The results are relevant for infinite-size systems.

Superlight small bipolarons

Abstract: Recent angle-resolved photoemission spectroscopy (ARPES) has identified that a finite-range Frohlich electron-phonon interaction (EPI) with c-axis polarized optical phonons is important in cuprate superconductors, in agreement with an earlier proposal by Alexandrov and Kornilovitch The estimated unscreened EPI is so strong that it could easily transform doped holes into mobile lattice bipolarons in narrow-band Mott insulators such as cuprates Applying a continuous-time quantum Monte-Carlo algorithm (CTQMC) we compute the total energy, effective mass, pair radius, number of phonons and isotope exponent of lattice bipolarons in the region of parameters where any approximation might fail taking into account the Coulomb repulsion and the finite-range EPI The effects of modifying the interaction range and different lattice geometries are discussed with regards to analytical strong-coupling/non-adiabatic results We demonstrate that bipolarons can be simultaneously small and light, provided suitable conditions on the electron-phonon and electron-electron interaction are satisfied Such light small bipolarons are a necessary precursor to high-temperature Bose-Einstein condensation in solids The light bipolaron mass is shown to be universal in systems made of triangular plaquettes, due to a novel crab-like motion Another surprising result is that the triplet-singlet exchange energy is of the first order in the hopping integral and triplet bipolarons are heavier than singlets in certain lattice structures at variance with intuitive expectations Finally, we identify a range of lattices where superlight small bipolarons may be formed, and give estimates for their masses in the anti-adiabatic approximation

Electronic excitations and metal-insulator transition in poly(3-hexylthiophene) organic field-effect transistors

Abstract: We carry out a comprehensive theoretical and experimentalstudy of charge injection in poly(3-hexylthiophene) (P3HT) to determinethe most likely scenario for metal-insulator transition in this system.Wecalculate the optical-absorption frequencies corresponding to a polaronand a bipolaron lattice in P3HT. We also analyze the electronicexcitations for three possible scenarios under which a first- or asecond-order metal-insulator transition can occur in doped P3HT. Thesetheoretical scenarios are compared with data from infrared absorptionspectroscopy on P3HT thin-film field-effect transistors (FETs). Ourmeasurements and theoretical predictions suggest that charge-inducedlocalized states in P3HT FETs are bipolarons and that the highest dopinglevel achieved in our experiments approaches that required for afirst-order metal-insulator transition.

Electron Spin Resonance Observation of Gate-induced Charge Carriers in Organic Field-effect Devices Fabricated on Silicon Substrates

Abstract: Electron spin resonance (ESR) measurements have been performed on metal–insulator–semiconductor (MIS) diode structures of regioregular poly(3-hexylthiophene), RR-P3HT, fabricated on silicon substrates with SiO2 layers as gate insulators. The conductivity of substrates was chosen so that it does not significantly lower the quality factor of the ESR cavity. Clear ESR signals due to field-induced polarons have been observed at g-values of around 2.002, consistent with those observed in the MIS devices of RR-P3HT fabricated on Al2O3 gate insulators. Carrier spins tend to saturate above the charge concentration of about 0.3%, suggesting the conversion of polarons with spin 1/2 to spinless bipolarons for higher carrier concentrations. The angular dependence of ESR signals exhibits distinct anisotropy, reflecting the fact that the surface of SiO2 is flatter than that of Al2O3. These results demonstrate that ESR can be performed on organic field-effect devices fabricated on silicon substrates.

Charge injection and transport in quantum confined and disordered systems

Abstract: Quantum dots and conducting polymers are modern semiconductors with a high potential for applications such as lasers, LEDs, displays, solar cells etc. These applications require the controlled addition of charge carriers into the material and knowledge of the details of charge transport. This thesis describes research into the charge-injection and charge-transport properties of films of quantum dots and of conducting polymers. Experiments include the synthesis and self-assembly of PbSe nanocrystals with various (odd) shapes and the evolution of the optical absorption of these nanocrystals as their size increases. The density of states (DOS) of quantum-dot solids made from CdSe and ZnO NCs was studied with electrochemical gating; CdSe suffers from surface states while ZnO is haunted by ghost electrons. The electronic conductivity was studied experimentally and with Monte Carlo simulations. Conductivity results from electrons that hop between quantum dots. The strange thing is that at low temperature the electrons often do not hop to the nearest nanocrystal, but to one that is further away. This is called variable-range hopping (VRH). To explain the experimentally observed T-dependence of conductivity the existing VRH models have been extended with a non-resonant tunneling expression based on Einstein fluctuations. This yields exactly the temperature dependence that is observed experimentally. The DOS of the conducting polymer PPV was determined over a wide energy range. It is shown, for the first time, that the DOS is Gaussian as a result of disorder. Two Gaussian levels were found in the DOS that were identified as polaron and bipolaron formation. At the highest doping levels the absorption spectrum has the characteristics of a metal, while the conductivity remains activated and follows the Mott variable-range hopping law. Interchain hopping is the limiting factor for the hole mobility in PPV.

Quantum Monte Carlo results for bipolaron stability in quantum dots

Abstract: Bipolaron formation in a two-dimensional lattice with harmonic confinement, representing a simplified model for a quantum dot, is investigated by means of quantum Monte Carlo simulations. This method treats all interactions exactly and takes into account quantum lattice fluctuations. Calculations of the bipolaron binding energy reveal that confinement opposes bipolaron formation for weak electron-phonon coupling, but abets a bound state at intermediate to strong coupling. Tuning the system from weak to strong confinement gives rise to a small reduction of the minimum Frohlich coupling parameter for the existence of a bound state.

Conductive, Magnetic, and Optical Properties of Sterically Hindered Dodecithiophenes. Evidence for the Coexistence of Bipolaron and π-Dimer

Abstract: Conductive, magnetic, and optical properties of sterically hindered dodecithiophenes bearing bulky 2,2-bis(butoxy-methyl)-1,3-propanediyl groups were examined. At high doping with FeCl 3 , a completely hindered dodecithiophene had only a poor conductivity of 5 x 10 -4 S cm -1 , but two partially hindered dodecithiophenes had relatively high conductivities of 1.9 and 0.6 S cm -1 , which are one or two orders lower than the conductivity (38 S cm -1 ) of the unhindered dodecithiophene. Despite having different structures and conductivities, all of the doped solids of these dodecithiophenes were magnetically inactive and demonstrated similar optical absorption spectra in the near infrared region, as generally observed for polythiophenes. On the other hand, their optical spectra in dichloromethane were quite different from one another. A solution spectrum of the unhindered dodecithiophene under high doping was characterized by two absorption transitions assignable to a π-dimer. On the other hand, a solution spectrum of the completely hindered dodecithiophene was characterized by an absorption transition due to a bipolaron. Furthermore, the solution spectra of two partially hindered dodecithiophenes could be explained as an overlap of both π-dimeric and bipolaronic bands. From these results, both π-dimer and bipolaron species appear to serve as active charge carriers in conductive poly(oligo)thiophenes.

Bipolaron formation in 1D–3D quantum dots: a lattice quantum Monte Carlo approach

Abstract: Polaron and bipolaron formation in the Holstein–Hubbard model with harmonic confinement potential, relevant to quantum dot structures, is investigated in one to three dimensions by means of unbiased quantum Monte Carlo simulations. The discrete nature of the lattice and quantum phonon effects are fully taken into account. The dependence on phonon frequency, Coulomb repulsion, confinement strength (dot size) and electron–phonon interaction strength is studied over a wide range of parameter values. Confinement is found to reduce the size of (bi-)polarons at a given coupling strength, to reduce the critical coupling for small-(bi-)polaron formation, to increase the polaron binding energy, and to be more important in lower dimensions. The present method also permits one to consider models with dispersive phonons, anharmonic confinement, or long-range interactions.

Bose–Einstein Condensation in the Pseudogap Phase of Cuprate Superconductors

Abstract: We have identified the unscreened Frohlich electron–phonon interaction (EPI) as the most essential for pairing in cuprate superconductors as now confirmed by isotope substitution, recent angle-resolved photoemission (ARPES), and some other experiments. Low-energy physics is that of mobile lattice polarons and bipolarons in the strong EPI regime. Many experimental observations have been predicted or explained in the framework of our “Coulomb–Frohlich” model, which fully takes into account the long-range Coulomb repulsion and the Frohlich EPI. They include pseudo-gaps, unusual isotope effects and upper critical fields, the normal state Nernst effect, diamagnetism, the Hall–Lorenz numbers, and a giant proximity effect (GPE). These experiments along with the parameter-free estimates of the Fermi energy and the critical temperature support a genuine Bose–Einstein condensation of real-space lattice bipolarons in the pseudogap phase of cuprates. On the contrary, the phase fluctuation (or vortex) scenario is incompatible with the insulating-like in-plane resistivity and the magnetic-field dependence of orbital magnetization in the resistive state of underdoped cuprates.

Ground and Excited States of Bipolarons in Two and Three Dimensions

Abstract: The properties of large bipolarons in two and three dimensions are investigated by averaging over the relative wavefunction of the two electrons and using the Lee-Low-Pines-Huybrechts variational method. The ground-state (GS) and excited-state energies of the Frohlich bipolaron for the whole range of electron-phonon coupling constants can be obtained. The energies of the first relaxed excited state (RES) and Franck–Condon (FC) excited state of the bipolaron are also calculated. It is found that the first RES energy is lower than the FC state energy. The comparison of our GS and RES energies with those in literature is also given.

Polaron and bipolaron dispersion curves in one dimension for intermediate coupling

Abstract: Bipolaron energies are calculated as a function of wave vector by a variational method of Gurari appropriate for weak or intermediate coupling strengths, for a model with electron-phonon interactions independent of phonon wave vector and a short-ranged Coulomb repulsion. It is assumed that the bare electrons have a constant effective mass. A two-parameter trial function is taken for the relative motion of the two electrons in the bipolaron. The energies of the bipolarons are compared with those of two single polarons as a function of wave vector for various parameter values. Results for the effective masses at the zone center are also obtained. Comparison is made with data of other authors for bipolarons in the Hubbard-Holstein model, which differs mainly from the present model in that it has a tight-binding band structure for the bare electrons.

Lone Pairs, Bipolarons and Superconductivity in Tellurium

Abstract: We have characterized a new type of bipolaron in the ambient pressure modification of tellurium, Te-I, which originates from Coulomb interactions instead of electron-phonon coupling as for the conventional Anderson bipolaron. The studies at the Hartree-Fock level and the constrained LDA calculations give an estimate (∼) for the stability of the bipolaron. A 3D-tight binding model has been proposed to explain the electronic structure obtained from first-principle calculations. The possible relevance of such bipolarons with superconductivity in the high pressure phase Te-II is discussed.

Superlight Small Bipolarons: A Route to Room Temperature Superconductivity

Abstract: Extending the BCS theory towards the strong electron-phonon interaction (EPI), a charged Bose liquid of small bipolarons has been predicted by us with a further prediction that the highest superconducting critical temperature is found in the crossover region of the EPI strength from the BCS-like to bipolaronic superconductivity. Later on we have shown that the unscreened (infinite-range) Frohlich EPI combined with the strong Coulomb repulsion create superlight small bipolarons, which are several orders of magnitude lighter than small bipolarons in the Holstein–Hubbard model (HHM) with a zero-range EPI. The analytical and numerical studies of this Coulomb–Frohlich model (CFM) provide the following recipes for room-temperature superconductivity: (a) The parent compound should be an ionic insulator with light ions to form high-frequency optical phonons, (b) the structure should be quasi two-dimensional to ensure poor screening of high-frequency phonons polarized perpendicular to the conducting planes, (c) a triangular lattice is required in combination with strong, on-site Coulomb repulsion to form the small superlight bipolaron, (d) moderate carrier densities are required to keep the system of small bipolarons close to the Bose-Einstein condensation regime. Clearly most of these conditions are already met in the cuprates.

Time-resolved polaron dynamics in molten solutions of cesium-doped cesium iodide.

Abstract: Temperature-dependent investigations of excess electrons in molten solutions of cesium-doped cesium iodide (Cs-CsI) (mole fraction of Cs approximately 0.003) were performed applying femtosecond pump-probe absorption spectroscopy. The pulse-limited induced bleach observed at probe wavelengths from 600 to 1240 nm was attributed to the excitation of equilibrated excess electrons which were initially formed by melting a Cs-CsI mixture. The interpretation of the relaxation process is based on strongly localized polarons that constitute the majority of defect states in this melt. As expected, the bipolaron contribution was insignificant. The time constants (tau1) were found to be temperature dependent confirming our earlier findings in Na-NaI melts that ionic diffusion almost exclusively controls the dynamics of excess electrons in high temperature ionic liquids. Apart from this temperature dependence, the relaxation dynamics of excess electrons do not differ irrespective of the excitation regime (blue or red part of the respective stationary spectra).

Resonating bipolaron driven pseudogap phase

Abstract: The spin as well as charge pseudogap features in the high T c cuprate can be traced back to the dynamical properties on small clusters (consisting of Cu ions together with their ligand O ions environments) which make up the overall structure of these materials. These ligand complexes being deformable, causes the itinerant electrons on the Cu ions to enter into resonating bipolaron states via binary collision processes, similar to the Feshbach resonance in ultra-cold fermionic gases. We illustrate such resonant pairing on an isolated cluster and discuss how, upon coupling together such clusters, the local physics evolves into global macroscopic features, such as lattice mediated driven superconducting, respectively insulating, states of such resonating bipolarons.

A computational study of regioisomers formed from the σ-dimerisation of asymmetric terthiophenes

Abstract: The structural and spectroscopic properties of the three possible regioisomers generated from the σ-dimerisation of 3′-styryl substituted terthiophene have been investigated theoretically using density functional theory calculations. The calculated geometries on the neutral species show that the thiophene chain is only slightly altered between isomers, however upon oxidation to the polaron or bipolaron species the bonding in the thiophene chain is sensitive to the regioisomerisation. The structural distortion caused by the polaron is confined between the substituents in the case of the HH isomer (where the substituents point towards the σ-link); this is not the case for the other two regioisomers. The calculated Raman spectra of the neutral species are similar for the three isomers, however, distinct spectral differences are predicted when the isomers are oxidised. For both the neutral and oxidised species the correlation of experimental and calculated spectra is superior for the HH isomer.

Localized Trions as Metastable Charged States in Conjugated Polymers

Abstract: We have shown that a charged trion consisting of two on-chain polarons of the same sign and a trapped polaron of the opposite sign can be metastable with respect to both dissociation and recombination A trion can be created upon fusion of a free polaron with a neutral pair, which eliminates a high Coulomb potential barrier for fusion of two charge carriers into a bipolaron We argue that trions can be created by photoexcitation of a conjugated polymer, in which illumination creates a high density of geminate pairs Since the metastable trion is anchored by a deep-trapped charge, it is immobile and can be considered as a coulombically trapped bipolaron, although the last may not exist as a free particle We employ the concept of localized trions in order to explain the reversible photoinduced fatigue of thermally stimulated luminescence in films of poly[methyl(phenyl)silylene]