3,599 research outputs found
Study of the Mechanisms of Flux Pinning in Type 2 Superconductors
Flux pinning mechanisms in type-2 semiconductors and specific heat measurements on annealed and deformed pure niobium sample
Thermally activated escape rates of uniaxial spin systems with transverse field
Classical escape rates of uniaxial spin systems are characterized by a
prefactor differing from and much smaller than that of the particle problem,
since the maximum of the spin energy is attained everywhere on the line of
constant latitude: theta=const, 0 =< phi =< 2*pi. If a transverse field is
applied, a saddle point of the energy is formed, and high, moderate, and low
damping regimes (similar to those for particles) appear. Here we present the
first analytical and numerical study of crossovers between the uniaxial and
other regimes for spin systems. It is shown that there is one HD-Uniaxial
crossover, whereas at low damping the uniaxial and LD regimes are separated by
two crossovers.Comment: 4 PR pages, 3 figures, final published versio
Field dependence of the temperature at the peak of the ZFC magnetization
The effect of an applied magnetic field on the temperature at the maximum of
the ZFC magnetization, , is studied using the recently obtained
analytic results of Coffey et al. (Phys. Rev. Lett. {\bf 80}(1998) 5655) for
the prefactor of the N\'{e}el relaxation time which allow one to precisely
calculate the prefactor in the N\'{e}el-Brown model and thus the blocking
temperature as a function of the coefficients of the Taylor series expansion of
the magnetocrystalline anisotropy. The present calculations indicate that even
a precise determination of the prefactor in the N\'{e}el-Brown theory, which
always predicts a monotonic decrease of the relaxation time with increasing
field, is insufficient to explain the effect of an applied magnetic field on
the temperature at the maximum of the ZFC magnetization. On the other hand, we
find that the non linear field-dependence of the magnetization along with the
magnetocrystalline anisotropy appears to be of crucial importance to the
existence of this maximum.Comment: 14 LaTex209 pages, 6 EPS figures. To appear in J. Phys.: Condensed
Matte
Monte Carlo simulation with time step quantification in terms of Langevin dynamics
For the description of thermally activated dynamics in systems of classical
magnetic moments numerical methods are desirable. We consider a simple model
for isolated magnetic particles in a uniform field with an oblique angle to the
easy axis of the particles. For this model, a comparison of the Monte Carlo
method with Langevin dynamics yields new insight in the interpretation of the
Monte Carlo process, leading to the implementation of a new algorithm where the
Monte Carlo step is time-quantified. The numeric results for the characteristic
time of the magnetisation reversal are in excellent agreement with asymptotic
solutions which itself are in agreement with the exact numerical results
obtained from the Fokker-Planck equation for the Neel-Brown model.Comment: 5 pages, Revtex, 4 Figures include
Integral Relaxation Time of Single-Domain Ferromagnetic Particles
The integral relaxation time \tau_{int} of thermoactivating noninteracting
single-domain ferromagnetic particles is calculated analytically in the
geometry with a magnetic field H applied parallel to the easy axis. It is shown
that the drastic deviation of \tau_{int}^{-1} from the lowest eigenvalue of the
Fokker-Planck equation \Lambda_1 at low temperatures, starting from some
critical value of H, is the consequence of the depletion of the upper potential
well. In these conditions the integral relaxation time consists of two
competing contributions corresponding to the overbarrier and intrawell
relaxation processes.Comment: 8 pages, 3 figure
Physical properties of the jet from DG Tauri on sub-arcsecond scales with HST/STIS
We derive the physical properties at the base of the jet from DG Tau both
along and across the flow and as a function of velocity. We analysed seven
optical spectra of the DG Tau jet, taken with the Hubble Space Telescope
Imaging Spectrograph. The spectra were obtained by placing a long-slit parallel
to the jet axis and stepping it across the jet width. The resulting
position-velocity diagrams in optical forbidden emission lines allowed access
to plasma conditions via calculation of emission line ratios.
We find at the base of the jet high electron density, 10, and
very low ionisation, , which combine to give a total
density up to 3 10. This analysis confirms previous reports of
variations in plasma parameters along the jet, (i.e. decrease in density by
several orders of magnitude, increase of from 0.05 to a plateau at 0.7
downstream at 2 from the star). Furthermore, a spatial coincidence is
revealed between sharp gradients in the total density and supersonic velocity
jumps. This strongly suggests that the emission is caused by shock excitation.
The position-velocity diagrams indicate the presence of both fast accelerating
gas and slower, less collimated material. We derive the mass outflow rate,
, in the blue-shifted lobe in different velocity channels, that
contribute to a total of 8 4 10 M
yr. We estimate that a symmetric bipolar jet would transport at the low
and intermediate velocities probed by rotation measurements, an angular
momentum flux of 2.9 1.5 10 M yr
AU km s.
The derived properties of the DG Tau jet are demonstrated to be consistent
with magneto-centrifugal theory. However, non-stationary modelling is required
in order to explain all of the features revealed at high resolution.Comment: 16 pages, 18 figure
Quantum Thermoactivation of Nanoscale Magnets
The integral relaxation time describing the thermoactivated escape of a
uniaxial quantum spin system interacting with a boson bath is calculated
analytically in the whole temperature range. For temperatures T much less than
the barrier height \Delta U, the level quantization near the top of the barrier
and the strong frequency dependence of the one-boson transition probability can
lead to the regularly spaced deep minima of the thermoactivation rate as a
function of the magnetic field applied along the z axis.Comment: 4 pages, no figures, rejected from Phys. Rev. Let
Investigating Biological Matter with Theoretical Nuclear Physics Methods
The internal dynamics of strongly interacting systems and that of
biomolecules such as proteins display several important analogies, despite the
huge difference in their characteristic energy and length scales. For example,
in all such systems, collective excitations, cooperative transitions and phase
transitions emerge as the result of the interplay of strong correlations with
quantum or thermal fluctuations. In view of such an observation, some
theoretical methods initially developed in the context of theoretical nuclear
physics have been adapted to investigate the dynamics of biomolecules. In this
talk, we review some of our recent studies performed along this direction. In
particular, we discuss how the path integral formulation of the molecular
dynamics allows to overcome some of the long-standing problems and limitations
which emerge when simulating the protein folding dynamics at the atomistic
level of detail.Comment: Prepared for the proceedings of the "XII Meeting on the Problems of
Theoretical Nuclear Physics" (Cortona11
Exploring the dimming event of RW Aur A through multi-epoch VLT/X-Shooter spectroscopy
RW Aur A is a CTTS that has suddenly undergone three major dimming events
since 2010. We aim to understand the dimming properties, examine accretion
variability, and derive the physical properties of the inner disc traced by the
CO ro-vibrational emission at NIR wavelengths (2.3 mic).
We compared two epochs of X-Shooter observations, during and after the
dimming. We modelled the rarely detected CO bandhead emission in both epochs to
examine whether the inner disc properties had changed. The SED was used to
derive the extinction properties of the dimmed spectrum and compare the
infrared excess between the two epochs. Lines tracing accretion were used to
derive the mass accretion rate in both states. The CO originates from a region
with physical properties of T=3000 K, N=1x10 cm and
vsini=113 km/s. The extinction properties of the dimming layer were derived
with the effective optical depth ranging from teff 2.5-1.5 from the UV to the
NIR. The inferred mass accretion rate Macc is Msun/yr and Msun/yr after and during the dimming respectively. By fitting the
SED, additional emission is observed in the IR during the dimming event from
dust grains with temperatures of 500-700K. The physical conditions traced by
the CO are similar for both epochs, indicating that the inner gaseous disc
properties do not change during the dimming events. The extinction curve is
flatter than that of the ISM, and large grains of a few hundred microns are
thus required. When we correct for the observed extinction, Macc is constant in
the two epochs, suggesting that the accretion is stable and therefore does not
cause the dimming. The additional hot emission in the NIR is located at about
0.5 au from the star. The dimming events could be due to a dust-laden wind, a
severe puffing-up of the inner rim, or a perturbation caused by the recent
star-disc encounter.Comment: Accepted by Astronomy & Astrophysic
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