980 research outputs found
On the Crystallization of Terbium Aluminium Garnet
Attempts to grow terbium aluminium garnet (Tb3Al5O12, TAG) by the Czochralski
method lead to crystals of millimeter scale. Larger crystals could not be
obtained. DTA measurements within the binary system showed that TAG melts
incongruently at 1840 deg. C. The perovskite (TbAlO3, TAP) with a congruent
melting point of 1930 deg. C is the most stable phase in this system. The
region for primary crystallization of TAP covers the chemical composition of
TAG and suppresses the primary crystallization of the terbium aluminium garnet.Comment: 6 pages, 2 figure
Development of Prototype Pixellated PIN CdZnTe Detectors
We report initial results from the design and evaluation of two pixellated
PIN Cadmium Zinc Telluride detectors and an ASIC-based readout system. The
prototype imaging PIN detectors consist of 4X4 1.5 mm square indium anode
contacts with 0.2 mm spacing and a solid cathode plane on 10X10 mm CdZnTe
substrates of thickness 2 mm and 5 mm. The detector readout system, based on
low noise preamplifier ASICs, allows for parallel readout of all channels upon
cathode trigger. This prototype is under development for use in future
astrophysical hard X-ray imagers with 10-600 keV energy response. Measurements
of the detector uniformity, spatial resolution, and spectral resolution will be
discussed and compared with a similar pixellated MSM detector. Finally, a
prototype design for a large imaging array is outlined.Comment: 10 pages Latex, 9 figures, to appear in Proc. of SPIE Vol. 3446 "Hard
X-ray and Gamma-Ray Detector Physics and Applications
Transport through open quantum dots: making semiclassics quantitative
We investigate electron transport through clean open quantum dots (quantum
billiards). We present a semiclassical theory that allows to accurately
reproduce quantum transport calculations. Quantitative agreement is reached for
individual energy and magnetic field dependent elements of the scattering
matrix. Two key ingredients are essential: (i) inclusion of pseudo-paths which
have the topology of linked classical paths resulting from diffraction in
addition to classical paths and (ii) a high-level approximation to diffractive
scattering. Within this framework of the pseudo-path semiclassical
approximation (PSCA), typical shortcomings of semiclassical theories such as
violation of the anti-correlation between reflection and transmission and the
overestimation of conductance fluctuations are overcome. Beyond its predictive
capabilities the PSCA provides deeper insights into the quantum-to-classical
crossover.Comment: 20 pages, 19 figure
Growth of Oxide Compounds under Dynamic Atmosphere Composition
Commercially available gases contain residual impurities leading to a
background oxygen partial pressure of typically several 10^{-6} bar,
independent of temperature. This oxygen partial pressure is inappropriate for
the growth of some single crystals where the desired oxidation state possesses
a narrow stability field. Equilibrium thermodynamic calculations allow the
determination of dynamic atmosphere compositions yielding such self adjusting
and temperature dependent oxygen partial pressures, that crystals like ZnO,
Ga2O3, or Fe{1-x}O can be grown from the melt.Comment: 4 pages, 3 figures, talk on CGCT-4 Sendai, May 21-24, 200
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Pulse Length Control in an X-Ray FEL by Using Wakefields
For the users of the high-brightness radiation sources of free-electron lasers it is desirable to reduce the FEL pulse length to 10 fs and below for time-resolved pump and probe experiments. Although it can be achieved by conventional compression methods for the electron beam or the chirped FEL pulse, the technical realization is demanding. In this presentation we study the impact of longitudinal wakefields in the undulator and how their properties can be used to reduced the amplifying part of the bunch to the desired length. Methods of actively controlling the wakefields are presented
Si-compatible candidates for high-K dielectrics with the Pbnm perovskite structure
We analyze both experimentally (where possible) and theoretically from
first-principles the dielectric tensor components and crystal structure of five
classes of Pbnm perovskites. All of these materials are believed to be stable
on silicon and are therefore promising candidates for high-K dielectrics. We
also analyze the structure of these materials with various simple models,
decompose the lattice contribution to the dielectric tensor into force constant
matrix eigenmode contributions, explore a peculiar correlation between
structural and dielectric anisotropies in these compounds and give phonon
frequencies and infrared activities of those modes that are infrared-active. We
find that CaZrO_3, SrZrO_3, LaHoO_3, and LaYO_3 are among the most promising
candidates for high-K dielectrics among the compounds we considered.Comment: 17 pages, 9 figures, 4 tables. Supplementary information:
http://link.aps.org/supplemental/10.1103/PhysRevB.82.064101 or
http://www.physics.rutgers.edu/~sinisa/highk/supp.pd
Supertransferred hyperfine interactions in layer LaSrGa0.995Cu0.005O4
The EPR allowing a direct observation of supertransferred hyperfine fields on the nuclei of the next-nearest cations to copper ion in the structure of LaSrGa0.995Cu0.005O4 isomorphous to the superconducting cuprates prove the far delocalisation of the spin density. The theoretical analysis of the value and mechanisms of this process by the method of configurational interaction leads to a good agreement with the experiment. It is shown that the cascade processes involving simultaneous electron transfers from the oxygen to the copper atom and from the neighbouring cation to the same oxygen, in the fragment considered bring a substantial contribution into polarisation of 3s-shell of gallium ion. A comparison of the supertransferred hyperfine fields and g-tensor values in the studied, diluted LaSrGa0.995Cu0.005O4 crystal and in the La2CuO4 cuprate confirms the validity of the local centre approach in the analysis of cuprate properties. © 2002 Elsevier Science B.V. All rights reserved
A strategy to characterize the LISA-Pathfinder cold gas thruster system
The cold gas micro-propulsion system that will be used during the LISA-Pathfinder mission will be one of the most important component used to ensure the "free-fall" of the enclosed test masses. In this paper we present a possible strategy to characterize the effective direction and amplitude gain of each of the 6 thrusters of this system
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