Anomalous flux quantization in a Hubbard ring with correlated hopping

We solve exactly a generalized Hubbard ring with twisted boundary conditions. The magnitude of the nearest-neighbor hopping depends on the occupations of the sites involved and the term which modifies the number of doubly occupied sites $t_{AB}=0$. Although $\eta$-pairing states with off-diagonal long-range order are part of the degenerate ground state, the behavior of the energy as a function of the twist rules out superconductivity in this limit. A small $t_{AB}$ breaks the degeneracy and for moderate repulsive $U$ introduce superconducting correlations which lead to ``anomalous'' flux quantization. PACS numbers: 74.20Mn, 71.27.+a, 71.30+h, 71.28.+dComment: 13 pages revtex, one figure in postscrip

Superconductivity with s and p-symmetries in an extended Hubbard model with correlated hopping

We consider a generalized Hubbard model with on-site and nearest-neighbour repulsions U and V respectively, and nearest-neighbour hopping for spin up (down) which depends on the total occupation n_b of spin down (up) electrons on both sites involved. The hopping parameters are t_{AA}, t_{AB} and t_{BB} for n_b=0,1,2 respectively. We briefly summarize results which support that the model exhibits s-wave superconductivity for certain parameters and extend them by studying the Berry phases. Using a generalized Hartree-Fock(HF) BCS decoupling of the two and three-body terms, we obtain that at half filling, for t_{AB}<t_{AA}=t_{BB} and sufficiently small U and V the model leads to triplet p-wave superconductivity for a simple cubic lattice in any dimension. In one dimension, the resulting phase diagram is compared with that obtained numerically using two quantized Berry phases (topological numbers) as order parameters. While this novel method supports the previous results, there are quantitative differences.Comment: Latex file, 14 pages, 2 postscript figure

Excitons in insulating cuprates

We study the electronic excitations near the charge-transfer gap in insulating CuO$_2$ planes, starting from a six-band model which includes $p_\pi$ and $d_{xy}$ orbitals and Cu-O nearest-neighbor repulsion $U_{pd}$. While the low lying electronic excitations in the doped system are well described by a modified $t-J$ model, the excitonic states of the insulator include hybrid $d_{xy}-$ $p_\pi$ states of $A_{2g}$ symmetry. We also obtain excitons of symmetries $B_{1g}$ and $E_u$, and eventually $A_{1g}$, which can be explained within a one-band model. The results agree with observed optical absorption and Raman excitations.Comment: 10 pages and 3 figures in postscript format, compressed with uufile

Magnetic Raman Scattering of Insulating Cuprates

We study the $B_{1g}$ and $A_{1g}$ Raman profiles of M$_{2}$Cu$O_{4}$ (with M= La, Pr, Nd, Sm, Gd), Bi$_{2}$Sr$_{2}$Ca$_{0.5}$Y$_{0.5}$Cu$_{2}$O$_{8+y}$%, YBa$_{2}$Cu$_{3}$O$_{6.2}$ and PrBa$_{2}$Cu$_{2.7}$Al$_{0.3}$O$_{7}$ insulating cuprates within the Loudon-Fleury theory, in the framework of an extended Hubbard model for moderate on-site Coulomb interaction $U$. We calculate the non-resonant contribution to these Raman profiles by using exact diagonalization techniques and analyze two types of contributing mechanisms to the line shapes: 4-spin cyclic exchange and spin-phonon interactions. Although these interactions contribute to different parts of the spectra, together, they account for the enhanced linewidth and asymmetry of the $B_{1g}$ mode, as well as the non-negligible intensity of the $A_{1g}$ Raman line observed in these materials.Comment: 8 pages, 2 eps figures. To be published in PR

A 94/183 GHz multichannel radiometer for Convair flights

A multichannel 94/183 GHz radiometer was designed, built, and installed on the NASA Convair 990 research aircraft to take data for hurricane penetration flights, SEASAT-A underflights for measuring rain and water vapor, and Nimbus-G underflights for new sea ice signatures and sea surface temperature data (94 GHz only). The radiometer utilized IF frequencies of 1, 5, and 8.75 GHz about the peak of the atmospheric water vapor absorption line, centered at 183.3 GHz, to gather data needed to determine the shape of the water molecule line. Another portion of the radiometer operated at 94 GHz and obtained data on the sea brightness temperature, sea ice signatures, and on areas of rain near the ocean surface. The radiometer used a multiple lens antenna/temperature calibration technique using 3 lenses and corrugated feed horns at 94 GHz and 183 GHz. Alignment of the feed beams at 94 GHz and 183 GHz was accomplished using a 45 deg oriented reflecting surface which permitted simultaneous viewing of the feeds on alternate cycles of the chopping intervals

Charge and spin excitations of insulating lamellar copper oxides

A consistent description of low-energy charge and spin responses of the insulating Sr_2CuO_2Cl_2 lamellar system is found in the framework of a one-band Hubbard model which besides $U$ includes hoppings up to 3^{rd} nearest-neighbors. By combining mean-field calculations, exact diagonalization (ED) results, and Quantum Monte Carlo simulations (QMC), we analyze both charge and spin degrees of freedom responses as observed by optical conductivity, ARPES, Raman and inelastic neutron scattering experiments. Within this effective model, long-range hopping processes flatten the quasiparticle band around $(0,\pi)$. We calculate also the non-resonant A_{1g} and B_{1g} Raman profiles and show that the latter is composed by two main features, which are attributed to 2- and 4-magnon scattering.Comment: 6 pages, 3 figures, To be published in PRB (july

Optical Conductivity of the Two-Dimensional Hubbard Model

Charge dynamics of the two-dimensional Hubbard model is investigated. Lancz$\ddot{\rm o}$s-diagonalization results for the optical conductivity and the Drude weight of this model are presented. Near the Mott transition, large incoherence below the upper-Hubbard band is obtained together with a remarkably suppressed Drude weight in two dimensions while the clearly coherent character is shown in one dimension. The two-dimensional results are consistent with previous results from quantum Monte Carlo calculations indicating that the Mott transition in this two-dimensional model belongs to the universality class characterized by the dynamical exponent of $z=4$.Comment: 4 pages LaTeX including 2 PS figures, to appear in J. Phys. Soc. Jp

Bond-charge Interaction in the extended Hubbard chain

We study the effects of bond-charge interaction (or correlated hopping) on the properties of the extended ({\it i.e.,} with both on-site ($U$) and nearest-neighbor ($V$) repulsions) Hubbard model in one dimension at half-filling. Energy gaps and correlation functions are calculated by Lanczos diagonalization on finite systems. We find that, irrespective of the sign of the bond-charge interaction, $X$, the charge--density-wave (CDW) state is more robust than the spin--density-wave (SDW) state. A small bond-charge interaction term is enough to make the differences between the CDW and SDW correlation functions much less dramatic than when $X=0$. For $X=t$ and fixed $V<2t$ ($t$ is the uncorrelated hopping integral), there is an intermediate phase between a charge ordered phase and a phase corresponding to singly-occupied sites, the nature of which we clarify: it is characterized by a succession of critical points, each of which corresponding to a different density of doubly-occupied sites. We also find an unusual slowly decaying staggered spin-density correlation function, which is suggestive of some degree of ordering. No enhancement of pairing correlations was found for any $X$ in the range examined.Comment: 10 pages, 7 PostScript figures, RevTeX 3; to appear in Phys Rev