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

Internal Stress Evolution and Subsurface Phase Transformation in Titanium Parts Manufactured by Laser Powder Bed Fusion—An In Situ X‐Ray Diffraction Study

Laser powder bed fusion (LPBF) is a metal additive manufacturing technology,which enables the manufacturing of complex geometries for various metals andalloys. Herein, parts made from commercially pure titanium are studied usingin situ synchrotron radiation diffraction experiments. Both the phase transformationand the internal stress buildup are evaluated depending on the processingparameters. For this purpose, evaluation approaches for both temperatureand internal stresses from in situ diffraction patterns are presented. Four differentparameter sets with varying energy inputs and laser scanning strategiesare investigated. A combination of a low laser power and scanning speed leads toa more homogeneous stress distribution in the observed gauge volumes. Theresults show that the phase transformation is triggered during the primarymelting and solidification of the powder and subsurface layers. Furthermore, thestress buildup as a function of the part height during the manufacturing processis clarified. A stress maximum is formed below the part surface, extending intodeeper layers with increasing laser power. A temperature evaluation approach forabsolute internal stresses shows that directional stresses decrease sharply duringlaser impact and reach their previous magnitude again during cooling