Bose–Einstein condensation of strongly correlated electrons and phonons in cuprate superconductors

Alexandrov, A. S.

doi:10.1088/0953-8984/19/12/125216

Cited by 21 publications

(21 citation statements)

References 112 publications

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“…Apart from the striking isotope effects explained quantitatively here, there is abundant independent evidence in favor of bipolarons and the BEC in underdoped cuprate superconductors [46]. In particular, the parameter-free estimates of the Fermi energy using the magnetic-field penetration depth [47] and the magnetic quantum oscillations [48] pointed to a very low value (below 50 meV), supporting the real-space pairing in underdoped cuprate superconductors.…”

Section: Discussionsupporting

confidence: 61%

Isotope effects in high-T_ccuprate superconductors as support for the bipolaron theory of superconductivity

Alexandrov

Zhao

2012

New J. Phys.

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We provide a unified parameter-free explanation of the observed oxygen-isotope effects on the critical temperature, the magnetic-field penetration depth and on the normal-state pseudogap for underdoped cuprate superconductors within the framework of the multi-(bi)polaron theory with strong Coulomb and Fröhlich interactions. We also quantitatively explain the measured critical temperature and the magnitude of the magnetic-field penetration depth. This paper thus represents an important support for the bipolaron theory of high-temperature superconductivity, compatible with many other independent observations. On the long journey towards a microscopic understanding of superconductivity, the observation of an isotope effect on the critical temperature, T c , in 1950 [1,2] provided an important clue to the microscopic mechanism of superconductivity. The presence of an isotope effect thus implies that superconductivity is not of purely electronic origin. In the same year, Fröhlich [3] pointed out that the electron-phonon interaction gave rise to an attractive interaction between electrons, which might be responsible for superconductivity. Fröhlich's theory played a decisive role in establishing the correct mechanism. Finally, in 1957, Bardeen, Cooper and Schrieffer (BCS) [4] developed the BCS theory that was the first successful microscopic theory of superconductivity. The BCS theory implies an isotope-mass dependence of T c , with an isotope-effect exponent α = −d ln T c /d ln M = 1/2, in excellent agreement with the reported isotope exponents in simple metallic superconductors such as Hg, Sn and Pb.The doping-dependent oxygen-isotope effect (OIE) on the critical temperature T c , α O = −d ln T c /d ln M O (where M O is the oxygen-isotope mass) [5] and the substantial OIE on the in-plane supercarrier mass m * * ab , α O m * = dm * * ab /d ln M O [6-11], provide direct evidence for a significant electron-phonon interaction (EPI) also in high-temperature cuprate superconductors. High-resolution angle-resolved photoemission spectroscopy (ARPES) [12] provides further evidence for strong EPI with c-axis-polarized optical phonons [13]. These results, along with optical [14], neutron scattering [15,16] and tunneling data [17][18][19], unambiguously show that lattice vibrations play a significant but unconventional role in high-temperature superconductivity. The interpretation of the optical spectra of high-T c materials as the polaron absorption [20,21] strengthens the view [22] that the Fröhlich EPI is important in those structures. Operating together with a shorter-range deformation potential and molecular-type (e.g. Jahn-Teller [23]) EPIs, the Fröhlich EPI can readily overcome the Coulomb repulsion at a short distance of about the lattice constant for electrons to form real-space inter-site bipolarons [24].Despite all these remarkable and well-performed experiments that lead to the consistent conclusion about the important role of EPI in high-temperature superconductors, there is no consensus on the microscopic origin of ...

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Section: Discussionsupporting

confidence: 61%

Isotope effects in high-T_ccuprate superconductors as support for the bipolaron theory of superconductivity

Alexandrov

Zhao

2012

New J. Phys.

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“…There is abundant independent evidence in favor of (bi)polarons [25] and 3D BEC in cuprate superconductors [71,72]. The substantial isotope effect on the carrier mass [73][74][75][76] predicted for (bi)polaronic conductors in [77] is perhaps the most compelling evidence for (bi)polaronic carries in cuprate superconductors.…”

Section: Concluding Remarks On Lattice (Bi)polarons In High-tempermentioning

confidence: 99%

High-temperature superconductivity: the explanation

Alexandrov

2011

Phys. Scr.

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Soon after the discovery of the first high temperature superconductor by Georg Bednorz and Alex Müller in 1986 the late Sir Nevill Mott answering his own question "Is there an explanation?" [Nature 327 (1987) 185] expressed a view that the Bose-Einstein condensation (BEC) of small bipolarons, predicted by us in 1981, could be the one. Several authors then contemplated BEC of real space tightly bound pairs, but with a purely electronic mechanism of pairing rather than with the electron-phonon interaction (EPI). However, a number of other researchers criticized the bipolaron (or any real-space pairing) scenario as incompatible with some angle-resolved photoemission spectra (ARPES), with experimentally determined effective masses of carriers and unconventional symmetry of the superconducting order parameter in cuprates. Since then the controversial issue of whether the electron-phonon 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 I outline some developments in the bipolaron theory suggesting that the true origin of high-temperature superconductivity is found in a proper combination of strong electron-electron correlations with a significant finite-range (Fröhlich) EPI, and that the theory is fully compatible with the key experiments.

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“…Analysis of the results yields an electron-phonon coupling parameter of an intermediate strength, α ≈ 4. Electron-phonon coupling in the perovskites is a subject of much recent interest due to the controversy over its relevance in the phenomena of multiferroicity, ferroelectricity, superconductivity and colossal magnetoresistance [1,2,3,4,5]. Despite much progress, full understanding of the physics of electron-phonon coupling in perovskites is still lacking because of additional crystallographic complexities of many materials involved (breathing, tilting and rotational distortions, ferroelectric symmetry breaking), magnetism, complex electronic effects (strong correlations), and also because of the lack of high-accuracy spectroscopic measurements specifically designed to probe electron-phonon coupling.…”

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Electron-Phonon Interaction and Charge Carrier Mass Enhancement inSrTiO3

Mechelen¹,

Marel²,

Grimaldi³

et al. 2008

Phys. Rev. Lett.

205

213

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We report a comprehensive THz, infrared and optical study of Nb doped SrTiO3 as well as DC conductivity and Hall effect measurements. Our THz spectra at 7 K show the presence of a very narrow (< 2 meV) Drude peak, the spectral weight of which shows approximately a factor of three enhancement of the band mass for all carrier concentrations. The missing spectral weight is regained in a broad 'mid-infrared' band which originates from electron-phonon coupling. We find no evidence of a particularly large electron-phonon coupling that would result in small polaron formation. Analysis of the results yields an electron-phonon coupling parameter of an intermediate strength, α ≈ 4.PACS numbers: 71.38.-k, 72.20.-i, 78.20.-e Electron-phonon coupling in the perovskites is a subject of much recent interest due to the controversy over its relevance in the phenomena of multiferroicity, ferroelectricity, superconductivity and colossal magnetoresistance [1,2,3,4,5]. Despite much progress, full understanding of the physics of electron-phonon coupling in perovskites is still lacking because of additional crystallographic complexities of many materials involved (breathing, tilting and rotational distortions, ferroelectric symmetry breaking), magnetism, complex electronic effects (strong correlations), and also because of the lack of high-accuracy spectroscopic measurements specifically designed to probe electron-phonon coupling.With this in mind, we have studied a prototypical perovskite oxide, SrTi 1−x Nb x O 3 with 0 ≤ x ≤ 0.02. SrTiO 3 is an insulator (∆ = 3.25 eV) with the conduction band formed by the Ti 3d states. These are split by the crystal field so that the three t 2g states become occupied when the material is electron-doped by substituting pentavalent Nb for tetravalent Ti. For 0.0005 ≤ x ≤ 0.02 SrTi 1−x Nb x O 3 becomes superconducting at a T c of typically ∼ 0.3 K [6,7], and at most 1.2 K [5]. Characterized by the threefold degeneracy of the conduction bands and the high lattice polarizability, electron doped SrTiO 3 provides a perfect opportunity for the study of electronphonon coupling and polaron formation in an archetypal perovskite [8,9].

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Bose–Einstein condensation of strongly correlated electrons and phonons in cuprate superconductors

Cited by 21 publications

References 112 publications

Isotope effects in high-T_ccuprate superconductors as support for the bipolaron theory of superconductivity

Isotope effects in high-T_ccuprate superconductors as support for the bipolaron theory of superconductivity

High-temperature superconductivity: the explanation

Electron-Phonon Interaction and Charge Carrier Mass Enhancement inSrTiO3

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Bose–Einstein condensation of strongly correlated electrons and phonons in cuprate superconductors

Cited by 21 publications

References 112 publications

Isotope effects in high-Tccuprate superconductors as support for the bipolaron theory of superconductivity

Isotope effects in high-Tccuprate superconductors as support for the bipolaron theory of superconductivity

High-temperature superconductivity: the explanation

Electron-Phonon Interaction and Charge Carrier Mass Enhancement inSrTiO3

Contact Info

Product

Resources

About

Isotope effects in high-T_ccuprate superconductors as support for the bipolaron theory of superconductivity

Isotope effects in high-T_ccuprate superconductors as support for the bipolaron theory of superconductivity