Large bipolarons and superconductivity

Superconductivity has long been speculated to result from charge carriers paired as mobile charged bosons. Although the pairing of carriers as small (single-site) bipolarons is known, small bipolarons readily localize. By contrast, large (multi-site) bipolarons, in analogy with large polarons, should be mobile. It is shown that large bipolarons can form in solids with very displaceable ions, e.g., many oxides. Large-polaronic (but not small-polaronic) carriers produce absorption spectra like the carrier-induced absorptions observed in cuprates. Redistribution of the self-trapped carriers of large bipolarons among sites of carriers' molecular orbitals in response to atomic motions lowers phonon frequencies. The dependence of the phonon zero-point energy on the spatial distribution of large bipolarons produces a phonon-mediated attraction between them. This dynamic quantum-mechanical attraction fosters the condensation of large bipolarons into a liquid. Superconductivity can result when the large-bipolarons' groundstate remains liquid rather than solidifying.     

On the absence of bipolarons in manganese perovskites

The paramagnetic phase of La_0.65Ca_0.35MnO₃ and La_0.65Pb_0.35MnO₃ powders was investigated by electron spin resonance. The resonance spectra, for a range of temperatures, was used to examine the presence of polarons and bipolarons. The resonance lines are compatible with the presence of polarons (g≈2), but the characteristic high-spin bipolaron lines (g≈4) are absent. The results are inconsistent with the presence of high-spin bipolarons. The evidence with respect to low-spin bipolarons is less conclusive, although the temperature dependence of the resonance line parameters speaks against the presence of low-spin bipolarons.     

Phase Separation of Electrons Strongly Coupled with Phonons in Cuprates and Manganites

Recent advanced Monte Carlo simulations have not found superconductivity and phase separation in the Hubbard model with on-site repulsive electron–electron correlations. We argue that microscopic phase separations in cuprate superconductors and colossal magnetoresistance (CMR) manganites originate from a strong electron–phonon interaction (EPI) combined with unavoidable disorder. Attractive electron correlations, caused by an almost unretarded EPI, are sufficient to overcome the direct inter-site Coulomb repulsion in these charge-transfer Mott–Hubbard insulators, so that low energy physics is that of small polarons and small bipolarons (real-space electron (hole) pairs dressed by phonons). They form clusters localized by disorder below the mobility edge, but propagate as the Bloch states above the mobility edge. I identify the Fröhlich finite-range EPI with optical phonons as the most essential for pairing and phase separation in superconducting layered cuprates. The pairing of oxygen holes into heavy bipolarons in the paramagnetic phase (current-carrier density collapse (CCDC)) explains also CMR of doped manganites due to magnetic break-up of bipolarons in the ferromagnetic phase. Here I briefly present an explanation of high- and low-resistance phase coexistence near the ferromagnetic transition as a mixture of polaronic ferromagnetic and bipolaronic paramagnetic domains due to unavoidable disorder in doped manganites.     

Bose–Einstein Condensation in the Pseudogap Phase of Cuprate Superconductors

We have identified the unscreened Fröhlich 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–Fröhlich” model, which fully takes into account the long-range Coulomb repulsion and the Fröhlich 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.     

From localized to itinerant bipolarons in a system of bipolarons hybridized with conduction electrons

Assuming a charge exchange coupling between localized bipolarons (located in certain regions of the crystalline lattice of the high-T _c cuprates) and correlated electrons (located in the CuO₂ layers), the intrinsically localized bipolarons acquire itinerancy below a certain characteristic temperatureT* and condense into a superfluid state below the critical temperatureT _c . The appearance of the itinerant bosonic states is linked to the opening of a pseudogap in the DOS due to strong local pairing between the conduction electrons of the electrons. As a result, the Fermi liquid properties of such a system are destroyed. These effects are manifest not only in the photoemission spectrum but also in the thermodynamic, transport, and magnetic properties.     

Bipolaron Formation in a Strong Coupling Polaronic Metal

We study the formation of bipolarons in the Holstein–Peierls–Hubbard model on a square lattice. This model, for sufficiently small Coulomb interaction, is known to describe a narrow band metallic state, in which the carriers are heavily dressed by the strong interaction with local, dispersionless (Einstein) phonons. To deal with the absence of a small expansion parameter, we use a combination of canonical transformations and a variational treatment in terms of squeezed phonon states. The coexistence of polarons and bipolarons is considered and found compatible for reasonable values of the system parameters, namely the electronic hopping term t, the phonon frequency and the electron–phonon coupling constant g. The possibility of bipolaron condensation is studied within this model and found to occur in the same parameter range as the polaron–bipolaron coexistence regime.     

Two dimensional excitons in thin films of thiophene oligomers

Transient and quasi-steady state photomodulation spectroscopy has been applied to films of substituted sexithiophene, namely , ωdihexylsexithiophene (, ωDHT₆), which is prototype two dimensional organic semiconductor. Two photoexcited states are seen in the ultrafast time domain characterized by photoinduced absorption bands at 1.7 eV and 1.5 eV respectively and different dynamics. We assign them to Frenkel excitons and inter-layer charge transfer excitons. The photoinduced absorption spectrum in the ms time domain shows a very narrow line at 0.7 eV and a broad structure around 1 eV which are assigned to doubly occupied states, π-dimers and bipolarons respectively.     

Key Pairing Interaction in Cuprate Superconductors

It has been now over 20 years since the discovery of the first high temperature superconductor by Georg Bednorz and Alex Müller and yet, despite intensive effort, no universally accepted theory exists about the origin of superconductivity in cuprates. A controversial issue on weather the electron?Cphonon interaction (EPI) is crucial for high-temperature superconductivity or weak and inessential has been one of the most challenging problems of contemporary condensed matter physics. Here, it is suggested that the true origin of high-temperature superconductivity is found in a proper combination of strong electron?Celectron correlations with a significant finite-range (Fröhlich) EPI so that low energy quasi-particles are small mobile polarons and bipolarons in cuprate superconductors. Our recent development of the bipolaron theory of tunnelling accounting for two energy scales, their temperature and doping dependencies, asymmetry and inhomogeneity of tunnelling spectra of cuprate superconductors is briefly discussed.     

Ti3+ Jahn–Teller Polarons and Bipolarons in BaTiO3

In high quality BaTiO₃ crystals without trapping defects conduction electrons form Ti³⁺ polarons, their stabilization energy being caused by a Jahn–Teller effect. Depending on locally fluctuating potentials, tunable by uniaxial stress, small, intermediate, and banding electrons are identified by EPR. There are indications that also bipolarons are present.     

Polarons, Bipolarons, and Possible High-Tc Superconductivity in Metal-Ammonia Solutions

We examine the nature of fluid metal-ammonia solutions with a special emphasis on the electronic structure and dynamics of polaronic and bipolaronic charge carriers. Importantly, we find that close to the compositionally-induced Nonmetal-to-Metal Transition in the fluid at low temperatures (ca. 240K), the vast majority (ca. 85% or above) of current carriers are highly mobile, diagmagnetic (S = 0) bipolarons. This raises the intriguing possibility, first proposed by R. A. Ogg in 1946, of a Bose–Einstein Condensation (BEC) of trapped electron pairs in vitreous, quenched metal-ammonia solutions. From a “modern” (2000) perspective we believe that there are important similarities to the situation in the crystalline layered cuprates, where we have argued elsewhere that High-T _c superconductivity derives from the BEC of bipolarons in the electronically active CuO₂ planes [A. S. Alexandrov and P. P. Edwards, Physica C 331, 97 (2000)]. We now propose that the search begins for high temperature superconductivity in quenched metal-ammonia and related solutions.     

Synthesis,characterization and low frequency a.c. conduction of polyaniline/fly ash composites

In situ polymerization of aniline was carried out in the presence of fly ash (FA) to synthesize polyaniline/ fly ash (PANI/FA) composites. The PANI/FA composites have been synthesized with various compositions (15, 20, 30 and 40 wt%) of FA in PANI. The composites, thus synthesized have been characterized by infrared spectroscopy and X-ray diffraction. The morphology of these samples was studied by scanning electron microscopy. Further the a.c. conductivity of these composites have been investigated in the frequency range 10²–10⁶ Hz. The presence of polarons and bipolarons are made responsible for frequency dependence of a.c. conductivity in these composites. The Cole-Cole plots indicate clear shift in the distribution of relaxation times as the wt% of FA in PANI changes. These composites show almost symmetric semicircles of Cole-Cole plots indicating the Debye-type relaxation in their polarization response.     

High-Tc Superconductivity with Bipolarons and Normal State Gap in Cuprates

The BCS theory is extended to a strong-coupling regime, λ≥1 with smallpolarons and bipolarons. The normal state gap is found in this regime whichis half of the bipolaron binding energy. In the framework of the bipolarontheory we explain the magnitude of the carrier specific heat andsusceptibility as well as their universal scaling with temperature in a widerange of doped cuprates. PACS numbers: 74.20.?z,74.65.+n,74.60.Mj

     

Polarons, Bipolarons, and Possible High-Tc Superconductivity in Metal-Ammonia Solutions

We examine the nature of fluid metal-ammonia solutions with a special emphasis on the electronic structure and dynamics of polaronic and bipolaronic charge carriers. Importantly, we find that close to the compositionally-induced Nonmetal-to-Metal Transition in the fluid at low temperatures (ca. 240K), the vast majority (ca. 85% or above) of current carriers are highly mobile, diagmagnetic (S = 0) bipolarons. This raises the intriguing possibility, first proposed by R. A. Ogg in 1946, of a Bose–Einstein Condensation (BEC) of trapped electron pairs in vitreous, quenched metal-ammonia solutions. From a modern (2000) perspective we believe that there are important similarities to the situation in the crystalline layered cuprates, where we have argued elsewhere that High-T _c superconductivity derives from the BEC of bipolarons in the electronically active CuO₂ planes [A. S. Alexandrov and P. P. Edwards, Physica C 331, 97 (2000)]. We now propose that the search begins for high temperature superconductivity in quenched metal-ammonia and related solutions.     

Electrically conducting polymers: from fundamental to applied research

A review of the state of the art of electrically conducting polymers is presented. Special emphasis is laid on the nature of the doping processes which induce high electrical conductivity in conjugated organic polymers and on the nature of charge carriers such as solitons, polarons and bipolarons produced in them as a result of such processes. Various factors affecting the electrical conductivity of these polymers and the various models proposed to explain charge transport in them are also discussed. The present status of the structure-property relationship in these conducting polymers and the various routes currently pursued to tailor novel conducting polymers are also briefly mentioned. Finally the various applications of these novel electronic materials are described.     

Electron-Phonon Interaction in the Presence of Strong Coulomb Repulsion

We consider the Hubbard-Holstein model on a 2D tilted eight-site square lattice at quarter fillings having on-site (λ ₁) and inter-site (λ ₂) electron-phonon (EP) interactions and inter-site Coulomb repulsion (V ). Mobility of electrons decreases, and hence, localization of electrons is favored by λ ₁ as well as inter-site Coulomb repulsion in the non-interacting limit (U/t = 0.0). In the interacting case (U/t = 8.0), an initial delocalization effect observed due to the competition between on-site Coulomb repulsion and EP interaction λ ₁ though V favors localization. For smaller λ ₁, the competition between λ ₂ and V increases the delocalization effect. As λ ₂ increases further, localization occurs. In the interacting case, localization occurs at much higher values of λ ₂ due to the additional delocalization effect of U. On-site (S0) and/or large bipolarons are formed with λ ₁ in the presence of V, and inter-site Coulomb repulsion effectively increases λ ₁. Thus, V favors the formation of S0 bipolarons. However, the formation of S1 bipolarons is suppressed by inter-site Coulomb repulsion. An existence of critical λ ₂ = λ _2c is observed for the formation of on-site (S0) and neighboring-site (S1) bipolarons.     

A New Approach to Transport Theory of High Tc Bismuth-Based Ceramic Superconductors

A new transport theory for high temperature superconductors is propos#ed and supported with extensive calculations.The Cooper-pairs while remaining on a pseudo Fermi surface behave like weak Fermions. When the Cooper-pairs become free they behave as spinless bosons. The polarons and bipolarons are highly localized quantum states with virtually no gap existing in between them and are considered as bosons with nondegenerate spining. The binding energies of Cooper-pairs both in a weak Fermion system and for a spinless boson are calculated.A kind of a semimetallic softening transition is responsible for high temperature superconductivity and is temperature independent. There is no electron–phonon coupling for high temperature superconductors. High temperature ceramic superconductors before the onset of superconductivity are found to be Mott–Hubbard dielectric insulating materials.     

Stability of bipolarons in conjugated polymers

The bipolarons appear to be the main entity for charge collection in conducting polymers. But their existence is challenged by the Coulomb repulsion. We study the stability of a bipolaron (BP) as a function of the strength of the long range Coulomb interaction U with and without impurities for both weak and strong electron-phonon coupling (). We find that in a free state the BP is stable only for small values of U for the weak coupling limit but stays stable until relatively large U in the strong coupling case. Being bound to a dopant, the bipolaron becomes stable in a wide range of U. Free BPs near the metal-polymer contact also are stabilized due to a screening by image charges. These results are important for charge injection at polymer-metal interfaces in the context of organic light-emitting diodes.     

Characterization and electrical properties of conductive polymer/colloidal graphite oxide nanocomposites

Conductive polymers such as polypyrrole (PPy) and poly(3,4-ethylenedioxythiophene) (PEDOT) have been synthesized in the presence of colloidal graphite oxide (CGO) prepared via Hummers and Offerman’s method, thus obtaining PPy/CGO and PEDOT/CGO nanocomposites. The resulted nanocomposites provide high and adjustable electrical conductivity when doped with different dopants, and a much better thermal stability than pristine CGO. For the case of doped PPy/CGO nanocomposites, CGO is confirmed experimentally to be exfoliated and thus could offer more “active sites” for the polymerization of pyrrole. For the case of doped PEDOT/CGO nanocomposites, X-ray diffraction analyses indicate the formation of PEDOT with the aid of CGO in aqueous media in spite of its monomeric water insolubility and a possible part intercalation of PEDOT between the layers of CGO. The temperature dependence of conductivity supports the three-dimensional Mott’s variable range hopping mechanism for doped PPy/CGO and PEDOT/CGO nanocomposites. The Fourier-transform infrared (FTIR) spectroscopy and electron spin resonance (ESR) spectra data, as evidence verifies that the charge carriers in doped PPy/CGO nanocomposites are polarons, while bipolarons serve as charge carriers in doped PEDOT/CGO nanocomposites.     

High-Tc Superconductivity with Bipolarons and Normal State Gap in Cuprates

The BCS theory is extended to a strong-coupling regime, 1 with smallpolarons and bipolarons. The normal state gap is found in this regime whichis half of the bipolaron binding energy. In the framework of the bipolarontheory we explain the magnitude of the carrier specific heat andsusceptibility as well as their universal scaling with temperature in a widerange of doped cuprates. PACS numbers: 74.20.–z,74.65.+n,74.60.Mj