Electronic interactions in strongly correlated systems: what is the "glue" for high temperature superconductivity?

Recent observation of a "kink" in single-particle dispersion in photoemission experiments on cuprate superconductors has initiated a heated debate over the issue of a boson that mediates the pairing in cuprates. If the "kink" is indeed caused by interaction with a bosonic excitation, then there are two possible candidates: phonons and spin fluctuations. Here, the role of anti-ferromagnetic spin fluctuations in shaping the phase diagram of cuprate superconductors will be discussed. By using the local (momentum-integrated) dynamic spin susceptibility, recently measured in neutron scattering experiments to high energies, the electronic self-energies are calculated that agree in many aspects with those measured directly in angle-resolved photoemission (ARPES) and optical spectroscopies. The spin fluctuations therefore seem to play a role typically played by phonons in renormalizing single particles. The key question emerging from this picture is whether the coupling detected in ARPES reflects the mediating boson, i.e. whether the spin fluctuations may be responsible for superconducting pairing.Comment: conference on "Strongly Correlated Electron Materials: Physics and Nanoengineering," part of the SPIE Optics & Photonics 2005 Symposiu

Termička stabilnost tankih slojeva TiO2 dobivenih kemijskim taloženjem iz pare. XPS i AES karakterizacija

Thermal stability of TiO2 thin polycrystalline films obtained by the very simple chemical vapour deposition method on the quartz, molybdenum and gold substrates have been examined by X-ray photoelectron and Auger spectroscopy (XPS and AES), respectively, before and after annealing in vacuum at temperatures from 298 K to 1200 K. In this temperature range we have found carbon, water, and O-H groups as the impurities at the surface of the films. The possible influence of both the annealing and the impurities on the film stoichiometry is discussed.Proučavana je termička stabilnost tankih, polikristaličnih slojeva TiO2 dobivenih jednostavnim kemijskim taloženjem iz pare na podloge iz kvarca, molibdena i zlata. Korištene su metode XPS i AES na uzorcima prije i poslije termičkog napuštanja u rasponu temperatura od 298 do 1200 K. Našli smo ugljik, vodu i O-H skupine kao nečistoće na površinama slojeva. Diskutirani su utjecaji termičkog napuštanja i nečistoća na stehiometriju slojeva

Symmetry Protected Josephson Supercurrents in Three-Dimensional Topological Insulators

Coupling the surface state of a topological insulator (TI) to an s-wave superconductor is predicted to produce the long-sought Majorana quasiparticle excitations. However, superconductivity has not been measured in surface states when the bulk charge carriers are fully depleted, i.e., in the true topological regime that is relevant for investigating Majorana modes. Here, we report measurements of DC Josephson effects in TI-superconductor junctions as the chemical potential is moved from the bulk bands into the band gap, or through the true topological regime characterized by the presence of only surface currents. We examine the relative behavior of the system at different bulk/surface ratios, determining the effects of strong bulk/surface mixing, disorder, and magnetic field. We compare our results to 3D quantum transport simulations to conclude that the supercurrent is largely carried by surface states, due to the inherent topology of the bands, and that it is robust against disorder