137 research outputs found
Vortex dynamics and second magnetization peak in PrFeAsOF superconductor
We have studied the vortex dynamics in the PrFeAsOF
superconducting sample by dc magnetization and dynamic magnetization-relaxation
rate measurements. The field dependence of the superconducting
irreversible magnetization reveals a second magnetization peak or
fishtail effect. The large value of is an indication of moderate vortex
motion and relatively weak pinning energy. Data analysis based on the
generalized inversion scheme suggests that the vortex dynamics can be described
by the collective pinning model. The temperature dependence of the critical
current is consistent with the pinning due to the spatial variation in the mean
free path near a lattice defect ( pinning). The temperature and field
dependence of indicates a crossover from elastic to plastic vortex creep
with increasing temperature and magnetic field. Finally, we have constructed
the vortex phase diagram based on the present data.Comment: 11 pages, 8 Figures, Accepted for publication in Journal of Applied
Physic
Fe-spin reorientation in PrFeAsO : Evidences from resistivity and specific heat studies
We report the magnetic field dependence of resistivity () and specific
heat () for the non-superconducting PrFeAsO compound. Our study shows a
hitherto unobserved anomaly at in the resistivity and specific heat
data which arises as a result of the interplay of antiferromagnetic (AFM) Pr
and Fe sublattices. Below the AFM transition temperature (), Pr
moment orders along the crystallographic c axis and its effect on the iron
subsystem causes a reorientation of the ordered inplane Fe moments in a
direction out of the plane. Application of magnetic field introduces
disorder in the AFM Pr sublattice, which, in turn, reduces the out-of-plane
Pr-Fe exchange interaction responsible for Fe spin reorientation. Both in
() and curves, the peak at broadens with the
increase of due to the introduction of the disorder in the AFM Pr
sublattice by magnetic field. In () curve, the peak shifts towards
lower temperature with and disappears above 6 T while in curve
the peak remains visible up to 14 T. The broadening of the anomaly at
in with increasing further confirms that magnetic
field induces disorder in the AFM Pr sublattice.Comment: 8 pages, 10 Figure
Quantm Magnetoresistance of the PrFeAsO oxypnictides
We report the observation of an unusual dependence of transverse
magnetoresistance (MR) in the PrFeAsO, one of the parent compound of pnictide
superconductors. Below the spin density wave transition, MR is large, positive
and increases with decreasing temperature. At low temperatures, MR increases
linearly with up to 14 T. For 40 K, MR vs curve develops a
weak curvature in the low-field region which indicates a crossover from
linear to dependence as 0. The linear MR originates
from the Dirac cone states and has been explained by the quantum mechanical
model proposed by Abrikosov.Comment: accepted for publication in Appl. Phys. Let
Machine learning with ensemble stacking model for automated sleep staging using dual-channel EEG signal
Abstract Sleep staging is an important part of diagnosing the different types of sleep-related disorders because any discrepancies in the sleep scoring process may cause serious health problems such as misinterpretations of sleep patterns, medication errors, and improper diagnosis. The best way of analyzing sleep staging is visual interpretations of the polysomnography (PSG) signals recordings from the patients, which is a quite tedious task, requires more domain experts, and time-consuming process. This proposed study aims to develop a new automated sleep staging system using the brain EEG signals. Based on a new automated sleep staging system based on an ensemble learning stacking model that integrates Random Forest (RF) and eXtreme Gradient Boosting (XGBoosting). Additionally, this proposed methodology considers the subjects' age, which helps analyze the S1 sleep stage properly. In this study, both linear (time and frequency) and non-linear features are extracted from the pre-processed signals. The most relevant features are selected using the ReliefF weight algorithm. Finally, the selected features are classified through the proposed two-layer stacking model. The proposed methodology performance is evaluated using the two most popular datasets, such as the Sleep-EDF dataset (S-EDF) and Sleep Expanded-EDF database (SE-EDF) under the Rechtschaffen & Kales (R&K) sleep scoring rules. The performance of the proposed method is also compared with the existing published sleep staging methods. The comparison results signify that the proposed sleep staging system has an excellent improvement in classification accuracy for the six-two sleep states classification. In the S-EDF dataset, the overall accuracy and Cohen's kappa coefficient score obtained by the proposed model is (91.10%, 0.87) and (90.68%, 0.86) with inclusion and exclusion of age feature using the Fpz-Cz channel, respectively. Similarly, the Pz-Oz channel's performance is (90.56%, 0.86) with age feature and (90.11%, 0.86) without age feature. The performed results with the SE-EDF dataset using Fpz-Cz channel is (81.32%, 0.77) and (81.06%, 0.76), using Pz-Oz channel with the inclusion and exclusion of the age feature, respectively. Similarly the model achieved an overall accuracy of 96.67% (CT-6), 96.60% (CT-5), 96.28% (CT-4),96.30% (CT-3) and 97.30% (CT-2) for with 16 selected features using S-EDF database. Similarly the model reported an overall accuracy of 85.85%, 84.98%, 85.51%, 85.37% and 87.40% for CT-6 to CT-2 with 18 selected features using SE-EDF database
Anomalous thermal expansion of SbTe topological insulator
We have investigated the temperature dependence of the linear thermal
expansion along the hexagonal c axis (), in-plane resistivity
(), and specific heat () of the topological insulator SbTe
single crystal. exhibits a clear anomaly in the temperature region
204-236 K. The coefficient of linear thermal expansion decreases
rapidly above 204 K, passes through a deep minimum at around 225 K and then
increases abruptly in the region 225-236 K. is negative in the
interval 221-228 K. The temperature dependence of both and can
be described well by the Debye model from 2 to 290 K, excluding the region
around the anomaly in
The magnetization of PrFeAsOF$_{0.12} sueprconductor
The magnetization of the PrFeAsOF polycrystalline sample
has been measured as functions of temperature and magnetic field . The
observed total magnetization is the sum of a superconducting irreversible
magnetization () and a paramagnetic magnetization (). Analysis of dc
susceptibility in the normal state shows that the paramagnetic
component of magnetization comes from the Pr magnetic moments. The
intragrain critical current density derived from the magnetization
measurement is large. The curve displays a second peak which shifts
towards the high-field region with decreasing temperature. In the low-field
region, a plateau up to a field followed by a power law
behavior of is the characteristic of the strong pinning. A vortex
phase diagram for the present superconductor has been obtained from the
magnetization and resistivity data.Comment: A revised version with modified title,8 pages, 7 figure
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