Nature of Correlated Motion of Electrons in the Parent Cobaltate Superconductors

Recently discovered class of cobaltate superconductors (Na0.3CoO2.nH2O) is a novel realization of interacting quantum electron systems in a triangular network with low-energy degrees of freedom. We employ angle-resolved photoemission spectroscopy to uncover the nature of microscopic electron motion in the parent superconductors for the first time. Results reveal a large hole-like Fermi surface (consistent with Luttinger theorem) generated by the crossing of super-heavy quasiparticles. The measured quasiparticle parameters collectively suggest a two orders of magnitude departure from the conventional Bardeen-Cooper-Schrieffer electron dynamics paradigm and unveils cobaltates as a rather hidden class of relatively high temperature superconductors.Comment: 5 pages, 4 figures, 1 tabl

Momentum-Resolved Inelastic X-ray Scattering as a Novel Tool to Study Charge Gap in Complex Insulators

We report particle-hole pair excitations in a cuprate insulator in the intermediate regimes of momentum-transfers using high energy inelastic x-ray scattering. The excitation spectra show dispersive features near the Mott edge which shed light on the momentum structure of the upper Hubbard band in cuprates. We briefly discuss the potential use of such a technique to study the momentum dependence of unoccupied bands and q-dependent charge fluctuations in complex insulators.Comment: 3 pages, 2 figures, Revise

Studies of Prototype CsI(Tl) Crystal Scintillators for Low-Energy Neutrino Experiments

Crystal scintillators provide potential merits for the pursuit of low-energy low-background experiments. A CsI(Tl) scintillating crystal detector is being constructed to study low-energy neutrino physics at a nuclear reactor, while projects are underway to adopt this technique for dark matter searches. The choice of the geometrical parameters of the crystal modules, as well as the optimization of the read-out scheme, are the results of an R&D program. Crystals with 40 cm in length were developed. The detector requirements and the achieved performance of the prototypes are presented. Future prospects for this technique are discussed.Comment: 32 pages, 14 figure

Pulse Shape Discrimination Techniques in Scintillating CsI(Tl) Crystals

There are recent interests with CsI(Tl) scintillating crystals for Dark Matter experiments. The key merit is the capability to differentiate nuclear recoil (nr) signatures from the background $\beta / \gamma$-events due to ambient radioactivity on the basis of their different pulse shapes. One of the major experimental challenges is to perform such pulse shape analysis in the statistics-limited domain where the light output is close to the detection threshold. Using data derived from measurements with low energy $\gamma$'s and nuclear recoils due to neutron elastic scatterings, it was verified that the pulse shapes between $\beta / \gamma$-events are different. Several methods of pulse shape discrimination are studied, and their relative merits are compared. Full digitization of the pulse shapes is crucial to achieve good discrimination. Advanced software techniques with mean time, neural network and likelihood ratios give rise to satisfactory performance, and are superior to the conventional Double Charge method commonly applied at higher energies. Pulse shape discrimination becomes effective starting at a light yield of about 20 photo-electrons. This corresponds to a detection threshold of about 5 keV electron-equivalence energy, or 40$-$50 keV recoil kinetic energy, in realistic experiments.Comment: 20 pages, 7 figure

Crossing w=-1 in Gauss-Bonnet Brane World with Induced Gravity

Recent type Ia supernovas data seemingly favor a dark energy model whose equation of state $w(z)$ crosses -1 very recently, which is a much more amazing problem than the acceleration of the universe. In this paper we show that it is possible to realize such a crossing without introducing any phantom component in a Gauss-Bonnet brane world with induced gravity, where a four dimensional curvature scalar on the brane and a five dimensional Gauss-Bonnet term in the bulk are present. In this realization, the Gauss-Bonnet term and the mass parameter in the bulk play a crucial role.Comment: Revtex 16 pages including 10 eps files, references added, to appear in Comm. Theor. Phy

From Fractional Chern Insulators to a Fractional Quantum Spin Hall Effect

We investigate the algebraic structure of flat energy bands a partial filling of which may give rise to a fractional quantum anomalous Hall effect (or a fractional Chern insulator) and a fractional quantum spin Hall effect. Both effects arise in the case of a sufficiently flat energy band as well as a roughly flat and homogeneous Berry curvature, such that the global Chern number, which is a topological invariant, may be associated with a local non-commutative geometry. This geometry is similar to the more familiar situation of the fractional quantum Hall effect in two-dimensional electron systems in a strong magnetic field.Comment: 8 pages, 3 figure; published version with labels in Figs. 2 and 3 correcte

Interface and electronic characterization of thin epitaxial Co3O4 films

The interface and electronic structure of thin (~20-74 nm) Co3O4(110) epitaxial films grown by oxygen-assisted molecular beam epitaxy on MgAl2O4(110) single crystal substrates have been investigated by means of real and reciprocal space techniques. As-grown film surfaces are found to be relatively disordered and exhibit an oblique low energy electron diffraction (LEED) pattern associated with the O-rich CoO2 bulk termination of the (110) surface. Interface and bulk film structure are found to improve significantly with post-growth annealing at 820 K in air and display sharp rectangular LEED patterns, suggesting a surface stoichiometry of the alternative Co2O2 bulk termination of the (110) surface. Non-contact atomic force microscopy demonstrates the presence of wide terraces separated by atomic steps in the annealed films that are not present in the as-grown structures; the step height of ~ 2.7 A corresponds to two atomic layers and confirms a single termination for the annealed films, consistent with the LEED results. A model of the (1 * 1) surfaces that allows for compensation of the polar surfaces is presented.Comment: 8 pages, 7 figure

Conformal mapping methods for interfacial dynamics

The article provides a pedagogical review aimed at graduate students in materials science, physics, and applied mathematics, focusing on recent developments in the subject. Following a brief summary of concepts from complex analysis, the article begins with an overview of continuous conformal-map dynamics. This includes problems of interfacial motion driven by harmonic fields (such as viscous fingering and void electromigration), bi-harmonic fields (such as viscous sintering and elastic pore evolution), and non-harmonic, conformally invariant fields (such as growth by advection-diffusion and electro-deposition). The second part of the article is devoted to iterated conformal maps for analogous problems in stochastic interfacial dynamics (such as diffusion-limited aggregation, dielectric breakdown, brittle fracture, and advection-diffusion-limited aggregation). The third part notes that all of these models can be extended to curved surfaces by an auxilliary conformal mapping from the complex plane, such as stereographic projection to a sphere. The article concludes with an outlook for further research.Comment: 37 pages, 12 (mostly color) figure

Signatures of the excitonic memory effects in four-wave mixing processes in cavity polaritons

We report the signatures of the exciton correlation effects with finite memory time in frequency domain degenerate four-wave mixing (DFWM) in semiconductor microcavity. By utilizing the polarization selection rules, we discriminate instantaneous, mean field interactions between excitons with the same spins, long-living correlation due to the formation of biexciton state by excitons with opposite spins, and short-memory correlation effects in the continuum of unbound two-exciton states. The DFWM spectra give us the relative contributions of these effects and the upper limit for the time of the exciton-exciton correlation in the unbound two-exciton continuum. The obtained results reveal the basis of the cavity polariton scattering model for the DFWM processes in high-Q GaAs microcavity.Comment: 11 pages, 1 figur