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

Abstract

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 ...

Unusual superconducting isotope effect in the presence of a quantum criticality

“…Further studies revealed the effect of isotopic replacement on many other quantities, such as the pseudogap [7], magnetic penetration depth [8], electron dynamics [9] and quasiparticle inelastic scattering [10] . This evidence motivated a variety of theoretical ideas for the pairing mechanism, such as the bipolaron theory [11], magnetically mediated SC [12,13] and valence fluctuation [14].…”

“…where J q = j J 0,j e iR j q is the Fourier transform of the coupling. The critical behavior can be seen by expanding J Γ , in (11) or (12), around M = M c . In both cases, we obtain Eq.…”

High-temperature superconductivity from realistic Coulomb and Fröhlich interactions

“…In particular the doping dependent oxygen-isotope effects on the critical temperature T c and on the in-plane supercarrier mass (ref. [7][8][9][10][11][12] ), provide direct evidence for a significant EPI and bipolaronic carriers 13 in high-temperature cuprate superconductors. Angle-resolved photoemission spectra (ARPES) 14,15 provide further evidence for the strong EPI apparently with c-axis-polarised optical phonons 16 .…”

The bipolaron model in transport properties of YBa₂Cu₃O_y superconductor prepared by adding Pr₂Co₇ magnetic nanoparticles

“…Depuis leur découverte par Bednorz et Muller [1], l'origine de la supraconductivité dans les supraconducteurs à haute température, notamment les cuprates, malgré des avancées [2,3] reste inconnue. La théorie BCS qui explique convenablement la supraconductivité des supraconducteurs ''conventionnels'' par une interaction attractive entre deux électrons via des phonons est mis en défaut par la supraconductivité à haute température.…”