Enhanced Pairing in the "Checkerboard" Hubbard Ladder

We study signatures of superconductivity in a 2--leg "checkerboard" Hubbard ladder model, defined as a one--dimensional (period 2) array of square plaquettes with an intra-plaquette hopping $t$ and inter-plaquette hopping $t'$, using the density matrix renormalization group method. The highest pairing scale (characterized by the spin gap or the pair binding energy, extrapolated to the thermodynamic limit) is found for doping levels close to half filling, $U\approx 6t$ and $t'/t \approx 0.6$. Other forms of modulated hopping parameters, with periods of either 1 or 3 lattice constants, are also found to enhance pairing relative to the uniform two--leg ladder, although to a lesser degree. A calculation of the phase stiffness of the ladder reveals that in the regime with the strongest pairing, the energy scale associated with phase ordering is comparable to the pairing scale.Comment: 9 pages, 9 figures; Journal reference adde

Transport in Two Dimensional Electronic Micro-emulsions

In two dimensional electron systems with Coulomb or dipolar interactions, a direct transition, whether first or second order, from a liquid to a crystalline state is forbidden. As a result, between these phases there must be other (microemulsion) phases which can be viewed as a meso-scale mixture of the liquid and crystalline phases. We investigate the transport properties of these new electronic phases and present arguments that they are responsible for the various transport anomalies that have been seen in experiments on the strongly correlated 2DEG in high mobility semiconductor devices with low electron densities

Quasiparticle scattering and local density of states in graphite

We determine the effect of quasiparticle interference on the spatial variations of the local density of states (LDOS) in graphite in the neighborhood of an isolated impurity. A number of characteristic behaviors of interference are identified in the Fourier transformed spectrum. A comparison between our results and scanning tunneling microscopy (STM) experiments could provide a critical test of the range (of energy) of applicability of the Fermi liquid description of graphite, where some evidence of the breakdown of Fermi liquid theory has recently been discussed. Moreover, given the similarity between the band structures of graphite and that of nodal quasiparticles in a d-wave superconductor, a comparison between results in the two materials is useful for understanding the physics of the cuprates.Comment: 5 pages, 4 figures, RevTex

High Temperature Superconductivity: Ineluctable Complexity

The discovery of charge-density-wave order in the high-temperature superconductor YBa$_2$Cu$_3$O$_{6+y}$ places charge order centre stage with superconductivity, suggesting they they are intertwined rather than competing.Comment: 3 pages, 1 figure, 19 references; News & Views article for Nature Physic