Fluctuation-free electron emission is obtained from MIM (Al-Al2O3-Au) cathodes. The Al2O3 layer is fabricated by anodic oxidation with a reduced electrolysis current density, i.e., a reduced oxidation rate. The slow oxidation process improves the insulating effect of the Al2O3 layer, and enables the MIM cathodes to operate in the non-formed state. The fluctuation-free emission is reproducible when the diode voltage is cut off instantaneously. With a thin Al2O3 layer, the diode voltage reguired for the cathode operation is reduced to values slightly above the work function of the top electrode.
Abstraci The recombinalion kinetics of photoinjected charge carriers in quasi-me-dimensional conducting polymers is studied. The dynamics of the correlated pair of photoexcitations is considered within Ihe model of one-dimensional diffusion of quasiparticles in segments of a polymer chain confined by defecls (traps). The main characteristics of the dynamics of a mmbining pair (disrribution function and survival prohability in a confined segment and the averaged survival probability in a polymer chain with random hap distribution) are calculated. The results obtained are associated with the observed characteristics, such as uansient phoUKunent decay, kinetics of photoinduced changes in optical absorption (or reflectivity) and delayed fluorescence caused by charge carrier recombination. It is shown thar the inRuence of the defects leads to acceleration of the photocurrent decay, namely. the usual power-law dependence cx (-'I2 describing Ihe geminate recombination kinetics in an intinite chain. at times 5 (l0n2W$)-' (W is the diffusion rate, c is the trap concentration), is changed to a &fer exponentiallke dependence 0: r'/6exp(-t'/3). In contrast, it is shown that the photoinduced absorption decay is slowed down due to trapping of photoexcitations by defects. The delayed fluorescence decay caused by recombination of photoexcitations also obeys an exponential-like dependence, cx rr1/2exp(-r1/3), unlike the case ofan infinire chain, when delayed fluorescence decay follows a power-law dependence a fM3l2. The theoretical results are compared with the experimenlal data on relaxation kinetics of photoencitations in nom-polyacetylene and restrictions on the model are considered.
Activity of the base metal is evaluated by identifying reaction products with the oxide layer, measuring Ba vapor pressure from the oxide, measuring of emission current properties, and by analyzing the surface composition of the oxide. Reducing capabilities of Zr, W, and Mo activators in Ni base metal are determined. Their activators are alloyed to the Ni core metal up to the solubility limits. The experimental results are evaluated systematically from the standpoint of base metal activity, together with the thermochemical treatment for the various reactions and the emission mechanism of the oxide coated cathode.
Temperature-dependent 0 K-edge absorption spectra of single-crystalline YBa2Cu306. 8 films were obtained across T"(77 K), using linearly polarized x rays. When the incident x rays were polarized parallel to the a(b) axis, absorption within -1.4 eV just above the Fermi level increased as the temperature was lowered from above T, (120 K) to below T, (68 K). The increase disappeared upon breaking the superconducting state by applying a dc current. When polarization was parallel to the c axis, lowering the temperature below T, had no effect on the absorption. The increase implies that, in the superconducting state, the 2pcr orbitals on the O(2) and O(3) sites in the Cu02 plane have more holes than in the normal conducting state.Doped holes play a key role in the mechanism of high-T, . superconductivity, because hole doping into the Cu02 plane induces superconductivity of cupreous oxides. ' Electron-energy-loss spectroscopy (EELS),x-rayabsorption spectroscopy (XAS), ' ' and photoelectron spectroscopy' ' indicate that hole doping creates 0 2p unoccupied electronic states above the Fermi level within the -2-eV (Refs. 14 and 15) charge-transfer gap.Wagener et al. ' have reported the increase of 0 2p holes at 60 K compared to 300 K for Y-Ba-Cu-0 and Bi-Sr-Ca-Cu-0 superconductors, using inverse photoelectron spectroscopy (IPS). They interpreted the increase as due to enhanced charge transfer from 0 to Cu at lower temperatures. Their IPS method, 's however, was unable to clarify whether the increase of 0 2p holes was correlated with the superconducting transition, and whether the increase was in the Cu02 plane. In this Rapid Comrnunication, we present experimental evidence that 0 2p unoccupied states of a single-crystalline YBa2Cu306.8 film increase anisotropically in the superconducting state compared with those in the normal conducting state. This suggests that 0 2pcr orbitals in the Cu02 plane have more holes as a direct consequence of the superconducting transition.We obtained the density of 0 2p unoccupied states by measuring 0 K-edge absorption spectra, using Auorescence yield (FY) detection, ' ' while monitoring the superconducting transition in situ. FY detection, ' "' ' which is bulk sensitive and probes to a depth of -0.2 pm, ' was indispensable, because the surface of superconducting YBa2Cu307y is unstable due to the oxygen deficiency and contamination.Thus, surface sensitive methods, such as total-secondary-electron-yield (TEY) detection and IPS, ' would leave ambiguity to their interpretation.Though EELS (Refs. 3-5) is also bulk sensitive, it is subject to electron radiation damage.Linearly polarized x rays from a synchrotron-radiation source was also essential for the present results, because the change of 0 K-edge FY spectrum due to the superconducting transition was only in the Cu02 plane.The experiments were performed using the soft-x-ray analysis system' at Branchline SA, Photon Factory, National Laboratory for High Energy Physics (KEK). We measured 0 K-edge FY spectra with a thin-window Si(Li) detector. ...
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