Room-Temperature Photoluminescence from Er3+ in Si-Er-O and Si-Ge-Er-O Thin Films at High Erbium Concentrations

Prior studies have shown that photoluminescence from Er3+ impurities in silicon is severely limited at room temperature by non-radiative relaxation and solid solubility; and room temperature emission from Er3+ in oxide-based hosts becomes diminished at high erbium concentrations. This work presents studies of thin films (0.2 micron thick) prepared by vacuum co-evaporation from elemental sources (Er, Si and Si/Ge) followed by vacuum annealing (600 degrees C); materials of this type, which are produced with high Er3+ concentrations, are shown to be capable of yielding strong room-temperature photoluminescence. Alloy films of Si-Er-O and Si-Ge-Er-O, containing (20 +/- 2) at. % Er and incorporating (16 +/- 2) at. % O (introduced via vacuum scavenging reactions), exhibit emission bands with dominant components at 1.51 and 1.54 micron (~0.04-micron overall spectral widths). Results are discussed in terms of Er-O complex formation and effects of local randomness on cooperative inter-Er3+ energy transfer among thermal-broadened and local-field Stark-split 4I13/2 to 4I15/2 transitions. Advantages of scalability and low-cost associated with this method of producing optically active silicon-based materials are discussed.Comment: 12 pages, 3 figure

Superconducting interaction charge in thallium-based high-Tc cuprates: Roles of cation oxidation state and electronegativity

Superconductivity in the Tl-based cuprates encompasses a notably broad range of measured optimal transition temperatures Tc0, ranging from lowest in the charge-depleted Tl-1201 compounds (Tl$_{1-x}$(Ba/Sr)$_{1+y}$La$_{1-y}$CuO$_{5-{\delta}}$), such as Tl$_{0.7}$LaSrCuO$_5$ (37 K) and TlBa$_{1.2}$La$_{0.8}$CuO$_5$ (45.4 K), to highest in the Tl-1223 compound TlBa$_2$Ca$_2$Cu$_3$O$_{9{\pm}{\delta}}$ (133.5 K). Seven Tl-based cuprates are considered and compared using the model of superconductive pairing via electronic interactions between two physically separated charge reservoirs, where Tc0 $\propto$ ({\sigma}{\eta}/A)$^{1/2}${\zeta}$^{-1}$ is determined by the superconducting interaction charge fraction {\sigma} the number {\eta} of CuO$_2$ layers, and the basal-plane area A, each per formula unit, and the transverse distance {\zeta} between interacting layers. Herein it is demonstrated that {\sigma} follows from the elemental electronegativity and the oxidation state of Tl, and other structurally analogous cations. The comparatively lower elemental electronegativity of Tl, in conjunction with its oxidation state, explains the higher {\sigma} and Tc0 values in the Tl-based compounds relative to their Bi-based cuprate homologues. A derivation of {\sigma} is introduced for the optimal Tl$_2$Ba$_2$Ca$_{{\eta}-1}$Cu$_{\eta}$O$_{2{\eta}+4}$ (for {\eta} = 1, 2, 3) compounds, which exhibit a Tl oxidation state at or near +3, obtaining the fundamental value {\sigma}$_0$ = 0.228 previously established for YBa$_2$Cu$_3$O$_{6.92}$. Also reported is the marked enhancement in {\sigma} associated with Tl$^{+1}$ and analogous inner-layer cations relative to higher-valence cations. For a model proposition of {\sigma} = {\sigma}$_0$, the fractional Tl$^{+1}$ content of the mixed-valence compound, TlBa$_2$Ca$_2$Cu$_3$O$_{9{\pm}{\delta}}$, is predicted to be 1/3 at optimization, in agreement ...Comment: 22 pages, 2 figure

Magnetoinductance of Josephson junction array with frozen vortex diffusion

The dependence of sheet impedance of a Josephson junction array on the applied magnetic field is investigated in the regime when vortex diffusion between array plaquettes is effectively frozen due to low enough temperature. The field dependent contribution to sheet inductance is found to be proportional to f*ln(1/f), where f<<1 is the magnitude of the field expressed in terms of flux quanta per plaquette.Comment: 5 pages, no figure

Coexisting Holes and Electrons in High-Tc Materials: Implications from Normal State Transport

Normal state resistivity and Hall effect are shown to be successfully modeled by a two-band model of holes and electrons that is applied self-consistently to (i) DC transport data reported for eight bulk-crystal and six oriented-film specimens of YBa2Cu3O7-{\delta}, and (ii) far-infrared Hall angle data reported for YBa2Cu3O7-{\delta} and Bi2Sr2CaCu2O8+{\delta}. The electron band exhibits extremely strong scattering; the extrapolated DC residual resistivity of the electronic component is shown to be consistent with the previously observed excess thermal conductivity and excess electrodynamic conductivity at low temperature. Two-band hole-electron analysis of Hall angle data suggest that the electrons possess the greater effective mass.Comment: 22 pages, 7 figures, 4 tables and 78 reference

Dynamical fluctuations in mode locking experiments on vortices moving through mesoscopic channels

We have studied the flow properties of vortices driven through easy flow mesoscopic channels by means of the mode locking (ML) technique. We observe a ML jump with large voltage broadening in the real part of the rf-impedance. Upon approaching the pure dc flow by reducing the rf amplitude, the ML jump is smeared out via a divergence of the voltage width. This indicates a large spread in internal frequencies and lack of temporal coherence in the dc-driven state.Comment: 2 pages, 2 figures, contribution to M2S-HTSC 2003, Ri