Showing papers on "Bipolaron published in 2012"

Synergistic Effects in the Gas Sensitivity of Polypyrrole/Single Wall Carbon Nanotube Composites

Abstract: Polypyrrole/single wall carbon nanotube composites were synthesized by in-situ chemical polymerization using pyrrole (PPy) as precursor and single wall carbon nanotubes (SWNTs) as additive component. Electron microscope images reveal that SWNTs component acts as nucleation sites for PPy growth in the form of spherical and cylindrical core-shell structures. The SWNTs/PPy core-shell results in thin n-p junctions which modify the PPy bandgap and reduce the work function of electrons. As a result of the strong coupling, Raman and IR spectra show that the PPy undergoes a transition from polaron to bipolaron state, i.e., indicating an increase in the conductivity. In the UV-Vis spectra, the 340 nm adsorption band (π*-π transition) exhibits a red shift, while the 460 nm adsorption band (bipolaron transition) experiences a blue shift indicating a change in electronic structure and a relocation of polaron levels in the band gap of PPy. The modification in PPy electronic structure brings in a synergistic effect in sensing feature. Upon exposure to oxygen (an oxidizing agent) and NH3 gas (a reducing agent), the PPy/SWNTs nanocomposite shows an enhancement in sensitivity exceeding ten folds in comparison with those of PPy or SWNTs.

Inelastic electron transport through mesoscopic systems: Heating versus cooling and sequential tunneling versus cotunneling processes

Abstract: Inelastic electron transport in quantum dot systems is studied via the hierarchical equations of motion combining a small polaron transformation approach, with differential conductance $dI/dV\ensuremath{\sim}V$ characteristics being evaluated accurately at the cotunneling level. We observe (i) the peak feature of phonon emission Franck-Condon sidebands to the zero-phonon peaks of both polaron and bipolaron in sequential electron transport; (ii) phonon absorption peaks occurring if the phonon temperature is sufficiently higher than that of the carrier electron; and (iii) the step feature of Raman sidebands in the cotunneling transport regime. We also evaluate the polaron transport response to a continuous-wave irradiation that induces bias-voltage oscillation. We observe, consistent with experimental results, that (iv) the photon-phonon-assisted tunneling enhances phonon absorptions while suppressing emissions. As the phonon absorption (emission) is associated with the process of absorbing (emitting) energy from (to) the phonon environment, an alternating or tailored field applied to contacts could be a practical means of cooling the mesoscopic quantum-transport device.

Dissociation of a Hubbard-Holstein bipolaron driven away from equilibrium by a constant electric field

Abstract: Using a variational numerical method we compute the time-evolution of the Holstein-Hubbard bipolaron from its ground state when at t=0 the constant electric field is switched on. The system is evolved taking into account full quantum effects until it reaches a quasi-stationary state. In the zero-field limit the current shows Bloch oscillations characteristic for the adiabatic regime where the electric field causes the bipolaron to evolve along the quasiparticle band. Bipolaron remains bound and the net current remains zero in this regime. At larger electric fields the system enters the dissipative regime with a finite steady-state current. Concomitantly, the bipolaron dissociates into two separate polarons. By examining different parameter regimes we show that the appearance of a finite steady-state current is inevitably followed by the dissociation of the bipolaron.

Investigation on organic magnetoconductance based on polaron-bipolaron transition

Abstract: We explore the magnetoconductance (MC) effect in an organic semiconductor device based on the magnetic field related bipolaron formation. We present the transition among spin-parallel spin-antiparallel polaron pairs and bipolarons with a group of dynamic equations. The transition rates are adjusted by the external magnetic field as well as the hyperfine interaction. By considering the mobility of polarons different from that of bipolarons, we obtain the MC in an organic semiconductor device. The theoretical calculation is well consistent to the experimental results. It is predicated that a maximum MC appears at a suitable branching ratio of bipolarons.

Investigation on organic magnetoconductance based on polaron-bipolaron transition

Abstract: We explore the magnetoresistance (MC) effect in an organic semiconductor device based on the magnetic field related bipolaron formation. By establishing a group of dynamic equations, we present the transition among spin-parallel, spin-antiparallel polaron pairs and bipolarons. The transition rates are adjusted by the external magnetic field as well as the hyperfine interaction of the hydrogen nuclei. The hyperfine interaction is addressed and treated in the frame work of quantum mechanics. By supposing the different mobility of polarons from that of bipolarons, we obtain the MC in an organic semiconductor device. The theoretical calculation is well consistent to the experimental data. It is predicated that a maximum MC appears at a suitable branching ratio of bipolarons. Our investigation reveals the important role of hyperfine interaction in organic magnetic effect.

Isotope effects in high-T c cuprate superconductors as support for the bipolaron theory of superconductivity

Abstract: We provide a unified parameter-free explanation of the observed oxygen-isotope effects on the critical temperature, the magnetic-field penetration depth and on the normal-state pseudogap for underdoped cuprate superconductors within the framework of the multi-(bi)polaron theory with strong Coulomb and Frohlich interactions. We also quantitatively explain the measured critical temperature and the magnitude of the magnetic-field penetration depth. This paper thus represents an important support for the bipolaron theory of high-temperature superconductivity, compatible with many other independent observations.

Mobile small bipolarons on a three-dimensional cubic lattice

Abstract: We use numerically exact quantum Monte Carlo (QMC) to compute the properties of three-dimensional bipolarons for interaction strengths where perturbation theory fails. For intermediate electron-phonon coupling and Hubbard $U$, we find that bipolarons can be both small and light, a prerequisite for bipolaron superconductivity. We use the QMC results to make estimates of transition temperatures, which peak at between 90--120 K and are demonstrated to be insensitive to Coulomb repulsion and impurities.

Bipolaron-SO(5) non-Fermi liquid in a two-channel Anderson model with phonon-assisted hybridizations

Abstract: We analyze non-Fermi liquid (NFL) properties along a line of critical points in a two-channel Anderson model with phonon-assisted hybridizations. We succeed in identifying hidden nonmagnetic SO(5) degrees of freedom for valence-fluctuation regime and analyze the model on the basis of boundary conformal field theory. We find that the NFL spectra along the critical line, which is the same as those in the two-channel Kondo model, can be alternatively derived by a fusion in the nonmagnetic SO(5) sector. The leading irrelevant operators near the NFL fixed points vary as a function of Coulomb repulsion U; operators in the spin sector dominate for large U, while those in the SO(5) sector do for small U, and we confirm this variation in our numerical renormalization group calculations. As a result, the thermodynamic singularity for small U differs from that of the conventional two-channel Kondo problem. Especially, the impurity contribution to specific heat is proportional to temperature and bipolaron fluctuations, which are coupled electron-phonon fluctuations, diverge logarithmically at low temperatures for small U.

Possibility of a (bi)polaron high-temperature superconductivity in Poly A/ Poly T DNA duplexes

Abstract: Dynamical equations for a polaron and bipolaron in a DNA duplex are obtained on the basis of Holstein-Hubbard Hamiltonian. It is shown that in a Poly A/Poly T duplex especially stable is a bipolaron state in which holes are localized on different chains of the duplex. With the use of the polaron and bipolaron free energy, the temperature of bipolaron decay is found to be Td ≈ 350 K which can serve as an approximate estimate of the superconducting transition temperature. The way of constructing superconducting nanowires on the basis of DNA is suggested.

Symmetry of bipolaron bound states for small Coulomb repulsion

Abstract: We consider the bipolaron in the Pekar--Tomasevich approximation and address the question whether the ground state is spherically symmetric or not. Numerical analysis has, so far, not completely settled the question. Our contribution is to prove rigorously that the ground state remains spherical for small values of the electron-electron Coulomb repulsion.

Heavy-Electron Formation and Polaron-Bipolaron Transition in the Anharmonic Holstein Model

Abstract: The emergence of the bipolaronic phase and the formation of the heavy-electron state in the anharmonic Holstein model are investigated using the dynamical mean-field theory in combination with the exact diagonalization method. For a weak anharmonicity, it is confirmed that the first-order polaron–bipolaron transition occurs from the observation of a discontinuity in the behavior of several physical quantities. When the anharmonicity is gradually increased, the polaron–bipolaron transition temperature is reduced as well as the critical values of the electron–phonon coupling constant for polaron–bipolaron transition. For a strong anharmonicity, the polaron–bipolaron transition eventually changes to a crossover behavior. The effect of anharmonicity on the formation of the heavy-electron state near the polaron–bipolaron transition and the crossover region is discussed in detail.

Improved results on the no-binding of bipolarons

Abstract: For the case of the bipolaron, it has been proved recently that for U ⩾ 53.2α, where U is the repulsion parameter of the electrons and α is the coupling constant of the polaron, no binding occurs. We show that actually for U ⩾ 52.1α, there is no binding. Furthermore, we obtain optimized results for small and large values of α: more specifically, we prove that for each e > 0, there is an α1 and an α2, such that if 0 < α ⩽ α1, a condition for no-binding becomes U ⩾ (40.4 + e)α, and if α ⩾ α2, it is U ⩾ (38.7 + e)α. We show that α1 can be computed with any desired accuracy, whereas we are merely able to prove the existence of such an α2.

Effects of Thermal Lattice Vibration on the Effective Potential of Weak-Coupling Bipolaron in a Quantum Dot

Abstract: Based on the Huybrechts' linear-combination operator, effects of thermal lattice vibration on the effective potential of weak-coupling bipolaron in semiconductor quantum dots are studied by using the LLP variational method and quantum statistical theory. The results show that the absolute value of the induced potential of the bipolaron increases with increasing the electron-phonon coupling strength, but decreases with increasing the temperature and the distance of electrons, respectively; the absolute value of the effective potential increases with increasing the radius of the quantum dot, electron-phonon coupling strength and the distance of electrons, respectively, but decreases with increasing the temperature; the temperature and electron-phonon interaction have the important influence on the formation and state properties of the bipolaron: the bipolarons in the bound state are closer and more stable when the electron-phonon coupling strength is larger or the temperature is lower; the confinement potential and coulomb repulsive potential between electrons are unfavorable to the formation of bipolarons in the bound state.

Pseudogap in high-temperature superconductors from realistic Fröhlich and Coulomb interactions

Abstract: It has been recently shown that the competition between unscreened Coulomb and Frohlich electron-phonon interactions can be described in terms of a short-range spin exchange Jp and an effective on-site interaction in the framework of the polaronic t-Jp- model. This model, that provides an explanation for high-temperature superconductivity in terms of Bose-Einstein condensation (BEC) of small and light bipolarons, is now studied as a charged Bose-Fermi mixture. Within this approximation, we show that a gap between bipolaron and unpaired polaron bands results in a strong suppression of low-temperature spin susceptibility, specific heat and tunnelling conductance, signalling the presence of a pseudogap regime in the normal state without any assumptions on pre-existing orders or broken symmetries in the normal state of the model.

Recombination of two triplet excitons in conjugated polymers

Abstract: Combining the one-dimensional tight-binding Su-Schrieffer-Heeger (SSH) model including interchain interactions and the extended Hubbard model (EHM), we investigate the recombination processes between two triplet excitons in conjugated polymers using a nonadiabatic evolution method. Due to the interchain coupling, the electron and/or hole in the two triplet excitons can exchange, and new species are formed. Depending on the interchain coupling and electron-electron interactions, it is found that the main product is excited-polaron state in which two electrons and a hole (or two holes and an electron) are bound together in a lattice deformation, with a small amount of singlet exciton, biexciton and bipolaron. In these products, the excited polaron, the singlet exciton and the biexciton can contribute to the emission. The results indicate that the recombination of two triplet excitons may play an important role in polymer light-emitting diodes.

Bipolaron formation in organic semiconductors at the interface with dielectric gates

Abstract: The formation of the electron-phonon induced bipolaron is shown to be feasible in organic semiconductors at the interface with dielectric gates due to the coupling of the carriers with interface vibrational modes and to the weak to intermediate strength of bulk electron-electron interaction. The polaronic bound states are found to be quite robust in a model with realistic strengths of electron coupling to both bulk and interface phonons. The crossover to nearly on-site bipolarons occurs for coupling values much smaller than those for nearly on-site polarons, but, on the other hand, it gives rise to an activated behavior of mobility with much larger activation energies. The results are discussed in connection with rubrene field-effect transistors.

The energy of polarons and bipolarons in relation to high Tc superconductivity

Abstract: In this thesis we investigate the possibility of the formation of a bipo- laron out of two polarons in a polarizable lattice. The formation of a bipolaron in such a lattice is thought to be a possible explanation for high Tc superconductivity. Based on classical electrostatics the energy of two mutually interacting polarizable particles is generalized to N interacting particles. This result has been used to device of a numerical simulation for the energy of a uniform square lattice and a CuO2 lattice. Through these calculations it is found that only for a highly unlikely set of mate- rial parameters (such as the polarizability of the particles) it is possible to assimilate a bipolaron without having an external force to keep it together.

Bipolaron formation in organic semiconductors at the interface with dielectric gates

Abstract: The formation of the electron-phonon–induced bipolaron is shown to be feasible in organic semiconductors at the interface with dielectric gates due to the coupling of the carriers with interface vibrational modes and to the weak to intermediate strength of bulk electron-electron interaction. The polaronic bound states are found to be quite robust in a model with realistic strengths of electron coupling to both bulk and interface phonons. The crossover to nearly on-site bipolarons occurs for coupling values much smaller than those for nearly on-site polarons, but, on the other hand, it gives rise to an activated behavior of mobility with much larger activation energies. The results are discussed in connection with rubrene field-effect transistors.

Bipolarons and polarons in the Holstein-Hubbard model: analogies and differences

Abstract: The single bipolaron problem is examined in the context of the 1D Holstein-Hubbard model, emphasizing analogies and differences with respect to the complementary single polaron physics. The bipolaron band structure below the phonon threshold is revealed, showing a complex relationship between numerous excited bands as the adiabatic limit is approached. Light bipolarons with significant binding energy, the stability of large bipolarons, the small to large bipolaron crossover as a function of the Hubbard repulsion, as well as the bipolaron dissociation, are investigated in detail, disentangling adiabatic, nonadiabatic and lattice coarsening effects. It is emphasized that condensation of bipolarons occurs in the dilute limit only at very low temperatures.

Electrical and Optical Properties of Phosphate Glasses Containing Multiple Transition Ions

Abstract: Electrical and optical properties of phosphate glasses containing multiple transition Ions have been studied In xP 2 O 5 - (100-x) (Fe 2 O 3 +MnO) (PFM) and xP 2 O 5 - [(100-x) (V 2 O 5 + MnO)] (PVM) glasses. DC conductivity results indicate that Mn Ions do not contribute to the conduction process. Small polarons, contributed by the reduced Fe and V Ions, have been identified to be the charge carriers in both glass systems. Although the activation energy for dc conduction, W, remains unchanged for all Fe concentrations in the PFM series, the resistivity shows an exponential increase when the concentration of Fe ions exceeds a critical value. In the PVM glasses, a decrease in the concentration of V Is accompanied with an Increase in W. However, as in the PFM system, an exponential Increase In resistivity was observed at a critical concentration of V Ions. The resistivity transition In these glasses has been tentatively explained by a (small) polaron to (small) bipolaron transition (PBT) which is supported by optical absorption data.

On the Possibility of Landau-Pekar Triplet Bipolaron Existance

Abstract: A theoretical analysis of a homogeneous magnetic field the action on the electronic terms of the bipolaron in the triplet 3 , 2 z x iy P ± state is made. It is shown that the action of the magnetic field, on the one hand, leads to displacement of the triplet terms to the low-energy region and thus to stabilization of the bipolaron and, on the other hand, appreciable weakens the criteria that limit the conditions of their existence. It is shown that the inclusion of electron-electron correlat ion leads to the stabilization of the triplet b ipolaron, as well as to expand the boundaries of his existence.

Pseudogap in high-temperature superconductors from realistic Fr\"ohlich and Coulomb interactions

Abstract: It has been recently shown that the competition between unscreened Coulomb and Frohlich electron-phonon interactions can be described in terms of a short-range spin exchange $J_p$ and an effective on-site interaction $\tilde{U}$ in the framework of the polaronic $t$-$J_p$-$\tilde{U}$ model. This model, that provides an explanation for high temperature superconductivity in terms of Bose-Einstein condensation (BEC) of small and light bipolarons, is now studied as a charged Bose-Fermi mixture. Within this approximation, we show that a gap between bipolaron and unpaired polaron bands results in a strong suppression of low-temperature spin susceptibility, specific heat and tunneling conductance, signaling the presence of normal state pseudogap without any assumptions on preexisting orders or broken symmetries in the normal state of the model.

Influence of temperature and LO phonon on the effective mass of bipolarons in polar semiconductor quantum dots

Abstract: The temperature and LO phonon effects of the bipolaron in polar semiconductor quantum dots (QDs) are studied by using the Tokuda modified linear-combination operator method and the Lee-Low-Pines variational method. The expressions for the mean number of LO phonons and the effective mass of the bipolaron are derived. Numerical results show that the mean number of LO phonons of the bipolaron decreases with increasing the temperature and the relative distance r between two electrons, but increases with increasing the electron-phonon coupling strength α. The effective mass of the bipolaron M* increases rapidly with increasing the relative distance r between two electrons when r is smaller, and it reaches a maximum at r ≈ 4.05rp, while after that, M* decreases slowly with increasing r. The effective mass of the bipolaron M* decreases with increasing the temperature. The electron-phonon coupling strength markedly influences the changes of mean number of LO phonons and the effective mass M* with the relative distance r and the temperature parameter γ.