Magnetic measurements have been carried out in the superconducting and normal states of the optimally doped nonmagnetic bismuthate superconductor Ba0.63K0.37BiO3. The magnetic data along with previous muSR, resistivity, and tunneling data consistentl
y show that there is a large polaronic enhancement in the density of states and effective electron-phonon coupling constant. The first-principle calculation within the density-functional theory indicates a small electron-phonon coupling constant of about 0.3-0.4, which can only lead to about 1 K superconductivity within the conventional phonon-mediated mechanism. Remarkably, the polaronic effect increases the electron-phonon coupling constant to about 1.4, which is large enough to leads to 32 K superconductivity. The present work thus uncovers the mystery of high-temperature superconductivity in bismuthate superconductors, which will also provide important insight into the pairing mechanism of other high-temperature superconductors.
Single-crystalline alpha-Fe2O3 nanorings (short nanotubes) and nanotubes were synthesized by a hydrothermal method. High-resolution transmission electron microscope and selected-area electron diffraction confirm that the axial directions of both nano
rings and nanotubes are parallel to the crystalline c-axis. What is intriguing is that the Morin transition occurs at about 210 K in the short nanotubes with a mean tube length of about 115 nm and a mean outer diameter of 169 nm while it disappears in the nanotubes with a mean tube length of about 317 nm and a mean outer diameter of 148 nm. Detailed analyses of magnetization data, x-ray diffraction spectra, and room-temperature Mossbauer spectra demonstrate that this very strong shape dependence of the Morin transition is intrinsic to hematite. We can quantitatively explain this intriguing shape dependence in terms of opposite signs of the surface magnetic anisotropy constants in the surface planes parallel and perpendicular to the c-axis (that is, K_parallel = -0.37 erg/cm^2 and K_perp = 0.42 erg/cm^{2}).
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.
High-temperature magnetic measurements have been carried out in hydrothermally synthesized greigite (Fe3S4). We show that the Curie temperature of greigite is significantly lower than that for its iron oxide counterpart Fe3O4. The lower TC value (abo
ut 677 K) of greigite is in quantitative agreement with that calculated using the exchange energy (3.25 meV) and the spin values of the two sublattices, which are inferred from the neutron and magnetization data of high-quality pure greigite samples. We further show that, with an effective on-site Hubbard energy Ueff = 1.16 eV, the lattice constant and two sublattice spins predicted from ab initio density-function theory are in nearly perfect agreement with the measured values. The parameter Ueff = 1.16 eV ensures Fe3S4 to be an excellent half-metallic material for spintronic applications.
Magnetic measurements up to 1000 K have been performed on hydrothermally synthesized $alpha$-Fe$_{2}$O$_{3}$ nanoparticles (60 nm) using a Quantum Design vibrating sample magnetometer. A high vacuum environment (1$times$10$^{-5}$ torr) during the mag
netic measurement up to 1000 K leads to a complete reduction of $alpha$-Fe$_{2}$O$_{3}$ to Fe$_{3}$O$_{4}$. This precludes the determination of the Neel temperature for the $alpha$-Fe$_{2}$O$_{3}$ nanoparticles. In contrast, coating $alpha$-Fe$_{2}$O$_{3}$ nanoparticles with SiO$_{2}$ stabilizes the $alpha$-Fe$_{2}$O$_{3}$ phase up to 930 K, which allows us to determine the Neel temperature of the $alpha$-Fe$_{2}$O$_{3}$ nanoparticles for the first time. The Neel temperature of the 60-nm $alpha$-Fe$_{2}$O$_{3}$ nanoparticles is found to be 945 K, about 15 K below the bulk value. The small reduction of the Neel temperature of the $alpha$-Fe$_{2}$O$_{3}$ nanoparticles is consistent with a finite-size scaling theory. Our current results also show that coating nanoparticles with SiO$_{2}$ can effectively protect nanoparticles from oxidation or reduction, which is important to technological applications.
Magnetic measurements have been performed on 40-nm sphere-like Fe3O4 nanoparticles using a Quantum Design vibrating sample magnetometer. Coating Fe3O4 nanoparticles with SiO2 effectively eliminates magnetic interparticle interactions so that the coer
cive field HC in the hightemperature range between 300 K and the Curie temperature (855 K) can be well fitted by an expression for noninteracting randomly oriented single-domain particles. From the fitting parameters, the effective anisotropy constant K is found to be (1.68 pm 0.17) times 105 erg/cm3, which is slightly larger than the bulk magnetocrystalline anisotropy constant of 1.35 times 105 erg/cm3. Moreover, the inferred mean particle diameter from the fitting parameters is in quantitative agreement with that determined from transmission electron microscope. Such a quantitative agreement between data and theory suggests that the assemble of our SiO2-coated sphere-like Fe3O4 nanopartles represents a good system of noninteracting randomly-oriented single-domain particles.
