Superconducting electronic state in optimally doped YBa2Cu3O7-d observed with laser-excited angle-resolved photoemission spectroscopy

Low energy electronic structure of optimally doped YBa2Cu3O7-d is investigated using laser-excited angle-resolved photoemission spectroscopy. The surface state and the CuO chain band that usually overlap the CuO2 plane derived bands are not detected, thus enabling a clear observation of the bulk superconducting state. The observed bilayer splitting of the Fermi surface is ~0.08 angstrom^{-1} along the (0,0)-(pi,pi) direction, significantly larger than Bi2Sr2CaCu2O8+d. The kink structure of the band dispersion reflecting the renormalization effect at ~60 meV shows up similarly as in other hole-doped cuprates. The momentum-dependence of the superconducting gap shows d_{x^2-y^2}-wave like amplitude, but exhibits a nonzero minimum of ~12 meV along the (0,0)-(pi,pi) direction. Possible origins of such an unexpected "nodeless" gap behavior are discussed.Comment: 9 pages, 10 figures; revised version accepted for publication in Phys. Rev.

Evidence of trap-assisted Auger recombination in low radiative efficiency MBE-grown III-nitride LEDs

By studying low radiative efficiency blue III-nitride light emitting diodes (LEDs), we find that the ABC model of recombination commonly used for understanding efficiency behavior in LEDs is insufficient and that additional effects should be taken into account. We propose a modification to the standard recombination model by incorporating a bimolecular nonradiative term. The modified model is shown to be in much better agreement with the radiative efficiency data and to be more consistent than the conventional model with very short carrier lifetimes measured by time-resolved photoluminescence in similar, low radiative efficiency material. We present experimental evidence that a hot carrier-generating process is occurring within these devices, in the form of measurements of forward photocurrent under forward bias. The forward photocurrent, due to hot carrier generation in the active region, is present despite the lack of any "efficiency droop"-the usual signature of band-to-band Auger recombination in high-quality III-nitride LEDs. Hot carrier generation in the absence of band-to-band Auger recombination implies that some other source of hot carriers exists within these low radiative efficiency devices, such as trap-assisted Auger recombination