Temperature dependent carrier lifetime studies of Mo in crystalline silicon

The capture cross sections of both electronsσn and holes σp were determined for interstitialmolybdenum in crystalline silicon over the temperature range of −110 to 150 °C. Carrier lifetimemeasurements were performed on molybdenum-contaminated silicon using a temperature controlled photoconductance instrument. Injection dependent lifetime spectroscopy was applied at each temperature to calculate σp and σn. This analysis involved a novel approach that independently determined the capture cross sections at each temperature assuming a known defect density and thermal velocity. Since the energy state is in the lower half of the bandgap, the determination of σp is unaffected by the defect energy at all temperatures, and σp is found to decrease with temperature in a fashion consistent with excitonic Auger capture. At temperatures below 0 °C, the determination of σn is also unaffected by the defect energy due to the suppression of thermal emission, and σn decreases with temperature as well. It is shown that a projection of σn to higher temperature suggests the defect has an energy of 0.375 eV above the valance band edge of silicon.D.M. likes to thank the Australian Research Council for fellowship and G.C. likes to thank “CrystalClear Integrated Project” Contract No. SES6-CT_2003-502583 funded by the European Commission

Generalized procedure to determine the dependence of steady-state photoconductance lifetime on the occupation of multiple defects

We present a procedure to determine the dependence of photoconductance lifetime on the occupation of multiple defects. The procedure requires numerical iteration, making it more cumbersome than the analytical equations available for single-defect and simplified two-defect cases, but enabling the following features: (i) it accounts for the defect concentration when calculating the equilibrium carrier concentrations, (ii) it permits recombination through any number of defects, (iii) it calculates the occupation fraction of all defects at any injection, and (iv) it promotes a good understanding of the role of defect occupation in photoconductance measurements. The utility of the numerical procedure is demonstrated on an experimental sample containing multiple defects. The dependence of the sample’s photoconductance on carrier concentration and temperature can be qualitatively described by the generalized procedure but not by either analytical model. The example also demonstrates that the influence of defect occupation on photoconductance lifetime measurements is mitigated at elevated temperatures—a conclusion of particular worth to the study of multicrystalline silicon.This work was funded by an Australian Research Council Linkage Grant between the Australian National University, SierraTherm Production Furnaces, and SunPower Corporation. D.M. is supported by an Australian Research Council fellowship

Neonatal tolerance to Mls-1a determinants: deletion or anergy of Vβ6 + T lymphocytes depending upon MHC compatibility of neonatally injected cells

Recent investigations in mice revealed that natural immunologlcal tolerance to endogenous minor lymphocyte-stimulating locus 1a (MIs-1a antigen correlates primarily with deletion of Mls-1aspeciflc Vβ6+ T lymphocytes In the thymus. Similar mechanisms account for acquired tolerance in some Instancessince the neonatal injection of Mls-1 a-expressing MHC compatible cells in neonatal mice within the first 24 hof life causes clonal deletion of Vβ6+ T cells. Here we demonstrate that Vβ6+ T cells are not deleted In mice neonatally treated with Mls-1a spleen cells expressing allogenelc H-2 molecules. However, when such non-deleted Vβ6+ T cells were tested In vitro, no interleukin 2 (IL-2) secretion or proliferation was observed after Mls-1a stimulation. This non-responsive state could be overcome by addition of exogenous IL-2, consistent with the fact that Vβ6+ cells enlarged and expressed IL-2 receptors upon Mls-1a stimulation. Furthermore, the same neonatally treated mice showed In vitro functional unresponsiveness of cytotoxic T cells but not of IL-2-secreting cells specific for the tolerated allogeneic MHC antigens. Taken together, our data Indicate that neonatal tolerance to Mls-1a can be accomplished by either clonal deletion or clonal anergy, and that it does not necessarily correlate with tolerance to MHC determinant

Spin Dynamics of the LAGEOS Satellite in Support of a Measurement of the Earth's Gravitomagnetism

LAGEOS is an accurately-tracked, dense spherical satellite covered with 426 retroreflectors. The tracking accuracy is such as to yield a medium term (years to decades) inertial reference frame determined via relatively inexpensive observations. This frame is used as an adjunct to the more difficult and data intensive VLBI absolute frame measurements. There is a substantial secular precession of the satellite's line of nodes consistent with the classical, Newtonian precession due to the non-sphericity of the earth. Ciufolini has suggested the launch of an identical satellite (LAGEOS-3) into an orbit supplementary to that of LAGEOS-1: LAGEOS-3 would then experience an equal and opposite classical precession to that of LAGEOS-1. Besides providing a more accurate real-time measurement of the earth's length of day and polar wobble, this paired-satellite experiment would provide the first direct measurement of the general relativistic frame-dragging effect. Of the five dominant error sources in this experiment, the largest one involves surface forces on the satellite, and their consequent impact on the orbital nodal precession. The surface forces are a function of the spin dynamics of the satellite. Consequently, we undertake here a theoretical effort to model the spin ndynamics of LAGEOS. In this paper we present our preliminary results.Comment: 16 pages, RevTeX, LA-UR-94-1289. (Part I of II, postscript figures in Part II

Bostonia: The Boston University Alumni Magazine. Volume 20

Founded in 1900, Bostonia magazine is Boston University's main alumni publication, which covers alumni and student life, as well as university activities, events, and programs

Double-Occupancy Errors, Adiabaticity, and Entanglement of Spin-Qubits in Quantum Dots

Quantum gates that temporarily increase singlet-triplet splitting in order to swap electronic spins in coupled quantum dots, lead inevitably to a finite double-occupancy probability for both dots. By solving the time-dependent Schr\"odinger equation for a coupled dot model, we demonstrate that this does not necessarily lead to quantum computation errors. Instead, the coupled dot ground state evolves quasi-adiabatically for typical system parameters so that the double-occupancy probability at the completion of swapping is negligibly small. We introduce a measure of entanglement which explicitly takes into account the possibilty of double occupancies and provides a necessary and sufficient criterion for entangled states.Comment: 9 pages, 4 figures include

Theory of the Quantum Hall Smectic Phase II: Microscopic Theory

We present a microscopic derivation of the hydrodynamic theory of the Quantum Hall smectic or stripe phase of a two-dimensional electron gas in a large magnetic field. The effective action of the low energy is derived here from a microscopic picture by integrating out high energy excitations with a scale of the order the cyclotron energy.The remaining low-energy theory can be expressed in terms of two canonically conjugate sets of degrees of freedom: the displacement field, that describes the fluctuations of the shapes of the stripes, and the local charge fluctuations on each stripe.Comment: 20 pages, RevTex, 3 figures, second part of cond-mat/0105448 New and improved Introduction. Final version as it will appear in Physical Review

ABCD of Beta Ensembles and Topological Strings

We study beta-ensembles with Bn, Cn, and Dn eigenvalue measure and their relation with refined topological strings. Our results generalize the familiar connections between local topological strings and matrix models leading to An measure, and illustrate that all those classical eigenvalue ensembles, and their topological string counterparts, are related one to another via various deformations and specializations, quantum shifts and discrete quotients. We review the solution of the Gaussian models via Macdonald identities, and interpret them as conifold theories. The interpolation between the various models is plainly apparent in this case. For general polynomial potential, we calculate the partition function in the multi-cut phase in a perturbative fashion, beyond tree-level in the large-N limit. The relation to refined topological string orientifolds on the corresponding local geometry is discussed along the way.Comment: 33 pages, 1 figur