Coulomb interactions between the carriers may provide the mechanism for enhanced unconventional superconductivity in the copper oxides. However, they simultaneously cause inelastic quasiparticle scattering that can destroy it. Understanding the evolution of this balance with doping is crucial because it is responsible for the rapidly diminishing critical temperature as the hole density p is reduced towards the Mott insulating state.

Exploring a recently developed mesoscale continuum theory of dislocation dynamics, we derive three predictions about plasticity and grain boundary formation in crystals. (1) There is a residual stress jump across grain boundaries and plasticity-induced cell walls as they form, which self-consistently acts to attract neighboring dislocations; residual stress in this theory appears as a remnant of the driving force behind wall formation under both polygonization and plastic deformation. We derive the predicted asymptotic late-time dynamics of the grain-boundary formation process.

We report on a pronounced magnetic anisotropy of magnetically doped quasi-two-dimensional charge density wave (CDW) Nb Se2 at doping concentrations near 1%, and on temperature dependent reflectance, both phenomena above Nb Se2 superconducting transition (7.2 K). Unusual spikes in magnetization reversal are noticeable near 20 K, below the CDW transition (33 K), and disappear as temperature nears the superconducting transition. In the far infrared region of the spectrum, we find two sudden jumps in reflectivity, one near the CDW transition, the other near 18 K.

Understanding the physics of low-dimensional systems and the operation of next-generation electronics will depend on our ability to measure the electrical properties of nanomaterials at terahertz frequencies (∼100 GHz to 10 THz). Single-walled carbon nanotubes are prototypical one-dimensional nanomaterials because of their unique band structure and long carrier mean free path. Although nanotube transistors have been studied at microwave frequencies (100 MHz to 50 GHz), no techniques currently exist to probe their terahertz response.

We have developed a technique for making low-resistance end-current injection contacts to geometrically, electronically, and mechanically anisotropic crystals of charge-density-wave (CDW) conductors. Transport measurements on Nb Se3 show that contact resistances are reduced by nearly 2 orders of magnitude compared with the standard side-contact geometry, and yield qualitatively similar results for the contact-sensitive phase-slip process.

We study vortex-lattice phases for a Bose gas trapped in a rotating optical-lattice near the superfluid-Mott-insulator transition. We find a series of abrupt structural phase transitions where vortices are pinned with their cores only on plaquettes or only on sites. We discuss connections between these vortex structures and the Hofstadter-butterfly spectrum of free particles on a rotating lattice. © 2008 The American Physical Society.

Many theoretical models of high-temperature superconductivity focus only on the doping dependence of the CuO2-plane electronic structure. However, such models are manifestly insufficient to explain the strong variations in superconducting critical temperature, Tc, among cuprates that have identical hole density but are crystallographically different outside of the CuO2 plane.

Electrons in atoms possess both spin and orbital degrees of freedom. In non-relativistic quantum mechanics, these are independent, resulting in large degeneracies in atomic spectra. However, relativistic effects couple the spin and orbital motion, leading to the well-known fine structure in their spectra. The electronic states in defect-free carbon nanotubes are widely believed to be four-fold degenerate, owing to independent spin and orbital symmetries, and also to possess electron-hole symmetry.

One of key challenges in high-Tc superconductivity research is to identify a predominant mechanism controlling superconductivity at atomic scale.

The problem of reconstructing an object from diffraction data that has been incoherently averaged over a discrete group of symmetries is considered. A necessary condition for such data to uniquely specify the object is derived in terms of the object support and the symmetry group. An algorithm is introduced for reconstructing objects from symmetry-averaged data and its use with simulations is demonstrated.