2,269 research outputs found
Pump-Probe Experiments on the Single-Molecule Magnet Fe8 : Measurement of Excited Level Lifetimes
We present magnetization measurements on the single molecule magnet Fe8 in
the presence of pulsed microwave radiation. A pump-probe technique is used with
two microwave pulses with frequencies of 107 GHz and 118 GHz and pulse lengths
of several nanoseconds to study the spin dynamics via time-resolved
magnetization measurements using a Hall probe magnetometer. We find evidence
for short spin-phonon relaxation times of the order of one microsecond. The
temperature dependence of the spin-phonon relaxation time in our experiments is
in good agreement with previously published theoretical results. We also
established the presence of very short energy diffusion times, that act on a
timescale of about 70 ns.Comment: submitted to Phys. Rev. Lett. (01 March 2007
An international survey of stress tests
In the summer of 2000, central banks from the Group of Ten countries surveyed large international banks about their use of stress tests_a risk management tool that measures a firm's exposure to extreme movements in asset prices. The survey findings highlight the risks that most concern financial institutions and clarify how these institutions use stress tests in their overall risk management programs.Risk management ; Risk assessment ; Financial services industry
Period spacings in red giants I. Disentangling rotation and revealing core structure discontinuities
Asteroseismology allows us to probe the physical conditions inside the core
of red giant stars. This relies on the properties of the global oscillations
with a mixed character that are highly sensitive to the physical properties of
the core. However, overlapping rotational splittings and mixed-mode spacings
result in complex structures in the mixed-mode pattern, which severely
complicates its identification and the measurement of the asymptotic period
spacing. This work aims at disentangling the rotational splittings from the
mixed-mode spacings, in order to open the way to a fully automated analysis of
large data sets. An analytical development of the mixed-mode asymptotic
expansion is used to derive the period spacing between two consecutive mixed
modes. The \'echelle diagrams constructed with the appropriately stretched
periods are used to exhibit the structure of the gravity modes and of the
rotational splittings. We propose a new view on the mixed-mode oscillation
pattern based on corrected periods, called stretched periods, that mimic the
evenly spaced gravity-mode pattern. This provides a direct understanding of all
oscillation components, even in the case of rapid rotation. The measurement of
the asymptotic period spacing and the signature of the structural glitches on
mixed modes are then made easy. This work opens the possibility to derive all
seismic global parameters in an automated way, including the identification of
the different rotational multiplets and the measurement of the rotational
splitting, even when this splitting is significantly larger than the period
spacing. Revealing buoyancy glitches provides a detailed view on the radiative
core.Comment: Accepted in A&
Probing the core structure and evolution of red giants using gravity-dominated mixed modes observed with Kepler
We report for the first time a parametric fit to the pattern of the \ell = 1
mixed modes in red giants, which is a powerful tool to identify
gravity-dominated mixed modes. With these modes, which share the
characteristics of pressure and gravity modes, we are able to probe directly
the helium core and the surrounding shell where hydrogen is burning. We propose
two ways for describing the so-called mode bumping that affects the frequencies
of the mixed modes. Firstly, a phenomenological approach is used to describe
the main features of the mode bumping. Alternatively, a quasi-asymptotic
mixed-mode relation provides a powerful link between seismic observations and
the stellar interior structure. We used period \'echelle diagrams to emphasize
the detection of the gravity-dominated mixed modes. The asymptotic relation for
mixed modes is confirmed. It allows us to measure the gravity-mode period
spacings in more than two hundred red giant stars. The identification of the
gravity-dominated mixed modes allows us to complete the identification of all
major peaks in a red giant oscillation spectrum, with significant consequences
for the true identification of \ell = 3 modes, of \ell = 2 mixed modes, for the
mode widths and amplitudes, and for the \ell = 1 rotational splittings. The
accurate measurement of the gravity-mode period spacing provides an effective
probe of the inner, g-mode cavity. The derived value of the coupling
coefficient between the cavities is different for red giant branch and clump
stars. This provides a probe of the hydrogen-shell burning region that
surrounds the helium core. Core contraction as red giants ascend the red giant
branch can be explored using the variation of the gravity-mode spacing as a
function of the mean large separation.Comment: Accepted in A&
Asteroseismic surface gravity for evolved stars
Context: Asteroseismic surface gravity values can be of importance in
determining spectroscopic stellar parameters. The independent log(g) value from
asteroseismology can be used as a fixed value in the spectroscopic analysis to
reduce uncertainties due to the fact that log(g) and effective temperature can
not be determined independently from spectra. Since 2012, a combined analysis
of seismically and spectroscopically derived stellar properties is ongoing for
a large survey with SDSS/APOGEE and Kepler. Therefore, knowledge of any
potential biases and uncertainties in asteroseismic log(g) values is now
becoming important. Aims: The seismic parameter needed to derive log(g) is the
frequency of maximum oscillation power (nu_max). Here, we investigate the
influence of nu_max derived with different methods on the derived log(g)
values. The large frequency separation between modes of the same degree and
consecutive radial orders (Dnu) is often used as an additional constraint for
the determination of log(g). Additionally, we checked the influence of small
corrections applied to Dnu on the derived values of log(g). Methods We use
methods extensively described in the literature to determine nu_max and Dnu
together with seismic scaling relations and grid-based modeling to derive
log(g). Results: We find that different approaches to derive oscillation
parameters give results for log(g) with small, but different, biases for
red-clump and red-giant-branch stars. These biases are well within the quoted
uncertainties of ~0.01 dex (cgs). Corrections suggested in the literature to
the Dnu scaling relation have no significant effect on log(g). However somewhat
unexpectedly, method specific solar reference values induce biases of the order
of the uncertainties, which is not the case when canonical solar reference
values are used.Comment: 8 pages, 5 figures, accepted for publication by A&
The connection between stellar granulation and oscillation as seen by the Kepler mission
The long and almost continuous observations by Kepler show clear evidence of
a granulation background signal in a large sample of stars, which is
interpreted as the surface manifestation of convection. It has been shown that
its characteristic timescale and rms intensity fluctuation scale with the peak
frequency (\nu_{max}) of the solar-like oscillations. Various attempts have
been made to quantify the observed signal, to determine scaling relations, and
to compare them to theoretical predictions. We use a probabilistic method to
compare different approaches to extracting the granulation signal. We fit the
power density spectra of a large set of Kepler targets, determine the
granulation and global oscillation parameter, and quantify scaling relations
between them. We establish that a depression in power at about \nu_{max}/2,
known from the Sun and a few other main-sequence stars, is also statistically
significant in red giants and that a super-Lorentzian function with two
components is best suited to reproducing the granulation signal in the broader
vicinity of the pulsation power excess. We also establish that the specific
choice of the background model can affect the determination of \nu_{max},
introducing systematic uncertainties that can significantly exceed the random
uncertainties. We find the characteristic background frequency and amplitude to
tightly scale with \nu_{max} for a wide variety of stars, and quantify a mass
dependency of the latter. To enable comparison with theoretical predictions, we
computed effective timescales and intensity fluctuations and found them to
approximately scale as \tau_{eff} \propto g^{-0.85}\,T^{-0.4} and A_{gran}
\propto (g^2M)^{-1/4}, respectively. Similarly, the bolometric pulsation
amplitude scales approximately as A_{puls} \propto (g^2M)^{-1/3}, which
implicitly verifies a separate mass and luminosity dependence of A_{puls}.Comment: 18 pages, 12 figures, accepted for A&
Seismic evidence for a weak radial differential rotation in intermediate-mass core helium burning stars
The detection of mixed modes that are split by rotation in Kepler red giants
has made it possible to probe the internal rotation profiles of these stars,
which brings new constraints on the transport of angular momentum in stars.
Mosser et al. (2012) have measured the rotation rates in the central regions of
intermediate-mass core helium burning stars (secondary clump stars). Our aim
was to exploit& the rotational splittings of mixed modes to estimate the amount
of radial differential rotation in the interior of secondary clump stars using
Kepler data, in order to place constraints on angular momentum transport in
intermediate-mass stars. We selected a subsample of Kepler secondary clump
stars with mixed modes that are clearly rotationally split. By applying a
thorough statistical analysis, we showed that the splittings of both
gravity-dominated modes (trapped in central regions) and p-dominated modes
(trapped in the envelope) can be measured. We then used these splittings to
estimate the amount of differential rotation by using inversion techniques and
by applying a simplified approach based on asymptotic theory (Goupil et al.
2013). We obtained evidence for a weak radial differential rotation for six of
the seven targets that were selected, with the central regions rotating
to times faster than the envelope. The last target was
found to be consistent with a solid-body rotation. This demonstrates that an
efficient redistribution of angular momentum occurs after the end of the main
sequence in the interior of intermediate-mass stars, either during the
short-lived subgiant phase, or once He-burning has started in the core. In
either case, this should bring constraints on the angular momentum transport
mechanisms that are at work.Comment: 16 pages, 8 figures, accepted in A&
Influence of antisymmetric exchange interaction on quantum tunneling of magnetization in a dimeric molecular magnet Mn6
We present magnetization measurements on the single molecule magnet Mn6,
revealing various tunnel transitions inconsistent with a giant-spin
description. We propose a dimeric model of the molecule with two coupled spins
S=6, which involves crystal-field anisotropy, symmetric Heisenberg exchange
interaction, and antisymmetric Dzyaloshinskii-Moriya exchange interaction. We
show that this simplified model of the molecule explains the experimentally
observed tunnel transitions and that the antisymmetric exchange interaction
between the spins gives rise to tunneling processes between spin states
belonging to different spin multiplets.Comment: 5 pages, 4 figure
Magnetization dynamics in the single-molecule magnet Fe8 under pulsed microwave irradiation
We present measurements on the single molecule magnet Fe8 in the presence of
pulsed microwave radiation at 118 GHz. The spin dynamics is studied via time
resolved magnetization experiments using a Hall probe magnetometer. We
investigate the relaxation behavior of magnetization after the microwave pulse.
The analysis of the experimental data is performed in terms of different
contributions to the magnetization after-pulse relaxation. We find that the
phonon bottleneck with a characteristic relaxation time of 10 to 100 ms
strongly affects the magnetization dynamics. In addition, the spatial effect of
spin diffusion is evidenced by using samples of different sizes and different
ways of the sample's irradiation with microwaves.Comment: 14 pages, 12 figure
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