75 research outputs found
Hochdruckuntersuchungen an Yb-Kondo-Gitter-Systemen in der Nähe einer magnetischen Instabilität
Tools for 3D scientific visualization in computational aerodynamics
The purpose is to describe the tools and techniques in use at the NASA Ames Research Center for performing visualization of computational aerodynamics, for example visualization of flow fields from computer simulations of fluid dynamics about vehicles such as the Space Shuttle. The hardware used for visualization is a high-performance graphics workstation connected to a super computer with a high speed channel. At present, the workstation is a Silicon Graphics IRIS 3130, the supercomputer is a CRAY2, and the high speed channel is a hyperchannel. The three techniques used for visualization are post-processing, tracking, and steering. Post-processing analysis is done after the simulation. Tracking analysis is done during a simulation but is not interactive, whereas steering analysis involves modifying the simulation interactively during the simulation. Using post-processing methods, a flow simulation is executed on a supercomputer and, after the simulation is complete, the results of the simulation are processed for viewing. The software in use and under development at NASA Ames Research Center for performing these types of tasks in computational aerodynamics is described. Workstation performance issues, benchmarking, and high-performance networks for this purpose are also discussed as well as descriptions of other hardware for digital video and film recording
Development of the critical exponent at the antiferromagnetic phase transition of YbRh2Si2 under chemical pressure
We present specific-heat measurements in the vicinity of the
antiferromagnetic phase transition on single crystals of the alloy
Yb(Rh_{1-x}Co_x)2Si2 for x<= 0.38. This study was motivated by the violation of
critical universality in the undoped YbRh2Si2 (Krellner et al., Phys. Rev.
Lett. 102, 196402) where we have found a large critical exponent a=0.38. For
Co-doped samples we observe a drastic change of the critical fluctuations
resulting in a negative a, explainable within the universality classes of phase
transitions. The development of a under chemical pressure gives strong
indication that the violation of critical universality in YbRh2Si2 is due to
the nearby quantum critical point.Comment: Accepted for the QCNP proceedings 200
Effect of chemical substitution and pressure on YbRh2Si2
We carried out electrical resistivity experiments on (Yb,La)Rh2Si2 and on
Yb(Rh,Ir)2Si2 under pressure and in magnetic fields. YbRh2Si2 exhibits a weak
antiferromagnetic transition at atmospheric pressure with a N\'eel temperature
of only T_N = 70 mK. By applying a small magnetic field T_N can be continuously
suppressed to T=0 at B_c = 60 mT (B_|_c) driving the system to a quantum
critical point (QCP). On applying external pressure the magnetic phase is
stabilized and T_N(p) is increasing as usually observed in Yb-based
heavy-fermion metals. Substituting Yb by La or Rh by Ir allows to create a
negative chemical pressure, La (Ir) being smaller than Yb (Rh), and eventually
to drive YbRh2Si2 to a pressure controlled QCP. In this paper we compare the
effect of external hydrostatic pressure and chemical substitution on the
ground-state properties of YbRh2Si2.Comment: 4 pages, 5 figures, proceedings paper of the QCNP0
FAST: A multi-processed environment for visualization of computational fluid dynamics
Three-dimensional, unsteady, multi-zoned fluid dynamics simulations over full scale aircraft are typical of the problems being investigated at NASA Ames' Numerical Aerodynamic Simulation (NAS) facility on CRAY2 and CRAY-YMP supercomputers. With multiple processor workstations available in the 10-30 Mflop range, we feel that these new developments in scientific computing warrant a new approach to the design and implementation of analysis tools. These larger, more complex problems create a need for new visualization techniques not possible with the existing software or systems available as of this writing. The visualization techniques will change as the supercomputing environment, and hence the scientific methods employed, evolves even further. The Flow Analysis Software Toolkit (FAST), an implementation of a software system for fluid mechanics analysis, is discussed
FAST: A multi-processed environment for visualization of computational fluid
Three dimensional, unsteady, multizoned fluid dynamics simulations over full scale aircraft is typical of problems being computed at NASA-Ames on CRAY2 and CRAY-YMP supercomputers. With multiple processor workstations available in the 10 to 30 Mflop range, it is felt that these new developments in scientific computing warrant a new approach to the design and implementation of analysis tools. These large, more complex problems create a need for new visualization techniques not possible with the existing software or systems available as of this time. These visualization techniques will change as the supercomputing environment, and hence the scientific methods used, evolve ever further. Visualization of computational aerodynamics require flexible, extensible, and adaptable software tools for performing analysis tasks. FAST (Flow Analysis Software Toolkit), an implementation of a software system for fluid mechanics analysis that is based on this approach is discussed
Scientific Visualization Using the Flow Analysis Software Toolkit (FAST)
Over the past few years the Flow Analysis Software Toolkit (FAST) has matured into a useful tool for visualizing and analyzing scientific data on high-performance graphics workstations. Originally designed for visualizing the results of fluid dynamics research, FAST has demonstrated its flexibility by being used in several other areas of scientific research. These research areas include earth and space sciences, acid rain and ozone modelling, and automotive design, just to name a few. This paper describes the current status of FAST, including the basic concepts, architecture, existing functionality and features, and some of the known applications for which FAST is being used. A few of the applications, by both NASA and non-NASA agencies, are outlined in more detail. Described in the Outlines are the goals of each visualization project, the techniques or 'tricks' used lo produce the desired results, and custom modifications to FAST, if any, done to further enhance the analysis. Some of the future directions for FAST are also described
The break up of heavy electrons at a quantum critical point
The point at absolute zero where matter becomes unstable to new forms of
order is called a quantum critical point (QCP). The quantum fluctuations
between order and disorder that develop at this point induce profound
transformations in the finite temperature electronic properties of the
material. Magnetic fields are ideal for tuning a material as close as possible
to a QCP, where the most intense effects of criticality can be studied. A
previous study on theheavy-electron material found that near a
field-induced quantum critical point electrons move ever more slowly and
scatter off one-another with ever increasing probability, as indicated by a
divergence to infinity of the electron effective mass and cross-section. These
studies could not shed light on whether these properties were an artifact of
the applied field, or a more general feature of field-free QCPs. Here we report
that when Germanium-doped is tuned away from a chemically induced
quantum critical point by magnetic fields there is a universal behavior in the
temperature dependence of the specific heat and resistivity: the characteristic
kinetic energy of electrons is directly proportional to the strength of the
applied field. We infer that all ballistic motion of electrons vanishes at a
QCP, forming a new class of conductor in which individual electrons decay into
collective current carrying motions of the electron fluid.Comment: Pdf files of article available at
http://www.physics.rutgers.edu/~coleman/online/breakup.pdf, pdf file of news
and views article available at
http://www.physics.rutgers.edu/~coleman/online/nvbreakup.pd
Divergence of the Grueneisen Ratio at Quantum Critical Points in Heavy Fermion Metals
We present low-temperature volume thermal expansion, , and specific
heat, , measurements on high-quality single crystals of CeNi2Ge2 and
YbRh2(SiGe) which are located very near to quantum
critical points. For both systems, shows a more singular temperature
dependence than , and thus the Grueneisen ratio
diverges as T --> 0. For CeNi2Ge2, our results are in accordance with the
spin-density wave (SDW) scenario for three-dimensional critical
spin-fluctuations. By contrast, the observed singularity in
YbRh2_{0.95}_{0.05}_2$ cannot be explained by the itinerant SDW
theory but is qualitatively consistent with a locally quantum critical picture.Comment: 11 pages, 4 figure
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