Evidence for very strong electron-phonon coupling in YBa_{2}Cu_{3}O_{6}

From the observed oxygen-isotope shift of the mid-infrared two-magnon absorption peak of YBa$_{2}$Cu$_{3}$O$_{6}$, we evaluate the oxygen-isotope effect on the in-plane antiferromagnetic exchange energy $J$. The exchange energy $J$ in YBa$_{2}$Cu$_{3}$O$_{6}$ is found to decrease by about 0.9% upon replacing $^{16}$O by $^{18}$O, which is slightly larger than that (0.6%) in La$_{2}$CuO$_{4}$. From the oxygen-isotope effects, we determine the lower limit of the polaron binding energy, which is about 1.7 eV for YBa$_{2}$Cu$_{3}$O$_{6}$ and 1.5 eV for La$_{2}$CuO$_{4}$, in quantitative agreement with angle-resolved photoemission data, optical conductivity data, and the parameter-free theoretical estimate. The large polaron binding energies in the insulating parent compounds suggest that electron-phonon coupling should also be strong in doped superconducting cuprates and may play an essential role in high-temperature superconductivity.Comment: 4 pages, 1 figur

BCS-BEC crossover on the two-dimensional honeycomb lattice

The attractive Hubbard model on the honeycomb lattice exhibits, at half-filling, a quantum critical point (QCP) between a semimetal with massless Dirac fermions and an s-wave superconductor (SC). We study the BCS-BEC crossover in this model away from half-filling at zero temperature and show that the appropriately defined crossover line (in the interaction-density plane) passes through the QCP at half-filling. For a range of densities around half-filling, the ``underlying Fermi surface'' of the SC, defined as the momentum space locus of minimum energy quasiparticle excitations, encloses an area which evolves nonmonotonically with interactions. We also study fluctuations in the SC and the semimetal, and show the emergence of an undamped Leggett mode deep in the SC. We consider possible implications for experiments on ultracold atoms and high temperature SCs.Comment: Revised - added section on the Fermi surface evolution, corrected error in superfluid density, added possible implications for cuprate

Spherical to deformed shape transition in the nucleon-pair shell model

A study of the shape transition from spherical to axially deformed nuclei in the even Ce isotopes using the nucleon-pair approximation of the shell model is reported. As long as the structure of the dominant collective pairs is determined using a microscopic framework appropriate to deformed nuclei, the model is able to produce a shape transition. However, the resulting transition is too rapid, with nuclei that should be transitional being fairly well deformed, perhaps reflecting the need to maintain several pairs with each angular momentum.Comment: 7 pages, 5 figure

Effect of oxygen stoichiometry on T(sub c) of Bi-based superconductors

The role of oxygen stoichiometry on T(sub c) is relatively well established on La2CuO(4+x) and the YBa2Cu3O(7-x) (123) superconductors, as compared to the Bi-based superconductors. Results are presented of investigations on the effects of oxygen stoichiometry on the transition temperature T(sub c) of Bi2Sr2CaCu2O(8+x) (2212 phase), and Pb-doped Bi2Sr2Ca2Cu3O(10+X) (2223 phase). It is shown that the effects of oxygen stoichiometry on T(sub c) of these two phases are very different. These results may be helpful in understanding the mechanism of superconductivity in the Bi-based superconductors