Showing papers on "Bipolaron published in 2010"

Bipolaron Formation in Organic Solar cells Observed by Pulsed Electrically Detected Magnetic Resonance

Abstract: We report the observation of a spin-dependent dark transport current, exhibiting spin coherence at room temperature, in a π -conjugated polymer-fullerene blend using pulsed electrically detected magnetic resonance. The resonance at g = 2.0028 ( 3 ) is due to polarons in the polymer, and exhibits spin locking at high microwave fields. The presence of an excess of fullerene, and the operating voltage (1 V) used, suppresses negative polaron formation in the polymer. It is concluded that spin-dependent transport is due to the formation of positive bipolarons.

Strong energy-momentum dispersion of phonon-dressed carriers in the lightly doped band insulator SrTiO$_3$

Abstract: Much progress has been made recently in the study of the effects of electron-phonon (el-ph) coupling in doped insulators using angle resolved photoemission (ARPES), yielding evidence for the dominant role of el-ph interactions in underdoped cuprates. As these studies have been limited to doped Mott insulators, the important question arises how this compares with doped band insulators where similar el-ph couplings should be at work. The archetypical case is the perovskite SrTiO$_3$ (STO), well known for its giant dielectric constant of 10000 at low temperature, exceeding that of La$_2$CuO$_4$ by a factor of 500. Based on this fact, it has been suggested that doped STO should be the archetypical bipolaron superconductor. Here we report an ARPES study from high-quality surfaces of lightly doped SrTiO$_3$. Comparing to lightly doped Mott insulators, we find the signatures of only moderate electron-phonon coupling: a dispersion anomaly associated with the low frequency optical phonon with a $\lambda'\sim0.3$ and an overall bandwidth renormalization suggesting an overall $\lambda'\sim0.7$ coming from the higher frequency phonons. Further, we find no clear signatures of the large pseudogap or small polaron phenomena. These findings demonstrate that a large dielectric constant itself is not a good indicator of el-ph coupling and highlight the unusually strong effects of the el-ph coupling in doped Mott insulators.

Bipolaron and N-polaron binding energies.

Abstract: The binding of polarons, or its absence, is an old and subtle topic. Here we prove two things rigorously. First, the transition from many-body collapse to the existence of a thermodynamic limit for N polarons occurs precisely at U=2α, where U is the electronic Coulomb repulsion and α is the polaron coupling constant. Second, if U is large enough, there is no multipolaron binding of any kind. Considering the known fact that there is binding for some U>2α, these conclusions are not obvious and their proof has been an open problem for some time.

Large-radius bipolaron and the polaron-polaron interaction

Abstract: Research on the polaron–polaron interaction and the theory of large-radius bipolarons are reviewed. The difference between the two-center and one-center continuum bipolaron models in isotropic and anisotropic crystals is discussed. It is shown that the inclusion of electron–electron correlations can significantly reduce the bipolaron and D−-center energies as well as the energies of exchange-bound pairs of shallow hydrogen-like centers. The two-center bipolaron configuration corresponds to a shallow secondary minimum and is unstable. The phonon-mediated exchange interaction between Pekar polarons has an antiferromagnetic nature and exceeds the ferromagnetic interaction due to the Coulomb interaction of electrons localized in polaron potential wells. The possibility that the superfluidity of bipolarons can give rise to high-temperature superconductivity is discussed and problems related to the Wigner crystallization of a polaron gas are examined.

Energy and critical ionic-bond parameter of a 3D large-radius bipolaron

Abstract: A theory of a strong-coupling large-radius bipolaron has been developed. The possibility of the formation of 3D bipolarons in high-temperature superconductors is discussed. For the bipolaron energy, the lowest variational estimate has been obtained at α > 8, where α is the electron-phonon coupling constant. The critical ionic-bond parameter ηc = ɛ∞/ɛ0, where ɛ∞ and ɛ0 are the high-frequency and static dielectric constants, has been found to be ηc = 0.2496.

Naphthobipyrrole: versatile synthesis and electropolymerization.

Abstract: A facile synthetic route to a new polycyclic pyrrole derivative, 3,8-diethyl-1,10-dihydro-benzo[e]pyrrolo[3,2-g]indole (1), is reported. This annulated bipyrrole acts as a monomer for electropolymerization and forms an electrochromic conducting polymer (poly1) when electrooxidized at low potentials (0.4 V vs Ag/Ag(+)) in acetonitrile. The presence of alkyl substituents at the 3 and 8 carbons (β-pyrrolic positions) induces regioselective 2,5'-coupling of the pyrrole repeat units and gives rise to the more uniform polymeric product, poly1. Poly1 exhibits globular morphology, as judged from SEM pictures. Its spectroelectrochemical features can be attributed to the formation of four possible states: neutral, polaron, bipolaron, and transverse bipolaron. The relatively low switching potentials (-0.6 to +0.9 V vs Ag/Ag(+) in MeCN) displayed by poly1 leads us to suggest that 1 has a role to play as a polymerizable moiety for the development of multicolor electrochromic materials.

Theoretical study on the emeraldine salt - impact of the computational protocol

Abstract: We report a theoretical study on the geometry characteristics and the electronic properties of bipolaron defects introduced in oligomers (tetramer to hexadecamer) of emeraldine salt. The main goal of the present investigation is to establish a quantum chemical method suitable for description of the target system and for determination of the oligomer segment perturbed by protonation. For the purpose, a series of first principles methods (HF and DFT) combined with different basis sets are tested and the influence of the medium (water) dielectric constant is estimated using PCM. It is shown that the optimized molecular geometry is critically dependent on the theoretical formalism applied. HF yields geometry of the tetramer substantially distorted from planarity and bipolaron, which is highly localized in the quinoid ring. The bipolaron produced by DFT tends to delocalize, thus enhancing the conjugation along the oligomer chain. The reliability of the separate methods is commented on the basis of a list of criteria such as reproducibility of available experimental geometry, spin contamination of wavefunctions and quantification of correlation effects. The BLYP/6-31G*/PCM method is outlined as the most appropriate. It is found that irrespective of the oligomer chain length and the degree of protonation (partial or full) a bipolaron spreads on three phenyl rings and the nitrogen atoms bound to them. © 2010 Elsevier B.V.

Light and stable triplet bipolarons on square and triangular lattices

Abstract: We compute the properties of singlet and triplet bipolarons on two-dimensional lattices using the continuous-time quantum Monte Carlo algorithm. Properties of the bipolaron including the total energy, inverse mass, bipolaron radius, and number of phonons associated with the bipolaron demonstrate the qualitative difference between models of electron-phonon interaction with long-range interaction (screened Fr\"ohlich) and those with purely local (Holstein) interaction. A major result of our survey of the parameter space is the existence of extra-light hybrid singlet bipolarons consisting of an on-site and an off-site component on both square and triangular lattices. We also compute triplet properties of the bipolarons and the pair dispersion. For pair momenta on the edge of the Brillouin zone of the triangular lattice, we find that triplet states are more stable than singlets.

Gain of the kinetic energy of bipolarons in the t-J -Holstein model based on electron-phonon coupling

Abstract: With increasing electron-phonon coupling as described within the t-J-Holstein model, bipolaron kinetic energy is lowered in comparison with that of the polaron. This effect is accompanied with "undressing" of bipolaron from lattice degrees of freedom. Consequently, the effective bipolaron mass becomes smaller than the polaron mass. Magnetic as well as lattice degrees of freedom cooperatively contribute to formation of spin-lattice bipolarons.

Hartree-Fock approximation of bipolaron state in quantum dots and wires

Abstract: The bipolaronic ground state of two electrons in a spherical quantum dot or a quantum wire with parabolic boundaries is studied in the strong electron-phonon coupling regime. We introduce a variational wave function that can conveniently conform to represent alternative ground state configurations of the two electrons, namely, the bipolaronic bound state, the state of two individual polarons, and two nearby interacting polarons confined by the external potential. In the bipolaron state the electrons are found to be separated by a finite distance about a polaron size. We present the formation and stability criteria of bipolaronic phase in confined media. It is shown that the quantum dot confinement extends the domain of stability of the bipolaronic bound state of two electrons as compared to the bulk geometry, whereas the quantum wire geometry aggravates the formation of stable bipolarons.

Bipolaron density-wave driven by antiferromagnetic correlations and frustration in organic superconductors

Abstract: We describe the paired electron crystal (PEC) which occurs in the interacting frustrated two-dimensional 1 4 - filled band. The PEC is a charge-ordered state with nearest-neighbor spin singlets separated by pairs of vacant sites, and can be thought of as a bipolaron density wave. The PEC has been experimentally observed in the insulating state proximate to superconductivity in the organic charge-transfer solids. Increased frustration drives a PEC-to-superconductor transition in these systems.

Bipolarons in Organic Electroluminescence

Abstract: We investigate the bipolaron channel B++ + B−− → BX. The dynamical evolutions and transition probabilities of this bipolaron luminescence channel are studied. It is found that this channel avoids the triplet and is able to enhance the efficiency. We suggest that choosing certain polymers and injection structures, in which B is easily formed, can improve the luminescence efficiency. It is believed that these results will motivate further experimental studies about the function of bipolarons in electroluminescence.

Exploring organic magnetoresistance : an investigation of microscopic and device properties

Abstract: Recently there has been much interest in combining the fields of organic electronics and spintronics. This has been motivated by the fact that low atomic mass of organic materials are predicted to have long spin lifetimes. Also, spintronic devices could benefit from the chemical tunability, ease of fabrication, and mechanical flexibility of organic semiconductors. The nascent field of organic spintronics has already presented many new phenomena which must be explained with novel physics, here we explore one of these phenomena, organic magnetoresistance (OMAR). OMAR is a room temperature spintronic effect in organic devices without any magnetic materials. OMAR is a large change in resistance (up to 25%) at low magnetic fields ( 20mT). OMAR represents a scientific puzzle since no traditional magnetoresistance mechanisms can explain the combination of properties listed above. Another one of the remarkable properties of OMAR is that the sign of the MR can change based operating conditions of the device, like temperature and voltage. In this dissertation we focused in particular on resolving the origin of the sign change since understanding this unique property should be a major step in unraveling the microscopic origin of OMAR. We have explored the properties of the sign change experimentally with bipolar semiconducting small molecule and polymer devices, in which we observed sign changes as functions of voltage and temperature. These devices showed a strong correlation between the sign change and the onset of minority charge carrier injection and we could describe the lineshape and MR(V) behavior as a superposition of two MR effects of opposite sign. From this work we concluded the separate MR effects were from the mobilities of holes and electrons having different responses to magnetic fields, which is best described by the bipolaron model for OMAR. To test this conclusion, we employed analytical and numerical device models assigning separate magnetomobilities to holes and electrons. The models show, counter-intuitively, that in the case when the minority charge carrier contact is injection limited, a decrease in minority charge carrier mobility increases the current. This is a result of the minority carrier contact acting like a constant current source, and of the compensation of the majority carrier space charge by the oppositely charged minority carriers. We show that these models describe the observed MR(V) behavior very well, and if one assumes the magnetic field acts to reduce the mobility of electrons and holes, we observe that our models can reproduce all the sign changes observed in literature. The device model also predicts how different device parameters affect the observed MR, to test its predictions we performed experiments in which we increased the charge recombination by dye doping the organic active layer, we also observed how changing the charge injection by altering the organic semiconductor/ metal contacts experimentally compared with the device model. The fact that the current can increase when the minority carrier mobility decreases may explain the fact that in experiments the magnitude of the negative MR features has been much larger than the positive MR features, even though, microscopically, the bipolaron model predicts the opposite. Therefore, the presence of both signs of magnetoresistance may be related only to the device physics and not to the microscopic mechanism which causes OMAR.

The bipolaron model in the normal state of Pr-doped GdBa2Cu3O7 superconductors

Abstract: The normal state resistivities of single phase polycrystalline Gd(Ba2 − xPrx)Cu3O7 + δ and (Gd1 − xPrx)Ba2Cu3O7 − δ samples with x = 0.0, 0.05, 0.10, and 0.15 were investigated. The bipolaron model can explain properly the normal state resistivity of the samples. Gd(Ba2 − xPrx)Cu3O7 + δ samples for the special cases x = 0.1 and 0.15 show different behavior. Based on some previous evidence, this behavior could originate from mis-substitution of Ba atoms at rare earth sites. In Gd(Ba2 − xPrx)Cu3O7 + δ samples, the number of localized carriers increases faster than in (Gd1 − xPrx)Ba2Cu3O7 − δ samples with Pr doping. This shows that the superconductivity suppression by Pr at the Ba site is more destructive than by Pr at the rare earth site.

Two-photon luminescence of small polarons in reduced LiNbO3 crystals

Abstract: The photoluminescence spectra of as-grown and chemically reduced nominally pure LiNbO3 crystals were measured. A powerful laser beam at 514.5 nm was used as pump to generate the small polarons, and a laser weak beam at 632.8 nm was used to probe the luminescence. The dependences of luminescence spectra on the pump beam power and crystal composition have been investigated. The luminescence intensity correlates with the so-called bipolaron absorption band in the chemically reduced crystal. We conclude that this gated luminescence at 800–950 nm is caused by the radiative transitions involving polaron states and it may be directly correlated with the two-color photorefractive sensitivity used for gated holographic recording.

Plastic spintronics : spin transport and intrinsic magnetoresistance in organic semiconductors

Abstract: A specific class of plastics and other organic materials is (semi)conducting and can be used for electronic applications. In this thesis, we investigated their use in certain applications using magnetic fields.These applications belong to the field of spin electronics, or ‘spintronics’, where next to the charge of the electrons, also their spin is utilized.This spin can be considered as a small magnetic moment, pointing either up or down. Organic semiconductors have several advantages that make them interesting for applications in spintronics.They are relatively cheap, are easy to process, and can be chemically adapted. Two different, but related, topics that combine organic materials with spintronics have been studied both experimentally and theoretically. The first topic is the recently discovered change in the current through an organic layer when a small magnetic field is applied. This effect is called organic magnetoresistance (OMAR). Large changes in the current (10–20%) have been found at relatively small magnetic fields (~10mT) and at room temperature.These properties make the effect interesting for applications like, for instance, cheap magnetic field sensors, opening the possibility to add magnetic functionality to existing organic electronic applications. Moreover, if its mechanisms are understood, the effect could be used to investigate processes in conventional organic electronic devices by studying their magnetic response. OMAR is observed in a wide range of organic materials, from small molecules to polymers. It is believed that OMAR originates from the interactions of a pair of charge carriers (for instance, electron–electron, hole–hole, or electron–hole).More specifically, it is believed that the possible interactions depend on the relative orientation of the spins of these two charges. Without an external magnetic field, small intrinsic magnetic fields in the organic layer (resulting from hyperfine coupling to nuclear spins) randomize the orientations of the two spins. This allows a change froma spin configuration that is less favorable for the current into amore favorable configuration. However, applying amagnetic field larger than these hyperfine fields results in a strong reduction of this spin randomization or spin mixing, causing a pair to remain locked in a less favorable spin configuration. Although there is agreement on the crucial role of hyperfine fields, the exact mechanisms behind OMAR are still heavily debated. Several models were proposed in literature that explain OMAR in terms of different charge pairs. In this thesis, we investigated a model based on pairs of equal carriers, called the bipolaron model. We used an elementary model of two neighboring sites, where, depending on the spins, one carrier might be preventing another one to pass. With this theoretical model we were able to successfully reproduce several characteristics of OMAR. Both a decrease and an increase in current, as found in experiments, could be obtained and also the universal shapes of the experimental OMAR curves could be reproduced. Additionally, we performed new types of experiments to gain better understanding of OMAR. We showed that when an oscillating magnetic field is applied,OMAR is reduced beyond a certain frequency threshold.This occurs when the slowest charges can no longer follow the oscillations, as we showed by measuring the frequency dependence of the capacitance. These findings are in agreement with recent interpretations in which these slowest carriers are expected to induce the largest OMAR effect. In literature, it was claimed that OMAR is independent of the orientation of the magnetic field. However, via sensitive measurements we demonstrated a small but systematic dependence on the angle between the magnetic field and the sample. We showed theoretically that this angle dependence can be explained in the different models by including an interaction between the spins. This interaction has to be direction dependent in order to explain the angle dependence.We identified dipole–dipole coupling or an anisotropy in the hyperfine fields as themost likely candidates. Furthermore,we outlined a first exploration of an alternative approach to describe OMAR curves. We introduced a function that allows us to extract information both about the hyperfine fields and about an additional broadening of the curves. Thereby, this approach could allow for amore quantitative analysis of changes in the OMAR curves resulting from changes in the operating conditions or the material properties. In the second topic, the spin of electrons is used in a different way. In many spintronics applications a difference between the number of spin-up and spin-down electrons, called spin polarization, is used to transport information through a device. For the functioning, it is essential that this polarization persists while transporting the charges.The main mechanism for loss of polarization inmost inorganic semiconductors, which is related to spin–orbit coupling, is negligible in organic materials. The absence of this loss mechanismmakes organic materials ideal candidates for these types of spintronics applications. However, there might still be other mechanisms that cause a smaller but non-zero loss of spin polarization. We conjecture that the hyperfine fields are the main source of polarization loss in organic materials, which results from mixing between the spin-up and spin-down electrons by precession of spins about these random fields. We theoretically investigated this effect of the hyperfine fields on the spin polarization. We explicitly included the hopping transport characteristic for organic semiconductors. Due to spatial and energetic disorder, the charges hop from one localized site to another. The longer the time they spend on a site, the larger the loss of spin polarization. We showed that an external magnetic field larger than the typical hyperfine-field strength reduces the loss of spin polarization. Hence, such an external field causes the polarization to persist over a larger distance, leading to an increase of the spin-diffusion length.We thus found a magnetic-field dependent spin-diffusion length. In addition, we found the spin-diffusion length to depend only weakly on temperature. A spintronics device that makes use of spin-polarized transport is the spin valve. Using the magnetic-field dependent spin-diffusion length obtained from our theory, we could very accurately fit experimental data on the magnetoresistance of organic spin valves reported in literature. However, there is still a hot debate on the interpretation of these and similar experiments.The question has been raised whether spins are indeed transported through the whole layer, or only through thin regions. A discriminating experiment would be the manipulation of spins during their transport through the organic layer.This can be done by applying a magnetic field perpendicular to the direction of the spin polarization. Using our spin-transport model, we made predictions about the results that can be expected from such an experiment. We showed that, in the case of transport through the organic layer, an effect of a perpendicular field should be observable, but that the strong oscillations in the signal that are typically seen in inorganic semiconductors will be absent. Finally, as an extension of the work presented in this thesis, we made predictions about possible future experiments in which spin polarization is combined with OMAR. Because for a spin-polarized current the majority of the spins point in the same direction, most charge pairs will have parallel spins.Therefore, within the bipolaron model, we expect an increase in the magnitude of OMAR when the injected current is spin polarized. Moreover, we showed that the shapes of the OMAR curves will also be changed.These experiments would provide a means to both prove spin-polarized transport and to validate the bipolaron model. In conclusion, both theoretical and experimental results on OMAR and spin polarized transport have been presented in this thesis. Contributions have been made to a new model for OMAR and new type of experiments have been performed that have added further insights to the puzzle of OMAR. The limiting role of the hyperfine fields on spin-polarized transport has been investigated theoretically, providing an explanation for the experimentally observed magnetoresistance curves of organic spin valves and providing suggestions for future experiments. Although the present work has led to better understanding of OMAR and spin-polarized transport, the field of organic spintronics still poses many theoretical and experimental challenges that should be resolved before a widespread emergence of organicspintronics applications will occur.

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.

Metal-polymer hybrid nanomaterials, method for preparing the same and optoelectronic device using the same

Abstract: PURPOSE: Metal-polymer hybrid nanomaterials are provided to multiple exciton of a conduction level of luminescent polymer nanomaterials through an energy transfer and an electron transport using SPR(Surface Plasmon Resonance) and to enable a user to easily control electrical and optical properties of the nanomaterials. CONSTITUTION: Metal-polymer hybrid nanomaterials comprise: a nanotube or a nanowire including a luminescent polymer having a n-conjugated structure; a dopant which forms a bipolaron band in a band gap of the nanoture or the nanowire by doping the luminescent polymer; and a metal layer having a surface plasmon energy level similar to the size of an energy band gap of the nanotube or the nanowire. Electrons existing in the bipolaron band moves to a Fermi level of the metal layer through the surface plasmon resonance.

Bipolaron in the t-J Model Coupled to Longitudinal and Transverse Quantum Lattice Vibrations

Abstract: We explore the influence of two different polarizations of quantum oxygen vibrations on the spacial symmetry of the bound magnetic bipolaron in the context of the t-J model by using exact diagonalization within a limited functional space. Linear as well as quadratic electron-phonon coupling to transverse polarization stabilize d-wave symmetry. The existence of a magnetic background is essential for the formation of a d-wave bipolaron state. With increasing linear electron-phonon coupling to longitudinal polarization the symmetry of a d-wave bipolaron state changes to a p wave. Bipolaron develops a large anisotropic effective mass.

Inter-site pairs of in-plane polarons of cuprates

Abstract: In this work pairing of polarons in quasi two-dimensional lattice is studied within the framework of extended Holstein–Hubbard model. Schrodinger equation is derived for two in-plane polarons that interact with apical ions of cuprates. Pairing thresholds of in-plane inter-site polaron pairs with s-, p- and d-symmetries of wave functions are obtained at Γ -point with truncated interaction potential.

Bipolaron mechanism of DX center in AlxGa1-xAs:Si

Abstract: The free-carrier concentration in Si-doped AlxGa1-xAs has been calculated by grand-canonical-ensemble statistics without any fitting parameters. Our results are quantitatively in agreement with various experimental data in the temperature range 77—300 K, which indicates that the physical picture of the ground state of DX center (DX-) is of an electronic bipolaron due to the interaction between excess electrons and lattice. When exposed to light, one bipolaron can turn into a polaron, meantime release one electron to the conduction band accompanied by lattice relaxations. Our calculations also prove that DX0 is unstable at thermal equilibrium, which further confirms our bipolaron mechanism.

Fermi Glass Versus High-Temperature Superconductivity Behavior

Abstract: Very recently BaSrNiO4 was reported to be a Fermi glass (Schilling et al. in J. Phys., Condens. Matter 21:015701, 2009). Its structure is essentially the one of K2NiF4 as is that of La2CuO4, in which the occurrence of high-temperature superconductivity (HTS) upon hole doping was first reported (Bednorz and Muller in Z. Phys. B 64:189, 1986; Adv. Chem. 100:757, 1988). The carriers in both have mainly eg character, and move in a stochastic potential as documented by a number of experiments. The difference of the two behaviors is mainly ascribed to the formation of intersite bipolarons (Kabanov and Mihailovic in J. Supercond. 13:950, 2000) , which is estimated to be up to two orders of magnitude larger in La2CuO4 than in BaSrNiO4. From this it follows that for HTS to occur, a large bipolaron formation energy in layered structures is required.

Spin Polaron-Bipolaron Scenario of Three Gap-Like Energy Scales in Superconducting Cuprates

Abstract: We argue that three gaps observed in underdoped cuprates can be attributed to the formation of antiferromagnetic spin polarons and bipolarons. Within the spin polaron scenario the antinodal pseudogap at he high energy scale originates from the change of the Fermi surface topology, induced by antiferromagnetic correlations. That change gives rise to the diminishing of the spectral weight at the antinodal region near the Brillouin zone boundary. We demonstrate that effect by analyzing effective models of doped antiferromagnets. The second type of pseudogap appearing at the intermediate energy scale originates from the phenomena which are precursory to superconductivity and predominantly concern the portion of the Fermi surface near the nodal region. In order to analyze the latter phenomenon we use the negative U Hubbard model, in which many details typical to spin polaron physics are neglected, but which contains the essential ingredient of it, that is the strong short range attraction. The lowest energy scale is related to the true superconducting gap which develops with doping, although both types of pseudogap diminish with doping. This behavior can be explained by the fact that the spin polaron band is empty in the undoped system and therefore the formation of the superconducting state in the system is forbidden. Due to a pedagogical character of this report, we present in the introduction a short overview of mostly recent experimental results which are related to the gap-pseudogap physics.