We have calculated the tunneling conductance of a superconductor-insulator-superconductor junction based on the polaron-bipolaron theory of superconductivity. The predicted incoherent hump features are in quantitative agreement with tunneling spectra
of optimally doped Bi2Sr2CaCu2O8+y and Bi2Sr2Ca2Cu3O10+y. We further show that angle-resolved photoemission spectra of underdoped cuprates are consistent with the Bose-Einstein condensation of inter-site bipolarons and that the superconducting gap symmetry is d-wave, which is determined by the anomalous kinetic process rather than by the pairing interaction. In the overdoped cuprates (BCS-like superconductors), the superconducting gap symmetry is the same as the pairing symmetry, which is found to be extended s-wave with eight line nodes in hole-doped cuprates and nodeless s-wave in electron-doped cuprates. The polaronic effect significantly enhances the density of states at the Fermi level and the effective electron-phonon coupling constant for low-energy phonon modes, which is the key to the understanding of high-temperature superconductivity.
We have analyzed scanning tunneling spectra of two electron-doped cuprates Pr0.88LaCe0.12CuO4 (Tc = 21 K and 24 K) and compared them with tunneling spectrum of hole-doped La1.84Sr0.16CuO4 and effective electron-boson spectral function of hole-doped L
a1.97Sr0.03CuO4 (extracted from angle-resolved photoemission spectrum). We have also analyzed tunneling spectra and angle-resolved photoemission spectra for hole-doped Bi2Sr2CaCu2O8. These results unambiguously rule out magnetic pairing mechanism in both electron- and hole-doped cuprates and support polaronic/bipolaronic superconductivity in hole-doped Bi2Sr2CaCu2O8.
We report high-temperature (300-1120 K) magnetization data of Fe and Fe3O4 nanoparticles embedded in multi-walled carbon nanotubes. The magnetic impurity concerntations are precisely determined by both high-energy synchrotron x-ray diffractometer and
inductively coupled plasma mass spectrometer. We unambiguously show that the magnetic moments of Fe and Fe3O4 nanoparticles are enhanced by a factor of about 3 compared with what they would be expected to have for free (unembedded) magnetic nanoparticles. The magnetization enhancement factor is nearly independent of the applied magnetic field but depends significantly on the cooling rate. What is more intriguing is that the enhanced moments were completely lost when the sample was heated up to 1120 K and the lost moments at 1120 K were completely recovered through several thermal cycles below 1020 K. Furthermore, there is a rapid increase or decrease in the magnetization below about 60 K. The anomalous magnetic properties cannot be explained by existing physics models except for the paramagnetic Meissner effect due to the existence of ultrahigh temperature superconductivity in the multi-walled carbon nanotubes.
In our recent paper entitled Pairing mechanism of high-temperature superconductivity: Experimental constraints (to be published in Physica Scripta, arXiv:1012.2368), we review some crucial experiments that place strong constraints on the microscopic
pairing mechanism of high-temperature superconductivity in cuprates. In particular, we show that phonons rather than spin-fluctuation play a predominant role in the microscopic pairing mechanism. We further show that the intrinsic pairing symmetry in the bulk is not d-wave, but extended s-wave (having eight line nodes) in hole-doped cuprates and nodeless s-wave in electron-doped cuprates. In contrast, the author of the Comment (to be published in Physica Scripta) argues that our conclusions are unconvincing and even misleading. In response to the criticisms in the Comment, we further show that our conclusions are well supported by experiments and his criticisms are lack of scientific ground.
Developing a theory of high-temperature superconductivity in copper oxides is one of the outstanding problems in physics. It is a challenge that has defeated theoretical physicists for more than twenty years. Attempts to understand this problem are h
indered by the subtle interplay among a few mechanisms and the presence of several nearly degenerate and competing phases in these systems. Here we present some crucial experiments that place essential constraints on the pairing mechanism of high-temperature superconductivity. The observed unconventional oxygenisotope effects in cuprates have clearly shown strong electron-phonon interactions and the existence of polarons and/or bipolarons. Angle-resolved photoemission and tunneling spectra have provided direct evidence for strong coupling to multiple-phonon modes. In contrast, these spectra do not show strong coupling features expected for magnetic resonance modes. Angle-resolved photoemission spectra and the oxygen-isotope effect on the antiferromagnetic exchange energy J in undoped parent compounds consistently show that the polaron binding energy is about 2 eV, which is over one order of magnitude larger than J = 0.14 eV. The normal-state spin-susceptibility data of holedoped cuprates indicate that intersite bipolarons are the dominant charge carriers in the underdoped region while the component of Fermi-liquid-like polarons is dominant in the overdoped region. All the experiments to test the gap or order-parameter symmetry consistently demonstrate that the intrinsic gap (pairing) symmetry for the Fermi-liquid-like component is anisotropic s-wave and the order-parameter symmetry of the Bose-Einstein condensation of bipolarons is d-wave.
We report magnetic susceptibility $chi(T)$ measurements on oxygen-isotope exchanged La$_{1-x}$Ca$_{x}$MnO$_{3+y}$ up to 700 K. The $1/chi(T)$ data show that the ferromagnetic exchange-energy $J$ depends strongly on the oxygen-isotope mass. The isotop
e effect on $J$ decreases with temperature up to 400 K and then increases again with temperature above 400 K. This unusual temperature dependence of the isotope effect cannot be explained by existing theories of the colossal magnetoresistance effect for doped manganites. The present results thus provide essential constraints on the physics of manganites.
