Spin-dependent transport in a quasiballistic quantum wire

We describe the transport properties of a 5 $\mu$m long one-dimensional (1D) quantum wire. Reduction of conductance plateaux due to the introduction of weakly disorder scattering are observed. In an in-plane magnetic field, we observe spin-splitting of the reduced conductance steps. Our experimental results provide evidence that deviation from conductance quantisation is very small for electrons with spin parallel and is about 1/3 for electrons with spin anti-parallel. Moreover, in a high in-plane magnetic field, a spin-polarised 1D channel shows a plateau-like structure close to $0.3 \times e^2/h$ which strengthens with {\em increasing} temperatures. It is suggested that these results arise from the combination of disorder and the electron-electron interactions in the 1D electron gas.Comment: 4 pages, 5 figures, latex to be published in Phys. Rev. B (15/3/2000

Anomalous spin-dependent behaviour of one-dimensional subbands

We report a new electron interaction effect in GaAs/AlGaAs quantum wires. Using DC-bias spectroscopy, we show that large and abrupt changes occur to the energies of spin-down (lower energy) states as they populate. The effect is not observed for spin-up energy states. At B=0, interactions have a pronounced effect, in the form of the well-known 0.7 Structure. However, our new results show that interactions strongly affect the energy spectrum at all magnetic fields, from 0 to 16T, not just in the vicinity of the 0.7 Structure.Comment: 4 pages, 2 figure

Major Galaxy Mergers Only Trigger the Most Luminous AGN

Using multiwavelength surveys of active galactic nuclei across a wide range of bolometric luminosities (10^{43}<L_{bol}(erg/s<5x10^{46}) and redshifts (0<z<3), we find a strong, redshift-independent correlation between the AGN luminosity and the fraction of host galaxies undergoing a major merger. That is, only the most luminous AGN phases are connected to major mergers, while less luminous AGN appear to be driven by secular processes. Combining this trend with AGN luminosity functions to assess the overall cosmic growth of black holes, we find that ~50% by mass is associated with major mergers, while only 10% of AGN by number, the most luminous, are connected to these violent events. Our results suggest that to reach the highest AGN luminosities -where the most massive black holes accreted the bulk of their mass - a major merger appears to be required. The luminosity dependence of the fraction of AGN triggered by major mergers can successfully explain why the observed scatter in the M-\sigma relation for elliptical galaxies is significantly lower than in spirals. The lack of a significant redshift dependence of the L_{bol}-f_{merger} relation suggests that downsizing, i.e., the general decline in AGN and star formation activity with decreasing redshift, is driven by a decline in the frequency of major mergers combined with a decrease in the availability of gas at lower redshifts.Comment: Accepted for publication by Astrophysical Journal Letters, 6 pages in emulateapj format, 3 figure

The Accuracy of Morphological Decomposition of Active Galactic Nucleus Host Galaxies

In order to assess the accuracy with which we can determine the morphologies of AGN host galaxies, we have simulated more than 50,000 ACS images of galaxies with z < 1.25, using image and noise properties appropriate for the GOODS survey. We test the effect of central point-source brightness on host galaxy parameter recovery with a set of simulated AGN host galaxies made by adding point sources to the centers of normal galaxies. We extend this analysis and also quantify the recovery of intrinsic morphological parameters of AGN host galaxies with a set of fully simulated inactive and AGN host galaxies. We can reliably separate good from poor fit results using a combination of reasonable error cuts, in the regime where L_{host}:L_{PS} > 1:4. We give quantitative estimates of parameter errors as a function of host-to-point-source ratio. In general, we separate host and point-source magnitudes reliably at all redshifts; point sources are well recovered more than 90% of the time, although spurious detection of central point sources can be as high as 25% for bulge-dominated sources. We find a general correlation between Sersic index and intrinsic bulge-to-total ratio, such that a host galaxy with Sersic n < 1.5 generally has at least 80% of its light from a disk component. Likewise, "bulge-dominated" galaxies with n > 4 typically derive at least 70% of their total host galaxy light from a bulge, but this number can be as low as 55%. Single-component Sersic fits to an AGN host galaxy are statistically very reliable to z < 1.25 (for ACS survey data like ours). In contrast, two-component fits involving separate bulge and disk components tend to over-estimate the bulge fraction by ~10%, with uncertainty of order 50%.Comment: 45 pages, 20 figures, submitted to ApJ ; Accepted Version -- additions to introduction and conclusions; title changed, was "Simulations of AGN Host Galaxy Morphologies

Quartz-kyanite pods in Proterozoic rocks in northern New Mexico : shear zone formation along an older hydrothermal alteration horizon

Large quartz-kyanite schist pods of unusual bulk composition enclosed by shells of sericite schist occur within the 1.7 Ga Vadito Group metarhyolite in northern New Mexico. These pods are discontinuous, lenticular, symmetrically zoned, and are stratiform within a map-scale sericitic horizon. Previous studies have not resolved whether the high-Al bulk composition of the quartz-kyanite pods was the result of weathering, hydrothermal alteration, or shearing. Geochemical differences between the quartz-kyanite/sericite schist pods and the sericite-rich layer that connects them suggest more than one alteration process. This study uses geochemical, structural, and metamorphic data to evaluate the origin and tectonic evolution of the quartz-kyanite rocks. Geochemical data from sampling traverses, mineral textures and map patterns indicate that the quartz-kyanite pods obtained their unusual (high Al, low K, Na, Ca, Fe) compositions through hydrothermal alteration associated with volcanism. Geochemical profiles are symmetrical, with depletion of Ca, Na, K, Fe, and enrichment of Si toward the center of the alteration zone. Higher K and Fe compositions in the sericite-rich layer that connects the pods suggests a different alteration process. Truncation of stratigraphic map units, grain-size reduction, S-C fabrics, and asymmetric prophyroblasts suggest that this second alteration process was related to a top-to-the-south shearing episode (D1) along a bedding-subparallel zone before D2 (N-vergent) deformation produced map-scale folds. Microstructural studies show that kyanite is an early (S1) metamorphic mineral produced prior to shearing of the previously altered volcanic rock, shown by alignment and grain-size reduction of kyanite within the earliest fabric (S1). Subsequent metamorphism and shearing may have enhanced the concentration of silica and aluminum in this zone, and linked the pods of altered rock into a map-scale sericite-rich (S1) shear zone. Other minerals that formed early in the deformational history of these rocks include staurolite, paragonite and albite, indicating peak P-T conditions of up to ~600 degrees, ~6kbar for S1 fabrics. S2 minerals in the quartz-kyanite pods also include staurolite, and chloritoid, and, in the politic schist layer, staurolite, garnet, biotite and garnet, indicating peak temperature for S2 of 575 degrees, and pressure sufficient for kyanite formation. These assemblages may produce looping P-T paths, a model which has been proposed for other regions of Proterozoic rocks in northern New Mexico

Ultra-dense phosphorus in germanium delta-doped layers

Phosphorus (P) in germanium (Ge) delta-doped layers are fabricated in ultra-high vacuum by adsorption of phosphine molecules onto an atomically flat clean Ge(001) surface followed by thermal incorporation of P into the lattice and epitaxial Ge overgrowth by molecular beam epitaxy. Structural and electrical characterizations show that P atoms are confined, with minimal diffusion, into an ultra-narrow 2-nm-wide layer with an electrically-active sheet carrier concentration of 4x10^13 cm-2 at 4.2 K. These results open up the possibility of ultra-narrow source/drain regions with unprecedented carrier densities for Ge n-channel field effect transistors

Major Mergers Host the Most Luminous Red Quasars at z ~ 2: A Hubble Space Telescope WFC3/IR Study

We used the Hubble Space Telescope WFC3 near-infrared camera to image the host galaxies of a sample of eleven luminous, dust-reddened quasars at z ~ 2 -- the peak epoch of black hole growth and star formation in the Universe -- to test the merger-driven picture for the co-evolution of galaxies and their nuclear black holes. The red quasars come from the FIRST+2MASS red quasar survey and a newer, deeper, UKIDSS+FIRST sample. These dust-reddened quasars are the most intrinsically luminous quasars in the Universe at all redshifts, and may represent the dust-clearing transitional phase in the merger-driven black hole growth scenario. Probing the host galaxies in rest-frame visible light, the HST images reveal that 8/10 of these quasars have actively merging hosts, while one source is reddened by an intervening lower redshift galaxy along the line-of-sight. We study the morphological properties of the quasar hosts using parametric Sersic fits as well as the non-parametric estimators (Gini coefficient, M_{20} and asymmetry). Their properties are heterogeneous but broadly consistent with the most extreme morphologies of local merging systems such as Ultraluminous Infrared galaxies. The red quasars have a luminosity range of log(L_bol) = 47.8 - 48.3 (erg/s) and the merger fraction of their AGN hosts is consistent with merger-driven models of luminous AGN activity at z=2, which supports the picture in which luminous quasars and galaxies co-evolve through major mergers that trigger both star formation and black hole growth.Comment: Submitted to ApJ. This version includes the response to the referee repor

Combinatorial Bounds and Characterizations of Splitting Authentication Codes

We present several generalizations of results for splitting authentication codes by studying the aspect of multi-fold security. As the two primary results, we prove a combinatorial lower bound on the number of encoding rules and a combinatorial characterization of optimal splitting authentication codes that are multi-fold secure against spoofing attacks. The characterization is based on a new type of combinatorial designs, which we introduce and for which basic necessary conditions are given regarding their existence.Comment: 13 pages; to appear in "Cryptography and Communications