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A S Alexandrov - One of the best experts on this subject based on the ideXlab platform.
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superlight small Bipolarons from realistic long range coulomb and frohlich interactions
Physical Review B, 2012Co-Authors: A S Alexandrov, J H Samso, G SicaAbstract:We report analytical and numerical results on the two-particle states of the polaronic t-Jp model derived recently with realistic Coulomb and electron-phonon (Fr¨ohlich) interactions in doped polar insulators. Eigenstates and eigenvalues are calculated for two different geometries. Our results show that the ground state is a Bipolaronic singlet, made up of two polarons. The Bipolaron size increases with increasing ratio of the polaron hopping integral t to the exchange interaction Jp but remains small in the whole range 0 t/Jp 1. Furthermore, the model exhibits a phase transition to a superconducting state with a critical temperature well in excess of 100K since the small Bipolarons are perfectly mobile. In the range t/Jp 1, there are distinct charge and spin gaps opening in the density of states, specific heat, and magnetic susceptibility well above Tc.
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long range electron phonon interactions lead to superlight small Bipolarons
Journal of Physics: Conference Series, 2007Co-Authors: J P Hague, P E Kornilovitch, J H Samso, A S AlexandrovAbstract:A finite-range Frohlich electron-phonon interaction (EPI) with c-axis polarized optical phonons has been identified in cuprate superconductors by photoemission spectroscopy, in agreement with an earlier proposal by Alexandrov and Kornilovitch [1]. In this article, we discuss the consequences of long-range interactions on phonon-mediated local pairing. First, we examine the effects of modifying interaction range and lattice geometries with regard to analytical strong-coupling/non-adiabatic results for ladder systems. To test the applicability of the analytic results to experimentally achievable couplings and phonon frequencies, we apply a continuous time quantum Monte-Carlo algorithm (CTQMC) to the computation of the effective mass and pairing radius of lattice Bipolarons. We demonstrate that Bipolarons can be simultaneously small and light due to a novel crab-like motion. Such light, small Bipolarons are a necessary precursor to high-temperature Bose-Einstein condensation in solids.
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superlight small Bipolarons
Journal of Physics: Condensed Matter, 2007Co-Authors: J P Hague, P E Kornilovitch, J H Samso, A S AlexandrovAbstract:Recent angle-resolved photoemission spectroscopy (ARPES) has identified that a finite-range Frohlich electron-phonon interaction (EPI) with c-axis polarized optical phonons is important in cuprate superconductors, in agreement with an earlier proposal by Alexandrov and Kornilovitch. The estimated unscreened EPI is so strong that it could easily transform doped holes into mobile lattice Bipolarons in narrow-band Mott insulators such as cuprates. Applying a continuous-time quantum Monte-Carlo algorithm (CTQMC) we compute the total energy, effective mass, pair radius, number of phonons and isotope exponent of lattice Bipolarons in the region of parameters where any approximation might fail taking into account the Coulomb repulsion and the finite-range EPI. The effects of modifying the interaction range and different lattice geometries are discussed with regards to analytical strong-coupling/non-adiabatic results. We demonstrate that Bipolarons can be simultaneously small and light, provided suitable conditions on the electron-phonon and electron-electron interaction are satisfied. Such light small Bipolarons are a necessary precursor to high-temperature Bose-Einstein condensation in solids. The light Bipolaron mass is shown to be universal in systems made of triangular plaquettes, due to a novel crab-like motion. Another surprising result is that the triplet-singlet exchange energy is of the first order in the hopping integral and triplet Bipolarons are heavier than singlets in certain lattice structures at variance with intuitive expectations. Finally, we identify a range of lattices where superlight small Bipolarons may be formed, and give estimates for their masses in the anti-adiabatic approximation.
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superlight small Bipolarons a route to room temperature superconductivity
arXiv: Superconductivity, 2007Co-Authors: A S AlexandrovAbstract:Extending the BCS theory towards the strong electron-phonon interaction (EPI), a charged Bose liquid of small Bipolarons has been predicted by us with a further prediction that the highest superconducting critical temperature is found in the crossover region of the EPI strength from the BCS-like to Bipolaronic superconductivity. Later on we have shown that the unscreened (infinite-range) Frohlich EPI combined with the strong Coulomb repulsion create superlight small Bipolarons, which are several orders of magnitude lighter than small Bipolarons in the Holstein–Hubbard model (HHM) with a zero-range EPI. The analytical and numerical studies of this Coulomb–Frohlich model (CFM) provide the following recipes for room-temperature superconductivity: (a) The parent compound should be an ionic insulator with light ions to form high-frequency optical phonons, (b) the structure should be quasi two-dimensional to ensure poor screening of high-frequency phonons polarized perpendicular to the conducting planes, (c) a triangular lattice is required in combination with strong, on-site Coulomb repulsion to form the small superlight Bipolaron, (d) moderate carrier densities are required to keep the system of small Bipolarons close to the Bose-Einstein condensation regime. Clearly most of these conditions are already met in the cuprates.
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superlight small Bipolarons in the presence of a strong coulomb repulsion
Physical Review Letters, 2007Co-Authors: J P Hague, J H Samso, P E Kornilovitch, A S AlexandrovAbstract:We study a lattice Bipolaron on a staggered triangular ladder and triangular and hexagonal lattices with both long-range electron-phonon interaction and strong Coulomb repulsion using a novel continuous-time quantum Monte Carlo algorithm to solve the two-particle Coulomb-Fro¨hlich model. The algorithm is preceded by an exact integration over phonon degrees of freedom, and as such is extremely efficient. The Bipolaron effective mass and radius are computed. Bipolarons on lattices constructed from triangular plaquettes have a novel crablike motion, and are small but very light over a wide range of parameters. We discuss the conditions under which such particles may form a Bose-Einstein condensate with high transition temperature, proposing a route to room temperature superconductivity
V. D. Lakhno - One of the best experts on this subject based on the ideXlab platform.
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superconducting properties of 3d low density translation invariant Bipolaron gas
Advances in Condensed Matter Physics, 2018Co-Authors: V. D. LakhnoAbstract:Consideration is given to thermodynamical properties of a three-dimensional Bose-condensate of translation-invariant Bipolarons (TI-Bipolarons). The critical temperature of transition, energy, heat capacity, and the transition heat of ideal TI-Bipolaron gas are calculated. The results obtained are used to explain experiments on high-temperature superconductors.
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translation invariant theory of polaron Bipolaron and the problem of quantizing near the classical solution
Journal of Experimental and Theoretical Physics, 2013Co-Authors: V. D. LakhnoAbstract:A physical interpretation of translation-invariant polarons and Bipolarons is presented, some results of their existence are discussed. Consideration is given to the problem of quantization in the vicinity of the classical solution in the quantum field theory. The lowest variational estimate is obtained for the Bipolaron energy E(η) with E(0) = -0.440636α2, where α is a constant of electron-phonon coupling, η is a parameter of ion binding.
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translation invariant theory of polaron Bipolaron and the problem of quantizing near the classical solution
arXiv: Other Condensed Matter, 2013Co-Authors: V. D. LakhnoAbstract:A physical interpretation of translation-invariant polarons and Bipolarons is presented, some results of their existence are discussed. Consideration is given to the problem of quantization in the vicinity of the classical solution in the quantum field theory. The lowest variational estimate is obtained for the Bipolaron energy E(\eta) with E(0)=-0,440636\alpha^2, where \alpha is a constant of electron-phonon coupling, \eta is a parameter of ion binding.
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translation invariant Bipolarons and the problem of high temperature superconductivity
Solid State Communications, 2012Co-Authors: V. D. LakhnoAbstract:Abstract It is shown that the Bipolaron ground state is described by a delocalized wave function. For a two-parameter wave function, the lowest variation estimate of the ground state energy in the strong coupling limit is found to be E = − 0 , 414125 α 2 . This is much lower than that derived with the use of the localized Bipolaron wave function. The results obtained testify to the possibility of a Bipolaron mechanism of high-temperature superconductivity.
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Energy and critical ionic-bond parameter of a 3D large-radius Bipolaron
Journal of Experimental and Theoretical Physics, 2010Co-Authors: V. D. LakhnoAbstract: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.
David Emin - One of the best experts on this subject based on the ideXlab platform.
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dynamic d symmetry bose condensate of a planar large Bipolaron liquid in cuprate superconductors
Philosophical Magazine, 2017Co-Authors: David EminAbstract:Planar-large-Bipolarons can form if the ratio of the surrounding mediums’ static to high-frequency dielectric constants is especially large, e0/e∞ >> 2. A large-Bipolaron in p-doped La2CuO4 is mode...
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dynamic d symmetry bose condensate of a planar large Bipolaron liquid in cuprate superconductors
arXiv: Superconductivity, 2017Co-Authors: David EminAbstract:Planar large-Bipolarons can form if the ratio of the surrounding mediums static to high-frequency dielectric constants is especially large. A large-Bipolaron in p-doped La2CuO4 is modeled as two electrons being removed from the out-of-plane orbitals of four oxygen ions circumscribed by four copper ions of a CuO2 layer. These oxygen dianions relax inwardly as they donate electrons to the surrounding outwardly relaxing copper cations. This charge transfer generates the strong in-plane electron-lattice interaction needed to stabilize a large-Bipolaron with respect to decomposing into polarons. The lowest-energy radial in-plane optic vibration of a large-Bipolarons four core oxygen ions with their associated electronic charges has d-symmetry. Electronic relaxation in response to multiple large-Bipolarons atomic vibrations lowers their frequencies to generate a phonon-mediated attraction among them which fosters their condensation into a liquid. This liquid features distinctive transport and optical properties. A large-Bipolaron liquids superconductivity can result when it undergoes a Bose condensation yielding macroscopic occupation of its ground-state. The synchronized vibrations of large-Bipolarons core-oxygen ions with their electronic charges generate this Bose condensates dynamic global d-symmetry.
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in plane conductivity of a layered large Bipolaron liquid
arXiv: Superconductivity, 2014Co-Authors: David EminAbstract:Distinctive normal-state properties of cuprate superconductors follow from their charge carriers forming a large-Bipolaron liquid. The very weak scattering of the slow-moving heavy-massed excitations of the liquid by acoustic phonons yields a scattering rate that is less than the Debye frequency. The moderate liquid mobility, greater than 1 cm2/V-sec at 300 K, results as the weak scattering of the liquid compensates for its large mass. In resolution of a long-standing dilemma, the dc resistivity resulting from scattering by acoustic phonons remains nearly proportional to temperature to well below the Debye temperature. Above the Debye frequency the frequency-dependent conductivity is dominated by excitation and photo-ionization of the self-trapped electronic carriers of the large-Bipolarons. Below the Debye frequency the frequency-dependent conductivity is dominated by the Drude-like collective motion of the large-Bipolaron liquid. The gap between these two domains sharpens with decreasing temperature as phonon scattering of the liquid diminishes. The high-frequency electronic excitations survive in the superconducting state.
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conductivities and seebeck coefficients of boron carbides softening Bipolaron hopping
Physical Review B, 2001Co-Authors: T. L. Aselage, David Emin, Steven S. MccreadyAbstract:The most conspicuous feature of boron carbides' electronic transport properties is their having both high carrier densities and large Seebeck coefficients. The magnitudes and temperature dependencies of the Seebeck coefficients are consistent with large contributions from softening Bipolarons: singlet Bipolarons whose stabilization is significantly affected by their softening of local vibrations. Boron carbides' high carrier densities, small activation energies for hopping ({approx} 0.16 eV), and anomalously large Seebeck coefficients combine with their low, glass-like thermal conductivities to make them unexpectedly efficient high-temperature thermoelectrics.
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singlet Bipolaron formation among degenerate electronic orbitals softening Bipolarons
Physical Review B, 2000Co-Authors: David EminAbstract:Bipolaron formation among degenerate electronic orbitals is considered. Symmetry-breaking deformations that lift the electronic degeneracy are shown to be required to stabilize a singlet with respect to a triplet. These symmetry-breaking deformations mix singlet states of the undeformed system. Mixing singlets of different energies causes the electronic energy of the lowest-energy singlet to fall nonlinearly with symmetry-breaking deformations. Through this nonlinearity, a singlet pair of the lowest energy not only fosters symmetry-breaking deformations but also reduces the stiffness constants of these symmetry-breaking distortions. The concomitant singlet-induced lowering of the vibrational frequencies reduces the vibrational free energy. Thus, singlet-induced atomic displacements and vibrational softening both contribute to lowering the free energy of a singlet Bipolaron below that of a triplet and of two separated polarons. At strong enough electron-lattice coupling, stabilization of a singlet Bipolaron is driven mainly by symmetry-breaking shifts of atoms' equilibrium positions. This singlet Bipolaron is a variant of a conventional Bipolaron whose stabilization with respect to a triplet requires symmetry-breaking deformations. However, stabilization of a singlet Bipolaron at moderate coupling results primarily from singlet-induced reductions of the frequencies of symmetry-breaking vibrations. This type of Bipolaron is defined as a ''softening'' Bipolaron. Distinctive properties of a softening Bipolaronmore » enable it to be identified. Most prominently, softening Bipolarons can lack the polaronic absorption bands that characterize conventional Bipolarons. In addition, the Seebeck coefficients of softening Bipolarons garner a distinctive contribution from singlet-induced softening of vibrational modes. Observation of these effects for boron carbides' singlet Bipolarons suggests that they are examples of softening Bipolarons. (c) 2000 The American Physical Society.« less
Noel C Giebink - One of the best experts on this subject based on the ideXlab platform.
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image enhanced Bipolaron formation at organic semiconductor electrode interfaces
Physical Review B, 2020Co-Authors: Jonathon R Schrecengost, Sukrit Mukhopadhyay, Noel C GiebinkAbstract:We explore the image charge interaction for organic semiconductor Bipolarons near a conducting interface and find that the cross term between one of the constituent charges and the image of its neighbor stabilizes the Bipolaron by up to \ensuremath{\sim}0.3 eV, dramatically increasing the concentration of this species near the interface. Using density functional theory calculations for the common hole transport molecule $N,N$\ensuremath{'}-bis(3-methylphenyl)-$N,N$\ensuremath{'}-diphenylbenzidine, we validate a simple point charge description of this effect and incorporate it within an interface energy level alignment model to predict the density of polarons and Bipolarons near the interface. We find that the image effect greatly enhances Bipolaron formation in the first few monolayers, leading to the expectation that Bipolarons account for more than 1% of the total interface charge in many cases of practical interest. This result reinforces the notion that Bipolarons are robust near the contacts of many organic semiconductor devices and thus helps to rationalize their involvement in the phenomenon of unipolar organic magnetoresistance.
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large Bipolaron density at organic semiconductor electrode interfaces
Nature Communications, 2017Co-Authors: Rijul Dhanker, Christopher L Gray, Sukrit Mukhopadhyay, Sean A Nunez, Chiao Yu Cheng, Anatoliy N Sokolov, Noel C GiebinkAbstract:Bipolaron states, in which two electrons or two holes occupy a single molecule or conjugated polymer segment, are typically considered to be negligible in organic semiconductor devices due to Coulomb repulsion between the two charges. Here we use charge modulation spectroscopy to reveal a Bipolaron sheet density >1010 cm−2 at the interface between an indium tin oxide anode and the common small molecule organic semiconductor N,N′-Bis(3-methylphenyl)-N,N′-diphenylbenzidine. We find that the magnetocurrent response of hole-only devices correlates closely with changes in the Bipolaron concentration, supporting the Bipolaron model of unipolar organic magnetoresistance and suggesting that it may be more of an interface than a bulk phenomenon. These results are understood on the basis of a quantitative interface energy level alignment model, which indicates that Bipolarons are generally expected to be significant near contacts in the Fermi level pinning regime and thus may be more prevalent in organic electronic devices than previously thought. Bipolarons - two electrons or holes localized on the same molecule - are generally considered negligible in organic electronic devices. Dhanker et al. show that large Bipolaron densities can exist near electrode interfaces and that they are linked to the phenomenon of unipolar organic magnetoresistance.
J H Samso - One of the best experts on this subject based on the ideXlab platform.
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superlight small Bipolarons from realistic long range coulomb and frohlich interactions
Physical Review B, 2012Co-Authors: A S Alexandrov, J H Samso, G SicaAbstract:We report analytical and numerical results on the two-particle states of the polaronic t-Jp model derived recently with realistic Coulomb and electron-phonon (Fr¨ohlich) interactions in doped polar insulators. Eigenstates and eigenvalues are calculated for two different geometries. Our results show that the ground state is a Bipolaronic singlet, made up of two polarons. The Bipolaron size increases with increasing ratio of the polaron hopping integral t to the exchange interaction Jp but remains small in the whole range 0 t/Jp 1. Furthermore, the model exhibits a phase transition to a superconducting state with a critical temperature well in excess of 100K since the small Bipolarons are perfectly mobile. In the range t/Jp 1, there are distinct charge and spin gaps opening in the density of states, specific heat, and magnetic susceptibility well above Tc.
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long range electron phonon interactions lead to superlight small Bipolarons
Journal of Physics: Conference Series, 2007Co-Authors: J P Hague, P E Kornilovitch, J H Samso, A S AlexandrovAbstract:A finite-range Frohlich electron-phonon interaction (EPI) with c-axis polarized optical phonons has been identified in cuprate superconductors by photoemission spectroscopy, in agreement with an earlier proposal by Alexandrov and Kornilovitch [1]. In this article, we discuss the consequences of long-range interactions on phonon-mediated local pairing. First, we examine the effects of modifying interaction range and lattice geometries with regard to analytical strong-coupling/non-adiabatic results for ladder systems. To test the applicability of the analytic results to experimentally achievable couplings and phonon frequencies, we apply a continuous time quantum Monte-Carlo algorithm (CTQMC) to the computation of the effective mass and pairing radius of lattice Bipolarons. We demonstrate that Bipolarons can be simultaneously small and light due to a novel crab-like motion. Such light, small Bipolarons are a necessary precursor to high-temperature Bose-Einstein condensation in solids.
-
superlight small Bipolarons
Journal of Physics: Condensed Matter, 2007Co-Authors: J P Hague, P E Kornilovitch, J H Samso, A S AlexandrovAbstract:Recent angle-resolved photoemission spectroscopy (ARPES) has identified that a finite-range Frohlich electron-phonon interaction (EPI) with c-axis polarized optical phonons is important in cuprate superconductors, in agreement with an earlier proposal by Alexandrov and Kornilovitch. The estimated unscreened EPI is so strong that it could easily transform doped holes into mobile lattice Bipolarons in narrow-band Mott insulators such as cuprates. Applying a continuous-time quantum Monte-Carlo algorithm (CTQMC) we compute the total energy, effective mass, pair radius, number of phonons and isotope exponent of lattice Bipolarons in the region of parameters where any approximation might fail taking into account the Coulomb repulsion and the finite-range EPI. The effects of modifying the interaction range and different lattice geometries are discussed with regards to analytical strong-coupling/non-adiabatic results. We demonstrate that Bipolarons can be simultaneously small and light, provided suitable conditions on the electron-phonon and electron-electron interaction are satisfied. Such light small Bipolarons are a necessary precursor to high-temperature Bose-Einstein condensation in solids. The light Bipolaron mass is shown to be universal in systems made of triangular plaquettes, due to a novel crab-like motion. Another surprising result is that the triplet-singlet exchange energy is of the first order in the hopping integral and triplet Bipolarons are heavier than singlets in certain lattice structures at variance with intuitive expectations. Finally, we identify a range of lattices where superlight small Bipolarons may be formed, and give estimates for their masses in the anti-adiabatic approximation.
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superlight small Bipolarons in the presence of a strong coulomb repulsion
Physical Review Letters, 2007Co-Authors: J P Hague, J H Samso, P E Kornilovitch, A S AlexandrovAbstract:We study a lattice Bipolaron on a staggered triangular ladder and triangular and hexagonal lattices with both long-range electron-phonon interaction and strong Coulomb repulsion using a novel continuous-time quantum Monte Carlo algorithm to solve the two-particle Coulomb-Fro¨hlich model. The algorithm is preceded by an exact integration over phonon degrees of freedom, and as such is extremely efficient. The Bipolaron effective mass and radius are computed. Bipolarons on lattices constructed from triangular plaquettes have a novel crablike motion, and are small but very light over a wide range of parameters. We discuss the conditions under which such particles may form a Bose-Einstein condensate with high transition temperature, proposing a route to room temperature superconductivity