Showing papers on "Bipolaron published in 2016"

Deep electron and hole polarons and bipolarons in amorphous oxide

Abstract: Amorphous (a)-${\mathrm{HfO}}_{2}$ is a prototype high dielectric constant insulator with wide technological applications. Using ab initio calculations we show that excess electrons and holes can trap in a-${\mathrm{HfO}}_{2}$ in energetically much deeper polaron states than in the crystalline monoclinic phase. The electrons and holes localize at precursor sites, such as elongated Hf-O bonds or undercoordinated Hf and O atoms, and the polaronic relaxation is amplified by the local disorder of amorphous network. Single electron polarons produce states in the gap at $\ensuremath{\sim}2$ eV below the bottom of the conduction band with average trapping energies of 1.0 eV. Two electrons can form even deeper bipolaron states on the same site. Holes are typically localized on undercoordinated O ions with average trapping energies of 1.4 eV. These results advance our general understanding of charge trapping in amorphous oxides by demonstrating that deep polaron states are inherent and do not require any bond rupture to form precursor sites.

A translation invariant bipolaron in the Holstein model and superconductivity.

Abstract: Large-radius translation invariant (TI) bipolarons are considered in a one-dimensional Holstein molecular chain. Criteria of their stability are obtained. The energy of a translation invariant bipolaron is shown to be lower than that of a bipolaron with broken symmetry. The results obtained are applied to the problem of superconductivity in 1D-systems. It is shown that TI-bipolaron mechanism of Bose-Einstein condensation can support superconductivity even for infinite chain.

Density Functional Theory Study of Selenium-Substituted Low-Bandgap Donor–Acceptor–Donor Polymer

Abstract: Theoretical study of an optically transparent, near-infrared-absorbing low energy gap conjugated polymer, donor–acceptor–donor (D-A-D), 2,1,3-benzosele-nadiazole (A) as acceptor and 3,4-ethylenedioxyselenophene (D) as donor fragments, with promising attributes for photovoltaic application is reported herein. The D and A moiety on the polymeric backbone has been found to be responsible for tuning the band gap, optical gap, open circuit (VOC), and short-circuit current density (JSC) in the polymers solar cells. D-A-D has a key role in charge separation and molecular architecture which ultimately influences the charge transport. Reduction in the band gap, high charge transformation, and enhanced visible light absorption in the D-A-D system is because of strong overlapping of molecular orbitals of D and A. The polaron and bipolaron effects are also investigated which has a direct relation with visible light photocurrent generation. In addition, the enhanced planarity and weak steric hindrance between adjacent...

Highly electrically conductive polyethylenedioxythiophene thin films for thermoelectric applications

Abstract: In this study, we report the utilization of protonic acids to treat polyethylene glycol-triblock-polypropylene glycol (PP) modified polyethylenedioxythiophene (PEDOT) doped with tosylate (Tos−) counter-ion (PP-modified PEDOT:Tos) thin films. Investigations using absorption, Raman, electron spin resonance and X-ray photoemission spectroscopies indicate that nearly 100% enhanced electrical conductivity and over a 30% improved power factor observed from the PP-modified PEDOT:Tos thin films with protonic acid treatment are attributed to the increased densities of the polaron state and the stabilized densities of the bipolaron state. Further studies by grazing incidence wide angle X-ray scattering and atomic force microscopy reveal that the crystal structure, amorphous regions, and the size of the exchanging counter ion have great influences on the electrical conductivities, the Seebeck coefficients and the power factors. Our studies provide a facile route to realize highly electrically conductive polymers for the development of effective organic thermoelectric devices.

A.C. conduction in glassy alloys of Se 90 Sb 10-x Ag x

Abstract: Temperature and frequency dependence of a.c. conductivity is studied in glassy Se90Sb10-xAgx (x=2, 4, 6 and 8) in the temperature range (288–358 K) and frequency range (1–500 kHz). An agreement between experimental and theoretical results suggests that the behavior of glassy Se–Sb–Ag system can be successfully explained by correlated barrier hopping (CBH) model. The results show that bipolaron hopping dominates over single polaron hopping in present case, which is explained in terms of lower value of potential barrier. By fitting the experimental data to CBH model, the density of charged defect states is calculated for all the glasses studied. The composition dependence of the density of these states is studied and found to show a maxima at 6 at% of Ag which is explained in terms of the mechanically stabilized structure at a particular average co-ordination number.

Phonon interaction of electrons in the translation-invariant strong-coupling theory

Abstract: A dependence of phonon interaction on the interelectronic distance is found for a translation-invariant (TI) strong-coupling bipolaron. It is shown that the charge induced by the electrons in a TI-bipolaron state is always greater than that in a bipolaron with spontaneously broken symmetry.

Unified percolation model for bipolaron-assisted organic magnetoresistance in the unipolar transport regime

Abstract: The fact that in organic semiconductors the Hubbard energy is usually positive appears to be at variance with a bipolaron model to explain magnetoresistance (MR) in those systems. Employing percolation theory, we demonstrate that a moderately positive $U$ is indeed compatible with the bipolaron concept for MR in unipolar current flow, provided that the system is energetically disordered, and the density of states (DOS) distribution is partially filled, so that the Fermi level overlaps with tail states of the DOS. By exploring a broad parameter space, we show that MR becomes maximal around $U=0$ and even diminishes at large negative values of $U$ because of spin independent bipolaron dissociation. Trapping effects and reduced dimension enhance MR.

Random free energy barrier hopping model for ac conduction in chalcogenide glasses

Abstract: The random free energy barrier hopping model is proposed to explain the ac conductivity (σac) of chalcogenide glasses. The Coulomb correlation is consistently accounted for in the polarizability and defect distribution functions and the relaxation time is augmented to include the overlapping of hopping particle wave functions. It is observed that ac and dc conduction in chalcogenides are due to same mechanism and Meyer-Neldel (MN) rule is the consequence of temperature dependence of hopping barriers. The exponential parameter s is calculated and it is found that s is subjected to sample preparation and measurement conditions and its value can be less than or greater than one. The calculated results for a − Se, As2S3, As2Se3 and As2Te3 are found in close agreement with the experimental data. The bipolaron and single polaron hopping contributions dominates at lower and higher temperatures respectively and in addition to high energy optical phonons, low energy optical and high energy acoustic phonons also contribute to the hopping process. The variations of hopping distance with temperature is also studied. The estimated defect number density and static barrier heights are compared with other existing calculations.

d-Wave Pairing Driven by Bipolaric Modes Related to Giant Electron-Phonon Anomalies in High-$T_{c}$ Superconductors

Abstract: Taking into account microscopic properties of most usual high-$T_{c}$ superconductors, like cuprates, we define a class of microscopic model Hamiltonians for two fermions (electrons or holes) and one boson (bipolaron) on the two-dimensional square lattice. We establish that these model Hamiltonians can show d-wave paring at the bottom of their spectrum, despite their space isotropy. This phenomenon appear when a "giant electron-phonon anomaly" is present at the boundaries of the Brillouin zone ("half breathing" bond-stretching mode), like in doped cuprates. Our results can be used to derive effective electron-electron interactions mediated by bipolarons and we discuss regimes where the corresponding model is relevant for the physics of high-temperature superconductivity and can be mathematically rigorously studied.

Impurity effects on charge transport and magnetoconductance in a single layer poly(3-hexyl-thiophene) device

Abstract: We have used the introduction of shallow hole traps in poly(3-hexyl-thiophene) to test one of the predictions of the bipolaron theory of magnetoconductance (MC). The results show that the introduction of shallow traps effectively increases the degree of energetic disorder in the transport states whilst not affecting the position of the Fermi level and that this results in an increase in the MC response. These results are demonstrated to be in qualitative agreement with the theory and suggest one mechanism through which trap states may affect the MC response of organic semiconductors. This work presents a controllable way of chemical doping to engineer a change in absolute current at a given bias depending on the choice of anodes. It also allows for tuning the magnitude of negative MC response and electroluminescence efficiency under different driving conditions.

Cooper pairs and bipolarons

Abstract: It is shown that Cooper pairs are a solution of the bipolaron problem for model Frohlich Hamiltonian. The total energy of a pair for the initial Frohlich Hamiltonian is found. Differences between the solutions for the model and initial two-particle problems are discussed.

Influence of the Rashba SOI and LO phonon effects on the interaction energy of the Fröhlich bipolaron in a quantum dot

Abstract: The influence of the Rashba spin-orbit interaction (SOI) and longitudinal optical (LO) phonon effect on the ground-state properties of the Frohlich bipolaron in a quantum dot are studied using the Tokuda-modified linear-combination operator method based on the Lee-Low-Pines unitary transformation. The results indicate that, under the condition of strong electron-phonon coupling (coupling strength a > 6), Eint < 0, the electron-phonon coupling body in quantum dot is mainly the bipolaron which is in a stably bound state. The bipolaron interaction energy Eint increases with increasing confinement strength of the quantum dot ω0, electron-phonon coupling strength α, and polaron velocity u and decreases with increasing Coulomb confinement potential s and Rashba spinobit coupling strength αR. In the bipolaron interaction energy Eint, the electron-phonon coupling energy Ee-ph plays the leading role, followed by the confinement potential energy of the quantum dot Ecoul and the Coulomb interaction energy between two electrons Ecouf. Though the additional energy ER-ph caused by the phonon effect accounts for a smaller percentage than the previous three, the electron-phonon coupling and the Rashba spin-obit coupling influence and infiltrate each other. Therefore, the influences of the bipolaron effect and the Rashba electron-spin interaction cannot be ignored when studying a quantum dot.

Small superlight bipolarons within t-Jp model

Abstract: The possibilities of the formation of superlight small bipolaron in doped polar insulators have been discussed within t-J p model derived from Frohlich and realistic long range Coulomb interaction. Increase of t/J p dresses the electrons forming polarons and bipolarons. Formation of inter-site (S1) bipolarons is favored by t/J p ratio. Entropy curves indicate that this formation of bipolarons lead to a more ordered state of the system.

Periodically Driven Adiabatic Bipolarons

Abstract: Small lattice bipolarons driven by external harmonic fields are considered in the adiabatic approximation. Resonant excitation of ions modulates the trapping potential and promotes hole transfer between neighboring atomic layers. It leads to a dramatic decrease of the apparent bipolaron mass compared to the undriven case. This effect offers an explanation for dynamic stabilization of superconductivity at high temperatures recently observed in layered cuprates.

Absence of Binding in the Nelson and Piezoelectric Polaron Models

Abstract: In the context of the massless Nelson model, we prove that two non-relativistic nucleons interacting with a massless meson field do not bind when a sufficiently strong Coulomb repulsion between the nucleons is added to the Hamiltonian. The result holds for both the renormalized and unrenormalized theories, and can also be applied to the so-called piezoelectric polaron model, which describes an electron interacting with the acoustical vibrational modes of a crystal through the piezoelectric interaction. The result can then be interpreted as well as a no-binding statement about piezoelectric bipolarons. The methods used allow also for a significant reduction of about 30% over previously known no-binding conditions for the optical bipolaron model of H. Frohlich.

Vibrational spectra of ternary organic conductors using DPPD

Abstract: The ternary charge transfer complexes of the well-known organic donor namely diphenyl-p-phenylenediamine (DPPD) have been studied with FTIR spectroscopy. There are two absorption edges – one at the Peierls gap (0.225 eV) and the other at the half of it, i.e. at 0.1125 eV. There is non-universal additional shrinkage of the second gap due to electron-electron scattering. The third gap at 0.056 eV is not observed as observed in the case of benzidine ternary complexes. Rather a free-carrier absorption revealing scattering of charge carriers by acoustic phonons is observed in 700-1000 cm-1 range. This also shows that bipolarons are the charge carriers in DPPD ternary complexes.

Efficient radical cation stabilization of PANI-ZnO and PANI-ZnO-GO composites and its optical activity

Abstract: Polyaniline (PANI) and its composites PANI-ZnO (Zinc oxide) and PANI-ZnO-GO (Graphene oxide) were successfully constructed. These materials were characterized by electron spin resonance (ESR) technique and ultraviolet visible spectrometry. The parameters such as line width, g-factor and spin concentration were deduced from ESR spectra, from the results the radical cation stabilization of PANI, PANI-ZnO and PANI-ZnO-GO composites were compared by the polaron and bipolaron formation. The absorption features obtained in the UV absorption spectra reveal the band gap of these modified PANI composites and also predicted the information of increasing and decreasing features of signal intensity and spin concentration.

Periodically driven adiabatic bipolarons

Abstract: Small lattice bipolarons driven by external harmonic fields are considered in the adiabatic approximation. Resonant excitation of ions modulates the trapping potential and promotes hole transfer between neighboring atomic layers. It leads to a dramatic decrease of the apparent bipolaron mass compared to the undriven case. This effect offers an explanation for dynamic stabilization of superconductivity at high temperatures recently observed in layered cuprates.

Cooper pairs and bipolarons

Abstract: It is shown that Cooper pairs are a solution of the bipolaron problem for model Frohlich Hamiltonian. The total energy of a pair for the initial Frohlich Hamiltonian is found. Differences between the solutions for the model and initial two-particle problems are discussed.

A translation invariant bipolaron in the Holstein model and superconductivity

Abstract: Large-radius translation invariant (TI) bipolarons are considered in a one-dimensional Holstein molecular chain. Criteria of their stability are obtained. The energy of a translation invariant bipolaron is shown to be lower than that of a bipolaron with broken symmetry. The results obtained are applied to the problem of superconductivity in 1D-systems. It is shown that TI-bipolaron mechanism of Bose-Einstein condensation can support superconductivity even for infinite chain.

Polaron and bipolaron of uniaxially strained one dimensional zigzag ladder

Abstract: An influence of the uniaxial strains in one dimensional zigzag ladder (1DZL) on the properties of polarons and bipolarons is considered. It is shown that strain changes all the parameters of the system, in particular, spectrum, existing bands and the masses of charge carriers. Numerical results obtained by taking into an account the Poisson effect clearly indicate that the properties of the (bi)polaronic system can be tuned via strain. Mass of bipolaron can be manipulated by the strain too which in turn leads to the way of tuning Bose–Einstein condensation temperature TBEC of bipolarons. It is shown that TBEC of bipolarons in strained 1DZL reasonably correlates with the values of critical temperature of superconductivity of certain perovskites.