3,956 research outputs found
Time domain study of frequency-power correlation in spin-torque oscillators
This paper describes a numerical experiment, based on full micromagnetic
simulations of current-driven magnetization dynamics in nanoscale spin valves,
to identify the origins of spectral linewidth broadening in spin torque
oscillators. Our numerical results show two qualitatively different regimes of
magnetization dynamics at zero temperature: regular (single-mode precessional
dynamics) and chaotic. In the regular regime, the dependence of the oscillator
integrated power on frequency is linear, and consequently the dynamics is well
described by the analytical theory of current-driven magnetization dynamics for
moderate amplitudes of oscillations. We observe that for higher oscillator
amplitudes, the functional dependence of the oscillator integrated power as a
function of frequency is not a single-valued function and can be described
numerically via introduction of nonlinear oscillator power. For a range of
currents in the regular regime, the oscillator spectral linewidth is a linear
function of temperature. In the chaotic regime found at large current values,
the linewidth is not described by the analytical theory. In this regime we
observe the oscillator linewidth broadening, which originates from sudden jumps
of frequency of the oscillator arising from random domain wall nucleation and
propagation through the sample. This intermittent behavior is revealed through
a wavelet analysis that gives superior description of the frequency jumps
compared to several other techniques.Comment: 11 pages, 4 figures to appear in PR
What can the SNO Neutral Current Rate teach us about the Solar Neutrino Anomaly
We investigate how the anticipated neutral current rate from will
sharpen our understanding of the solar neutrino anomaly. Quantitative analyses
are performed with representative values of this rate in the expected range of
. This would provide a signal for transition
into a state containing an active neutrino component. Assuming this state to be
purely active one can estimate both the neutrino flux and the
survival probability to a much higher precision than currently possible.
Finally the measured value of the rate will have profound implications for
the mass and mixing parameters of the solar neutrino oscillation solution.Comment: Brief discussion on the first NC result from SNO added; final version
to be published in the MPL
A graphene transmon operating at 1 T
A superconducting transmon qubit resilient to strong magnetic fields is an
important component for proposed topological and hybrid quantum computing (QC)
schemes. Transmon qubits consist of a Josephson junction (JJ) shunted by a
large capacitance, coupled to a high quality factor superconducting resonator.
In conventional transmon devices, the JJ is made from an Al/AlO/Al tunnel
junction which ceases operation above the critical magnetic field of Al, 10 mT.
Alternative junction technologies are therefore required to push the operation
of these qubits into strong magnetic fields. Graphene JJs are one such
candidate due to their high quality, ballistic transport and electrically
tunable critical current densities. Importantly the monolayer structure of
graphene protects the JJ from orbital interference effects that would otherwise
inhibit operation at high magnetic field. Here we report the integration of
ballistic graphene JJs into microwave frequency superconducting circuits to
create the first graphene transmons. The electric tunability allows the
characteristic band dispersion of graphene to be resolved via dispersive
microwave spectroscopy. We demonstrate that the device is insensitive to the
applied field and perform energy level spectroscopy of the transmon at 1 T,
more than an order of magnitude higher than previous studies.Comment: attached supplementary materia
Finding cool subdwarfs using a V-J reduced proper-motion diagram: Stellar parameters for 91 candidates
We present the results of a search for cool subdwarfs for which our
candidates were drawn from a V-J reduced proper-motion diagram constructed by
Salim & Gould (2002). Kinematic (U, V, and W) and self-consistent stellar
parameters (Teff, log g, [Fe/H], and V_t) are derived for 91 candidate
subdwarfs based on high resolution spectra. The observed stars span 3900K <
Teff < 6200K and -2.63 < [Fe/H] < 0.25 including only 3 giants (log g < 4.0).
Of the sample, 77 stars have MgH lines present in their spectra. With more than
56% of our candidate subdwarfs having [Fe/H] < -1.5, we show that the V-J
reduced proper-motion diagram readily identifies metal-poor stars.Comment: PASP (in press
Impact of very massive stars on the chemical evolution of extremely metal-poor galaxies
Context. In recent observations of extremely metal-poor, low-mass, starburst galaxies, almost solar Fe/O ratios are reported, despite N/O ratios consistent with the low metallicity.
Aims: We aim to investigate if the peculiar Fe/O ratios can be a distinctive signature of an early enrichment produced by very massive objects dying as pair-instability supernova (PISN).
Methods: We ran chemical evolution models with yields that account for the contribution by PISN. We used both the non-rotating stellar yields from a recent study and new yields from rotating very massive stars calculated specifically for this work. We also searched for the best initial mass function (IMF) that is able to reproduce the observations.
Results: We can reproduce the observations by adopting a bi-modal IMF and by including an initial burst of rotating very massive stars. Only with a burst of very massive stars can we reproduce the almost solar Fe/O ratios at the estimated young ages. We also confirm that rotation is absolutely needed to concomitantly reproduce the observed N/O ratios.
Conclusions: These results stress the importance of very massive stars in galactic chemical evolution studies and strongly support a top-heavy initial mass function in the very early evolutionary stages of metal-poor starburst galaxies
Phenomenology of Neutrino Oscillations
The phenomenology of solar, atmospheric, supernova and laboratory neutrino
oscillations is described. Analytical formulae for matter effects are reviewed.
The results from oscillations are confronted with neutrinoless double beta
decay.Comment: 11 pages, 2 figures, latex, Plenary talk given at Workshop in High
Energy Particle Physics-6, Chennai, Indi
Muon-anti-neutrino <---> electron-anti-neutrino mixing: analysis of recent indications and implications for neutrino oscillation phenomenology
We reanalyze the recent data from the Liquid Scintillator Neutrino Detector
(LSND) experiment, that might indicate anti-nu_muanti-nu_e mixing. This
indication is not completely excluded by the negative results of established
accelerator and reactor neutrino oscillation searches. We quantify the region
of compatibility by means of a thorough statistical analysis of all the
available data, assuming both two-flavor and three-flavor neutrino
oscillations. The implications for various theoretical scenarios and for future
oscillation searches are studied. The relaxation of the LSND constraints under
different assumptions in the statistical analysis is also investigated.Comment: 17 pages (RevTeX) + 9 figures (Postscript) included with epsfig.st
The evolutionary status of the semiregular variable QYSge
Repeated spectroscopic observations made with the 6m telescope of yielded new
data on the radial-velocity variability of the anomalous yellow supergiant
QYSge. The strongest and most peculiar feature in its spectrum is the complex
profile of NaI D lines, which contains a narrow and a very wide emission
components. The wide emission component can be seen to extend from -170 to +120
km/s, and at its central part it is cut by an absorption feature, which, in
turn, is split into two subcomponents by a narrow (16km/s at r=2.5) emission
peak. An analysis of all the Vr values leads us to adopt for the star a
systemic velocity of Vr=-21.1 km/s, which corresponds to the position of the
narrow emission component of NaI. The locations of emission-line features of
NaI D lines are invariable, which point to their formation in regions that are
external to the supergiant's photosphere. Differential line shifts of about
10km/s are revealed. The absorption lines in the spectrum of QYSge have a
substantial width of FWHM~45 km/s. The method of model atmospheres is used to
determine the following parameters: Teff=6250K, lg g=2.0, and microturbulence
Vt=4.5km/s. The metallicity of the star is found to be somewhat higher than the
solar one with an average overabundance of iron-peak elements of [Met/H]=+0.20.
The star is found to be slightly overabundant in carbon and nitrogen,
[C/Fe]=+0.25, [N/Fe]=+0.27. The alpha-process elements Mg, Si, and Ca are
slightly overabundant [alpha/H]=+0.12. The strong sodium excess, [Na/Fe]=+0.75,
is likely to be due to the dredge-up of the matter processed in the NeNa cycle.
Heavy elements of the s-process are underabundant relative to the Sun. On the
whole, the observed properties of QYSge do not give grounds for including this
star into the group of RCrB or RVTau-type type objects.Comment: 29 pages, 8 figures, 4 tables; accepted by Astrophys. Bulleti
Neutron-proton pairing in the BCS approach
We investigate the BCS treatment of neutron-proton pairing involving
time-reversed orbits. We conclude that an isospin-symmetric hamiltonian,
treated with the help of the generalized Bogolyubov transformation, fails to
describe the ground state pairing properties correctly. In order for the np
isovector pairs to coexist with the like-particle pairs, one has to break the
isospin symmetry of the hamiltonian by artificially increasing the strength of
np pairing interaction above its isospin symmetric value. We conjecture that
the np isovector pairing represents part (or most) of the congruence energy
(Wigner term) in nuclear masses.Comment: 9 pages, RevTex, submitted to Phys. Rev.
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