Randall-Sundrum with Kalb-Ramond field: return of the hierarchy problem?

We show that when the antisymmetric Kalb-Ramond field is included in the Randall-Sundrum scenario, although the hierarchy problem can be solved, it requires an extreme fine tuning of the Kalb-Ramond field (about 1 part in $10^{62}$). We interpret this as the return of the problem in disguise. Further, we show that the Kalb-Ramond field induces a small negative cosmological constant on the visible brane.Comment: 8 pages, latex, 4 figures. Contributed talk at `Recent Developments in Gravity' (NEB XII), Nafplion, Greece, 29 June 200

Supergravity with Self-dual B fields and Instantons in Noncommutative Gauge Theory

We study Type IIB supergravity in the presence of (euclidean) D3 branes and nonzero self-dual B-fields. We point out that the Einstein frame metric is identical to the full geometry for D3 branes without B fields turned on. Furthermore, in a decoupling limit in which the theory is conjectured to be dual to noncommutative Yang-Mills theory, the entire Einstein metric remains intact, and in particular, is asymptotically flat. We construct D-instanton solutions in this geometry. We show that in the decoupling limit the D-instanton action agrees with the action of the corresponding instanton in the noncommutative Yang-Mills theory and is expressed in terms of the open string coupling. Some other aspects of this correspondence, which have unusual features because the underlying metric is asymptotically flat, are explored.Comment: 25 pages, harvma

Reconstructing the bulk Fermi surface and the superconducting gap properties from Neutron Scattering experiments

We develop an analytical tool to extract bulk electronic properties of unconventional superconductors through inelastic neutron scattering (INS) spectra. Since the spin excitation spectrum in the superconducting (SC) state originates from Bogoliubov quasiparticle scattering associated with Fermi surface nesting, its energy-momentum relation--the so called `hour-glass' feature--can be inverted to reveal the Fermi momentum dispersion of the single-particle spectrum as well as the corresponding SC gap function. The inversion procedure is analogous to the quasiparticle interference (QPI) effect in scanning tunneling microscopy (STM). Whereas angle-resolved photoemission spectroscopy (ARPES) and STM provide surface sensitive information, our inversion procedure provides bulk electronic properties. The technique is essentially model independent and can be applied to a wide variety of materials.Comment: 8 pages, 4 figure

Nonmonotonic dx2−y2d_{x^2-y^2} superconducting gap in electron-doped Pr0.89_{0.89}LaCe0.11_{0.11}CuO4_4: Evidence of coexisting antiferromagnetism and superconductivity?

Recent experiments on Pr$_{0.89}$LaCe$_{0.11}$CuO$_4$ observe an anisotropic spin-correlation gap and a nonmonotonic superconducting (SC) gap, which we analyze within the framework of a $t-t^{\prime}-t^{\prime\prime}-t^{\prime\prime\prime}-t^{iv}-U$ model with a $d_{x^2-y^2}$ pairing interaction including a third harmonic contribution. By introducing a realistic broadening of the quasiparticle spectrum to reflect small-angle scattering, our computations explain the experimental observations, especially the presence of a maximum in the leading edge gap in the vicinity of the hot-spots. Our analysis suggests that the material behaves like a {\it two-band} superconductor with the d-wave third harmonic acting as the {\it interband pairing gap}, and that the anti-ferromagnetic (AFM) and SC orders co-exist in a uniform phase