High-resolution soft X-ray emission spectrograph at advanced light source

The progress of valence electronic spectroscopy with X-rays has been largely limited by the challenge of fine instrumentation. Relatively weak scattering cross-section and/or limited (coarse) energy resolution restrict X-ray spectroscopy to address fundamental issues in condensed-matter electron physics. Depending on the nature of excitations, high brightness photon beam from third-generation synchrotron facilities helps raise the count rates to a detectable limit over the noise level. Over the past decade, it has been realized that high-resolution resonant inelastic soft X-ray scattering has the potential to play an important role in understanding complex phenomena observed in highly correlated systems. Driven by such demand, we have developed a soft X-ray emission spectrograph based on variable line spacing (VLS) gratings to work in the soft X-ray and deep UV (the M edge of transition metals) regime. The slit-less design coupled with high quality optical elements and a high quantum efficiency (QE) in-vacuum CCD detector greatly improves the overall throughput. The M edge spectrograph has been demonstrated to have a resolving power better than 10,000 and will be used in conjunction with meV-resolution beamline (MERLIN) at the Advanced Light Source at Lawrence Berkeley National Laboratory.     

Variable-range hopping in a silicon inversion layer

The low temperature conductance in the inversion layer of an MNOSFET shows a transition from activated to metallic conduction as the Fermi level is raised. The temperature dependence in the activated region is consistent with two-dimensional variable-range hopping and the minimum metallic conductivity, 4500 Ω^-1m^-1, agrees with a theoretical estimate based on Mott's formula.     

Interaction effects at crossings of spin-polarized one-dimensional subbands

We report conductance measurements of ballistic one-dimensional (1D) wires defined in GaAs/AlGaAs heterostructures in an in-plane magnetic field, B. When the Zeeman energy is equal to the 1D subband energy spacing, the spin-split subband N upward arrow intersects (N+1) downward arrow, where N is the index of the spin-degenerate 1D subband. At the crossing of N=1 upward arrow and N=2 downward arrow subbands, there is a spontaneous splitting giving rise to an additional conductance structure evolving from the 1.5(2e(2)/h) plateau. With further increase in B, the structure develops into a plateau and lowers to 2e(2)/h. With increasing temperature and magnetic field the structure shows characteristics of the 0.7 structure. Our results suggest that at low densities a spontaneous spin splitting occurs whenever two 1D subbands of opposite spins cross.     

Evaluation of protein modification during anti-viral heat bioprocessing by electrospray ionization mass spectrometry

During the preparation of therapeutic plasma and recombinant protein biopharmaceuticals heat-treatment is routinely applied as a means of viral inactivation. However, as most proteins denature and aggregate under heat stress, it is necessary to add thermostabilizing excipients to protein formulations destined for anti-viral heat-treatment in order to prevent protein damage. Anti-viral heat-treatment bioprocessing therefore requires that a balance be found between the bioprocessing conditions, virus kill and protein integrity. In this study we have utilized a simple model protein, beta-lactoglobulin, to investigate the relationship between virucidal heat-treatment conditions (protein formulation and temperature) and the type and extent of protein modification in the liquid state. A variety of industrially relevant heat-treatments were undertaken, using formulations that included sucrose as a thermostabilizing excipient. Using liquid chromatography/electrospray ionization mass spectrometry (LC/ESI-MS) we show here that protein modifications do occur with increasingly harsh heat-treatment. The predominant modification under these conditions was protein glycation by either glucose or fructose derived from hydrolyzed sucrose. Advanced glycation end products and additional unidentified products were also present in beta-lactoglobulin protein samples subjected to extended heat-treatment. These findings have implications for the improvement of anti-viral heat-treatment bioprocesses to ensure the safety and efficacy of protein biopharmaceuticals. CopyrightCopyright 2001 John Wiley & Sons, Ltd.     

Weak localization, hole-hole interactions, and the "Metal"-insulator transition in two dimensions

A detailed investigation of the metallic behavior in high-quality GaAs-AlGaAs two-dimensional hole systems reveals the presence of quantum corrections to the resistivity at low temperatures. Despite the low density ( r(s)>10) and high quality of these systems, both weak localization (observed via negative magnetoresistance) and weak hole-hole interactions (giving a correction to the Hall constant) are present in the so-called metallic phase where the resistivity decreases with decreasing temperature. If these quantum corrections persist down to T = 0, the results suggest that even at high r(s) there is no metallic phase in two dimensions.