14,073 research outputs found
Magnetic Properties of Pd_(0.996)Mn_(0.004) Films for High Resolution Thermometry
We have previously reported on the temperature and magnetic field dependence of the magnetic susceptibility of thin Pd_(1−x)Mn_x alloy films. Extensive new measurements on sputtered films show that a commercial quality sputtering process produces a film with the same dependence of Curie temperature on x as previously reported for bulk samples of the same material. These measurements and parameters from the Renormalization Group theory for a Heisenberg ferromagnet, yield an estimate for T_c of 1.16 ± 0.01 K when x − 0.004, consistent with previously reported bulk result
Measurement of the SOC State Specific Heat in ^4He
When a heat flux Q is applied downward through a sample of liquid 4He near the lambda transition, the helium self organizes such that the gradient in temperature matches the gravity induced gradient in Tlambda. All the helium in the sample is then at the same reduced temperature tSOC = ((T[sub SOC] - T[sub lambda])/T[sub lambda]) and the helium is said to be in the Self-Organized Critical (SOC) state. We have made preliminary measurements of the 4He SOC state specific heat, C[del]T(T(Q)). Despite having a cell height of 2.54 cm, our results show no difference between C[del]T and the zero-gravity 4He specific heat results of the Lambda Point Experiment (LPE) [J.A. Lipa et al., Phys. Rev. B, 68, 174518 (2003)] over the range 250 to 450 nK below the transition. There is no gravity rounding because the entire sample is at the same reduced temperature tSOC(Q). Closer to Tlambda the SOC specific heat falls slightly below LPE, reaching a maximum at approximately 50 nK below Tlambda, in agreement with theoretical predictions [R. Haussmann, Phys. Rev. B, 60, 12349 (1999)]
Effect of Inhomogeneous Heat Flow on the Enhancement of Heat Capacity in Helium-II by Counterflow near Tλ
In 2000 Harter et al. reported the first measurements of the enhancement of the heat capacity ΔCQ[equivalent]C(Q)-C(Q=0) of helium-II transporting a heat flux density Q near Tλ. Surprisingly, their measured ΔCQ was ~7–12 times larger than predicted, depending on which theory was assumed. In this report we present a candidate explanation for this discrepancy: unintended heat flux inhomogeneity. Because C(Q) should diverge at a critical heat flux density Qc, homogeneous heat flow is required for an accurate measurement. We present results from numerical analysis of the heat flow in the Harter et al. cell indicating that substantial inhomogeneity occurred. We determine the effect of the inhomogeneity on ΔCQ and find rough agreement with the observed disparity between prediction and measurement
Implementation and evaluation of simultaneous video-electroencephalography and functional magnetic resonance imaging
The objective of this study was to demonstrate that the addition of simultaneous and synchronised video to electroencephalography (EEG)-correlated functional magnetic resonance imaging (fMRI) could increase recorded information without data quality reduction. We investigated the effect of placing EEG, video equipment and their required power supplies inside the scanner room, on EEG, video and MRI data quality, and evaluated video-EEG-fMRI by modelling a hand motor task. Gradient-echo, echo-planner images (EPI) were acquired on a 3-T MRI scanner at variable camera positions in a test object [with and without radiofrequency (RF) excitation], and human subjects. EEG was recorded using a commercial MR-compatible 64-channel cap and amplifiers. Video recording was performed using a two-camera custom-made system with EEG synchronization. An in-house script was used to calculate signal to fluctuation noise ratio (SFNR) from EPI in test object with variable camera positions and in human subjects with and without concurrent video recording. Five subjects were investigated with video-EEG-fMRI while performing hand motor task. The fMRI time series data was analysed using statistical parametric mapping, by building block design general linear models which were paradigm prescribed and video based. Introduction of the cameras did not alter the SFNR significantly, nor did it show any signs of spike noise during RF off conditions. Video and EEG quality also did not show any significant artefact. The Statistical Parametric Mapping{T} maps from video based design revealed additional blood oxygen level-dependent responses in the expected locations for non-compliant subjects compared to the paradigm prescribed design. We conclude that video-EEG-fMRI set up can be implemented without affecting the data quality significantly and may provide valuable information on behaviour to enhance the analysis of fMRI data
Reversibility in the Extended Measurement-based Quantum Computation
When applied on some particular quantum entangled states, measurements are
universal for quantum computing. In particular, despite the fondamental
probabilistic evolution of quantum measurements, any unitary evolution can be
simulated by a measurement-based quantum computer (MBQC). We consider the
extended version of the MBQC where each measurement can occur not only in the
(X,Y)-plane of the Bloch sphere but also in the (X,Z)- and (Y,Z)-planes. The
existence of a gflow in the underlying graph of the computation is a necessary
and sufficient condition for a certain kind of determinism. We extend the
focused gflow (a gflow in a particular normal form) defined for the (X,Y)-plane
to the extended case, and we provide necessary and sufficient conditions for
the existence of such normal forms
On the Spin History of the X-ray Pulsar in Kes 73: Further Evidence For an Utramagnetized Neutron Star
In previous papers, we presented the discovery of a 12-s X-ray pulsar in the
supernova remnant Kes 73, providing the first direct evidence for an
ultramagnetized neutron star, a magnetar, with an equivalent dipole field of
nearly twenty times the quantum critical magnetic field. Our conclusions were
based on two epochs of measurement of the spin, along with an age estimate of
the host supernova remnant. Herein, we present a spin chronology of the pulsar
using additional GINGA, ASCA, XTE, & SAX datasets spanning over a decade.
Timing and spectral analysis confirms our initial results and severely limit an
accretion origin for the observed flux. Over the 10 year baseline, the pulsar
is found to undergo a rapid, constant spindown, while maintaining a steady flux
and an invariant pulse profile. Within the measurement uncertainties, no
systematic departures from a linear spin-down are found - departures as in the
case of glitches or simply stochastic fluctuations in the pulse
times-of-arrival (e.g. red timing noise). We suggest that this pulsar is akin
to the soft gamma-ray repeaters, however, it is remarkably stable and has yet
to display similar outbursts; future gamma-ray activity from this object is
likely.Comment: 6 pages with 3 embedded figures, LaTex, emulateapj.sty. Submitted to
the ApJ Letter
Heat transport in Bi_{2+x}Sr_{2-x}CuO_{6+\delta}: departure from the Wiedemann-Franz law in the vicinity of the metal-insulator transition
We present a study of heat transport in the cuprate superconductor
Bi_{2+x}Sr_{2-x}CuO_{6+\delta} at subkelvin temperatures and in magnetic fields
as high as 25T. In several samples with different doping levels close to
optimal, the linear-temperature term of thermal conductivity was measured both
at zero-field and in presence of a magnetic field strong enough to quench
superconductivity. The zero-field data yields a superconducting gap of
reasonable magnitude displaying a doping dependence similar to the one reported
in other families of cuprate. The normal-state data together with the results
of the resistivity measurements allows us to test the Wiedemann-Franz(WF) law,
the validity of which was confirmed in an overdoped sample in agreement with
previous studies. In contrast, a systematic deviation from the WF law was
resolved for samples displaying either a lower doping content or a higher
disorder. Thus, in the vicinity of the metal-insulator cross-over, heat
conduction in the zero-temperature limit appears to become significantly larger
than predicted by the WF law. Possible origins of this observation are
discussed.Comment: 9 pages including 7 figures, submitted to Phys. Rev.
Influence of the photon - neutrino processes on magnetar cooling
The photon-neutrino processes ,
and are investigated
in the presence of a strongly magnetized and dense electron-positron plasma.
The amplitudes of the reactions and
are obtained. In the case of a cold degenerate
plasma contributions of the considering processes to neutrino emissivity are
calculated. It is shown that contribution of the process to neutrino emissivity is supressed in comparision with the
contributions of the processes and
. The constraint on the magnetic field strength in the
magnetar outer crust is obtained.Comment: 8 pages, LaTeX, 2 PS figures, based on the talk presented by D.A.
Rumyantsev at the XV International Seminar Quarks'2008, Sergiev Posad, Moscow
Region, May 23-29, 2008, to appear in the Proceeding
The Fission Fragment Rocket Engine for Mars Fast Transit
In this paper we discuss the advantages and challenges of utilizing Fission
Fragment Rocket Engines (FFREs) to dramatically reduce transit time in space
travel, for example, traveling to Mars. We discuss methods to decrease the size
and weight of FFREs. These include utilizing metallic deuterides as moderators,
driving the engines with electron beam bremsstrahlung, and operating the FFREs
as subcritical assemblies, not as nuclear reactors. We discuss these and other
new innovations based upon improved materials and technology that may be
integrated into a revolutionary nuclear rocket technology.Comment: 10 pages, 2 figures, 2 table
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