Showing papers on "Bipolaron published in 2017"

Tuning electrical conductivity, charge transport, and ferroelectricity in epitaxial BaTiO3 films by Nb-doping

Abstract: The electrical conductivity, charge transport behavior, and ferroelectricity of epitaxial BaNbxTi1-xO3 films (BNTO, 0.0 ≤ x ≤ 0.5) prepared by pulsed laser deposition are investigated. It is found that Nb-doping can tune the conventional insulating BaTiO3 films from an insulating to highly conductive semiconducting or metallic state, resulting in a variation of the electrical conductivity of the BNTO films over 105. For x ≤ 0.25, the charge transport is dominated by the small polaron hopping mechanism, while the charge transport for x = 0.5 transits from the bipolaron to the small-polaron, and then the thermal phonon scattering mechanisms with increasing temperature. Interestingly, the piezo-force microscopy imaging reveals the presence of ferroelectricity in the properly Nb-doped conductive BNTO films (x ≤ 0.25) deposited in the presence of a small amount of oxygen (3 × 10−3 Pa). Our work provides additional technical roadmaps to manipulate the conductivity and charge transport behaviors in ferroelectric...

Dynamic d-symmetry Bose condensate of a planar-large-bipolaron liquid in cuprate superconductors

Abstract: Planar-large-bipolarons can form if the ratio of the surrounding mediums’ static to high-frequency dielectric constants is especially large, e0/e∞ >> 2. A large-bipolaron in p-doped La2CuO4 is mode...

Large bipolaron density at organic semiconductor/electrode interfaces

Abstract: Bipolaron states, in which two electrons or two holes occupy a single molecule or conjugated polymer segment, are typically considered to be negligible in organic semiconductor devices due to Coulomb repulsion between the two charges. Here we use charge modulation spectroscopy to reveal a bipolaron sheet density >1010 cm−2 at the interface between an indium tin oxide anode and the common small molecule organic semiconductor N,N′-Bis(3-methylphenyl)-N,N′-diphenylbenzidine. We find that the magnetocurrent response of hole-only devices correlates closely with changes in the bipolaron concentration, supporting the bipolaron model of unipolar organic magnetoresistance and suggesting that it may be more of an interface than a bulk phenomenon. These results are understood on the basis of a quantitative interface energy level alignment model, which indicates that bipolarons are generally expected to be significant near contacts in the Fermi level pinning regime and thus may be more prevalent in organic electronic devices than previously thought. Bipolarons - two electrons or holes localized on the same molecule - are generally considered negligible in organic electronic devices. Dhanker et al. show that large bipolaron densities can exist near electrode interfaces and that they are linked to the phenomenon of unipolar organic magnetoresistance.

Bipolarons bound by repulsive phonon-mediated interactions

Abstract: When dressed particles (polarons) exchange quantum phonons, the resulting interactions are generally attractive. If the particles have hard-core statistics and the coupling to phonons is through the kinetic energy terms, phonon-mediated interactions are repulsive. Here, we show that such repulsive phonon-mediated interactions bind dressed particles into bipolarons with unique properties. These bipolaron states appear in the gap between phonon excitations, above the two-polaron continuum. While thermodynamically unstable, the bipolaron is protected by energy and momentum conservation and represents a quasiparticle with a large dispersion and a negative effective mass near zero momentum. We discuss possible experimental implementation of the conditions for the formation of such repulsively bound bipolarons.

From positive to negative magnetoresistance behavior at low applied magnetic fields for polyaniline:titania quantum dot nanocomposites

Abstract: Here, we report the tuning from the positive to negative magnetoresistance response at room temperature and low applied magnetic fields (H ∼ 200 mT) for polyaniline nancomposites prepared via in situ growth of titanium oxide quantum dots. In addition, we showed experimental Raman evidence revealing that the positive magnetoresistance response in these polyaniline nanocomposites is mediated by the bipolaron mechanism. Confocal Raman spectroscopy under applied magnetic field analysis showed the decrease of the polaron population to form bipolarons of polyaniline when exposed to an applied magnetic field for the TiO2 quantum dot diluted regime. Negative magnetoresistance, observed for the TiO2 quantum dot higher concentration regime, was attributed to the suppression of polyaniline polarons probably associated with its partial chemical functionalization at the interface due to the increasing concentration of TiO2 quantum dots.

The Properties of the Ground State of the Fröhlich Bipolaron with Rashba Spin-Orbit Coupling in a Quantum Dot ∗

Abstract: The properties of the ground state of bipolarons with Rashba spin-orbit (SO) coupling in a quantum dot (QD) are studied by using the Lee-Low-Pines-Tokuda variational method. The results of numerical calculation indicate that the condition to form the stable bipolaron structure in the QD (binding energy E b > 0) is naturally satisfied with the electron-phonon strong coupling (coupling constant α > 6). The binding energy of bipolarons E b increases with the increase of the confinement strength ω 0 of the QD, electron-phonon coupling strength α, Coulomb bound potential β and velocity u of polarons. The ground energy of bipolarons E splits into E(+) and E(−), corresponding to spin both “up” and both “down” of two electrons, respectively, and $\left | {E(-)} \right |>\left | {E(+)} \right |$ . The grouenergy of bipolarons E in the QD is composed of the electron-phonon coupling energy E e-ph, confinement potential of the QD E couf, Coulomb energy between two electrons E coul and Rashba SO coupling energy E SO. E e-ph is always negative and plays a dominant role. The weights of E coul and E couf are only next to E e-ph. Though the weight of the Rashba SO coupling energy E SO is the smallest one, it can influence other parts of the ground-state energy through interacting with the phonon. Therefore, the bipolaron effect and Rashba SO coupling must not be ignored when investigating the QD.

Peculiarities in the concentration dependence of the superconducting transition temperature in the bipolaron theory of Cooper pairs

Abstract: It is shown that the bipolaron theory of Cooper pairs suggests that there is a possibility for a superconducting phase to exist at low and high levels of doping and be absent at the intermediate level of doping. The results obtained possibly to imply the universal character of 1/8 anomaly.

Spin-dependent electrical conduction in a pentacene Schottky diode explored by electrically detected magnetic resonance

Abstract: Reported is the observation of dark spin-dependent electrical conduction in a Schottky barrier diode with pentacene (PSBD) using electrically detected magnetic resonance at room temperature. It is suggested that spin-dependent conduction exists in pentacene thin films, which is explored by examining the anisotropic linewidth of the EDMR signal and current density–voltage (J–V) measurements. The EDMR spectrum can be decomposed to Gaussian and Lorentzian components. The dependency of the two signals on the applied voltage was consistent with the current density–voltage (J–V) of the PSBD rather than that of the electron-only device of Al/pentacene/Al, indicating that the spin-dependent conduction is due to bipolaron formation associated with hole polaronic hopping processes. The applied-voltage dependence of the ratio of intensity of the Gaussian line to the Lorentzian may infer that increasing current density should make conducting paths more dispersive, thereby resulting in an increased fraction of the Gaussian line due to the higher dispersive g-factor.

Ultrasmall magnetic field-effect and sign reversal in transistors based on donor/acceptor systems.

Abstract: We present magnetoresistive organic field-effect transistors featuring ultrasmall magnetic field-effects as well as a sign reversal. The employed material systems are coevaporated thin films with different compositions consisting of the electron donor 2,2',7,7'-tetrakis-(N,N-di-p-methylphenylamino)-9,9'-spirobifluorene (Spiro-TTB) and the electron acceptor 1,4,5,8,9,12-hexaazatriphenylene hexacarbonitrile (HAT-CN). Intermolecular charge transfer between Spiro-TTB and HAT-CN results in a high intrinsic charge carrier density in the coevaporated films. This enhances the probability of bipolaron formation, which is the process responsible for magnetoresistance effects in our system. Thereby even ultrasmall magnetic fields as low as 0.7 mT can influence the resistance of the charge transport channel. Moreover, the magnetoresistance is drastically influenced by the drain voltage, resulting in a sign reversal. An average B0 value of ≈2.1 mT is obtained for all mixing compositions, indicating that only one specific quasiparticle is responsible for the magnetoresistance effects. All magnetoresistance effects can be thoroughly clarified within the framework of the bipolaron model.

Polaron and bipolaron stability on paraphenylene polymers.

Abstract: Polyparaphenylene is the prototypical conjugated polymer containing phenyl rings and its properties are good references for a family of derived polymers. We investigate the structure, stability, and dynamics of polarons and bipolarons in polyparaphenylene chains under an applied electric field. To do this, we use a bidimensional SSH Hamiltonian model with the Hubbard extension, i.e., with local and nearest-neighbor Coulomb interaction, which has been designed to work with general hexagonal lattices, from which polyparaphenylene can be seen as a prominent case. Using the time-dependent Hartree–Fock approximation, we calculate the structural characteristics, the maximum field strength, supported before polarons and bipolarons gets unstable, and the maximum velocity achieved by these charge carriers. We obtained the polaron and bipolaron terminal velocity to be 0.51 A/fs and 1.15 A/fs, respectively. The maximum field strength determined by our calculations is 0.54 mV/A and 0.80 mV/A, respectively. Our results are in good agreement with other theoretical methods and experiments.

The effects of temperature on the formation and stability of bipolarons in conjugated polymers

Abstract: Within the one-dimensional tight-binding Su-Schrieffer-Heeger (SSH) model modified to include thermal effects and electron-electron interactions, the transition from polaron to bipolaron and the formation of bipolaron induced by injecting charges are separately simulated using a nonadiabatic evolution method. For the transition process, it is found that with the temperature effects taken into account, two separate polarons with the same charge and antiparallel spins can recombine into a bipolaron. The results show that with the temperature increasing, the time taken for the recombination of two polarons decreases. The effects of different distances between the polarons are also numerically simulated. For the bipolaron formation, we investigate the evolution of two charges injected into a polymer chain with the thermal effects. We find that the bipolaron is always quickly formed and its dynamical stability is less sensitive to the temperature change. Thermal effects can only affect the degree of the charges localization.

Dynamic dissociation of trions in conjugated polymers

Abstract: Within a one-dimensional tight-binding model, the dynamic dissociation processes of trions under an external electric field have been investigated using a nonadiabatic evolution method. We demonstrate that the dissociation of the trion under an applied electric field is more difficult than in the case for either the polaron or the bipolaron. Considering the effect of the electron-electron interactions, it is found that the on-site Coulomb interactions reduce while the nearest-neighbor interactions enhance the stability of the trion. Furthermore, due to the competition between the on-site Coulomb and the nearest-neighbor interactions, the static trion presents two typical configurations in general, i.e., a bound state of an exciton and a polaron or a bound state of a bipolaron and an opposite charged polaron. Accordingly, the outcome of the trion dissociation is closely related to its static configuration.

Influence of the Rashba Effect on the Ground-State Properties of the Fröhlich Bipolaron in a Quantum Dot

Abstract: The influence of the Rashba effect on the ground-state properties of the Frohlich bipolaron in a quantum dot is first studied using the variational method of Pekar type based on the Lee–Low–Pines unitary transformation. The results indicate that, under the condition of strong electron–phonon coupling (coupling strength $$\alpha >6$$ ), the condition of forming bipolaron in a quantum dot (binding energy $$E_{\mathrm{b}>0} )$$ is naturally met; the bipolaron binding energy $$E_\mathrm{b} $$ increases with increasing confinement strength of the quantum dot $$\omega _0 $$ , dielectric constant ratio of medium $$\omega _0$$ and electron–phonon coupling strength $$\alpha $$ and increases or decreases linearly with increasing Rashba spin–orbit coupling strength $$\alpha _\mathrm{R} $$ . The bipolaron in quantum dot is in a bound state, and the contribution of the Rashba effect to the ground-state energy consists of $$E(\uparrow \uparrow )$$ , $$E(\downarrow \downarrow )$$ and $$E(\uparrow \downarrow )$$ , corresponding to three spin states of two electrons as follows, spin-parallel and antiparallel; the absolute value of the ground-state energy increases with increasing $$\eta $$ and $$\alpha $$ and increases or decreases linearly with increasing $$\alpha _\mathrm{R} $$ ; in the interaction energy $$E_\mathrm{int} $$ of the ground-state bipolaron, the electron–phonon coupling energy $$E_{\mathrm{e}-\mathrm{ph}}$$ obviously takes a larger ratio than the Rashba spin–orbit coupling energy $$E_{\mathrm{SO}} $$ , but the electron–phonon coupling and the Rashba spin–orbit coupling influence and infiltrate each other.

Intimacy Between Local Lattice and High Temperature Superconductivity: Perspective View on Undeniable Facts

Abstract: Intimate relation between local lattice and high-temperature superconductivity is one of undeniable evidences for the lattice-driven mechanism of superconductivity that has been totally omitted in the previous discussions of possible pairing mechanism because researchers anticipated novel mechanism that does not seamlessly connected to conventional phonon-driven mechanism. We have found that local lattice shows unusual temperature-dependent deviation from equilibrium lattice that always maximizes at the onset superconducting critical temperature and quickly disappears right below. Taking an example of most popular doped cuprate La1.85Sr0.15Cu1−xO4 having a layered CuO2 plane, in-plane local distortion is shows a signature of polaron (bipolaron). The nature of dynamic lattice response in relation to the onset of superconductivity was probed by polarized x-ray absorption spectroscopy (XAS). Starting from La1.85Sr0.150.15Cu1−xO4, we studied pure and magnetic impurity-doped MxLa1.85Sr0.15Cu1−xO4 (M = Mn, Ni, Co, x < 0.05) single crystals and more recently Fe pnictides. The results confirmed that the distorted domains maximize at Tdmax ~ Tconset, which is described by the disappearance of the in-plane Cu-O bond alternation upon the completion of phase coherence (onset of superconductivity). Together with the less significant anomalies in Fe pnictides reflecting superfluid density, the results suggest that the in-plane distortion with either Q2 or pseudo JT symmetry is a prerequisite for high temperature superconductivity in cuprates. This strongly suggests the electronic pairing mechanisms ignore the contribution of lattice and a proper treatment of lattice is a missing element that is actually deeply involved in the pairing mechanism. Here we describe our perspective view on the intimacy between local lattice and high-temperature superconductivity.

Dynamic d-symmetry Bose condensate of a planar-large-bipolaron-liquid in cuprate superconductors

Abstract: Planar large-bipolarons can form if the ratio of the surrounding mediums static to high-frequency dielectric constants is especially large. A large-bipolaron in p-doped La2CuO4 is modeled as two electrons being removed from the out-of-plane orbitals of four oxygen ions circumscribed by four copper ions of a CuO2 layer. These oxygen dianions relax inwardly as they donate electrons to the surrounding outwardly relaxing copper cations. This charge transfer generates the strong in-plane electron-lattice interaction needed to stabilize a large-bipolaron with respect to decomposing into polarons. The lowest-energy radial in-plane optic vibration of a large-bipolarons four core oxygen ions with their associated electronic charges has d-symmetry. Electronic relaxation in response to multiple large-bipolarons atomic vibrations lowers their frequencies to generate a phonon-mediated attraction among them which fosters their condensation into a liquid. This liquid features distinctive transport and optical properties. A large-bipolaron liquids superconductivity can result when it undergoes a Bose condensation yielding macroscopic occupation of its ground-state. The synchronized vibrations of large-bipolarons core-oxygen ions with their electronic charges generate this Bose condensates dynamic global d-symmetry.

The Possible Mechanisms of Conductivity in Polyene-Like Polymers and Types of Conductivity in Maximally Feeble External Fields

Abstract: Linear conjugated systems are the systems of connected p-orbitals with delocalized electrons in molecular compounds with the alternation of simple and multiple bonds. In general, the linear conjugated systems, which will be discussed here, can be represented by the formula [R1 – (CH)m – R2], where R1 and R2 are terminal group connected to the polymethine chain. These systems can be either electric neutral and charged positively or negatively, depending on what they injected – electron or hole. Methine carbon atom of (CH) in the chain is in sp2-hybridized state. To explain the properties of some organic compounds such as quasi-metallic conductivity, a significant change in the spectral properties of ionic dyes that absorb and emit light in the near infrared region of the spectrum [1, 2, 3, 4] managed, including through the use of the concept of solitons. It is established that injection of electrons/holes in the conduction band leads to soliton level appearance inside the restricted area and this is accompanied by marked shift of the valence band top and the conduction band bottom, when electron injected – downward energy, in the case of hole injection – in the direction of energy increasing [1, 2, 3, 4]. In this work we use the charge transfer model based on the concept of solitons, according to Davidov A. [4] which complements and develops other models [1, 2, 3, 4]. 1. Mishra А. Cyanine during 1990s: А review. Chem. Rev., 2000, 100, p.1973–2011. 2. F.Meyers, S.R.Marder, J.W.Perry, Introducing to Nonlinear Optical Properties Organic Materials. In Chemistry Advanced Materials. An Overreviw. Chapt. 6. Ed. L. V. Interrante, J.Hampden-Smith. 1998. Wiley-VCH. Inc. New York-Chicherster-Weinheim-Brisbane-Singapore-Toronto P.207–268. 3. Bredas J.L., Street G.B. Polaron, bipolaron and solitons in conductiong polymers. Acc.Chem.Res. 1985, 18, 309–315. 4. 4. Davydov A. S. (1973). “The theory of contraction of proteins under their excitation”. Journal of Theoretical Biology 38 (3): 559–569. doi:10.1016/0022-5193(73)90256-7. PMID 4266326.

THE BIPOLARON MODEL IN TRANSPORT PROPERTIES OF YBa2Cu3Oy SUPERCONDUCTOR PREPARED BY ADDING Pr2Co7 MAGNETIC NANOPARTICLES

Abstract: Samples with the addition of ferromagnetic nanoparticles of Pr 2 Co 7 on YBa 2 Cu 3 O y (YBCO) polycrystalline superconductor were prepared by the standard solid state reaction method. The analysis of x-ray diffraction patterns by Rietveld refinement indicates orthorhombique to tetragonal phase transformation and oxygen rate reduction from x=0.3% of Pr 2 Co 7 content. Resistivity at room temperature increases as Pr 2 Co 7 content increases. The normal state resistivity versus temperature was analyzed properly by the bipolaron model. It suggested the increase of mobile charge localization with Pr 2 Co 7 content. Keywords: Pr 2 Co 7 ferromagnetic nanoparticles, high temperature superconductor, bipolaron model, localization of mobile charge

Peculiarities in the concentration dependence of the superconducting transition temperature in the bipolaron theory of Cooper pairs

Abstract: It is shown that the bipolaron theory of Cooper pairs suggests that there is a possibility for a superconducting phase to exist at low and high levels of doping and be absent at intermediate level of doping. The results obtained imply possibly universal character of 1/8 anomaly.