לוח שנה משולב

After briefly reviewing past works on holographic computations of transport coefficients, I will show how one can derive a new and simple formula for bulk viscosity in the setting of the fluid-gravity correspondence using the null focusing equation for the horizon. The formula involves derivatives of the horizon values of bulk scalar fields with respect to the entropy and charge density. Using this formula one can straightforwardly reproduce several results in the literature that previously required numerical methods. I will conclude by showing our formula is exact, even though it apparently only involves horizon data. Our proof uses the fact that the hydrodynamics equations and transport coefficients are the same on any constant r holographic screen in the bulk.

Recently, anomalous superdiusion of ultra cold 87Rb atoms in an optical lattice has been observed along with a fat-tailed, Levy type, spatial distribution (Sagi et al PRL 108, 093002 (2012)). The anomalous exponents were found to depend on the depth of the optical potential. We find, within the framework of the semiclassical theory of Sisyphus cooling, three distinct phases of the dynamics, as the optical potential depth is lowered: normal diusion; Levy diusion; and x t3=2 scaling, the latter related to Obukhov's model (1959) of turbulence. The process can be formulated as a Levy walk, with strong correlations between the length and duration of the excursions. We show how an infinite covariant density describes the the momentum distribution, and explain how this non normalizable density plays a dual role to the stationary Boltzmann like equilibrium density.

Joint work with David Kessler.

Organic Photovoltaics offer the promise of low cost high performance photovoltaics. One of the key areas for improvement is in developing selective contacts that improve efficiency and lifetime. We will report on the development of novel inorganic hole transport layers (HTL) and the related electron transport layers (ETL) for organic photovoltaics (OPV). All the studied materials belong to the general class of wide-bandgap p-type oxide semiconductors. Coupled to the “conventional TCO’s” the pairing of new materials by design for the HTL and ETL potentially can improve efficiency and stability tailored to a particular bulk heterojunction. How we can begin to to design such materials and then realize them experimentally is the topic of the talk. Potential candidates suitable for HTL applications include SnO, NiO, MO3, Cu2O (and related CuAlO2, CuCrO2, SrCu2O4 etc) and Co3O4 (and related ZnCo2O4, NiCo2O4, MgCo2O4 etc.). Materials have been optimized by high-throughput combinatorial approaches. The thin films were deposited by RF sputtering and pulsed laser deposition at ambient and elevated temperatures. Performance of the inorganic HTLs and that of the reference organic PEDOT:PSS HTL were compared by measuring the power conversion efficiencies and spectral responses of the P3HT/PCBM- and PCDTBT/PCBM-based OPV devices. Preliminary results indicate that Co3O4-based HTLs have performance comparable to that of our previously reported NiOs and PEDOT:PSS HTLs, leading to a power conversion efficiency of about 4 percent. The effect of composition and work function of the ternary materials on their performance in OPV devices is under investigation.

High-Tc cuprate superconductors display startling nanoscale inhomogeneity in essential properties such as pseudogap energy scale, Fermi surface, and even superconducting
critical temperature. The direct cause of this inhomogeneity has remained mysterious; theoretical explanations have ranged from chemical disorder to spontaneous electronic phase separation. We extend the energy range of scanning tunneling spectroscopy, allowing the first complete mapping of all three types of oxygen dopants in Bi(2+y)Sr(2-y)CaCu2O(8+x) with maximum superconducting Tc ~ 90K. We show that a subset of these dopants are indeed the hidden variable at the root of the nanoscale disorder. We explain how the spatial variations in competing electronic orders, such as the pseudogap and the charge density wave, are governed by the disorder in the dopant concentrations, which suggests a possible avenue
to raise Tc in this material.