Sounding Rocket XUV Observations of Capella

Capella (d = 13pc) was observed in the 170 Å to 450 Å band by a sounding rocket borne spectrometer with a spectral resolution of 35 Å FWHM. Minimum flux sensitivity varies with wavelength and a 2σ upper limit is 0.25 and 0.6 photons cm-2s-1 at 180 Å and 304 Å respectively. The prominent stellar emission line in this bandpass is He II 304 Å. The Capella flux at earth in this line is expected to be in the range 2.8 to 5.4 photons cm-2s-1 in the absence of interstellar extinction. Interstellar absorption was computed from the Copernicus satellite measure of the neutral hydrogen column density, N(H I) = 1.2 x 1018cm-2, towards Capella and a ratio of N(He I)/N(H I) = 0.1. The 50% attenuation at 304 Å results in a flux at earth of 1.4 to 2.7 phcm-2s-1. No significant Capella flux was detected in the bandpass despite inflight verification of instrument's sensitivity obtained from observations of the He II 304 Å geocoronal emission. The most reasonable explanation for the absence of the 304 Å line is that absorption in the intervening medium exceeds that predicted from N(He I) = 0.1N(H I). A lower limit to the Capella line of sight N(He I) is determined from the 304 Å observation. We find that N(He I)/N(H I) is greater than 0.4. A helium to hydrogen abundance ratio of 0.1 is retained if the hydrogen ionization in the local interstellar medium (LISM) exceeds 60%. Several models of the LISM which incorporate this ionization fraction are treated for self consistency, as well as compatibility with observations of the temperature and ionization fraction of the interplanetary medium. The most plausible model consists of a local, warm (10,000 K), gas embedding the solar system, surrounded by a hot (105 → 106 K) plasma. Ionization of hydrogen in the warm cloud must be maintained by a local interstellar radiation field.</p

Two nonmagnetic impurities in the DSC and DDW state of the cuprate superconductors as a probe for the pseudogap

The quantum interference between two nonmagnetic impurities is studied numerically in both the d-wave superconducting (DSC) and the d-density wave (DDW) state. In all calculations we include the tunnelling through excited states from the CuO$_2$ planes to the BiO layer probed by the STM tip. Compared to the single impurity case, a systematic study of the modulations in the two-impurity local density of states can distinguish between the DSC or DDW states. This is important if the origin of the pseudogap phase is caused by preformed pairs or DDW order. Furthermore, in the DSC state the study of the LDOS around two nonmagnetic impurities provide further tests for the potential scattering model versus more strongly correlated models.Comment: 6 pages, 6 figure

Quantum impurity dynamics in two-dimensional antiferromagnets and superconductors

We present the universal theory of arbitrary, localized impurities in a confining paramagnetic state of two-dimensional antiferromagnets with global SU(2) spin symmetry. The energy gap of the host antiferromagnet to spin-1 excitations, \Delta, is assumed to be significantly smaller than a typical nearest neighbor exchange. In the absence of impurities, it was argued in earlier work (Chubukov et al. cond-mat/9304046) that the low-temperature quantum dynamics is universally and completely determined by the values of \Delta and a spin-wave velocity c. Here we establish the remarkable fact that no additional parameters are necessary for an antiferromagnet with a dilute concentration of impurities, n_{imp} - each impurity is completely characterized by a integer/half-odd-integer valued spin, S, which measures the net uncompensated Berry phase due to spin precession in its vicinity. We compute the impurity-induced damping of the spin-1 collective mode of the antiferromagnet: the damping occurs on an energy scale \Gamma= n_{imp} (\hbar c)^2/\Delta, and we predict a universal, asymmetric lineshape for the collective mode peak. We argue that, under suitable conditions, our results apply unchanged (or in some cases, with minor modifications) to d-wave superconductors, and compare them to recent neutron scattering experiments on YBCO by Fong et al. (cond-mat/9812047). We also describe the universal evolution of numerous measurable correlations as the host antiferromagnet undergoes a quantum phase transition to a Neel ordered state.Comment: 36 pages, 12 figures; added reference

The doping dependence of T* - what is the real high-Tc phase diagram?

Underdoped high-Tc superconductors are frequently characterised by a temperature, T*, below which the normal-state pseudogap opens. Two different "phase diagrams" based on the doping (p) dependence of T* are currently considered: one where T* falls to zero at a critical doping state and the other where T* merges with Tc in the overdoped region. By examining the temperature dependence of the NMR Knight shift and relaxation rate, entropy, resistivity, infrared conductivity, Raman scattering, ARPES and tunnelling data it is concluded that the second scenario is not at all supported. Neither can one distinguish a small and a large pseudogap as is often done. T* is an energy scale which falls abruptly to zero at p=0.19.Comment: 13 pages, 11 figures, a response to confusion at M^2S Conference, Houston, regarding the phase behaviour of the HTS cuprates. Submitted to Physica C, 2 May 2000. More references added as well as section on c-axis resistivit

Dynamic Placement of Virtual Machines for Managing SLA Violations

{bobroffJakochut,kirkbeaty} @ us. ibm.com Abstract- A dynamic server migration and consolidation al-gorithm is introduced. The algorithm is shown to provide sub-stantial improvement over static server consolidation in reducing the amount of required capacity and the rate of service level agreement violations. Benefits accrue for workloads that are variable and can be forecast over intervals shorter than the time scale of demand variability. The management algorithm reduces the amount of physical capacity required to support a specified rate of SLA violations for a given workload by as much as 50 % as compared to static consolidation approach. Another result is that the rate of SLA violations at fixed capacity may be reduced by up to 20%. The results are based on hundreds of production workload traces across a variety of operating systems, applications, and industries. I

Scalability Analysis of Job Scheduling using Virtual Nodes

Abstract. It is important to identify scalability constraints in existing job scheduling software as they are applied to next generation parallel systems. In this paper, we analyze the scalability of job scheduling and job dispatching functions in the IBM LoadLeveler job scheduler. To enable this scalability study, we propose and implement a new virtualization method to deploy different size LoadLeveler clusters with minimal number of physical machines. Our scalability studies with the virtualization show that the LoadLeveler resource manager can comfortably handle over 12,000 compute nodes, the largest scale we have tested so far. However, our study shows that the static resource matching in the scheduling cycle and job object processing during the hierarchical job launching are two impediments for the scalability of LoadLeveler.