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Showing papers on "Bipolaron published in 2019"


Journal ArticleDOI
TL;DR: In this article, the critical temperature of the transition of a translation-invariant bipolaron Bose gas into the superconducting state, its energy, heat capacity and heat of transition were investigated.
Abstract: The properties of a Bose gas of translation-invariant (TI) bipolarons analogous to Cooper pairs are considered. As in the BCS theory, the description of a TI-bipolaron gas is based on the electron-phonon interaction and Froehlich Hamiltonian. As distinct from the BCS theory, when the correlation length greatly exceeds the mean distance between the pairs, here we deal with the opposite case when the correlation length is much less than the distance between the pairs. We calculate the critical temperature of the transition of a TI-bipolaron Bose-gas into the superconducting state, its energy, heat capacity and heat of the transition. The results obtained are used to explain the experiments on high-temperature superconductors. Possible ways of raising the critical temperature of high-temperature superconductors are discussed.

23 citations


Journal ArticleDOI
TL;DR: In this paper, a three-orbital Su-Schrieffer-Heeger model defined on a two-dimensional Lieb lattice and in the negative charge transfer regime using determinant quantum Monte Carlo was studied.
Abstract: We study a three-orbital Su-Schrieffer-Heeger model defined on a two-dimensional Lieb lattice and in the negative charge transfer regime using determinant quantum Monte Carlo. At half-filling (1 hole/unit cell), we observe a bipolaron insulating phase, where the ligand oxygen atoms collapse and expand about alternating cation atoms to produce a bond-disproportionated state. This phase is robust against moderate hole doping but is eventually suppressed at large hole concentrations, leading to a metallic polaron-liquid-like state with fluctuating patches of local distortions. Our results suggest that the polarons are highly disordered in the metallic state and freeze into a periodic array across the metal-to-insulator transition. We also find an $s$-wave superconducting state at finite doping that primarily appears on the oxygen sublattices. Our approach provides an efficient, non-perturbative way to treat bond phonons in higher dimensions and our results have implications for many materials where coupling to bond phonons is the dominant interaction.

20 citations


Journal ArticleDOI
TL;DR: In this article, a comprehensive computational investigation concerning the polaron properties of a high electron conductivity π-conjugated ladder-type polymer, namely polybenzimidazobenzophenanthroline (BBL), is presented.
Abstract: Electronic charged states (i.e., polarons) play a crucial role in governing charge transfer, spin, thermo-electric and redox mechanisms in organic functional materials. An accurate description at the quantum-chemical level is mandatory to understand their response and transport properties. We report a comprehensive computational investigation concerning the polaron properties of a high electron conductivity (n-type) π-conjugated ladder-type polymer, namely polybenzimidazobenzophenanthroline (BBL). We show how spin polarized unrestricted Density Functional Theory (UDFT) and restricted (RDFT) methods can lead to solutions of the polaron and bipolaron electronic wavefunctions which are not the most stable ones. This aspect can be traced back to the multiconfigurational character of the electronic charged states’ wavefunction. We demonstrate how broken symmetry DFT (BS-UDFT) can circumvent this issue, well describing the polaron/bipolaron localization in terms of spin densities and structural deformations, thus providing a correct assessment of the electron transport parameters (e.g., reorganization energy), otherwise incorrectly computed at the UDFT/RDFT levels. Our calculations are further validated by comparing the IR spectra of polaronic species with the experimental one, as measured on doped BBL films. Our study calls for an urgent and careful computational assessment of the electronic charged states (e.g., polaron, bipolaron, etc.), in high performance π-conjugated materials, such as ladder-type polymers and other donor–acceptor derivatives, for a correct understanding of their charge, heat, and spin transport mechanisms.

20 citations


Journal ArticleDOI
TL;DR: This work uses a two-dimensional tight-binding approach to reveal the formation of bipolarons in GNRs and shows that the formed bipolarons are dynamically stable even for high electric field strengths when it comes to GNRs.
Abstract: Graphene nanoribbons (GNRs) are two-dimensional structures with a rich variety of electronic properties that derive from their semiconducting band gaps. In these materials, charge transport can occur via a hopping process mediated by carriers formed by self-interacting states between the excess charge and local lattice deformations. Here, we use a two-dimensional tight-binding approach to reveal the formation of bipolarons in GNRs. Our results show that the formed bipolarons are dynamically stable even for high electric field strengths when it comes to GNRs. Remarkably, the bipolaron dynamics can occur in acoustic and optical regimes concerning its saturation velocity. The phase transition between these two regimes takes place for a critical field strength in which the bipolaron moves roughly with the speed of sound in the material.

15 citations


Journal ArticleDOI
TL;DR: A two-dimensional version of the Su–Schrieffer–Heeger model is used to investigate the stability of bipolarons in armchair graphene nanoribbons (AGNRs) and shows how bipolaron stability is dependent on the strength of the electron–phonon interactions.
Abstract: Graphene nanoribbons are 2D hexagonal lattices with semiconducting band gaps. Below a critical electric field strength, the charge transport in these materials is governed by the quasiparticle mechanism. The quasiparticles involved in the process, known as polarons and bipolarons, are self-interacting states between the system charges and local lattice distortions. To deeply understand the charge transport mechanism in graphene nanoribbons, the study of the stability conditions for quasiparticles in these materials is crucial and may guide new investigations to improve the efficiency for a next generation of graphene-based optoelectronic devices. Here, we use a two-dimensional version of the Su–Schrieffer–Heeger model to investigate the stability of bipolarons in armchair graphene nanoribbons (AGNRs). Our findings show how bipolaron stability is dependent on the strength of the electron–phonon interactions. Moreover, the results show that bipolarons are dynamically stable in AGNRs for electric field strengths lower than 3.0 mV/A. Remarkably, the system’s binding energy for a lattice containing a bipolaron is smaller than the formation energy of two isolated polarons, which suggests that bipolarons can be natural quasiparticle solutions in AGNRs.

14 citations


Journal ArticleDOI
25 Apr 2019
TL;DR: In this article, the thermodynamic properties of a three-dimensional Bose-condensate of translation-invariant bipolarons (TI-bipolarons) in magnetic field were investigated.
Abstract: Consideration is given to thermodynamical properties of a three-dimensional Bose-condensate of translation-invariant bipolarons (TI-bipolarons) in magnetic field. The critical temperature of transition, critical magnetic fields, energy, heat capacity and the transition heat of TI-bipolaron gas are calculated. Such values as maximum magnetic field, London penetration depth and their temperature dependencies are calculated. The results obtained are used to explain experiments on high-temperature superconductors.

12 citations


Journal ArticleDOI
TL;DR: In this article, two electropolymerizable monomers were synthesized, namely dithieno[3,2-b:2′,3′-d]pyrrole N-functionalized with 4-(2-heptylthiazol-4-yl)phenyl (DTP1) and 4-(5-octylthiophen-2-yl)-phenyl(DTP2) groups.

11 citations


Journal Article
TL;DR: In this article, the electronic structure and optical properties of double-stranded benzimidazo-benzophenanthroline ladder (BBL) were studied using the density functional theory (DFT) and the time-dependent DFT method.
Abstract: Theoretical understanding of the electronic structure and optical transitions in n-doped conducting polymers is still controversial for polaronic and bipolaronic states and is completely missing for the case of a high doping level. In the present paper, the electronic structure and optical properties of the archetypical n-doped conducting polymer, double-stranded benzimidazo-benzophenanthroline ladder (BBL), are studied using the density functional theory (DFT) and the time-dependent DFT method. We find that a polaronic state in the BBL chain is a spin-resolved doublet where the spin degeneracy is lifted. The ground state of two electrons corresponds to a triplet polaron pair, which is in stark contrast to a commonly accepted picture where two electrons are postulated to form a spinless bipolaron. The total spin gradually increases until the reduction level reaches cᵣₑd = 100% (i.e., one electron per monomer unit). With further increase of the reduction level, the total spin decreases until it becomes 0 for the reduction level cᵣₑd = 200%. The calculated results reproduce the experimentally observed spin signal without any phenomenological parameters. A detailed analysis of the evolution of the electronic structure of BBL and its absorption spectra with increase in reduction level is presented. The calculated UV–vis–NIR spectra are compared with the available experimental results. The electronic structure and optical absorption for different reduction levels presented here are generic to a wide class of conducting polymers, which is illustrated by the corresponding calculations for another archetypical conducting polymer, poly(3,4-ethylenedioxythiophene) (best known as PEDOT).

11 citations


Journal ArticleDOI
TL;DR: In this article, the relationship between electrical properties and type of carrier, polarons and/or bipolarons, in unannealed and annealed PBTTT-C16 in an ionic-liquid-gated transistor (ILGT) configuration using Raman spectroscopy and electrochemical measurements was studied.

9 citations


Journal ArticleDOI
11 Feb 2019
TL;DR: In this paper, a comprehensive study of the UV-vis-NIR spectra of all oxidation states and doped forms of a series of oligo(aniline)s of varying lengths (dimer, tetramer and octamer) was performed using a computationally inexpensive DFT method that is particularly suited to molecules with charge-transfer character.
Abstract: The oxidation states and doped forms of oligo(aniline)s are readily interconverted, and each state has characteristic UV-vis-NIR absorptions, making this spectroscopic technique ideal for in situ analysis of oligo(aniline) behaviour. However, experimental isolation of some of these states can be challenging and quantitative agreement between experimental and calculated spectra has been poor, making it difficult to identify the exact structure(s) and properties of each state. Here we report a comprehensive study of the UV-vis-NIR spectra of all oxidation states and doped forms of a series of oligo(aniline)s of varying lengths (dimer, tetramer and octamer), using a computationally inexpensive DFT method that is particularly suited to molecules with charge-transfer character. The computational study suggests that doped oligo(aniline)s form mixtures of spin isomers (polaronic and bipolaronic forms) in solution, and we have been able to evaluate and compare the most likely electronic configurations, as well as supporting our insights experimentally, by ESR spectroscopy. This doping approach enables tuning of the spin isomer equilibrium position by varying the concentration of protonic dopant, offering a new pathway to explore the electronic structure of π-conjugated molecules more generally, and opening up new approaches to the design of spintronic materials.

5 citations


Journal ArticleDOI
TL;DR: The Su-Schrieffer-Heeger model with Hubbard extensions solved within of the time-dependent Hartree-Fock approximation is used to account for electron-electron interactions, developed over a hexagonal lattice, and results are elastic scattering in most cases.
Abstract: We study the dynamics of scattering of two positive polarons moving toward each other with parallel spins and of a polaron and a bipolaron both with positive charges and different velocities, in single chains of cis-polyacetylene and polyparaphenylene. We use the Su–Schrieffer–Heeger model with Hubbard extensions solved within of the time-dependent Hartree–Fock approximation to account for electron–electron interactions, developed over a hexagonal lattice. The main results are elastic scattering in most cases. This behavior changes to transmission of the polaron through the bipolaron when the ratio of their velocities in the same direction is in the range of 3:1 to 5:1 when no external electric field is present. The velocities ratio changes to of 6:1 to 8:1 under external electric field. This suggests that these systems may present an abrupt increase in the charge transport, under specific conditions, in an otherwise smoothly and monotonically increasing relation to the applied field. Their effective mass...

Journal ArticleDOI
TL;DR: In this paper, the structure of a one-dimensional bipolaron was studied, taking into account three types of correlation effects: such as interelectronic correlations caused by the direct dependence of the wave function on the distance between electrons as well as the one-center and two-center correlations.

Journal ArticleDOI
TL;DR: In this paper, the formation of the optical polaron and bipolaron in two-dimensional (2D) systems is studied in the intermediate electron-phonon coupling regime.

Posted Content
TL;DR: In this paper, it is shown that in high-temperature superconductors (HTSC) the Little-Parks effect based on bipolaron mechanism which in HTSC is the equivalent of Cooper pairing can be anomalously high.
Abstract: As is known, Little-Parks effect concerned with oscillations of the critical temperature of a superconducting transition was one of the first effects which suggested the existence of Cooper pairing in conventional superconductors. It is shown that in high-temperature superconductors (HTSC) the Little-Parks effect based on bipolaron mechanism which in HTSC is the equivalent of Cooper pairing can be anomalously high. The results obtained can be used as a new method for detecting soft phonon modes in nanostructures on the basis of HTSC. The results can be also used to enhance the critical temperature of a superconducting transition.

Journal ArticleDOI
TL;DR: In this article, the electric current of pyrrole oligomers, as a model of p-type doped polypyrrole, was studied theoretically using the first-principles calculation method.

Journal ArticleDOI
TL;DR: In this paper, a phase diagram for the charge and spin dynamics of an interacting gas of small (bi)polarons across the discontinuous conducting transition is proposed for understanding the long-standing conundrum of Mooij correlations.
Abstract: We performed electron spin-resonance (ESR), magnetic, and optical absorption studies of a model system used to investigate the metal-insulator transition in a deformable lattice. The ESR and magnetic studies revealed that the small polarons have no correlation to the conductivity below or above the mobility edge. The conducting transition is marked by the appearance of an asymmetric ESR signal simultaneously with a significant suppression of the small bipolaron optical absorption band indicating that the extended states result from an abrupt dissociation of the small bipolarons. Immediately above the mobility edge competing phases of extended states, small polarons, and small bipolarons coexist. We propose a phase diagram for the charge and spin dynamics of an interacting gas of small (bi)polarons across the discontinuous conducting transition that has implications for understanding the long-standing conundrum of Mooij correlations.

Journal ArticleDOI
TL;DR: In this article, the authors investigated the effect of the intermolecular interaction on the rotational state of two diatomic molecules immersed in a bath of bosons and proposed the biangulon quasiparticle.
Abstract: Inspired by the possibility to experimentally manipulate and enhance chemical reactivity in helium nanodroplets, we investigate the effective interaction and the resulting correlations between two diatomic molecules immersed in a bath of bosons. By analogy with the bipolaron, we introduce the \emph{biangulon} quasiparticle describing two rotating molecules that align with respect to each other due to the effective attractive interaction mediated by the excitations of the bath. We study this system in different parameter regimes and apply several theoretical approaches to describe its properties. Using a Born-Oppenheimer approximation, we investigate the dependence of the effective intermolecular interaction on the rotational state of the two molecules. In the strong-coupling regime, a product-state ansatz shows that the molecules tend to have a strong alignment in the ground state. To investigate the system in the weak-coupling regime, we apply a one-phonon excitation variational ansatz, which allows us to access the energy spectrum. In comparison to the angulon quasiparticle, the biangulon shows shifted angulon instabilities and an additional spectral instability, where resonant angular momentum transfer between the molecules and the bath takes place. These features are proposed as an experimentally observable signature for the formation of the biangulon quasiparticle. Finally, by using products of single angulon and bare impurity wave functions as basis states, we introduce a diagonalization scheme that allows us to describe the transition from two separated angulons to a biangulon as a function of the distance between the two molecules.

Journal ArticleDOI
TL;DR: In this article, a generalized variation method was developed to calculate the bipolaron size in the ground normal state of superconductors at various doping conditions, revealing a possible physical reason of strongly different doping behavior of the CO wave vector in different cuprates.
Abstract: A model resulting in charge ordering (CO) similar to that observed in cuprate superconductors is under study. It includes strong long-range electron-phonon interaction (EPI) and high density of correlated carriers. Coexistence of large bipolarons and delocalized carriers is a feature of such system. We develop generalized variation method to calculate the bipolaron size (CO period) in the ground normal state of such system at various doping. The approach allows the revealing of a possible physical reason of strongly different doping behavior of the CO wave vector in different cuprates. Obtained doping dependences of the CO period and temperature of the CO decay demonstrate quantitative agreement with those observed in cuprates. Predicted in the suggested approach ratio of the CO wave vector to the wave vector of the high-energy anomaly (HEA) in ARPES spectrum is in consent with that in cuprates. Calculated resonant x-rays scattering on the CO emerging in the model is in good agreement with experiments on cuprates including the asymmetry of the CO peaks' cross-section. A gap arises in the spectrum of delocalized carriers near antinodal direction due to their scattering on the periodic potential created by autolocalized carriers, analogously to photon crystal effect.

Journal ArticleDOI
TL;DR: The type of carrier, polaron and bipolaron, generated in PBTTT-C16 ionic-liquid-gated transistors fabricated with [EMIM][TFSI] or as mentioned in this paper, was identified using Raman spectroscopy.
Abstract: The type of carrier, polaron and bipolaron, generated in PBTTT-C16 ionic-liquid-gated transistors fabricated with [EMIM][TFSI] or [EMIM][FAP] was identified using Raman spectroscopy. Doping levels ...

Journal ArticleDOI
TL;DR: In this paper, it is demonstrated that according to translation-invariant bipolaron theory of high temperature superconductors, the different types of experiments measure for the gap different values, thus tunneling experiments determine the bipolaron energy for a superconducting gap.
Abstract: The explanation of the nature of superconducting gap in high temperature superconductors (HTSC) is a fundamental task which solution can lead to the understanding of superconducting mechanism. However, it has not been fully solved yet. From the mid of the twentieth century when Bardeen, Cooper and Schrieffer constructed their theory it has been believed that a superconducting gap is a collective phenomenon of electron excitations. In this work it is demonstrated that according to translation-invariant bipolaron theory of HTSC the different types of experiments measure for the gap different values. Thus tunneling experiments determine the bipolaron energy for a superconducting gap. On the other hand, the angle - resolved photoemission spectroscopy method measures the phonon frequency for which the electron-phonon interaction is maximum. Such effects as kinks in spectral measurements of gap, its angular dependence, existence of pseudogap and others have got natural explanations.