51 research outputs found
Two Band Model Interpretation of the p to n Transition in Ternary Tetradymite Topological Insulators
The requirement for large bulk resistivity in topological insulators has led
to the design of complex ternary and quaternary phases with balanced donor and
acceptor levels. A common feature of the optimized phases is that they lie
close to the p to n transition. The tetradymite Bi2Te3_xSex system exhibits
minimum bulk conductance at the ordered composition Bi2Te2Se. By combining
local and integral measurements of the density of states, we find that the
point of minimum electrical conductivity at x=1.0 where carriers change from
hole-like to electron-like is characterized by conductivity of the mixed type.
Our experimental findings, which are interpreted within the framework of a two
band model for the different carrier types, indicate that the mixed state
originates from different type of native defects that strongly compensate at
the crossover point
Geometric representation of interval exchange maps over algebraic number fields
We consider the restriction of interval exchange transformations to algebraic
number fields, which leads to maps on lattices. We characterize
renormalizability arithmetically, and study its relationships with a
geometrical quantity that we call the drift vector. We exhibit some examples of
renormalizable interval exchange maps with zero and non-zero drift vector, and
carry out some investigations of their properties. In particular, we look for
evidence of the finite decomposition property: each lattice is the union of
finitely many orbits.Comment: 34 pages, 8 postscript figure
Sum of Lyapunov exponents of the Hodge bundle with respect to the Teichmuller geodesic flow
We compute the sum of the positive Lyapunov exponents of the Hodge bundle
with respect to the Teichmuller geodesic flow. The computation is based on the
analytic Riemann-Roch Theorem and uses a comparison of determinants of flat and
hyperbolic Laplacians when the underlying Riemann surface degenerates.Comment: Minor corrections. To appear in Publications mathematiques de l'IHE
An instrument for in situ time-resolved X-ray imaging and diffraction of laser powder bed fusion additive manufacturing processes
In situ X-ray-based measurements of the laser powder bed fusion (LPBF) additive manufacturing process produce unique data for model validation and improved process understanding. Synchrotron X-ray imaging and diffraction provide high resolution, bulk sensitive information with sufficient sampling rates to probe melt pool dynamics as well as phase and microstructure evolution. Here, we describe a laboratory-scale LPBF test bed designed to accommodate diffraction and imaging experiments at a synchrotron X-ray source during LPBF operation. We also present experimental results using Ti-6Al-4V, a widely used aerospace alloy, as a model system. Both imaging and diffraction experiments were carried out at the Stanford Synchrotron Radiation Lightsource. Melt pool dynamics were imaged at frame rates up to 4 kHz with a ∼1.1 μm effective pixel size and revealed the formation of keyhole pores along the melt track due to vapor recoil forces. Diffraction experiments at sampling rates of 1 kHz captured phase evolution and lattice contraction during the rapid cooling present in LPBF within a ∼50 × 100 μm area. We also discuss the utility of these measurements for model validation and process improvement
Subsurface Cooling Rates and Microstructural Response during Laser Based Metal Additive Manufacturing
Laser powder bed fusion (LPBF) is a method of additive manufacturing characterized by the rapid scanning of a high powered laser over a thin bed of metallic powder to create a single layer, which may then be built upon to form larger structures. Much of the melting, resolidification, and subsequent cooling take place at much higher rates and with much higher thermal gradients than in traditional metallurgical processes, with much of this occurring below the surface. We have used in situ high speed X-ray diffraction to extract subsurface cooling rates following resolidification from the melt and above the β-transus in titanium alloy Ti-6Al-4V. We observe an inverse relationship with laser power and bulk cooling rates. The measured cooling rates are seen to correlate to the level of residual strain borne by the minority β-Ti phase with increased strain at slower cooling rates. The α-Ti phase shows a lattice contraction which is invariant with cooling rate. We also observe a broadening of the diffraction peaks which is greater for the β-Ti phase at slower cooling rates and a change in the relative phase fraction following LPBF. These results provide a direct measure of the subsurface thermal history and demonstrate its importance to the ultimate quality of additively manufactured materials
Early development and gill function in freshwater fish
SIGLEAvailable from British Library Document Supply Centre-DSC:DXN005682 / BLDSC - British Library Document Supply CentreGBUnited Kingdo
Combined effects of water exchange regimes and calcium carbonate additions on growth and survival of hatchery-reared juvenile spotted babylon (Babylonia areolata Link 1807) in recirculating grow-out system
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