1,302 research outputs found
Spin-glass like dynamics of ferromagnetic clusters in LaBaCoO
We report the magnetization study of the compound
LaBaCoO where Ba doping is just above the critical
limit for percolation of ferromagnetic clusters. The field cooled (FC) and zero
field cooled (ZFC) magnetization exhibit a thermomagnetic irreversibility and
the ac susceptibility show a frequency dependent peak at the ferromagnetic
ordering temperature (T203~K) of the clusters. These features
indicate about the presence of a non-equilibrium state below T. In the
non-equilibrium state, the dynamic scaling of the imaginary part of ac
susceptibility and the static scaling of the nonlinear susceptibility clearly
establish a spin-glass like cooperative freezing of ferromagnetic clusters at
200.9(2)~K. The existence of spin-glass like freezing of ferromagnetic clusters
is further substantiated by the ZFC aging and memory experiments. We also
observe certain dynamical features which are not present in a typical
spin-glass, such as, initial magnetization after ZFC aging first increases and
then decreases with the wait time and an imperfect recovery of relaxation in
negative temperature cycling experiments. This imperfect recovery transforms to
perfect recovery on concurrent field cycling. Our analysis suggests that these
additional dynamical features have their origin in inter-cluster exchange
interaction and cluster size distribution. The inter-cluster exchange
interaction above the magnetic percolation gives a superferromagnetic state in
some granular thin films but our results show the absence of typical
superferromagnetic like state in LaBaCoO.Comment: 10 pages, 8 figure
Large linear magnetoresistance from neutral defects in BiSe
The chalcogenide BiSe can attain the three dimensional (3D) Dirac
semimetal state under the influence of strain and microstrain. Here we report
the presnece of large linear magnetoresistance in such a BiSe crystal.
The magnetoresistance has quadratic form at low fields which crossovers to
linear above 4 T. The temperature dependence of magnetoresistance scales with
carrier mobility and the crossover field scales with inverse of mobility. Our
analysis suggest that the linear magnetoresistance in our system has a
classical origin and arises from the scattering of high mobility 3D Dirac
electrons from crystalline inhomogeneities. We observe that the charged
selenium vacancies are strongly screened by high mobility Dirac electrons and
the neutral crystalline defects are the main scattering center for transport
mechanism. Our analysis suggests that both the resistivity and the
magnetoresistance have their origin in scattering of charge carriers from
neutral defects.Comment: 6 pages, 5 figure
Coexistence of interacting-ferromagnetic and small-antiferromagnetic clusters in LaBaCoO
We report detailed dc magnetization, linear and non-linear ac susceptibility
measurements on the hole doped disordered cobaltite
LaBaCoO. Our results show that the magnetically ordered
state of the system consists of coexisting non-ferromagnetic phases along with
percolating ferromagnetic-clusters. The percolating ferromagnetic-clusters
possibly start a magnetic ordering at the Curie temperature of 201.5(5)~K. The
non-ferromagnetic phases mainly consist of antiferromagnetic-clusters with size
smaller than the ferromagnetic-clusters. Below Curie temperature the system
exhibits an irreversibility in the field cooled and zero field cooled
magnetization and frequency dependence in the peak of ac susceptibility. These
dynamical features indicate towards the possible coexistence of spin-glass
phase along with ferromagnetic-clusters similar to LaSrCoO
(x0.18), but the absence of field divergence in third harmonic of ac
susceptibility and zero field cooled memory clearly rule out any such
possibility. We argue that the spin-glass phase in LaSrCoO
(x0.18) is associated with the presence of incommensurate
antiferromagnetic ordering in non-ferromagnetic phases which is absent in
LaBaCoO. Our analysis show that the observed dynamical
features in LaBaCoO are possibly due to progressive thermal
blocking of ferromagnetic-clusters which is further confirmed by the
Wohlfarth's model of superparamagnetism. The frequency dependence of the peak
of ac susceptibility obeys the Vogel-Fulcher law with s.
This together with the existence of an AT line in H-T space indicates the
presence of significant inter-cluster interaction among these
ferromagnetic-clusters.Comment: 20 pages, 9 figure
A critical examination of magnetic states of LaBaCoO: non-Griffiths phase and interacting ferromagnetic-clusters
We report detailed dc magnetization, linear and non-linear ac susceptibility
measurements on the hole doped disordered cobaltite
LaBaCoO. Our results show that the magnetically ordered
state of the system consists of coexisting non-ferromagnetic phases along with
percolating ferromagnetic-clusters. The percolating ferromagnetic-clusters
possibly undergo a 3D Hisenberg like magnetic ordering at the Curie temperature
of 202(3) K. In between 202 and 220 K, the linear and non-linear ac
susceptibility measurements show the presence of magnetic correlations even
when the spontaneous magnetization is zero which indicates the presence of
preformed short range magnetic-clusters. The characteristics of these short
range magnetic-clusters that exist above Curie temperature are quite distinct
than that of Griffiths phase e.g the inverse dc susceptibility exhibits an
field independent upward deviation, and the second harmonic of ac
susceptibility is non-negative. Below Curie temperature the system exhibit
spin-glass like features such as irreversibility in the field cooled and zero
field cooled magnetization and frequency dependence in the peak of ac
susceptibility. The presence of a spin or cluster -glass like state is ruled
out by the absence of field divergence in third harmonic of ac susceptibility
and zero field cooled memory. This indicates that the observed spin-glass like
features are possibility due to progressive thermal blocking of
ferromagnetic-clusters which is further confirmed by the Wohlfarth's model of
superparamagnetism. The frequency dependence of the peak of ac susceptibility
obeys the Vogel-Fulcher law with s. This together with
the existence of an AT line in H-T space indicates the existence of significant
inter-cluster interaction among these ferromagnetic-clusters.Comment: 10 pages, 10 figure
Observation of three-dimensional Dirac semimetal state in topological insulator Bi2Se3
The three dimensional (3D) topological insulators are predicted to exhibit a
3D Dirac semimetal state in critical regime of topological to trivial phase
transition. Here we demonstrate the first experimental evidence of 3D Dirac
semimetal state in topological insulator Bi2Se3 with bulk carrier concentration
of ~ 10^19 cm^{-3}, using magneto-transport measurements. At low temperatures,
the resistivity of our Bi2Se3 crystal exhibits clear Shubnikov-de Haas (SdH)
oscillations above 6T. The analysis of these oscillations through
Lifshitz-Onsanger and Lifshitz-Kosevich theory reveals a non-trivial pi Berry
phase coming from 3D bands, which is a decisive signature of 3D Dirac semimetal
state. The large value of Dingle temperature and natural selenium vacancies in
our crystal suggest that the observed 3D Dirac semimetal state is an outcome of
enhanced strain field and weaker effective spin-orbit coupling.Comment: 5 pages, 6 figures, This work has been presented at 8th India
Singapore Symposium in Condensed Matter Physics Feb 25-Feb 27, 2015 at Indian
Institute of Technology, Kanpur, Indi
Observation of quantum Hall effect in a microstrained BiSe single crystal
We report the observation of quantum Hall effect (QHE) in a BiSe
single crystal having carrier concentration ()
cm, three dimensional Fermi surface and bulk
transport characteristics. The plateaus in Hall resistivity coincide with
minima of Shubnikov de Haas oscillations in resistivity. Our results
demonstrate that the presence of perfect two dimensional transport is not an
essential condition for QHE in BiSe. The results of high resolution
x-ray diffraction (HRXRD), energy-dispersive x-ray spectroscopy (EDX), and
residual resistivity measurements show the presence of enhanced crystalline
defects and microstrain. We propose that the formation of localized state at
the edge of each Landau level due to resonance between the bulk and defect band
of BiSe causes the quantum Hall effect.Comment: 5 pages, 2 figure
Generation of non-classical optical fields by a beam splitter with second-order nonlinearity
We propose quantum-mechanical model of a beam splitter with second-order
nonlinearity and show that non-classical features such as squeezing and
sub-Poissonian photon statistics of optical fields can be generated in output
fundamental and second harmonic modes when we mix coherent light beams via such
a nonlinear beam splitter
Class Vectors: Embedding representation of Document Classes
Distributed representations of words and paragraphs as semantic embeddings in
high dimensional data are used across a number of Natural Language
Understanding tasks such as retrieval, translation, and classification. In this
work, we propose "Class Vectors" - a framework for learning a vector per class
in the same embedding space as the word and paragraph embeddings. Similarity
between these class vectors and word vectors are used as features to classify a
document to a class. In experiment on several sentiment analysis tasks such as
Yelp reviews and Amazon electronic product reviews, class vectors have shown
better or comparable results in classification while learning very meaningful
class embeddings
Sequence to Sequence Learning for Optical Character Recognition
We propose an end-to-end recurrent encoder-decoder based sequence learning
approach for printed text Optical Character Recognition (OCR). In contrast to
present day existing state-of-art OCR solution which uses connectionist
temporal classification (CTC) output layer, our approach makes minimalistic
assumptions on the structure and length of the sequence. We use a two step
encoder-decoder approach -- (a) A recurrent encoder reads a variable length
printed text word image and encodes it to a fixed dimensional embedding. (b)
This fixed dimensional embedding is subsequently comprehended by decoder
structure which converts it into a variable length text output. Our
architecture gives competitive performance relative to connectionist temporal
classification (CTC) output layer while being executed in more natural
settings. The learnt deep word image embedding from encoder can be used for
printed text based retrieval systems. The expressive fixed dimensional
embedding for any variable length input expedites the task of retrieval and
makes it more efficient which is not possible with other recurrent neural
network architectures. We empirically investigate the expressiveness and the
learnability of long short term memory (LSTMs) in the sequence to sequence
learning regime by training our network for prediction tasks in segmentation
free printed text OCR. The utility of the proposed architecture for printed
text is demonstrated by quantitative and qualitative evaluation of two tasks --
word prediction and retrieval.Comment: 9 pages (including reference), 6 figures (including subfigures), 5
table
Tuning the phase transition dynamics by variation of cooling field and metastable phase fraction in Al doped PrCaMnO
We report the effect of field, temperature and thermal history on the time
dependence in resistivity and magnetization in the phase separated state of Al
doped PrCaMnO. The rate of time dependence in resistivity
is much higher than that of magnetization and it exhibits a different cooling
field dependence due to percolation effects. Our analysis show that the time
dependence in physical properties depends on the phase transition dynamics
which can be effectively tuned by variation of temperature, cooling field and
metastable phase fraction. The phase transition dynamics can be broadly divided
into the arrested and un-arrested regimes, and in the arrested regime, this
dynamics is mainly determined by time taken in the growth of critical nuclei.
An increase in cooling field and/or temperature shifts this dynamics from
arrested to un-arrested regime, and in this regime, this dynamics is determined
by thermodynamically allowed rate of formation of critical nuclei which in turn
depends on the cooling field and available metastable phase fraction. At a
given temperature, a decrease in metastable phase fraction shifts the crossover
from arrested to un-arrested regimes towards lower cooling field. It is rather
significant that inspite of the metastable phase fraction calculated from
resistivity being somewhat off from that of magnetization, their cooling field
dependence exhibits a striking similarity which indicate that the dynamics in
arrested and un-arrested regimes are so different that it comes out vividly
provided that the measurements are done around percolation threshold.Comment: 7 pages, 5 figure
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