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Future colliders such as NLC and JLC will require a highly-polarized macropulse with charge that is more than an order of magnitude beyond that which could be produced for the SLC. The maximum charge from the SLC uniformly-doped GaAs photocathode was limited by the surface charge limit (SCL). The SCL effect can be overcome by using an extremely high (>1019 cm-3) surface dopant concentration. When combined with a medium dopant concentration in the majority of the active layer (to avoid depolarization), the surface concentration has been found to degrade during normal heat cleaning (1 hour at 600 C). The Be dopant as typically used in an MBE-grown superlattice cathode is especially susceptible to this effect compared to Zn or C dopant. Some relief can be found by lowering the cleaning temperature, but the long-term general solution appears to be atomic hydrogen cleaning., 11 pages, 3 figures, 1 table, contributed to 10th Workshop on Polarized Sources and Targets, Novosibirsk, Sept. 22-26, 2003, to be submitted to Nucl. Instrum. and Meth. A

A high-gradient-doping technique is applied to strained polarized photocathodes. A 5.0– 7.5 nm p-type surface layer doped to 5×10 19 cm −3 is found sufficient to overcome the surface charge limit while maintaining high beam polarization. This technique can be employed to meet the charge requirements of the Next Linear Collider with a polarization approaching 80%.

The deformation potentials a, b and d for GaAs have been determined from polarized photoluminescence measurements upon a set of epitaxially grown strained GaAs structures possessing varying levels of compressive biaxial lattice strain. X-ray diffraction measurements yield values for the degree of lattice strain while the polarized photoluminescence measurements permit a separate determination to be made of the heavy hole and light hole band energies. Correlation of these data allows for the determination of the deformation potentials within the context of the orbital-strain Hamiltonian.

An anisotropy in the quantum efficiency (QE) has been observed in photoemission from strained GaAs photocathodes excited by linearly polarized light. The wavelength dependence of the anisotropy is closely correlated with that of the electron-spin polarization. Based on a theoretical analysis, we show that the QE anisotropy is caused by an in-plane strain anisotropy arising from anisotropic strain relaxation. The QE anisotropy calculated from the in-plane strain anisotropy measured with X-ray diffraction and the measured QE anisotropy are in good agreement.

Superconducting rf surface resistance measurements were made on oxidized Nb cavities that had been warmed for a few minutes to temperatures between 250 and 300° C. Warmed oxide layers were further studied using X-ray photoelectron spectroscopy, Auger electron spectroscopy and secondary electron emission yield measurements. Warming usually reduced the temperature-dependent (BCS) part of the surface resistance by 10–20% while, for warming temperatures near 300° C, the low-temperature residual surface resistance increased by as much as 70 nΩ. Surface spectroscopy measurements showed that, at temperatures between 200 and 250° C, the oxide layer was chemically altered but remained surface-segregated; between 250 and 300° C, however, surface oxygen dissolved into the Nb bulk. Changes in the BCS resistance were consistent with a model based on a shortening of the electron mean free path by oxygen that had diffused into the Nb metal and based on the theoretical dependence of BCS resistance on electron mean free path. No specific cause for the increased residual resistance could be positively identified, although oxygen-induced surface roughening is a possibility.

Valence band x-ray photoelectron spectra from Fe(100) have been measured as a function of temperature to above the Curie temperature, Tc. The room temperature data can be reconciled with the theoretical one-particle density of states (DOS). At T = 1.034Tc, the data do not resemble the paramgnetic DOS of Fe as calculated in the disordered-local-moment limit.

During 4 years of on line operation of the SLAC polarized electron gun (PEGGY) and polarized LEED (PLEED) system, we have observed and characterized the failure modes of the GaAs (100) photocathodes (PC's) used in these systems. Several modes are observed. Gradual decreases in electron polarization and intensity are attributed to the physisorption of CO 2 on the PC's during running at LN 2 temperatures. Such PC's can be rejuvenated by warming to 90 K, i.e. above the CO 2 desorption temperature. These PC's recover 90% of their original intensity. A second well-characterized failure mode results from overheating the PC during in situ heat cleaning prior to activation. In this mode, As is preferentially evaporated from the GaAs, leaving a Ga 2 O 3 layer on the surface. This effect has been studied by AES sputter profiling which indicates that the substantial thickness of the oxide layer blocks photoemission. These PC's may only be recovered by chemically removing the oxide layer. A third mode which is not as well characterized appears for thin Ga oxide layers. Properties of these PC's include reduced emission and the presence of a cutoff bias level. Such PC's are also not recoverable in situ .

Dynamical calculations are performed to determine the differential cross section of low energy electrons scattered from antiferromagnetic NiO (111) surfaces. We find that the spin-dependence of this quantity with respect to the incident electron polarization depends strongly on: (1) the magnetization of the topmost layer, (2) the exchange potential model used, and (3) the incident beam angle.

The higher energies planned for the next generation of particle accelerators and storage rings makes the use of superconducting hi-Q RF cavities highly desirable. Past efforts to produce reliable cavities for such projects have met with limited success. Among the barriers to achieving the maximum electric field gradient are oxide layer charging, single surface multipactoring and field emission. These are suface effects. At SLAC, a multi-technique surface analysis system has been constructed to examine possible sources of these problems and to suggest processes or surface coatings which will reduce or eliminate them. As one component of this analysis, we have investigated the time evolution of species on anodized Nb/sub 2/O/sub 5/-on-Nb surfaces as a function of electron bombardment. The surface concentration of C increases at an anomalously high rate under the exciting electron beam. Examination of the surface and gas phase indicate that the C source is in the underlying material. Estimates of the penetration depth of the beam are in agreement with the fact that there is a significant rate increase in the surface C buildup when the beam penetrates the anodized layer into the Nb bulk. Bulk analytical methods indicate, however, that the C concentration in the Nbmore » is very low. Grain boundary diffusion of C to the surface and/or an enhancement of the C diffusion coefficient due to localized beam pipe heating are examined as possible explanations of this effect.« less