1,267 research outputs found
Group-wise penalized estimation schemes in model-based clustering
Gaussian mixture models provide a probabilistically sound clustering approach.
However, their tendency to be over-parameterized endangers their utility
in high dimensions. To induce sparsity, penalized model-based clustering strategies
have been explored. Some of these approaches, exploiting the link between
Gaussian graphical models and mixtures, allow to handle large precision matrices,
encoding variables relationships. By assuming similar components sparsity levels,
these methods fall short when the dependence structures are group-dependent. Our
proposal, by penalizing group-specific transformations of the precision matrices, automatically
handles situations where under or over-connectivity between variables
is witnessed. The performances of the method are shown via a real data experimen
Penalized Model-Based Clustering with Group-Dependent Shrinkage Estimation
Gaussian mixture models (GMM) are the most-widely employed approach to perform model-based clustering of continuous features. Grievously, with the increasing availability of high-dimensional datasets, their direct applicability is put at stake: GMMs suffer from the curse of dimensionality issue, as the number of parameters grows quadratically with the number of variables. To this extent, a methodological link between Gaussian mixtures and Gaussian graphical models has recently been established in order to provide a framework for performing penalized model-based clustering in presence of large precision matrices. Notwithstanding, current methodologies do not account for the fact that groups may be under or over-connected, thus implicitly assuming similar levels of sparsity across clusters. We overcome this limitation by defining data-driven and component specific penalty factors, automatically accounting for different degrees of connections within groups. A real data experiment on handwritten digits recognition showcases the validity of our proposal
Angular dependence of the magnetization of isotropic superconductors: which is the vortex direction?
We present studies of the dc magnetization of thin platelike samples of the
isotropic type II superconductor PbTl(10%), as a function of the angle between
the normal to the sample and the applied magnetic field . We determine
the magnetization vector by measuring the components both parallel
and normal to in a SQUID magnetometer, and we further decompose it in
its reversible and irreversible contributions. The behavior of the reversible
magnetization is well understood in terms of minimization of the free energy
taking into account geometrical effects. In the mixed state at low fields, the
dominant effect is the line energy gained by shortening the vortices, thus the
flux lines are almost normal to the sample surface. Due to the geometrical
constrain, the irreversible magnetization remains locked to the
sample normal over a wide range of fields and orientations, as already known.
We show that in order to undestand the angle and field dependence of the
modulus of , which is a measure of the vortex pinning, and to
correctly extract the field dependent critical current density, the knowledge
of the modulus and orientation of the induction field is required.Comment: 11 pages, 6 figure
Primary gas thermometry by means of laser-absorption spectroscopy: Determination of the Boltzmann constant
We report on a new optical implementation of primary gas thermometry based on
laser absorption spectrometry in the near infrared. The method consists in
retrieving the Doppler broadening from highly accurate observations of the line
shape of the R(12) transition in
CO gas at thermodynamic equilibrium. Doppler width measurements as a
function of gas temperature, ranging between the triple point of water and the
gallium melting point, allowed for a spectroscopic determination of the
Boltzmann constant with a relative accuracy of .Comment: Submitted to Physical Review Letter
Equilibrium tuned by a magnetic field in phase separated manganite
We present magnetic and transport measurements on La5/8-yPryCa3/8MnO3 with y
= 0.3, a manganite compound exhibiting intrinsic multiphase coexistence of
sub-micrometric ferromagnetic and antiferromagnetic charge ordered regions.
Time relaxation effects between 60 and 120K, and the obtained magnetic and
resistive viscosities, unveils the dynamic nature of the phase separated state.
An experimental procedure based on the derivative of the time relaxation after
the application and removal of a magnetic field enables the determination of
the otherwise unreachable equilibrium state of the phase separated system. With
this procedure the equilibrium phase fraction for zero field as a function of
temperature is obtained. The presented results allow a correlation between the
distance of the system to the equilibrium state and its relaxation behavior.Comment: 13 pages, 5 figures. Submited to Journal of Physics: Condensed Matte
Persistent X-Ray Photoconductivity and Percolation of Metallic Clusters in Charge-Ordered Manganites
Charge-ordered manganites of composition exhibit persistent photoconductivity upon
exposure to x-rays. This is not always accompanied by a significant increase in
the {\it number} of conduction electrons as predicted by conventional models of
persistent photoconductivity. An analysis of the x-ray diffraction patterns and
current-voltage characteristics shows that x-ray illumination results in a
microscopically phase separated state in which charge-ordered insulating
regions provide barriers against charge transport between metallic clusters.
The dominant effect of x-ray illumination is to enhance the electron {\it
mobility} by lowering or removing these barriers. A mechanism based on magnetic
degrees of freedom is proposed.Comment: 8 pages, 4 figure
Properties of charge density waves in LaBaCuO
We report a comprehensive x-ray scattering study of charge density wave
(stripe) ordering in , for which the
superconducting is greatly suppressed. Strong superlattice reflections
corresponding to static ordering of charge stripes were observed in this
sample. The structural modulation at the lowest temperature was deduced based
on the intensity of over 70 unique superlattice positions surveyed. We found
that the charge order in this sample is described with one-dimensional charge
density waves, which have incommensurate wave-vectors (0.23, 0, 0.5) and (0,
0.23, 0.5) respectively on neighboring planes. The structural
modulation due to the charge density wave order is simply sinusoidal, and no
higher harmonics were observed. Just below the structural transition
temperature, short-range charge density wave correlation appears, which
develops into a large scale charge ordering around 40 K, close to the spin
density wave ordering temperature. However, this charge ordering fails to grow
into a true long range order, and its correlation length saturates at , and slightly decreases below about 15 K, which may be due to the onset
of two-dimensional superconductivity.Comment: 11 pages, 9 figure
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